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Development of Biobased Poly(L-Lactide)/Polyamide Blends with Improved Interfaces and Thermo-Mechanical Properties for High-Performance Applications Amulya Raj

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Amulya Raj. Development of Biobased Poly(L-Lactide)/Polyamide Blends with Improved Interfaces and Thermo-Mechanical Properties for High-Performance Applications. Polymers. Ecole nationale supérieure Mines-Télécom Lille Douai, 2019. English. NNT : 2019MTLD0011. tel-03039646

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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. NETN° d’ordre : 2019MTLD0011

THÈSE

présentée en vue d’obtenir le grade de

DOCTEUR

Génie des matériaux par

Amulya RAJ

DOCTORAT DE L’UNIVERSITÉ DE LILLE DELIVRÉ PAR IMT LILLE DOUAI

DEVELOPMENT OF BIOBASED POLY(L-LACTIDE)/POLYAMIDE BLENDS WITH IMPROVED INTERFACES AND THERMO-MECHANICAL PROPERTIES FOR HIGH-PERFORMANCES APPLICATIONS

Soutenance le 3 Décembre 2019 devant le jury d’examen :

CERI Matériaux et Procédés IMT Lille Douai Ecole Doctorale SMRE 104 (Lille I, Lille II, Artois, ULCO, UVHC, Centrale Lille, Chimie Lille, IMT Lille Douai)

Chapter 1

NETN° d’ordre : 109378

THÈSE

présentée en vue d’obtenir le grade de

DOCTEUR

Génie des matériaux par

Amulya RAJ

DOCTORAT DE L’UNIVERSITÉ DE LILLE DELIVRÉ PAR IMT LILLE DOUAI

DEVELOPMENT OF BIOBASED POLY(L-LACTIDE)/POLYAMIDE BLENDS WITH IMPROVED INTERFACES AND THERMO-MECHANICAL PROPERTIES FOR HIGH-PERFORMANCES APPLICATIONS

Soutenance le 3 Décembre 2019 devant le jury d’examen :

President GAUCHER Valérie, Pr. Université de Lille Rapporteur RAQUEZ Jean-Marie, Pr. Université de Mons Rapporteur CHALAMET Yvan, Pr. Université Jean Monnet Saint-Etienne Examinateur FATYEYEVA Kateryna, Dr.HDr Université de Rouen Directeur de PRASHANTHA Kalappa, Dr.HDr IMT Lille Douai thèse Encadrant SAMUEL Cedric, Dr. IMT Lille Douai Laboratoire d’accueil CERI Matériaux et Procédés IMT Lille Douai Ecole Doctorale SMRE 104 (Lille I, Lille II, Artois, ULCO, UVHC, Centrale Lille, Chimie Lille, IMT Lille Douai)

Chapter 1

ACKNOWLEDGEMENTS

I would like to express my deep gratitude to Dr.Prashantha Kalappa and Dr.Cedric Samuel for providing me a wonderful platform to conduct my thesis in IMT-Lille-Douai, France. My special thanks to my thesis supervisor Dr.Cedric Samuel for his guidance, suggestions, encouragement and the time he invested throughout my thesis. Thank you for the Friday meetings, that helped me improve my presentation skills further and the continuous evaluation and scientific discussion further enhanced my interest in thesis. I offer my sincere thanks to Dr. Patricia KRAWCKZAK head of the department of TPCIM, IMT-Lille-Douai for welcoming me to her team and her encouragement during my thesis. I would like to thank Dr.Valerie Gauchier for being the president of the jury of my theis defense. I would like to thank Dr.Jean-Marie Racquez and Dr.Yvan Chalamet for accepting our invitation to be the reporters in jury for my thesis defense. I would like to thank for their valuable time. I would like to thank Dr.Kateryna Fatyeyeva for accepting our invitation to be the examinater in the jury for my thesis defense. I offer my sincere thanks to Carlo ANGOTZI for helping me several times while working with Coperion extruder and all the other times I needed technical help. I would like to thank Laurent CHARLET, Pierre BEDART, Jean-Pierre MIMARD, Maxime DORCHIES and Xavier DORCHIES for their technical help throughout this work. My special thanks to Murielle DELSERT and Dominique KOCZOROWSKI for being so kind, for their quick responses. I would even like to thank them for all the help they provided during these 3 years. Thank you Murielle for speaking French slowly so that I understood and for bearing my French. I would also like to thank Dr. Malladi NAGALAKSHMAIAH for his guidance and mentoring me during the final phase of my thesis. Your suggestions and support helped me a lot to improve myself and my work. I would like to thank my dear friend Anurag for being there from the day one, helping me, listening to all my problems and encouraging me throughout this tedious journey. My special thanks to Keerthi Krishna for being an amazing friend, a food partner and for always listening to all my stories. I would like to thank Aniket for his friendship, patiently listening and encouraging me during my final year. I would like to thank Antoine for being the best office neighbor ever. I would also like to thank Helene for her friendship and the amazing talks we had during these 3 years. I would also like to thank Sebastien, Nazim and Jennifer for their warm welcome and help. My special thanks to Axel for helping me learn French, support and his 5

Chapter 1 friendship. My special thanks to Caroline for her friendship, cheering me up with her positive attitude and inspiring me never to stop achieving my goal. She has been my confidante here and I thank her from heart for that. I would like to thank Julie for her friendship, harry potter marathon and support in my first year of thesis. I would like to thank Anuprita for her friendship, constant support and her help towards the end of my thesis and reducing my stress levels. Special thanks to Sutapa, Bheems and Rakesh for being just a phone call away when I needed them. Finally last but not the least I would like to thank my parents for their everlasting support, encouragment and love which made all this possible. I would like to thank my fiancé Abhilash for his constant support and for all the strength when I was discouraged or stressed. Thank you for helping me throughout my thesis without complaining, thank you for the sensible advices and unconditional love without which it would be very difficult to finish this thesis.

Chapter 1

Contents Abbreviations ...... 9 1 Introduction ...... 11 2 State of the art ...... 15 PLA overview ...... 15 PLA markets and trends ...... 17 Strategies to overcome drawbacks of PLA ...... 18 2.3.1 Plasticization ...... 18 2.3.2 Co-polymerisation...... 20 2.3.3 Nucleating agents and nanocomposites ...... 20 2.3.4 Impact modifiers ...... 21 2.3.5 Stereocomplexation of Poly(Lactide) ...... 22 2.3.6 Miscellaneous approaches ...... 22 Overview of Polymer blends ...... 23 2.4.1 Thermodynamics of blends ...... 23 2.4.2 Processing methods of polymer blends...... 24 PLA based blends ...... 25 2.5.1 PLA/PMMA blends ...... 27 2.5.2 PLA/Polycarbonate blends...... 33 2.5.3 PLA/semi-aromatic Polyester blends ...... 40 2.5.4 PLA/Polyamide blends ...... 45 Conclusions ...... 55 Objectives and Workplan (Article organization) ...... 56 3 Compatibility in Biobased Poly(L-Lactide)/Polyamide Binary Blends: From Melt-State Interfacial Tensions To (Thermo)Mechanical Properties ...... 59 Introduction ...... 60 Experimental section ...... 62 3.2.1 Materials ...... 62 3.2.2 Processing of PLA/PA Blends ...... 63 3.2.3 Characterizations...... 63 Results and Discussions ...... 66 3.3.1 Melt-State Compatibility of PLA/PA Blends ...... 66 Tensile Mechanical Properties ...... 72 Impact Properties and Thermal Resistance of PLA/PA Blends ...... 75 Conclusions ...... 78 4 Tuning of properties of PLA/PA12 blends with the addition of compatibilizer...... 81 Introduction ...... 81

Chapter 1

Experimental Section ...... 83 4.2.1 Materials ...... 83 4.2.2 Processing of PLA/PA Blends ...... 84 4.2.3 Characterizations...... 84 Results and Discussions ...... 86 4.3.1 Morphology...... 86 4.3.2 Mechanical Properties ...... 88 4.3.3 Morphology of injection molded samples-correlation with mechanical properties 91 4.3.4 (Thermo)mechanical properties ...... 94 4.3.5 FTIR studies ...... 97 Conclusions ...... 98 5 High Shear Extrusion Processing of Poly(L-Lactide) and Polyamide-12 Blends: Effect of Screw Speed and Feed Rate on (Thermo)Mechanical Properties ...... 101 Introduction ...... 102 Experimental Section ...... 104 5.2.1 Materials ...... 104 5.2.2 Processing of PLA/PA Blends ...... 104 5.2.3 Characterization methods...... 105 Results and Discussion ...... 108 5.3.1 Tensile properties of as-produced PLA/PA12 blends ...... 108 Impact properties of as-produced PLA/PA12 blends ...... 111 Thermomechanical properties of as –produced PLA/PA12 blends ...... 112 Morphology of as-produced PLA/PA12 blends ...... 114 Discussion on the effect of extreme screw speed ...... 118 Conclusions ...... 120 6 Conclusions ...... 123 Perspectives ...... 125 7 APPENDIX ...... 127 List of figures ...... 135 List of tables ...... 138 8 References ...... 140

Chapter 1

Abbreviations

ABS/PC-Acrylonitrile-butadiene-styrene/polycarbonate ADR random copolymer of styrene and glycidyl methacrylate ASAA - alkenyl-succinide-anhydride-amide ASAI - alkenyl-succinic-anhydride-imide BH – Benoylhydrazide BN - boron nitride

CaCO3 - Calcium Carbonate CL - Caprolactum CNT - Carbon nanotubes DMBH - Decamethylenedicarboxylic dibenzoylhydrazide EBS - N,N’-ethylene bis-stearamide EOR-MAH - poly(ethylene-co-octene) ruber maleic anhydride EGMA - poly(ethylene-co-glycidyl methacrylate) ESAC - epoxy functionalized styrene-acrylate copolymer ECE - multifunctional epoxies G-POSS - glycidyll isooctyl polyhedral oligomeric silsesquioxane HDT - Heat deflection temperature OMBH - Octamethylenedicarboxylic dibenzoylhydrazide OMMT - Organo-Montmorillonite PLA - Poly(Lactide) or Polylactic acid PLLA - Poly(L-Lactide) PDLA - Poly(D-Lactide) PDLLA - Poly(D, L-Lactide) PEG - Polyethyleneglycol P[CL-co-LA] - Polycaprolactum-co-lacticacid (40,41,44) P[Cl-co-VA] - Polycaprolactum-co-valarectone PMMA - Polymethyl methacrylate PA - Polyamide PS - polystyrene PBA - polybutylacrylate PBSL - poly(butylene succinate-co-l-lactate)

Chapter 1

PBT - poly(butylene terephthalate) PC - polycarbonate PES - poly(ethylene succinate) PET - polyethylene terephthalate PVDF - polyvinylidene fluoride PVP - poly(vinylpyrrolidone) PVPh - poly(vinylphenol) PEBA-GMA - poly(ethylene n-butylene acrylate glycydyl methacrylate PTT - polytrimethylene terephthalate PA6 - polyamide 6 PA (6-10) - polyamide 6-10 PA 11 - polyamide 11 PA12- polyamide 12 PA (10-10) - polyamide 10-10 PA (6-6) - polyamide 6-6 PCD – polycarbodiimide ® PLArex - modified PLA with Jonacryl-ADR-4300 epoxy-based resin POE-g-MAH - polyethylene octene grafted maleic anhydride ROP - Ring-opening polymerization SAN-g-MAH - poly(styrene-co-acrylonitrile)-g-maleic anhydride SEM - scanning electron microscopy SMA - styrene maleic anhydride copolymer T-POSS - tetrasilanol phenyl polyhedral oligomeric silsesquioxane TGDDM - N,N,N’,N’=tetraglycidyl-4,4’-diaminodiphenyl methane TPU - thermoplastic polyurethane TsOH - p-toulene sulfonic acid

Tg - Glass transition temperature UCST - upper critical solution temperature VLδ Valarectone

Chapter 1

1 Introduction The innate need to produce biopolymers has greatly increased in the last decade and this is mainly due to the depletion of fossil fuels and environmental concerns that have led to the quest for development of bioplastics. Bioplastic is a broad term and it can be classified into three categories; biobased, biodegradable or both biobased and biodegradable [1,2]. According to European bioplastics, the production of bioplastics is nearly 2.11 million tons of the plastics in the market every year. It accounts for less than 1% of the 335 million tons of synthetic plastics produced every year [3]. Nonetheless, the bioplastic market is mounting by 20–30% each year due to versatile applications such as packaging, cosmetic products, agriculture, and horticulture. The applications of bioplastics are slowly shifting into engineering applications such as electronic components (computer casings, mobile casings, mouse and keyboard elements) and in automotive sector partly biobased plastics are being used for interior and exterior features (dashboard components, steering wheel, seat, and airbag) [4]. Poly (lactic acid) (PLA) has gained significant attention due to its propitious properties such as renewability, biodegradability, biocompatibility and comparable stiffness and strength [5–12]. All these attributes of PLA lead to development of green packaging, medical implants and biomedical applications which are currently the focus of industrial and scientific research throughout the world. Intensive and elaborate reviews about PLA synthesis, properties and applications have been previously reported [9,13–18].

Despite the accelerating advancements of PLA, it has some shortcomings such as low ductility, poor toughness, low heat deflection temperature, slow degradation rate and hydrophobicity [10,19–22]. These limitations hinder the application of PLA in engineering and high-end applications. High-end or durable applications are those in which the material possesses properties enabling them to perform for prolonged use in structural applications, over a wide temperature range, under mechanical stress, and in difficult chemical and physical environment. Acrylonitrile-butadiene-styrene/polycarbonate (ABS/PC) blends are widely used in electronic and automotive industries due to their set of balanced properties; heat resistance, toughness, and easier processing. The HDT of this material ranges from 90-110°C, Izod impact strength at room temperatures ranges from 54-60kJ/m2, tensile strength of 41MPa and elongation at break above 45%. These properties may vary according to applications. Nevertheless, as this material has a wide range of engineering applications, these properties can be considered as specifications required for high-value applications. [23,24]. Various strategies have been proposed previously to overcome these drawbacks such as physical and

Chapter 1 chemical modifications, toughening mechanisms and polymer blending. A comprehensive general review about PLA toughening mechanisms, physical and chemical modifications have been widely discussed in the past [8,17,25–29]. Although there are various strategies to improve the properties of PLA, polymer blending stands out owing to its cost-effectiveness, ease of commercial production, viability, and solvent-free process. Recently, few reviews have been reported on PLA based blends by Nofar.M et al. and Hamad.K et al [10,20]. However, the key focus of the present study is on development of PLA blends with polyamide in order to improve the ductility, toughness and thermal resistance of properties of PLA; making it suitable for industrial and high-end applications with engineering polymers. This thesis highlights the use of commercially available biobased polymers which emerges as a new range of products for different applications. Special emphasis is made on melt blending of PLA with bio sourced polyamides to develop an understanding of the melt processability, physical and thermo-mechanical properties.

The major objectives of the thesis were to develop polymer blends of biobased PLA and PA. The initial focus was on the screening of different polyamides among the various bio-based polyamides (PA10-10, PA10-12, and PA11) and PA12. To further enhance the (thermo)mechanical properties of the PLA and the selected PA blends, two different methodologies were attempted; classical compatibilization and optimising the extrusion parameters such as screw speed and feed rate.

The subject material of the thesis is presented in the following chapters.

Chapter 1 is introductory in nature. In this chapter, an attempt was made to briefly explain the general context of the present work and our objectives.

Chapter 2 provides a detailed literature survey emphasizing on the importance of improving PLA properties via melt blending with various engineering thermoplastics with supportive literature.

In Chapter 3, a comprehensive description about the compatibility of biobased PA10-10, PA10-12, PA11 and non biobased PA12 with PLA and its effect on (thermo)mechanical properties are presented.

Chapter 4 highlights the enhancement of PLA/selected PA blend properties via compatibilization strategy. Particular attention has been given to the improvement in properties such as ductility, toughness and thermal resistance.

Chapter 1

Chapter 5 concentrates on the second strategy employed for the improvement of PLA/selected PA blend properties; extrusion parameters (screw speed and feed rate). The effect of varying screw speed and feed rate on PLA/selected PA blend’s ductility, toughness, and thermal resistance.

Chapter 6 summarizes the entire findings of the thesis and some perspectives are presented

All the references are presented in Chapter 7.

A list of Figures and Tables are presented at the end of the thesis and some additional information is presented in Appendix.

Chapter 1

Chapter 2

2 State of the art This literature survey covers a comprehensive review of PLA based blends with engineering polymers with special focus on properties such as ductility, impact strength, and thermal resistance. Brief gist of PLA overview, polymer blending and other strategies to overcome the drawbacks of PLA have been discussed.

PLA overview Several bioplastics have been introduced in the market and among these Poly(lactic acid) (PLA) holds a major part close to10.3% (Figure 2.1) [3]. Albeit the discovery of PLA was in 1932 by Carothers (Dupont), the inability to achieve high molecular weight inhibited further progress. However, today there are numerous ways to synthesize PLA. The major ways to produce PLA is either by ring-opening polymerization (ROP) of lactides or by polycondensation of the lactic acid monomers.

Figure 2.1. Global production of bioplastics (Slightly modified and printed from [3])

The monomers to synthesize PLA are obtained by the fermentation of corn, beet-sugar and cane-sugar [10,12,15,17–19,21,30–32]. The parameters such as temperature, pH and pressure to synthesize PLA require rigorous controlling making it a difficult process [15,33]. The detailed procedure of the PLA production is shown in the schematic diagram in Figure 2.2.

Chapter 2

Figure 2.2. Poly(Lactide) life cycle

Poly(Lactide) is an aliphatic polyester; due to the presence of the chiral molecule in the monomer (Lactic acid:2-hydroxy propionic acid), two enantiomers are present namely D-lactic acid and L-Lactic acid. Hence, PLA exists as three different stereoisomers viz. PDLA, PLLA, and PDLLA with respect to the enantiomer. The previously mentioned promising properties such as biocompatibility, biodegradability, and resorbability led to significant applications in medical science and biotechnology (for e.g. sutures, surgical implants, and drug delivery systems) [10,17,33,34] and intensive research is being carried out about PLA for tissue engineering scaffolds [13,35–37]. The ease of removal of PLA by the body system and retention of shape in the system has made it suitable for these applications. Another potential application of PLA is packaging, which has been beneficial to solve the ecological problem of plastic waste accumulation [14,38–40]. The use of PLA in food packaging has already received significant attention [39–41] from industries and currently, PLA-based containers for water, juice, and yogurt are used in Europe, Japan, and North America as supermarket products. PLA based packaging materials have been Generally Recognized As Safe (GRAS) which put it in an exceptional position for food applications [42]. The application of PLA based materials extends to waste-composting bags, mulch films, controlled release matrices for fertilizers, pesticides, and herbicides [21,43]. Over the past few years PLA has ventured into niche applications such as advanced textiles filaments and fibers, shape memory materials, 3D

Chapter 2 printing and piezoelectric devices due to its unique set of properties. The properties such as good mechanical strength, biocompatibility and low refractive index which is advantageous for producing brighter shades make it well suited for, advanced textiles filaments and fabrics [44,45]. Shape memory polymers are another distinctive application in which PLA was found to be suitable due to its crystallization behavior and physical entanglements which can correspond to shape fixing and shape recovering parts [46]. Some limitations such as deformation above 10% strain and glass transition temperature of 60°C make it unsuitable for its application [46]. However, by melt blending it with polymers such as PMMA [47–49], PEG [50–52] and TPU [53–55] and by block copolymerization with other monomers [46] has led its successful application in shape memory materials. Since the advent of 3-D printing it has made remarkable progress in various fields wherein polymers are the most utilized class of materials [56]. PLA is one of the widely used materials for 3 D printing due to its biocompatibility, processability, and rigidity [56–61]. PLA is well for piezoelectric device due to its transparency, flexibility, and biodegradability. PLA exhibits piezoelectricity by thermal stretching leading to the alignment of molecular chains along the stretched direction [62–64]. In general, the fabrication of other piezoelectric materials requires a complex process known as poling, whereas for PLA this step can be omitted making it more advantageous for industrial applications [64]. A schematic representation of PLA source, preparation, properties, and applications is depicted in Figure 2.2.

PLA markets and trends After Dupont patented the production of PLA in 1954 sincere efforts were made to commercialize PLA by numerous companies such as Nature Works, Nova Evonik industries, Rodenburg Biopolymers, Cargill Inc, Carbion, Pyramid Technologies, WeforYou, Dupont Corporation and Braskem [21]. However, Nature Works LLC stands as a major producer of PLA. Over the past two decades, Nature Works Company developed and produced the different grades of PLA such as Poly(L-Lactide)(PLLA), Poly(D-Lactide)(PDLA) and PLA fibers. Today numerous grades of each type are available and are being used for wide range of applications. Due to the growing demand and interest of PLA, the production of PLA has grown rapidly over the years. The statistical progress of PLA from 2011 to 2020 is shown in Figure 2.3. According to Nova Institute’s statistics, PLA production is expected to reach 800,000 tons by 2020 [3]. PLA based products have already been commercialized in packaging and biomedical devices and recently they are being explored in durable applications as well. NatureWorks has recently produced a new series targeted specifically for 3D printing market

Chapter 2 namely Ingeo series, these series of materials are suitable material for durable applications due to its low thermal shrinkage, good processability, printability, improved impact strength and heat resistance [65].

Figure 2.3. Evolution of PLA production from 2011 to 2020 adapted from [3]

Strategies to overcome drawbacks of PLA Despite enormous research interest and production sites, the usage of PLA in durable applications is limited in terms of mechanical and thermal properties. In order to overcome the fore-mentioned issues, numerous strategies have been investigated to improve the properties of PLA such as impact resistance, ductility, crystallinity, heat resistance, hydrophilicity, heat deflection temperature and biodegradation rate [25,28] by physical and chemical modification. In particular, toughening of PLA has gained significant attention in recent years and is attained by different methods namely i) Plasticization (adding toughening agents), ii) copolymerization, iii) nucleating agents iv) PLA nanocomposites v)stereo complexation and vi) polymer blending [8,26,66,67]. The detailed description of these strategies is discussed in the following sections.

2.3.1 Plasticization One of the well-versed materials used to increase the toughness of PLA are plasticizers. According to the literature two different kinds of plasticizers often used i) low molecular weight /oligomers(short-chain) ii) polymeric plasticizers [8,27]. In addition to enhancement of the processability, plasticizers improve the flexibility and ductility of glassy polymers. The choice of plasticizer is a crucial step; a preferred plasticizer for PLA should significantly reduce the glass transition temperature (Tg) of PLA, should be bio compostable, nonvolatile, nontoxic and exhibit minimal leaching or migration during aging [27]. Several citrate esters including

Chapter 2 triethyl citrate, tributyl citrate, acetyltriethyl citrate, and acetyltributyl citrate, glycerin triacetate and bis(2-ethyl-hexyl) adipate have been used as plasticizers to enhance the toughness of PLA [27,68,69]. It was reported that for the low molecular weight plasticizers, 10-20 wt.% of plasticizer brought about the maximum improvement in mechanical properties. However, phase separation is observed above 20 wt.% and diminishing the mechanical performance [27]. Although the efficiency of low molecular weight plasticizers is much better, they tend to evaporate during melt processing and migrate to the surface causing an issue for storage. Polymeric plasticizers such as polyethylene glycol of different molecular weight, poly(ether glycol) methyl ether acrylate, poly(1,3-butanediol), dibutylsebacate, acetyl glycerol monolaurate, poly(1,3-butylene glycoladipate) can also be used as plasticizers [8,27]. Polyethylene glycol was found to be the most efficient polymer plasticizer. Nevertheless above 20% of this plasticizer decreased the properties including ductility, impact strength, and phase separation was observed [8]. This might be due to the threshold limit of the filler presence in the matrices. Since both low molecular weight plasticizers and polymeric plasticizers have their demerits, combinations of both were also attempted. A 1/1(w/w) mixture of glycerin triacetate and oligomeric poly(1,3-butylene glycoladipate was utilized as a plasticizer in PLA and it was reported that a ductile behavior was achieved above 9wt.% of plasticizer. However, a decrease in tensile strength was observed with the gradual increase of this mixed plasticizer [70]. Lemmouchi et al. made an interesting observation upon using a combination of 1/1(w/w) tributyl citrate and poly(D,L-LA)-b-poly(ethyleneglycol) copolymer (PLA-b-PEG) with various molecular architecture as plasticizer in PLA [71]. Balanced mechanical properties were observed without the major reduction in the tensile strength and modulus with increase in ductility of >220% at 20wt.% of plasticizer. In addition to this a depression in Tg and an improvement in impact strength was also observed [71].Green plasticizers such as epoxidized soybean oil, cardanol, wood flour and oligomers obtained from sunflower biodiesel have also been incorporated as plasticizers in PLA [72–76]. The inclusion of these green plasticizers resulted in the improvement of ductility and impact strength. The addition of epoxidized soybean oil as a plasticizer improved the fore mentioned properties; this toughening behavior was found to be due to partial miscibility of the two phases. However, above 5wt.% of epoxidized soybean oil significant reduction in the modulus and tensile strength was observed, this decrease in the properties is due to the plasticization effect which in turn reduced the Tg of the system [75]. In conclusion, plasticizers have proven to be efficient in increasing the ductility and toughness. To achieve a significant increase in these properties a higher content of plasticizer (>20wt.%) is required and this accompanies with a major reduction in tensile 19

Chapter 2 strength and modulus. Phase separation and faster cold crystallization are also observed after plasticization of PLA. Hence, an effort has to be made to design an apt and optimal system of plasticizers for maintaining balanced thermo-mechanical properties of PLA.

2.3.2 Co-polymerisation Copolymerization is another extensively investigated technique to improve hydrophilicity, impact, ductility and tensile properties of PLA. Copolymerization of PLA can be obtained either through polycondensation of lactic acid with other monomers or ring-opening copolymerization of lactic acid with other cyclic monomers. As the latter synthesis route gives more precise control of chemistry and higher molecular weight of copolymers, it is a preferred method [27].The hydrophilicity of the PLA was improved by copolymerizing with polyethylene glycol (PEG) [17]. PEG is a highly biocompatible, nontoxic and nonimmunogenic polymer with excellent hydrophilicity. Mainly, copolymerization of PLA with PEG is used in tissue engineering and drug delivery applications [17,77,78]. It is known to decrease the attractive forces between solid surfaces and proteins because of its highly hydrated polymer chains, steric stabilization forces, as well as chain mobility [17]. Such properties can make a surface highly resistant to biological fouling and reduce protein adsorption and resistance to bacterial and animal cell adhesion. Since PEG is highly soluble in water and many organic solvents, it can also be easily removed from the tissue [79,80]. In addition, since it has two hydroxyl groups with reactive ends, the usage of PEG as the macromonomer to improve the hydrophilicity and the biocompatibility of PLA is a good choice [17].

2.3.3 Nucleating agents and nanocomposites Nucleating agents were mainly used to improve the toughness, heat resistance and crystallinity of PLA. In this context different nucleating agents namely talc, N,N′-ethylene bis-stearamide (EBS), carbon nanotubes, metal salts of phenylphosphonic acid, multiamide and hydrazide compounds, barium sulfate, titanium dioxide, calcium carbonate (CaCO3) and nano-CaCO3 can further improve the toughness of PLA [8]. Numerous investigations have been conducted on improving crystallization of PLA with the help of nucleating agents. In an another study commercial heterogeneous multiamide nucleating agent was used to enhance the heat resistance of PLA at a very small concentration of 0.2% [81]. Other organic nucleating agents such as Benoylhydrazide (BH) compounds, octamethylenedicarboxylic dibenzoylhydrazide (OMBH) and decamethylenedicarboxylic dibenzoylhydrazide (DMBH) are known to impart enhancement in the crystallization of PLA [82]. In order to improve the crystallinity, annealing

Chapter 2 has been directly tried in the molding process such as in injection molding, this technique can be called as an in-mold annealing process, where the cooling time is increased to facilitate effective demolding of the samples. This process eliminates the post annealing step thereby saving the excess time and energy consumed by this. Higher injection mold temperature (110°C) led to PLA molded articles with high percentage of crystallinity at high heat deflection temperature (HDT). However, the problem with this step is molding cycle time of around 2 min is required due to higher cooling time. Hence, demolding of the processed components would be difficult with short cooling cycle [83]. Biobased nucleating agents including cellulose nanocrystals, silylated nanocrystals and chemically modified thermoplastic starch have been incorporated in PLA and improvement in crystallinity, modulus and tensile strength were reported [84,85].

Nanofillers (cellulose nanofibers and crystals, chitosan nanocrystals, etc) and nanometric scale PLA copolymers as fillers are often used to prepare the PLA nanocomposites. Initially the application of PLA nanocomposites was only limited to biomedical, biotechnological and packaging [86]. However, with the increasing demand to find an alternative source even for the high-end applications, several PLA nanocomposites have been prepared. Depending on the end use requirement PLA nanocomposites have proven to be an excellent method to improve synthetic bone substitute and repair, tissue engineering, drug delivery systems along with thermal, mechanical and impact properties. Several nanofillers such as CNT, hallosites, OMMT, and graphene have been utilized in PLA or PLA copolymers [87–89].

2.3.4 Impact modifiers Impact modifiers have been used in PLA to improve the toughness, various impact modifiers have already been commercialized [86]. Impact modifiers used for PLA can be elastomers or linear polymers with low Tg, polymers with core and shell wherein the core is the soft rubbery part and is responsible for toughness and shell is the rigid part and is responsible for good compatibilization and interfacial interactions. Poly(ether-block-amide) (PEBA) is a commercial copolyester elastomer obtained from Arkema. It is an effective impact modifier and exhibits excellent resistance to impact even at subzero temperatures up to -40°C. However, higher content up to 30wt.% is required to bring about a major increase in the impact strength of PLA [90]. In another study, PEBA grafted GMA was utilized as an impact modifier in PLA and thermoplastic starch acetate blends (TPSA). A notched Izod impact strength of around 60kJ/m2 was observed for the PLA/TPSA/PEBA-g-GMA of 70/15/15 compositions [91]. Another commercial impact modifier incorporated in PLA was Biomax® strong 100. The

Chapter 2 impact strength improved considerably for semicrystalline PLA when compared to amorphous PLA [86]. Random biodegradable copolyesters; caprolactum (CL) with D,L-lactide, (P[CL-co- LA]) and CL with δ-valarectone (VL), (P[CL-co-VL]) have been used as impact modifiers for PLA and impact strength of 39.7kJ/m2 [66,67,92]. Several other elastomeric polymers such as POE catalyzed with matellocene grafted with GMA, Sukano® and PolyOne have also been proven to impart toughness in PLA [86]. Despite the significant increase in impact strength by adding impact modifiers in PLA, impact modifiers have some major limitations such as reduction of optical clarity, modulus and are mostly non-biodegradable [86,90]. To attain a proper toughness stiffness balance several parameters such as dispersion of impact modifier in PLA, interfacial adhesion and size of the dispersed phase need to be optimized to find a suitable impact modifier [86,90].

2.3.5 Stereocomplexation of Poly(Lactide) The interaction between polymers possessing varying tacticities leading to stereoselective association is known as stereocomplexation or stereocomplex formation. Lactic acid is a chiral molecule hence PLA exists as stereo enantiomers (D and L-form). The Stereo-complexation of PLLA and PDLA has led to enhanced physical properties, mechanical properties, thermal resistance and hydrolytic stability [93]. In a recent study, new stereo-chemical crystal structure was developed by blending equimolar ratios of PLLA and PDLA. The stereo-complexation of these blends enhances the melting temperature from 180 to 230°C. It is worth to note that the individual form of PLLA and PDLA have a melting temperature 50°C lower compared to fore mentioned [94]. Modulation of wide range of properties was achieved by stereo complexation of PLA compared to neat PLA. The recent advances in polymerization techniques have given rise to wide array of new functional PLA-based copolymers. The stereo complex formation between the enantiomers in these newly developed PLA based copolymers have brought about tailored properties for biomedical, tissue engineering and engineering plastics applications [93,95]. Comprehensive reviews on this topic have already been reported [93,96–98].

2.3.6 Miscellaneous approaches Mixed strategies such as the addition of both nucleating agents with plasticizers, stereocomplexation of PLA and plasticizers have been tried by several researchers over recent years. Moser K et al. studied the effect of a combination of sorbitol and PEG in PLA, this combination of nucleating agent and plasticizer improved HDT to 75°C and Charpy impact strength of the un-notched samples to 131kJ/m2 [99]. However, a decrease in modulus and tensile strength was observed for higher content of PEG, the best stiffness toughness balance

Chapter 2 was observed for 1wt.% of sorbitol and 10wt.% of PEG [99]. Other nucleating agents such as

CaCO3, talc, sulfates, and HNT in combination with PEG as plasticizer were added in PLA. The stiffness toughness balance was achieved by these combinations as nucleating agents act as reinforcement and PEG enhances the chain mobility [100]. PLA stereocomplex crystallites (SC) can be used as nucleating agents in PLA. These improve the crystallinity and thermal resistance of PLA. SC can be used as nucleating agents in PLLA/PDLA blends and it was observed that faster crystallization rates of PLLA were achieved [84]. Further research is required to create an apt combination of strategies to improve the drawbacks of PLA.

Overview of Polymer blends A polymer blend is a mixture of two or more polymers to create a new material with versatile properties. Polymer blending technology has grown tremendously over the past decade. The multiphase polymer blends depend on two major concepts; interface between the two entities and the morphology. Morphology of a polymer blend indicates the size, shape and spatial distribution of the component phases with respect to each other and interface between blends plays a crucial role in the final properties [101,102]. Polymer blending is an ideal process to attain better mechanical and physical properties compared to several other strategies such as physical and chemical modification of PLA. Over the recent years, polymer blending has attracted considerable interest since the process is cost-effective meaning economically, it enables the dilution of engineering resin with a low-cost polymer, and it is less expensive than the development of new products by synthesis and can be easily upscaled by the industries commercially for various applications. Polymer blend offers an attractive way to develop new generation plastic materials. Accordingly, blends of PLA with both commodity and engineering polymers have been increasingly investigated for several applications.

2.4.1 Thermodynamics of blends Polymer blends are mostly immiscible leading to phase separation occurring on a size of several microns, which can induce weakening of material properties due to poor interfacial adhesion. The shape, as well as the size, of the dispersed phase, is strongly dependent on several parameters such as interfacial and rheological properties and the composition of the blend [103–105]. Blending has several advantages as mentioned previously such as the blend system attains all the desired properties, the engineering polymer’s cost can be reduced by incorporating inexpensive polymers while maintaining the performance, specific properties such as flame retardance (acrylics with polyvinyl chloride), solvent resistance (polyalkenes with polyphenylene ether), and biodegradability can be improved by blending, processability

Chapter 2

can be improved by blending a polymer with low Tg to process it well below the degradation temperature and blending could also be used for recycling industrial plastic waste [106].

Polymer blends can be broadly classified as miscible blends and immiscible blends. In miscible blends, the polymers are miscible at molecular level. Miscible polymer blends exhibit a single

Tg which is generally in between the Tg of the components. Miscibility in terms of Gibbs free energy can be explained with the equation 2.1 given below.

GM = HM - TSM (2.1)

Where ΔGM = change in free energy, ΔHM = change in enthalpy, ΔSM = change in entropy, T = absolute temperature. The blend is considered a homogeneous miscible blend if the Gibbs free energy of mixing has a negative value. A few examples of miscible polymer blends are polystyrene and poly(phenylene oxide), poly(methyl methacrylate) (PMMA) and poly(vinylidene difluoride) and PLLA/PMMA. Immiscible blends on the other hand exhibit two phases with coarse morphology, poor adhesion, and high interfacial tension. Unlike miscible blends these exhibit two different Tg of its blend components. In terms of Gibbs free energy, a blend is considered immiscible if ΔGM has a positive value. Some commonly known immiscible blends are polyamide (PA) and acrylonitrile butadiene styrene, PA and polyphenylene oxide, PA and ethyl propylene diene monomer and PA and polypropylene blends. Polyethylene and polypropylene blends though structurally similar are immiscible in nature [101,102,106]. These blends have discrete phase structure. Morphology of blends represents the shape and spatial distribution of the blend components phases with respect to one another. Droplet in matrix morphology enhances toughness and impact properties, laminar morphology enhances the barrier properties, fibrillar morphology improves the tensile properties and co-continuous morphology improves the electrical conductivity and impact properties. The type of morphology formed during blending depends on nature of the blend components, viscosity ratio, interfacial tension and the processing conditions [105].

2.4.2 Processing methods of polymer blends Polymer blending is one of the most efficient ways to improve the drawbacks of PLA. The two different methods can be used for the preparation of PLA blends- solution blending (solvent casting) and melt blending. Solution blending method of PLA is carried out by the following steps; choosing a suitable solvent for dissolving the polymers, followed by mechanical mixing and eventually the solvent evaporation [20,107]. However certain drawbacks limits the process for commercial production, due to the difficulties in the evaporation of solvents and the high

Chapter 2 cost of solvents. Importantly, the process is not eco-friendly due to their great amounts of petro- based solvents. However, solution blending is considered to be a good technique especially for biomedical and biotechnological applications where the pre-requisition is to avoid degradation and inter-chain chemical reactions that can occur during the melt blending of PLA with some natural polymers [29,103,108].

Melt blending is a process in which the two individual components are mixed above the melting temperatures within a heated mixer, such as an internal blender, single-screw extruder, and twin-screw extruder. Twin-screw extruder has been proven to be a better suitable method as it is more efficient and yield productivity is higher when compared to the other mixers. This method is predominantly interesting due to its simplicity, cost-effectiveness, and availability at an industrial level. The processing parameters such as screw speed, feed rate, residence time and temperature profile should be optimized for maximum yield and better properties [10,20,29,103,104]. The melt blending method, however, has been suggested to be more effective than solution blending since better miscibility can be attained. However, for durable applications melt blending can be more effective. Nevertheless, PLA blends processed by either solution blending or melt blending methods can be subjected to different processes to obtain various structures, such as films, fibers, and porous structures [10,20,103,104].

PLA based blends PLA based blends have gained considerable attention from past decade. The academic awareness and industrial research have grown exponentially ever since. The remarkable properties obtained by polymer blending have led to a tremendous increase in research articles in the last decade and this is depicted in Figure 2.4. Multifarious polymers have been blended with PLA depending on the end-use applications such as biodegradable polymers (starch, lignin, chitosan, polycaprolactone, polyhydroxyalkanoates, poly(butylene adipate-co- terephthalate), poly(butylene succinate-co-adipate), poly(butylene succinate), polyvinyl acetate, poly(vinyl alcohol) and poly(propylene carbonate)), commodity plastics (polyolefins, polystyrene (PS)), elastomers (natural rubber, styrene butadiene styrene, acrylonitrile butadiene, and isoprene rubber), thermoset polyurethane and engineering polymers PMMA, polyamide (PA), poly(butyl acrylate) (PBA), poly(butylene succinate-co-l-lactate) (PBSL), poly(butylene terephthalate) (PBT), polycarbonate (PC), poly (D-lactic acid) or poly D-lactide (PDLA), poly (D, L-lactic acid) (PDLLA), poly(ethylene succinate) (PES), polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), Poly(vinylpyrrolidone) (PVP) and poly(vinylphenol) (PVPh)).

Chapter 2

Table 2.1. General properties of PLA and other engineering polymers [109–117]

Properties PLA PMMA PC PET PBT

Density (g/cm3) 1.21-1.30 1.17-1.20 1.2 1.41 1.33

Tensile Strength 21-150 48-76 62.1 48-54 45-56 (MPa) Youngs Modulus 0.35-3.45 3-4 2.4 1.7-2.3 2.6 (GPa) Elongation at 2.5-10 2-10 80-120 70-230 200-300 break (%) Impact strength 2-3 16-27 Jm-1 850 Jm-1 26-90 Jm-1 53 Jm-1 (notched) (notched) (notched) Tg (°C) 45-65 114-140 150 69-115 30-60

Melting 150-200 160-170 160-220 265 220-230 temperature (°C) Heat deflection 49-55 68.1-98.7 132-140 63 (at 150 temperature (°C) 264psi) (0.45MPa)

Biobased 100 - Partially 30 - Counterparts and bio-based (BioPET content (%) (Durabio™) 001)

The scope of the article is limited to the discussions about blends of PLA and engineering polymers aiming at high-end applications. It is interesting to observe that the studies on PLA/engineering polymers blends have rapidly increased over the years (Figure 2.4).

PLA has comparable tensile strength and young modulus to engineering polymers such as PC, PET, and PBT the detailed properties of engineering polymers are reported from the literature in Table 2.1. However, properties such as impact strength, ductility and heat deflection temperature are significantly lower than PMMA, PC, polyesters, and PA; the reported values are tabulated in Table 2.1 and Table 2.5. Blending PLA with engineering polymers provides a potential platform to obtain materials with higher ductility, toughness and thermal resistance. These properties play a pivotal role in fabricating products with high value, durable applications.

In the blending process, few parameters play vital roles, such as type of polymer, blend composition, processing method, compatibilizers and reactive blending on the final properties. The influence of each parameter is discussed in the following sections.

Chapter 2

Figure 2.4. Evolution of PLA based blends with engineering polymers over the years

2.5.1 PLA/PMMA blends Interesting studies were reported on PLA/PMMA blends, this can be ascribed to interesting properties of PMMA such as high heat deflection temperature, ductility, optically transparency, ultra-violet light resistance and has long term durability compared to PLA [47,118–124]. Initial studies on PLLA/PMMA blends were reported by Eguiburu J.L et al. [118]. In this study, various compositions of PLLA/PMMA were prepared by solvent casting.

The miscibility in the blends was observed with a single Tg in the second heating cycle, Depression of melting point was observed as the content of PMMA increased which is another sign of miscibility. Multiphase system was observed for precipitated blends, whereas, a miscible system was observed when thermal treatment (200°C) was done with subsequent cooling [118]. Several researchers studied the immiscible-miscible transformations of PLA/PMMA blends, where miscibility was obtained after annealing or melt processing, whereas existence of immiscibility when solution blended was intriguing which could be observed by SEM in Figure 2.5. These blends exhibited upper critical solution temperature (UCST) behavior and several parameters such as molecular weight of PMMA, tacticity of PMMA, annealing temperature, processing method affected this behavior [48,119,120,122,123,125]. As mentioned before, solvent blending is not an apt technique for durable or high-performance applications. Melt blending of PLA and PMMA have been carried out in various mixing equipment including internal mixer, internal mixer with Banbury rotors and twin screw extruder at a temperature ranging from 170-210°C with a speed between the

Chapter 2 range of 50-100rpm and a residence time of 3-10 minutes [47,119]. The PLA/PMMA blends show miscibility with controlled parameters such as annealing temperature, molecular weight of PMMA and tacticity of PMMA, however without it, the blends are compatible and the PMMA droplets were found to be in the range of 0.5-0.6µm depending on the blend composition [126]. The compatibility of the blends was improved considerably with addition of non-reactive compatibilizers such as copolymers, clay, impact modifiers [124,127,128]. In some other study, Anakabe J et al. reported that the PMMA droplets were formed lesser than 410 nm with addition of poly(styrene-co-glycidyl methacrylate) copolymer in PLA/PMMA blends. The formation of droplets was observed during SEM characterization and the results are shown in Figure 2.6 for different compositions [127]. Interestingly, brittle to ductile transformation was observed in PLA/PMMA blends and impact strength was also found to be higher in the blends. However, a slight decrease in the tensile strength was observed as this property is low for neat PMMA [124,127].

Chapter 2

Figure 2.5. SEM micrographs (×3000; left) and OM images (×800;right) for as-cast PMMA- 100k/PLLA=50/50 (mass ratio) blends from phase separation to homogeneous phase [120]

Bouzouita A et al. detailed that the addition of commercially available impact modifier Biomax® Strong 120 (BS) 17wt.% was added to neat PLA, improving the notched impact strength considerably from 3.4 kJ/m2 to ~24 kJ/m2 [124]. Further the addition of PMMA to PLA/BS blends the impact strength was improved to 42.8 kJ/m2 up to 30%, and further increase in above 50 wt.% PMMA content reduced the impact toughness due to lack of affinity between impact modifier and PMMA [124].

Figure 2.6. SEM micrographs at 10 000 magnifications of PLA and PLA/PMMA blends at different ratios containing 3 pph of P(S-co-GMA) copolymer: (a) 100/0, (b) 95/5, (c) 90/10, (d) 95/15, and (e) 80/20 [127].

Chapter 2

Similar studies reported that the addition of copolymer increased the ductility and impact strength of PLA/PMMA blends [119,124,125,127]. The blends processed by melt blending were found to be more transparent in nature when compared to the blends prepared by solvent blending and the results are shown in Figure 2.7. Interestingly with increasing PMMA content the transparency also improved [119].

Figure 2.7. PLA/PMMA blends (50/50) melt blending (left), solvent blending (right) [119]

The thermal resistance of PLLA was improved considerably with 20 wt.% of PMMA, addition of copolymer poly(styrene-co-glycidyl methacrylate) of 3pph improved the thermal stability of the blends by 10°C, Tg of the blends was found to be improved to 65°C with addition of this copolymer and it further increased with higher content of PMMA above 30 wt.% [119,124,127]. Samuel C et al. observed the improvement of barrier properties in the case of miscible PLA/PMMA blends [119]. Samuel C et al. investigated the shape memory behavior of PLLA/PMMA blends and observed “temperature memory effect”, where there is a selective activation of specific nanodomains containing different amount of PLLA and PMMA. A broad glass transition was detected for a miscible 50/50wt.% of PLLA/PMMA blend composition with an efficient “temperature memory effect” causing a triple memory effect within the chain

Chapter 2 entanglements. The schematic representation of the high entangled network is shown in Figure 2.8 [49]. Similar studies have been done by Hao X et al. on the shape memory ability of PLA/PMMA blends, the effects of crystalline region and chain entanglement on the shape memory performances were investigated by thermo-mechanical studies and rheology [47]. This shape memory behavior can be clearly seen in Figure 2.9 wherein initially the specimen is stretched upon heating and by cooling the shape is fixed, and this specimen recovers its original shape and size upon heating it to the initial temperature. The crystallinity seemed to drop at 50wt.% of PMMA, this is due to the transition from semicrystalline to amorphous phase. For the semi-crystalline blends with < 50 wt.% PMMA increase in entanglement density was observed which leads to decreased crystallites and an enhanced shape recovery ratio for the blends.

Figure 2.8. Schematic and potential structure of the symmetric blend stretched at various temperatures within the glass transition to explain the observed “temperature-memory effect” (black points represent entanglements or “hard domains”) [49]

This demonstrates that the molecular entanglement has a positive influence on the shape memory performance, whereas crystallites exhibit an opposite impact which may arise from the strain-induced crystallization and the molecular slippage between the crystalline and amorphous chains occurs upon long-term stress [47].

Chapter 2

Figure 2.9. Schematic diagram for shape memory test of PLA/PMMA blends [47]

To summarize, PLA/PMMA blends show miscibility when processing parameters such as temperature and annealing time are controlled. In general PLA/PMMA blends are compatible and addition of copolymers poly(styrene-co-glycidyl methacrylate), impact modifiers (Biomax® Strong 120) and mineral (clay, calcium carbonate) improves the interface between

Table 2.2. Overview of PLA/PMMA blends

Blend (Name, grade, Properties Prospective Reference and composition) and applications additives PLAL210, PLAL207S Immiscible-droplet size 5- Biomedical [121] & PMMA 50µm (28/72) applications PLLA/PMMA Miscible - [129]

PLA (Ingeo 4032D)/ Miscible Shape [47] PMMA (Plexiglas 7N) Improvement in shape memory recovery properties materials

PLLA Miscible blends - [130] (Lacty#5000)/PMMA PLA (Ingeo Immisicible-PMMA droplet Semi- [127] 3051D)/PMMA + P(s- size <400nm, ductility and durable co-GMA) thermal stability improved applications

PLA(4032D)/PMMA of Immiscible droplet diameter Automotive [124] Biomax® Strong 120 (<600nm), improvement in applications (BS) HDT ,ductility and toughness

Chapter 2

PLA(Ingeo Miscible blends - [128] 4032D)/PMMA + clay, Clay- Improvement of T1GPa, quartz and calcium thermal decomposition carbonate temperature Quartz- improvement of, thermal decomposition temperature CaCO3-deterioration of properties due to PLA chain scission PLA(Ingeo Immiscible blends (<800nm) Automotive 4032D)/PMMA(90/10, for all compositions, good and 70/30, 50/50, 30/70, tensile properties electronic 10/90) applications PLA(4032D)/PMMA Miscible blends. Increase in Automotive, [119] and PLA crystallinity, barrier electronic PLA(4032D)/PMMA2 properties (O2, CO2, N2 casing and (90/10, 80/20, 70/30, permeability) and Izod Impact technical 60/40, 50/50, 20/80) strength packaging. PLA(Ingeo™ Improvement in flame - [131] 3051D)/PMMA (80/20) resistance and thermal + 20phr of flame stability and no char formation retardant (Reofos®) observed after burning.

PLA and PMMA. PLA/PMMA blends improve thermal resistance, thermal stability, optical transparency, ductility, impact strength, and barrier properties. A few studies on the shape memory behavior of PLA/PMMA have also been done. These blends have potential applications in automotive industry and shape memory materials. Different studies reported in this context on PLA/PMMA blends were summarized in Table 2.2 along with processing methods, properties, and prospective applications.

2.5.2 PLA/Polycarbonate blends Polycarbonate (PC) is an engineering thermoplastic with high thermal stability, impact resistance, high voltage insulation, transparency, and durability, leading to beneficial applications in electronic, automotive, electrical and packaging [10,20,132–138]. PLA/ PC blends have gained significant attention in the past decade due to their potential to be utilized in industrial applications [10,20,132–138], PLA/PC blends have the potential to replace PC/ABS due to their higher bio-based content and their cost-effectiveness but the durability of PLA/PC blends have to be improved to make them perfectly suitable for industrial applications [138]. These blends are prepared by melt blending in a twin-screw extruder or internal mixer at a melt temperature ranging from 190-230°C at various speeds of 70-150 rpm for a residence

Chapter 2 time of 3-10 minutes. PLA and PC are immiscible blends with little compatibility which can be observed in Figure 2.10 as the interface is weak with poor interfacial adhesion and large droplet size of the dispersed phase), hence, various compatibilizers have been used to improve the compatibility [10,20,132–138]. The interface between PLA and PC was found to be improved with addition of compatibilizers such as styrene-acrylic multi-functional-epoxide oligomeric agent (SAmfE, Joncryl® ADR4300-F), styrene maleic anhydride copolymer (SMA, Joncryl® ADR3400), ethylene-maleic anhydride-glycidyl methacrylate terpolymer, poly(styrene-co-acrylonitrile)-g-maleic anhydride (SAN-g-MAH), poly(ethylene-co-octene) rubber-maleic anhydride (EOR-MAH), poly(ethylene-co-glycidyl methacrylate) (EGMA), poly(butylene succinate-co-lactate) (PBSL) and epoxy [134,135,137,139] . The dispersed phase (PC) was found to be spherical in shape with the addition of PBSL as a compatibilizer and it was even observed that PBSL was finely dispersed in the PC spheres as seen in Figure 2.11. However as the content of PBSL used was higher (20phr) large aggregates were observed in Figure 2.11 probably due to coalescence of PC and PBSL phases [134].

Chapter 2

Figure 2.10. Blend morphology of PLA/PC a) 70/30 (twin-screw extruder), b) 50/50, (c-f) (90/10, 70/30, 50/50, 30/70, 10/90) (internal mixer) [126,134,139]

Yemisci F et al. observed lowest droplet size in PLA/PC/SAmfE at 1wt.% concentration of SAmfE [139]. By adding the other compatibilizers such as tetrasilanol phenyl polyhedral oligomeric silsesquioxane (T-POSS) and glycidyl isooctyl polyhedral oligomeric silsesquioxane (G-POSS) increased the agglomeration of particles was and the results can be observed at Figure 2.12 [139]. In another study, Izod impact strength drastically increased to 65J/m with 10phr of PBSL in PLA/PC (50/50) blends when compared to the unmodified blend in which the impact strength observed was as low as ~10J/m [134]. However, when the content of PBSL was further increased to 20phr a reduction of impact strength to ~50J/m was observed owing to formation of large aggregates [134]. Lee J B et al. studied the effect of (SAN-g- MAH), poly(ethylene-co-octene) rubber-maleic anhydride (EOR-MAH) and EGMA on PLA/PC blends and found that the least droplet size of 0.1µm was found for PLA/PC/SAN-g- MAH blends as seen in Figure 2.13 [137].

Figure 2.11. Morphology of PLA/PC/ PBSL of compositions (a) 50/50/0 , (b) (50/50/5phr) , (c) (50/50/10 phr), (d) (50/50/20phr) [134]

Chapter 2

The interfacial tension between blends was measured by palierne’s equation and it was found that the PLA/PC blends without the compatibilizer had much higher interfacial tension of 3.34 mN/m when compared to PLA/PC with 5 phr of SAN-g-MAH with an interfacial tension of 0.08 mN/m. Whereas in the case of PLA/PC /EOR-MAH, PLA/PC/EGMA blends, the values of interfacial tension were found to be 1.03 and 1.82mN/m respectively which is lower than that of PLA/PC blends without any compatibilizer. However, the interfacial tension obtained by using EOR-MAH and EGMA as compatibilizer was higher than the PLA/PC/SAN-g-MAH [137]. Interestingly, chain extenders such as a random copolymer of styrene and glycidyl methacrylate (ADR) and N,N,N’,N’-tetraglycidyl-4,4’-diaminodiphenyl methane (TGDDM) were added to PLA/PC blends, there was no drastic improvement of interface between the polymers [136].

PLA/PC blends showed poor mechanical properties, in particular, tensile strength, ductility,

Chapter 2

Figure 2.12. Morphology of PLA/PC(70/30) with 1wt.% of (a) SAmfE, (b) SMA, (c) T-POSS and (d) G-POSS [139] flexural strength, and impact strength without compatibilizers owing to their poor interface and phase separation. With addition of compatibilizers considerable improvement of mechanical properties was observed. Among the various compatibilizers used SAmfE, SMA, T-POSS and G-POSS improved the tensile strength of PLA/PC blends. However, deterioration in elongation at break was observed with addition of these compatibilizers due Figure 2.13. Morphology of

PLA/PC blends with 5 phr of SAN-g-MAH (left), EOR-MAH (middle) and EGMA (right) [137] to the phase separation owing to the poor compatibility between PLA and PC phase [139]. The ductility of PLA/PC blends was found to be better when ADR of 0.1, 0.3 and 0.5 phr and TGDDM of 0.1, 0.3 and 0.5 phr were used as the compatibilizers as observed in Figure 2.14. The authors claim that the improvement in properties was due to the coupled reaction of PC and PLA with the aid of the chain extenders at the interface. The maximum elongation at break of 120% was obtained for 0.3phr of ADR as observed in Figure 2.14 [136]. Lee J B et al. observed that among SAN-g-MAH, EOR-MAH and EGMA; SAN-g-MAH at 5phr showed 2 2 maximum tensile strength of ~650kgf/cm , flexural strength of ~1150kgf/cm and impact strength of ~38kgfcm/cm which can be clearly observed in Figure 2.15. It was even observed that EOR-MAH and EGMA only improved mechanical properties up to 2 phr after which a linear decline was observed and results can be seen in Figure 2.15 [137].

Chapter 2

Figure 2.14. Stress-strain curves of PLA/PC with ADR and TGDMM as compatibilizers [136]

Heat deflection temperature (HDT) of PLA/PC (50/50) was found to be increased from 80°C to ~95°C by adding 5phr of PBSL. However, with further increase in content of PBSL to 10 and 20phr, HDT dropped to ~77°C and ~61°C, respectively. There was a drastic increase in HDT of 129°C with the addition of epoxy of 10phr with tetrabutylammonium bromide (1.0phr) as catalyst. This might be due to the ring opening reaction of epoxy groups with the carboxyl groups in PLA which improved the interfacial adhesion [134]. Thermal resistance was improved in the PLA/PC (70/30) blends containing ADR and TGDMM [136]. The storage modulus was found to be higher for PLA/PC blends containing SMA when compared to SAmfE, T-POSS, and G-POSS [139].

Hashima K et al. investigated the properties of the quarternary blend system including toughened PLA (two grades injection (l) and film extrusion (h)) with hydrogenated styrene butadiene styrene block copolymer, EGMA (compatibilizer) and PC. Izod impact strength and HDT improved for this blend system and maximum value of ~60kJ/m2 and ~89°C respectively obtained for PLA/PC/SEBS/EGMA (40/40/15/5) compositions. Elongation at break was also found to be much higher for PLAh/PC/SEBS/EGMA (60/20/15/5) to 120% without major decrease in stiffness [133].

Chapter 2

Figure 2.15. Mechanical properties of PLA/PC (70/30) blend with different compatibilizers [137]

Yuryev Y et al. studied the hydrolytic degradation of PLA/PC blends with acrylic impact modifier poly(ethylene n-butylene acrylate glycydyl methacrylate) (EBA-GMA) in deionized water [138], it was observed that the presence of this modifier slowed the degradation rate of the blends due to the hydrophobic nature of EBA-GMA. Hence, it can be said that blending PC with PLA with a suitable compatibilizer improves the impact strength, ductility, and thermal resistance and even degradation rate of the blends can be slowed. The blend composition, processing parameters and amount of compatibilizer need to be optimized for attaining the best properties. PLA/PC blends have the potential for its usage in electronic cases and automotive parts thereby decreasing the usage of petroleum-based polymers. Blend type, blend compositions, properties and prospective applications for PLA/PC blends have been summarised in Table 2.3.

Table 2.3. Overview of PLA/PC blends

Chapter 2

Blend Properties Prospective Reference (Compositions) applications PLA/PC/SAN-g- SAN-g-MAH-improvement in - [137] MAH impact, flexural strength. The PLA/PC/EOR- Interfacial tension at the droplet MAH diameter of PC 0.38 µm (5 phr) PLA/PC/EGMA

PLA (4032D)/ Improvement in ductility, impact - [136] PC/ ADR strength and heat resistance. PLA (4032D)/ Compatibilizers show best results PC/ TGDDM at 0.3 phr. PLA (3001D)/ Improvement in toughness, HDT, - [133] PC & ductility for PLA (4032D)/ PC/ PLA (4032D)/ SEBS/ EGMA 40:40:15:5 PC/ With SEBS and EGMA PLA (2002D)/ For PBSL impact strength - [134] PC with PBSL improved and HDT decreased. For and EP EP addition of catalyst TBAB reduced PC droplet size and increased HDT. A combination of PBSL and EP increased toughness and HDT. PLA (2002D)/ SAmfE showed better mechanical, - [139] PC with SAmfE thermo-mechanical and lowest PC and SMA, T- droplet size. poss and G-poss. PLA (Ingio Higher mechanical properties, Competitive [138] 3251D)/ PC/ toughness and heat resistance. for PC-ABS EBA-GMA blends. Possibility for performance applications. PLA/ PC/ TPU Improvement in toughness, - [132] with DBTO ductility with deterioration in tensile strength. DBTO improves the interface and increase in tensile, impact, ductility and crystallinity of PLA and thermal stability.

2.5.3 PLA/semi-aromatic Polyester blends Polyesters are a group of polymers containing ester functional group in the backbone; they can be classified based on the structure as aliphatic, semi-aromatic and aromatic polyesters. Semi- aromatic polyesters are those in which one of the monomers contains an aromatic group such

Chapter 2 as polyethylene terephthalate (PET), polybutylene terephthalate (PBT) and polytrimethylene terephthalate (PTT). These are important engineering thermoplastics that are suitable for blending with PLA due their higher thermal resistance, better mechanical properties, good dimensional stability and chemical resistance [10,20,140–146].

PLA/PET blends are prepared in single screw extruder, melt mixer and injection moulding at a temperature ranging from 190-270°C, with a speed of 40-200rpm depending on the processing methods and for a residence time for 3-8 min. Miscible, immiscible and compatible PLA/PET blends have been observed by several studies and they are dependent on the processing parameters [141,142,147]. Torres-Huerta et al. observed that with inclusion of 1wt.% of PLA in PET gives matrix-droplet morphology but whereas just a little higher amount >5wt.% brings about a co-continuous morphology (Figure 2.16) [142].

Figure 2.16. Morphology of PLA/PET blends of compositions (a) 1/99, (b) 2.5/97.5, (c) 5/95, (d) 7.5/92.5 [142]

The thermal stability of the PLA/PET blends increased when a moderate amount of 5-20 wt.% of PLA is maintained in the blends [141,144]. McLauchlin A R et al. observed that above 2wt.% of PLA in the blends decreased the impact strength drastically and the Young’s modulus was higher for 0.5 wt.% but decreased with the further increase of PLA content due to its poor interfacial adhesion [148]. Similar behavior was observed by Torres-Huerta A M et al., where higher concentrations of PLA >7.5 wt.% reduced the ductility and impact strength in PLA/PET blends. The optimum wt.% of PLA was found to be 2.5% as it enhanced ductility of ~78% but

Chapter 2 deteriorated impact strength of the blends due to poor compatibility between the polymers [142]. Blends of PLA/PET glycol-modified (80/20) with PLA-g-MA as a compatibilizer were studied by Jiang et al [149]. From the rheology data and emulsion models such as the Palierne model, they showed that the use of 3 wt% PLA-g-MA significantly reduced the interfacial tension between PLA and PET. The droplet size was reduced, and the size distribution was improved, causing significant enhancements of ductility without sacrificing the tensile strength and modulus.

PBT is an important engineering polyester due to its high crystallinity, higher service temperature, good mouldability, and easy processability. PLA and PBT blends are compatible in nature due to the interactions between the polyester groups. However, melt processing of these blends is difficult due to their difference in melting point, PBT melts at around 240-260°C while PLA at 170-190°C. Hence, it is necessary to use chain extenders such as polycarbodiimide, tris(nonylphenyl) phosphite, para-phenylene diisocyanate and epoxy functionalized styrene-acrylate copolymer [150,151]. Melt blending of PLA and PBT at 230- 250°C is carried out in Brabender, twin-screw micro compounder, internal batch mixer and twin-screw extruder [150–154]. PLA/PBT blends without any compatibilizer show poor interactions and weak adhesion. However, in the presence of compatibilizers or chain extenders such as epoxy functionalized styrene-acrylate copolymer (ESAC), poly(ethylene-n-butyl- acrylate- co-glycidyl methacrylate), para-phenyline diisocyanate improve the interface between the polymers. The blends show matrix-droplet morphology until 30 wt.% of PLA and >30 wt.% droplet coalescence and co-continuous morphologies are observed and this can be seen in Figure 2.17. Chang B et al. measured the co-continuity using solvent extraction method using chloroform as the solvent [150]. The crystallinity of PLA was found be to higher in the PLA/PBT blends when compared to the neat PLA due to synergy effect and interactions between the functional groups of the polyesters reasons [151,152]. The crystallization rate of PBT is higher than that of PLA, addition of para-phenylene diisocyanate increased the crystallization rate of PLA thereby decreasing the cycle time of the injection moulding [152].

Chapter 2

Figure 2.17. Quantitative analysis of the PLA percentage co-continuity development in PLA/PBT blends using the solvent extraction method (schematic diagram: PLA phase (red in color), PBT phase (blue in color)) [150].

Chang B et al. observed that the mechanical properties of PLA/PBT blends were poor in the absence of compatibilizer, but with the presence of ESAC, the tensile strength and tensile modulus increased as seen in Figure 2.18. The branching effect of ESAC brought about the improvement of ductility in the blends, PLA/PBT (60/40) with 1.0phr of ESAC showed around 150% of elongation at break as seen in Figure 2.18. With addition of a second compatibilizer (EBA-GMA) the impact strength was further improved from 30 J/m to 95 J/m which can be clearly observed in Figure 2.19 [155]. To summarise PLA/PET blends show improvement in thermal stability, but an appropriate amount of PLA is necessary to maintain the impact strength and modulus of the blends. The use of compatibilizers such as PLA-g-MA for modified PET reduced the interfacial tension between the blends and an improvement in ductility was observed without sacrificing tensile strength and modulus. PLA/PBT blends showed an improvement in PLA crystallinity, compatibilizers are necessary to observe an improvement in toughness and ductility. Chain extenders are also necessary as there is a vast difference in melting point. Further investigations on these blends are necessary to identify an apt engineering application.

Chapter 2

Figure 2.18. Mechanical properties of PLA/PBT blends with ESAC as compatibilizer [150]

Some interesting studies reported on PLA/PET/PBT/PTT blends are reported along with the processing technique and properties are tabulated in Table 2.4.

Figure 2.19. Notched impact strength of PLA/PBT blends with ESAC and EBA-GMA[155]

Chapter 2

Table 2.4. Overview of PLA/semi-aromatic polyester blends

Blend Properties Reference PLA(2002D)/PET Lower droplet size-270-657nm, [156] improvement in ductility, reduction of impact strength PLA Lower droplet size (<500nm), lower [149] (Ingeo™2003D)/PETG interfacial tension and higher ductility +PLA-g-MAH with PLA-g-MAH, PLA(Hisun Revode 201 Impact strength and tensile properties [148] Injection grade)/ reduced >2wt.% PLA

PLA (2002D)/ PET and Improvement in thermal stability [144] PLA/PET (recycled) PLA /PET (collected Decrease in mechanical properties > [146] from post-consumer 5wt.% PLA, thermal stability unaffected bottles) with the presence of PLA PLA/PET +cyanowood Improvement in thermal stability, [141] and starch reduction in mechanical properties - improper dispersion of the filler PLA(2003D)/PBT Improvement in crystallinity of PLA [154] PLA(Ingeo 2351D)/PBT Improvement in tensile strength, tensile [155] + ESAC and EBA-GMA modulus, and ductility with ESAC (1phr) Synergistic effect of ESAC and EBA- GMA on improvement in impact strength PLA(4032D)/PBT +PPDI Improvement in crystallinity of PLA, [151] interfacial adhesion, decrease in biodegradation rate PLA(4032D)/PBT Improvement in ductility with significant [152] reduction in stress at break (40 wt.% PBT)

2.5.4 PLA/Polyamide blends Polyamides are class of engineering polymers in which the repeating units are linked with amide bonds. Polyamides have good dimension stability, good barrier properties, high heat deflection temperature, high ductility, chemical resistance, impact resistance and abrasion resistance which make it a suitable polymer to blend with PLA (Table 2.5). Polyamides such as polyamide 6 (PA6), polyamide 6-10 (PA6-10), polyamide 11 (PA 11), polyamide 10-10 (PA10-10) and polyamide 6-6 (PA6-6) have been blended with PLA. PLA/PA blends are compatible in nature due to hydrogen bonding between the amide groups and ester groups of PA and PLA, respectively.

Chapter 2

Table 2.5. General properties of PLA and different PA polymers [109–111,157]

Properties PLA PA6 PA6-10 PA11 PA10-10

Density (g/cm3) 1.21-1.30 1.13 1.16 1.15

Tensile Strength 21-150 79 83 70 (MPa) Young’s Modulus 0.35-3.45 2.9 1-2 1.2 1.5 (GPa) Elongation at break 2.5-10 70 120-300 160 150-170 (%) Impact strength 53 (Jm-1) 50 (Jm-1) 27 (Jm-1)

Tg (°C) 45-65 47-57 67 42

Melting temperature 150-200 220 220 185 200-205 (°C) Heat deflection 65 157-175 temperature (°C)

Biobased 100 - 60 100 Counterparts and content (%) PLA/PA6 blends have been prepared in Brabender, plasti-corder kneader and twin-screw extruder at temperature within the range of 210-250°C and speed ranging from 25-150rpm [158–163]. Similar to PLA/PBT the melt temperature difference exists even in the case of PLA/PA6 blends and hence chain extenders such as multifunctional epoxies (ECE) and polycarbodiimide (PCD) are used [161]. PLA/PA6 blends are compatible in nature and show matrix-droplet morphology up to 30 wt.% of PLA and >30 wt.% of PLA co-continuous morphology is observed. Figure 2.20 depicts the SEM images of the PLA/PA6 blends where the morphology can be clearly observed. PLA/PA6 blends without compatibilizer show good dispersion but it is better in the presence of compatibilizers such as alkenyl-succinide- anhydride-amide (ASAA), alkenyl-succinic-anhydride-imide (ASAI), polyethelene octene grafted maleic anhydride (POE-g-MAH). The SEM images depicted in Figure 2.21 shows the better dispersion with the use of compatibilizers.

Chapter 2

Figure 2.20. SEM images of PLA/PA6 a) 80/20, b) 70/30, c) 60/40 and d) 50/50 at different magnifications [159,163].

Mechanical properties of PLA/PA6 blends without compatibilizer were poor due to their incompatibility, poor adhesion, and weak interface. However, with inclusion of compatibilizers and chain extenders such as ASAI, ASAA, POE-g-MAH, thermoplastic polyurethane (TPU), PCD and ECE the mechanical properties were found to be better [159,161–163]. Khankrua R et al. observed that with 0.5phr of ECE as chain extender, the elongation at break improved to ~58% when compared to PLA/PA6 blends (4.5%) and it was even observed that the tensile strength was improved to ~58 MPa from ~47MPa for PLA/PA6/ECE blends [161]. An interesting observation was made by Chen Yi et al. about the inclusion of TPU as toughening agent in PLA/PA6 blends. The elongation at break for these blends improved considerably and the highest value was found to be for 20 % TPU (~270%) in PA6/PLA (70/30). Conversely, there was a decrease in tensile strength and improvement of impact strength was not prominent [162].

Chapter 2

Figure 2.21. SEM images of PLA/PA6 blends with compatibilizers a) POE-g-MAH, b) ASAA and c) ASAI at different magnifications [159,163].

Khankrua R et al. observed that with the addition of PCD and ECE, the thermal stability of the PLA/PA6 blends improved. The onset and deflection temperature of PLA with PCD and ECE was found to be higher when compared to neat PLA. This increase might be due to the reaction between the end groups of PLA and PCD and the epoxy groups of ECE could react with both hydroxyl and carboxyl groups of PLA[161].

To the best of the author’s knowledge, for the first time, PLA/PA6-10 blends were prepared by Pai F C et al [164]., Interestingly the PA6-10 utilized in these blends is bio-based. As PLA/PA6-10 blends show poor compatibility without a compatibilizer, bisphenol-A type epoxy resin was used as a compatibilizer to improve the interface between PLA and PA6-10. The blends were prepared in a twin-screw extruder with a screw speed of 120rpm at 200-230°C. Droplet-matrix morphology was observed for PLA/PA6-10 blends, the droplet size of PA6-10 was observed to be ~3µm without the compatibilizer. However, with the addition of epoxy the droplet size decreased, and it was found lowest for 1 phr epoxy wherein a droplet size of ~1µm was obtained. Better tensile strength, elongation at break, flexural strength and notched and un- notched impact strength are reported with high concentrations of epoxy compatibilizer of approximately 3phr. Although a decrease in properties is observed with further increase in content of epoxy of around 5phr and this might be due to the agglomeration of PA6-10 particles as a result of crosslinking reaction [164]. The same authors prepared thermally conductive compatibilized PLA/PA6-10 blends by the addition of boron nitride (BN). PLA/PA6-10/BN blends showed improvement in thermal conductivity 0.86Wm-1.K-1 at 50phr BN due to good interactions between PLA/PA6-10 blends and BN leading to efficient heat transfer.

Improvement in thermal stability T5wt%-334°C at 40phr BN and Young’s modulus ~3460 MPa at 40phr BN was observed due to the reinforcing effect and flaky shape of BN. The impact properties deteriorated due to rigidity of BN fillers. These blends could find applications in 3C

Chapter 2 parts and lighting enclosures [165]. Lately, researchers are more focused on PLA/PA11 blends due to some advantages such as i) polymers are 100% bio-based systems. ii) due to their close proximity of melting points facilitating the removal of further usage of the chain extenders. One of the earliest PLA/PA11 blends was prepared by Stoclet G et al., the morphological, thermal and mechanical behavior of the blends were discussed elaborately and it gives us a clear understanding of PLA/PA11 blend behavior at different compositions (20/80,40/60,55/45,60/40,65/35,75/25,80/20 and 90/10) [166]. PLA/PA11 blends are generally compatible in nature. Different processing equipments were used for the preparation of PLA/PA11 blends such as twin-screw extruder, twin-screw micro-compounder, internal mixer (Haake rheomix) with a speed ranging from 50-100rpm at a melt temperature ranging from 170-215°C. The nature of the dispersed phase depends on the composition of the blends and different morphologies such as matrix-droplet, fibrillar and co-continuous structures can be observed in PLA/PA11 blends as seen in Figure 2.22 [117,166–172]. With addition of nanoparticles such as halloysites, modified sepiolites, carbon nanotubes in the PLA/PA11 blends, nanometric range of the droplet size of the dispersed phase was observed [117,172]. Rashmi B J et al. observed the localization of halloysites (HNT) particles in the PA11 phase and forming a salami structure and this improved the interface to a great extent and reduced the droplet size <500nm drastically of the PA11 phase. observed the maximum improvement in elongation at break of ≈155% and un-notched impact strength of ≈37 kJ/m2 for 2wt.% HNT, increase in HNT content above 2wt.% deteriorated the properties of the blends and this is due to agglomeration of HNT particles or are damaging the PA11 fibrils which might be leading to premature fracture of the samples [117]. Gug J et al. studied the reactive compatibilization of PLA/PA11 with p-toluene sulfonic acid (TsOH) at higher screw speed up to 3000 rpm, effect of high shear rate on ester-amide interchange reactions was studied [173]. PLA/PA11 (50/50) blends were ground to powder prior to extrusion, extrusion was carried out in a twin-screw extruder at different screw speeds (250, 500, 1000, 2000 and 3000 rpm) at 205°C, TsOH catalyst solution (0.01 g/mL) in anhydrous ethanol was introduced using a peristaltic pump, the flow rate maintained was 2.7 mL/min so that 0.5 wt % catalyst was introduced in to the extruder.

Chapter 2

Figure 2.22. Morphology of PLA/PA11 blends of different compositions at different magnifications (20/80, 40/60, 60/40, 80/20, 90/10 at 5μm and 30/70, 50/50, 70/30 at 10μm)

[166,169,170]

Chapter 2

It is interesting to observe that the addition of catalyst modified the morphology from droplet to co-continuous for the same speed (250 rpm) as seen in Figure 2.23. This could be due to the inability of the random copolymer to localize at the interface and it disperses in the either PLA or PA11 region causing continuous structure. Mechanical properties showed improvement with the addition of catalyst, elongation at break improved drastically at 0.5wt.% of TsOH. However, the variation of screw speed did not have a major influence on the mechanical properties [173].

Figure 2.23. Morphology of PLA/PA11 (50/50) blends (a) PLA/PA11 (250rpm), (b) PLA/PA11 (2000 rpm), (c) PLA/PA11/TsOH 0.5% (250rpm), (d) PLA/PA11/TsOH 0.5% (2000rpm) [173]

It was even observed that higher screw speed led to a decrease in mechanical properties, and this could be due to the high shear stress leading to depolymerization and degradation of the blend system [173]. New bio-based polyamides are available in the market such PA10-10 and PA10-12 which are yet to be ventured in detail for blending with PLA. There are some preliminary studies on these blends. Callioux J et al. studied the effect of viscosity ratio on the morphology and mechanical properties of PLA/PA10-10 blends [157]. The blends were prepared in a twin-screw extruder, two types of PLA were utilized including PLA and PLArex (modified PLA with Joncryl-ADR-4300F® epoxy-based resin). Different wt.% (10-50%) of PA10-10 were blended with PLA and PLArex. Morphologically PLA/PA10-10 blends showed droplet-matrix morphology with weak interface and larger droplet size of PA10-10 and even

Chapter 2 at higher content of PA10-10 (50wt.%) co-continuous structure was not observed as seen in SEM images in Figure 2.24 [157].

Figure 2.24. SEM images of cryofractured samples of PLA/PA10-10 and PLArex/PA10-10 (scale-2 μm) [157]

Chapter 2

On the contrary, for PLArex/PA10-10 blends the interface between the polymers was strong and dispersion was much better when compared to PLA/PA10-10 blends as depicted in Figure 2.24. At 20wt.% PA10-10 droplet size was reduced to around 200nm for PLArex/PA10-10 blend when compared to droplet size of 1.75µm in PLA/PA10-10 blends. Mechanical properties such as tensile tests and tensile-impact tests were done on the blends and it was observed that stiffness for PLA/PA10-10 and PLArex/PA10-10 (90/10) was improved to ~2.9 and ~2.7GPa respectively when compared to neat PLA with a stiffness value of ~2.3GPa. This could be due to a small PLA crystal region present for this composition which was absent for higher PA10-10 content. Ductility considerably improved for PLArex/PA10-10 blends and the maximum strain at break of ~173% was achieved for 50wt.% PA10-10 as observed in Figure 2.25a. Toughening behavior of the blends was evaluated by tensile-impact tests and it was observed that for PA10-10 of 20 wt.% a gradual increase in the tensile-impact strength was observed and the highest being for 50wt.% for both PLA/PA10-10 and PLArex/PA10-10 blends. However, PLArex/PA10-10 showed better tensile-impact strength comparatively owing to the better interface between the polymers as observed in SEM images (Figure 2.24, Figure 2.25b) [157].

Figure 2.25. Ductile (a) and tensile-impact behavior (b) of PLA/PA10-10 blends [157]

In conclusion, PLA/PA blends are important for durable applications as these blends show higher ductility, impact resistance, and higher thermal stability. As polyamides are a group of different polymers with amide group in the backbone with varying chain length, each type of polyamide has a significant influence on the final properties of the blend system. PLA/PA6 blends with the presence of compatibilizers show strong interface, good adhesion and better mechanical properties (elongation at break and tensile strength). The addition of chain extenders is necessary owing to the large difference in melting points between PLA and PA6

Chapter 2 and it was noticed that the addition of chain extenders improved the thermal stability of the blends. Similarly, for PLA/PA6-10 blends addition of appropriate amount of compatibilizer improved the interface between PLA and PA6-10 thus reducing the droplet size of the dispersed phase. Compatibilizers imparted better mechanical properties such as higher tensile strength, flexural strength, toughness, and ductility.

Table 2.6. A general overview of PLA/PA blends

Blend Properties Prospective Reference applications

PLA/PA6 Improvement in ductility, Potential to replace [159,161– ASAA, ASAI, toughness and thermal parts which do not 163] POE-g-MAH, stability (only in the presence need high load TPU, PCD and of chain extender) bearing capabilities ECE

PLA(4060D0)/PA6- Reduction of droplet size of 3C parts, lighting [164,165] 10+Bisphenol-A PA6-10 (~1µm-1phr), enclosures type epoxy resin & improvement in ductility, BN flexural strength, toughness and improvement in thermal conductivity, young’s modulus and thermal stability PLA/PA11 + Improvement in ductility and - [117,166– halloysites, toughness, reduction in 172,174] modified sepolites, droplet size (<500nm) with carbon nanotubes, halloysites joncryl (epoxy- based resins) and TsOH

PLA (Ingeo Better dispersion with - [157] 4032D)/PA1010 PLArex, improvement in and PLArex/PA10- ductility and toughness 10 In the last few years, there has been a growing interest in PLA/PA11 blends as its 100% biobased blend system. The close proximity of melting points of PLA and PA11 excludes the need for chain extender. PLA/PA11 blends are self-compatible in nature but have weak interface and poor adhesion. The mechanical properties show an improvement at higher content of PA11 above 40wt.%. Nevertheless addition of compatibilizer and nanoparticles improves the mechanical properties (ductility and toughness) at lower content of PA11 of 20wt.%. A first comparative study on ultra-high speed (up to 3000rpm) extrusion and low speed extrusion for reactive compatibilization of PLA/PA11 blends was reported and it was found that high speed

Chapter 2 leads to good dispersion and better properties. However, very high screw speed (>2000rpm) can bring about depolymerization and degradation of polymer.

As previously mentioned, the novel promising bio-based polyamides available in today’s market are PA10-10 and PA10-12. The melting points of these polyamides are also in proximity to the melting point of PLA (Table 2.5). There have been very few studies on PLA/PA10-10 blend and it was observed that rheologically modifying PLA with epoxy-based resin improved the interface between the polymers thereby improving ductility and toughness in the blends.

Therefore PLA/PA blends are a potential blend system with exorbitant properties such as ductility, toughness and higher thermal resistance. PLA/PA blends can find applications in electronic cases, lighting enclosures, and 3C parts. An overview of PLA/PA blends has been tabulated in Table 2.6.

Conclusions Recent studies reveal that the interest of developing PLA-based blends with desired properties is growing rapidly and intense research is being carried out to produce durable products based on PLA. The threats posed by the conventional polymers to the environment are alleviated with the use of biobased polymers such as PLA and its blends; hence this is an intriguing subject not only for researchers but also for plastics manufacturing companies.

This chapter focuses on the various attempts made on PLA blends with engineering polymers. The inherent brittleness, poor toughness and lower thermal resistance of PLA have been tried to improve by blending with various engineering polymers such as PMMA, PC, PET, and polyamides. Research on these blends has grown rapidly in the last few years as these blends have the potential to replace their petroleum counterparts for high-end applications. Further comprehensive research on PLA/engineering polymer blends is necessary to make it suitable and commercialize.

PLA blends with PMMA, Polyesters, polycarbonate, and polyamides brought about an improvement in the ductility, toughness and thermal resistance. However, for most of them use of compatibilizers is necessary to improve the interface between polymers. The strong interface between PLA and engineering polymers is essential to achieve desired properties. It is also important to bear in mind that the improvement in mechanical properties such as ductility and toughness should be achieved without losing other properties of PLA such as modulus and

Chapter 2 tensile strength. The addition of nanoparticles also improved the interface and had an impact on the blend morphology and properties of PLA due to their selective localization.

The promising applications of PLA/PMMA blends are in injection moulded articles, shape memory applications and automotive applications. PLA/PC blends are well suited electronic casing and electronic packaging; PLA/PC (40/60) blend was successfully used by the Samsung Company for manufacturing the casing of the mobile phone Samsung ReclaimTM. However, these blends are not durable for automotive applications and PMMA is considered better for such applications. PLA/PA blends can also find potential applications in electronic casing and 3D printability of this blend is ongoing research.

Hence several petroleum based plastics could be replaced within years by PLA compounds for various engineering applications. Besides energy and environmental concerns, efforts to solve PLA's shortcomings via blending or other possible routes and the introduction of PLA-based products with better performance and desired properties is the new thrust for researchers. Nevertheless, extensive research still needs to be carried out to develop PLA-based compounds/blends with superior performance, fewer shortcomings, lower cost, and desirable multifunctional properties.

Among the various blends polyamides steal the limelight not only due to their exceptional properties such as higher HDT, ductility, and toughness but even due to the fact there are various new biobased polyamides in the market. A blend system of PLA/PA with almost 100% biobased content for durable applications could be of high interest. Hence, in the thesis effort has been made to develop a novel biobased PLA/PA system with enhanced morphologies and interface and is suitable for high-value applications.

Objectives and Workplan (Article organization) Based on the intensive literature survey we concluded that blends of PLA/PA could be of great interest as polyamide could improve the heat resistance, ductility and toughness thereby compensating the limitations of PLA and making it suitable for durable applications.

As mentioned previously new biobased polyamides are available in the market namely PA10- 10 and PA10-12. PA11 is relatively not new but is a good candidate to blend with PLA. PA12 is not a biobased polymer; nevertheless, intensive research is being carried out by Evonik industries to develop a biobased PA12. The close proximity of melting points of these polyamides and PLA eliminates the necessity to use chain extenders and facilitates easier extrusion. In the first step PLA/PA blends were prepared in a twin-screw extruder and thorough

Chapter 2 quantitative and qualitative analysis was done such as blend composition, morphological, thermal, rheological, interfacial properties and mechanical properties (ductility and toughness) characterizations. Depending on the results, a blend system with an apt blend composition showing the best combination of interfacial, thermal and mechanical properties was selected. Hence, it can be said that the apt polyamide was screened based on the thorough analysis of the blends. The findings of this study let to an article entitled “Compatibility in Biobased Poly(L-Lactide)/Polyamide Binary Blends: From Melt-State Interfacial Tensions To (Thermo)Mechanical Properties” and this article was published in Journal of Applied Polymer Science in August 2019.

In the next step, further improvement of the blends was aimed. The classical way to improve the properties of a blend is to add compatibilizer which improves the interface and interfacial adhesion leading to better thermal and mechanical properties. PLA grafted maleic anhydride was used as a compatibilizer. The content of compatibilizer was optimized by analysis various properties such as morphological, thermal and mechanical. These results led to an article entitled “Tuning the (thermo)mechanical properties of PLA/PA12 blends with the addition of PLA-g-MA as a compatibilizer”

Optimizing the processing parameters of the extruder was the next step; it is a limited explored area and achieving improvement in properties with this method eliminates the usage of compatibilizers thereby reducing the cost of the overall process. In this step high-speed extrusion was done and the screw speed and feed rate of the extruder were optimized keeping the PLA/PA blend composition constant. Initially different screw speeds including 200, 500, 800 and 1100 rpm were tried and after thorough analysis of properties a screw speed was selected. This selected screw speed was kept constant and various feed rates were tried namely 2, 3.5, 4 and 5 kg/h. The analysis included morphological, thermal, rheological and mechanical property characterizations. A comprehensive text on this is presented in the form of article “High Shear Extrusion Processing of Poly(L-Lactide) and Polyamide-12 Blends: Effect of Screw Speed and Feed Rate on (Thermo)Mechanical Properties”. A schematic representation of the work plan is presented in Figure 2.26.

Finally, the highlights of the work and the conclusions drawn from the study and perspectives are briefly discussed.

Chapter 2

Screening of apt PA (PLA/PA blends, morphology, interfacial , rheological, thermal & mechanical properties)

Optimising the extrusion Addition of compatibilizer parameters (high speed extrusion)

Morphology, thermal, Morpology, thermal & rheological & mechanical mechanical properties properties

Figure 2.26. Schematic representation of the workplan

Chapter 3

3 Compatibility in Biobased Poly(L-Lactide)/Polyamide Binary Blends: From Melt-State Interfacial Tensions To (Thermo)Mechanical Properties Amulya Raj1,2, Kalappa Prashantha1,2 and Cédric Samuel1,2*

1 Ecole Nationale Supérieure Mines Telecom Lille Douai, Institut Mines Telecom Lille Douai (IMT Lille Douai), Département Technologie des Polymères et Composites & Ingénierie Mécanique (TPCIM), 941 rue Charles Bourseul, Douai, F-59508, France 2Université de Lille, Lille, F-59000, France

Paper published in Journal of Applied Polymer Science, 2019, 136, 48440

Correspondence to: Cédric Samuel ([email protected])

Abstract: The melt compatibility between poly(L-Lactide) and polyamides with related thermomechanical properties are addressed. A particular attention is paid to four commercial polyamides with extrusion processing temperatures close to PLA (PA10-10 to PA12). PLA/PA blend morphologies without compatibilizer are first revealed by SEM. PA12 displays the best droplet dispersion into PLA (Dn 700 nm) whereas a poor interfacial adhesion is attested for

PLA/PA10-10 blends. Interfacial tensions corroborate the PLA/PA10-10 incompatibility (12

9 mN/m, 240°C) with decreasing 12 in the order PLA/PA10-10 > PLA/PA11 > PLA/PA12 (12 2 mN/m). Surface tensions confirm the highest compatibility between PLA and PA12. Ductilities, toughnesses and thermal resistances of PLA/PA blends are evaluated up to 40 wt% PA. Brittle-to-ductile transitions are observed for PA content higher than 30 wt% with the highest ductility for PLA/PA12, in accordance with their enhanced compatibility. Impact strengths display similar trends with a 2-fold increase for PLA/PA12. An outstanding synergy between PLA and PA is highlighted by DMA with heat deflection up to 130°C for PLA/PA blends. The synergy arises from a peculiar crystallization of PLA in the presence of PA. PLA/PA morphologies/interfaces can be consequently tuned by an appropriate PA choice with interesting improvements of thermomechanical properties for high-performance/durable applications.

Chapter 3

Introduction Poly(L-Lactide) (PLA) is one of the most promising biobased polymer due to its inherent renewability, biodegradability, good transparency, high modulus and biocompatibility [8,10,19,21,65,175–178]. These pronounced properties make PLA a potential replacement for many of the engineering polymers. Some of the already implemented applications include resorbable sutures, textiles, food and beverage containers, coated papers and other moulded articles [8,10,19,21,65,175–178]. However, there are few disadvantages in PLA such as low ductility, low heat deflection temperature and poor barrier properties that clearly restrict PLA market penetration into high-added value applications (automotive, electronics and smart applications) [8,10,19,21,65,175–178]. Polymer blends processed by extrusion technologies represents one of the most effective approach to overcome these limitations at reasonable development costs and, in this respect, various PLA-based complex formulations with polyethylene, aliphatic polyesters, polyamides or polycarbonates were successfully developed [10,177]. Polyamides (PA) are engineering plastics of high interest for PLA-based blends due to their intrinsic high ductility, stiffness, impact strength and thermal stability. Interestingly, many new biobased PA are now available from renewable resources at industrial scale (Arkema, Evonik, DSM and EMS Grivory mainly) such as polyamide10-10 (PA10-10) and polyamide10-12 (PA10-12) derived from sebacic acid. Also, longer aliphatic polyamides such as polyamide-12 (PA12) are also expected be derived from a biobased source [179]. These new classes of biobased PA could theoretically represent ideal blending partners with PLA for high- performance applications while optimizing the biobased content in the final blend system. Several types of PLA/PA blends have been reported in the recent years such as PLA/PA6, PLA/PA6-10, PLA/PA10-10 and PLA/PA11 [117,157,160,164,166,167,180]. Immiscible blends with characteristic matrix/droplet or co-continuous morphologies are encountered and various compatibilizers have been successfully employed such as poly(ethylene-co-octene) elastomer grafted with pendant maleic anhydride moieties, poly(ethylene-co-glycidyl methacrylate-graft-styrene-co-acrylonitrile) and epoxide-based compounds [164,180,181]. This later strategy has been efficiently developed on PLA blends with 50 wt% of partly- biobased PA6-10 to form an in-situ copolymer at the PLA/PA6-10 interface during melt blending in an extrusion process [164]. Improved tensile strength, elongation at break, flexural strength and un-notched impact strength are reported with high concentrations of epoxy compatibilizer (approx. 3phr). PLA/PA11 blends have been widely considered in the recent years as both are renewable and form a 100% biobased system in addition to interesting final

Chapter 3 properties [117,129,166–169,172,173,180,182]. Compatibilization strategies include esteramide exchange reactions using titanium(IV)isopropoxide and p-toluenesulfonic acid or incorporation of nanoparticles such as halloysites, organo-modified montmorillonite and sepiolite to get refined morphologies and improved mechanical properties [117,167,168,171,172]. The case of PLA/PA11 blends was thoroughly investigated and interestingly, several authors concluded about an inherent good compatibility between these two materials without any compatibilizer [166]. Micronic to sub-micronic dispersion of PA11 into PLA was found together with considerable improvements in the PLA mechanical properties [166]. A low interfacial energy was suspected, indicating that compatibilizers are not always mandatory in PLA/PA systems. Taking into consideration the high impact of the interfacial tension (together with rheological and processing considerations) on morphology formation (droplet sizes, droplet dispersion/distribution, coalescence dynamics, interface qualities/widths, etc.) and final properties[183–186], a specific attention to interfacial properties between PLA and PA is required to develop new PLA/PA formulations with enhanced properties at lower costs. In particular, high interfacial tensions (12) tend to generate weak interfaces and might result in a poor blend with detrimental properties whereas low 12 values induce broad interfaces and optimal interfacial adhesion.[166] Interfacial tension values for PLA/PA systems in the melt state remains largely unknown and only PLA/PA11 has been recently considered by various authors [117,129,169,170,187]. Conflicting results are obtained ranging between 0.8–5.6 mN/m at 200 – 220°C indicating a significant level of compatibility between PLA and PA11 blends in the melt state, in agreement with solid-state properties [166]. However, PLA/PA interfacial tensions need to be refined and extended to various PA grades/types to get reliable information about PLA/PA melt compatibility. Taking into account the recent developments of biobased PA with melting/processing temperatures close to the one of PLA (in particular PA10-10, PA11, PA10-12 and PA12), significant modifications of the PLA/PA melt compatibility are also probable in PLA/PA associations and interfacial tensions in the melt state could help to select PLA/PA grades for future applications. Hence, in an effort to improve (thermo)mechanical properties of PLA by melt-blending with PA using extrusion technologies, the melt-state compatibility between PLA and (biobased) PA is first investigated. Several PLA/PA blends are prepared (i.e. PLA/PA10-10, PLA/PA10-12, PLA/PA11 and PLA/PA12) without the use of any compatibilizers. The dispersion of PA droplets within a PLA matrix (i.e. 20 wt% PA) is addressed by scanning

Chapter 3 electron microscopy and interfacial/surface tensions in the melt state are subsequently assessed for these associations to get quantitative analysis of the melt compatibility. Related (thermo)mechanical properties of PLA/PA blends up to 40 wt% PA are then evaluated to detect improvements in the PLA ductility, impact toughness and thermal resistance/heat deflection temperature (HDT). Results are discussed based on various hypotheses with a specific focus on the PLA/PA compatibility level.

Experimental section

3.2.1 Materials Poly(L-lactide) (PLA, grade 4032D, NatureWorks®, biobased content 100%), poly(amide10-10) (PA10-10, Vestamid Terra DS16, Evonik, biobased content 100%) and poly(amide10-12) (PA10-12, Vestamid Terra DD16, Evonik, biobased content 45%) in pellet form were procured from Natureplast (Ifs, France). Poly(amide-11) (PA11, grade BMNO, biobased content 100%) in pellet form was procured from Arkema. Two poly(amide-12) grades (PA12HV and PA12LV, Rilsamid AESNO & AMNO respectively) in the pellet form were also procured from Arkema. Densities at ambient temperature, melting temperatures and melt volume rates are tabulated in Table 3.1. Table 3.1. Specific densities at ambient temperature, melting temperature and melt volume index of as-selected materials (manufacturer datas).

Density (g/cm3) Melting temperature (°C) Melt volume index

(cm3/10min)

PLA 1.24 171 6.5 (210°C)

PA10-10 1.05 204 n.a.

PA10-12 1.03 190 n.a.

PA11 1.01 189 36 (235°C)

PA12HV 1.01 179 2.1 (235°C)

PA12LV 1.01 179 57 (235°C) n.a. not available

Chapter 3

3.2.2 Processing of PLA/PA Blends Before extrusion trials, all materials were dried under vacuum at 80°C overnight. PLA/PA10-12, PLA/PA10-10, PLA/PA11 and PLA/PA12 of various compositions (PLA/PA wt% 90/10, 80/20, 70/30 and 60/40) were processed by twin-screw extrusion using a Haake Rheomex PTW 16 OS twin-screw extruder (Thermoscientific, Germany, screw diameter 16 mm, L/D 40). Blends were extruded with an average speed of 80 rpm, a total mass flow rate close to 500 g/h and a mean residence time of approx. 2 minutes. Similar temperature profiles were used from the hopper to the die (180°C, 190°C, 200°C, 210°C, 215°C, 215°C, 215°C, 215°C, 210°C and 195°C). The extruded blends were cooled in a water bath, pelletized and dried under vacuum at 80°C overnight before being used for subsequent molding processes. Neat polymers were also processed through a similar procedure. Tensile test specimens (ISO-527-1 standards, gauge length of 50 mm, width of 5 mm and thickness of 1.5 mm) were fabricated by injection-molding using a Haake Minijet II molding machine (chamber temperature 215°C, mold temperature 45°C, injection pressure 550 bar, cooling time 5s). Impact test specimens (parallelepiped shape, length of 80 mm, width of 10 mm, thickness of 4 mm) and DMA specimens (parallelepiped shape, length of 80mm, width of 10 mm and thickness of 4 mm) were fabricated by injection-molding using a Babyplast 6/10P injection molding machine (injection temperature 200 – 210°C, mold temperature 35 – 50°C, injection/consecutive pressure 60/80 bar, cooling time of 10s). Specimens for rheology experiments (diameter 40 mm, thickness 2 mm) were fabricated by compression-molding using a Dolouet molding machine at 210°C (melting time and low pressure cycle 8 min, high pressure cycle 5 min).

3.2.3 Characterizations

3.2.3.1 Scanning Electron Microscopy (SEM) The morphology of the compression-molded and injection-molded PLA/PA blends was observed using a scanning electron microscope (Neoscope II, JEOL, magnification x60,000). Morphologies were revealed by cryo-fracture in liquid nitrogen and fractured surfaces were sputtered with gold under vacuum in order to make them conductive. Droplet sizes were measured by image analysis using Image J software on at least 50 droplets followed by size classification into 7/8 classes. Volume/number average droplet radius diameters (Dv, Dn) with the droplet diameter polydispersity (ID) were calculated using equations 3.1-3.3. Two samples per blend were used to get standard deviations on droplet size measurements.

∑ 푛푖× 푅푖 퐷푛 = 2 × ∑ 푛푖 (3.1)

Chapter 3

4 ∑ 푛푖× 푅푖 퐷푣 = 2 × 3 ∑ 푛푖× 푅푖 (3.2)

퐷푣 퐼퐷 = 퐷푛 (3.3) with Dn the droplet number-average diameter, Dv the droplet volume-average diameter, Ri the average droplet radius of the i-class and ni the number of droplets into the i-class.

3.2.3.2 Dynamic Rheology Dynamic rheology experiments were performed on compression-molded PLA/PA blends using a dynamic rheometer (Haake Mars III, ThermoScientific) at 220°C (temperature stabilization time 3 min) using plate – plate geometry (diameter 35 mm, gap 1 mm) under nitrogen atmosphere to limit thermal degradation. Strain sweep tests were first carried out with an angular frequency of 1 rad.s-1 starting from an initial deformation of 0.01% to 10% to evaluate the linear viscoelasticity domain. Frequency sweep tests were subsequently carried out over an angular frequency range between 100 and 0.4 rad/s (optimal strain 10%).

3.2.3.3 Melt-State Surface Tension Surface tensions of neat PLA and neat PA were evaluated by the pendant drop method in the melt state using a DSA100 goniometer (Krüss, Germany) equipped with a specific test chamber for high-temperature measurements. Neat polymers were extruded through a 2mm- diameter metal syringe and pendant drops of molten polymers with volumes ranging from 10 to 15 µL were formed at the die exit in a nitrogen atmosphere. Data were processed with the DSA software to fit pendant drop profiles with a Young-Laplace model and access surface tension. Two measurements were performed for each polymer.

3.2.3.4 Tensile and Impact Tests Tensile tests were carried out on injection-molded PLA/PA dumbbell specimens using a tensile testing machine (Instron 3110, U.K, 1kN force cell). Tensile properties were determined at 25°C and 50% RH according to ISO 527-1 (pre-load of 0.5N, strain rate 1mm/min for Young’s modulus and 10mm/min for general properties. At least five dumbbell specimens were tested. Charpy notched/un-notched impact tests were carried out on injection- molded PLA/PA rectangular specimens using a pendulum impact machine (Model 5101, Zwick, Germany, impact energy of 7.1J). Impact strengths were determined at 25°C, 50% RH according to ISO 179-1 standard. At least seven dumbbell specimens were tested.

3.2.3.5 Dynamic Mechanical Analysis (DMA) Dynamic mechanical analyses were carried out on injection-molded PLA/PA rectangular specimens using a Metravib DMA+150 in dual cantilever mode. The samples were

Chapter 3 heated from 25°C to 130°C at a heating rate of 2°C/min and frequency of 10 Hz. At least two rectangular specimens were tested. Before DMA experiments, specimens were annealed at 110°C for 6h. Thermal resistance/heat deflection temperatures (HDT) were estimated at specific values of the storage modulus (400 MPa and 250 MPa for HDTa/HDTb respectively), a method partially implemented in several studies.[188,189] These storage modulus values were first calibrated using known values on polyamides and temperatures where storage modulus drops down to 400MPa and 250MPa were named T400MPa-HDTa and T250MPa-HDTb respectively [190].

3.2.3.6 Selective Dissolution Studies Morphologies and PA continuity in PLA/PA the blends were also revealed by selective dissolution of blends into chloroform as an excellent solvent for PLA and a non-solvent for polyamides. Predetermined weights of PLA/PA blends were dissolved in chloroform and left for dissolution for a week at ambient temperature. Solutions were filtered, thoroughly washed with chloroform and dried at ambient temperature. This procedure was repeated several times until a constant weight was reached. The PA continuity was determined by weighing the PA residue collected after dissolution/filtration experiments and calculated by the equation 3.4. 푚 % 푃퐴 퐶표푛푡푖푛푢푖푡푦 = 100 × ( 푟푒푐표푣푒푟푒푑푃퐴) (3.4) 푚푡표푡푎푙푃퐴 with mrecoveredPA the amount of polyamide recovered after dissolution/filtration and mtotalPA is the initial amount of polyamide in the blend.

3.2.3.7 Differential Scanning Calorimetry (DSC) Differential scanning calorimetry (DSC Mettler Toledo) was used to measure the PLA crystallinity in neat PLA and PLA/PA blends, in particular for injection-molded specimens after an annealing step at 110°C. The analysis was carried out under nitrogen atmosphere and a single heating scan was used for this purpose at various heating rates (5°C/min, 10°C/min and 20°C/min). PLA melting temperatures and associated enthalpy of fusion (Tm and ΔHm) were extracted and the PLA crystallinity index (Xc-PLA) was calculated according to equation 3.5. Note that PLA crystallinities were normalized to the amount of PLA in PLA/PA blends.

훥퐻푚−푃퐿퐴−훥퐻푐푐−푃퐿퐴 푋푐−푃퐿퐴 = 0 × 100 (3.5) 훥퐻푚 with Xc is the degree of crystallinity, ΔHm-PLA the melting enthalpy of PLA, ΔHcc-PLA the cold 0 crystallization enthalpy of PLA during heating and ΔHm the melting enthalpy of fusion for 100% crystalline PLA (93 J/g) [191].

Chapter 3

Results and Discussions

3.3.1 Melt-State Compatibility of PLA/PA Blends The first major focus of this study was the investigation of the melt-state compatibility between PLA and various commercially-available (biobased) polyamides (PA10-10, PA10-12, PA11 and PA12). Actually, interfacial tension data between PLA and as-selected PA could not be found explicitly in literature and only PLA/PA11 were investigated with some contradictory results [117,129,169,170,187]. In this context, interfacial tensions were addressed for further selection of an appropriate PA grade/type and development of highly-compatible PLA/PA blends with optimized morphologies/interfaces [117]. From previous studies by our group, a rheological method based on the emulsion model for viscoelastic fluids was used to measure the interfacial tension with a precision close to 20%.[192,193] Interfacial tension (12) can be evaluated from the equation 3.6.[194,195]

푅η (19퐾+16)[2퐾+3−2∅(퐾−1)] 푚 훾12 = (3.6) 4휆푓[10(퐾+1)−2∅(5퐾+2)] with ηm the newtonian viscosity of matrix phase, K the newtonian viscosity ratio (between the dispersed PA phase and the PLA matrix phase) and ϕ the volume fraction of the PA dispersed phase. Two parameters need a specific attention: (i) the droplet radius R of the PA dispersed phase and (ii) the droplet form relaxation time f at the considered temperature. Other parameters such as the newtonian viscosity of neat polymers (extrapolated viscosity to zero shear rate) and newtonian viscosity ratio are constant and readily accessible by dynamic rheology (Figure A1). PLA/PA blends with 20 wt% PA were specifically produced by twin- screw extrusion to insure matrix/droplet morphologies. It should be noted that specimens were also post-processed by compression-molding to get stabilized PA droplet morphologies as ideal microstructures for the analysis of the interfacial tension by microscopy coupled to dynamic rheology. Scanning electron microscopy (SEM) was first used to measure PA droplet size (droplet diameter and size polydispersity) into the PLA matrix and Figure 3.1 displays SEM images of cryogenically-fractured PLA/PA samples. PLA represents the continuous matrix phase and PA droplets are unambiguously observed. PA droplet sizes with standard deviations are tabulated in Table 3.2 with respective Dn, Dv, Ip and blend viscosity ratio. The highest Dn is obtained for PLA/PA10-10 blend with Dn close to 1.4 µm. All other blends displayed droplet diameters in the range 1.0 – 1.2 µm, except for PLA/PA12HV with sub-micronic PA12HV dispersion into PLA (Dn ≈ 700 nm).

Chapter 3

a) b)

c) d)

Figure 3.1. Morphologies of (a) PLA/PA10-10, (b) PLA/PA10-12, (c) PLA/PA11, (d) PLA/PA12HV and (e) PLA/PA12LV blends with 20 wt% PA processed by twin-screw extrusion followed by compression-molding (for sake of clarity, PA droplets embedded in a PLA matrix).

PA droplet sizes and dispersions within the micron-scale for PA10-10, PA10-12, PA11 and PA12LV are in agreement with previous results from literature [157,166] and also consistent with the viscosity ratios close to the unity (between 0.4 and 1.4, indicating optimal PA grades for PLA from a melt viscosity point of view). By comparision with PA12LV, a surprising sub- micronic dispersion is obtained for PA12HV and, based on droplet/particle coalescence and sintering models [184,196–198], this effect could be attributed to the elevated viscosity of

Chapter 3

PA12HV (and elevated viscosity ratio, p ≈ 5) that increases droplet coalescence time at zero shear rate (during the compression molding process).

Table 3.2. Number-average and volume-average PA droplet diameters (Dn and Dv), PA droplet size polydispersity (Ip) and PA/PLA viscosity ratio (p) as a function of the PA type (standard deviations into brackets).

1 Blend Dn (µm) Dv (µm) Ip p

PLA/PA10-10 1.4 (0.1) 2.0 (0.4) 1.4 0.7

PLA/PA10-12 1.0 (0.1) 1.2 (0.1) 1.2 1.4

PLA/PA11 1.2 (0.1) 1.6 (0.1) 1.2 0.5

PLA/PA12HV 0.7 (0.1) 0.9 (0.1) 1.2 4.9

PLA/PA12LV 1.2 (0.1) 1.4 (0.1) 1.2 0.4

1 * * Evaluated by dynamic rheology at 100 rad/s and 220°C (p =  PA/ PLA), see Figure A1. Such conclusions are also in agreement with previous studies on polyolefin/polystyrene blends regarding the effect of the droplet viscosity on coalescence dynamics [185,199]. It could be concluded that the sub-micronic morphology obtained for PLA/PA12HV blend is probably in an unstable state due to excessive coalescence time. Droplet coalescence/sintering models also indicate that interfacial tension partly controls the coalescence dynamics and thus confirming the importance of such datas for morphology control. Some qualitative analysis regarding the PLA/PA interface quality could be discussed from SEM. In particular, PLA/PA10-10 blends are prone to an intensive interfacial debonding with large interfacial voids and significant PA10-10 particle pull-out during sample preparation (Figure 3.1a). An extremely weak interface is thus observed for PLA/PA10-10 and stronger interfaces are detected for all other blends (Figure 3.1b-e). Reduced interfacial debonding in PLA/PA12 blends seems to be observed with the best interface for PLA/PA12HV blends.

Chapter 3

a) b)

Figure 3.2. Storage modulus (a) and imaginary part of the complex viscosity (b) as a function of the angular frequency for neat PLA and PLA/PA blends (20 wt% PA) at 220°C (compression-molded samples).

Based on above morphological results, a quantitative evaluation of the interfacial tension was then attempted in as-prepared PLA/PA blends using dynamic rheology. Droplet form relaxation times need to be extracted from rheological data to address the interfacial tension in PLA/PA blends. Blends were subsequently subjected to dynamic frequency sweeps at 220°C to evaluate the storage modulus as a function of the frequency and the associated imaginary part of the complex viscosity (Figure 3.2). Neat PLA exhibits a classical melt-state behavior with a low frequency slope of 2 indicating fully-relaxed chains at 220°C. For PLA/PA blends, the droplet form relaxation process could easily be observed at low frequencies with an extra-elasticity compared to neat PLA. Table 3.3. Average interfacial tension in as-studied PLA/PA blends at 220°C (average values, standard deviation into brackets).

Blend Interfacial tension 12 (mN/m)

PLA/PA10-10 9.0 (0.8)

PLA/PA10-12 6.0 (0.5)

PLA/PA11 4.9 (0.3)

PLA/PA12LV 2.0 (0.1)

The extraction of droplet relaxation time could be performed by two methods, (i) the weighted relaxation time spectrum obtained by a regularization method and/or (ii) the imaginary part of the complex viscosity [194,200,201]. Related protocols with their reliability

Chapter 3 are documented elsewhere[193]. Droplet relaxation times are tabulated in Table A1-A2. Average values of the interfacial tension for all blends are tabulated in Table 3.2.

The interfacial tension is found to be the lowest for PLA/PA12LV (12 = 2.0 mN/m) and the highest for PLA/PA10-10 (12 = 9.0 mN/m), such trend was found in accordance with interfacial debonding phenomena observed by SEM (Figure 3.1d-e). Intermediate values are detected for PLA/PA10-12 (12 = 6.0 mN/m) and for PLA/PA11 (12 = 4.9 mN/m). This latter is consistent with recent works (12 = 5.6 mN/m) on PLA/PA11 blend and could attest for a good reliability of our interfacial values [117,129,170]. It should be also noted that PLA/PA12HV was not tested due to overlapping of the droplet relaxation process with the terminal relaxation of neat PA12HV. Consequently, a clear trend appears in terms of interfacial tension in the melt state and subsequent melt compatibility with the highest compatibility between PLA and PA12. An effect of the PA aliphatic chain length is also suspected here with potential linkage to thermodynamic properties of neat components. To strengthen and validate this trend, surface tension in the melt state of neat materials was then accessed by the pendant drop method in N2.

Figure 3.3. Pendant drops profiles at 180°C for neat PLA and at 240°C for neat PA10-10, neat PA11 and neat PA12LV (from left to right).

The compatibility in the melt state could also be approached by a careful evaluation of the surface tension of neat polymers with indirect correlations to their interfacial tension.37

Surface tensions were subsequently evaluated by the pendant drop technique in N2 and pendant drop profiles are displayed in Figure 3.3. It should be noted that surface tension of neat PA was measured at 240°C to (i) optimize the stabilization time of the pendant drop (approx. 5 min) and (ii) get comparative between as-selected all PA. However, to avoid excessive thermal degradation of PLA, surface tension of PLA was only measured at 180°C (followed by extrapolation to 240°C). Excellent drop shape fittings with the so-called Young-Laplace model were observed for all polymers (see Figure A2) with drop shape parameter B close to the

Chapter 3 optimum value (B = 0.65 – 0.67) insuring a reliable surface tension value. Surface tensions at 240°C with relative standard deviations lower than 3% were extracted and values are tabulated in Table 3.4. The surface tensions of PA10-10, PA11 and PA12 were found to be 31.2, 29.4 and 27.5 mN/m respectively.

Table 3.4. Specific melt density and surface tension (s) of neat PLA and neat PA at 240°C obtained by the pendant drop method. 3 Specific melt density (g/cm ) s (mN/m)

PLA 1.05b 21.9 (0.5)c

PA10-10 0.88 a 31.2 (0.2)

PA10-12 0.86 a n.m.

PA11 0.85 a 29.4 (0.3)

PA12LV 0.84 a 27.5 (0.2)

PA12HV 0.84 a n.m. a Evaluated at 240°C and 0.1 MPa by PvT experiments. b Evaluated at 180°C and 0.1 MPa by PvT experiments. c Extrapolated value from 180°C with a ds/dT coefficient of 0.06 mN/m/°C.[186]

Experimental s 25.5 mN/m (+/- 0.5 mN/m) at 180°C. n.m. not measured, viscosity too high. The PA surface tension clear decreases with the length of its aliphatic sequence. Such trends/values seem consistent with some studies on PA melt-state surface tensions and a decrease of the PA polarity with the length of the aliphatic sequence [186]. It should be noted that PA10-12 was not evaluated due to the high viscosity of this PA grade. The surface tension of PLA is clearly lower than all PA (s 21.9 mN/m) in agreement with several studies on the PLA surface tension at elevated temperatures [202–206]. Surface tensions of PLA and PA12 were subsequently found to be the closest (s-PLA – sPA12 = 5.6 mN/m) than other PLA/PA associations. The clear correlation with previous interfacial tensions confirms the enhanced compatibility between PLA and PA12 in the melt state. This obvious correlation also tends to

Chapter 3 prove the absence of intensive exchange reactions during extrusion leading to in-situ compatibilization.

Tensile Mechanical Properties The second major focus of this study deals with the evaluation of final (thermo)mechanical properties of PLA/PA blends as a function of PA type and for PA content up to 40 wt% into PLA. These results are of high technological interest for final applications but could also bring interesting correlations with the compatibility level previously evaluated for PLA/PA blends (and the related quality of PLA/PA interfaces developed during extrusion). In this respect, blends were injection-molded and mechanical properties were first assessed by means of tensile tests. Typical stress – strain curves of PLA/PA12HV blends as a function of PA12HV content (up to 40 wt%) are plotted in Figure 3.4 and stress-strain curves obtained for other PLA/PA associations could be found in Figure A3-A4-A5. Young’s modulus, stress at yield and strain at break were evaluated from tensile tests and their evolutions with the amount of PA (by weight) are tabulated in Table A3, Table A4 and Figure 3.5 respectively. PLA is a brittle polymer with poor ductility (strain at break 3.4%) and high stiffness (Young’s modulus 2885 MPa and stress at yield 66.6 MPa) in accordance with many reports on PLA mechanical properties [8,10,19,21,65,175–178]. Mechanical properties of PLA are obviously improved upon blending with PA12HV with a clear brittle-to-ductile transition for PA12HV content higher than 30 wt% (Figure 3.4). The

Figure 3.4. Typical stress – strain curves evaluated by tensile tests for neat PLA (a), neat PA12HV (e) and PLA/PA12HV blends with 20 wt% PA12HV (b), 30 wt% PA12HV (c) 40 wt% PA12HV (d) (insert: zoom between 0 – 50% strain).

Chapter 3 strain at break reached a maximal value of approx. 170% in PLA/PA12HV blends with 40 wt% PA12HV representing a remarkable ductility for a PLA-based blends at such elevated PLA content [8,10,65,176,177]. Brittle-to-ductile transitions are also observed upon blending other PA but maximal strain only reached 65% for PA10-10 or PA10-12 and 90% for PA11 (Figure 3.5). In this respect, PLA/PA12HV blends displayed the best ductility among all PLA/PA associations with highest strains at break (Figure 3.5). It should be noted that other mechanical parameters (Young’s modulus and stress at yield) display classical evolutions with balanced stiffness and tensile strength but no clear trend could be extracted in term of PLA/PA compatibility [117,166–169]. As a conclusion on tensile properties of injection-molded PLA/PA, brittle-to-ductile transitions are observed for all PLA/PA for PA content higher than 30 wt% but a specific enhancement is highlighted with the use of PA12HV. This effect seems to be consistent with previous trends on PLA/PA compatibility (PA12HV displayed the best compatibility with PLA), indicating that the interface quality could play an important role on tensile mechanical properties.

To stress the influence of the PLA/PA compatibility on tensile mechanical properties, additional experiments were performed to test various hypotheses (i.e. modification of the PA continuity and presence of elongated structures). The morphology of the PA phase in injection- molded tensile specimens was first investigated using selective dissolution in chloroform (an excellent solvent for PLA and a non-solvent for PA) to observe the continuity extent of PA. Chloroform solutions of PLA/PA blends with 30 and 40 wt% PA could be found in Figure A6 and Figure 3.6a displays the evolution of the PA continuity as a function of the amount of PA in all blends. No PA continuity is detected up to 20 wt% PA. PA continuity starts to increase for PA content higher than 30 wt% and a close correlation with brittle-to-ductile transitions could be concluded here.

Chapter 3

Figure 3.5. Evolution of the tensile strain at break with the amount of PA (by weight) for PLA/PA10-10, PLA/PA10-12, PLA/PA11 and PLA/PA12HV blends.

Only PLA/PA10-12 reached a full PA continuity at 40 wt% PA and all other blends are marked by mixed morphologies between PA droplets and continuous PA structures without clear trends regarding the effect of the PA type. Interestingly, PLA/PA10-10 and PLA/PA12HV blends reached similar PA continuity levels (80%) indicating that PA continuity does not control the specific ductility enhancement observed with PA12HV. PLA/PA blends are also known to produce elongated/fibrillar morphologies by injection-molding [129]. The presence of such structures in injection-molded specimens was also checked by SEM on transversal cross- sections of tensile specimens (Figure 3.6b and c). According to previous morphological analyses in Figure 3.1, refined elongated morphologies are detected for PLA/PA12HV blends processed by injection-molding compared to PLA/PA10-10 blends.

a) 74

Chapter 3

b) c)

Figure 3.6. Evolution of the PA continuity with the amount of PA for injection-molded blends (a). Morphology of injection-molded PLA/PA10-10 (b) and PLA/PA12HV (c) blends as observed by SEM (transversal cross-sections).

However, all blends clearly developed highly-elongated morphologies within the molding and the tensile drawing direction. In this context, the formation of highly-elongated and partially- continuous PA structures probably controlled the brittle-to-ductile transition but PLA/PA compatibility and interface quality also plays a major role on the final ductility of PLA/PA blends with a maximal impact using PA12HV

Impact Properties and Thermal Resistance of PLA/PA Blends For further developments of PLA/PA blends into high-performances applications, impact strength and thermal resistance represents key properties to overcome. Injection- molded PLA/PA blends were subsequently subjected to impact tests and dynamic mechanical analysis (DMA). Figure 3.7 first shows the un-notched Charpy impact strength of injection- molded PLA/PA blends. The impact strength of neat PLA was evaluated to 18.1 kJ/m² (±0.6 kJ/m²) in accordance with many previous reports about its high brittleness [8,10,19,21,65,175– 178]. PLA/PA10-10 and PLA/PA11 blends do not display any improvement of the impact strength in agreement with previous studies [8,117,168]. However, significant increases in impact strength are observed with the incorporation of PA10-12 and PA12HV with impact strengths up to 33 – 38 kJ/m² for 40 wt% PA12HV and PA10-12 respectively. The origin of such improvements using PA12HV and PA10-12 into PLA remains complex to address. The PLA/PA compatibility could play a key role on the final impact strength of PLA/PA blends but, here, highly-elongated morphologies with an orientation perpendicular to the impact test could also bring beneficial effects. Although the impact strength has been improved by a factor 2, actual results are not as eminent as the ductility enhancement observed on previous tensile tests. It should be also noted that notched impact tests were also performed but poor improvements of the impact strength upon blending with PA with unclear trends related to the

Chapter 3

PA type (Table A5). The storage modulus as a function of the temperature was evaluated by DMA for injection-molded PLA/PA blends (40 wt% PA) to evaluate their thermal resistances (i.e. heat deflection temperature, HDT) for high-temperature applications. It should be noted that annealed materials at 110°C were used to avoid the cold-crystallization of PLA and maximize subsequent thermal resistances.

Figure 3.7. Evolution of the unnotched impact strength with the amount of PA (by weight) for PLA/PA10-10, PLA/PA10-12, PLA/PA11 and PLA/PA12HV blends.

The storage modulus (E') gradually decreased with temperature with significant drop for temperature higher than 70°C corresponding to glass transition of annealed PLA (Figure 3.8). Interestingly, higher values of the storage modulus were observed for all PLA/PA blends compared to neat PLA (40 wt% PA) and HDT estimations are tabulated in Table 3.5 (T400MPa-

HDTa and T250MPa-HDTb, temperatures corresponding to storage moduli of 400 and 250MPa respectively). The thermal resistance of neat PLA was evaluated to 78°C and 84°C for T400MPa-

HDTa and T250MPa-HDTb respectively in accordance with several studies related to the heat deflection temperature of annealed PLA [178,188]. For neat PA, T400MPa-HDTa and T250MPa-HDTb values are found in the range 51 – 62°C and 71 – 93°C respectively (Table 3.5).

Chapter 3

Figure 3.8. Storage modulus as a function of temperature (DMA) for neat PLA and PLA/PA10-10, PLA/PA10-12, PLA/PA11 and PLA/PA12HV blends (40 wt% PA).

However, the thermal resistance of injection-molded PLA/PA blends obviously increases by approx. 30 to 45°C above the value of neat PLA with HDTb up to 130°C. Thus, an outstanding synergy between PLA and PA is revealed on final (thermo)mechanical properties. It should be noticed that the highest T400MPa-HDTa/T250MPa-HDTb are reported for PLA/PA11 blends. To the best of our knowledge, this effect was never identified and the synergy between PLA and PA is believed to arise from the annealing step of the specimens. In this context, annealed blends were subjected to DSC analysis for an evaluation of the PLA crystallinity after injection- molding and annealing. DSC first heating scans at various heating rates and related PLA melting temperatures/enthalpies could be found in Figure A7 and Table A6. Corresponding PLA crystallinities are tabulated in Table 3.5. The crystallinity of neat PLA was evaluated to 46%, in agreement with the annealing temperature/time used in this study[178,188]. All PLA/PA blends are marked by an important increase of the PLA crystallinity in the range 54 – 66% with a close correlation to the resultant thermal resistance.

Table 3.5. Thermal resistance (T400MPa-HDTa and T250MPa-HDTb) and PLA crystallinity (Xc-PLA, normalized to PLA content) of injection-molded and annealed neat PLA and PLA/PA blends (40 wt% PA) as evaluated by DMA and DSC (standard deviation into brackets).

a Blend T400MPa-HDTa (°C) T250MPa-HDTb (°C) Xc-PLA (%)

Neat PLA 78 (1) 84(1) 46 (1)

PLA/PA10-10 85 111 54 (4)

Chapter 3

PLA/PA10-12 85 (2) 118 (2) 65 (2)

PLA/PA11 95 (3) 130 (4) 66 (3)b

PLA/PA12HV 86 (4) 115 (3) 61 (4)

Neat PA10-10 51 93 n. a.

Neat PA10-12 59 74 n. a.

Neat PA11 62 86 n. a.

Neat PA12HV 56 71 n. a. a PLA crystallinity evaluated according to equation 3.5. b Approximate value due to melting peak overlapping n. a. not applicable

The highest PLA crystallinity is observed with the specific use of PA11, in agreement with the thermal resistance of this blend. In this respect, it could be concluded that the synergy between PLA and PA on thermomechanical properties is provoked by a specific enhancement of PLA crystallinity induced by the annealing step in the presence of PA. The PLA/PA compatibility clearly plays a minor role on the final thermomechanical properties and a careful investigation of the peculiar/additional crystallization of PLA in the presence of PA is required in near future.

Conclusions The study focused on the development of PLA/PA binary blends using twin-screw extrusion technologies in the absence of compatibilizer for high-performance applications. Several polyamides (PA10-10, PA10-12, PA11 or PA12) were selected and the PLA/PA compatibility in the melt state was first investigated. PLA/PA10-10 binary blends clearly displayed the lowest compatibility (high PA droplet sizes, intensive interfacial debonding and a high interfacial tension in the melt state close to that of 9mN/m). On the contrary, PLA/PA12 blends showed enhanced interfacial adhesion and low PA12 droplet size (Dn ≈ 700 nm) but also the minimal interfacial tension (12 close to 2 mN/m) and the closest surface tension. The PLA/PA compatibility level clearly increased with the length of the PA aliphatic sequence and the best compatibility was undoubtly concluded for PLA/PA12 binary blends. This interesting result about the enhanced PLA/PA12 compatibility probably found a thermodynamic origin (free volume, PA polarity, etc.) and future investigations in that way will be conducted for an advanced tuning/optimization of the PLA/PA compatibility.

Chapter 3

Concerning final properties linked to high-performance applications, tensile mechanical properties of injection-molded PLA/PA blends revealed a brittle-to-ductile transition above 30 wt% PA into PLA for all PA, a phenomenon primarily attributed to the formation of highly-elongated and partially-continuous PA structures. However, the highest ductilities were obtained for PLA/PA12 blends (approx. 170% at 40 wt% PA12) indicating that the PLA/PA compatibility played a key role on tensile mechanical properties. Similar trends were observed for the impact strength of un-notched PLA/PA blends. However, the impact strength only increased by a factor 2 compared to neat PLA and the PLA/PA compatibility seemed to play a minor role on impact properties. Future investigations related to PLA/PA formulation and processing are required to optimize their impact strength and reach tough PLA/PA blends. Finally, an outstanding synergy between PLA and PA was observed on thermal resistance of PLA/PA blends. Annealed PLA/PA specimens are marked by a spectacular increase of the resultant HDT up to 30 – 45°C than neat PLA. Such synergy between PLA and PLA was found to arise from a specific increase of the PLA crystallinity up to 65% during the annealing treatment in the presence of PA. The best thermal resistance was observed for PLA/PA11 blends. Despite a minor impact of the PLA/PA compatibility on final thermomechanical properties, such high crystallinity value is clearly of prime importance for engineering high-temperature applications and detailed scientific/technological investigations will be conducted in a near future on the crystallization behavior of PLA in the presence of PA and on creep/fatigue behavior at temperatures above 100°C.

Among all investigated blends, PLA/PA12 consequently represents the most promising association with the highest compatibility, good interface and highest tensile mechanical properties. Optimization of the impact strength and thermal resistance still need some efforts but high-performance applications are clearly accessible for this potentially 100% biobased PLA-based materials.

Acknowledgements Authors gratefully acknowledge both the International Campus on Safety and Intermodality in Transportation (CISIT, France), the European Community (FEDER funds) as well as the Hauts-de-France Region (France) for their contributions to funding extrusion equipments and characterization tools (dynamic rheometers, microscopes, goniometers, calorimeters and tensile benches).

Chapter 3

Chapter 5

4 Tuning of properties of PLA/PA12 blends with the addition of compatibilizer Amulya Raj1,2, Kalappa Prashantha1,2 and Cédric Samuel1,2* 1 Ecole Nationale Supérieure Mines Telecom Lille Douai, Institut Mines Telecom Lille Douai (IMT Lille Douai), Département Technologie des Polymères et Composites & Ingénierie Mécanique (TPCIM), 941 rue Charles Bourseul, Douai, F-59508, France 2Université de Lille, Lille, F-59000, France

Correspondence to: Cédric Samuel ([email protected])

Abstract

The effect of poly(L-Lactide) (PLA) grafted maleic anhydride (PLA-g-MA) as a compatibilizer on the properties of PLA and polyamide blends is addressed. The blends with compatibilizers are extruded in a twin-screw extruder. Various compositions of the blends are evaluated to optimize the content of PLA-g-MA. Morphology of the blends reveals good dispersion with strong interfacial adhesion, a droplet diameter of 1μm (for 2wt.% of compatibilizer) is obtained for the compression-molded samples. Ductility of the compatibilized blends shows remarkable improvement, PLA/PA12/PLA-g-MA (69/30/1) displayed the highest ductility (291%). The impact strength of compatibilized blends showed similar 2-fold improvement. Morphology of injection molded samples revealed finer dispersion and stronger interface. The droplet size (0.8μm), morphology of the transversal section displayed fibrillation of PA12 for 1 and 2 wt.% of PLA-g-MA. Considerable synergy was detected in DMA of the compatibilized blends; the thermal resistance of the blends showed improvement, this behavior could be due to the formation of copolymer leading to additional crystallization. Overall studies indicated that PLA-g-Ma is an efficient compatibilizer for PLA/PA12 blends as it enhances the (thermo)mechanical properties.

Introduction The replacement of petroleum-based polymers to biobased polymers is the need of the hour due to the detrimental effects of the former on the environment [5,7,207,208]. Among the various biobased polymers Poly(Lactide) (PLA), also known as Poly(Lactic acid) has gained special attention in recent years due to its remarkable properties such as renewability, biocompatibility, transparency, high modulus, and biodegradability [10,11,20,21,110,175,178]. These properties led to the commercialization of PLA thereby

Chapter 5 making it suitable for various applications such as resorbable sutures, food, and beverage packaging, surgical implants, scaffolds and textiles [9,10,13,14,16,33]. Despite its propitious properties and applications, the intricacy of implementing PLA in high value or durable applications is restricted due to its limitations as arising from low melt strength, inherent brittleness, poor impact strength and low heat resistance [10,11,20,21,90]. Various strategies have been investigated to improve the drawbacks of PLA including toughening mechanisms, copolymerization, stereocomplexation of PLA, addition of nanoparticles and nucleating agents and polymer blending [10,20,25,26,65,90,93,100]. Amongst all the strategies, polymer blending stands out owing to its feasible way to develop new material with versatile and tailor- made properties with additional benefit of being cost-effective [10,20,101,102]. Blending PLA with engineering polymers is an efficient way to overcome majority of limitations of PLA. Over the recent years intensive research is being carried on PLA based blends with engineering polymers such as polycarbonates, poly(methylmethacrylate), polyesters and polyamides [119,124,149,209,210].

Properties such as high ductility, impact strength, stiffness, and higher thermal resistance make polyamides one of the most suitable engineering polymers to blend with PLA. In general, PLA/PA blends are immiscible but compatible in nature. Different types of polyamides have been blended with PLA including PA6, PA11, PA6-10, PA10-10, PA10-12 and PA12 [157,163,165,166,210]. In order to achieve better final properties, it is necessary improve the compatibility further. One of the classic methods to improve the compatibility is by use of compatibilizers [101,177]. Multifarious compatibilization strategies have been employed in PLA/PA blends by addition of compatibilizers such as polyalkenyl-poly-maleic-anhydride- imide/amide agents, maleic anhydride fraction grafted on polyethylene-octene elastomer, epoxy based compounds and poly(ethylene-co-glycidyl methacrylate-graft-styrene-co- acrylonitrile) [157,159,165,169,174,180,181]. Walha F et al. studied the effect of multifunctional epoxide (Joncryl ADR®-4368) on PLA and PA11 blends, two routes of compatibilization were investigated: (i) One step mixing of adding all the components at once to the extruder, (ii) two-step process where initially PLA and Joncryl are premixed and PA11 is introduced later [169]. Rheological studies revealed good compatibility between the phases and formation of PLA-joncryl-PA11 copolymer was also confirmed. It was observed that both the processes improved the interfacial adhesion thus reducing droplet size (<1µm). Significant improvement in ductility was also observed with the highest being for PLA/PA11(80/20) and 0.7 wt.% of Joncryl ADR®-4368 (~355%) [174]. In another interesting study, maleic anhydride

Chapter 5 grafted on polyethylene octene elastomer was utilized as a compatibilizer in PLA/PA6 [163]. The blends showed good dispersion with lower domain size, good ductility, and impact strength. This is due to the formation of POE-co-PA6 and POE-co-PLA copolymers during the melt blending thereby improving the compatibility between the blend components [163]. There have been some recent studies about reactive compatibilization of PLA/PA11, catalysts such as p-toluenesulfonic acid and titanium isopropoxide have been incorporated to induce ester amide interchange reactions between the blend components [168,173].

PLA grafted maleic anhydride (PLA-g-MA) is another potential compatibilizer, it has been incorporated in PLA based starch, poly(ethylene terephthalate glycol) (PETG) and Poly(butylene adipate terephthalate) (PBAT) blends [177]. In PLA/starch blends the addition of PLA-g-MA has brought about an improvement in the interface by reducing the droplet size of the dispersed phase, significant improvement in tensile strength and ductility were also observed [177,211,212]. Additional improvement in morphology, tensile properties and ductility were found in PLA/PBAT and PLA/PETG blends in the presence of PLA-g-MA as the compatibilizer [177].

Maleic anhydride based coupling agents have been previously used in PLA/PA blends [159,181]. However, there are no significant studies of PLA-g-MA as a compatibilizer in the PLA/PA blend system. In our previous work we have reported various PLA blends with long- chain aliphatic polyamides. It was found that PA12 showed the best compatibility with PLA with low interfacial tensions, good dispersion and exceptional mechanical properties (ductility>170% for 40 wt.% PA12), good impact properties and thermal resistance [210]. Hence, in an attempt to obtain further improvement in compatibility, thermal resistance and mechanical properties we present the effect of compatibilizer on the properties of PLA/PA12 blends.

Experimental Section

4.2.1 Materials Poly(L-lactide) (PLA, grade 4032D, biobased content 100%, density 1.24 g/cm3, melting temperature 171°C, melt volume index 6.5 cm3/10 min at 210°C, according to manufacturer data) was supplied by NatureWorks®, USA. Poly(amide-12) (PA12, grade Rilsamid AESNO, density 1.01 g/cm3, melting temperature 179°C, melt volume index 2.1 cm3/10 min at 235°C, according to manufacturer data) was supplied by Arkema, France. PLA and PA12 were used as received in the pellet form. PLA grafted maleic anhydride (PLA-g-

Chapter 5

MA) was kindly provided to us by the University of Mons and the preparation method can be referred in the research work published by Quintana R et al.[213]. It has a maleic anhydride content of 0.45%.

4.2.2 Processing of PLA/PA Blends Before extrusion trials, all materials were dried under vacuum at 80°C overnight. PLA/PA12/PLA-g-MA (wt.% 70/30/0, 69/30/1, 68/30/2, 67/30/3, 66/30/4, 65/30/5) were processed by twin-screw extrusion using a Haake Rheomex PTW 16 OS twin-screw extruder (Thermoscientific, Germany, screw diameter 16 mm, L/D 40). Blends were extruded with an average speed of 80 rpm, a total mass flow rate close to 500 g/h and a mean residence time of approx. 2 minutes. Similar temperature profiles were used from the hopper to the die (180°C, 190°C, 200°C, 205°C, 210°C, 210°C, 205°C, 200°C, 195°C and 185°C). The extruded blends were cooled in a water bath, pelletized and dried under vacuum at 80°C overnight before being used for subsequent molding processes. Neat polymers were also processed through a similar procedure. Tensile test specimens (ISO-527-1 standards, gauge length of 50 mm, width of 5 mm and thickness of 1.5 mm) were fabricated by injection-molding using a Haake Minijet II molding machine (chamber temperature 215°C, mold temperature 45°C, injection pressure 550 bar, cooling time 5s). Impact test specimens (parallelepiped shape, length of 80 mm, width of 10 mm, thickness of 4 mm) and DMA specimens (parallelepiped shape, length of 80mm, width of 10 mm and thickness of 4 mm) were fabricated by injection-molding using a Babyplast 6/10P injection molding machine (injection temperature 200 – 210°C, mold temperature 35 – 50°C, injection/consecutive pressure 60/80 bar, cooling time of 10s). Specimens for rheology experiments (diameter 40 mm, thickness 2 mm) were fabricated by compression-molding using a Dolouet molding machine at 210°C (melting time and low-pressure cycle 8 min, high- pressure cycle 5 min).

4.2.3 Characterizations

4.2.3.1 Scanning Electron Microscopy (SEM) The morphology of the compression-molded and injection-molded PLA/PA blends was observed using a scanning electron microscope (Neoscope II, JEOL, magnification x60,000). Morphologies were revealed by cryo-fracture in liquid nitrogen and fractured surfaces were sputtered with gold under vacuum in order to make them conductive. Droplet sizes were measured by image analysis using Image J software on at least 50 droplets followed by size classification into 7/8 classes. Volume/number average droplet radius diameters (Dv, Dn) with

Chapter 5

the droplet diameter polydispersity (ID) were calculated using equations 4.1-4.3. Two samples per blend were used to get standard deviations on droplet size measurements.

∑ 푛푖× 푅푖 퐷푛 = 2 × ∑ 푛푖 (4.1) 4 ∑ 푛푖× 푅푖 퐷푣 = 2 × 3 ∑ 푛푖× 푅푖 (4.2)

퐷푣 퐼퐷 = 퐷푛 (4.3) with Dn the droplet number-average diameter, Dv the droplet volume-average diameter, Ri the average droplet radius of the i-class and ni the number of droplets into the i-class.

4.2.3.2 Tensile and Impact Tests Tensile tests were carried out on injection-molded PLA/PA dumbbell specimens using a tensile testing machine (Instron 3110, U.K, 1kN force cell). Tensile properties were determined at 25°C and 50% RH according to ISO 527-1 (pre-load of 0.5N, strain rate 1mm/min for Young’s modulus and 10mm/min for general properties. At least five dumbbell specimens were tested. Charpy notched/un-notched impact tests were carried out on injection- molded PLA/PA rectangular specimens using a pendulum impact machine (Model 5101, Zwick, Germany, impact energy of 7.1J). Impact strengths were determined at 25°C, 50% RH according to ISO 179-1 standard. At least seven dumbbell specimens were tested.

4.2.3.3 Dynamic Mechanical Analysis (DMA) Dynamic mechanical analyses were carried out on injection-molded PLA/PA rectangular specimens using a Metravib DMA+150 in dual cantilever mode. The samples were heated from 25°C to 130°C at a heating rate of 2°C/min and frequency of 10 Hz. At least two rectangular specimens were tested. Before DMA experiments, specimens were annealed at 110°C for 6h. Thermal resistance/heat deflection temperatures (HDT) were estimated at specific values of the storage modulus (400 MPa and 250 MPa for HDTa/HDTb respectively), a method partially implemented in several studies [188,189]. These storage modulus values were first calibrated using known values on polyamides and temperatures where storage modulus drops down to 400MPa and 250MPa were named T400MPa-HDTa and T250MPa-HDTb respectively [190].

4.2.3.4 Differential Scanning Calorimetry (DSC) Differential scanning calorimetry (DSC Mettler Toledo) was used to measure the PLA crystallinity in neat PLA and PLA/PA blends, in particular for injection-molded specimens after an annealing step at 110°C. The analysis was carried out under a nitrogen atmosphere and

Chapter 5 a single heating scan was used for this purpose at various heating rates (5°C/min, 10°C/min and 20°C/min). PLA melting temperatures and associated enthalpy of fusion (Tm and ΔHm) were extracted and the PLA crystallinity index (Xc-PLA) was calculated according to equation 4.5. Note that PLA crystallinities were normalized to the amount of PLA in PLA/PA blends.

훥퐻푚−푃퐿퐴−훥퐻푐푐−푃퐿퐴 푋푐−푃퐿퐴 = 0 × 100 (4.5) 훥퐻푚 with Xc is the degree of crystallinity, ΔHm-PLA the melting enthalpy of PLA, ΔHcc-PLA the cold 0 crystallization enthalpy of PLA during heating and ΔHm the melting enthalpy of fusion for 100% crystalline PLA (93 J/g) [191].

4.2.3.5 Extraction/separation experiments by centrifugation The PLA matrix and the PA12 dispersed phase of PLA/PA12 blends were selectively extracted/separated by PLA dissolution followed by centrifugation in a centrifugal rotary machine (Hettich D7200, Germany). Predetermined weights (approx. 1g) of extruded pellets of PLA/PA12 blends were dissolved in dichloromethane (1 mL) and acetone (9 mL). The solution was then submitted to centrifugation. Centrifugation time was set 1 hour at a rotation speed of 4000 rpm. After every centrifugation cycle, the dissolved phase was carefully transferred in a beaker and a fresh solvent system (dichloromethane/acetone 1:9) was added to centrifugation tube containing the insoluble PA12 fraction. The dissolution – centrifugation was repeated to 5 – 6 times. The PLA solution and settled PA12 particle were dried for 48 hours prior to further characterizations.

4.2.3.6 Fourier Transform Infrared studies The chemical structure of the neat PLA, PA12 and extracted PA12 phase was analyzed by Fourier transform infrared (FTIR) spectroscopy (Nicolet 380 FTIR, ThermoScientific, France). Injection-molded samples are analyzed directly in reflection mode. The absorption spectra were recorded using a microscope in reflection or transmission mode. The spectra range is 4000–800 cm-1 with the resolution of 16 cm-1 and using 62 scans.

Results and Discussions

4.3.1 Morphology PLA/PA12/PLA-g-MA blend samples were compressed and cryogenically fractured; the SEM images of the same are represented in Figure 4.1. The continuous phase is represented by PLA and PA12 droplets are dispersed in the PLA matrix (Figure 4.1). The blends displayed good dispersion and the dispersion seems to improve with the addition of PLA-g-MA. The Dn, Dv, and Ip are tabulated in Table 4.1, where it can be observed that the polydispersity index remains

Chapter 5 constant (≈1.2) for all the compositions. The domain size of PA12 decreased with the addition of PLA-g-MA (≈1μm) until 2 wt.%, further addition increased the droplet size of PA12. The droplet size of PA12 in blends with highest concentration of PLA-g-MA (5wt.%) is ≈1.5μm, it could be said that the blends reached saturation at 2wt.% of PLA-g-Ma as further increase in the content results in the bigger droplet size of dispersed phase (PA12) (Figure 4.2). The reduction in the droplet size could be related to compatibilization effect of PLA-g-MA on PLA/PA12 blends at the interface resulting in decrease of coalescence of droplets. However, the interfacial adhesion is strong between the blend components and remains unaltered with the increase in concentration of PLA-g-MA suggesting no interfacial debonding occurs even at higher concentrations of PLA-g-MA.

a) b)

c) d)

e) f)

Figure 4.1. Morphology of PLA/PA/PLA-g-MA of various compositions (a)70/30/0 (b) 69/30/1 (c) 68/30/2 (d) 67/30/3 (d) 66/30/4 (e) 65/30/5

Chapter 5

Figure 4.2. Emulsification curve of PLA/PA12 blends with varying PLA-g-MA content

4.3.2 Mechanical Properties The major focus of this study was to investigate the effect of compatibilizer on the (thermo)mechanical properties. The blends were processed by injection molding and subjected to tensile testing; the related stress-strain curves are plotted in Figure 3. The values of stress at yield, elongation at break and Young’s modulus evaluated from the tensile tests are tabulated in Table 2. The poor ductility of PLA (3.4%) drastically improved by the addition of 30 wt.% PA12 (153%) indicating good compatibility as confirmed in our previous studies [210]. PLA- g-MA was incorporated in the blends to observe the potential evolution of compatibility and its effect on mechanical properties.

Table 4.1. Number-average and volume-average PA12 droplet diameters (Dn and Dv), PA droplet size polydispersity (Ip) with standard deviation in brackets of PLA/PA12 /PLA-g-MA blends (compression molded samples)

Blend Dn (µm) Dv (µm) Ip

70/30/0 1.1 (0.1) 1.4 (0.03) 1.2 69/30/1 1 (0.06) 1.2 (0.05) 1.2 68/30/2 1 (0.04) 1.2 (0.06) 1.2 67/30/3 1.3 (0.1) 1.5 (0.09) 1.1 66/30/4 1.3 (0.1) 1.6 (0.06) 1.2 65/30/5 1.5 (0.07) 1.8 (0.05) 1.2

There was a significant improvement in the ductility with the addition of PLA-g-MA as a compatibilizer in PLA/PA12 blends as can be seen in Figure 4.3. Even at low concentrations

Chapter 5 of PLA-g-MA (1wt.%), the improvement in ductility (≈291%) was noteworthy. Hence, it could be said that very little amount of PLA-g-MA is sufficient to further improve the compatibility and leading to higher ductility.

However, with further increase in content of PLA-g-MA a slight decrease in the ductility is observed, the lowest ductility obtained for 5wt.% of PLA-g-MA (260%). The decrease in ductility is linear with PLA-g-MA content, it should be noted that the decrease in ductility is not drastic suggesting that even at higher concentrations there is no interfacial debonding leading to poor ductility and this in accordance with the morphological observations (Figure 4.1).

Figure 4.3. Stress-strain curves of PLA/PA12 blends with varying concentrations of compatibilizer (PLA-g-MA)

The values of Young’s modulus and stress at yield are in the range of 2.0-2.1 GPa and 54- 55MPa respectively for all the compositions. These values are intermediate of PLA (2.9GPa, 67MPa) and PA12 (1.4GPa, 39.5MPa) in accordance with previous studies of PLA and PA [117,166,167].

The evolution of ductility with varying content of PLA-g-MA in PLA/PA12 blends is plotted in Figure 4.3. It can be observed that the optimum content of PLA-g-MA required to improve the ductility remarkably (291%) is as low as 1wt.% and a further increase in the content does not have incremental effect on ductility.

Chapter 5

Figure 4.4. Ductility of PLA/PA12 blends with varying PLA-g-MA content

The impact strength of the injection molded blend samples was evaluated. The Charpy un- notched impact strength of the PLA/PA12/PLA-g-MA blends are tabulated in Table 4.2. The variation of impact strength with different concentrations of PLA-g-MA is plotted in Figure 4.5 for a clear depiction of the influence of compatibilizer on PLA/PA blends. The toughening effect of PA12 can be clearly seen in the PLA/PA12 (70/30, 28.3 kJ/m2) blends without compatibilizer. It is interesting to observe that with the incorporation of PLA-g-MA there is further increase in impact strength suggesting the positive effect of compatibility in blends. The maximum impact strength (45 kJ/m2) achieved was for the blends containing 2wt.% of PLA-g-MA. Further addition of PLA-g-MA in blends showed a slight linear decrease in impact strength and the least value (32.1 kJ/m2) being for 5wt.% of PLA-g-MA. Ductility and toughness are considered important properties for durable applications. The addition of compatibilizer (PLA-g-MA) in PLA/PA12 blends has a positive effect on ductility and toughness. Hence, it can be said that these blends could be suitable for high value applications with high ductility requirements. However, the reasoning for this improvement is not very evident in the morphological observations of compressed samples.

Chapter 5

Figure 4.5. Impact strength of the PLA/PA12 blends with varying concentration of PLA-g- MA

The SEM images of injection molded samples could give us a better understanding of the improvement in properties. The morphologies of injection molded samples of blends and their correlation with mechanical properties are discussed in the following section.

Table 4.2. Stress at yield, strain at break and impact strength for as-produced PLA/PA12/PLA- g-MA blends

PLA/PA12/ Young’s Impact strength Stress at yield Strain at PLA-g-MA Modulus (un-notched) (MPa) break (%) compositions (GPa) (kJ/m2) 100/0/0 66.6 (1.8) 3.4 (1.5) 2.9(0.06) 18.1 (0.6) 0/100/0 39.5 (1.2) 138.3 (2.3) 1.4(0.05) Non break 70/30/0 56.5 (1.6) 153.0 (3.3) 2.2(0.06) 28.3 (0.7) 69/30/1 55.3(1) 291.2(16.6) 2.1(0.05) 45.4 (0.8) 68/30/2 53.9(2.4) 282.2(16.2) 2.1(0.04) 46.8 (0.8) 67/30/3 52.8(3.1) 268.7(22.9) 2.1(0.09) 40.5(0.2) 66/30/4 52.4(4.5) 268.2(15) 2.1(0.1) 40.4(0.2) 65/30/5 53.7(0.9) 260.7(5.04) 2.0(0.3) 32.1(2.4)

4.3.3 Morphology of injection molded samples-correlation with mechanical properties Morphology of injection-molded samples was observed to get an in-depth understanding of the effect of compatibilizer on the mechanical properties of the blends. The SEM images of

Chapter 5 transversal and cross-sectional part of the injection-molded samples are depicted in Figure 4.6.

The blends display a fine dispersion of PA12 droplets in PLA matrix. Dn, Dv, and Ip of the PLA/PA12/PLA-g-MA blends are tabulated in Table 4.3. The droplet diameter of PA12 ranges from micronic to submicronic with lowest being for 2wt.% of PLA-g-MA (0.8μm) and the polydispersity index is ≈1.1 for all the blend compositions.

a) b)

c) d)

e) f) Figure 4.6. Cross-sectional SEM analysis of injection-molded PLA/PA12/PLA-g-MA blends a)70/30/0, b)69/30/1, c)68/30/2, d)67/30/3, e)66/30/4, f)65/30/5

The evolution of droplet diameter as a function of PLA-g-MA content has been plotted in Figure 4.7. It can be clearly seen that the droplet diameter decreases linearly up to 2wt.% and an increase in the droplet size was observed with a further increase in PLA-g-MA concentration. The lower droplet diameters, better interfacial adhesion and stronger interface between the blend components of injection molded samples (Figure 4.6) when compared to compression molded samples (Figure 4.1) could be the reason for higher ductility in blends

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[166,169,210]. It is interesting to observe that for injection molded samples, the droplet size of 5wt.% PLA-g-MA(0.96μm) is lower than PLA/PA12(70/30) (1.1μm), this fine dispersion of PA12 droplets even at higher concentration of PLA-g-MA could be the reason for the ductility behavior of the blends at 5wt.% of PLA-g-MA(260%).

Table 4.3. Number-average and volume-average PA12 droplet diameters (Dn and Dv), PA droplet size polydispersity (Ip) with standard deviation in brackets of PLA/PA12 /PLA-g-MA blends

Blend Dn (µm) Dv (µm) Ip 70/30/0 1.1(0.09) 1.3(0.09) 1.2 69/30/1 0.94(0.07) 1.06(0.04) 1.1 68/30/2 0.83(0.04) 0.9(0.03) 1.1 67/30/3 0.9(0.05) 1(0.03) 1.1 66/30/4 0.9(0.03) 1.02(0.04) 1.1 65/30/5 0.96(0.06) 1.04(0.09) 1.1

The tensile properties of the blends could be corroborated by the PA12 fibrillation in the blends and the effect of PLA-g-MA on this behavior. The SEM images of the transversal section of injection molded samples are depicted in Figure 4.8, it can be observed that the blends show fibrillation of PA12 up to 3wt.% of PLA-g-MA. At higher concentrations of PLA-g-MA the fibrillation of PA12 appears to be hindered (Figure 4.8), this could be due to higher concentration of compatibilizer at the interface limiting the formation of elongated PA12 in the injection molded samples [214,215].

Figure 4.7. Droplet diameter of PLA/PA12 blends with varying concentration of PLA-g-MA

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This behavior of hindered elongation of PA12 could be related to the slightly lower ductility and impact strength values (260%, 32kJ/m2) obtained for the blends containing a higher concentration of PLA-g-MA (5wt.%).

4.3.4 (Thermo)mechanical properties Thermomechanical properties including thermal resistance or heat deflection temperature (HDT) are prominent for high value applications. Thermomechanical properties were evaluated using DMA to obtain the evolution of the storage modulus/loss factor with temperature.

a) b)

c) d)

e) f)

Figure 4.8. Transversal section SEM analysis of injection-molded PLA/PA12/PLA-g-MA blends a)70/30/0, b)69/30/1, c)68/30/2, d)67/30/3, e)66/30/4, f)65/30/5

The HDT (T400MPa-HDTa and T250MPa-HDTb) were subsequently extracted from storage modulus with a specific experimental procedure previously reported [210]. The HDT of

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PLA/PA12 blends under load was evaluated by dynamic mechanical analysis. The influence of varying concentrations of compatibilizer on the HDT of blends was investigated through these tests. The variation of storage modulus with temperature has been plotted in Figure 4.9. It should be noted that the blends were annealed at 110°C to eliminate the cold crystallization of PLA. The storage modulus (E’) gradually decreases with temperature with a classical drop for temperatures higher than 70°C associated with the glass transition/ -relaxation of PLA (Figure 4.9). The -relaxation temperature of PLA/PA12(70/30) blends is noticed close to 70°C. Higher storage modulus is observed for 2wt.% of PLA-g-MA blends in comparison with the other compositions. This behavior is still indeterminate but an additional PLA crystallization at PLA/PA12 interface could induce a significant increase in PLA crystallinity [210,216,217]. The impact of the concentration of compatibilizer on storage modulus is not significant in the glassy state. However, in rubbery state (T > 70–80°C), a prominent influence of 2 wt.% of PLA-g-MA is observed. The other concentrations did not show this synergism.

Figure 4.9. Storage modulus as a function of the temperature of PLA/PA12 blends with varying content of PLA-g-MA(wt.%) Such thermomechanical modifications in the glassy/rubbery state can be correlated with

HDT/thermal resistance improvements. Heat deflection temperature (T400MPa-HDTa and T250MPa-

HDTb) of blends were subsequently evaluated from DMA graphs and tabulated in Table 4.4. PLA/PA12/PLA-g-MA blends showed improvement in HDT when compared to uncompatibilized blends with HDT values close to 81-95°C. Significant improvement of HDT was observed for 2wt.% of PLA-g-MA (84°C-T400MPa-HDTA and 99°C-T250MPa-HDTB). The HDT values show a linear increase until 2wt.% of PLA-g-MA, corresponding to the similar trends that were observed in mechanical properties. Upon increasing the content of PLA-g-MA

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gradual decrease in HDT values were observed with the least being for 5wt.% (79°C-T400MPa-

HDTA , 86°C-T250MPa-HDTB). In conclusion, the addition of compatibilizer to the PLA/PA12 blends is beneficial to improve thermal resistance with a considerable increase in the thermal resistance for 2wt.% of PLA-g- MA. Similar to tensile and impact properties, the optimum content of PLA-g-MA to enhance thermal resistance of PLA/PA12 blends is found to be at 2wt.%.

Figure 4.10. DSC cooling(left) and heating(right) thermogram of PLA/PA12 blends with PLA- g-MA

Differential scanning calorimetry was performed on PLA/PA12 blends to investigate various thermal properties such as glass transition temperature, melting temperature, and cold crystallization behavior. The Tg of all the blends is around 64°C, which is an intermediate value between PLA and PA12. This is a typical additive rule behavior and it could be said that the compatibilizer does not affect Tg of the blend. It can be observed in heating thermograms (Figure 4.10) that the melting point of the blends with compatibilizer is slightly lower than that of uncompatibilized blends, this could be due to the reaction of anhydride group of PLA-g-MA and amino group PA12, suggesting a formation of copolymer at the interface. This copolymer could be a hindrance to the motion of segments causing difficulty in polymer chain arrangement[181]. However, there is a minor increase in the melting point at 4 and 5wt.% of PLA-g-MA. The cooling thermograms of the blends show a typical droplet and bulk crystallization behavior of PLA/PA blends [117,166,210]. The temperature at which droplet crystallization occurs remains almost unaltered (97°C) for uncompatibilized and compatibilized blends. However, the bulk crystallization behavior shows an effect of compatibilizer (PLA-g-MA), with an increase in temperature of bulk crystallization as the

Chapter 5 content of PLA-g-MA increases. This might be due to larger droplets or the copolymer at interface delaying the bulk crystallization of the blends.

Table 4.4. Thermal properties of PLA/PA12 blends with varying content of PLA-g-MA

PLA/PA12/PLA-g-MA T400Mpa T250Mpa Melting point Tg (°C) compositions HDTA (°C) HDTB (°C) (°C) (PLA) (PA12 100/0/0 78(0.5) 84(1) 168.0 - 61.0 70/30/0 81(2.5) 95(3.7) 169.4 177.8 64.7 69/30/1 80(0.5) 92(0.0) 168.6 176.7 64.7 68/30/2 84(1) 99(0.5) 168.3 176.4 64.2 67/30/3 80.5(1) 86(1) 168.1 176.4 64.2 66/30/4 81(1.5) 89(1.5) 169.3 176.7 64.2 65/30/5 79(1) 86(0.8) 169.8 177.2 64.0

4.3.5 FTIR studies FTIR was performed to study the possible grafting or copolymer formation between PLA and PA due to the presence of compatibilizer. FTIR was done on the extracted PA12 phase from centrifugation. The relative absorptions at 3286 cm−1 and 1751 cm−1 correspond to characteristic stretching vibration absorptions of NH amide groups and C=O ester groups of PA12 and PLA respectively (Figure 4.11) [218,219]. The existence of absorption peak corresponding to carbonyl group of PLA in the extracted phase of PA12 could be considered as evidence to grafting/copolymer formation in low amounts. Additionally, the absorption peak of the carbonyl group appears to have shifted from 1751 cm−1 to 1758 cm−1 for the blend (Figure 4.11), indicating hydrogen bonding interaction between the molecules of PLA and PA resulting from the grafting/copolymer formation of PLA and PA12 [218]. Overall the improvement in mechanical properties could be due to the combined effects of grafting, PA12 droplet size and PA12 fibrillation. However, PA12 fibrillation and PA12 droplet size could be a predominant factor for the improvement in ductility and toughness. The amount of PLA grafting could represent a key parameter to control in order to avoid losing PA12 fibrillated structures.

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Figure 4.11 FTIR studies of PLA/PA12 blends with varying content of compatibilizers

Conclusions The major emphasis of this study is to investigate the compatibilization effects of PLA-g-MA on PLA/PA12(70/30) blends in a twin-screw extruder. Various concentrations of PLA-g-MA were incorporated in the blends to assess the optimum content of compatibilizer required to achieve maximum (thermo)mechanical properties. The positive effect of PLA-g-MA is evident in morphology, mechanical properties, and thermal resistance.

Morphology of the compression molded blend samples with different concentrations of PLA- g-MA revealed fine dispersion of PA12 in the PLA matrix with a strong interface and good interfacial adhesion. The lowest droplet diameter of PA12 was around 1μm at 2wt.% of PLA- g-MA.

Concerning the mechanical properties, remarkable improvement in ductility was observed in the blends with varying content of PLA-g-MA. The highest ductility (291%) was achieved for blends containing 1wt.% of PLA-g-MA, a further increase in the concentration of compatibilizer leads to minor decrease in ductility. The impact properties of the compatibilized blends showed similar trends, and the toughening behavior of PA12 is observed. However, the maximum impact strength (46.8 kJ/m2) was attained at 2wt.% of PLA-g-MA. Hence it could be said that the optimum content of PLA-g-MA required to improve the mechanical properties (ductility and impact strength) is between 1 and 2wt.% of PLA-g-MA.

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The influence of compatibilizer on PLA/PA12 blends on the thermal resistance of the blends (HDT) was not as significant as mechanical properties. The improvement in thermal resistance of the blends was linear with increasing content of PLA-g-MA. The highest HDT values (84°C-

T400MPa-HDTA and 99°C-T250MPa-HDTB) were obtained for 2wt.% of PLA-g-MA.

To get a better in-depth understanding of the improvement in mechanical properties the morphology of injection molded specimens was examined. The cross-sectional SEM images revealed finer dispersion, stronger interface, and better interfacial adhesion when compared to compression molded samples. This could be the reason for the improvement in ductility and toughness of the blends. The lowest droplet diameter (0.8μm) obtained was for 2wt.% of PLA- g-MA. Transversal sections of injection molded samples were also examined to assess the relation between fibrillation of PA12 and mechanical properties of the blends, the fibrillation of PA12 is evident in the SEM images. However, for higher content of PLA-g-MA (>3wt.%) and hindrance in the fibrillation was observed and this might due to the higher concentration of PLA-g-MA at the interface impeding the elongation of PA12. FTIR studies were also done to study the probable grafting and its correlation with mechanical properties. The absorption peak corresponding to carbonyl group of PLA in the extracted phase of PA12 could be considered as indication of grafting/copolymer formation. Hence, grafting of PLA and PA12 in the presence of PLA-g-MA could be considered as a contributing factor for the improvement in ductility. However, at higher concentrations of PLA-g-MA the ease of PA12 fibrillations are obstructed thereby reducing the mechanical properties.

In conclusion, PLA-g-MA is a suitable compatibilizer for PLA/PA12 blends. Remarkable improvement in ductility was observed and considerable improvement in toughness and heat resistance was attained. These blends could be well suited for durable applications which require high ductility and toughness. Further studies related to particular applications are required to discover the apt application for these blends.

Acknowledgments

Authors gratefully acknowledge both the International Campus on Safety and Intermodality in Transportation (CISIT, France), the European Community (FEDER funds) as well as the Hauts-de-France Region (France) for their contributions to funding extrusion equipment and characterization tools (dynamic rheometers, microscopes, goniometers, calorimeters, and tensile benches).

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5 High Shear Extrusion Processing of Poly(L-Lactide) and Polyamide-12 Blends: Effect of Screw Speed and Feed Rate on (Thermo)Mechanical Properties Raj Amulya1,2, Samuel Cédric1,2*, Prashantha Kalappa1,2 1Institut Mines Telecom Lille Douai (IMT Lille Douai), Département Technologie des Polymères et Composites & Ingénierie Mécanique (TPCIM), 941 rue Charles Bourseul, F- 59508 Douai, France. 2Université de Lille, F-59000 Lille, France

*Corresponding Author: [email protected] ABSTRACT Melt-blending poly(L-lactide) (PLA) with engineering polymers using extrusion technologies represents one of the most efficient strategies to overcome various PLA drawbacks and achieve tailor-made properties for high-performance applications. Compatible PLA/polyamide-12 (PA12) blends (30 wt-% PA12) are here processed by twin-screw extrusion at high shear and the effect of several processing parameters (screw speed and feed rate) on final properties (tensile strength, ductility, impact strength, and thermal resistance) is specifically addressed. High tensile strengths with values close to neat PLA could be maintained for these blends with slight positive effects of the screw speed and feed rate. Up to screw speed of approx. 800 rpm, ductilities/strains at break and impact toughnesses significantly increase with maximal ductility (approx. 225%) and impact strengths (48 kJ/m²). However, extreme screw speeds higher than 1100 rpm dramatically reduce both ductility and impact strength. Concerning thermal resistance, a constant increase of the heat deflection temperature is observed with the screw speed and thermal resistance up to 123°C could be obtained. In this respect, processing conditions of PLA/PA12 blends have a profound effect on all (thermo)mechanical properties with positive impact of the screw speed/feed rate on tensile strength and thermal resistance but a limit value of 800 rpm is detected for ductility and toughness. Actual results on PLA/PA12 blends are then discussed in terms of induced morphology and macromolecular degradation at extreme screw speed, in particular for the PLA matrix. As a consequence, PLA/PA12 blends could represent interesting candidates for high- performance applications (automotive, electronics, additive manufacturing, etc…) and their optimization could be easily done by playing on processing conditions.

Keywords: Biobased polymers, Poly(L-lactide), Polyamide-12, Polymer blends, High-Speed Extrusion

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Introduction Biobased polymers and in particular Poly(L-lactide) (PLA, also called polylactic acid) have gained considerable attention these recent years due to sustainability considerations (monomers from renewable resources) and final properties close to conventional plastics [6,7,207,220–225]. PLA displays interesting properties such as high transparency, high modulus (> 3GPa) and biocompatibility suitable for packaging applications, food/liquid containers, surgical sutures/implants and coated papers [9,19,31,39,226]. However, PLA market penetration into high-performances and durable applications (such as automotive and electronics) are still quite poor due to severe limitations such as low heat deflection temperature, low ductility, and impact strength when compared to engineering polymers [10,26]. Various solutions have been proposed to overcome PLA drawbacks, among them PLA-based blends with engineering polymers could represent a cost-effective route to obtain tailor-made PLA materials.

Several blends of PLA with other polymers such as poly(methyl methacrylate), polycarbonates, polyamides, polyolefins, and polyesters have been reported [10,20,48,123,181,210,227,228]. PLA/polyamide blends have been widely investigated in the recent years as polyamides have high ductility, high heat deflection temperatures and could be partly produced from renewable chemicals [10,20,164,166,167,180,210] Our previous study confirms a high compatibility between PLA and polyamides with long aliphatic chains and poly(amide-12) (PA12) was found to be the most compatible with PLA in the melt state due to lowest interfacial tension between the two components [210]. However, the domain size of the PA12 dispersed phase into PLA was not as low as expected and need to be improved to maximize final properties such as ductility, toughness and heat resistance. The known strategies to lower the domain size of the dispersed phase are using compatibilizers (classical approach), in-situ polymerization of dispersed phase in the matrix and high shear extrusion processing [171,229–233]. High shear extrusion (also called high-speed extrusion) refers to the processing/formulation of plastics using continuous and industrially-relevant twin-screw extruders at elevated screw speeds to apply high shear/elongation strains/rates on polymer melts. The extrusion screw speed is generally between 1000 – 4000 rpm and such processes have been mainly developed to enhance melt-state dispersions of rigid fillers into thermoplastic matrices and chemical modifications of polymers [173,231]. The main issue regarding the use of elevated screw speeds concerns thermal and mechanical degradation of polymer materials with potentially dramatic effects on final properties [173,234]. However, extreme shearing

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Chapter 5 conditions could be of very high interest for the production of polymer blends with improved mixing, refined morphologies, enhanced final properties and potential in-situ generation of suitable compatibilizers.

High shear/high-speed processing techniques are clearly less explored than other classical compatibilization techniques for polymer blends and only a limited amount of research has been conducted on high shear extrusion of polymer blends [173,230–237]. Shimizu H. et al [232]. have prepared poly(vinylidene fluoride) (PVDF) and polyamide 11 (PA11) blends by high shear processing . The authors observed that the compatibility between PA11 and PVDF was improved to a great extent with PA11 droplet sizes down to 100 nm [232]. The screw speed maintained was 1200 rpm and the average shear rate was 1760 s-1. The ductility was also remarkably improved in these blends in agreement with the reduction of PA11 droplet size. Calderon B A et al. also studied the compatibility between poly(butylene succinate) and poly(propylene carbonate) via high shear extrusion at different screw speeds ranging from 200 to 2000 rpm. The authors reported that high screw speeds yielded better morphology through droplet breakup in the blend inducing lower droplet sizes. However, as mentioned previously high screw speeds of 2000 rpm proned thermal and mechanical degradation of polymer components [173,234,235]. Li.Y et al. improved the optical and mechanical properties of PMMA and polycarbonate (PC) blends by high shear extrusion [233]. A rare combination of high transparency and mechanical properties was observed with PMMA droplet size in PC matrices lower than 50 nm and a single glass transition temperature. Teyssandier.F et al. prepared polyamide 12 (PA12) and thermoplastic starch blends by high shear processing [238] with various screw speeds ranging between 300 rpm to 1200 rpm. The diameter of thermoplastic starch in PA12 matrix was found to be lower than 300 nm and the ductility of the blends was improved remarkably with an increase in impact strength without major modifications in tensile modulus [238]. Gug et al. prepared PLA PA11 blends in the presence of p-toluene sulphonic acid (TsOH) as catalyst in a twin-screw speed at the following speeds (250, 500, 1000, 2000 and 3000 rpm). Reactive copolymerization of PLA and PA11 via ester-amide exchange reaction (aminolysis) was observed in NMR. Copolymers acted as compatibilizers, decreased the interfacial tension between the two phases and improved the ductility significantly. High catalyst level (over 2 wt %) and shear stress (higher than 2000 rpm) showed degradation and deterioration of the blend properties [173]. These works clearly suggest that high shear extrusion of polymer blends could significantly improve their morphologies (down to nanoscale) and final mechanical properties (tensile modulus,

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Chapter 5 mechanical strength, impact strength, etc.) without using any additional compatibilizers. In this respect, such processing technique potentially represents an industrially-relevant and cost- effective tool for the development of high-performance polymer blends.

In our previous work, it was found that PLA/PA12 showed the best compatibility with low interfacial tension. As low as 30wt.% of PA12 was suffice to increase the ductility of the blends (~150%) [210]. In order to further enhance the (thermo)mechanical properties the effect of extrusion parameters on PLA/PA12 blends was studied. Hence, in this work we present the effect of screw speed and feed rate on ductility, toughness, morphology and thermal properties of PLA/PA12 blends via high-speed extrusion. The influences of screw speed and feed rate on properties such as morphology, thermal and mechanical properties are discussed in detail in the following sections.

Experimental Section

5.2.1 Materials Poly(L-lactide) (PLA, grade 4032D, biobased content 100%, density 1.24 g/cm3, melting temperature 171°C, melt volume index 6.5 cm3/10 min at 210°C, according to manufacturer data) was supplied by NatureWorks®, USA. PA12 (grade Rilsamid AESNO, density 1.01 g/cm3, melting temperature 179°C, melt volume index 2.1 cm3/10 min at 235°C, according to manufacturer data) was supplied by Arkema, France. PLA and PA12 were used as-received in the pellet form.

5.2.2 Processing of PLA/PA Blends Before compounding, all materials were dried under vacuum at 80°C for 8 hours to remove any traces of moisture in the materials. PLA/PA12 blends with a fixed composition (PLA/PA 70/30 wt-%) were prepared using a co-rotating twin-screw extruder (ZSK26MC, Coperion, Germany) with a screw diameter of 26 mm (L/D ratio 35). The barrel temperature regulation is carried out on 10 zones and the temperature profile was maintained constant for all experiments (200 – 205 – 210 – 210 – 210 – 210 – 210 – 210 – 200 - 185°C, from the hopper to the die). A specific screw profile was used for PLA/PA12 according to previous studies by our group [129] (Figure 5.1). Dry-blends of PLA and PA12 pellets were feed into the extruder using a K-Tron gravimetric feeder. The extrudates were cooled in a water bath and pelletized followed by drying under vacuum at 80°C overnight before being reused for subsequent molding processes. Two sets of experiments were tested, (i) different screw speeds (200 rpm, 500 rpm, 800 rpm and 1100 rpm) at a constant feed rate of 5 kg/h and (ii) different feed rate (2 kg/h, 3 kg/h, 4 kg/h, 5 kg/h) at a constant speed of 800 rpm. 104

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Figure 5.1. As-selected screw profile for twin-screw high shear extrusion processing of PLA/PA12 blends. Extrusion direction from right (hopper) to left (die) with the following screw elements: 216/18D – 108/KB45°D – 12/12R – 24/KB45°D – 72/KB30°D – 24/KB45°D – 36/12D – 72/KB45°D – 36/18D – 108/KB45°D – 12/KB90°R – 108/18D – 24/12D – 24/MixD – 72/18D – 108/12D (D: Direct, R: Reverse, KB: Kneading blocks, Mix: Mixing elements). Screw profile adapted from [129]. Tensile test specimens (ISO-527-1 standards, gauge length 50 mm, a width of 5 mm and thickness of 1.5 mm were processed by injection-molding using a Haake Minijet II molding machine. The following processing conditions were used for the preparation of injection- moulded specimens (mould temperature 45°C, chamber temperature 215°C, injection pressure 550 bar, cooling time 5s). Impact specimens were prepared by injection moulding (Babyplast 6/10P, Spain). The processing parameters maintained during the preparation of samples were as follows, the temperature maintained ranged from 200 to 210°C and the mould temperature was kept at 35-50°C. The injection and the consecutive pressure were 60 and 80 bars respectively with a cooling time of 10 seconds. Specimens for morphological characterization were processed by compression-molding using Dolouet molding press. The parameters and conditions were used for all compression-moulded specimens (melt temperature: 205°C, melting time and low pressure cycle: 8 min, high pressure cycle: 5 min).

5.2.3 Characterization methods

5.2.3.1 Tensile mechanical properties and impact tests Tensile tests were carried out on PLA/PA blends and neat polymers with a tensile testing machine (Instron 3110, U.K) and a 1kN force cell to obtain stress-strain curves. Tensile properties were determined according to ISO 527-1 (pre-load of 0.5N, strain rate 1mm/min for Young’s modulus and 10 mm/min for general properties. At least five dumbbell specimens were tested. Charpy un-notched impact tests were carried out as per ISO 179-1 standard by using a pendulum impact machine (Model 5101, Zwick, Germany) at 25°C and 50% RH and impact

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Chapter 5 energy of 7.1J. All the reported values were calculated as averages over seven impact specimens.

5.2.3.2 Dynamic Mechanical Analysis (DMA) The dynamic mechanical analysis of the specimens was carried out on a dual cantilever beam in the flexion mode (DMA+150, MetraviB, France). The samples were heated from 25°C to 130°C at a heating rate of 2°C/min and frequency of 10 Hz. At least two rectangular specimens were tested. To eliminate the cold crystallization peak of PLA the specimens were annealed at 110°C for 6 hours. The approximate thermal resistance of blends which could correspond to heat deflection temperature (HDTa and HDTb) at two different storage moduli (400 and 250MPa) respectively was evaluated by the DMA graphs. This measurement procedure for HDT was based on a previous by our group [210].

5.2.3.3 Scanning Electron Microscopy (SEM) The morphology of the compression-moulded PLA/PA blends was observed using a scanning electron microscope (SEM, Neoscope II, JEOL, magnification x60,000). Morphologies were examined by cryo-fracturing the samples in liquid nitrogen and fractured surface were sputtered with gold under vacuum in order to make them conductive. Droplet sizes were measured by image analysis using Image J software on at least 50 droplets followed by size classification into 7/8 classes. Volume/number average droplet radius diameters (Dv,

Dn) with the droplet diameter polydispersity (ID) were calculated using equations 5.1-5.3. Two samples per blend were used to get standard deviations on droplet size measurements.

∑ 푛푖 × 푅푖 (5.1) 퐷푛 = 2 × ∑ 푛푖 4 ∑ 푛푖 × 푅푖 (5.2) 퐷푣 = 2 × 3 ∑ 푛푖 × 푅푖

퐷푣 (5.3) 퐼퐷 = 퐷푛

with Dn the droplet number-average diameter, Dv the droplet volume-average diameter, Ri the average droplet radius of the i-class and ni the number of droplets into the i-class.

5.2.3.4 Selective Dissolution Studies Selective dissolution studies were used to detect phase morphology in PLA/PA blends. Predetermined weights of PLA/PA blends were dissolved in chloroform, a solvent for PLA and a non-solvent for polyamides. The solutions were kept for dissolution in a test tube for a week

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Chapter 5 at room temperature. Solutions were washed with chloroform thoroughly, filtered and dried at room temperature. This procedure was repeated until the weight of the sample was constant.

5.2.3.5 Differential Scanning Calorimetry (DSC) Differential scanning calorimetry (DSC Mettler Toledo) was used to measure the PLA crystallinity in neat PLA and PLA/PA12 blends, in particular for injection-molded specimens after an annealing step. The analysis was carried out under a nitrogen atmosphere and a single heating scan at 10°C/min was used for this purpose. PLA melting temperatures and associated enthalpy of fusion (Tm and ΔHm) were extracted and the PLA crystallinity index (Xc-PLA) was calculated according to equation 5.4. Note that PLA crystallinities were normalized to the amount of PLA in PLA/PA blends.

훥퐻푚−푃퐿퐴 − 훥퐻푐푐−푃퐿퐴 푋푐−푃퐿퐴 = 0 × 100 (5.4) 훥퐻푚 with Xc is the degree of crystallinity, ΔHm-PLA the melting enthalpy of PLA, ΔHcc-PLA the cold 0 crystallization enthalpy of PLA during heating and ΔHm the melting enthalpy of fusion for 100% crystalline PLA (94 J/g) [166].

5.2.3.6 Extraction/separation experiments by centrifugation The PLA matrix and the PA12 dispersed phase of PLA/PA12 blends were selectively extracted/separated by PLA dissolution followed by centrifugation in a centrifugal rotary machine (Hettich D7200, Germany). Predetermined weights (approx. 1g) of extruded pellets of PLA/PA12 blends were dissolved in dichloromethane (1 mL) and acetone (9 mL). The solution was then submitted to centrifugation. Centrifugation time was set 1 hour at a rotation speed of 4000 rpm. After every centrifugation cycle, the dissolved phase was carefully transferred in a beaker and a fresh solvent system (dichloromethane/acetone 1:9) was added to centrifugation tube containing the insoluble PA12 fraction. The dissolution – centrifugation was repeated to 5 – 6 times. The PLA solution and settled PA12 particle were dried for 48 hours prior to further characterizations.

5.2.3.7 Dynamic rheology Dynamic rheology experiments were performed on compression-molded PLA/PA12 blends using a dynamic rheometer (Haake Mars III, ThermoScientific) at 220°C (temperature stabilization time 3 min) using plate – plate geometry (diameter 35 mm, gap 1 mm) under nitrogen atmosphere to limit thermal degradation. Strain sweep tests were first carried out with an angular frequency of 1 rad.s-1 starting from an initial deformation of 0.01% to 10% to evaluate the linear viscoelasticity domain. Frequency sweep tests were subsequently carried

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Chapter 5 out over an angular frequency range between 100 and 0.4 rad/s (optimal strain 10%). Dynamic rheology experiments were also performed on PLA matrices extracted from blends using the above-mentioned procedure to evaluate PLA weight-average molecular weight from the zero- shear viscosity 0 (Carreau-Yasuda fitting). For these experiments, plate – plate geometry (diameter 25 mm, gap 0.4 mm) under nitrogen atmosphere was used and frequency sweep tests were carried out at 185°C over an angular frequency range between 100 and 1 rad/s (optimal strain 10%).

Results and Discussion

5.3.1 Tensile properties of as-produced PLA/PA12 blends The major objective of this work is to enhance (thermo)mechanical properties (ductility, toughness and heat resistance) of biobased PLA by blending with PA12, a highly compatible polymer with PLA [210]. Based on a previous investigation by our group, blend compositions were fixed to 30 wt-% PA12 into PLA as the lowest PA12 content to induced significant ductility/toughness improvements (strain at break 153%, impact strength 28 kJ/m²) [117,166,182,210]. Blends were previously prepared using a lab scale twin-screw extruder at a screw speed of 80 rpm and advanced processing methods could largely improve dispersion state of PA12 into PLA with potential improvements of final (thermo)mechanical properties [173,230]. In this respect, the efficiency of high shear extrusion processing without using additional compatibilizers is investigated here for PLA/PA12 blends (70/30 wt-%) with a specific focus on the influence of processing conditions (screw speed 200 – 1100 rpm and feed rate 2 – 5kg/h). Tensile mechanical properties were first examined and typical tensile stress/strain curves of PLA/PA12 blends (70/30 wt-%) are displayed in Figure 5.2 with varying screw speeds from 200 – 1100 rpm and feed rates from 2 – 5 kg/h. A high impact of the extrusion screw speed on PLA/PA12 tensile behavior is detected, in particular on blend ductility. It can be clearly observed that the strain at break/ductility of PLA/PA12 blends significantly increases with the screw speed to reach interesting strains at break above 200%. An optimal screw speed of 800 rpm is consequently detected and a further increase in screw speed to 1100 rpm dramatically deteriorated PLA/PA12 ductility. The feed rate seems to produce only limited effects on PLA/PA12 tensile properties with only minor modification of the tensile behavior.

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1111 1111

Figure 5.2. Typical tensile stress-strain curves obtained for PLA/PA12 blends (70/30 wt-%) processed by high shear extrusion with varying screw speeds (left, constant feed rate 5 kg/h) and varying feed rates (right, constant screw speed 800 rpm). Young’ modulus, stress at yield and strain at break were extracted from tensile behaviors to perform a quantitative analysis regarding the impact of the screw speed/feed rate. Average values are tabulated in Table 5.1 and Table 5.2 with trends/evolutions reported in Figure 5.3 and Figure 5.4. For Young’s modulus, values ranging from 2.0 to 2.4 GPa are observed for as-produced PLA/PA12 blends (Table 5.1 and Table 5.2). These intermediate values between neat PLA (2.9 GPa) and neat PA12 (1.3 GPa) are in accordance with classical mechanical models and with several studies on PLA/PA blends [117,166,167]. A similar situation is noticed for the stress at yield of PLA/PA12 blends with values ranging from 52.1 to 59.6 MPa (Table 5.1 and Table 5.2) as intermediate values between neat PLA (66.6 MPa) and neat PA12 (39.5 MPa), also in agreement with several mechanical models [117]. Taken into account standard deviations on stresses at yield, it could be stated out that processing conditions do not modify significantly tensile strengths of our blends and only a slight positive effect of the screw speed and the feed rate is concluded (Figure 5.3a and Figure 5.4a). As previously observed on tensile stress/strain curves of as-produced PLA/PA12 blends, screw speed and feed rate induce significant effects on the blend ductility. At low screw speeds, strains at break close to 140 – 150% are observed (near the value of neat PA12 ductility) in agreement without our previous investigation [210]. PLA/PA12 ductility clearly increases with the screw speed and a maximal ductility (strain at break 224.2 ± 8.3%) is obtained for an optimal screw speed of 800 rpm (Figure 5.3b). In this respect, the efficiency of the high shear extrusion is clearly demonstrated and higher ductility could be obtained with this processing method. A surprising synergy is also obtained with blend ductility higher than neat components that also represents a powerful and innovative effect arising from high shear extrusion

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Chapter 5 processing of PLA/PA12. However, a dramatic drop is then detected at 1100 rpm indicating that processing conditions should be carefully controlled for ductility optimization of PLA/PA12 blends by high shear extrusion. The feed rate also induces significant positive effects on blend ductility (Figure 5.4b) and optimal extrusion processing conditions were found close to 800 rpm and 5kg/h. As a conclusion on tensile properties and subsequent blend ductility, high shear extrusion without any compatibilization is highly beneficial to enhance PLA/PA12 ductility. Table 5.1. Stress at yield, strain at break and impact strength for as-produced PLA/PA12 blends (70/30 wt-%) with varying screw speeds (constant feed rate 5 kg/h) (standard deviations into brackets). Young’s Impact strength Screw Speed Stress at yield Strain at break Modulus (un-notched) (rpm) (MPa) (%) (GPa) (kJ/m2) Neat PLA 66.6 (1.8) 3.4 (1.5) 2.9 (0.1) 18.1 (0.6) Neat PA12 39.5 (1.2) 138.3 (2.3) 1.4 (0.1) Non break 80 56.5 (1.6) 153.0 (3.3) 2.2 (0.1) 28.3 (0.7) 200 55.6 (1.8) 154.5 (5.2) 2.4 (0.1) 33.5 (0.7) 500 57.6 (1.7) 183.8 (6.7) 2.3 (0.1) 36.9 (0.7) 800 58.1 (2.4) 224.2 (8.3) 2.1 (0.1) 48.2 (0.3) 1100 59.6 (3.8) 18.1 (4.2) 2.1 (0.1) 31.8 (0.1)

Table 5.2. Stress at yield, strain at break and impact strength for as-produced PLA/PA12 blends (70/30 wt-%) with varying feed rates (constant screw speed 800 rpm) (standard deviations into brackets). Young’s Impact strength Stress at yield Strain at break Feed rate (kg/h) Modulus (un-notched) (MPa) (%) (GPa) (kJ/m2) 5 58.1 (2.4) 224.2 (8.3) 2.1 (0.1) 48.2 (0.3) 4 54.6 (5.6) 216.3 (15.4) 2.1 (0.1) 33.2 (0.7) 3 55.7 (4.3) 210.1 (10.4) 2.1 (0.1) 32.5 (0.6) 2 52.1 (4.8) 201.4 (15.3) 2.2 (0.1) 27.7 (0.5) Optimal processing conditions are here clearly detected and a careful control is required to avoid dramatic deterioration of the blend ductility.

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a) b) Figure 5.3. Tensile strength (a) and strain at break (b) as a function of the screw speed for as- produced PLA/PA12 blends (70/30 wt-%) (constant feed rate 5 kg/h).

Impact properties of as-produced PLA/PA12 blends Impact properties are also of major importance for high-performances applications and, in this respect, the impact of processing conditions on the resultant impact strength of PLA/PA12 blends was also examined. The un-notched impact strength of neat polymers and PLA/PA12 blends are tabulated in Table 5.2 and Table 5.3. For sake of clarity, impact strengths were also plotted as a function of the screw speed and feed rate in Figure 5.5. PA12 significantly improved PLA impact strength when compared to the brittle PLA (19 kJ/m²) and this effect is primarily due the toughening efficiency of the PA12 phase (no break of neat PA12 specimens in impact tests) [168]. However, processing conditions also induced significant positive effects on final toughness of PLA/PA12 blends. The impact toughness clearly increases with screw speed to reach the maximal impact toughness of 48.2 kJ/m² at 800 rpm (Figure 5.5a).

a) b)

Figure 5.4. Tensile strength (a) and strain at break (b) as a function of the feed rate for as- produced PLA/PA12 blends (70/30 wt-%) (constant screw speed 800 rpm). Interestingly, by comparison with our previous lab scale extrusion method at low screw speed (impact strength 28 kJ/m²) [210], the efficiency of high shear extrusion to produce PLA/PA12

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Chapter 5 blends with superior toughness is clearly demonstrated. However, as previously detected on tensile behavior, impact strength dramatically decreases at a higher screw speed of 1100 rpm (Figure 5.5a). A significant positive impact of feed rate is also noticed (Figure 5.5b). As a conclusion, impact properties of PLA/PA12 blends produced by high shear extrusion display similar trends as tensile properties. Optimal processing conditions are detected (800 rpm and 5 kg/h) to induce a maximal toughness of 48 kJ/m² for PLA/PA12 blends (70/30 wt-%) but a careful control of the processing conditions is also required to avoid toughness deterioration.

a) b)- Figure 5.5. Un-notched impact strength as a function the screw speed (a constant feed rate 5 kg/h) and feed rate (b, constant screw speed 800 rpm) for as-produced PLA/PA12 blends (70/30 wt-%).

Thermomechanical properties of as –produced PLA/PA12 blends Thermomechanical properties with related thermal resistance (or heat deflection temperature, HDT) are also of major importance for high-performances applications. In this respect, the impact of processing conditions on the thermal resistance of PLA/PA12 blends was also investigated. Based on previous mechanical results, the effect of the screw speed was only examined here to test the efficiency of the high shear extrusion. Thermomechanical properties were evaluated using DMA to obtain the evolution of the storage modulus/loss factor with temperature. The thermal resistance or HDT (T400MPa-HDTa and T250MPa-HDTb) were subsequently extracted from storage modulus with a specific experimental procedure [210]. Note that annealed samples at 110°C for 8 hours were used to eliminate PLA cold crystallization events during DMA heating. The storage modulus (E’) gradually decreases with temperature with a classical drop for temperatures higher than 70°C associated with the glass transition/α-relaxation of PLA (Figure 5.6). The α -relaxation temperature of neat PLA is recorded close to 70°C without significant modifications in as-produced PLA/PA12 blends. The α -relaxation temperature of

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Chapter 5 neat PA12 is observed at 58°C and this relaxation could be also detected in PLA/PA12 blends. However, its exact temperature is difficult to catch due to a high reduction of the PA12 - relaxation intensity in blends. In the glassy state (T < 70 – 80°C), higher storage modulus is observed for blends than neat components. Such effect is still unclear but an additional PLA crystallization at PLA/PA12 interface is suspected inducing a significant increase in PLA crystallinity [210,216,217]. Concerning the impact of the screw speed, no significant modifications of storage modulus are detected in the glassy state.

Figure 5.6. Storage modulus and loss factor as a function of the temperature for injection- molded and annealed PLA/PA12 blends (70/30 wt-%) prepared with various screw speeds.

However, in rubbery state (T > 70 – 80°C), a major influence of the screw speed is noticed with enhanced rubbery modulus of PLA/PA12 for screw speeds higher than 800 rpm and the highest values recorded at 1100 rpm. Such thermomechanical modifications in the glassy/rubbery state were previously correlated with HDT/thermal resistance improvements. Heat deflection temperature (T400MPa-HDTa and

T250MPa-HDTb) of as-produced blends were subsequently extracted from DMA graphs and tabulated in Table 5.3. Annealed neat PLA displayed HDT values close to 78 – 84 °C and significant improvements are recorded for PLA/PA12 blends. Thermal resistances of PLA/PA12 blends clearly increase with the screw speed and HDT reached 86.5 – 114°C for

800 rpm (T400MPa-HDTA and T250MPa-HDTB respectively). Note that a spectacular improvement is noticed for 1100 rpm (T400MPa-HDTA ≈ 92° C, T250MPa-HDTB ≈ 123°C), such trends being in agreement with the evolution of the rubbery modulus. Interestingly, a clear correlation is also obtained between thermal resistance (T400MPa-HDTA and T250MPa-HDTB) and PLA crystallinity extent after annealing (Table 5.3), in agreement with our previous study on PLA/PA blends [210]. The presence of PA12 into PLA tends to increase PLA crystallinity and a further enhancement could be induced by high shear extrusion processing, reaching very high Xc

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Chapter 5 values close to 59 – 66% for 800 – 1100 rpm respectively. Slight modifications of PLA melting temperature are also detected (Figure A8 and Table A7) that could be linked to thermomechanical degradation during extrusion, an effect developed in a later section.

Table 5.3. Thermal resistance (T400MPa-HDTa and T250MPa-HDTb) and PLA crystallinity (normalized value to PLA weight content) of injection-molded and annealed neat PLA and PLA/PA12 blends prepared with various screw speeds (70/30 wt-%) (standard deviation into brackets).

T400Mpa – HDTA (°C) T250Mpa – HDTB (°C) Xc-PLA (%) Neat PLA 78.0 (0.5) 84.0 (1) 46 (1) PLA/PA12 – 80rpm 81.0 (2.5) 95.0 (3.7) n.d. PLA/PA12 – 200 rpm 83.0 (0.5) 102.0 (0.6) 53a PLA/PA12 – 500 rpm 84.5 (0.2) 103.0 (0.5) 54a PLA/PA12 – 800 rpm 86.5 (0.8) 114.0 (1.5) 59a PLA/PA12 – 1100 rpm 92.5 (1.2) 123.0 (1.8) 66 a a Calculated using equation 5.4 and PLA melting enthalpy estimated from the melting peak deconvolution n.d. not determined In conclusion, high shear extrusion is also highly beneficial on thermal resistance of PLA/PA12 blends with remarkable increase of the thermal resistance for screw speed higher than 800 rpm. Compared to tensile/impact properties, no optimum screw speed is detected and extreme screw speeds tend to boost thermal resistance of PLA/PA12 blends.

Morphology of as-produced PLA/PA12 blends High shear extrusion clearly represents an interesting route to produce high- performances PLA/PA12 blends with enhanced ductility, toughness, and thermal resistance. Several trends were highlighted concerning the effect of processing conditions. A major impact of the screw speed is detected with an optimal screw speed of 800 rpm and higher screw speeds tend to induce complex effects (deterioration of ductility/toughness and enhancement of thermal resistance). In an effort to understand the origin of such effects for further optimization, PLA/PA12 blend morphologies induced by high-speed extrusion were investigated to provide information about PA12 dispersion into PLA. The existence of PA12 droplets, fibrils or continuous structures are possible in PLA/PA [210] with various levels of interfacial adhesion to the matrix [105] and such structures can be studied by SEM or dissolution studies.

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Detection of PA12 continuous structures was first attempted using the dissolution of PLA/PA12 into chloroform, a solvent for PLA and non-solvent for PA12. Colloidal solutions are clearly obtained for all PLA/PA12 blends without residue (Figure 5.7) attesting for the absence of any PA continuous structure. Cross-sectional SEM analysis of injection-molded blends prepared with different screw speeds is displayed in Figure 5.8. Matrix-droplet morphologies are clearly confirmed from SEM images with PA12 domain sizes within the sub- micronic/nanometric range as previously observed for various PLA/PA blends [117,129,166,210]. Cross-sectional PA12 droplet sizes are tabulated in Table 5.4. For screw speeds up to 500 rpm, the PA12 droplet size remained nearly constant with Dn close to 0.72 – 0.75 µm and low polydispersity index of approx. 1.1. Interestingly, a significant decrease of the PA12 droplet size is observed for 800 rpm with Dn down to 600 nm and minor modification of the polydispersity index. However, such trend does not hold for higher screw speeds and a drastic increase of PA12 droplet size is observed for 1100 rpm with Dn up to 1.1 µm and a noticeable increase of polydispersity index to 1.27. Actual results are consistent with the good PLA/PA12 compatibility (low droplet sizes and low dispersity index) [210] and an interesting correlation with tensile/impact properties is detected. The optimum screw speed of 800 rpm with maximal ductility/toughness clearly matches with the minimal PA12 droplet size (Dn 600 nm) and, on the contrary, the dramatic drop of impact/tensile properties seems to arise from the high PA12 droplet size (Dn 1.1 µm).

Figure 5.7. Selective dissolution studies in chloroform of injection-molded PLA/PA12 blends (70/30 wt-%) prepared with screw speeds of 200 rpm, 500 rpm, 800 rpm and 1100 rpm (from left to right).

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a) b)

c) d)

Figure 5.8. Cross-sectional SEM analysis of injection-molded PLA/PA12 blends (70/30 wt-%) prepared with screw speeds of (a) 200 rpm, (b) 500 rpm, (c) 800 rpm and (d) 1100 rpm.

In this respect, the dimension of PA12 droplets seems to control mechanical performances of PLA/PA12 blends and high shear extrusion at elevated screw speed play a key role to enhance PA12 dispersion into PLA due to severe mixing conditions (high shear/strain rates applied on the melt) [173,230,235,238,239]. However, complex phenomenons are observed at extreme screw speed with detrimental effects on the resultant PA12 droplet size and polydispersity index. To strengthen the previous correlations between PA12 domain size and tensile/impact properties with a deeper insight on the effect of extreme screw speed, the presence of fibrillated PA12 structures into injection-molded PLA/PA12 blends was investigated, such elongated structures were previously reported in previous articles by our group with interesting correlation to final properties [129,210].

Table 5.4. Cross-sectional number-average and volume-average PA12 droplet diameters (Dn and Dv), PA12 droplet size polydispersity (Ip) and PA12 morphology type as a function of the screw speed (standard deviations into brackets).

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Screw speed Dn (µm) Dv (µm) Ip PA12 Morphology type 80 rpm 0.75 (0.02) 0.85 (0.03) 1.13 Fibrillar 200 rpm 0.73 (0.02) 0.80 (0.03) 1.09 Fibrillar 500 rpm 0.72 (0.04) 0.80 (0.03) 1.10 Fibrillar 800 rpm 0.60 (0.02) 0.70 (0.03) 1.16 Fibrillar 1100 rpm 1.10 (0.02) 1.40 (0.03) 1.27 Ellipsoidal

SEM images of transversal sections for PLA/PA12 blends produced with various screw speeds are displayed in Figure 5.9. For screw speed up to 800 rpm, a high level of PA12 fibrillation into PLA is observed with an intensive generation of PA12 fibrils during the injection-molding process. Such morphologies are in agreement with the rheological properties of neat components as previously investigated in recent studies on PLA/PA blends [129,210]. Interestingly, refined fibrillar PA12 structures clearly appear for a screw speed of 800 rpm but a higher screw speed up to 1100 rpm is clearly detrimental to the fibrillation process of PA12 during injection-molding with only ellipsoidal PA12 dispersed phase obtained at extreme screw speed. In this respect, it could be stated out that (i) high shear processing significantly modifies rheological properties of neat components and (ii) the PA12 fibrillation seems to control mechanical properties in terms of final ductility/impact toughness of as-produced PLA/PA12 blends. The presence of fibrillar PA12 structures with low cross-sectional diameters obtained during high shear extrusion at screw speed close to 800 rpm could explain the PLA/PA12 synergism for the resultant blend ductility and impact properties. However, the fibrillar-to-ellipsoidal shape transition of PA12 observed for extreme screw speeds still remain unclear and could potentially indicate an intensive macromolecular degradation during processing with severe rheological modifications of PLA and PA12.

a) b)

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c) d)

Figure 5.9. SEM images of transversal sections obtained for injection-molded PLA/PA12 blends (70/30 wt-%) prepared with screw speeds of (a) 200 rpm, (b) 500 rpm, (c) 800 rpm and (d) 1100 rpm.

Discussion on the effect of extreme screw speed Within the previous sections, a strong positive impact of the extrusion screw speed is demonstrated on final (thermo)mechanical properties of PLA/PA12 (70/30 wt-%) prepared by high shear extrusion. For screw speeds up to 800 rpm, ductilities/toughnesses/thermal resistances increase in accordance with refined PA12 fibrillar morphologies into PLA matrices obtained after injection-molding. However, extreme screw speeds of 1100 rpm obviously deteriorate ductility and toughness of PLA/PA12, such effects being in close correlations with a fibrillar-to-ellipsoidal PA12 shape transition. Based on previous work by our group, the development of fibrillar morphologies in PLA/PA blends using the injection-molding process is linked to the PA12/PLA viscosity and elasticity ratio. A strong impact of PLA melt elasticity at elevated shear rate (approx. 104 s-1) was identified [129]. In this respect, the fibrillar-to- ellipsoidal PA12 shape transition observed for extreme screw speeds could arise from PLA macromolecular degradation during extrusion inducing strong modifications of the PLA melt rheology. Such phenomenon was previously detected for classical twin-screw extrusion of PLA [173,234] and could also explain the unexpected high thermal resistance/crystallinity extent of PLA/PA12 blends at 1100 rpm (Table 5.3). In the first approach, as-produced PLA/PA12 blends (70/30 wt-%) were submitted to dynamic rheology at 220°C in the melt state (Figure A9) and a negative impact of screw speed is obviously observed on all rheological parameters (storage modulus, loss modulus, and complex viscosity). High shear extrusion consequently tends to degrade rheological properties of PLA/PA12. PA12 is known to be quite stable during high shear extrusion [238] and a test was performed on the degradation of PLA matrices using an extraction method by centrifugation (removal of PA12 fibrils/droplets from as-produced

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Chapter 5 blends). Dynamic rheology experiments are presented in Figure 5.10 for extracted PLA matrices. PLA elasticity at the maximal frequency of 100 rad/s (i.e. 100 s-1) linearly decreases with the screw speed as detected by higher phase angle (52.9° at 200 rpm and 63.9° at 1100 rpm) (Figure 5.10 and Table 5.5). A simultaneous decrease of the zero-shear complex viscosity 0 is also observed.

Figure 5.10. Dynamic rheology experiments (complex viscosity and phase angle, left and right respectively) performed at 185°C on PLA matrices extracted from PLA/PA12 blends produced at various screw speeds.

In this respect, high shear extrusion clearly reduces PLA melt elasticity at high shear rates, an effect linked to PLA thermomechanical degradation (corresponding estimated Mw 90,000 g/mol at 200 rpm and 56,000 g/mol at 1100 rpm [240], it should be noted that PLA extraction seems to induce some side effects, see Appendix Figure A10).

Table 5.5. PLA elasticity/phase angle (δ), PLA zero-shear viscosity (η0-PLA) and PLA weight- average molecular weight (Mw) as a function of the extrusion screw speed.

a b c δ (°) η0-PLA (Pa.s) Mw-PLA (g/mol) PLA/PA12 200rpm 52.9 3900 90,000 PLA/PA12 500rpm 56.3 1780 70,000 PLA/PA12 800rpm 61.4 1010 63,000 PLA/PA12 1100rpm 63.7 860 56,000 a Determined on the extracted PLA matrices, dynamic rheology at 100 rad/s b Determined on the extracted PLA matrices, Carreau-Yasuda fittings of dynamic rheology results c Estimated using [240]

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As a conclusion, high shear extrusion induces a significant thermomechanical degradation of the PLA matrix leading to lower PLA molecular weights and lower PLA melt elasticity at high shear relevant for injection-molding. Such effects could fit with above-mentioned (thermo)mechanical properties and PLA/PA12 morphology, in particular with the existence of a threshold/critical value for PLA elasticity (and PLA molecular weights or zero-shear viscosity) to maintain PA12 fibrillar structures into PLA by injection-molding.

Conclusions This study primarily targets the study of extrusion parameters on the improvement of thermo (mechanical) properties of PLA/PA12 blends. The blends were processed in a twin- screw extruder with high screw speeds ranging from 200 rpm to 1100 rpm and varying feed rates ranging from 2kg/h to 5kg/h, this generates a high shear on the melt thereby leading to better dispersion. The blends were further investigated for their morphological, thermal and mechanical properties. After a through analysis of these properties it was observed that distinctive feed rates did not show any major influence on the properties. Hence, it can be said that feed rate is not a governing factor for the improvemen in properties.

As-produced PLA/PA12 blends showed a drastic improvement in the ductility without hampering the stress at yield. The maximum elongation at break was observed for a screw speed of 800 rpm (224.2±8.3%). The impact strength of the PLA/PA12 blends showed a linear increase with screw speed, but the improvement was not as eminent as the ductility.

SEM images of the blends showed strong interface and fewer voids for varying screw speed, the droplet size of PA12 was not as low as found in the previous studies [232,233,235]. However, the droplet size was much lower than the screw speed 80 rpm (750 nm). The lowest droplet size was for screw speed 800 rpm (~600 nm) and a strong interface was observed.

The thermal properties under dynamic load were assessed by DMA and it was found that the storage modulus of the blends was much higher when compared to the neat PLA and PA12. The increase in storage modulus was directly proportional to the increase in screw speed and variation of low rate led to minimal change in the storage modulus. Annealing of the materials seems to have increased the global crystallinity of the blends resulting in high storage modulus. The approximate HDT values of the blends with varying screw speed were observed to be higher than for neat PLA thus making these blends suitable for durable applications.

In order to get a better understanding of the improvement in mechanical properties with varying screw speed, the morphology of injection moulded specimens was observed. SEM images of

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Chapter 5 the cross-sectional side revealed very fine dispersion and strong interface. Whereas, in the transversal SEM images displayed fibrillation of PA12. However, at extreme screw speed (1100rpm) drastic decrease in the ductility was observed and the fibrillation of PA12 seemed to be reduced. To further study this effect, rheology on extracted PLA phase was performed and it was found that at elevated screw speed due to the thermomechanical degradation of PLA, reduction in PLA melt elasticity was observed. Molecular weights were estimated and a significant drop of molecular weight was observed (Mw 90,000 g/mol at 200 rpm and 56,000 g/mol at 1100 rpm [240]). The reduction in molecular weight confirms that at extreme screw speed degradation of PLA occurs leading to low melt elasticity. The low melt elasticity of PLA hinders the fibrillation of PA12 thereby decreasing the ductility of the blends.

Hence, high-speed extrusion of PLA/PA12 blends with an optimum screw speed is an effective way to improve the mechanical properties and morphology between immiscible blend systems retaining higher biobased content in the blend.

Acknowledgments

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6 Conclusions The major focus of the study was to enhance (thermo)mechanical properties of PLA thus making it suitable for high value or durable applications. After a thorough literature survey, polymer blending with engineering polymers was found to be one of the most beneficial and efficient ways to improve PLA (thermo)mechanical properties. Among the engineering polymers suitable for blending with PLA, polyamides stand out due to their high ductilities, impact strengths, thermal resistances and potential production from renewable feedstocks. New biobased polyamides are available in the market such as PA10-10 (Vestamid Terra DS16) and PA10-12 (Vestamid Terra DD16) which seemed to be ideal candidates for PLA. Other polyamides such as biobased PA11 and non-biobased PA12 were also explored to confirm actual trends on PLA/PA blend in terms of melt-state compatibility and related (thermo)mechanical properties.

The first chapter presents an investigation of the melt-state compatibility between PLA and as- selected polyamides (PA10-10, PA10-12, PA11, and PA12) and its effect on (thermo)mechanical properties. PLA/PA12 blends showed enhanced interfacial adhesion with the lowest PA12 droplet size, the lowest interfacial tension and the closest surface tension to PLA. The PLA/PA compatibility level evidently increased with the length of the PA aliphatic sequence and the best compatibility was undoubtedly concluded for PLA/PA12 binary blends. Tensile properties of injection-molded PLA/PA blends exhibited a transition from brittle to ductile nature for above 30 wt% PA into PLA for all PA types. This behavior is due to the formation of highly elongated and partially continuous PA structures. The highest ductility was obtained for PLA/PA12 blends signifying that the compatibility between PLA/PA is an important factor for tensile mechanical properties. Similar results were observed for the impact strength of un-notched PLA/PA blends. However, the impact strength only increased by a factor 2 compared to neat PLA and the PLA/PA compatibility seemed to play a minor role in impact properties. Annealed PLA/PA specimens displayed a remarkable improvement in the resultant HDT up to 30 – 45°C than neat PLA. Such synergy between PLA and PA was found to arise from a specific increase of the PLA crystallinity during the annealing treatment in the presence of PA. Among all investigated blends, PLA/PA12 is the most promising blends with maximum compatibility, good interface, and better mechanical properties.

The second phase of this work consists in enhancing (thermo)mechanical properties of these highly-compatible PLA/PA12 blends. In order to achieve this, two strategies were explored (i)

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Chapter 6 classical compatibilization by a reactive PLA-graft-maleic anhydride and (ii) optimization of extrusion parameters, in particular screw speed and feed rate. As previously mentioned 30 wt% of PA was an adequate concentration to improve the (thermo)mechanical properties and this ratio PLA/PA12 (70/30) was fixed for both approaches.

In the former strategy regarding classical compatibilization, the effect of PLA-g-MA as a compatibilizer on PLA/PA12 (70/30) blends was studied. The blends were prepared in a twin- screw extruder with various concentrations of PLA-g-MA incorporated in the blends to evaluate the optimum content of compatibilizer required to obtain higher (thermo)mechanical properties. The positive effect of PLA-g-MA is evident on blend morphology, mechanical properties, and thermal resistance. The mechanical properties of the compatibilized showed outstanding improvement in ductility. The highest ductility was achieved for blends containing 1 wt.% of PLA-g-MA and further increase in the concentration of compatibilizer leads to minor decrease in ductility. The impact properties of the compatibilized blends showed similar trends with a significant increase to the PLA's toughness. However, the maximum impact strength was obtained for 2 wt.% of PLA-g-MA. Hence, the optimum content of PLA-g-MA required to improve the mechanical properties lies between 1 and 2wt.% of PLA-g-MA. To get a better in-depth understanding of the improvement in mechanical properties, the morphology of injection molded specimens were examined. The cross-sectional SEM images reveal finer dispersion, stronger interfaces, and better interfacial adhesion when compared to compression molded samples. This could be the reason for previous improvements in ductility and toughness. Transversal sections of injection molded samples were also examined to assess the relation between fibrillation of the PA12 phase and mechanical properties of the blends. Fibrillation of PA12 is evident on SEM images but, for higher content of PLA-g-MA (> 3wt.%), a hindrance in the PA12 fibrillation was observed and this might be due to higher concentrations of PLA-g-MA at the interface impeding the elongation of PA12. To further corroborate this FTIR studies were performed on extracted PA12 phase, this study revealed the presence of PLA absorption peak confirming low amount of grafting. Overall, the improvement in mechanical properties could be due to combined effect of grafting and PA12 fibrillation. However the latter seems to be governing factor for the major improvement in mechanical properties. The influence of compatibilizer on PLA/PA12 blends on the thermal resistance of the blends (HDT) was not as significant as mechanical properties. The highest thermal resistance was observed for 2 wt.% of PLA-g-MA. These blends could be well suited for durable applications which require high ductility and toughness.

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The final strategy mainly involved the optimization of extrusion parameters including screw speed and feed rate for PLA/PA12 (70/30) blends. The blends were extruded in a twin-screw extruder with high screw speeds ranging from 200 rpm to 1100 rpm (constant feed rate) and feed rates ranging from 2 kg/h to 5 kg/h (constant screw speed). Drastic improvement in ductility was observed for PLA/PA12 without hampering the stress at yield with varying screw speeds and the maximum elongation at break was observed for screw speed of 800 rpm. The impact strength of the PLA/PA12 blends depicted a linear increase with screw speed but the improvement was not as significant as the ductility. However, distinctive feed rates did not show any major influence on mechanical properties. Morphology of the blends displayed strong interface and fewer voids for varying screw speed. The lowest droplet size was observed for screw speed 800 rpm (~600 nm) with a strong interface. However, varying feed rates did not show any effect on the droplet size. The thermal properties under dynamic load were assessed by DMA and it was found that the storage modulus of the blends displayed high synergy levels when compared to the neat PLA and PA12. The increase in storage modulus was correlated to the increase in screw speed and variation of feed rate led to minimal change in the storage modulus. Annealing of the materials seems to have increased the global crystallinity of the blends resulting in high storage modulus. The approximate HDT values of the blends with varying screw speed were observed to be significantly higher than neat PLA and potentially competitive with ABS or ABS/PC.

Amongst the two strategies, high-speed extrusion resulted in a better combination of higher mechanical properties and thermal resistance whereas compatibilized blends are marked by phenomenal ductilities without considerable improvements in thermal resistance. High-speed extrusion has an additional benefit in eliminating the necessity to use an additional compatibilizing agent thereby minimizing the related material costs. Hence, it can be concluded that high-speed extrusion of PLA/PA12 blends is a better method with good (thermo)mechanical properties suitable for high performance or durable applications.

Perspectives In the preliminary studies related to melt-state compatibility between PLA and various polyamides, additional characterizations of interfaces could be done using other methods such as embedded fiber retraction and breaking thread methods. These studies about interfacial tension would further validate the results obtained by rheology and surface tension measurements. Synthesis of PA13 or PA14, longer aliphatic PA and their partial miscibility

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Chapter 6 with PLA could be of interest. Origin of fibrillation, a peculiar morphology observed in PLA/PA12 blends could be studied in depth.

In the next step regarding compatibilitization, a lower amount of compatibilizer (0.5wt.%) could be incorporated to detect an eventual optimal concentration for enhancing the blend ductility. Additionally, a two-step extrusion could be performed wherein initially PA12 and predetermined content of PLA-g-MA could be premixed by extrusion. Later the blends of PLA and the premixed PA12/PLA-g-MA could be extruded. And the detection of grafted PLA-graft- PA by other characterizations such as NMR could be of interest.

For the next strategy of high-speed extrusion, scaling up of the actual pilot-scale process and the evaluation of the properties for larger specimens could be of high interest to conclude about the reliability of this high shear extrusion process and the consistency of PLA/PA12 properties.

Although the PLA/PA12 blends are a potential replacement for its petrosourced counterparts in a high performance application, further investigations of long term properties such as creep, aging and fatigue could be conducted. The blends could be well suited for electronic components such as laptop enclosures, keyboard, and mobile phone casing. However, to corroborate this, a product prototype could be manufactured and subsequent product testing could be done. Similarly, the feasibility of 3D printing of these blends for suitable products could be explored.

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7 APPENDIX

Figure A1: Complex viscosity with angular frequency for neat materials obtained by dynamic frequency. Extraction of the Newtonian viscosity ( 0, zero-shear viscosity) by extrapolation to low frequency and evaluation of the viscosity at extrusion shear rate ( 100 s-1) at an angular frequency of 100 rad/s.

Table A1: Droplet form relaxation time at η” peak (λf), Newtonian viscosity ratio (K), matrix

Newtonian viscosity (ηm), dispersed phase Newtonian viscosity (ηd), number-average droplet radius (Rn, see Table 2) and subsequent interfacial tension (γ12 , calculated from equation 6) for all PLA/PA blends..

Blend λf (s) K ηm (Pa.s) ηd (Pa.s) Rn (µm) γ12 (mN/m)

PLA/PA10-10 0.10 0.82 500 410 0.7 8.3

PLA/PA10-12 0.17 2.9 500 1450 0.5 6.5

PLA/PA11 0.12 0.52 500 260 0.6 5.1

PLA/PA12LV 0.30 0.37 500 185 0.6 1.9

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Table A2: Droplet form relaxation time evaluated using the weighted relaxation spectra (λf),

Newtonian viscosity ratio (K), matrix Newtonian viscosity (ηm), dispersed phase Newtonian viscosity (ηd), volume-average droplet radius (Rv, see Table 2) and subsequent interfacial tension (γ 12 , calculated from equation 6) for all PLA/PA blends.

Blend λf (s) K ηm (Pa.s) ηd (Pa.s) Rv (µm) γ12 (mN/m)

PLA/PA10-10 0.12 0.82 500 410 1.0 9.8

PLA/PA10-12 0.24 2.9 500 1450 0.6 5.5

PLA/PA11 0.18 0.52 500 260 0.8 4.6

PLA/PA12LV 0.33 0.37 500 185 0.7 2.0

Figure A2: Drop shape fittings for pendant drops at 180°C of neat PLA and at 240°C of neat PA10-10, neat PA11 and neat PA12LV (from left to right).

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Figure A3: Typical stress – strain curves evaluated by tensile tests for neat PLA (a), neat PA10- 10 (e) and PLA/PA10-10 blends with 20 wt-% PA10-10 (b), 30 wt-% PA10-10 (c), 40 wt-% PA10-10 (d) (insert: zoom between 0 – 50% strain).

Figure A4: Typical stress – strain curves evaluated by tensile tests for neat PLA (a), neat PA10- 12 (e) and PLA/PA10-12 blends with 20 wt-% PA10-12 (b), 30 wt-% PA10-12 (c), 40 wt-% PA10-12 (d) (insert: zoom between 0 – 50% strain).

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Figure A5: Typical stress – strain curves evaluated by tensile tests for neat PLA (a), neat PA11 (e) and PLA/PA11 blends with 20 wt-% PA11 (b), 30 wt-% PA11 (c) and 40 wt-% PA11 (d).

Table A3: Young’s modulus of neat PLA, neat PA and all PLA/PA blends as a function of PA (by weight) evaluated by tensile tests (standard deviation into brackets).

Young’s Modulus PLA PA Blend Blend Blend Blend (MPa) (90/10) (80/20) (70/30) (60/40)

PLA/PA10-10 2890 1130 2390 2150 2260 2040

(60) (40) (60) (50) (70) (50)

PLA/PA10-12 2890 1200 2510 2280 2140 2000

(60) (60) (110) (100) (70) (40)

PLA/PA11 2890 1100 2655 2460 2280 2030

(60) (50) (50) (50) (70) (50)

PLA/PA12HV 2890 1280 2880 2460 2190 2060

(60) (50) (75) (60) (60) (50)

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Table A4: Stress at a yield of neat PLA, neat PA and all PLA/PA blends as a function of PA (by weight) evaluated by tensile tests (standard deviation into brackets).

Stress at yield Neat PLA Neat PA Blend Blend Blend Blend

(MPa) (90/10) (80/20) (70/30) (60/40)

PLA/PA10-10 66.6 (1.8) 34.9 59.1 58.6 56.5 52.4 (1.4) (0.3) (3.0) (1.6) (3.2)

PLA/PA10-12 66.6 (1.8) 33.3 60.3 59.1 55.6 52.6 (1.7) (2.5) (2.3) (2.0) (1.9)

PLA/PA11 66.6 (1.8) 32.7 56.1 52.4 50.2 48.3 (2.2) (1.5) (2.4) (1.8) (2.6)

PLA/PA12 66.6 (1.8) 39.5 58.6 57.6 56.5 55.8 (1.3)

(1.2) (1.5) (1.6) (1.6)

a) b)

Figure A6: PLA/PA solutions of tensile specimens processed by injection-molded tensile specimens after 1 week dissolution in chloroform. PLA/PA blends with 30 wt-% PA (a) and PLA/PA blends with 40 wt-% PA (b).

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Table A5: Evolution of the notched impact strength with the amount of PA (by weight) for PLA/PA10-10, PLA/PA10-12, PLA/PA11 and PLA/PA12HV blends.

Impact Strength PLA PA 90/10 80/20 70/30 60/40

(kJ/m2)

PLA/PA10-12 2.3±0.2 10.1±0.3 2.3±0.2 2.3±0.2 3.2±0.4 5.01±0.4

PLA/PA10-10 2.3±0.2 5.1±0.3 2.6±0.3 3.2±0.4 4.2±0.2 4.3±0.2

PLA/PA11 2.3±0.2 7.6±0.2 3.0±0.5 3.3±0.3 2.9±0.3 2.8±0.3

PLA/PA12 2.3±0.2 11.1±1.2 2.3±0.2 2.5±0.3 2.1±0.3 2.3±0.3

Figure A7: DSC first heating scan at 10°C/min for injection-molded and annealed neat PLA (a), PLA/PA10-10 (b), PLA/PA10-12 (c), PLA/PA11 (d) and PLA/PA12HV blends (e) (40 wt-% PA).

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Table A6: PLA melting enthalpy (ΔHm-PLA) and PLA melting temperature evaluated by DSC at various heating rates (first heating scan) for neat PLA, PLA/PA10-10, PLA/PA10-12, PLA/PA11 and PLA/PA12HV blends (40 wt-% PA).

ΔHm-PLA ΔHm-PLA ΔHm-PLA

(J/g) Tm-PLA (°C) (J/g) Tm-PLA (°C) (J/g) Tm-PLA (°C)

Heating rate (°C/min) 20 10 5 Neat PLA 43.8 168.8 43.3 168.0 41.4 168.7 PLA/PA10-10 26.8 170.7 31.1 169.6 32.8 168.6 PLA/PA10-12 36.9 170.8 37.2 169.1 35.1 168.5 PLA/PA11 37.8 170.3 39.1 167.5 35.1 167.8 PLA/PA12HV 32.0 170.6 36.6 169.0 34.4 168.5

Figure A8. DSC first heating scans at 10°C/min for injection-molded and annealed PLA/

PA12 blends at various screw speeds (injection-molded samples).

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Table A7. Total melting enthalpy, PLA normalized melting enthalpy and PLA melting temperature of injection-molded and annealed neat PLA and PLA/PA12 blends prepared with various screw speeds (70/30 wt-%) (DSC first heating scan).

Screw speed ΔHm-total (J/g) ΔHm-PLA (J/g-PLA) Tm-PLA (°C)

200 rpm 43.5 50.1 171.1

500 rpm 44.0 50.7 171.1

800 rpm 47.2 54.7 169.2

1100 rpm 52.1 61.7 170.0

Figure A9. Complex modulus, loss modulus, and complex viscosity as a function of the frequency at 220°C for as-produced PLA/PA12 blends by high shear extrusion using various screw speed.

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Figure A10. Complex viscosity and phase angle as a function of the frequency at 185°C for neat PLA. Zero-Shear viscosity of PLA is found in between PLA/PA12 produced at 200 rpm and 500 rpm. This effect seems to indicate that PLA extraction by centrifugation induces side effects such as removal of undesirable PA12 nanoparticles and/or PLA post-condensation.

List of figures Figure 2.1. Global production of bio plastics 13

Figure 2.2. Poly(Lactide) life cycle 14

Figure 2.3. Evolution of PLA production from 2011 to 2020 16

Figure 2.4. Evolution of PLA based blends with engineering polymers over the years 25

Figure 2.5. SEM micrographs (×3000; left) and OM images (×800;right) 27

Figure 2.6. SEM micrographs at 10 000 magnifications of PLA and PLA/PMMA 28

Figure 2.7. PLA/PMMA blends (50/50) melt blending (left), solvent blending 29 (right)

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Figure 2.8. Schematic “temperature-memory effect” 30

Figure 2.9. The schematic diagram for shape memory test of PLA/PMMA blends 30

Figure 2.10. Blend morphology of PLA/PC 33

Figure 2.11. Morphology of PLA/PC/ PBSL of compositions 34

Figure 2.12. Morphology of PLA/PC(70/30) 35

Figure 2.13. Morphology of PLA/PC blends 36

Figure 2.14. Stress-strain curves of PLA/PC 36

Figure 2.15. Mechanical properties of PLA/PC (70/30) 37

Figure 2.16. Morphology of PLA/PET blends of compositions 39

Figure 2.17. Quantitative analysis of the PLA percentage 41 co-continuity development in PLA/PBT

Figure 2.18. Mechanical properties of PLA/PBT blends 42

Figure 2.19. Notched impact strength of PLA/PBT blends 42

Figure 2.20. SEM images of PLA/PA6 45

Figure 2.21. SEM images of PLA/PA6 blends 46

Figure 2.22. Morphology of PLA/PA11 blends 48

Figure 2.23. Morphology of PLA/PA11 (50/50) blends 49

Figure 2.24. SEM images of cryofractured samples 50

Figure 2.25. Ductile and tensile-impact behavior of PLA/PA10-10 blends 51

Figure 3.1. Morphologies of PLA blends with various PA 65

Figure 3.2. Rheology of neat PLA and PLA/PA blends (20 wt% PA) at 220°C 67

Figure 3.3. Pendant drops profiles at 180°C for neat PLA and at 240°C for PA 68

Figure 3.4. Typical stress-strain curves for neat PLA (a), neat PA12HV (e) and 70 PLA/PA12HV blends

Figure 3.5. Evolution of the tensile strain at break with the amount of PA 71

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Figure 3.6. Evolution of the PA continuity with the amount of PA for injection- 72 molded blends (a). Morphology of injection-molded PLA/PA10-10 (b) and PLA/PA12HV (c) blends as observed by SEM (transversal cross-sections).

Figure 3.7. Evolution of the unnotched impact strength with the amount of PA 74

Figure 3.8. Storage modulus as a function of temperature (DMA) for neat PLA and 74 PLA/PA blends

Figure 4.1. Morphology of PLA/PA/PLA-g-MA of various compositions 85

Figure 4.2. Emulsification curve of PLA/PA12 blends with varying PLA-g-MA 86 content

Figure 4.3. Stress-strain curves of PLA/PA12 blends with varying concentrations of 87 compatibilizer (PLA-g-MA)

Figure 4.4. Ductility of PLA/PA12 blends with varying PLA-g-MA content 88

Figure 4.5. Impact strength of the PLA/PA12 blends with varying concentration of 89 PLA-g-MA

Figure 4.6. Cross-sectional SEM analysis of injection-molded PLA/PA12/PLA-g- 90 MA blends

Figure 4.7. Droplet diameter of PLA/PA12 blends with varying concentration of 91 PLA-g-MA

Figure 4.8. Transversal section SEM analysis of injection-molded PLA/PA12/PLA- 92 g-MA blends

Figure 4.9. Storage modulus as a function of the temperature of PLA/PA12 blends 93 with varying content of PLA-g-MA(wt.%)

Figure 4.10. DSC cooling(left) and heating(right) thermogram of PLA/PA12 blends 94 with PLA-g-MA

4.11 FTIR studies of PLA/PA12 blends with varying content of compatibilizers 96

Figure 5.1. As-selected screw profile for twin-screw high shear extrusion processing 103 of PLA/PA12 blends.

Figure 5.2. Typical tensile stress-strain curves obtained for PLA/PA12 blends 107 (70/30 wt-%)

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Figure 5.3. Tensile strength (a) and strain at break (b) as a function of the screw 109 speed for as-produced PLA/PA12 blends (70/30 wt-%) (constant feed rate 5 kg/h).

Figure 5.4. Tensile strength (a) and strain at break (b) as a function of the feed rate 110 for as-produced PLA/PA12 blends (70/30 wt-%) (constant screw speed 800 rpm).

Figure 5.5. Un-notched impact strength as a function the screw speed 110

Figure 5.6. Storage modulus and loss factor as a function of the temperature for 111 injection-molded and annealed PLA/PA12 blends (70/30 wt-%) prepared with various screw speeds.

Figure 5.7. Selective dissolution studies in chloroform of injection-molded 114 PLA/PA12 blends (70/30 wt-%) prepared with screw speeds

Figure 5.8. Cross-sectional SEM analysis of injection-molded PLA/PA12 blends 114 (70/30 wt-%) prepared with screw speeds

Figure 5.9. SEM images of transversal sections obtained for injection-molded 116 PLA/PA12 blends (70/30 wt-%) prepared with screw speeds

Figure 5.10. Dynamic rheology experiments (complex viscosity and phase angle, 117 left and right respectively) performed at 185°C on PLA matrices extracted from PLA/PA12 blends produced at various screw speeds.

List of tables

Table 2.1. General properties of PLA and other engineering polymers 24 Table 2.2. Overview of PLA/PMMA blends 31 Table 2.3. Overview of PLA/PC blends 38 Table 2.4. Overview of PLA/semi-aromatic polyester blends 41 Table 2.5. General properties of PLA and different PA polymers 44 Table 2.6. General A general overview of PLA/PA blends 52 Table 3.1. Specific densities at ambient temperature, melting temperature and melt 60 volume index of as-selected materials (manufacturer datas).

Table 3.2. Number-average and volume-average PA droplet diameters (Dn and 66 Dv), PA droplet size polydispersity (Ip) and PA/PLA viscosity ratio (p) as a function of the PA type (standard deviations into brackets).

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Table 3.3. Average interfacial tension in as-studied PLA/PA blends at 220°C 67 (average values, standard deviation into brackets).

Table 3.4. Specific melt density and surface tension (s) of neat PLA and neat PA 69 at 240°C obtained by the pendant drop method.

Table 3.5. Thermal resistance (T400MPa-HDTa and T250MPa-HDTb) and PLA crystallinity 75 (Xc-PLA, normalized to PLA content) of injection-molded and annealed neat PLA and PLA/PA blends (40 wt% PA) as evaluated by DMA and DSC (standard deviation into brackets).

Table 4.1. Number-average and volume-average PA12 droplet diameters (Dn and 86 Dv), PA droplet size polydispersity (Ip) with standard deviation in brackets of PLA/PA12 /PLA-g-MA blends (compression molded samples) Table 4.2. Stress at yield, strain at break and impact strength for as-produced 89 PLA/PA12/PLA-g-MA blends

Table 4.3. Number-average and volume-average PA12 droplet diameters (Dn and 91 Dv), PA droplet size polydispersity (Ip) with standard deviation in brackets of PLA/PA12 /PLA-g-MA blends Table 4.4. Thermal properties of PLA/PA12 blends with varying content of PLA- 95 g-MA Table 5.1. Stress at yield, strain at break and impact strength for as-produced 109 PLA/PA12 blends (70/30 wt-%) with varying screw speeds (constant feed rate 5 kg/h) (standard deviations into brackets). Table 5.2. Stress at yield, strain at break and impact strength for as-produced 110 PLA/PA12 blends (70/30 wt-%) with varying feed rates (constant screw speed 800 rpm) (standard deviations into brackets).

Table 5.3. Thermal resistance (T400MPa-HDTa and T250MPa-HDTb) and PLA crystallinity 113 (normalized value to PLA weight content) of injection-molded and annealed neat PLA and PLA/PA12 blends prepared with various screw speeds (70/30 wt-%) (standard deviation into brackets). Table 5.4. Cross-sectional number-average and volume-average PA12 droplet 116 diameters (Dn and Dv), PA12 droplet size polydispersity (Ip) and PA12 morphology type as a function of the screw speed (standard deviations into brackets).

Table 5.5. PLA elasticity/phase angle (), PLA zero-shear viscosity (0-PLA) and 119 PLA weight-average molecular weight (Mw) as a function of the extrusion screw speed

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Content Poly(lactic acid), (2001). doi:10.1021/MA001173B.

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ABSTRACT The major objective of this thesis is to enhance the (thermo)mechanical properties of biobased poly(lactic acid) (PLA) to reach high performance/durable applications. Polymer blending with engineering polymers is an effective approach to improve these properties and polyamides (PA) were selected for blending with PLA due to their inherent high ductilities, high impact toughnesses, high thermal resistances coupled with a potential production from renewables feedstocks. Various biobased polyamides such as polyamide10-10, polyamide10-12, polyamide11 and polyamide12 were blended with PLA in a twin-screw extruder. Thorough characterizations such as morphology, rheology, surface tension, mechanical testing and thermal properties of PLA/PA blends were performed to assess the apt polyamide for PLA. Polyamide12 (PA12) was found to be the ideal candidate for PLA with a low PLA/PA12 interfacial tension, very good PA12 dispersion into PLA and enchanced (thermo)mechanical properties for PLA/PA12 blends compared to other PLA/PA blends (higher ductility and impact strength with significant increase in thermal resistance). To further enhance (thermo)mechanical properties, two different strategies were attempted namely compatibilization with a reactive PLA-graft-maleic anhydride (PLA-g-MA) and optimization of the extrusion processing parameters. In the former classical compatiblization strategy, PLA-g-MA was incorporated as a compatibilizer in PLA/PA12 blends and PLA-g-MA content was optimized with respect to various properties such as morphology and (thermo)mechanical. It was observed that 1 – 2 wt.% PLA-g-MA could induce significant improvements of the PLA/PA12 ductility, impact strength and thermal resistance. The latter strategy involved the optimization of extrusion parameters such as screw speed and feed rate. PLA/PA12 blends were extruded at various screw speeds (200 – 1100rpm) and feed rate (2 – 5 kg/h) on a pilot twin-screw extrusion line. The screw speed has a profound impact of the blend properties. The best (thermo)mechanical properties were achieved for an optimal screw speed of 800 rpm followed by a dramatic deterioration of the blend properties at extreme screw speed. Such effects are discussed based on the generation of fibrillary PA12 structures and PLA macromolecular degradation during high shear extrusion. In conclusion, remarkable improvements in (thermo)mechanical properties were achieved by blending PLA with PA12. Amongst the strategies employed to further enhance the properties, the optimization of extrusion parameters represent a cost-effective approach compared to classical compatibilization. PLA/PA12 blends could be a potential candidate for the replacement of petrosourced counterparts used in high performance applications, in particular electronic casing applications.

RESUME L'objectif principal de cette thèse est d'améliorer les propriétés (thermo)mécaniques de l’acide polylactique (PLA) biosourcé pour atteindre des applications hautes performances/durables. Les mélanges avec des polymères techniques sont des approches efficaces pour améliorer ces propriétés et les polyamides (PA) ont été sélectionnés pour être mélangés avec du PLA en raison de leur ductilité élevée, de leur résistance aux chocs élevée, de leurs résistances thermiques élevées associées à une production potentielle à partir de matières premières renouvelables.Divers polyamides biosourcés tels que le polyamide10-10, le polyamide10-12, le polyamide11 et le polyamide12 ont été mélangés avec du PLA par extrusion bivis. Des caractérisations approfondies (morphologie, rhéologie, tension superficielle, tests mécaniques et propriétés thermiques) ont été effectuées sur les mélanges PLA / PA pour évaluer le polyamide le plus approprié pour le PLA. Le polyamide12 (PA12) s'est révélé être le candidat idéal pour les PLA avec une tension interfaciale PLA/PA12 faible, une très bonne dispersion du PA12 dans le PLA et des propriétés (thermo)mécaniques améliorées pour les mélanges PLA / PA12 par rapport aux autres mélanges PLA/PA (ductilité et résistance aux chocs supérieures avec augmentation significative de la résistance thermique). Afin d'améliorer ces propriétés (thermo)mécaniques, deux stratégies différentes ont été tentées, à savoir la compatibilisation avec un PLA greffé anhydride maleique (PLA-g-MA) et l'optimisation des paramètres d’extrusion. Dans la stratégie de compatibilization classique, le PLA-g-MA a était incorporé en tant que compatibilisant dans les mélanges de PLA/PA12 et la teneur en PLA-g- MA a était optimisée vis-à-vis des diverses propriétés morphologiques et (thermo)mécaniques. Il a été observé que 1 à 2% de PLA-g-MA induit des améliorations significatives de la ductilité, de la résistance au choc et de la résistance thermique du mélange PLA/PA12. La dernière stratégie implique l'optimisation des paramètres d'extrusion tels que la vitesse de la vis et le débit massique d’alimentation. Les mélanges de PLA/PA12 ont été extrudés à différentes vitesses de vis (200 à 1100 tr/min) et le débit massique d’alimentation (2 à 5 kg/h) sur une ligne d’extrusion bivis pilote. La vitesse de la vis a un impact profond sur les propriétés du mélange. Les meilleures propriétés (thermo)mécaniques ont été obtenues pour une vitesse de vis optimale de 800 tr/min suivie d'une détérioration considérable des propriétés du mélange à une vitesse de vis extrême. Ces effets sont discutés en fonction de la génération de structures PA12 fibrillaires et de la dégradation macromoléculaire du PLA au cours de l’extrusion à fort cisaillement. En conclusion, des améliorations remarquables des propriétés (thermo)mécaniques ont été obtenues en mélangeant du PLA avec du PA12. Parmi les stratégies utilisées pour améliorer ces propriétés, l'optimisation des paramètres d'extrusion représente une approche rentable par rapport à la compatibilisation classique. Les mélanges de PLA/PA12 pourraient donc constituer des candidats potentiels pour remplacer leurs homologues à base de ressources pétrolières utilisés dans les applications à hautes performances, en particulier les applications de type boîtier électronique.

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