Polyamide 11/Poly(Butylene Succinate) Bio-Based Polymer Blends

Home , Polyamide, Polybutylene succinate

materials

Article Polyamide 11/Poly(butylene succinate) Bio-Based Polymer Blends

Maria Laura Di Lorenzo 1,*, Alessandra Longo 1,2 and René Androsch 3

1 Institute of Polymers, Composites and Biomaterials (CNR), Via Campi Flegrei, 34, 80078 Pozzuoli (NA), Italy 2 Department of Chemical, Materials and Production Engineering, University of Naples “Federico II”, P.le Tecchio 80, 80125 Napoli, Italy 3 Interdisciplinary Center for Transfer-oriented Research in Natural Sciences, Martin Luther University Halle-Wittenberg, D-06099 Halle/Saale, Germany * Correspondence: [email protected]

Received: 24 July 2019; Accepted: 29 August 2019; Published: 3 September 2019

Abstract: The manuscript details the preparation and characterization of binary blends of polyamide 11 (PA 11) and poly(butylene succinate) (PBS), with PA 11 as the major component. The blends are fully bio-based, since both components are produced from renewable resources. In addition, PBS is also biodegradable and compostable, contrarily to PA 11. In the analyzed composition range (up to 40 m% PBS), the two polymers are not miscible, and the blends display two separate glass transitions. The PA 11/PBS blends exhibit a droplet-matrix morphology, with uniform dispersion within the matrix, and some interfacial adhesion between the matrix and the dispersed droplets. Infrared spectroscopy indicates the possible interaction between the hydrogens of the amide groups of PA 11 chains and the carbonyl groups of PBS, which provides the compatibilization of the components. The analyzed blends show mechanical properties that are comparable to neat PA 11, with the beneﬁt of reduced material costs attained by addition of biodegradable PBS.

Keywords: polyamide 11; poly(butylene succinate); polymer blends; bio-based polymers; biodegradable polymers; mechanical properties; thermal analysis; morphology; spectroscopy

1. Introduction Nowadays, synthetic polymers have replaced many traditional materials, such as wood, stone, metal, and ceramics, and they are used in all areas of daily life and application: they protect food and prevent spoilage, insulate electric cables, save fuel by making cars lighter and safer, are used as fabric for clothing, in contact lenses, etc. Out of about the 335 million tons of plastics produced annually, 99% are produced from petroleum [1], and it is expected that, by 2050, the plastics industry will account for 20% of the total oil consumed annually [2]. Such large use of synthetic polymers resulted in increasing environmental concern, which, coupled to the realization that petroleum resources are finite, led to considerable search for alternative sources of raw materials for the production of new polymers. Moving away from petrochemical feedstocks toward short-term renewable resources will moderate the detrimental influence of plastics on the environment. It is expected that the polymers that are produced from short-term, e.g., annually renewable feedstocks, which are often addressed as bio-based polymers, might fully replace those produced from fossil sources [3]. At present, several bio-based polymers are industrially produced, with applications that mainly include food packaging and agriculture, as well as biomedical devices [4–6]. Properties and the performance/cost ratio of bio-based polymers need to be improved in order to expand the variety of potential fields and enter new markets. A popular route applied for decades, also for classical polymers, is to tailor the properties of a given polymer for specific applications by blending [7–9].

Materials 2019, 12, 2833; doi:10.3390/ma12172833 www.mdpi.com/journal/materials Materials 2019, 12, 2833 2 of 14

This approach of enhancing properties is relatively inexpensive in comparison to developing new polymerization routes to attain novel polymers. In other words, research and development of new polymer blends play a crucial role in increasing the competitiveness of bio-based polymers. One of the most important bio-based polymers is polyamide 11 (PA 11), which is a commercial aliphatic polyamide that is produced from castor oil [2,10,11]. Even if only representing a small fraction of the worldwide polyamide production, PA 11 finds applications in a wide range of fields, thanks to its biocompatibility, good oil and salt water resistance, excellent piezoelectric and cryogenic properties, and lower hydrophilicity as compared to the more widely used polyamides 6 and 6.6 [11,12]. Therefore, PA11 is used as an engineering polymer in a large range of industries, including automotive, offshore applications, as well as food packaging [13,14]. Many research efforts have been devoted to improve the properties of PA 11, mainly through incorporation of inorganic fillers [12–18], or by blending with other, especially bio-based polymers, like poly(l-lactic acid), or polyhydroxyalcanoates to further expand the application range [19,20]. Among the various bio-based polymers, blends of PA 11 with poly(butylene succinate) (PBS) have barely been investigated. To our knowledge, only a single study about PA 11/PBS blends was performed, where a composition-dependent improvement of the impact strength of PA 11 was reported, without affecting other mechanical properties, like the flexural modulus [21]. From these results, it appears that the addition of PBS has potential to improve properties of PA 11, also taking into account that PBS is not only bio-based, but also biodegradable and compostable [22,23], contrarily to PA 11. In addition, the production of PBS only requires half the costs as the production of an equivalent amount of PA 11 [24–26]. Therefore, PA 11/PBS blends appear as promising new polymeric materials, with reduced material/production costs as compared to plain PA 11, and expected to be also partially biodegradable. The biodegradation of PBS chains is initiated by the hydrolysis of ester bonds, which leads to the formation of water-soluble fragments with a molar mass lower than 500 Da. These short PBS chain segments can be assimilated by microorganisms, and then turned to carbon dioxide, water, and biomass [22,23]. Conversely, PA 11 is not easily decomposable in the environment. Hence, PA 11 formulations containing PBS may be partially biodegradable, with the extent of biodegradation of these blends currently under investigation. PA 11 and PBS are both semi-crystalline polymers. PA 11 is rigid at room temperature, with a glass transition temperature of the mobile amorphous fraction (MAF) of Tg = 43 ◦C[27], and a rigid amorphous fraction (RAF) that devitrifies at higher temperatures [28]. It also exhibits crystal polymorphism that depends on thermo-mechanical history, with six different crystal modifications being reported in the literature [29–32]. After melt processing, like injection-molding [33], the polymer is semi-crystalline at room temperature, often containing lamellar crystals, which were grown to form a spherulitic superstructure. The maximum crystal fraction, similar as in case of other linear polyamides, is well below 50%, however, contributing to its balanced property profile [27]. Further dedicated studies about semi-crystalline morphologies of PA 11 forming, e.g., at high supercooling of the melt, or developed upon self-nucleation are available in the literature [34,35]. At room temperature, PBS has a rubbery mobile amorphous fraction, with a glass transition below 30 C[36], as well as sizable RAF, − ◦ whose vitrification/devitrification was quantified in [37] as a function of the cooling rate. In addition, o the melting points of the two polymers largely differ, with the equilibrium melting temperature (Tm) of o PBS in the range of 127.5–146.5 ◦C[38], whereas PA 11 has a Tm = 203–220 ◦C[39,40]. PBS is already used as additive in other bio-based polymer formulations, for instance, in blends with poly(lactic acid) (PLA), to improve the flexibility, toughness, and heat resistance of PLA [41], and it is likely that the addition of PBS also has potential for improving properties of PA 11. As mentioned above, PBS may improve the impact strength of PA 11 while maintaining the flexural modulus, as recently published in a short communication [21]. However, information regarding these blends are limited to quantitative data on impact strength and modulus, supported by analysis of morphology of the blends [21]. The improvement of mechanical properties of PA 11 upon the addition of PBS, as probed in [21], deserves a more detailed investigation, to fully exploit the potential of these blends Materials 2019, 12, 2833 3 of 14 and optimize both composition and processing. For these reasons, a detailed investigation of the influence of PBS on the thermal and mechanical properties of PA 11 is needed, which is reported in this manuscript for blends containing the polyamide as the main component. This will be completed by a thorough analysis of the influence of thermal history on the crystallization kinetics of both polymers, to be presented in a forthcoming manuscript.

2. Experimental Part

2.1. Materials A heat- and light-stabilized PA 11, extrusion grade Rilsan® BESNO TL from Arkema 3 (Colombes, France), was used. The melt volume index of the polymer is 1 cm /10 min (235 ◦C, 1 2.16 kg) [42] and the number-average molar mass and polydispersity are 17.2 kg mol− and around 2, respectively [35]. Further material data are available at the Arkema website [42]. PBS Bionolle 1001MD was kindly received by Showa Denko K. K. (Japan). This polymer grade has a melt-ﬂow index of 1.4 g/10 min (190 ◦C, 2.16 kg). The number-average molar mass and polydispersity 1 are 57.7 kg mol− and 2, respectively, as measured by gel permeation chromatography. Before melt mixing, the PA 11 and PBS pellets were dried in a vacuum oven at 80 ◦C for 4 h, or 60 ◦C for 16 h, respectively.

2.2. Blend Preparation Binary PA 11/PBS blends with a composition of 100/0, 90/10, 80/20, 60/40 m/m% were prepared by melt mixing in a Brabender-like apparatus Rheocord EC of Haake Inc. (Vreden, Germany) at 210 ◦C and 32 rpm for 8 min.

2.3. Preparation of Compression-Molded Sheets PA 11/PBS blends were compression-molded with a Collin Hydraulic Laboratory Forming Press P 200 E. The blends were heated to 210 ◦C and then kept at this temperature for 2 min without application of any pressure, to allow for complete melting. After this period, a load of 0.5 tons was applied for 2 min, and then the sample was cooled to room temperature in less than 3 min by means of cold water circulating in the plates of the press. Compression-molded sheets with a thickness of about 300 µm were obtained. A compression-molded sheet of plain PBS was also prepared, by pre-melting at 135 ◦C for 2 min, followed by molding with a load of 0.5 ton for 2 min, and then cooling to room temperature via cold water circulating in the plates of the press. A lower molding temperature was used to attain sheets with the same thickness as the PA11/PBS blends. However, the varied thermal history did not lead to signiﬁcant variation in the analyzed material properties, which were limited to thermal stability (thermal degradation of PBS initiates above 300 ◦C, as shown below), glass transition, and crystallization/melting behavior (the cooling rate from the relaxed melt was the same as the blends), nor aﬀected infrared spectroscopy analysis.

2.4. Differential Scanning Calorimetry Differential scanning calorimetry (DSC) was performed with a Perkin-Elmer Pyris Diamond DSC (Waltham, MA, USA) that was equipped with an Intracooler II as cooling system. The instrument was calibrated in temperature and energy with a high purity indium standard, using dry nitrogen as purge 1 gas at a flow rate of 30 mL min− . The compression-molded blends were analyzed upon heating at 20 K min 1, from 60 to 210 C. The molded samples had a thickness of 300 µm, which corresponds − − ◦ to about 6 mg when cut to fit into DSC sample pans. The experimentally measured heat-flow-rate raw data were corrected for instrumental asymmetry by subtraction of a baseline, measured under identical conditions as the samples, including a close match of the masses of the aluminum pans. All of the experiments were repeated three times to ensure reproducibility. Materials 2019, 12, 2833 4 of 14

2.5. Thermogravimetry Thermogravimetric analyses (TGA) were carried out with a Perkin Elmer Pyris Diamond TG-DTA instrument under nitrogen atmosphere. Measurements were performed on samples of about 5 mg, 1 and then heated from room temperature to 600 ◦C at 10 K min− in nitrogen atmosphere, with a 1 nominal gas ﬂow rate of 30 mL min− .

2.6. Fourier-Transform Infrared Spectroscopy Fourier-transform infrared spectroscopy (FTIR) spectroscopy was performed in reflection mode while using a PerkinElmer FTIR Spectrometer Model Spectrum 100 equipped with a PerkinElmer Universal Attenuated Total Reflectance (ATR) sampling accessory with a diamond 1 crystal. Each spectrum is an average of 16 individual scans, recorded at a resolution of 2 cm− . The compression-molded blends were analyzed by directly placing the films on the diamond crystal. Each spectrum was repeated three times.

2.7. Scanning Electron Microscopy Morphological analysis of cryogenically fractured PA 11/PBS blends was performed while using a FEI Quanta 200 FEG environmental scanning electron microscope (ESEM) (Eindhoven, The Netherlands) in low vacuum mode, while using a Large Field Detector (LFD) and an accelerating voltage of 30 kV. Before analysis, the samples were sputtered-coated with an Au–Pd alloy using a Baltech Med 020 Sputter Coater System and mounted on aluminum stubs by means of carbon adhesive disks.

2.8. Tensile Tests Dumbbell-shaped specimens with a length and thickness of the gauge section of 25 and 4 mm, respectively, were cut from the compression molded sheets and then used for tensile measurements. Stress-strain curves were obtained with an Instron machine, Model 4505 (Norwood, MA, USA) at 1 a cross-head speed of 5 mm min− . Young’s modulus, stress, and strain at yield and at break were calculated from an average of seven specimens.

3. Results and Initial Discussion

Figure1 illustrates the apparent specific heat capacity ( cp) plots of PA 11/PBS compression-molded 1 sheets, as measured upon heating at 20 K min− , one day after preparation. Plain PA 11 (black curve) displays a glass transition temperature (Tg) centered at 41 ◦C, as typical for PA 11 [27,39], with the corresponding heat-capacity increment overlapping with a small enthalpy-recovery peak that is caused by the short storage at room temperature (i.e., below Tg), as well as by the different cooling/heating rates used [43]. The material shows double melting due to crystal reorganization on slow heating, which is in agreement with literature data [27]; the final melting peak is detected at 188 ◦C. The DSC plot of plain PBS (magenta curve) shows a Tg of 36 C, with a cp-jump that points to a − ◦ mobile amorphous content of wA = 0.22, as calculated by comparison the measured cp-step to the heat capacity step at Tg of the fully amorphous polymer [37,44]. This is followed by a small endotherm peaked at 42 ◦C, coupled with a sizable increase of cp, and then by a broad endotherm, a sharp recrystallization, and a final melting peak. The overall crystal fraction, as measured by a comparison of the experimentally observed enthalpy of fusion and the heat of fusion of 100% crystalline PBS of 1 220 J g− [45] indicates a crystal fraction wC = 0.28. The rigid amorphous fraction, wRA, as calculated by difference [46–48], amounts to wRA = 0.50. Small endotherms at temperatures close to 40 ◦C have been reported in the literature for PBS, and ascribed to the melting of small crystals that is caused by annealing at temperatures above Tg [45]. However, the integration of the endotherm peaked at 42 ◦C in Figure1 leads to a small enthalpy of transition that may be linked to melting of only 1% of PBS crystals, a too low amount to justify the marked increase in cp, which instead reveals the mobilization of 0.35 of annealing at temperatures above Tg [45]. However, the integration of the endotherm peaked at 42 °C Materials 2019, 12, 2833 5 of 14 in Figure 1 leads to a small enthalpy of transition that may be linked to melting of only 1% of PBS crystals, a too low amount to justify the marked increase in cp, which instead reveals the mobilization ofsolid 0.35 fraction. of solid It fraction. may be speculated It may be thatspeculated the endotherm that the at endotherm 42 ◦C is linked at 42 to enthalpy°C is linked relaxation to enthalpy of the relaxationRAF, which of partlythe RAF, mobilizes which inpartly this mobilizes temperature in this range temperature [37]. range [37].

-40 -20 0 20 40

p c

5 J/(K g) PA 11/PBS 100/0 90/10 80/20 60/40 0/100

0 50 100 150 200 Temperature (°C)

FigureFigure 1. 1. ApparentApparent specific speciﬁc heat capacitycapacity ((ccpp)) ofof compression-moldedcompression-molded PA PA 11 11/PBS/PBS sheets sheets as as a functiona function of 1 −1 oftemperature, temperature, measured measured upon upon heating heating at 20at 20 K minK min− . Data. Data are are shifted shifted vertically vertically for for clarity. clarity.

TheThe DSC DSC plots plots of of PA PA 11/PBS 11/PBS blends containing between 10 and 40 m% PBS display multiple thermal events, which are close to the algebraic sum of the c curves of the two polymers. The glass thermal events, which are close to the algebraic sum of the cp curves of the two polymers. The glass transition of PBS, centered at 36 C[36,49], is hardly detectable in the 90/10 blend, but it becomes transition of PBS, centered at −−36 °C◦ [36,49], is hardly detectable in the 90/10 blend, but it becomes more intense with the increasing content of PBS in the blend, as seen in the insert of Figure1. T of PA more intense with the increasing content of PBS in the blend, as seen in the insert of Figure 1.g Tg of PA11 overlaps11 overlaps with with the smallthe small endotherm endotherm peaked peaked at 42 at◦C 42 in °C plain in plain PBS, withPBS, thewith latter the intensifyinglatter intensifying with a withhigher a higher PBS content. PBS content. When When compared compared to neat to polymers, neat polymers, the melting the melting of PA 11of crystalsPA 11 crystals in the blendsin the blendsseems scalingseems scaling in size within size composition, with composition, whereas whereas the recrystallization the recrystallization and melting and of melting PBS crystals of PBS in T crystalsthe blends in the also blends display also a small display shift a small in temperature. shift in temperature. The detection The of detection two separate of twog separate’s in the blends,Tg’s in theoccurring blends, at occurring the same at temperatures the same temperatures as in the plain as polymers,in the plain indicates polymers, the indicates immiscibility the immiscibility of PA 11 and ofPBS. PA This11 and is confirmedPBS. This is by confirmed the occurrence by the ofoccurrence separate crystallizationof separate crystallization and melting and events melting of the events two ofpolymers, the two polymers, but with some but with influence some of influence PA 11 on of crystallization PA 11 on crystallization and melting and of PBSmelting chains. of PBS chains. TheThe crystal crystal fractions fractions of of PA PA 11 and PBS in the blen blendsds were determined by by the integration of of the the meltingmelting endothermsendotherms and and normalization normalization to theto contentthe content of the of respective the respective blend component. blend component. The measured The 1 melting enthalpy of PA 11 fraction is 40 1 J g , independent−1 of the blend composition. PA 11 is a measured melting enthalpy of PA 11 fraction± is− 40 ± 1 J g , independent of the blend composition. PApolymorphic 11 is a polymorphic polymer and polymer it exhibits and six it exhibits different six crystalline different phases crystalline [29–31 phases]. The [29–31]. melting The profile melting of PA profile11 that of is shownPA 11 that in Figure is shown1 indicates in Figure presence 1 indicates of the presenceα-modification, of the α as-modification, it is expected as for it theis expected selected forpathway the selected of preparation pathway [ 27of]. preparation Literature data[27]. on Literatu the bulkre data enthalpy on the of bulk melting enthalpy of PA 11of αmelting-crystals of vary PA 1 11from α-crystals 189 to 244vary J gfrom− [27 189], whichto 244 leadsJ g−1 [27], to a which crystal leads fraction to a ofcrystal the PA fraction 11 part of (thewC,PA PA) of11 0.16part to(w 0.21,C,PA) ofrespectively. 0.16 to 0.21, Additionally, respectively. the Additionally, heat of fusion the of theheat PBS of fraction,fusion of normalized the PBS fraction, to its content normalized in the blends, to its 1 contentseems to in be the una blends,ffected seems by the to blend be unaffected composition, by th ande blend it amounts composition, to 55 and0.5 it J gamounts− . A comparison to 55 ± 0.5 of J ± 1 gthe−1. A measured comparison heat of of the fusion measured with the heat heat of fusion of fusion with of the 100 heat % crystalline of fusion PBSof 100 of 220% crystalline J g− [45] PBS results of 220in a J crystal g−1 [45] fraction results ofin thea crystal PBS parts fraction in the of blendsthe PBS (w partsC,PBS )in of the 0.26. blends As such, (wC,PBS the) of total 0.26. crystal As such, fraction the total(wC =crystalwC,PA fraction+ wC,PBS ()w ofC = the wC,PA compression + wC,PBS) of the molded compression sheets linearly molded increases sheets linearly with the increases addition with of PBS, the additiondue to its of larger PBS, due crystallinity, to its larger from crystallinity, 16% in plain from PA 16% 11 to in 20% plain in thePA 11 PA to 11 20%/PBS in blend the PA 60 /11/PBS40. blend 60/40.To gain information about the phase structure and morphology of PA 11/PBS blends, scanning electron microscopy (SEM) analyses were performed, with the results presented in Figure2. Figure2a illustratesTo gain the information cryogenically-fractured about the phase surfaces structure of compression-molded, and morphology of PA plain 11/PBS PA 11, blends, which scanning appears electronsmooth, microscopy as expected. (SEM) The analyses addition were of PBS performed, (Figure2b–d) with results the results in formation presented of in a Figure matrix-particle 2. Figure structure, that is, there occurred phase separation. The size of the particles increases with PBS content in5 the blends, which, coupled to the small voids surrounding the minor phase inclusions, seems to suggest 2a illustrates the cryogenically-fractured surfaces of compression-molded, plain PA 11, which appears smooth, as expected. The addition of PBS (Figure 2b–d) results in formation of a matrix- Materials 2019, 12, 2833 6 of 14 particle structure, that is, there occurred phase separation. The size of the particles increases with PBS content in the blends, which, coupled to the small voids surrounding the minor phase inclusions, seemsa scarce to interfacial suggest a adhesion.scarce interfacial Otherwise, adhesion. the PBS Otherwise, particles appearthe PBS homogeneously particles appear distributed homogeneously within distributedthe matrix, andwithin most the of matrix, them were and most not pulled of them out we duringre not the pulled cryogenical out during fracture the cryogenical process, rather fracture than process,remained rather attached than to remained the matrix, attached perhaps to indicatingthe matrix, compatibility perhaps indicating between compatibility the blend components. between the blend components.

(a) (b)

Figure 2. ScanningScanning electron electron micrographs micrographs of of cryogenically cryogenically fractured fractured surfaces surfaces of of PA PA 11/PBS 11/PBS blends: ( (aa)) 100/0;100/0; (b) 9090/10;/10; (c) 8080/20;/20; and,and, ((d)) 6060/40./40.

Close inspection of of the cryogenically fractured fractured su surfacesrfaces revealed revealed the the presence of small fibrils fibrils between the dispersed PBS particles and the PA 11 matr matrix.ix. This This is is illustrated illustrated in in Figure Figure 33,, whichwhich showsshows thethe morphology of the cryogenically fractured 6060/40/40 blend atat higherhigher magnification.magnification. PBS droplets appear to to be bonded to PA 11 via small fibrils,fibrils, which join the two phases, however leaving some voids. Such fibrilsfibrils areare observedobserved in in all all of of the the analyzed analyzed blends blends of diofff differenterent composition, composition, and and they they are alsoare alsoreported reported in [21 ].in These [21]. fibrilsThese/voids fibrils/voids may form may during form fracture, during due fracture, to the release due ofto residualthe release/internal of residual/internalstress imposed to stress the blends imposed by crystallization to the blends/volume by crystallization/volume contraction of the components contraction at dioffferent the componentstemperatures, at by different different temperatures, thermal shrinkage by different of the PA thermal 11 and shrinkage PBS, or by of cooling-rate the PA 11 gradients and PBS, during or by cooling-ratemelt-processing gradients [50]. In during fact, the melt-processing fibrils indicate [50]. coupling In fact, of the matrixfibrils indicate and particles, coupling which of the is perhaps matrix anddue toparticles, interaction which between is perhaps the functional due to interaction groups of PAbetween 11 and the PBS functional macromolecules, groups establishedof PA 11 and during PBS macromolecules,melt mixing, which established might provide during partial melt compatibilizationmixing, which might of the provide blend partial components. compatibilization of the blend components.

Materials 2019, 12, 2833 7 of 14

Figure 3. Scanning electron micrograph of a cryogenically fractured surface of the PA 11/PBS 60/40 FigureFigure 3. Scanning electronelectron micrograph micrograph of of a cryogenicallya cryogenically fractured fractured surface surface of the of PA the 11 PA/PBS 11/PBS 60/40 blend.60/40 blend. blend. The type of interaction was investigated by FTIR-ATR analysis. Figure4a shows the FTIR-ATR The type of interaction was investigated by FTIR-ATR analysis. Figure 4a shows the FTIR-ATR spectraThe of type plain of PAinteraction 11 (black), was plain investigated PBS (magenta), by FTIR-ATR and the analysis. 60/40 blend Figure (green) 4a shows in the the wavenumber FTIR-ATR spectra of plain PA 11 (black),1 plain PBS (magenta), and the 60/40 blend (green) in the wavenumber spectrarange from of plain 3500 PA to 11 1400 (black), cm− .plain The FTIR-ATRPBS (magenta spectrum), and the of plain 60/40 PBS blend has (green) a complex in the band wavenumber at around range from1 3500 to 1400 cm−1. The FTIR-ATR spectrum of plain PBS has a complex band at around range1712cm from− , which3500 to is 1400 the convolutioncm−1. The FTIR-ATR of three spectrum diﬀerent Cof= Oplain stretching PBS has modes a complex at around band 1736,at around 1720, 1712 cm−1, which1 is the convolution of three1 different C=O stretching modes at around 1736, 1720, and 1712and 1714cm−1,cm which− [51 is]. the The convolution band at 1736 of three cm− differentcorresponds C=O to stretching the stretching modes mode at around of C= O1736, in the 1720, mobile and 1714 cm−1 [51]. The band at 1736 cm−1 1corresponds to the stretching mode of C=O in the mobile 1714amorphous cm−1 [51]. fraction, The band the one at at1736 1720 cm cm−1− correspondsis assigned toto thethe stretching stretching in mode the rigid of C=O amorphous in the fractionmobile amorphous fraction, the1 one at 1720 cm−1 is assigned to the stretching in the rigid amorphous fraction amorphousand the one fraction, at 1714 cm the− oneregards at 1720 the cm crystalline−1 is assigned phase. to Thethe stretching FTIR-ATR in spectrum the rigid of amorphous PA 11 has a fraction band at and the one1 at 1714 cm−1 regards the crystalline phase. The FTIR-ATR spectrum 1of PA 11 has a band and1635 the cm one− that at 1714 corresponds cm−1 regards to the the stretching crystalline mode phase. of The C=O, FTIR-ATR a band at spectrum 3300 cm− ofthat PA 11 is assignedhas a band to at 1635 cm−1 that corresponds to the stretching mode of1 C=O, a band at 3300 cm−1 that is assigned to atthe 1635 N–H cm stretching−1 that corresponds and the amide to the II stretching band at 1540 mode cm of− .C=O, This a latter band band at 3300 results cm−1 from that anis assigned interaction to the N–H stretching and the amide II band at 1540 cm−1. This latter band results from an interaction thebetween N–H N–Hstretching bending and and the theamide C–N II stretchingband at 1540 of the cm C–N–H−1. This latter group. band results from an interaction between N–H bending and the C–N stretching of the C–N–H group. 1 betweenFocusing N–H bending on the FTIR-ATRand the C–N spectra stretching of the of blendsthe C–N–H in the group. range between 1450 and 1800 cm− Focusing on the FTIR-ATR spectra of the blends in the range between1 1450 and 1800 cm−1 (Figure (FigureFocusing4b), the on C the=O FTIR-ATR stretching spectra band in of plain the blends PBS shows in the arange shift between of 3 cm− 1450on theand addition 1800 cm− of1 (Figure 10 m% 4b), the C=O stretching band in plain PBS shows a shift of 3 cm−1 on the addition of 10 m% PA 11. At 4b),PA 11.the AtC=O the stretching same time, band a shift in plain of the PBS N–H shows stretching a shift modeof 3 cm band−1 on andthe addition the amide ofII 10 band m% ofPA PA11 11. At is the same time, a shift of the N–H stretching mode band and the 1amide II band of PA11 is observed.1 theobserved. same time, The a amide shift of II bandthe N–H progressively stretching shiftsmode fromband 1540and cmthe− amidein plain II band PA 11 of toPA11 1544 is cm observed.− in the The amide II band progressively shifts from1 1540 cm−1 in plain PA 11 to 1544 cm−1 in the1 60/40 blend, The60/40 amide blend, II band together progressively with a shift shifts of 2 from cm− 1540on the cm N–H−1 in plain stretching PA 11 band to 1544 at 3300cm−1 in cm the− , 60/40 as shown blend, in together with a shift of 2 cm−1 on the N–H stretching band at 3300 cm−1, as shown in Figure 4c. togetherFigure4c. with a shift of 2 cm−1 on the N–H stretching band at 3300 cm−1, as shown in Figure 4c.

100 100

80 80

60 60

PA 11/PBS 40 PA 11/PBS 40 100/0 100/0 60/40 60/40 % Transmittance 0/100 % Transmittance 20 0/100 20 (a) (a) 0 03500 3000 2500 2000 1500 3500 3000 2500 2000 1500 -1 Wavenumbers (cm-1 ) Wavenumbers (cm )

Figure 4. Cont.

Materials 2019, 12, 2833 8 of 14

100

40 PA 11/PBS 100/0

% Transmittance 90/10 20 80/20 60/40 (b) 0/100 0 1800 1700 1600 1500 Wavenumbers (cm-1)

100

90 PA 11/PBS 100/0 85 60/40 0/100 % Transmittance

80 (c)

3500 3400 3300 3200 310 -1 Wavenumbers (cm )

FigureFigure 4. 4. Fourier-transformFourier-transform infrared infrared spectroscopy-Attenu spectroscopy-Attenuatedated TotalTotal Reﬂectance Reflectance (FTIR-ATR) (FTIR-ATR) spectra spectra of ofcompression-molded compression-molded samples samples of of PA PA 11 11/PBS/PBS blends: blends: ((aa)) plainplain polymers compared compared to to the the 60/40 60/40 blend; blend; (b) the whole analyzed composition range, in the wavenumber range from 1800 to 1450 cm−1; and,1 (c) (b) the whole analyzed composition range, in the wavenumber range from 1800 to 1450 cm− ; and, enlargement(c) enlargement of the of theplots plots shown shown in ( ina) (toa) tohighlight highlight changes changes in inthe the wave wavenumbernumber range range of of stretching stretching of of N–HN–H band band of of PA PA 11. 11.

TheseThese spectral spectral changes changes can be rationalizedrationalized whilewhile takingtaking interaction interaction between between the the carbonyl carbonyl group group of ofPBS PBS and and the the N–H N–H group group of theof the amide amide in PAin PA 11, established11, established upon upon melt melt mixing, mixing, into account.into account. Even Even if the ifshifts the shifts are low are in low terms in of terms frequencies, of frequencies, those are those reproducible, are reproducible, hence not arehence connected not are to connected instrumental to instrumentalerrors. With errors. the available With the experimental available experimental data, such data, small such variations small variations in wavenumbers in wavenumbers can only can be onlyascribed be ascribed to hydrogen to hydrogen bonding, whilebonding, also while taking also into accounttaking into that account the shift ofthat the the C= shiftO stretching of the C=O band stretchingof PBS is tooband low of to PBS be explainedis too low with to be a changeexplained in thewith crystalline a change structurein the crystalline of the polymer. structure However, of the polymer.the latter However, was shown the not latter to vary was byshown DSC not analyses to vary of by Figure DSC1 .analyses of Figure 1. TheThe nominal nominal stress-strain stress-strain curves curves of of PA PA 11 11 and and PA PA 11/PBS 11/PBS blends blends that that were were obtained obtained at at room room temperaturetemperature are are presented presented in Figure in Figure 5. Table5. Table 1 summarizes1 summarizes the Young’s the Young’s modulus modulus (E), yield ( stressE), yield and strain,stress and stress strain, and and strain stress at and break. strain All atof break.the analyzed All of materials the analyzed show materials ductile behavior. show ductile The deformationbehavior. The of deformationplain polyamide of plain occurs polyamide in three occursstages, inas threetypical stages, for semi-crystalline as typical for semi-crystalline polymers [52– 54]:polymers a first stage,[52–54 where]: a ﬁrst the stress stage, sharply where increase the stresss with sharply strain, increases followed with by a strain,homogeneous followed plastic by a deformationhomogeneous stage, plastic then deformation a necking phenomenon stage, then o accurs, necking which phenomenon shows that occurs,the deformation which shows becomes that heterogeneous,the deformation until becomes the sample heterogeneous, breaks. Plain until PA the 11 sample displays breaks. a Young’s Plain modulus PA 11 displays of E = a930 Young’s MPa, yieldingmodulus occurs of E = at930 strain MPa, εy yielding= 25%, and occurs stress at strainσy = 29ε yMPa,= 25%, and and rupture stress atσ yelongation= 29 MPa, ε andr = 300%, rupture and at stress σr = 52 MPa. The measured parameters are in line with literature data [15,55,56]. The addition 8

Materials 2019, 12, 2833 9 of 14

elongation εr = 300%, and stress σr = 52 MPa. The measured parameters are in line with literature data [15,55,56]. The addition of PBS results in a small variation of the measured mechanical parameters, withof PBS a smallresults progressive in a small variation decrease ofof thethe Young’smeasured modulus mechanical with parameters, the addition with of PBS, a small as well progressive as of the yieldingdecrease andof the break Young’s parameters. modulus with the addition of PBS, as well as of the yielding and break parameters.

100/0 20 90/10

Stress (MPa) 80/20 60/40

0 0 100 200 300

Strain (%)

Figure 5. Engineering tensile stress-strain plot plot of of PA PA 11/PBS 11/PBS blends, measured at room temperature, −11 with a crosshead speed 5 mm min− ..

TableTable 1. Tensile parameters of PA 1111/PBS/PBS blends.blends.

PA 11/PBSPA E (MPa) 11/PBS E (MPa)σy (MPa) σy (MPa) εy ε(%)y (%) σr (MPa) σrε(MPa)r (%) εr (%) 100/0 930100/040 930 ± 40 29 1 29 ± 1 25 25 ± 2 2 52 ± 3 300 52 ± 320 300 20 ± ± ± ± ± 90/10 91090/1030 910 ± 30 29 1 29 ± 1 25 25 ± 3 3 50 ± 3 290 50 ± 320 290 20 ± ± ± ± ± 80/20 89080/2040 890 ± 40 28 2 28 ± 2 23 23 ± 5 5 49 ± 5 280 49 ± 520 280 20 ± ± ± ± ± 60/40 87060/4030 870 ± 30 27 2 27 ± 2 20 20 ± 3 3 39 ± 5 270 39 ± 530 270 30 ± ± ± ± ±

Literature datadata of of Young’s Young’s modulus modulus of plainof plain PBS rangePBS range from 500from to 500 590 MPato 590 [57 MPa,58], i.e.,[57,58], the modulus i.e., the modulus is lower than in PA 11. PBS is more ductile than PA 11, with εr = 950% [59], although some is lower than in PA 11. PBS is more ductile than PA 11, with εr = 950% [59], although some authors authorsalso reported also reported PBS as being PBS aas brittle being polymer a brittle [60polymer,61], probably [60,61], dueprobably to material due to degradation material degradation [62]. All of [62].the analyzed All of the PA analyzed 11/PBS PA blends 11/PBS display blends ductile display behavior, ductile exhibitingbehavior, exhibiting elongation elongation values that values resemble that resemblethose of the those plain of polyamide.the plain polyamide. It is well known that in semi-crystallinesemi-crystalline polymerspolymers Young’s modulus increases with the crystal fraction [[63–65].63–65]. TheThe slight slight decrease decrease of Young’sof Young’s modulus modulus in the in blends the blends is caused is bycaused the lower by the modulus lower modulusof PBS, which of PBS, overwhelms which overwhelms the somewhat the highersomewhat overall higher crystallinity overall crystallinity of the material. of the Being material. measured Being at measuredlow strain, at Young’s low strain, modulus Young’s is not significantlymodulus is anotffected significantly by the blend affected morphology, by the whereasblend morphology, large-strain properties,whereas large-strain like stress properties, and strain like at stress break, and strongly strain dependat break, on strongly the morphology, depend on the including morphology, phase includingseparation phase in the separation amorphous in phase, the amorphous as well as thephase, homogeneity as well as ofthe dispersion homogeneity and adhesionof dispersion between and adhesionthe phases. between Good tensile the phases. properties, Good suchtensile as properties tensile strength, such andas tensile yield andstrength ultimate and parameters,yield and ultimate can be parameters,expected if the can domains be expected are small if the in domains size and wellare small dispersed in size in and the surroundingwell dispersed matrix in the [66 surrounding]. As seen in matrixFigure2 [66]., the As PA seen 11 /PBS in Figure blends 2, present the PA an 11/PBS island-matrix blends present morphology an island-m with PBSatrix particles morphology surrounded with PBS by particles surrounded by small voids, and with a particle size that varies with blend composition. small voids, and with a particle size that varies with blend composition. However, the PBS particles However, the PBS particles are not decoupled from the matrix, but are kept connected with fibrils, are not decoupled from the matrix, but are kept connected with fibrils, which is probably due to which is probably due to interaction between the amide and carbonyl groups of the two polymers, interaction between the amide and carbonyl groups of the two polymers, as suggested by FTIR-ATR as suggested by FTIR-ATR analysis. Such fibrils that bridge the PA 11 and PBS phases allow the analysis. Such fibrils that bridge the PA 11 and PBS phases allow the samples to bear high elongation, samples to bear high elongation, comparable to that of pure PA 11, with a small decrease only comparable to that of pure PA 11, with a small decrease only observed in the blend containing 40 m% observed in the blend containing 40 m% of the dispersed component. of the dispersed component.

Materials 2019, 12, 2833 10 of 14

The inﬂuence of PBS on the thermal stability of PA 11 was investigated by thermogravimetry (TGA). 1 The TGA plot of PA 11/PBS blends, analyzed upon heating at 10 K min− in nitrogen atmosphere, is presented in Figure6a, while the derivative thermogravimetry curves are shown in Figure6b. The TGA curve of plain polyamide displays a slow mass loss between 200 and 380 ◦C, with an overall mass decrease of about 5%. A further increase of the temperature leads to a steep decrease of the polymer mass, with full decomposition of the polymer just below 500 ◦C[25], and a maximum degradation rate being observed at 455 ◦C, as seen in Figure6b. Pure PBS also undergoes thermal degradation in a single step, with no signiﬁcant mass loss until about 300 ◦C and complete decomposition around 430 ◦C, which is in agreement with the literature data [67,68]. The primary mass loss is caused by the volatilization of small molecules, including succinic acid and butylene glycol, followed by major thermal degradation of PBS chains, due to random chain scission at the ester bonds, with the formation of carboxylic end groups and vinyl groups [38].

100 2.0 )

80 -1 1.5 100/0 60 90/10 100/0 80/20 90/10 1.0 60/40 80/20 40 0/100

Mass (%) 60/40 0/100 0.5 20 Rate of mass loss (% K 0.0 0 100 200 300 400 500 200 300 400 500 Temperature (°C) Temperature (°C) (a) (b)

FigureFigure 6. Thermogravimetry (TGA)(TGA) plots plots of of PA PA 11 /11/PBSPBS blends, blends, measured measured in nitrogen in nitrogen atmosphere atmosphere upon 1 −1 uponheating heating at 10 at K min10 K− min:(a): normalized (a) normalized mass mass as function as function of temperature;of temperature; and, and, (b) ( rateb) rate of massof mass loss loss as asfunction function of of temperature. temperature.

TheIn the influence blends, thermalof PBS on degradation the thermal is initiatedstability atof the PA same 11 was temperature investigated as the by degradation thermogravimetry of plain (TGA).PBS, with The all TGA of the plot analyzed of PA compositions 11/PBS blends, displaying analyzed two majorupon stepsheating of massat 10 loss, K min mostly−1 in resembling nitrogen atmosphere,those of the pureis presented components. in Figure The 6a, peaks while in the degradationderivative thermogravimetry rate of the PBS particles curves are of theshown blends in Figureappear 6b. unchanged The TGA ascurve compared of plain to polyamide the pure polymer, displays witha slow a minormass lo shiftss between of the degradation 200 and 380 of°C, the with PA an11 overall portions mass of the decrease blends, of which about undergo 5%. A further degradation increase at of slightly the temperature higher temperatures. leads to a steep This decrease suggests ofsome the minorpolymer inﬂuence mass, with of the full degradation decomposition products of the of polymer PBS on thejust thermalbelow 500 degradation °C [25], and of a PA maximum 11. degradation rate being observed at 455 °C, as seen in Figure 6b. Pure PBS also undergoes thermal degradation4. Final Discussion in a single and Conclusionsstep, with no significant mass loss until about 300 °C and complete decompositionPrevious researcharound 430 indicated °C, which that is PBS in canagreement be used with to improve the literature mechanical data [67,68]. properties The ofprimary PA 11, massspeciﬁcally loss is thecaused impact by the strength. volatilization These of data small are molecules, now completed including by asuccinic full analysis acid and of thebutylene blend glycol,morphology, followed the by phase major structure, thermal thermal, degradation and tensileof PBS properties chains, due of PAto random 11/PBS blends,chain scission with PA at 11 the as estermajor bonds, component. with the formation of carboxylic end groups and vinyl groups [38]. InIn the analyzedblends, thermal composition degradation range from is initiated 0 to 40 m%at the PBS, same PA 11temperature and PBS are as immiscible the degradation and form of plaina four-phase PBS, with morphology all of the madeanalyzed of two compositions crystal phases, displaying plus two two amorphous major steps phases. of mass The loss, components mostly resemblingarrange in athose particle-matrix of the pure morphology,components. withThe peaks roughly in the spherical degradation PBS particles rate of the that PBS are particles suspended of thein the blends PA 11appear matrix unchanged and small as voids compared surrounding to the pure the minority-phasepolymer, with a inclusions.minor shift Increasingof the degradation the PBS ofcontent the PA from 11 portions 10 to 40 m%of the results blends, in anwhich increase undergo of the degradation average particle at slightly size. Despite higher temperatures. the immiscibility This of suggeststhe blend some components, minor influence the matrix of the and degradation particles display products interfacial of PBS adhesion, on the thermal as most degradation PBS particles of PA are 11.not removed upon cryogenical fracture, but they remain anchored to the matrix. Interaction between the amide groups of PA 11 chains and the carbonyl groups of PBS was proven by FTIR-ATR analysis, 4. Final Discussion and Conclusions Previous research indicated that PBS can be used to improve mechanical properties of PA 11, specifically the impact strength. These data are now completed by a full analysis of the blend morphology, the phase structure, thermal, and tensile properties of PA 11/PBS blends, with PA 11 as major component. In the analyzed composition range from 0 to 40 m% PBS, PA 11 and PBS are immiscible and form a four-phase morphology made of two crystal phases, plus two amorphous phases. The components arrange in a particle-matrix morphology, with roughly spherical PBS particles that are suspended in the PA 11 matrix and small voids surrounding the minority-phase inclusions. Increasing the PBS content from 10 to 40 m% results in an increase of the average particle size. Despite the immiscibility of the blend components, the matrix and particles display interfacial adhesion, as most PBS particles are not removed upon cryogenical fracture, but they remain anchored to the matrix. Interaction between the amide groups of PA 11 chains and the carbonyl groups of PBS was proven by FTIR-ATR analysis, which may rationalize the adhesion between the phases. This is revealed also by the SEM 10

Materials 2019, 12, 2833 11 of 14 which may rationalize the adhesion between the phases. This is revealed also by the SEM micrographs of the fractured surfaces of the blend that display small fibrils that bridge the PBS inclusions with the PA 11 matrix. It may be advisable, and the subject of further investigation, to investigate the effect of addition of a compatibilizer that may further improve the interfacial adhesion between the phases. At the level of compatibility that is only attained by melt blending PA 11 and PBS, the PA 11-rich binary blends display mechanical properties that are comparable to neat PA 11. There is only observed minor variation of the Young’s modulus and of the yielding and break parameters, despite that PBS alone can sustain much higher elongation, up to 900% of the initial value, when compared to the 300% typical of PA 11 and of the analyzed blends. In other words, it is likely that further increase of interfacial adhesion, which may be provided by a compatibilizer, or by further promoting reaction between the functional groups of the two polymers, might lead to a blend formulation with improved mechanical properties as compared to pure PA 11, and that is also partially biodegradable. The analysis of the thermal properties indicated little influence of the blend composition on thermal degradation, with only a minor shift of the degradation temperature of the PA 11 matrix upon the addition of PBS. The amorphous phases have separate glass transitions whose temperature seems not be affected by blending, at least for the analyzed composition range. Blending seems not to influence the crystallization of PBS and PA 11, which might appear in contrast with the reported interaction between PA 11 and PBS phases. However, the data that are presented in this manuscript are limited to the processing history imparted upon melt-mixing using a specific extruder and compression molding. The condition of crystallization of PA 11 can influence crystallization kinetics of PBS parts of the blends, which may help in modulating the crystallinity and crystal morphology, and, in turn, material properties, as will be detained in a forthcoming manuscript that illustrates a thorough analysis of crystallization kinetics of both polymers in the blends.

Author Contributions: Conceptualization, M.L.D.L.; methodology, M.L.D.L., A.L. and R.A.; data curation, M.L.D.L. and A.L.; writing—original draft preparation, M.L.D.L.; writing—review and editing, M.L.D.L., A.L. and R.A.; supervision, M.L.D.L. Funding: This research received no external funding. Acknowledgments: The authors wish to thank Salvatore Mallardo, Barbara Immirzi, and Maria Cristina Del Barone of IPCB-CNR for their assistance with blend preparation, GPC and SEM analysis, respectively. The authors gratefully acknowledge Showa Denko K. K. (Japan), and Ines Kühnert of Leibniz-Institut für Polymerforschung e.V. Dresden (Germany) for kindly providing PBS and PA 11, respectively. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Bioplastic Market Data. Available online: www.european-bioplastics.org/market/ (accessed on 30 May 2019). 2. Scott, J.L.; Buchard, A. Polymers from plants: Biomass ﬁxed carbon dioxide as a resource. In Managing Global Warming—An Interface of Technology and Human Issues; Letcher, T.M., Ed.; Academic Press: Cambridge, MA, USA, 2019; pp. 503–525. 3. Imre, B.; Pukánszky, B. Compatibilization in bio-based and biodegradable polymer blends. Eur. Polym. J. 2013, 49, 1215–1233. [CrossRef] 4. Di Lorenzo, M.L.; Raimo, M.; Cascone, E.; Martuscelli, E. Poly(3-hydroxybutyrate)-based copolymers and blends: Inﬂuence of a second component on crystallization and thermal behavior. J. Macromol. Sci.—Phys. 2001, 40, 639–667. [CrossRef] 5. Nakajima, H.; Dijkstra, P.; Loos, K. The Recent Developments in Biobased Polymers toward General and Engineering Applications: Polymers That Are Upgraded from Biodegradable Polymers, Analogous to Petroleum-Derived Polymers, and Newly Developed. Polymers 2017, 9, 523. [CrossRef][PubMed] 6. Di Lorenzo, M.L.; Androsch, R. (Eds.) Industrial Applications of Poly(lactic acid); Springer International Publishing: Cham, Switzerland, 2018; p. 282. 7. Paul, D.R.; Newman, S. Polymer Blends; Academic Press: New York, NY, USA, 1978. Materials 2019, 12, 2833 12 of 14

8. Sionkowska, A. Current research on the blends of natural and synthetic polymers as new biomaterials: Review. Progr. Polym. Sci. 2011, 36, 1254–1276. [CrossRef] 9. Hamad, K.; Kaseem, M.; Ayyoob, M.; Joo, J.; Deri, F. Polylactic acid blends: The future of green, light and tough. Progr. Polym. Sci. 2018, 85, 83–127. [CrossRef] 10. Martino, L.; Basilissi, L.; Farina, H.; Ortenzi, M.A.; Zini, E.; Di Silvestro, G.; Scandola, M. Bio-based polyamide 11: Synthesis, rheology and solid-state properties of star structures. Eur. Polym. J. 2014, 59, 69–77. [CrossRef] 11. Patel, R.; Ruehle, D.A.; Dorgan, J.R.; Halley, P.; Martin, D. Biorenewable blends of polyamide-11 and polylactide. Polym. Eng. Sci. 2014, 54, 1523–1532. [CrossRef] 12. He, X.; Yang, J.; Zhu, L.; Wang, B.; Sun, G.; Lv, P.; Phang, I.Y.; Liu, T. Morphology and melt rheology of nylon 11/clay nanocomposites. J. Appl. Polym. Sci. 2006, 102, 542–549. [CrossRef] 13. Liu, T.; Lim, K.P.; Tjiu, W.C.; Pramoda, K.P.; Chen, Z.K. Preparation and characterization of nylon 11/organoclay nanocomposites. Polymer 2003, 44, 3529–3535. [CrossRef] 14. Mancic, L.; Osman, R.F.M.; Costa, A.M.L.M.; d’Almeida, J.R.M.; Marinkovic, B.A.; Rizzo, F.C. Thermal and mechanical properties of polyamide 11 based composites reinforced with surface modified titanate nanotubes. Mater. Des. 2015, 83, 459–467. [CrossRef] 15. Sahnoune, M.; Taguet, A.; Otazaghine, B.; Kaci, M.; Lopez-Cuesta, J.M. Effects of functionalized halloysite on morphology and properties of polyamide-11/SEBS-g-MA blends. Eur. Polym. J. 2017, 90, 418–430. [CrossRef] 16. Yu, M.; Zhang, Q.; Fu, Q. Preparation and characterisation of polyamide 11/clay nanocomposites. Chin. J. Polym. Sci. 2004, 22, 43–47. 17. Halim, K.A.A.; Farrell, J.B.; Kennedy, J.E. Preparation and characterisation of polyamide 11/montmorillonite (MMT) nanocomposites for use in angioplasty balloon applications. Mater. Chem. Phys. 2013, 143, 336–348. [CrossRef] 18. Jariyavidyanont, K.; Focke, W.; Androsch, R. Crystallization kinetics of polyamide 11 in presence of sepiolite and montmorillonite nanofillers. Coll. Polym. Sci. 2016, 294, 1143–1151. [CrossRef] 19. Tercjak, A.; Haponiuk, J.T.; Masiulanis, B. Study of thermal property changes of biopol/polyamide 11 blends during biodegradation in compost. J. Therm. Anal. Calor. 2003, 74, 605–608. [CrossRef] 20. Walha, F.; Lamnawar, K.; Maazouz, A.; Jaziri, M. Rheological, Morphological and Mechanical Studies of Sustainably Sourced Polymer Blends Based on Poly(Lactic Acid) and Polyamide 11. Polymers 2016, 8, 61. [CrossRef][PubMed] 21. Moriyama, T.; Sumiya, N.; Saito, T. Impact strength improvement of polyamide 11 without flexural modulus reduction by dispersing poly(butylene succinate) particles. Polym. J. 2016, 48, 221–224. [CrossRef] 22. Ichikawa, Y.; Mizukoshi, T. Bionolle (Polybutylenesuccinate). Adv. Polym. Sci. 2012, 245, 285–313. 23. Wu, Y.; Xiong, W.; Zhou, H.; Li, H.; Xu, G.; Zhao, J. Biodegradation of poly(butylene succinate) film by compost microorganisms and water soluble product impact on mung beans germination. Polym. Degrad. Stab. 2016, 126, 22–30. [CrossRef] 24. Chanoine, P. Specialty Polyamides High Performance Materials Segment. Available online: https://www.arkema.com/export/sites/global/.content/medias/downloads/investorrelations/en/finance/ arkema-investor-day-2012-specialty-polyamides-va.pdf (accessed on 1 July 2019). 25. Schütte, A. Processing of Bioplastics: A Guideline. Available online: https://www.ifbb-hannover.de/files/ IfBB/downloads/EV_Processing-of-Bioplastics-2016.pdf (accessed on 1 July 2019). 26. Van den Oever, M.; Molenveld, K.; van der Zee, M.; Bos, H. Bio-Based and Biodegradable Plastics: Facts and Figures: Focus on Food Packaging in the Netherlands (Rapport nr. 1722); Wageningen Food Biobased Research: Wageningen, The Netherlands, 2017; ISBN 9789463431217. 27. Jariyavidyanont, K.; Focke, W.; Androsch, R. Thermal Properties of Biobased Polyamide 11. In Advances in Polymer Science; Springer: Berlin/Heidelberg, Germany, 2019. 28. Xenopoulos, A.; Wunderlich, B. Thermodynamic properties of liquid and semicrystalline linear aliphatic polyamides. J. Polym. Sci. Part B Polym. Phys. 1990, 28, 2271–2290. [CrossRef] 29. Mathias, L.J.; Powell, D.G.; Autran, J.P.; Porter, R.S. Nitrogen-15 nmr characterization of multiple crystal forms and phase transitions in polyundecanamide (nylon 11). Macromolecules 1990, 23, 963–967. [CrossRef] 30. Zhang, G.; Li, Y.; Yan, D. Polymorphism in nylon-11/montmorillonite nanocomposite. J. Polym. Sci. Part B Polym. Phys. 2004, 42, 253–259. [CrossRef] 31. Nair, S.S.; Ramesh, C.; Tashiro, K. Crystalline phases in nylon-11: Studies using HTWAXS and HTFTIR. Macromolecules 2006, 39, 2841–2848. [CrossRef] Materials 2019, 12, 2833 13 of 14

32. Rhoades, A.M.; Wonderling, N.; Schick, C.; Androsch, R. Supercooling-controlled heterogeneous and homogenous crystal nucleation of polyamide 11 and its effect onto the crystal/mesophase polymorphism. Polymer 2016, 106, 29–34. [CrossRef] 33. Jariyavidyanont, K.; Williams, J.L.; Rhoades, A.M.; Kühnert, I.; Focke, W.; Androsch, R. Crystallization of polyamide 11 during injection molding. Polym. Eng. Sci. 2018, 58, 1053–1061. [CrossRef] 34. Mollova, A.; Androsch, R.; Mileva, D.; Schick, C.; Benhamida, A. Effect of supercooling on crystallization of polyamide 11. Macromolecules 2013, 46, 828–835. [CrossRef] 35. Jariyavidyanont, K.; Janke, A.; Androsch, R. Crystal self-nucleation of polyamide 11. Thermochim. Acta 2019, 677, 139–143. [CrossRef] 36. Di Lorenzo, M.L.; Androsch, R.; Righetti, M.C. Low-temperature crystallization of poly(butylene succinate). Eur. Polym. J. 2017, 94, 384–391. [CrossRef] 37. Signori, F.; Pelagaggi, M.; Bronco, S.; Righetti, M.C. Amorphous/crystal and polymer/filler interphases in biocomposites from poly(butylene succinate). Thermochim. Acta 2012, 543, 74–81. [CrossRef] 38. Gan, Z.; Abe, H.; Kurokawa, H.; Doi, Y. Solid-state microstructures, thermal properties, and crystallization of biodegradable poly(butylene succinate) (PBS) and its copolyesters. Biomacromolecules 2001, 2, 605–613. [CrossRef] 39. Pyda, M. (Ed.) Nylon 11 (NYLON11) Heat Capacity, Enthalpy, Entropy, Gibbs Energy; Datasheet from “The Advanced THermal Analysis System (ATHAS) Databank—Polymer Thermodynamics”; Springer Materials, Springer-Verlag GmbH: Heidelberg, Germany, 2014. 40. Zhang, Q.; Mo, Z.; Liu, S.; Zhang, H. Influence of annealing on structure of Nylon 11. Macromolecules 2000, 33, 5999–6005. [CrossRef] 41. Verespej, M. NatureWorks Launches Biopolymer Alloys. Plast. Technol. 2012, 4. Available online: https://www.ptonline.com/articles/natureworks-launches-biopolymer-alloys (accessed on 2 July 2019). 42. Rilsan BESNO TL Product Information. Available online: https://www.extremematerials-arkema.com/en/ materials-database/ (accessed on 28 June 2019). 43. Wunderlich, B. Thermal Analysis of Polymeric Materials; Springer: Berlin/Heidelberg, Germany, 2005. 44. Androsch, R.; Wunderlich, B. Scanning Calorimetry. In Macromolecular Engineering; Matyjaszewski, K., Gnanou, Y., Leibler, L., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2011; Volume 3, Chapter 12. 45. Papageorgiou, D.G.; Zhuravlev, E.; Papageorgiou, G.Z.; Bikiaris, D.; Chrissafis, K.; Schick, C. Kinetics of nucleation and crystallization in poly(butylene succinate) nanocomposites. Polymer 2014, 55, 6725–6734. [CrossRef] 46. Righetti, M.C.; Tombari, E.; Di Lorenzo, M.L. Crystalline, mobile amorphous and rigid amorphous fractions in isotactic polystyrene. Eur. Polym. J. 2008, 44, 2659–2667. [CrossRef] 47. Righetti, M.C.; Laus, M.; Di Lorenzo, M.L. Temperature dependence of the rigid amorphous fraction in poly(ethylene terephthalate). Eur. Polym. J. 2014, 58, 60–68. [CrossRef] 48. Di Lorenzo, M.L.; Righetti, M.C. Crystallization-induced formation of rigid amorphous fraction. Polym. Cryst. 2018, 1, e10023. [CrossRef] 49. Papageorgiou, G.Z.; Bikiaris, D.N. Crystallization and melting behavior of three biodegradable poly(alkylene succinates). A comparative study. Polymer 2005, 46, 12081–12092. [CrossRef] 50. Wu, S. Chain structure, phase morphology, and toughness relationships in polymers and blends. Polym. Eng. Sci. 1990, 30, 753–761. [CrossRef] 51. Yao, S.F.; Chen, X.T.; Ye, H.M. Investigation of Structure and Crystallization Behavior of Poly(butylene succinate) by Fourier Transform Infrared Spectroscopy. J. Phys. Chem. B 2017, 121, 9476–9485. [CrossRef] 52. Hong, K.; Rastogi, A.; Strobl, G. A Model Treating Tensile Deformation of Semicrystalline Polymers: Quasi-Static Stress Strain Relationship and Viscous Stress Determined for a Sample of Polyethylene. − Macromolecules 2004, 37, 10165–10173. [CrossRef] 53. Patlazhan, S.; Remond, Y. Structural mechanics of semicrystalline polymers prior to the yield point: A review. J. Mater. Sci. 2012, 47, 6749–6767. [CrossRef] 54. Pawlak, A.; Galeski, A.; Rozanski, A. Cavitation during deformation of semicrystalline polymers. Progr. Polym. Sci. 2014, 39, 921–958. [CrossRef] Materials 2019, 12, 2833 14 of 14

55. Stoclet, G.; Sclavons, M.; Devaux, J. Relations between structure and property of polyamide 11 nanocomposites based on raw clays elaborated by water-assisted extrusion. J. Appl. Polym. Sci. 2013, 127, 4809–4824. [CrossRef] 56. Ruehle, D.A.; Perbix, C.; Castañeda, M.; Dorgan, J.R.; Mittal, V.; Halley, P.; Martin, D. Blends of biorenewable polyamide-11 and polyamide-6,10. Polymer 2013, 54, 6961–6970. [CrossRef] 57. Li, H.; Chang, J.; Cao, A.; Wang, J. In vitro evaluation of biodegradable poly(butylene succinate) as a novel biomaterial. Macromol. Biosci. 2005, 5, 433–440. [CrossRef][PubMed] 58. Lin, N.; Yu, J.; Chang, P.R.; Li, J.; Huang, J. Poly(butylene succinate)-based biocomposites filled with polysaccharide nanocrystals: Structure and properties. Polym. Compos. 2011, 32, 472–482. [CrossRef] 59. Yun, I.S.; Hwang, S.W.; Shim, J.K.; Seo, K.H. A Study on the Thermal and Mechanical Properties of Poly (Butylene Succinate)/Thermoplastic Starch Binary Blends. Int. J. Prec. Eng. Manuf.—Green Technol. 2016, 3, 289–296. [CrossRef] 60. Totaro, G.; Sisti, L.; Celli, A.; Askanian, H.; Verney, V.; Leroux, F. Poly(butylene succinate) bionanocomposites: A novel bio-organo-modified layered double hydroxide for superior mechanical properties. RSC Adv. 2016, 6, 4780–4791. [CrossRef] 61. Xu, J.; Guo, B.H. Microbial Succinic Acid, Its Polymer Poly(butylene succinate), and Applications. In Plastics from Bacteria; Microbiology Monographs Volume 14; Chen, G.Q., Ed.; Springer: Berlin/Heidelberg, Germany, 2010; Volume 14. 62. Puchalski, M.; Szparaga, G.; Biela, T.; Gutowska, A.; Sztajnowski, S.; Krucínska, I. Molecular and Supramolecular Changes in Polybutylene Succinate (PBS) and Polybutylene Succinate Adipate (PBSA) Copolymer during Degradation in Various Environmental Conditions. Polymers 2018, 10, 251. [CrossRef] 63. Di Lorenzo, M.L.; Righetti, M.C. The three-phase structure of isotactic poly(1-butene). Polymer 2008, 49, 1323–1331. [CrossRef] 64. Mileva, D.; Zia, Q.; Androsch, R. Tensile properties of random copolymers of polypropylene with ethylene and 1-butene: Effect of crystallinity and crystal habit. Polym. Bull. 2010, 65, 623–634. [CrossRef] 65. Glüge, R.; Altenbach, H.; Kolesov, I.; Mahmood, N.; Beiner, M.; Androsch, R. On the effective elastic properties of isotactic polypropylene. Polymer 2019, 160, 291–302. [CrossRef] 66. Cimmino, S.; Di Lorenzo, M.L.; Di Pace, E.; Silvestre, C. Isotactic Poly(1-butene)/Hydrogenated Oligo(cyclopentadiene) Blends: Miscibility, Morphology, and Thermal and Mechanical Properties. J. Appl. Polym. Sci. 1998, 67, 1369–1381. [CrossRef] 67. Chrissafis, K.; Paraskevopoulos, K.M.; Bikiaris, D.N. Thermal degradation mechanism of poly(ethylene succinate) and poly(butylene succinate): Comparative study. Thermochim. Acta 2005, 435, 142–150. [CrossRef] 68. Mallardo, S.; De Vito, V.; Malinconico, M.; Volpe, M.G.; Santagata, G.; Di Lorenzo, M.L. Poly(butylene succinate)-based composites containing β-cyclodextrin/d-limonene inclusion complex. Eur. Polym. J. 2016, 79, 82–96. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).