(PHBV) and Poly(Lactic Acid) (PLA) Blends Were Studied

Assessing the Feasibility of Poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)

and Poly-(lactic acid) for Potential Food Packaging Applications.

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

Sunny Jitendra Modi

Graduate Program in Food Science and Nutrition

The Ohio State University

2010

Master's Examination Committee:

Dr. Yael Vodovotz, Advisor

Dr. Kurt Koelling

Dr. Sudhir Sastry

Sunny Jitendra Modi

2010

Abstract

Poly (hydroxybutyrate) (PHB) is biodegradable aliphatic polyester that is

produced by a wide range of microorganisms. Basic PHB has relatively high glass

transition and melting temperatures. To improve flexibility for potential food packaging applications, syntheses of PHB with various co-polymers such as Poly-(3- hydroxyvalerate) (HV) decreases the glass and melting temperatures as well as broadens

the processing window since there is improved melt stability at lower processing

temperatures. In this study, PHB was synthesized with different valerate contents (5, 12,

and 20%) and molecular weights ranging from 150 to 600 kDa. Several objectives of this

study were to first characterize the thermal, mechanical, rheological, and barrier

properties of PHB synthesized with different valerate contents, and second to compare

these properties in PHBV with similar hydroxyvalerate content but different molecular

weight. Finally, the properties obtained were compared against commonly used thermoplastic packaging materials. All PHBV materials displayed a glass transition between -10 to 20ºC. The two melting transitions found for Aldrich 5%, 12%, and

Tianan 20%, resulted from crystals formed during cooling of the samples. The melt

rheology suggested thermal instability of samples as the complex viscosity decreased

with increasing temperature due to a decrease in molecular weights of the materials.

These results suggest that processing the copolymer below 160ºC would be beneficial

with low screw speed. The mechanical results indicate all PHBV materials had high ii elastic modulus and flexural strength with low tensile strength and elongation at break.

The WVTR results indicated the polymer to be very hydrophilic, which resulted in higher transmission rates.

Individually PHBV and PLA polymers have serious disadvantages when compared to thermoplastics that are currently used. To address high costs and thermal instability, blends of PHBV with PLA were explored as an alternative way of acquiring novel materials with desired properties. At the start of this work, three grades of PLAs were commercially available for purchase with different D- and L- lactide ratios and molecular weights used in extruding to injection molding. Miscibility of PHBV with PLA was studied using Differential Scanning Calorimetry (DSC) and Thermogravimetric

Analysis (TGA). Three different blends of commercially available PLAs and one grade of PHBV were blended using a micro-compounder at 175ºC. The composition of PHBV in blends ranged from 50 to 80%. Therefore, the aim of this study was to characterize thermal properties of three PLA resins blended with PHBV, and to assess the effect of varying PHBV concentrations on these properties in the blends. DSC analysis indicated the blends were immiscible due to separate melting temperatures representing individual polymers. However, minimal changes in the glass transitions were witnessed for both

PHBV and PLA materials. These changes in temperature were due to weak hydrogen bonding and spherulites overlapping. The TGA analysis showed two degradation peaks for individual PHBV and PLA polymers, thus supplementing the results observed for

DSC analysis. The mechanical properties of the blends were also investigated using an

Instron and Rheological Solids Analyzer units. The results showed significant

iii improvements in the elastic modulus, flexural strength, and elongation at break. In general, the viscosity decreased with increasing rotational frequency due to break-up of entanglements between polymers. At various concentrations, the melt viscosity of PHBV was improved with the addition of PLAs at 160 and 170ºC. At melting temperatures, the addition of PLA caused no significant improvements in the complex viscosity.

Dedication

To my parents, my little sister, and good friend Heather Ann Stewart.

Acknowledgments

This thesis would not have been possible without the combined effort of my

graduate committee members Dr. Yael Vodovotz, Dr. Kurt Koelling, and Dr. Sudhir

Sastry. They provided counsel and feedback on coursework and thesis. I would like to

express my sincerest gratitude to Dr. Yael Vodovotz for being more than an advisor but

also being a wonderful friend.

In addition, I would like to thank the Center for Advanced Processing and

Packaging Studies (CAPPS), the Institute for Materials Research (IMR), for funding this research project. Furthermore, I would like to thank Stephen Myers of The Ohio

BioProducts Innovation Center (OBIC) for supplying the lab with TA Instruments used for this study. Finally, the materials used for study were obtained from Tianan Biologic

Material Company and Jamplast Inc., and I would like to thank representatives from both companies for countless technical assistants.

Finally, I would like to thank my “joint” lab mates in Food Science for their help.

Thanks to Rachel Crockett, Ruth Lucius, and Alex Siegwein for passing their valuable knowledge on thermal and rheology analysis. In addition, I would like to thank Luca

Serventi for keeping the lab joyful and stress free, “you wanna eat.” I would also like to thank Alex Suter and Amber Simmons for talking many classes together, so we could understand the material better. In addition, I am deeply in debt to Amber for her help

proofreading and editing the thesis. I would like to thank Merliana Winardi Leim “Lia”

for her help during lab experiments and extrusions. Furthermore, I would like to acknowledge Jennifer Ahn-Jarvis for having our “5-minutes power conversation” and sharing her hints/experiences. Finally, I would like to thank Michael Boehm, Lu Feng,

Christopher Kagarise, Koki Miyazono, and Bin Zhu for sharing their extrusion, rheology, and general chemical engineering knowledge.

vii

Vita

August 2003 ...... Collins Hill High School Suwannee,

Georgia

May 2007 ...... B.A. Food Science and Technology, The

University of Georgia

May 2007 ...... Minor in Chemistry, The University of

Georgia

2007-2010 ...... Graduate Research Assistant at The Ohio

State University. Master of Science in Food

Science and Technology emphasizing

potential applications of bio-based polymer

in food packaging.

Publications

S. Modi, K. Koelling, Y. Vodovotz, “Thermal and Rheological Properties of PHB

Synthesized with Various Hydroxyvalerate Content for Potential Use in Food

Packaging”, Proceedings of SPE – ANTEC, (2009)

viii

S. Modi, K. Koelling, Y. Vodovotz, “Thermal and Rheological Properties of Poly-(3-

hydroxybutyrate-co-3-hydroxyvalerate) and Poly (Lactic Acid) blends for Food

Packaging Applications”, Proceedings of SPE – ANTEC, (2010)

Fields of Study

Major Field: Food Science and Nutrition

Table of Contents

Abstract ………………………………………………………………………………… ii

Dedication ……………………………………………………………….……………… v

Acknowledgments ………………………………………...…………………………… vii

Vita …….…………………………………………………………………..………….. viii

List of Tables ………………………………………………………………………..… xiv

List of Figures ……………………………………………………...……………..…… xvi

Chapter 1: Introduction……………………………………………...…………………… 1

Chapter 2: Statement of the Problem ……………………………………………………. 4

Chapter 3: Literature Review …………………………………………………………… 6

3.1: Introduction to thermoplastic polymer ……………………………………... 6

3.1.1: Problems with thermoplastics ……………………………………. 8

3.2: Introduction to biodegradable polymer ……………………………………... 9

3.3 Polyhydroxyalkanoates (PHA) …………………………………………….. 10

3.3.1 Polyhydroxyalkanoates (PHA) synthesis …………………….…... 12

3.3.2: Physicochemical properties of PHA, specifically PHB ……….… 15

3.3.3: PHB plasticized with Poly-(3-hydroxyvalerate) (HV) ………... 17

3.4: Introduction to Poly(lactic acid) (PLA) ...... 19

3.4.1: Poly(lactic acid) (PLA) production ...... 20

3.4.2: Poly(lactic acid) (PLA) structure ...... 23

3.4.3: Poly(lactic acid) (PLA) physical properties ...... 23

3.4.4: Poly(lactic acid) (PLA) rheological behavior ...... 25

3.4.5: Poly(lactic acid) (PLA) mechanical properties ...... 25

3.5: Importance of Food Packaging ...... 27

3.6: Importance of Thermal Analysis ...... 27

3.7: Importance of Mechanical Analyses ...... 29

3.8: Importance of Rheological and Barrier Analysis ...... 30

Chapter 4: Assessment of PHB with Varying Hydroxyvalerate content for Potential Packaging Applications ...... 32

4.1: Introduction ...... 33

4.2: Materials ...... 34

4.3: Sample Preparations ...... 35

4.3.1: Mechanical and Permeation Samples ...... 35

4.3.2: Rheological Samples ...... 35

4.4: Thermal Properties ...... 36

4.4.1: Thermal Decomposition ...... 36

4.4.2: State of Material Components ...... 36

4.5: Rheological Properties ...... 36

4.6: Mechanical Properties ...... 37

4.6.1: Tensile strength and Elongation at Break ...... 37

4.6.2: Flexural Strength and Elastic Modulus ...... 37

4.6.3: Glass Transitions, Modulus, and Tan Delta Properties ...... 38 xi

4.7: Water Vapor Transmission Rate ...... 38

4.8: Results and Discussion ...... 39

4.8.1: Thermal Analysis ...... 39

4.8.2: Rheological Analysis ...... 44

4.8.3: Mechanical Properties ...... 48

4.8.4: Barrier Properties ...... 53

4.9: Conclusion ...... 55

Chapter 5: Miscibility of Poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) and Poly (Lactic Acid) blends Determined by Thermal Analysis ...... 57

5.1: Introduction ...... 58

5.2: Materials ...... 61

5.3: Blend Preparations ...... 61

5.3.1: Degradation Temperature of Blends ...... 61

5.3.2: Phase transition of Blends ...... 62

5.4: Results and Discussion ...... 62

5.4.1: Phase Transitions of Blends ...... 62

5.4.2: Thermal Decompositions of Blends ...... 68

5.5: Conclusion ...... 71

Chapter 6: Mechanical and Rheological Properties Poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) and Poly (Lactic Acid) blends ...... 72

6.1: Introduction ...... 73

6.2: Materials ...... 75

6.3: Blend and Sample Preparations ...... 76

xii

6.4: Mechanical Properties ...... 76

6.4.1: Tensile Properties ...... 76

6.4.2: Flexural Properties ...... 76

6.5: Rheological Properties ...... 77

6.6: Results and Discussion ...... 77

6.6.1: Mechanical Analysis ...... 76

6.6.2: Rheological Analysis ...... 80

6.7: Conclusions ...... 86

Chapter 7: Conclusions ...... 88

Chapter 8: Future works ...... 91

References ...... 92

xiii

List of Tables Table Page

3.1 Thermal and mechanical properties of PHB with different PHV content at 25˚C .. 18

3.2 Mechanical properties of poly(98% L-lactide), poly(94% L-lactide) compared to PS and PET films. ………..……………………….. 26

4.1 Thermal degradation temperature of PHBV obtained via TGA ………..………… 39

4.2 Average melting temperatures of PHBV obtained via DSC ……………………… 42

4.3 Average glass transition temperatures of PHBV obtained via DSC ……………… 43

4.4 Average glass transition temperatures from loss modulus (E’) and storage modulus change in (E’) obtained from DMA .………………………. 50

4.5 Mechanical properties of PHBV materials ………………………………………... 51

4.6 Water vapor transmission rates of PHBV films………………………………….… 54

5.1 Average melting temperatures of T5%-3051D at four concentrations obtained via DSC ………………………………………………… 63

5.2 Average glass transitions of T5%-3051D at four concentrations obtained via DSC ……………………………………………….……... 65

5.3 Average melting transitions of T5%-4042D at four concentrations obtained via DSC ………………………………………….……... 66

5.4 Average glass transitions of T5%-4042D at four concentrations obtained via DSC………………………………………………….…… 67

5.5 Average melting transitions of T5%-6202D at four concentrations obtained via DSC …………………………………………….…… 67

5.6 Average glass transitions of T5%-6202D at four concentrations obtained via DSC ……………………………………………….…….... 68

5.7 Thermal degradation temperature of blended polymers with four different concentrations obtained via TGA …………………………….……..68 xiv

6.1 Mechanical properties of Tianan 5% (T5%) blended with PLA grades 3051D, 4042D, and 6202D ………………………………………….….… 79

List of Figures

Figure Page

3.1 Structures of widely used thermoplastic polymers in packaging industry ……….… 8

3.2 Electron micrograph of 90% dry cell of P(3HB-co-3HHx) ………………………... 12

3.3 Metabolic pathway in the synthesis of PHB ……………..………………………… 13

3.4 Metabolic pathways of PHA synthesis from different carbon sources within a bacterial cell ………………………………………... 14

3.5 Chemical structure of Poly-(3-hydroxybutyrate) (PHB) …………………………... 16

3.6 Various stuctures of PHA ………………………………………………………….. 19

3.7 Chemical structure of Poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) .... 19

3.8 Chemical structure of DD-, LL-, Meso- (one D- and L-) lactide ……………………..... 21

3.9 Synthesis for obtaining high molecular PLA ...... 22

3.10 Current manufacturing procedure for commercial PLA production ...... 22

3.11 Chemical structure of PLA ...... 23

3.12 Different states of high molecular weight amorphous PLA with increasing temperatures...... 24

4.1 Thermograms of four PHBV materials obtained from TGA ...... 40

4.2 DSC melting transition scans of PHBV at 10ºC/min ...... 42

4.3 DSC glass transition scans of PHBV at 20 ºC/min, second melting behavior ...... 44

4.4 Complex viscosity-frequency sweeps of four PHBV materials ...... 47

4.5 Complex viscosity-time sweeps at 1Hz for PHBV materials ...... 48

4.6 Modulus (E’), Damping -temperature scans of PHBV at 5 ºC/min ...... 50 xvi

5.1 First melting thermograms of Tianan 5% blended with PLA 3051D at 10ºC/min obtained via DSC...... 64

5.2 Second melting thermograms of Tianan 5% blended with PLA 3051D at 10ºC/min obtained via DSC ...... 64

5.3 TGA thermograms of Tianan 5% blended with PLA 3051D at four concentrations (20, 30, 40, and 50% PLA) ...... 69

6.1 Complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 3051D at 160ºC ...... 80

6.2 Complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 3051D at 170ºC ...... 81

6.3 Complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 4042D at 160ºC ...... 82

6.4 complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 4042D at 170ºC ...... 82

6.5 complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 6202D at 160ºC ...... 83

6.6 complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 6202D at 170ºC ...... 84

6.7 Complex viscosity-time at 1Hz for 1 hour at 160ºC for Tianan 5% blended with PLA 3051D ...... 85

6.8 Complex viscosity-time at 1Hz for 1 hour at 170ºC for Tianan 5% blended with PLA 3051D ...... 86

xvii

Chapter 1

Introduction

In the chemical processing industry, fossil fuel feedstock such as gas and oil are by far the most important raw materials. According to the Energy Balances of OECD

Countries, the chemical processing industry is the third largest consumer of fossil fuel

(12%) behind energy generations (54%) and transportation (34%) [1]. The success of fossil fuel over the past 60 years can be attributed to various factors such as abundant amounts of supply, thermoplastic process ability, and numerous applications [2].

However, prior oil crisis, formation of green house gases, and saturation of landfills with thermoplastic waste has lead to exploration in more environmentally friendly polymeric materials.

Poly (hydroxyalkanoate) (PHA’s) is bio-derived polymers that are produced by wide ranges of microorganisms. PHAs are high molecular polyesters that have similar characteristics to conventional thermoplastic, yet are fully biodegradable under normal composting conditions. Poly (hydroxybutyrate) (PHB) are widely studied by researchers and considered a type of PHA [3]. Commercial use of PHBs is limited by availability and cost of the product compared to conventional thermoplastic, narrow processing temperatures due to degradation, and brittleness at room temperature. For food packaging applications, flexibility of polymer is required at low (room and refrigeration)

temperatures. To improve these properties, incorporation of 3-hydroxyvalerate (HV)

during the fermentation process results in lower melting points of the copolymer and

decrease brittleness [4,5]. Increasing the amount of HV content in the homo-polymer

helps in decreasing the melting point of the copolymer, thus broadening the processing

window. In addition, incorporation of HV content in the backbone helps in improving

the melt stability at lower processing temperatures.

Fermentation of agriculture crops and waste products yields Poly (L-lactic acid)

(PLA) polymer, a semi-crystalline structure composed of three carbon monomers chain with hydroxyl and carboxyl groups at each end [6,7]. PLAs are produced from ring opening polymerization of lactides unit resulting in D and L lactic acid monomers. The

ratio of L to D- monomer units affects the degree of crystallinity, melting temperature and

machinability [8]. Potential advantages of PLA are high mechanical strength,

thermoplastic characteristics, biocompatibility, and being derived from renewable

recourses [6,9,10]. On the other hand, the brittleness, thermal instability, and poor water

vapor barrier properties are potential drawbacks for PLA resins [6,9,11].

Individually PHBV and PLA polymers have serious disadvantages when

compared to thermoplastics that are currently used. To address high costs and thermal

instability, blends of PHBV with PLA were explored as an alternative way of acquiring

novel materials with desired properties. Numerous researchers have studied blends of

these polymers as potential applications in food packaging to medical sutures. For

example, Zhang et al. prepared films by dissolving PLA and PHB in chloroform for film

applications, Wang et al. studied the physical properties of co-extruded PLA and PHBV

polymer blends, and Ferreira et al. studied PLA/PHBV blended pins for stabilizing bone fractures [9,10,12]. These researchers focused on two key factors: the miscibility of PLA and PHBV polymers and the potential improvements of the physical properties.

Although PHBV has a high polarity that allows its carboxylic groups to confer a certain polarity with PLA, the problem faced is that the PLA polymers fail to align properly due to the low polarity of PLA in blends [13].

Chapter 2

Statement of Problem

Previous researchers have studied Poly-(3-hydroxybutyrate-co-3-hydroxyvalerate)

(PHBV) polymers and Poly (L-lactic acid) (PLA); however, many of those polymers are

not being manufactured. In addition, many of these researchers have only tested PHB

polymer with only one concentration of hydroxyvalerate units. Although hydroxyvalerate

contents varied between these studies, the effect of molecular weight on thermal and

mechanical properties of same hydroxyvalerate content polymers were not discussed.

This study focused on analyzing various concentrations of hydroxyvalerate units in the

PHB homo-polymer. Furthermore, the effects of molecular weights between polymers

were also examined in PHBV, and different properties observed for both polymers can be

attributed to the molecular weight. It was hypothesized that the addition of

hydroxyvalerate units will improve several properties of the PHB polymer, and blending

PHBV with PLA will improve numerous properties of PHBV co-polymer.

Aim 1: Understand the physicochemical behavior of PHB synthesized with different

valerate contents (5, 12, and 20%) from two manufacturers (Tianan and Sigma-Aldrich)

• Determine the effect of valerate content on PHB using thermal analysis.

• Investigate the effect of four valerate concentrations on rheological behavior.

• Determine the mechanical properties (tensile strength, elongation at break,

flexural strength and flexural or Young’s modulus) and barrier properties of PHB

with different valerate content.

• Compare and contrast properties of two 5% valerate materials with different

molecular weight

• Compare properties from this work with other conventional thermoplastic used

for packaging.

Aim 2: To improve physicochemical properties of Tianan 5% with the addition of three

grades of commercially available PLAs at 20 to 50% in concentrations.

• Determine the effects of blending PLA on PHBV using thermal analysis.

• Investigate rheological properties of blends.

• Determine the mechanical properties of blends (tensile strength, elongation at

break, flexural strength and flexural or Young’s modulus.

Aim 3: Extrude blown films of pure PHBV and PLA materials as well as 50/50 blends of both materials using a single screw extruder with a blown film dye.

Chapter 3

Literature review

3.1: Introduction to thermoplastic polymer

According to Robertson, packaging is defined as an industrial and marketing technique for containing, protecting, identifying and facilitating the sale and distribution of agricultural, industrial and consumer products [14]. In addition the Packaging Institute

International defines packaging as the enclosure of “products, items or packages in a wrapped pouch, bag, box, cup, tray, can, tube, bottle, or other container” that performs any on the following “functions: containment, protection, and/or preservation; communication; and utility or performance” [14,15].

In modern society, the term “plastic” is used to describe many long chain molecules that are manmade [16,17]. The term plastic originated from Greek word

“plastiko’s,” meaning capable of being molded into different shapes [18]. The basic materials used to make plastics are carbon, silicon, hydrogen, nitrogen, oxygen, and chloride, which are extracted from oil, coal, and natural gas [19]. Over the past 60 years, these synthetic plastics have routinely substituted natural materials used for packaging of products, thereby becoming an indispensable part of daily life. In 2005, 250 million tons of petroleum-based plastics and synthetic polymers were produced worldwide, with a

12% per year expansion rate predicted [20,21]. As reported by Shah et al., synthetic plastics are extensively used in 30% of all packaging needs such as food, pharmaceuticals, cosmetics, detergents, and chemicals [17]. These plastics have replaced paper and other cellulose-based products for packaging because of their superior physical and chemical properties such as strength, lightness, water and microbial resistance, and environmental influences [17]. Most widely used thermoplastics for packaging include polyethylene (PE, LLDPE, LDPE, MDPE, HDPE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyurethane

(PU), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyurethane

(PU), poly-(ethylene terephthalate) (PET), polyvinyl alcohol (PVA) and nylon (Figure

3.1) [14,17,19]. In addition to superior mechanical and thermal properties, these thermoplastics have superior durability and stability. However due to their durability, these polymers have attracted more public and media scrutiny than any other solid waste because of their visibility in the waste system [17,22]. Ultimately, many of these polymers are introduced back into the ecosystem as waste products after single-use.

Figure 3.1: Structures of widely used thermoplastic polymers in packaging industry. polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyurethane (PU) [17,23].

3.1.1: Problems with thermoplastics

Many packaging experts will argue that synthetic plastics are the wonder material of today’s society. Unfortunately, these polymers’ useful characteristics are overshadowed by their steady contribution to the waste disposal problem and their negative impact on the environment worldwide. Since, these polymers are not readily broken down resulting in their accumulation as littler. Many consumers are unaware that

8 synthetic polymers disposed of today may still be around for hundreds of years (200-400 years), and these polymers are becoming a problem for many municipal waste management companies due to rapidly shrinking capacities of landfills in the United

States and Europe [21]. In many under-developing nations, the lack of municipal waste management practices and unregulated disposal of plastic packaging is affecting the quality of life for the local populations and causing health concerns [21]. According to the US Environmental Protection Agency, approximately 24.2 million metric tons of municipal solid waste was deposited into the US landfills made from non-renewable resources in 2005 [21]. In addition, every American generates roughly 1,500 pounds of waste per year; of this waste, 40-60% (600-900 pounds) is from single-use food packaging made from synthetic polymers [21]. This single-use food packaging waste make-up roughly 20-30% of landfill space [21]. The ultimate consequence of such irresponsible behavior will be depletion of natural resources. Ideally, these plastics would be recycled as infinite number of times, preventing them from ending up as part of landfills. However, recycling only accounts for 3.4% of waste [21]. Therefore, utilization of biodegradable polymers may prove as more plausible alternate.

3.2: Introduction to biodegradable polymer

Biodegradable polymers have hydrolyzable backbones that can easily biodegrade upon microbial attack. Despite many challenges (availability, cost, narrow processing temperatures, and brittleness) faced with these biopolymers, many scientists worldwide believe that biodegradable polymers with improved properties can provide a positive role in the development of environmentally friendly single-use consumer packaging [21]. In

9 addition, the consumer willingness to pay premiums for greener products, and their demand for environmentally friendly packaging materials have further helped to build momentum to seek new uses for biodegradable polymers [21]. Finally, the heightened fuel price of recent years, the rising cost of petroleum-derived chemical additives, and problems associated with disposal of plastic products have also helped in new investigations of bio-based packaging [21].

Biodegradable polymers can be classified by a particularly complicated system

[21]. However, for this study, the biodegradable polymers were divided into three categories: chemically synthesized polymers, starch-based biodegradable plastics, and polyhydroxyalkanoates (PHA) [24]. The chemically synthesized polymers are derived from petroleum-based stocks that are susceptible to enzymatic and microbial attack causing them to bio-degrade [17,21,24]. Examples of chemically synthesized polymers include polyglycolic acid, polylactic acid, poly-(ε-caprolactone), and polyvinyl alcohol

[24]. According to Khanna et al., these polymers are not commercially suitable to replace conventional thermoplastics [24]. The second category is starch-based biodegradable plastics. In this classification, starch is added as filler and a cross-linking additive to produce a blend of starch and plastic. However, these blends are not fully biodegradable compared to others because the thermoplastic fragments are recalcitrant and remain after the polymer matrix is broken down by microorganism. Some examples include starch blended with polyethylene, low-density polyethylene, ethylene acrylic acid, poly(vinyl alcohol) (PVOH), and vinyl acetate [17,21,24]. The final category is polyhydroxyalkanoates (PHA), which are completely biodegradable polymers [24].

3.3 Polyhydroxyalkanoates (PHA)

Polyhydroxyalkanoates (PHA) are a family of biodegradable polymers that can

replace conventional thermoplastic used for packaging. PHAs were first discovered by

Maurice Lemoigne in 1923 when cultures of Bacillus subtilis were allowed to lyse in distilled water resulting in poly-(3-hydroxybutyric acid) (PHB) decreasing the pH of the water [24,25]. PHB’s are most the common groups of PHA’s and widely studied by researchers. The PHA polymers exist naturally in microorganisms when limited amount of nutrients are available [24,26]. More specifically, PHAs are biopolymers that are synthesized by bacteria as intracellular carbon and energy storage granules under limited nutrients in the presence of an excess carbon source, Figure 3.2 [24,27]. The molecular weight of this polymer ranges from 200 to 3,000 kDa depending on the microorganism, nutrients, and growth conditioned [28]. In microorganisms, PHA exists as discrete lipid inclusions that are 0.2 mm in diameter and localized in the cytoplasm, Figure 3.2 [29].

This inclusion can be visualized by any light microscopy technique with Sudan Black B

or Nile Blue A stains [29-32]. The PHA polymers are divided into two categories

depending on the number of carbon atoms in the chain. The short chain (SCL) category

consist of 3-5 carbon atoms, while medium chain (MCL) consist of 6-14 carbon atoms

[33]. The variability in the number of carbon atoms is due to different synthesis

pathways, substrates used, and fermentation parameters.

Figure 3.2: Electron micrograph of 90% dry cell of P(3HB-co-3HHx) from Ralstonia eutropha. Bar represents 0.5 µm. [34]

3.3.1 Polyhydroxyalkanoates (PHA) synthesis

Khanna et al. and Sudesh et al. have discussed four pathways for synthesis of

PHAs [24,34]. These pathways are discussed briefly below. The first pathway (Figure

3.3) is used by Ralstonia eutropha, where β-ketothiolase is used in the condensation of

two molecules of acetyl-CoA to acetoacetyl-CoA. An NADPH-dependent acetoacetyl-

CoA reductase then carries out its conversion to 3-hydroxybutyryl-CoA. The third and

the final step is the polymerization reaction catalyzed by PHB synthesis [24,33]. The

second pathway is utilized by Rhodopsuedomonas rubrum. This pathway differs from

that of Ralstonia eutropha after the second step where the acetoacetyl-CoA formed by β- ketothiolase is reduced by a NADH dependent reductase to l-(+)-3-hydroxybutyryl-CoA, which is then converted to d-(−)-3-hydroxybutyryl-CoA by two enoyl-CoA hydrates [24].

Third pathway uses Pseudomonas species belonging to rRNA homology group I. P. 12 oleovorans and other Pseudomonas species accumulate PHA consisting of 3- hydroxyalkanoic acid of MCL if cells are cultivated on alkanes, alcohols or alkanoic acids [35-37]. The fourth pathway has not been studied in great details, but is also utilized by the Pseudomonas species belonging to rRNA homology group II. This pathway involves the synthesis of co-polyesters consisting of MCL PHAs from acetyl-

CoA, which has the highest cost out to four pathways used to synthesize PHAs. Another cost factor that has hindered the growth and popularity of PHA is the carbon substrate.

For comparison, the cost of PHA polymer is about 5 to10 times more expensive than the petroleum-derived polymers [38]. Manufacturers have tried using cheaper carbon sources

(glucose, fructose, and acetic acid), but the polymer concentration and content obtained were considerably lower than pure carbon source, Figure 3.4.

Figure 3.3: Metabolic pathway in the synthesis of PHB using Ralstonia eutropha [24].

Figure 3.4: Metabolic pathways of PHA synthesis from different carbon sources within a bacterial cell [34].

Fermentation strategies also allow for different chain length PHA polymers.

There are two groups of bacteria used to produce PHA. The first group requires limitation

of essential nutrients such as nitrogen, phosphorous, magnesium, or sulfur in an excess carbon source environment. The bacteria included in this group are Ralstonia eutropha,

Protomonas extorquens, and Protomonas oleovorans [24]. The second group of bacteria

does not require limitations in the nutrient intake for the accumulation of PHA polymers

in cytoplasm. These bacteria include Alcaligenes latus, Azotobacter vinelandii (mutant

strain), and recombinant E. coli. Many commercial PHA and PHB are produced either by

fed-batch or continuous fermentation. The fed-batch system requires bacteria from the

first group, while continuous requires bacteria from the second group [24]. According to

Khanna et al., the fed-batch technique is a two-step cultivation process in which a desired

concentration of biomass is obtained without nutrient limitation in the first stage [24].

Subsequently, the essential nutrient is kept in limiting concentration in the second stage, thus allowing intracellular accumulation of PHA. For cultivation of these bacteria, an optimal ratio of carbon source and nutrient to be limited should be fed to produce PHA with high efficiency. For example Kim et al., accumulated 80% PHA in dry cell weight by using Ralstonia eutropha under limited nitrogen or phosphorus nutrients [39].

However, several key factors must be considered for mass production of PHA such as: microorganism ability to utilize an inexpensive carbon source, growth rate, polymer synthesis rate, and the maximum extent of polymer accumulation [24].

During the last century, most PHA production, especially PHB, was done using

Ralstonia eutropha due to its ability to produce large quantities of PHB from simple carbon sources such as glucose, fructose, and acetic acid. Currently, many manufacturers are using recombinant E. coli strain to produce PHB. Some of the advantages of employing recombinant E. coli strain include: its ability to synthesize extremely high intracellular levels of PHB, being open to specific genetic strategies such as genetically mediated lyses, ability to use several inexpensive carbon sources, easy purification of

PHB, and the utilization of mutants to metabolically engineer strains that produce

P(3HB-co-3HV) copolymers [40,41]. In addition, a recombinant E. coli strain does not require limitation of nutrients but is dependent on the amount of acetyl-CoA produced

[40,41]. Kim et al., reported that 80 to 90% PHB accumulation could be reached using recombinant E. coli strain within 30 to 50 hours in a fed-batch fermentation system [42].

3.3.2: Physicochemical properties of PHA, specifically PHB

When PHB is isolated from bacteria, it has 55 to 80% crystallinity with perfectly

isotactic structure in (R)-configuration, Figure 3.5 [43]. Maximum crystallization and

growth rate of PHB spherulites occurs in the temperature range of 50-60˚C. Other

researchers have reported a higher temperature of 90˚C for overall growth and crystallization of PHB spherulites [44]. PHB polymer typically forms spherulites when crystallized from a melt state in bulk materials [44]. In addition, a single crystal of PHB

forms a lath-shaped crystal with dimensions of around 0.3 to 2 µm for the short axis and

5 to 10 µm for the long axis [32]. The single crystal formation in the long axis depends on various factors such as molecular weight, solvent, and crystallization temperature.

Finally, single crystals with well-defined structures are mono-lamellar systems, while films and plates (bulk materials) are usually multi-lamellar systems that have aggregated into multi-oriented lamellar crystals.

Figure 3.5: Chemical structure of Poly-(3-hydroxybutyrate) (PHB) [45].

The molecular weight of PHB ranges from 1x104 to 3x106 g/mol with a

polydispersity of around two [46]. The glass transition temperature (Tg) is approximately

4˚C, and the melting temperate (Tm) is approximately 180˚C. The mechanical properties are similar to isotactic polypropylene (PP) with Young’s modulus (3.5 GPa), tensile strength (43 MPa), and elongation at break (5%) [33]. As a result, PHB polymer is a brittle material with higher stiffness than conventional thermoplastic polymers. Other 16

researchers have tried to understand the brittle nature of PHB polymer. For example,

Barham and coworkers suggested that the PHB homo-polymer grown from the melt state

results in large-scale cracks that are often visible in the spherulites leading to

embrittlement [46-48]. De Koning and Lemstra et al., observed embrittlement of PHB

during storage after initial crystallization from the melt state, this secondary

crystallization results in the reorganization of lamellar crystals formed during the initial

crystallization, which limits the number of amorphous chains between crystals [45]. De

Koning and Lemstra et al. suggested that this limitation could be overcome by annealing

the polymer after initial crystallization [50]. The newer PHB polymers are made from

recombinant E. coli gene harboring PHA synthesis gene from R. eutropha that can

produce ultra-high molecular weight PHB polymers [24,34]. Kusaka et al. reported

weight average molecular weights in the range of 3x106 to 1.1x107 using recombinant E. coli genes under special fermentation conditions [51]. In addition, they tested the mechanical properties of these ultra-high molecular weight PHB polymers and reported elongation at break value of 58%, Young’s modulus of 1.1 GPa, and tensile strength of

62 MPa. They further studied this polymer by annealing the PHB at 190˚C and

determined the mechanical properties were enhanced [51].

3.3.3: PHB plasticized with Poly-(3-hydroxyvalerate) (HV)

To overcome the brittleness issues, many PHB manufacturers will plasticize PHB

homo-polymer internally using 3-hydroxyvalerate, 4-hydroxybutyrate, 3-hydrohexonoate,

or 3- hydroxypropionate, Figure 3.6 [17,52]. In this study, the supplied materials

consisted of poly-(3-hydroxyvalerate) (HV) or (PHV) units incorporated into PHB,

17 resulting into poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) or P(3HB-co-

3HV), Figure 3.7. The final structure is a nine-carbon monomer with hydroxyl and carboxyl end groups [6]. The PHBV co-polymers have approximately same amount of crystallinity (50-80%) as PHB homo-polymer. A structural characteristic of PHBV is isodimorphism, meaning they display a melting temperature minimum for PHV content of 30 mol% and co-crystallization of the two monomer units in either of the homo- polymer crystal lattices of PHB and PHV, depending on whether the (R)-PHV composition is above or below 40 mol% [24,34]. The added co-polymer decreased the glass and melting temperature, Table 3.1. With increasing HV content, the co-polymer becomes tougher (increase in impact strength), more flexible (decrease in Young’s modulus), and absorb more strain (increase in elongation at break). Furthermore, the processing window is broadened since there is improved melt stability at lower processing temperatures with increasing HV content [4,6,24]. Therefore, this plastic has the potential to be tailored to specific applications that are similar to conventional thermoplastics that are commonly used.

Table 3.1: Thermal and mechanical properties of PHB with different PHV content at 25˚C [54-56].

Figure 3.6: Various stuctures of PHA [53].

Figure 3.7: Chemical structure of Poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) [4].

3.4: Introduction to Poly(lactic acid) (PLA)

Another biopolymer resin that received great interest as a substitute to thermoplastic packaging is poly(lactic acid) (PLA). PLA is derived from lactic acid (2- hydroxy propionic acid) using ring-opening polymerization resulting in a cyclic dimer

[27,57]. During the later part of last the century, PLA-based packaging was focused on high value films, rigid thermoforms, and coated papers due to higher cost of productions

[57]. However, as emerging technology lowers production costs, broader array of packaging applications may be found. With increasing production of PLA, numerous advantages arise such as manufacturing from renewable agricultural resources (corn and

soybean), consuming more carbon dioxide, significant energy saving, producing renewable and compostable packaging, and manipulating the physical and mechanical properties through ring polymerization and condensation [58-63]. Currently, the use of

PLA in food packaging is mostly for short shelf life products such as clamshell containers, drinking cups, sundae and salad cups, overlaps and lamination films, and blister packages [64-68]. In addition, manufacturers are testing thermoformed PLA containers used to store and transport fresh fruit and vegetables to retail markets [57].

3.4.1: Poly(lactic acid) (PLA) production

The basic material for PLA production is lactic acid manufactured from carbohydrate via fermentation or chemical synthesis. Lactic acid (2-hydroxy propionic acid) has a central, asymmetric carbon atom with two optically active configuration, L(+) and D(-) isomers, Figure 3.8. The majority of lactic acid manufacturers used fermentation

with Lactobacilli genes to produce high rate of lactic acid. However, the final lactic acid

polymer has low molecular weight that is not suitable for packaging. As a result, Cargill

Inc. in 1992 patented chemical reactions to produce PLA with higher molecular weight of

100 kDa [59,67]. In general, there are three different methods to produce higher molecular weight PLA (100 kDa): direct condensation polymerization, azeotropic condensation, and polymerization through lactide formation, latter of which is patented by Cargill Inc.

Briefly, direct condensation polymerization uses chain coupling agents and adjuvants to produce higher molecular weight PLA. However, without these agents, it very difficult to obtain a solvent free PLA, but this process is the least expensive out to

three known methods [68-72]. Azeotropic condensation is based on reducing the distillation pressure of lactic acid, which causes water to condense and be removed.

Afterward, catalyst and diphenyl esters are added to the reaction vessel to form the PLA.

Polymerization through lactide formation uses dextrose fermentation to produce an

intermediate low molecular mass of poly(lactic acid). Using low pressure, this

poly(lactic acid) is converted into a mixture of lactide stereoisomers. Lactide is a cyclic

dimer of lactic acid formed during condensation of two (L),(D), or meso-lactic acid

molecules, Figure 3.8. After vacuum distillation, a high molecular PLA mass is formed

by either cationic or anionic ring-opening polymerization of the lactides [59,72-74]. The

cationic ring-opening polymerization is initiated by methyl trifluoromethanesulfonic acid

or trifluoromethanesulfonic acid (triflic acid), while the anionic ring-opening

polymerization is initiated by nucleophilic reaction of the anion with the carbonyl group

and subsequent acyl-oxygen cleavage [74-76]. In Figure 3.9, the three known methods

for high molecular weight production are summarized, and Figure 3.10 shows the current

process for PLA production. More through explanations can be found in Auras et al. and

Garlotta et al [57,77].

Figure 3.8: Chemical structure of DD-, LL-, Meso- (one D- and L-) lactide [57].

Figure 3.09: Synthesis for obtaining high molecular PLA [77].

Figure 3.10: Current manufacturing procedure for commercial PLA production [57]. 22

3.4.2: Poly(lactic acid) (PLA) structure

The structure of PLA is a chain of three-carbon monomers with hydroxyl and

carboxyl groups at each end of each monomer. (Figure 3.11) PLA polymer structure is

determined by the stereo-chemical composition of L- or D- lactide units. PLA derived

from greater than 93% L-lactic acid is a semi-crystalline polymer, while PLA with 50 to

93% L-lactic acid is amorphous polymer [57]. The further addition of D- and meso-lactide

introduces coils in a very regular poly(L-lactide) structure. These molecular

imperfections are responsible for decreasing the crystallization of PLA [57]. The majority

of all PLAs are made up of L- and D,L-lactide copolymers because small amounts of meso- lactide impurities are produced during ring opening polymerization [57].

Figure 3.11: Chemical structure of PLA [57].

3.4.3: Poly(lactic acid) (PLA) physical properties

The physical properties of polymers depend on molecular characteristics such as crystal thickness, degree of crystallinity, spherulite size, morphology, and chain orientation [57]. These characteristics are also influenced by the physical properties of

PLA such as the stereoisomers of lactic acid. Regular structure and crystalline properties are observed in homo-polymers of poly(D-lactide), poly(L-lactide), or high ratios of D- or

L-lactide copolymers. Depending on the amounts of L, D, and meso-lactide, high

molecular weight PLAs are either amorphous or semi-crystalline at room temperature

[57,77]. For amorphous polymers, the glass transition temperature (Tg) is an important

parameter because increasing chain mobility occurs around the glass transition temperature. In Figure 3.12, amorphous PLA polymers are completely brittle below the

beta-relaxation temperature (Tβ) of -45ºC, and near Tg (58ºC) PLA undergoes physical

aging, thus transitioning from brittle to rubbery state [57]. At temperatures of 110-150˚C,

the polymer transitions from rubbery to viscous state with the exact temperature depending on the molecular weight and shear stress. The polymer starts to decompose at

215˚C with complete degradation at 285˚C [47]. On the other hand, the Tg and melting

temperature (Tm) are important physical characteristic for predicting semi-crystalline

PLA behavior.

Figure 3.12: Different states of high molecular weight amorphous PLA with increasing temperatures [57].

For semi-crystalline PLA polymers, the Tg indicates the thermal transition from brittle to rubbery state. The melting temperature is determined from the stereochemistry of the PLA polymer, thus increasing concentration of meso-lactide units reduces the

melting temperature. Pure L- or D- PLA has a melting temperature of 180˚C, and the

melting temperature increases with increasing molecular weight. Commercially available

PLA exhibits a Tg around 50 to 80˚C, while the melting temperature ranges from 130 to

180˚C [57,78]. For example, PLAs with molecular weight of 430 and 22,730 Da has a Tg

of -8.0 and 55.5˚C, respectively [79]. Finally, the physical properties are greatly affected

by the thermal history of PLA due to changes in the crystallinity.

3.4.4: Poly(lactic acid) (PLA) rheological behavior

Rheological properties of PLAs are dependent on molecular weight, L- to D- lactide ratio, shear rate, melt processing, amount of plasticizers, and the amount of energy put into system [57]. The shear viscosity of PLA polymers affects thermal processing in extrusion, injection molding, blown films, sheet forming, fiber spinning and others

[57,77]. The rheology of PLA is non-Newtonian, pseudoplastic melting, where the

viscosity parameter is a function of the shear rate (shear dependent). Most commercial

semi-crystalline PLA has a higher shear viscosity than amorphous PLA. However, in

both compositions, shear viscosity decreases with increasing temperature. Dorgan and

co-workers have studied wide range of PLAs with different stereoisomers. They have

found that the melt viscosity is not dependent on the optical composition of the polymer,

thus contradicting claims made by Auras et al.[80]. Finally, the melt flow index provides

insights in the liquid polymers extrusion capabilities. The values of melt flow index

range from 8.51 g/10 min and 7.83 g/10 min for poly(98% L-lactide) and poly(94% L- lactide) at 200˚C [57,77].

3.4.5: Poly(lactic acid) (PLA) mechanical properties

Mechanical properties of PLA are affected by degree of orientation, stereochemistry of isomers, and annealing time. In Table 3.2, mechanical properties of poly(98% L-lactide and 94% L-lactide), PS and PET are summarized. Generally, the

higher concentration of L-lactide units provides higher tensile strength, while the lower

concentration of L-lactide units affects elongation at break values. Most commercially available PLAs have similar mechanical properties to PS using the same testing conditions [57]. Finally, the impact resistance of poly(98% L-lactide ) films is around

360 g, which is significantly affected by the crystallinity of PLA polymers. Auras and others have reported that crystallization kinetics are strongly dependant on the copolymer composition [57,81]. For example, if D-lactide units are added to PLA with high

L-lactide units, the polymer structure of PLA becomes more disordered with increasing

spherulites as crystallization time increases [82].

Table 3.2: Mechanical properties of poly(98% L-lactide), poly(94% L-lactide) compared to PS and PET films [57].

Finally, commercial PLAs can be processed using conventional thermoplastic

injection molding, blown film molding, sheet extrusion, thermoforming, and film

forming. Ingeo brand PLA commercialized by Cargill LLC (Minnetonka, Minnesota) can

be used for extrusion, thermoforming, cast film, blown films, and injection stretch blow

molded bottles and containers [83,84]. Unlike many thermoplastics, PLA resin loses

thermal stability when heated 10ºC above the melting temperature [72,77]. This thermal

instability can be attributed to chain splitting, hydrolysis, and residual monomer

26 concentration, thus reducing the molecular weight of polymer [85]. For example, PLA with 100% L-lactide units has a narrow processing window of 12ºC, while addition of

10% D-lactide units widens the processing conditions by 40ºC due to the composition’s lower melting temperature [77]. To improve PLA’s properties, many scientists has plasticized PLA with different lactide isomers, glycerol, citrate ester, polyethylene glycol

(PEG), PEG monolaurate, and oligomeric lactic acid to improve the brittle nature of PLA

[8]. Out of these plasticizers, PEG is the most efficient in reducing the glass transition temperature from 58 to 12ºC, while glycerol is the least effective of these plasticizing agents. Finally, based on thermal gravimetric analysis (TGA), commercially available

PLA polymers start to degrade near 300ºC and complete degradation occur around

400ºC.

3.5 Importance of Food Packaging:

In the food industry, packaging materials are often made from various plastics to achieve optimal barrier properties. To protect foods, various unique requirements need to be met when considering a material for food packaging applications that encompass the dynamic interaction between food, packaging material, and the environment [3].

Additionally, other more common properties such as gas and water vapor permeability, mechanical changes, sealing and thermoforming capabilities, machinability, transparency, anti-fogging, printability, resistance to light, water, acid, grease, availability and of course cost all are important factors [86].

3.5.1: Importance of thermal testing

Many packaging environments range from sub-freezing to elevated temperatures

near melting points of many packaging materials for few seconds to years in storage. By

performing thermal analysis using Differential Scanning Calorimetry (DSC) and

Thermogravimetric Analysis (TGA) specific temperature profile of a polymeric material

can be determined. Briefly, DSC measure the difference in the amount of heat required

to increase the temperature of a polymer and reference sample (empty pan) as function of

temperature or time [87]. On the other hand, TGA is a sensitive balance that measures changes in the weight (gained or lost) of the sample as a function of temperature or time

[87]. The DSC thermogram can be useful in providing phase transitions (glass, crystallization, and melting) of polymers, while the TGA provide insight on thermal stability and decomposition temperature of a polymer.

The glass, crystallization, and melting temperature are dependent on the physical state of the polymers ranging from amorphous (lacking positional order), crystalline

(positional order) to semi-crystalline (containing both order and disorder within the same polymeric region). Non-crystalline (amorphous) polymers are characterized by the glass transition temperature (Tg), while crystalline polymers are characterized by the crystalline melting temperature (Tm) [14]. Finally, the semi-crystalline polymer is characterized by both glass and melting temperatures. The glass transition temperature

(Tg) is when the polymeric material changes from solid to rubbery state, which is also known as softening point. Below Tg, the polymer backbone sequences are frozen and brittle, which is not useful state for many packaging applications. At or near Tg, the

polymer backbone begins to move and slides pass one another; this movement causes the

28 polymeric material to become flexible and very useful for packaging applications.

Furthermore, many commercially used packaging materials have a low molecular weight plasticizers added to polymeric structure, which lowers Tg temperatures [14]. On the other hand, the melting temperature (Tm) is a very indefinite rubber to liquid transition sometimes referred to as flow transition. Finally, the packaging will start to fail, as the temperature gets closer to the melting temperature due to the polymer turning liquid.

The physical properties of a conventional plastic depend greatly on the Tg and Tm relative to the room temperature. If both Tg and Tm are below room temperature, the polymer is a liquid. However if the room temperature lies between Tg and Tm, the polymer is a very viscous super cooled liquid, which is desired for packaging materials. If both Tm and Tg are above room temperature a glassy, brittle, and amorphous polymer is observed [16]. For many commercially thermoplastic, the glass and melting temperatures vary between polymers such as PET (73-80 and 245-265ºC), PS (70-80 and 100ºC),

LDPE (-100 and 95-110ºC), and PP(0 and 176˚C) [88, 89].

3.5.2: Importance of mechanical testing

Many plastics exhibit wide ranges of properties, which are employed in designing packages. For packaging, strength, stiffness, and extensibility parameters are reflected in the tensile strength, flexural strength, and elongation at break using a mechanical analyzer. Specifically, the tensile strength is the stress necessary to cause appreciable plastic deformation or the maximum stress the material can withstand [90]. Flexural strength is a measure of resistance to rapture for materials that are under load. Elongation at break is the maximum strain the material can withstand. Lastly, elastic modulus is

defined as degree of elasticity of a material when force is applied. This is usually

measured by Young’s modulus, as the ratios of stress to strain in the range were Hooke’s

law hold true (low strains). The elastic modulus of traditional thermoplastic polymers are:

PET (2.8-4.1 GPa), PS (2.3-3.3 GPa), LDPE (0.2 GPa), and PP (1.7 GPa), while

elongation at break value of PET (30-300%), PS (2.50%), LDPE (100-1000%), and PP

(400%) [88,89]. Finally, the tensile strength value to LDPE (8-20 MPa), PET (48-72

MPa), PS (34 to 50 MPa), and PP (38 MPa) [88,89]. Strong packaging materials with toughness are susceptible to the abuses of shipping and handling. Furthermore, the packaging materials vary because different requirements are needed to adequately protect the product. During storage of products, the packaging material is under stress due to dead loads from stacked pallets. Stress relaxation testing is performed to understand how much deformation a packaging can take before the stress overcome the packaging and causes shrink band or shrink label [91].

3.5.3: Importance of rheological and barrier testing

Rheological testing of polymers is conducted to determine flow and deformation properties of polymers under stress at various temperatures of transport and storage [92].

In addition, flow testing can predict molecular changes between entanglements that occur when the polymer is exposed to various strains and stresses, thus affecting the structural capabilities of the packaging [6,92]. Rheological evaluation simulates numerous capabilities of polymers in processing equipments used to make bag, bottles, films, pouches. Finally, rheological testing is necessary in developing equations that describe behaviors of the polymers in melt state with various deformations history. Gases, water

and water vapor are potential migrant into and out of packaging that can spoil food items.

As a result, barrier testing is performed to determine the permeation of these components

into or out of the packaging. Traditional thermoplastic are highly favored due to their low

transmission rate such as poly(ethylene terephthalate) (PET, 3.48 g/m2/day), oriented

polystyrene (OPS, 5.18 g/m2/day), and low-density polyethylene (LDPE, 7.90 g/m2/day)

[93-95]

Chapter 4

Assessment of PHB with Varying Hydroxyvalerate content for Potential Packaging Applications

Sunny Modi1, Kurt Koelling2, Yael Vodovotz1

1Ohio State University, Department of Food Science & Technology, 110 Parker Food Science & Technology Bldg. 2015 Fyffe Road, Columbus, Ohio 43210 USA 2 Ohio State University, Department of Chemical and Biomolecular Engineering, 125 Koffolt Laboratories, 140 West 19th Avenue, Columbus, Ohio 43210 USA

Abstract

Poly (hydroxybutyrate) (PHB) is biodegradable aliphatic polyester that is

produced by a wide range of microorganisms. Basic PHB has relatively high glass

the processing window since there is improved melt stability at lower processing

temperatures. In this study, PHB synthesized with different valerate contents (5, 12, and

20%) was characterized. All PHBV materials displayed a glass transition between -10 to

20ºC. The two melting transitions found for Aldrich 5%, 12%, and Tianan 20%, resulted

from crystals formed during cooling of the samples. The melt rheology suggested

degradation of samples as the complex viscosity decreased with increasing temperature

due to a decrease in molecular weights of the samples. These results suggest that

processing the copolymer below 160ºC would be beneficial with low screw speed. The

mechanical results indicate all PHBV materials had high elastic modulus and flexural 32 strength with low tensile strength and elongation at break. The WVTR results indicated the polymer to be very hydrophilic, which resulted in higher transmission rates.

Key words: Poly (3-Hydroxybutyrate-co-3-hydroxyvalerate) (PHBV); biomaterials; thermal; mechanical; rheological; barrier properties; processing.

4.1: Introduction

Bio-based materials for packaging applications are being sought to replace synthetic, non-degradable, thermoplastics due to rapid growth in municipal waste, consumer awareness and stricter government regulations [4,6,9]. Furthermore, synthetic polymer waste used for food and consumer packaging constitutes about 600-900 lbs per

American, which is 20-30% of landfill space [9]. Currently, the potential for aliphatic polyesters derived from fermentation such as poly-(ε-caprolactone) (PCL), poly(L-lactic acid) (PLA), and poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) can be used to alleviate demands on diminishing landfill space as they are fully biodegradable due to their hydrolyzable backbones [4,6,9].

One potential biopolymer family is that of polyhydroxyalkanoates (PHA’s) that are biosynthesized from variety of microorganisms. Amongst PHA’s, poly-(3- hydroxybutyrate) (PHB) are studied most frequently and easiest to produce [96]. The

PHB polymer, being saturated polyesters, behaves similarly to conventional thermoplastics. However, it has relatively high glass transition and melting temperatures; therefore, increased brittleness resulting in a poor processing window and higher cost limiting their use [96,97].

To overcome these drawbacks, the PHB homo-polymer can be plasticized internally by bacterial fermentation using 3-hydroxyvalerate, 4-hydroxybutyrate, 3- hydrohexonoate, or 3-hydroxypropionate [17,99]. In this study, poly-(3-hydroxyvalerate)

(HV) units were incorporated into PHB, resulting in poly-(3-hydroxybutyrate-co-3- hydroxyvalerate) (PHBV). The added co-polymer decreased the glass and melting temperature as well as broadens the processing window, since there is improved melt stability at lower processing temperatures [4,98]. The final structure is a nine-carbon monomer with hydroxyl and carboxyl end groups; therefore, allowing the plastic to be tailored to specific applications that are similar to conventional thermoplastics [6].

Previous researchers have mostly tested the PHB polymer with only one concentration of hydroxyvalerate units and from a single manufacturer. For example

Verhoogt et al., studied thermal, rheological, and mechanical properties of Biopol PHB with 12% HV units (Mw of 539 kDa), while Wang et al. studied Tianan PHB with 1% HV

units (Mw of 130 kDa) and determined similar characterization as Verhoogt [9,96].

Although hydroxyvalerate contents varied between these studies, the effect of molecular weight on thermal and mechanical properties of same hydroxyvalerate content polymers were not discussed. For many polymers, the molecular weight effect thermal, mechanical, rheological, and other properties during dwell times in an extruder or injection molding units, thus it very important to understand molecular weight effects and other properties studied. Therefore, the aims of this study were to first characterize the thermal, mechanical, rheological, and barrier properties of PHB synthesized with

different valerate contents, and second to compare these properties in PHBV with similar

hydroxyvalerate content but different molecular weight.

4.2: Materials

Biodegradable Poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) was obtained from two manufacturers: Tianan Biologic Material Co., Ltd. (Ningbo, China) with HV content

of 5 and 20%, and Sigma-Aldrich (St. Louis, Missouri) manufactured PHBV with 5 and

12% hydroxyvalerate content. Tianan 5% (T5%) is available in bulk quantities with a

molecular weight of 280 kDa, and Tianan 20% (T20%) is in developing stages with a

molecular weight of 270 kDa. Sigma-Aldrich PHBV is available in limited amounts with

a molecular weight of 150 kDa for HV content of 5% (A5%) and 400-600 kDa for HV

content of 12% (A12%), according to manufacturers’ specifications.

4.3: Sample Preparations

4.3.1: Mechanical and Permeation Samples

PHBV polyesters were pre-treated in a vacuum oven for 24 hrs at 60ºC. For

mechanical and permeation characterizations, the sample films were prepared by

compression molding sheets (94 x 94 x 0.20 mm3) using a Carver Heated Press Model

3851-D (Wabash, Indiana) at 10ºC above their melting temperature. The Aldrich 5%

material resulted in a very brittle and porous film, thus solution casting was used to make

the films. 5 g of Aldrich 5% polyester was added to 40 g of chloroform and mixed for

four hours at 40ºC until clear solution was achieved. The solution was uniformly spread

on glass plates, and placed in a desiccator for two weeks for drying. The resulted films

were vacuum dried for additional two weeks at 60ºC.

4.3.2: Rheological Samples

For rheological characterizations, disks were compression molded to 25.0 mm in diameter and approximately 1.0 mm thickness.

4.4: Thermal Properties

4.4.1: Thermal Decomposition

Thermogravimetric analysis (TGA) was used to study thermal decomposition properties using a TA Instruments TGA 5000 (New Castle, Delaware). The samples were heated under nitrogen environment from 150 to 500ºC with a heating rate of

20ºC/min. Samples were vacuum dried at 60ºC for 24 hours prior to any testing.

4.4.2: State of Material Components

Differential Scanning Calorimetry (DSC) was used to determine the state of the material components as a function of temperature. TA Instruments Q100 (New Castle,

Delaware) was used to perform DSC analysis. The pure materials were vacuum dried at

60ºC for 24 hours prior to any testing and stored in a desiccator during testing. The melting transitions were determined using 5 to 10 mg samples, and heated from -20 to

200ºC with a heating rate of 10ºC/min. The samples were allowed to anneal at 200ºC for

5 min to remove the thermal history of the polymers. Subsequently, the same samples were cooled to -20ºC, held for 5 minutes, and reheated from -20 to 200ºC with a heating rate of 10ºC/min. The glass transitions were determined using the same temperature profile with a different heating and cooling rates of 20ºC/min and a sample size of 10 to

20 mg. The glass transitions reported were determined from the second heating, and a similar procedure is described elsewhere [4,6,96].

4.5: Rheological Properties

Rheological behaviors were obtained using a TA Instrument Ares 2000 (New

Castle, Delaware). Samples were vacuum dried at 63ºC for 24 hours prior to testing.

Rheological experiments were performed at the melting temperature range of the

polymers determined from DSC results using 25 mm parallel plate system. The sample

was equilibrated for five minutes before the gap was set to the testing position of approximately 0.9 mm or until the top plate made contact with the top of sample using

Leica light. The furnace was opened to remove excess material. Dynamic viscosity was

measured with increasing time and frequency. Frequency sweeps ranged from 0.1 to 100

rad/sec by conducting four sequential sweeps and measuring the changes in viscosity with respect to frequency. Time sweep was performed for one hour using constant frequency of 1 Hz. For each frequency and time sweeps, the linear viscoelasticity limits were determined [7,92].

4.6: Mechanical Properties

4.6.1: Tensile strength and Elongation at Break

Tensile properties were determined according to ASTM D638-08. Prior to any

tensile testing, the samples were vacuumed dried for 24 hours at 60ºC. The tensile data

was obtained using an Instron 5542 with Bluehill v. 2.17 software package (Instron Corp.

Norwood Massachusetts). For tensile testing, dumbbell shaped samples (62 x 10 x 1.0 mm3) with the gage section of 3.0 mm, and a crosshead speed of 5 mm/min at 26.0ºC.

The reported values are averages of at least five samples.

4.6.2: Flexural Strength and Elastic Modulus

Flexural properties were determined according to D790-07. Prior to any flexural testing, the samples were vacuumed dried for 24 hours at 60ºC. The specimens were rectangular bars (51 x 7.0 x 2.0 mm3) that were tested using TA Instrument RSA3 unit

(New Castle, Delaware) with a crosshead speed of 2.5 mm/min at 26.0ºC. The reported values are averages of at least five samples.

4.6.3: Glass Transitions, Modulus, and Tan Delta Properties

The mechanically induced glass transitions of the polyesters were studied using a

TA Instrument Dynamic Mechanical Analyzer Q800 (New Castle, Delaware). The specimens were equilibrated at -30.0ºC and subsequently heated to 160ºC with a heating rate of 5ºC/min under liquid nitrogen. The film dimensions were approximately 30.0 x

7.00 x 0.5 mm3 with amplitude of 15.00 µm, preload of 1 N, and frequency of 1 Hz using the tension film clamps.

4.7: Water Vapor Transmission Rate

Water vapor transmission rates (WVTR) of PHBV films were determined using a

Mocon Permatran-W 3/31 (Minneapolis, Minnesota). The procedure described in the

ASTM guide F1249-06 was followed. The Mocon unit was set to 100% relative humidity (r.h.) by using a wet sponge in each cell. Prior to testing each material, the instrument was calibrated using supplied “red” calibration film (203 µm) at 37.8ºC with a flow rate of 100 cm3/min. To prevent the infrared detector from being overloaded with water vapor, the sample area was reduced to 5 cm2 using the aluminum foil supplied by

Mocon. The samples were analyzed at 37.8ºC with a flow rate of 100 cm3/min at 100% r.h. in 30-minute cycles. The reported values were obtained after the samples had

reached steady state (4-8 hours). The averages of at least three trials are reported. The

WVTR values (g/m2/day) were normalized using equation 4.1 [99,100].

WVTR = WVTR (raw) * t/25, where t (µm) is the thickness of specimen. [4.1]

4.8: Results and Discussion

4.8.1: Thermal Analysis

The thermogravimetric analysis (TGA) results for the PHBV materials are

summarized in Table 4.1, and the TGA curves are shown in Figure 4.1. TGA analysis

provides insight as to when the polymer starts to decompose from thermal stress, thus

losing its functionality as an adequate packaging polymer. Tianan 5% had the narrowest

degradation temperature range, while Aldrich 12% showed a wider temperature range.

The curves show minimal separation between the PHB homo-polymer and PHV

plasticizing units. Previous researchers have stated that the thermal degradation begins

around 250ºC for Biopol and Tianan PHBV materials [9,96,101,102]. This degradation is

caused by chain scission and hydrolysis, which results in a lower molecular weight

polymer and formation of crotonic acid [9,96]. The higher concentrations of HV units in

Tianan 20% did not improve thermal stability of the polymer, when compared to Tianan

5%.

Table 4.1: Thermal degradation temperature of PHBV obtained via TGA. Tonset (ºC) Tpeak (ºC) T20% 196 284 T5% 238 288 A12% 177 296 A5% 187 276 Tonset = beginning mass loss; Tpeak = maximum mass loss

Figure 4.1: Thermograms of four PHBV materials obtained from TGA. T20% = Tianan 20%, T5% = Tianan 5%, A12%= Aldrich 12%, A5% = Aldrich 5%.

The thermal events of glass, crystallization, and melting transitions of PHBV materials were studied using differential scanning calorimetry (DSC). The DSC transitions are summarized in Table 4.2, and the thermograms are shown in Figures 4.2.

Numerous researchers have reported that PHB is susceptible to thermal degradation, which results in a shift of the highest melting peak to a lower melting peak and wider endotherms upon heating [9,103,104]. Based on these findings, the thermograms of first and second heating were compared, and the second heating was used to determine the glass transition temperature. The first melting transitions ranged from 132 to 171ºC. The higher HV content samples (Aldrich 12% and Tianan 20%) resulted in two melting transitions. Similar results were observed by Wang et al., with two melting transitions at

165 and 182˚C during their first melting [9]. Both samples of 5% HV resulted in a single

melting curve with Tianan sample having a uniform melting peak. The Aldrich material

exhibited a broader melting range (130ºC to 166ºC), due to the decreasing molecular

weight of the polymer from the thermal treatment [96].

When the first heating scans were compared to the rescan of the sample, some degradation was observed after annealing the materials for three minutes at 200ºC, Figure

4.2. The changes in the melting behaviors of Tianan 20%, Aldrich, 12%, and 5% were

significant. In Tianan 20%, the higher temperature transition was transformed into a

shoulder rather than a separate peak as observed in the first melting thermogram. Aldrich

5% had a broader melting transition in the first heating behavior at 166ºC, which was

transformed into two separate melting transitions in the second heating behavior at 144

and 159ºC. Similarly in the second melting behavior, Aldrich 12% resulted in two

melting transitions at 149, 158ºC. The lower melt represented the original crystal of HV,

while the higher transitions originated from the crystal of PHB [5]. In addition, the

second transition represents the crystal melting during the heating cycle in the DSC that

occurred from crystal thickening or recrystallization. Tianan 5% results showed a

uniform melting peak, suggesting an optimum balance between the content of PHB and

HV. The two melting transitions seen with Aldrich 5%, 12%, and Tianan 20% suggest

the polymer is changing from solid, to semi-solid, and to liquid as the individual

polymers begin to melt, thus impairing their functionality for adequate packaging

applications. Furthermore, processing of these polymers becomes difficult due to

multiple melting temperatures, thus an optimal die temperature is difficult to reach

without introducing draw resonance and necking problems.

Table 4.2: Average thermal parameters of PHBV obtained via DSC. Samples Crystallization Melting Transitions Melting Transitions Transition (First Melting) (Second Melting) Tpc (˚C) Tpm1 (˚C) Tpm2 (˚C) Tpm1 (˚C) Tpm2 (˚C) T20% 66.1 132 171 130 163 T5% * - 171 - 165 A12% * 143 156 149 158 A5% * - 166 144 159 *Material did not show results during first heating Tpm = melting peak temperature. T20% = Tianan 20%, T5% = Tianan 5%, A12%= Aldrich 12%, A5% = Aldrich 5%.

Figure 4.2: DSC melting transition scans of PHBV at 10 ºC/min. Solid lines = first melting, dotted lines = second melting behavior. T20% = Tianan 20%, T5% = Tianan 5%, A12% = Aldrich 12%, A5% = Aldrich 5%.

The glass transitions were obtained from the second heating cycle. The data is

summarized in Table 4.3, and the scans are shown in Figure 4.3. Results showed that all

PHBV materials displayed a glass transition between -10 to 20ºC. The comparison

between Aldrich 5% and Tianan 5% PHBV showed the Aldrich material had a more

pronounced glass transition, Figure 4.3. The degradation noticed at higher temperatures

with DSC (Tianan 20%, Aldrich 5 and 12%) could be attributed to chain scission, which caused PHBV molecules to break down [9,96,101,102]. The poor thermal stability observed with TGA and DSC may lead to inferior mechanical properties, thus processing of Tianan 5%, PHBV should be performed below 160ºC. Overall, the uniform melting and lower glass transition makes Tianan 5% ideal for packaging applications. The thermal properties of Tianan 5% indicate similar glass and melting temperature with PP

(0 and 176ºC) [89]. However, the glass and melting temperatures differ greatly with other traditional thermoplastic such as PET (73-80 and 245-265ºC), PS (70-80 and

100ºC), LDPE (-100 and 95-110ºC) [88,89].

Table 4.3: Average glass transition temperatures of PHBV obtained via DSC. Samples Glass Transitions (Second Melting) Teig (ºC) Tmg (ºC) T20% -3.1 -0.050 T5% -2.6 -0.40 A12% -7.4 -2.7 A5% -0.89 2.0 Teig = extrapolated onset temperature, Tmg = midpoint temperature, T20% = Tianan 20%, T5% = Tianan 5%, A12%= Aldrich 12%, A5% = Aldrich 5%.

Figure 4.3: DSC glass transition scans of PHBV at 20 ºC/min, second melting behavior.

4.8.2: Rheological Analysis

Summary of the melt rheology is shown in Figures 4.4 and 4.5. In figure 4.4, the complex viscosity-frequency sweeps of the PHBV materials are shown at 150 (A), 160

(B), and 170ºC (C) respectively. The results indicate a decrease in complex viscosity with

increasing rotational frequency. At 160 and 170ºC, Tianan 20%, Aldrich 5%, and 12%

demonstrate smaller decreases in complex viscosity as frequency increased. The

complex viscosity for Tianan 5% was very stable at low frequencies for all temperatures

studied, while complex viscosities of Tianan 20%, Aldrich 5%, and 12% decreased by

five orders of magnitude at low frequencies from 150 to 160ºC. Subsequent scans were performed at the conclusion of the first scan, and these results show no change in

complex viscosity at 150ºC over the frequency sweeps. Additionally at 160ºC, the

viscosity decreased marginally for the subsequent second, third, and fourth scans for

Tianan 20%, Aldrich 5%, and 12% at low frequencies. At 170ºC, the decreases in the 44 complex viscosity were greater for subsequent scans of Tianan 20%, Aldrich 5%, and

12% throughout the frequency sweep. The changes noticed in complex viscosity for subsequent scans can be attributed to slight degradation within the polymer and decreases in the molecular structure. At the melting temperature of 150-170˚C, the Tianan 5% showed marginal to no change in the complex viscosity for the subsequent scans; thus, indicating greater melt stability than other PHBV polymers.

In Figure 4.5, the complex viscosity-time was measured at 1 Hz for all materials in the melting temperature range. Tianan 5% was determined to be a stable material because the complex viscosity held steady during one hour of testing, with only minor decreases in the initial complex viscosity observed between 150 and 170ºC. The Aldrich

12% material showed severe degradation in the initial viscosity as the temperatures were increased from 150 and 170ºC. These results differ from Verhoogt et al., who found

PHBV with 12% HV content degrades in complex viscosity by 10% within eight minutes at 160 and 170ºC [96]. From the rheology testing, Tianan 5% was found to be very stable material, thus the polymer could be used for potential packaging where draw resonance and necking might be a problem during processing.

One explanation for the lower complex viscosity at higher temperatures is the inability of the polymer molecule to withstand the melting temperatures. The methylene protons in the homo-polymer are in close proximity with two ester groups, which causes thermal instability observed in DSC and decreases in complex viscosity observed with rheological work for Tianan 20%, Aldrich 5 and 12%. As summarized by Ramkumar et al., these protons can easily be cleaved resulting in the breakdown of the molecular

45 weight and chain [6,96]. Furthermore, the thermal and rheological changes observed with

PHBV polymers can be attributed to their low molecular weights. Similar results were also found by Verhoogt and co-workers, and they concluded that lower molecular weight

PHBV polymer is susceptible to chain scission that strongly influences polymer properties [96]. The Aldrich 5% polymer has a molecular weight of 150 kDa, which is lower than commercially available Tianan 5%, 280 kDa. Based on these findings desirable properties for injection molding or blown film extrusion can be achieved by processing Tianan 5% and other PHBV polymers around 160ºC or below. In addition to low temperatures, the screw speed should be minimized to prevent shearing of the polymer entanglements, which would lower viscosity by sheering the polymer.

Figure 4.4: complex viscosity-frequency sweeps of four PHBV materials. A = 150ºC, B =160ºC, C = 170ºC. Pluses = Tianan 20%, Triangles = Tianan 5%, Stars = Aldrich 12%, Circles = Aldrich 5%.

Figure 4.5: complex viscosity-time sweeps at 1Hz for PHBV materials. A = 150ºC, B =160ºC, C = 170ºC. Pluses = Tianan 20%, Triangles = Tianan 5%, Stars = Aldrich 12%, Circles = Aldrich 5%.

4.8.3: Mechanical Properties

Dynamic Mechanical Analysis (DMA) storage modulus (E’) was used to characterize the viscoelastic transition from a brittle to a rubbery state, as shown in Table

4.4 and Figure 4.6. A low frequency of 1 Hz was used since it provides better insight

into physico-chemical interactions at the molecular level of the materials. The storage

moduli transitions were observed between -7 to 10ºC for all PHBV materials, Figure 4.6.

Like conventional thermoplastics, E’ decreased significantly with rising temperature.

Tianan 20% had the highest storage modulus of all the materials followed by Tianan 5%,

Aldrich 12% then Aldrich 5%. These results were surprising since, the higher concentration of HV units in Tianan 20% were expected to decrease the stiffness parameter (storage modulus). One possible explanation for higher stiffness factor is due to limited interaction between PHB and HV units. Although Tianan 5% showed a higher modulus change than Aldrich 12%, this change is likely due to nucleating and anti- oxidizing aids added to Tianan 5% material (according to manufacturer). The small peak

in Tianan 20% (approximately 70ºC) was indicative of the material entering the cold

crystallization region, thus decreasing the storage modulus [101].

Tan delta (δ) is expressed as the ratio of loss modulus (E’’) to storage modulus

(E’) and is highly affected by the material stiffness and density (damping), Figure 6. The

damping peak occurred between 15 to 30ºC, approximately 20ºC above the glass

transition of the PHBV materials. Increased stiffness in the Aldrich 5% and Tianan 5%

are reflected in a decrease in the tan δ peak. Furthermore, the lower HV content limited

the mobility of the polymer chains, thus reducing the damping capacity of the polymer.

Widening of tan δ indicates heterogeneity of the materials, which was also observed with

DSC curves for Tianan 20%, Aldrich 5 and 12%. Tianan 5% had narrower damping peak

suggesting uniform distribution and crystallization effect between PHB homo-polymer

and HV units in the matrix [101]. The DMA results suggest that possible application for

Tianan 5% would be between 20 to 80ºC, since in this temperature range the polymer provides optimal protection against fracturing. Bucci et al. observed similar results noting that PHB provides better protection than polypropylene in this temperature range; whereas, PHB performance was inferior to polypropylene at normal freezing and refrigeration conditions [105].

Table 4.4: Average glass transition temperatures from loss modulus (E’) and storage modulus change in (E’) obtained from DMA. Glass Transition (E’) ∆ Storage Modulus (MPa) Teig (ºC) Tmg (ºC)

Aldrich 5% 0.85 9.83 372.0 Aldrich 12% -6.38 3.65 1491 Tianan 5% 7.27 21.5 1732 Tianan 20% -3.49 7.98 2612

Figure 4.6: Storage modulus (E’), Damping -temperature scans of PHBV at 5 ºC/min.

The mechanical properties of PHBV materials are shown in Table 4.5. Aldrich

5% mechanical properties were not determined due to very porous and brittle samples,

which fractured during cooling off the samples. In general, the results of the polymer

studied indicate that PHBV is a brittle material that is strong and hard based on high

elastic modulus and lower tensile strength observed. The lower tensile strength observed

for Tianan 20% can be attributed to limited interaction between PHB and HV units. On

the other hand, opposite results were observed with Aldrich 12% (4.49%) having higher

elongation at break compared to Tianan 5 and 20% (1.39 and 1.26%). Overall

mechanical properties were better for Aldrich 12% (400-600 kDa) compared to Tianan

materials (270 and 280 kDa), which can be attributed to differences in molecular weight

as found by Kusaka and co-workers [106]. The difference in elongation at break between

Aldrich 12% and Tianan 5% is due to more embrittlement of Tianan 5%. Barham et al.,

suggested that micro-fractures and cracks are present when spherulites are grown from the melt state [107,108]. Storage is another condition leading to embrittlement in PHBV material after initial crystallization from the melt [109]. This embrittlement causes the reorganization of lamellar crystals formed during the initial crystallization, which tightly constrains the amorphous chains between crystals. Annealing the polymer after the initial crystallization will produce a better polymer with improved mechanical properties and decreased embrittlement [106,110].

Table 4.5: Mechanical properties of PHBV materials [9,96,101] Samples Tensile Elongation at Flexural Elastic Strength Break (%) Strength Modulus (MPa) (MPa) (GPa) T20% 15.2 1.26 29.6 8.37 T5% 22.1 1.39 52.3 13.0 A12% 20.9 4.49 154 8.67 Biopol 12% HV 23.0 18.8 30.0 1.25 Tianan 1% HV 19.7 0.170 39.1 3.65 *Aldrich 5% material was not tested

Similar tensile strength to that of the Biopol material was observed for Aldrich

12% (Table 4.5), but the elongation at break and flexural strength varied significantly for

Aldrich 12% [96,101]. In addition Wang et al. tested PHBV with 1% HV content (Mw of

130 kDa), and there results are summarized in Table 4.5 [9]. Khanna et al. reported that

an increase in HV content results in decreasing tensile strength with 20% HV content

having a tensile strength of 32 MPa [24]. This value is double the tensile strength value

observed for Tianan 20% (15.2 MPa) [24]. The differences in the mechanical, thermal,

and rheological characteristics observed in this work with previous research may be due

to differences in the polymer molecular weight. Such disparity can be attributed to genre

of bacteria, carbon source, fermentation technique, downstream processing, and other

factors that result in different PHBV polymer with unique molecular weight and structure

[24].

Mechanical and barrier properties represent major challenges for scientists to

overcome for the development of a successful biopolymer. The hydrophilic nature of

PHBV makes it a poor water vapor barrier, reflected in the high water vapor transmission

rates observed (Table 4.5). Next to the barrier properties, the mechanical properties of the

PHBV polymers are also important. It has been reported that HV units reduces

crystallinity, melting temperature, and stiffness but increase toughness and impact

resistances [111]. By manipulating the PHB to HV ratio in the random copolymer, the

copolymer can resemble either polypropylene (PP) (low HV content) or LDPE (high HV

content) in flexibility and tensile strength. The elastic modulus of Tianan 5% (13.0 GPa) was higher compared to traditional thermoplastic such as PET (2.8-4.1 GPa), PS (2.3-3.3

GPa), LDPE (0.2 GPa), and PP (1.7 GPa), suggesting that the PHBV polymer is flexible

above the glass transition temperature to provide potential applications quick serve restaurants [88,89]. However, these plastics can absorb higher stain before breaking as measured by elongation at break. Tianan 5% had elongation at break value of 1.39% compared to PET (30-300%), PS (2.50%), LDPE (100-1000%), and PP (400%) [88,89].

Finally, Tianan 5% (22.1 MPa) had similar tensile strength value to LDPE (8-20 MPa); but, the tensile strength differed greatly from PET (48-72 MPa), PS (34 to 50 MPa), and

PP (38 MPa) [88,89].

4.8.4: Barrier Properties

The average water vapor transmission rates (WVTR) of five samples are summarized in Table 4.6. The values of WVTR ranged from 63.1 to 440 g/m2/day. The

5% HV materials showed large disparity with Tianan 5% (372 g/m2/day) having a transmission rate significantly higher than Aldrich 5% (120 g/m2/day). The methyl and

ethyl groups present in the PHBV polymer reduced the water vapor transmission rate due

to the steric hindrance effects causing the chain flexibility to decrease [100,112,113].

However, these hydrocarbon groups did not lower the WVTR; the transmission rates

increased with increasing concentration of the hydroxyvalerate units in the Tianan

materials.

Shogen et al. observed increasing transmission rates for three PHBV materials

with 6, 12, and 18% hydroxyvalerate content and reported transmission rates of 124, 204,

and 245 g/m2/day, which was significantly lower than 327 and 440 g/m2/day for Tianan 5

and 20% [100]. The discrepancy in these results may be attributed to differences in

crystallinity between these materials, with Shogen et al. reporting crystallinity from 74 to

62% for PHB with 6, 12, and 18% hydroxyvalerate content [100]. Increasing the amounts

of 3-hydroxyvalerate units will lead to a decrease in the crystallinity of the PHB homo-

polymer and an increase in the WVTR. Crystals in a polymer reduce the water

transmission due to their small cross-sectional and low permeability restricting chain

mobility, thus lowering water permeability [100,112,113]. A semi-crystalline polymer

has a higher WVTR above Tg since polymer chains can slide past one another increasing

the free volume and thus allowing the water molecules to diffuse through the materials.

The percentage of crystallinity was not determined in this study.

Table 4.6: Water vapor transmission rates of PHBV films. Samples WVTR (g/m2/day) T20% 440 (±126) T5% 372 (±142) A12% 63.1 (±22.7) A5%* 120 (33.0) * A5% was performed using solution casted films.

The Aldrich 12% showed good mechanical and barrier properties that are required

for various packaging applications. Although other researchers have reported lower

WVTR, such discrepancies may be attributed to different manufacturers of PHBV 54

polymer and wide range of processing conditions from solution casting to commercially extruding the films [100,112,113]. WVTR for Tianan 5% is very high compared to low

WVTR for traditional thermoplastic such as poly (ethylene terephthalate) (PET, 3.48 g/m2/day), oriented polystyrene (OPS, 5.18 g/m2/day), and low-density polyethylene

(LDPE, 7.90 g/m2/day) [93-95,111].

Tianan 5% polymer was processed in a lab scale extruder at 175ºC. The

brittleness and low melt strength made it difficult to extrude blown films. The polymer

began to fracture immediately when exiting the blown film die. In addition, the low melt

strength caused the molten polymer to sag as the polymer was cooled by the reel system.

The use of PHBV packaging applications has been limited due to its performance,

processing and cost compared to traditional thermoplastics [105]. However, there is

continuing effort to find successful applications of PHBV in packaging as suggested by

Bucci et al. [105]. In addition, this work has spawn interest in blending PHBV with commercially available poly(lactic acid) (PLA) as a viable way to improve PHBV thermal, mechanical, and rheological properties.

4.9: Conclusion

The ultimate aim of this work was to understand the thermal, rheological, mechanical, and barrier properties of PHBV from various manufacturers with different valerate content. The difference in valerate content showed two melting transitions for non-commercially available PHBV co-polymers (Aldrich 5, 12% and Tianan 20%), suggesting an immiscible blend between the PHB homo-polymer and HV co-monomer.

However, the TGA curves showed minimal separation between the PHB homo-polymer

and PHV plasticizing units. The rheological data showed that the Tianan 5% polymer is

thermally stable in the melting temperatures. However, Aldrich 5, 12 % and Tianan 20%

PHBV polymers are thermally unstable at temperatures higher than 160ºC. This

instability can be attributed to the breakdown of the polymer chain via the random chain

scission process resulting into lower molecular weight that resulted in decreasing

complex viscosity at higher testing temperatures.

The mechanical and barrier properties observed were greatly affected by the

molecular weight of the polymers. The PHBV material is brittle with high elastic modulus and low tensile strength, thus making it strong and hard material. Overall,

mechanical properties of Aldrich 12% were better than the Tianan materials due to the

higher molecular weight of the Aldrich polymer. Additionally, commercially available

Tianan 5% had higher modulus of elasticity, while lower tensile strength and elongation

at break compared to traditional thermoplastic used for packaging. Furthermore, the water vapor transmission rate was very high compared to thermoplastic used for packaging, which is due to wide range of processing conditions, HV units, and molecular weights.

Chapter 5

Miscibility of Poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) and Poly (Lactic

Acid) blends Determined by Thermal Analysis.

Sunny Modi1, Kurt Koelling2, Yael Vodovotz1

1Ohio State University, Department of Food Science & Technology, 110 Parker Food Science & Technology Bldg. 2015 Fyffe Road, Columbus, Ohio 43210 USA. 2 Ohio State University, Department of Chemical and Biomolecular Engineering, 125 Koffolt Laboratories, 140 West 19th Avenue, Columbus, Ohio 43210 USA.

Abstract:

Miscibility of poly (3-Hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with poly

(L-lactic acid) (PLA) were studied using differential scanning calorimetry (DSC) and

thermogravimetric analysis (TGA). Three different grades of commercially available

PLAs and one type of PHBV were blended in different ratios of 50/50, 60/40, 70/30 and

80/20 of PHBV/PLA using a micro-compounder at 175ºC. According to Natureworks,

PLA grades 3051D and 4042D are designed for injection molding and extrusion

equipments. In addition, PLA 4042D was designed to be used in biaxially oriented films

with great clarity, machinability, good barrier properties. PLA 6202D grade is designed

for fiber spinning and drawing, and this grade can be used to make various fiber products.

DSC and TGA analysis indicated the blends were immiscible, yet some compatibility

was observed due to influence on the respective Tg’s of the individual materials.

Key words: Poly (3-Hydroxybutyrate-co-3-hydroxyvalerate) (PHBV); Poly (L-lactic

acid) (PLA); biomaterials; Differential Scanning Calorimetry (DSC); Thermogravimetric

Analysis (TGA)

5.1: Introduction

Biopolymers have generated vast interest during this decade due to rapid growth

in municipal waste, consumer awareness, and stricter government regulations [6,24].

Currently, aliphatic polyesters derived from fermentation such as poly-(ε-caprolactone)

(PCL), poly(L-lactic acid) (PLA), and poly-(3-hydroxybutyrate-co-3-hydroxyvalerate)

(PHBV) have potential to alleviate the waste disposal problem of diminishing landfill

space [4,6,9]. These polymers are well known to have the ability to fully biodegrade

because of their hydrolyzable backbones. In addition, being aliphatic polyesters, these

polymers provide a range of mechanical properties that can be manipulated from

packaging to medical applications.

Fermentation of agriculture crops and waste products yields poly (L-lactic acid)

(PLA) polymer, a semi-crystalline structure composed of three carbon monomers chain with hydroxyl and carboxyl groups at each end [6,7]. PLAs are produced from ring opening polymerization of lactide units resulting in D- and L- lactic acid monomers. The

ratio of L to D- monomer units affects the degree of crystallinity, melting temperature and

machinability [8]. Potential advantages of PLA are high mechanical strength,

thermoplastic characteristics, biocompatibility, and being derived from renewable

recourses [6,9,10,11]. On the other hand, the brittleness, thermal instability, and poor

water vapor barrier properties are potential drawbacks for these resins [6,9,11].

The physical properties of PLA polymers are related to their enantiomeric purity

of the lactic acid stereo-copolymer. Depending on the amounts of L-, D-, and meso- lactide, high molecular weight PLAs are either amorphous or semi-crystalline at room temperature. [57,77] Amorphous PLA polymers are completely brittle below the beta- relaxation temperature (Tβ) of -45ºC, and near Tg (58ºC) PLA undergoes physical aging,

thus transitioning from brittle to rubbery state. [57] At temperatures of 110-150˚C, the

polymer transitions from rubbery to viscous the exact temperature depending on the

molecular weight and shear stress. For semi-crystalline PLA polymers, the Tg indicates

the thermal transition from brittle to rubbery state. The melting temperature is determined from the stereochemistry of the PLA polymer. With increasing concentration of meso-

lactide units, the melting temperature decreases. Pure L- or D- PLA has a melting temperature of 180˚C, and the melting temperature increases with increasing molecular weight. Commercially available PLA exhibits a Tg around 50 to 80˚C, while the melting

temperature ranges from 130 to 180˚C. [57,78]

Bacterially synthesized poly(hydroxybutyrate) (PHB) are widely studied by researchers and considered a type of poly(hydroxyalkanoate) (PHA’s) [4]. Commercial use of PHB is limited by low availability, cost of the polymer compared to conventional thermoplastics, narrow processing temperatures due to degradation, and brittleness at

room temperature [4,10,11]. For many applications, flexibility of a polymer is required at

low temperatures. Incorporation of 3-hydroxyvalerate (HV) during the fermentation

process improves these properties and results in lower melting points of the copolymer.

This broadens the processing window and decreases brittleness [5,24]. A nine-carbon

59 polymer with hydroxyl and carboxyl groups at the ends of the structure is produced [6].

Therefore, the polymer could be tailored to specific applications that are similar to the conventional thermoplastics [6]. In addition, incorporation of HV content in the backbone helps in improving the melt stability at lower processing temperatures.

Individually PHBV and PLA polymers have serious disadvantages when compared to thermoplastics that are currently used. To address high costs and thermal instability, blends of PHBV with PLA were explored as an alternative way of acquiring novel materials with desired properties. Numerous researchers have studied blends of these polymers as potential applications in packaging to medical sutures. For example,

Zhang et al. prepared films by dissolving PLA and PHB in chloroform for film applications, Wang et al. studied the physical properties of co-extruded PLA and PHBV polymer blends, and Ferreira et al. studied PLA/PHBV blended pins for stabilizing bone fractures [9,10,12]. These researchers focused on two key factors: the miscibility of PLA and PHBV polymers and the potential improvements of the physical properties [9,10,12].

Low molecular weight PLA (mw < 18,000) blended with PHB at 200ºC resulted in a miscible blend [114-116]. In addition, miscible blends of PLA and PHB (10% PHB by wt) resulted in a new type of polymer alloy improved properties compared to individual polymers [10,117]. In these studies, low molecular weight PLA (1.8 and 43 kDa) produced by microbial fermentation was used [115,120]. Current commercial grades of PLA are made by ring-opening polymerization that yields higher molecular weight polymers. Little is known about blends of PHBV with these new commercially available PLA’s. Therefore, the aim of this study was to characterize thermal properties

of three PLA resins blended with PHBV, and to assess the effect of varying PHBV

concentrations on these properties in the blends.

5.2: Materials

Commercially available PHBV with 5% HV content and a molecular weight of

280 kDa (according to the manufacturer) was obtained from Tianan Biologic Material

Co. (Ningbo, China). Ingeo brand poly(L- lactic acid) was obtained from Natureworks

LLC (Minnetonka, Minnesota) in three different concentrations of D, L -lactide units.

PLA resin 3051D had a weight average molecular weight of 160 kDa and 96% L-Lactide

to 4% D-Lactide units. PLA grade 4042D had a weight average molecular weight of 200

kDa and 92% L-Lactide to 8% D-Lactide units. The final grade of PLA was 6202D had a

weight average molecular weight of 140 kDa with 98% L-Lactide to 2% D-Lactide units,

according to Natureworks LLC.

5.3: Blend Preparations

Blends for each grade of PLA with Tianan 5% PHBV were made consisting of

20, 30, 40, and 50% PLA. The two polymers were blended using a Daca instrument

micro-compounder (Santa Barbara, California) at 175ºC for 10 minutes with a screw

speed of 100 rpm. The blended rods were subjected to thermal characterization.

5.3: Thermal Analysis

5.3.1: Degradation Temperature of Blends

Thermogravimetric analysis (TGA) was used to study the weight loss properties

as a function of temperature for all the blended rods using a TGA 5000 model from TA

Instruments (New Castle, Delaware). The blended samples were heated under nitrogen

61 environment from 150 to 500ºC with a heating rate of 20ºC/min. Samples were vacuum dried at 60ºC for 24 hours prior to any testing. The reported values are averages of at least three samples

5.3.2: Phase transition of Blends

Differential Scanning Calorimetry (DSC) was used to determine the glass and melting behaviors. DSC analysis was performed on a TA Instruments Model Q100 (TA

Instruments New Castle, Delaware). Blended samples were vacuum dried at 60ºC for 24 hours prior to any testing. The melting transitions were determined using 5 to 10 mg samples, and heated from -20ºC to 200ºC with a heating rate of 10ºC/min. The samples were allowed to anneal at 200ºC for five minutes to remove the thermal history of the polymers. Subsequently, samples were cooled to -20ºC, held for five minutes, and reheated from -20ºC to 200ºC with a heating rate of 10ºC/min. The glass transitions were determined from the second heating with a sample size of 10 to 20 mg, and a heating rate of 20ºC/min. The reported values are averages of at least three samples

5.4: Results and Discussion

5.4.1: Phase Transitions of Blends

Previous researchers have shown that PHBV is susceptible to thermal degradation, which causes broader endotherm and shift of the highest melting peak to lower temperature in the DSC thermograms [9,103,104]. Based on these findings, comparison between the first and second heating thermograms was studied. (Table 5.1)

For the first melting thermograms, T5%-3051D at a 50/50 concentration had two melting transitions, while other blend concentrations (60/40, 70/30, and 80/20) had three

transitions, Figure 5.1. The melting transitions near 150 and 168ºC were attributed to pure PLA and PHBV, respectively, since the pure polymer melts occurred at similar temperatures. (Figure 5.1) The highest temperature peak of 181ºC can be attributed to

PHB homo-polymer, as was noted previously by Chen and co-workers [5]. It is interesting to note that as the concentration of Tianan 5% in the blend was increased, more of the PHV polymer phase separated as indicated by increasing melting peak observed near 168ºC. These results suggest that a higher concentration of PLA is required to achieve a more stable blend by preventing the PHBV from disassociating. A rescan (Figure 5.2) was performed after annealing the sample for five minutes at 200ºC and results showed one large melting peak at 165ºC and a residual peak (150ºC) from the remaining PLA crystals for all concentrations of T5%-3051D. The large peak observed was similar to the pure Tianan 5% with a shoulder at 171ºC, indicating some separation of the PHBV co-polymer into individual PHV and PHB crystals. However, 50/50 blend showed two melting peaks suggesting that the polymers melted as individual polymers rather than a homogenous mixture of both polymers.

Table 5.1: Average melting temperatures of T5%-3051D at four concentrations obtained via DSC. T5-3051D First Scan (˚C) Second Scan (˚C) Cry. (˚C) Tpm1 Tpm2 Tpm3 Tpm1 Tpm2 Shoulder 0/100 162 ------50/50 150 - 182 149 166 172 122 60/40 150 168 183 149 166 171 120 70/30 151 169 184 149 165 171 121 80/20 152 169 183 148 165 170 116 100/0 171 - - 165 - - - Tpm = peak melting temperature.

Figure 5.1: First melting thermograms of Tianan 5% blended with PLA 3051D at 10ºC/min obtained via DSC.

Figure 5.2: Second melting thermograms of Tianan 5% blended with PLA 3051D at 10ºC/min obtained via DSC

The glass transitions obtained from the second heating scan showed two distinct

transitions attributed to PHBV and PLA that are independent of blend compositions when

cooling the sample from 200ºC (data summarized in Table 5.2). The materials showed

relatively no change in the glass transition temperature when compared to the pure

Tianan 5% material. On the other hand, the glass transitions for PLA component of the blend decreased by 5°C when compared to pure PLA material. Therefore, separate glass and melting transitions suggest that Tianan 5% and PLA 3051D blends are immiscible in

their amorphous state. However, it is interesting to note that the glass transition of 50/50

blend was affected more than the other combinations suggesting that some interaction

occurred between the polymers at this concentration. Zhang et al. determined that these

interactions between PHB and PLA are due to weak bonding between alpha methylene

groups of PHB to carboxylic group of PLA [10]. Matsuura et al. reported similar types

of hydrogen bonding between the two polymers [118].

Table 5.2: Average glass transitions of T5%-3051D at four concentrations obtained via DSC. T5-3051D Glass Transition (PHBV) Glass Transition (PLA) Onset (˚C) Mid-Point (˚C) Onset (˚C) Mid-Point (˚C) 0/100 - - 57.8 61.5 50/50 -1.55 3.43 50.7 55.7 60/40 -1.90 3.52 51.5 55.4 70/30 0.498 6.67 52.2 57.1 80/20 1.02 6.62 53.9 58.4 100/0 -2.60 4.90 - -

For all concentrations of the Tianan 5% and PLA 4042D blends (T5%-4042D), three distinctive peaks were observed with increasing concentrations of PHBV in the first melting thermograms using DSC (Table 5.3). The melting peaks were observed near 150,

170, and 182ºC, and as discussed previously for the T5%-3051D, were attributed to PLA,

PHV, and PHB respectively. Similar results were observed by Long et al, for Tianan

PHB with 15% HV content [5]. When the first and second heating thermograms were

compared the results showed two unique melts (150 and 165ºC) with a shoulder (170ºC)

for three concentrations (50/50, 60/40, and 70/30) in the second heating. The 80/20 melt

showed three distinguishable melts at 149, 163, and 170ºC, suggesting that the co-

polymer separated into three pure polymers. With the addition of PLA, lesser affect on

the melt stability of PHBV was observed. Due to the heterogeneous nature of these

blends, they are most likely difficult to process in a conventional extruder.

Table 5.3: Average melting transitions of T5%-4042D at four concentrations obtained via DSC. T5-4042D First Scan (˚C) Second Scan (˚C) Cry. (˚C) Tpm1 Tpm2 Tpm3 Tpm1 Tpm2 Shoulder 0/100 148 ------50/50 149 168 183 149 166 171 124 60/40 151 169 183 149 165 171 124 70/30 148 169 186 149 166 171 124 80/20 149 170 183 149 163 170 101 100/0 171 - - 165 - - - Tpm = peak melting temperature.

The glass transition of Tianan 5% PHBV blended with PLA 4042D also showed

similar results to T5%-3051D. The PHBV glass transition increased slightly (3ºC)

approaching the glass transition of PLA, as seen in Table 5.4. The glass transition of the

PLA component decreased by 8ºC as the composition of PLA was decreased from 100%

to 20%. Therefore, it can be surmised that some compatibility of these polymers was achieved in the amorphous regions.

Table 5.4: Average glass transitions of T5%-4042D at four concentrations obtained via DSC. T5-4042D Glass Transition (PHBV) Glass Transition (PLA) Onset (˚C) Mid-Point (˚C) Onset (˚C) Mid-Point (˚C) 0/100 - - 58.4 62.0 50/50 0.418 6.54 51.0 56.4 60/40 -0.0700 4.96 52.3 55.6 70/30 -0.407 4.96 50.2 55.6 80/20 0.093 4.54 53.5 57.7 100/0 -2.60 4.90 - -

Tianan 5% blended with PLA 6202D (T5%-6202D) polymers showed different melting temperatures at various concentrations of PLA (Table 5.5). The first heating scan for 50/50 and 70/30 concentrations showed three melting temperatures near 163, 170, and

183ºC for PLA, PHV, and PHB respectively. However, 60/40 and 80/20 showed two melting transitions near 162 and 181ºC for PLA and PHB respectively. These fewer melting transitions implies greater compatibility of the polymers. On the second heating scan after annealing for five minutes at 200ºC, the 60/40 and 80/20 concentrations had separated into two separate peaks compared to a large heterogeneous peak with a shoulder at 165ºC for 50/50 and 70/30 concentrations. The two separate peaks were attributed to the PHBV co-polymer separating into individual units as was observed previously for the other PLA types. These results indicate that the 50/50 blend may lead to best compatibility. The glass transition of T5%-6202D blends showed similar patterns to the other blended polymers, Table 5.6. The first glass transition increased for PHBV portion, while PLA portion of the blends decreased.

Table 5.5: Average melting transitions of T5%-6202D at four concentrations obtained via DSC. T5-6202D First Scan (˚C) Second Scan (˚C) Cry. ( ˚C) Tpm1 Tpm2 Tpm3 Tpm1 Tpm2 0/100 162 - - - - - 50/50 163 170 183 166 170 104 60/40 162 - 181 157 167 104 70/30 163 171 183 165 171 98.6 80/20 165 - 182 163 170 97.6 100/0 171 - - 165 - - Tpm = peak melting temperature.

Table 5.6: Average glass transitions of T5%-6202D at four concentrations obtained via DSC. T5-6202D Glass Transition (PHBV) Glass Transition (PLA) Onset (˚C) Mid-Point (˚C) Onset (˚C) Mid-Point (˚C) 0/100 - - 58.0 61.0 50/50 0.130 7.67 52.0 57.7 60/40 0.355 6.06 50.9 56.0 70/30 0.298 7.27 50.6 56.9 80/20 -0.435 7.83 54.5 59.5 100/0 -2.60 4.90 - -

5.4.2: Thermal Decompositions of Blends

Thermogravimetric analysis (TGA) curves are shown in Figure 5.3 for the T5%-

3051D blends and summarized for all the blends in Table 5.7. The TGA results indicated that PHBV and PLA polymers tend to degrade individually with peak degradation occurring approximately at 280ºC and 350ºC respectively. In addition, the peak degradation decreased for PLA with increasing concentration of PHBV. On the other hand, no significant changes in the peak degradation temperature were observed for the

PHBV polymer. Thus, the TGA data supports the DSC analysis that these polymers are mostly immiscible.

Table 5.7: Thermal degradation temperature of blended polymers with four different concentrations obtained via TGA. PHBV PLA Tonset (ºC) Tpeak (ºC) Tonset (ºC) Tpeak (ºC) T5%-3051D 0/100 - - 293 381 50/50 220 280 305 362 60/40 223 279 303 355 70/30 228 279 301 350 80/20 228 282 302 341 100/0 238 288 - - T5%-4042D 0/100 - - 303 390 50/50 227 283 309 365 60/40 239 284 308 362 70/30 231 284 307 354 80/20 227 285 306 341 100/0 238 288 - - T5-6202D 0/100 - - 293 390 50/50 229 287 312 364 60/40 229 286 310 361 70/30 227 287 308 354 80/20 223 284 307 337 100/0 238 288 - - Tonset = beginning mass loss; Tpeak = maximum mass loss

Figure 5.3: TGA thermograms of Tianan 5% blended with PLA 3051D at four concentrations (20, 30, 40, and 50% PLA).

Results for this study are consistent with by Zhang et al., and demonstrate that commercially available PHBV and PLA blends are most compatible for PLA 6202D and higher PLA concentrations for the conditions and materials tested. However, in practical terms, these materials would be difficult to process in an extruder, injection molding, or other processing units. For example, if the higher melting temperature were used then the lowest melting temperature for PLA would start to thermally decompose. According to

Hartmann et al., PLA quickly loses its thermal stability when heated above it melting point. They found significant molecular weight deterioration when PLA was held 10ºC above the melting point for significant amounts of time [72]. According to Migliaresi and co-workers, the molecular weight deterioration is attributed to PLA’s chain splitting [85].

In addition Auras et al. summarized PLA’s poor thermal stability due to: hydrolysis by trace amounts of water that is catalyzed by hydrolyzed monomer (lactic acids); zipper like depolymerization, catalyzed by the remaining polymerization catalyst; oxidative random-chain scission; intermolecular transesterification to monomer and oligomeric esters; intermolecular tranesterification resulting in the formation of monomer and oligomeric lactides of low molecular weight [57].

Yet other researchers have argued that blending these polymers at a higher temperature of 200ºC resulted in greater miscibility due to transesterification between

PHBV and PLA chains [119]. Ikehara et al. believed that miscible crystalline blends are

created from a homogenous melts, where the melting temperatures of the two polymers

are very similar [119]. However, immiscible blends are observed when the differences

between the melting temperatures of the two polymers are large. For immiscible blends,

the higher melting polymer crystallizes first causing the spherulites to fill-in the available volume [10,119,120]. The lower melting polymer is spatially limited inside the spherulites of other polymer [10,119,120]. When the polymers have similar melting

temperatures, both polymers have the ability to co-crystallize [10,120]. Zhang and

Ikehara et al. reported that the melting point difference in most miscible crystalline blends is less than 100ºC [10,119]. More specifically, the melting point difference between PLA and PHBV in their work was 47ºC, which caused simultaneous crystallization [10,119,120]. In this study, the differences between PLA and PHBV melting temperatures were 9 and 23ºC for PLA 3051D and 6202D respectively, yet no simultaneous crystallization occurred possibly due to higher molecular weight of the

PLA’s in this study compared to Zhang and Ikehara et al.[10,119]

5.5: Conclusion

The aim of this study was to characterize the thermal properties of commercially available polymers Tianan 5% blended with 0 to 50% PLA grades 3051D, 4042D, and

6202D at 175ºC using a micro-compounder. Melting, glass transition and decomposition data indicate a significant immiscibility of these various blends. However, promising results were found with the 50/50 blends of T5%-6202D. Future work will focus on adding melt stabilizers by various manufacturers to improve compatibility and process ability of this blend.

Chapter 6

Mechanical and Rheological Properties Poly-(3-hydroxybutyrate-co-3- hydroxyvalerate) and Poly (Lactic Acid) blends.

Sunny Modi1, Kurt Koelling2, Yael Vodovotz1

Abstract

The mechanical and rheological properties of poly-(3-hydroxybutyrate-co-3-

hydroxyvalerate) (PHBV) and poly(Lactic Acid) (PLA) blends were studied. Three

different blends of commercially available PLAs and one grade of PHBV were blended

using a micro-compounder at 175ºC. The composition of PHBV in blends ranged from

50 to 80%. With the addition of PLA, the flexural strength and elastic modulus of the

polymer increased significantly. However, the addition of PLA caused the elongation at

break and tensile strength to decrease compared to pure PHBV material. The complex viscosity of the blends was measured with respect to frequency and time at 160 and

170ºC. Like many conventional plastics, the complex viscosity decreased with increasing rotational frequency due to decreasing entanglements and molecular weight. In addition,

four subsequent scans were performed and no changes in the initial complex viscosity

were seen for many of the blends at 160 and 170ºC. The complex viscosity with respect

to time was very stable for the blends, but no improvements in the PHBV viscosity were

seen with the addition of PLA at 170ºC.

Key words: Poly (3-Hydroxybutyrate-co-3-hydroxyvalerate) (PHBV); Poly (L-lactic

acid) (PLA); biomaterials; blends; mechanical properties; rheological properties

6.1: Introduction

The success of fossil fuel over the past 60 years can be attributed to various factors such as abundant supply, thermoplastic processing ability, and numerous applications [2]. However, in recent years, formation of green house gases and saturation of landfills with non-degradable thermoplastic wastes has lead to exploration of more environmentally friendly polymeric materials. Aliphatic polyesters such as polyglycolic acid, polybutylene succinate, polycaprolactone, polylactic Acid, and poly-(3- hydroxybutyrate) can be used to alleviate landfill saturation due to their hydrolyzable backbones, which allows bacteria and enzymes to degrade these polymers [4,6,9,13]. In addition, these polymers have similar properties to conventional thermoplastic, which enables their use in numerous applications ranging from packaging to medical applications [4,6,13].

Polylactide or Poly(L-lactic acid) (PLA) are interchangeable terms used to

describe aliphatic polyesters, but the difference in terminology indicates the synthesis

method chosen to produce PLA from the lactic acid monomer [27]. In general, PLA is

produced from esterification of lactic acid during fermentation via direct and indirect

condensation and chain extending reactions resulting in D and L lactic acid monomers

[57,77]. According to Auras et al., polymerization through lactide formation is widely

used to make commercial PLAs. This fermentation process yields a semi-crystalline structure composed of three-carbon monomers chain with hydroxyl and carboxyl groups

at each end [6,7]. The ratio of L to D- monomer units affects the degree of crystallinity, melting temperature, and machinability [8]. Potential advantages of PLA are its high mechanical strength, thermoplastic characteristics, biocompatibility, and being derived from renewable recourses [6,9,10]. On the other hand, brittleness, thermal instability,

and poor water vapor barrier properties are potential drawbacks for these resins [5,6,9].

Bacterially synthesized poly-(3-hydroxybutyrate) (PHB) are widely studied by

researchers and considered a type of poly(hydroxyalkanoate) (PHA’s) [4]. Commercial

use of PHB is limited by low availability, cost of the polymer compared to conventional thermoplastics, narrow processing temperatures due to degradation, and brittleness at

room temperature [4,11,12,120]. For many applications, flexibility of polymer is required

at low temperatures. Incorporation of 3-hydroxyvalerate (HV) during the fermentation

process improves these properties and results in lower melting points of the copolymer.

This broadens the processing window and decreases brittleness [4,5]. A nine-carbon

polymer with hydroxyl and carboxyl groups at the ends of the structure is produced [6].

Therefore, the polymer would be tailored to specific applications that are similar to the conventional thermoplastics [6]. In addition, incorporation of HV content in the backbone helps in improving the melt stability at lower processing temperatures.

Previous researchers have shown these blend provides more optimal physical and

mechanical properties not attainable from individual polymers [4,11]. Zhang et al.

investigated PHBV blended with PLA and determined that PLA can improve toughness

properties of PHBV. Gao et al. determined that PLA helped reduce the crystallinity of

PHBV [121]. However, many of the PHBV polymers tested are not commercially available, or the manufacturing of these polymers is no longer available. Although numerous researchers have tested pure PHBV and PLA materials, little information is known regarding the rheological behavior of PLA blended with PHBV materials [6,7,92].

Rheological studies are conducted to determine flow properties of polymers using various

geometries [92]. In addition, flow testing can predict molecular changes between

entanglements that occur when the polymer is exposed to various strains and stresses

[7,92]. Such characterization is critical for assessing the utility of these polymers in

various processing conditions and in developing equations that describe behaviors of the

polymers in melt state with various deformations history [6]. Thus, the objective of this

study was to assess the effect of combining 20-50% PLA on the mechanical and

rheological properties of PHBV

6.2: Materials

Commercially available PHBV with 5% HV content and a molecular weight of

280 kDa was obtained from Tianan Biologic Material Co. (Ningbo, China), according to

the manufacturer. Ingeo brand Poly (L- lactic acid) was obtained from Natureworks LLC

(Minnetonka, Minnesota) in three different concentrations of D, L- Lactide units. PLA

resin 3051D had a weight average molecular weight of 160 kDa and 96% L-Lactide to

4% D-Lactide units. PLA grade 4042D had a weight average molecular weight of 200

kDa and 92% L-Lactide to 8% D-Lactide units. The final grade of PLA 6202D had a

weight average molecular weight of 140 kDa with 98% L-Lactide to 2% D-Lactide units,

according to Natureworks LLC.

6.3: Blend and Sample Preparations

Blends for each grade of PLA with Tianan 5% PHBV were made consisting of

20, 30, 40, and 50% PLA using a Daca instrument micro-compounder (Santa Barbara,

California) at 175ºC for 10 minutes with a screw speed of 100 rpm. The blended rods were compression molded (Carver Heated Press Model 3851-D Wabash, Indiana) into

dumbbell and rectangular shaped bars at 183ºC. For rheological characterizations, 25.0

mm disks with 1.0 mm thickness were compression molded at 183ºC using the same

Carver Heated Press (Wabash, Indiana).

6.4: Mechanical Properties

6.4.1: Tensile Properties

Tensile properties were determined according to ASTM D638-08. Prior to any

tensile testing, the samples were vacuumed dried for 24 hours at 60ºC. The tensile data

was obtained using an Instron 5542 with Bluehill v. 2.17 software package (Instron Corp.

Norwood Massachusetts). Dumbbell shaped samples (62 x 10 x 1.0 mm3) with the gage

section of 3.0 mm, and a crosshead speed of 5 mm/min at 26.0ºC. In addition, a pre-load

of two MPa was applied to prevent slippage of the samples.

6.4.2: Flexural Properties

Flexural properties were determined according to ASTM D790-07, and prior to

any testing, the samples were vacuumed dried for 24 hours at 60ºC. Rectangular bars (51

x 7.0 x 2.0 mm3) were tested using TA Instrument RSA3 unit (New Castle, Delaware)

with an approximate crosshead speed of 2.2 mm/min at 26.0ºC. The reported values are

averages of at least five samples.

6.5: Rheological Properties

Rheological behaviors were obtained using a TA Instrument Ares 2000 (New

Castle, Delaware). Sample disks were vacuum dried at 63ºC for 24 hours prior to testing.

Rheological experiments were performed at 160 and 170ºC using 25 mm parallel plate

system. The disks were equilibrated for five minutes before the gap was set to the testing

position of approximately 0.9 mm or until the top plate made contact with the top of disk using Leica light. The furnace was opened to remove excess material. Dynamic viscosity was measured with increasing time and frequency. Frequency sweeps ranged from 0.1 to 100 rad/sec by conducting four sequential sweeps and measuring the changes in viscosity with respect to frequency. Time sweep was performed for one hour using a constant frequency of 1 Hz. For each frequency and time sweeps, the linear viscoelasticity limits were determined.

6.6: Results and Discussion

6.6.1: Mechanical Analysis

The mechanical properties are summarized in Table 6.1. In general, the PHBV

polymer is hard and strong based on high elastic modulus and low elongation at break.

For the Tianan 5% blended with PLA 3051D (T5-3051D), the 50/50 and 70/30

concentrations displayed better tensile properties compared to Tianan 5% material, Table

6.1. Similarly Wang et al., reported higher tensile strength (49.7 MPa) and elongation at

break (2.07%) for 70/30 blends of PHB with 1-2% HV content blended with Natureworks

PLA The flexural strength decreased for all concentrations. Wang et al. reported a higher flexural strength (75.0 MPa) for 70/30 concentration, while this study observed a significantly higher flexural modulus of 17.9 GPa compared to theirs (3.51 GPa) [9]. The elastic modulus was significantly improved (38 and 170%) by the addition of 30 and 50%

PLA. The lower concentration of PLA had an adverse effect on PHBV polymer by decreasing the tensile (37%) and flexural properties (71%). Overall, the 50/50 concentration of Tianan 5% and PLA 3051D showed improvement in tensile and flexural properties compared to other concentration and Tianan 5% polymer.

Tianan 5% blended with PLA 4042D (T5-4042D) results are displayed in Table

6.1. The blended polymers showed no significant improvements in tensile strength when compared to Tianan 5%. The 50/50 and 60/40 blends resulted in significant improvements in flexural strength (121 and 23%) and elastic modulus (200 and 210%).

In addition, a 21% improvement in the 50/50 and 17% improvements in 60/40 blends over pure PHBV material were observed for elongation at break. Similar to Tianan 5% blended with PLA 3051D, the 50/50 showed better mechanical properties compared to pure PHBV material and other blends.

The last PLA grade 6202D blended with Tianan 5% showed similar properties as

T5-4042D, and these results are summarized in Table 6.1. The 50/50 blend had

significant impact on the flexural strength and elastic modulus. The flexural strength

improved by 117% with the addition of PLA, while the elastic modulus increased by 26%

over the pure PHBV materials. The addition of PLA did not improve the tensile

properties of PHBV for all concentrations. However, the 70/30 concentration resulted in

a 10% increase in elongation at break. As seen with the other blends, the 50/50 blend

showed significant improvement in the flexural properties, while the 70/30 blend showed

similar properties to Tianan 5%. The addition of different PLA grades at lower

concentration had adverse effect on Tianan 5% by decreasing the flexural and tensile

properties of the PHBV polymer, as seen with 80/20 concentration in T5-6202D and T5-

4042D blends. Similarly, Ferreira and Iannace et al. observed that the addition of PLA in lower concentrations decreased the mechanical properties [4,11,122]. These deleterious effects could be attributed to PLA and PHBV spherulites that interpenetrate the growth fronts, resulting into poor mechanical properties [4,11,115,122].

Table 6.1: Mechanical properties of Tianan 5% (T5%) blended with PLA grades 3051D, 4042D, and 6202D. T5-3051D Tensile Elongation at Flexural Elastic Strength Break (%) Strength Modulus (MPa) (MPa) (GPa) 50/50 23.0 (±4.10) 1.60 (±0.120) 42.3 (±11.7) 35.1 (±3.53) 60/40 9.97 (±2.75) 0.895 (±0.452) 12.9 (±7.33) 17.9 (±7.00) 70/30 23.9 (±3.96) 1.74 (±0.185) 20.1 (±6.81) 34.4 (±4.92) 80/20 13.9 (±6.15) 0.97 (±0.330) 15.0 (±8.82) 12.3 (±1.19) 100/0 22.1 (±7.93) 1.39 (±0.326) 52.3 (±1.91) 13.0 (±0.761) T5-4042D 50/50 19.1 (±4.05) 1.42 (±0.240) 116 (±8.80) 39.0 (±3.23) 60/40 18.1 (±7.13) 1.62 (±0.195) 64.7 (±25.8) 40.3 (±12.6) 70/30 17.7 (±5.14) 1.27 (±0.449) 31.07 (±11.0) 29.8 (±11.0) 80/20 13.5 (±4.55) 1.05 (±0.474) 30.3 (±9.07) 32.7 (±4.07) 100/0 22.1 (±7.93) 1.39 (±0.326) 52.3 (±1.91) 13.0 (±0.761) T5-6202D 50/50 19.4 (±3.83) 1.40 (±0.200) 113 (±11.1) 16.4 (±1.39) 60/40 13.7 (±2.60) 1.10 (±0.14) 22.9 (±2.19) 13.9 (±2.23) 70/30 21.8 (±3.63) 1.53 (±0.243) 38.4 (±2.17) 14.9 (±1.63) 80/20 10.3 (±0.755) 0.87 (±0.232) 17.4 (±2.80) 15.2 (±3.45) 100/0 22.1 (±7.93) 1.39 (±0.326) 52.3 (±1.91) 13.0 (±0.761) Values are average, ± Standard Deviation, n = 5.

6.6.2: Rheological Analysis

In contrast, Figures 6.1 and 6.2 summarizes the complex viscosity vs. frequency

sweeps of Tianan 5% blended with PLA 3051D at 160 and 170ºC. In general, the

viscosity decreased with increasing frequency due to break-up of entanglements between

the polymers. With the addition of 40% PLA (60/40), the initial viscosity was higher than

pure PHBV polymer, while the other concentrations had lower initial viscosity compared

to Tianan 5%. At 170ºC, similar results were seen with 60/40 and 80/20 concentrations having higher initial viscosity than Tianan 5%. When subsequent scans were performed at the conclusion of the first scan, these results show minimal to no change in complex viscosity at 160 and 170ºC over the frequency range. With the addition of 40% PLA

3051D, the melt viscosity was improved compared to pure PHBV polymer.

Figure 6.1: complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 3051D at 160ºC.

Figure 6.2: complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 3051D at 170ºC.

The rheological results for Tianan 5% blended with PLA 4042D (T5-4042D) are shown in Figures 6.3 and 6.4. At 160ºC, the addition of PLA did not improve melt viscosity of PHBV. (Figures 6.3) The Tianan material had highest initial and final complex viscosity. However, at 170ºC, the addition of 20, 40, and 50% PLA improved initial and final viscosity of PHBV polymer. (Figures 6.4) When the subsequent test were performed, no change in the initial viscosity was seen for 60/40 concentration at 170ºC, while the other concentrations showed an increase in initial viscosity from the start of second, third and fourth scans. Furthermore, at 160ºC, the subsequent scans were very stable and no changes in the complex viscosity were observed at 160ºC. Similar to PLA

3051D, the addition of 20 and 50% PLA 4042D improved the melt viscosity of Tianan

5% at 170ºC, while no significant improvements were seen at 160ºC.

Figure 6.3: complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 4042D at 160ºC.

Figure 6.4: complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 4042D at 170ºC.

The melt rheology of Tianan 5% blended with PLA 6202D is shown in Figures

6.5 and 6.6. The addition of 40 and 50% PLA improved the initial and final viscosity, while 70/30 and 80/20 showed similar viscosities as Tianan 5% at 160ºC and 170ºC.

However, at 170ºC, the initial viscosity decreased for subsequent second, third, and fourth scans for 60/40 and 70/30 concentrations. The remaining two concentrations

(80/20 and 50/50) showed no changes in complex viscosity. In addition, no viscosity changes were observed for subsequent scans at 160ºC. Thus, the 50/50 and 60/40 concentrations improved rheological properties of Tianan 5%.

Figure 6.5: complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 6202D at 160ºC.

Figure 6.6: complex viscosity-frequency sweeps of Tianan 5% blended with four concentrations of PLA 6202D at 170ºC.

The viscosity vs. time was measured at 1 Hz for Tianan 5% blended with PLA

3051D at 160 and 170ºC, Figures 6.7 and 6.8. At both temperatures, the results showed no significant improvements in the complex viscosity with the addition of PLA.

However, the melt viscosity was very stable throughout the one-hour time sweep. Tianan

5% blended with PLA 4042D at 170ºC, improvements in the Tianan 5% complex viscosity were observed with the addition of 20 and 30% PLA after 16 minutes. The final blend of Tianan 5% and PLA 6202D showed no improvements in the PHBV polymer viscosity. These blends viscosity scans were very similar to Tianan 5% and

PLA 3051D as shown in Figures 6.7 and 6.8. During the start of testing, many blends showed an increasing viscosity due to the polymers aligning. Overall, no significant

84 improvement in the viscosity was seen for all three blends and their concentrations over

Tianan 5%.

Figure 6.7: Complex viscosity-time at 1Hz for 1 hour at 160ºC for Tianan 5% blended with PLA 3051D.

Figure 6.8: Complex viscosity-time at 1Hz for 1 hour at 170ºC for Tianan 5% blended with PLA 3051D.

6.7: Conclusions

The ultimate aim of this study was to improve the mechanical and rheological properties of PHBV polymer by blending PLA grades 3051D, 4042D, and 6202D at

175ºC using a micro-compounder. The results showed improvements in the elongation at

break, tensile strength, and elastic modulus in numerous concentrations of Tianan 5% blended with PLA grades 3051D, 4042D, and 6202D. However, no significant improvements in the tensile strength were observed for many of the blends. The rheological analysis showed improvements in the complex viscosity of PHBV with the addition of PLAs. In addition, less thermal instability was seen with blends compared to

pure PHBV material at 160 and 170ºC for subsequent second, third, and fourth scans. The

complex viscosity of blends were very stable throughout the one hour time sweeps, but 86 no increases in the viscosity were observed compared to the complex viscosity for Tianan

5%.

Chapter 7

Conclusions

It was hypothesized that the addition of hydroxyvalerate units will improve

thermal, mechanical, rheological, and barrier properties of PHB homo-polymer. The

thermal properties were improved with the addition of HV units, as noted in Table 4.2.

Tianan 20% (highest HV content) had the lowest onset melting temperature of 132ºC,

while Tianan 5% had the highest melting temperature 171ºC. However, the different

concentrations of the hydroxyvalerate units did not affect glass transition temperature

significantly. The glass transition temperature observed for Tianan 20% was very similar

to Tianan and Aldrich 5%. In addition, the rheological impact of HV units was minimal.

For all the non-commercial polymers (Tianan 20%, Aldrich 5, and 12%), the viscosity

decreases were attributed to thermal instability within the polymer and decreases in the

molecular structure and weight due to chain scission. Tianan 5% was thermally stable at the melting temperatures of polymer, and it had the highest complex viscosity for all four

polymers. The addition of hydroxyvalerate units improved the mechanical properties

with Aldrich 12%. The Aldrich material had less embrittlement and higher molecular

weight out of the four polymers that resulted in better elongation at break, flexural

strength, and elastic modulus. The water vapor transmission rates were improved with the addition of HV units. Between the Aldrich materials, the higher HV content Aldrich

12% resulted in lower water vapor transmission rates, as noted in Table 4.6. The Tianan

materials observed opposite results with increasing HV units, the Tianan 5% had lower

water vapor transmission compared to Tianan 20%. When Tianan 5% and Aldrich 12%

were compared, the transmission rate was significantly better for Aldrich due to less

spherulites with cracks.

Finally, the results observed for commercially available Tianan 5% were compared to thermoplastic materials used for packaging. The elastic modulus of Tianan

5% (13.0 GPa) was higher compared to traditional thermoplastic such as PET (2.8-4.1

GPa), PS (2.3-3.3 GPa), LDPE (0.2 GPa), and PP (1.7 GPa), suggesting that the PHBV

polymer is flexible above the glass transition temperature. However, thermoplastics can

absorb higher stain before breaking as measured by elongation at break. Tianan 5% had

elongation at break value of 1.39% compared to PET (30-300%), PS (2.50%), LDPE

(100-1000%), and PP (400%). Finally, Tianan 5% (22.1 MPa) had similar tensile strength value to LDPE (8-20 MPa); but, the tensile strength differed greatly from PET

(48-72 MPa), PS (34 to 50 MPa), and PP (38 MPa). The thermal properties of Tianan 5% where compared to polypropylene, and the results indicate similar glass and melting temperatures with PP (0 and 176ºC). However, the glass and melting temperatures differ greatly with other traditional thermoplastic such as PET (73-80 and 245-265ºC), PS (70-

80 and 100ºC), LDPE (-100 and 95-110ºC). Tianan 5% polymer was processed in a lab scale extruder at 175ºC. The brittleness and low melt strength made it difficult to extrude blown films. The polymer began to fracture immediately exiting the blown film die. In

89 addition, the low melt strength caused the molten polymer to sag as the polymer was cooled by the reel system.

Therefore, blends of PHBV with commercially available poly(lactic acid) (PLA) were studied as a viable way to improve PHBV thermal, mechanical, and rheological properties. The PLA grades ranged in different D- and L-lactide ratios and molecular weights. The composition of PHBV in the blends ranged from 50 to 80%. DSC analysis indicated the blends were immiscible due to separate melting temperatures representing individual polymers. However, minimal changes in the glass transitions were witnessed for both PHBV and PLA materials. These changes in temperature were due to weak hydrogen bonding and spherulites overlapping. The TGA analysis showed two degradation peaks for individual PHBV and PLA polymers, thus supplementing the results observed for DSC analysis. The mechanical properties of the blends showed significant improvements in the elastic modulus, flexural strength, and elongation at break. In general, the viscosity decreased with increasing rotational frequency due to break-up of entanglements between polymers. At various concentrations, the melt viscosity of PHBV was improved with the addition of PLAs at 160 and 170ºC. At melting temperatures, the addition of PLA caused no significant improvements in the complex viscosity. The 50/50 blends of each PLA with PHBV polymers were extruded using similar extruder and processing conditions to pure PHBV blown films. The blown film had very low melt viscosity, which resulted in blends sagging after exiting the die. In addition, the blends started to fracture due to intermediate cooling rate within the reel system.

Chapter 8

Future Studies

The characterizations of PHBV and PLA/PHBV blends provided valuable insight in the potential applications of these polymers and a point of reference for future work.

As an exploratory study, it is merely a starting point in the assessment of PHBV as potential packaging material. To expand PHBV applications, significant improvements in the melt viscosity of PHBV should be performed using biodegradable grade of titanate coupling additive in 2 to 5% by weight. In addition, using thermoplastic starch or ethylene vinyl alcohol (EVOH) as a tie-layer system with PLA and PHBV films on either side would help in decreasing the barrier properties of blends and further improvements in mechanical properties. Finally, heat deflection temperature testing (HDT) should be performed to determine the proper temperature were the polymers start to lose their mechanical strengths.

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