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Perspective pubs.acs.org/journal/ascecg

Perspective on Polylactic Acid (PLA) based Sustainable Materials for Durable Applications: Focus on Toughness and Heat Resistance Vidhya Nagarajan,†,‡ Amar K. Mohanty,*,†,‡ and Manjusri Misra†,‡ †

School of Engineering, Thornborough Building, University of Guelph, Guelph, N1G2W1 Ontario, Canada Bioproducts Discovery and Development Centre, Department of Plant Agriculture, Crop Science Building, University of Guelph, Guelph, N1G2W1 Ontario, Canada

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ABSTRACT: Evolution of the bioplastics industry has changed directions dramatically since the early 1990s. The latest generation is moving toward durable bioplastics having high biobased content. The main objective is to replace “fossil carbon” with “renewable carbon”, a holistic strategy to mitigate climate change by minimizing the environmental impact of a product throughout its life cycle. Durable bioplastics is desired for multiuse long-term application in automotive, electronics and other industries. One necessary requirement for them is to be both tough and strong, yet the two attributes are often mutually exclusive. Does this mean a biobased and biodegradable polymer as polylactic acid (PLA) with its high strength but low toughness cannot be adopted for durable applications? Well, not exactly; this is where the concept of tailoring the properties of PLA to achieve stiffness−toughness balance along with acceptable heat resistance comes into play. In this perspective, we summarize the recent research progress in addressing the toughness vs strength and heat resistance conflict inherent in PLA. Blends having super toughness and composites based on the toughened PLA blends formulated to obtain desired material properties are covered. Morphology and crystallinity that individually contribute to toughness and heat resistance have also been elucidated. KEYWORDS: Super toughened, Heat resistant, Impact strength, HDT, Reactive blending, Compatibilization, Morphology, Crystallinity, Nucleating agent, Copolymers

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INTRODUCTION The past is prologue for durable bioplastics; the quest for materials with properties similar to those of engineering plastics but derived from renewable resources is becoming a reality in the 21st century. Although several biobased engineering plastics are already available in the market, the idea here is to take advantage of the cost competiveness and unique properties of polylactic acid (PLA). The past decade has seen a remarkable surge of research interest in developing PLA based blends and composites for durable applications in automotive, electronics and semistructural parts. The diversity of the approaches, and the specialty additives and toughening agents has increased our knowledge on controlling the performance of PLA for longterm durable product applications. This perspective provides a critical review of the literature in the field of super toughened, heat resistant PLA blends and biocomposites followed by recommendations for future work.

emissions and end of life options, PLA is superior to many petroleum based polymers.1 PLA already serves as an alternative to certain petroleum based plastics in commercial applications. It is available in the market at a price on a par with that of common plastics like polypropylene. Market demand for PLA has grown dramatically over the past decade, with much of it being in the packaging industry.2 PLA was initially promoted for single use packaging applications, given the key benefit of short life cycle due to its compostable nature. The application areas for PLA are widening with usage in durable structural parts generating particular high demand. According to European trade association for the bioplastics industry, the global production of durable bioplastics is forecasted to increase by 535% from 2014 to 2019.3 Besides property enhancement with suitable additives, when the final formulations are intended for compostable applications, the materials should satisfy the compostability standards set forth in ASTM D6400 or EN 13432. On the flip side, there are not many cost-effective and compostable additives that are available to raise substantially the performance level of PLA

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GETTING OUR RESEARCH BEARINGS FOR AN ERA OF DURABLE BIOPLASTICS PLA is a biodegradable thermoplastic polyester produced by condensation polymerization of lactic acid, which is derived by fermentation of sugars from carbohydrate sources such as corn, sugarcane or tapioca.1,2 From energy consumption, CO2 © 2016 American Chemical Society

Received: February 14, 2016 Revised: March 29, 2016 Published: May 17, 2016 2899

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depends on many extrinsic (notch, temperature, loading mode, specimen geometry, fracture behavior) and intrinsic (phase morphology, chain structure and entanglements) variables. The responsiveness of a particular polymer to be rubber toughened is also said to depend on entanglement density (νe) and characteristic chain ratio (C∞); these two will decide the fracture behavior of crazing and yielding.15 See Wu’s work15 for detailed understanding of these concepts. Toughening mechanisms including shear yielding, multiple crazing and a combination of both have been reported for toughened PLA blends.9 According to toughening theories,15−19 stress concentration due to the presence of spherical rubbery particles is the first step to complex toughening process. Multiple crazing occurs when the stress required for craze initiation is less than the yield stress. In this situation, maximum triaxial stress concentrations at the dispersed particles initiate crazes. Craze termination is the next natural step in this mechanism through the formation of small multiple crazes leading to crack propagation. New surfaces generated during the creation of multiple crazes consume more energy than a small number of large crazes. Shear yielding occurs when the stress required for craze initiation is greater than yield stress. Toughening by this mechanism is usually achieved by hydrostatic tension in the dispersed particles acting as shear band initiators. When the yield stress and craze initiation stress are comparable or when there are interactions between the shear bands and crazes formed in the matrix, the combination of shear yielding and multiple crazing becomes the predominant mechanism. Cavitation is another important precursor phenomenon to any toughening mechanism. Two types of cavitation have been observed in PLA toughened with a rubbery phase: (i) internal cavitation, which occurs when the interfacial bonding is strong between the rubber domains and matrix; (ii) debonding cavitation, which occurs when there is poor interfacial bonding strength. To prevent the localization of strain, cavities formed either in the rubber particle (internal) or the matrix (debonding) alters the triaxial stress state and favors the formation of shear bands ultimately leading to shear yielding of the matrix. Combination of internal and debonding cavitation is also a possible mechanism. The particle size, shape and distribution of toughening agent can be tailored to reduce substantially the amount of impact modifiers or elastomers required for a desired toughness. Multicomponent blends containing reactive copolymers are therefore being developed to tune the phase morphology in interesting ways and obtain blends with moderate stiffness and sufficient toughness by employing techniques like in situ reactive compatibilization and dynamic vulcanization. These processes increase interfacial strength by promoting chemical reactions between blending components establishing strong bridge for transmission of stresses. Resulting PLA blends with drastic improvement in impact toughness are being referred to as “super toughened” PLA. This term was first known to be used by Wu15 for convenience to denote arbitrarily blends having notched impact strength higher than 10 ft/lb or ∼530 J/ m (energy lost per unit width, North American standard), which is approximately equal to 53 kJ/m2 (energy lost per unit cross-sectional area, European standard) depending on the dimension of the sample. Research work specifically focused on achieving super toughened PLA blends (impact strength beyond 35 kJ/m2) is reviewed in this section. They are categorized according to the type of reactive toughening polymers and techniques used. This is followed by recom-

while retaining its compostability. Industries are therefore seeing a major shift in the marketplace from “compostability” to “renewability”. However, being compostable and being renewable are not dependent or in conflict with each other, each has its own advantages. Preference for “renewable carbon” instead of “fossil carbon” stems from the very realization of our need to reduce nonrenewable resource consumption, and greenhouse gas (GHG) emissions. The Kyoto Protocol was the first critical step taken toward a truly sustainable future; it mandates emission cuts for industrialized nations. Ratified by 145 nations around the world, the protocol entered into force in February 2005.4 At the 10 year mark, United Nation Framework Convention on Climate Change (UNFCCC) announced those countries who took on the targets of the protocol have collectively reduced the emissions over 20% as opposed to the aimed target of 5%.5 A successor climate change agreement approved in Paris COP21 Conference, December 2015, has set a goal to keep the world under 1.5 °C temperature rise.6 A Japanese government directive says by 2020, 20 wt % of all plastics used in Japan must be derived from renewable resource.7 Leadership in Energy and Environmental Design (LEED) certifications, carbon tax and other local regulations are also driving the demand for durable biopolymers. Current research around the world on PLA modification and application is focused on producing high performance partially renewable materials that can compete with conventional plastics. However, much like other synthetic plastics, PLA has its own inherent weakness that prevents it from being widely adopted for durable applications, in particular its low toughness and heat resistance. PLA has a very slow crystallization rate, whereas a high level of crystallinity is desirable in finished products as it dictates most of the mechanical and thermal properties. The toughness and ductility of PLA have been improved with multiple strategies including plasticization, copolymerization, and melt blending with different tough polymers, rubbers and thermoplastic elastomers. Research progress in toughening PLA based on these strategies can be found in several recent review articles.8−14 However, none of these articles have articulated the efforts taken toward achieving a PLA based material with improved short-term heat resistance. This perspective summarizes the most recent developments in achieving super toughened and heat resistant PLA blends and composites. Exhaustive literature available on these topics are organized based on the strategies and approaches taken to resolve the material problems. Various factors governing the toughness and heat resistance of the blends and composites are also discussed.

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SUPER TOUGHENED BLENDS: CURRENT TREND SEEKING TOUGHER PLA Toughness: Definitions and Mechanisms. Toughness is a complicated property; it is defined in terms of “impact strength/toughness”, the ability to absorb sudden impact energy without breaking and “tensile toughness”, the ability to absorb energy while being pulled apart or stretched. Emphasis is on the ability to absorb energy before fracture. A good combination of strength and ductility is the key to toughness. PLA is a brittle polymer with low crack initiation energy (measured by unnotched impact test) and low crack propagation energy (measured by notched impact test); it fails by crazing. Although it may be relatively easy to improve the ductility (elongation at break), it is much more challenging to increase the impact toughness of PLA. Impact toughness 2900

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ACS Sustainable Chemistry & Engineering Table 1. Impact Strength Results for Super Toughened PLA Blends technique/additives

range of impact strength achieved

GMA based copolymers, thermoplastic elastomers and GMA grafted copolymers: EGMA, POE, POE-g-GMA, PEBA, PEE EMAGMA, PEBA-g-GMA compatibilizers and chain extenders for PLA blends with PBS, PBSA, PBAT: LTI, Joncryl, TPP

notched notched notched notched

acrylic impact modifiers and acrylic copolymer with GMA: MBS, ABS-g-GMA, AcrylPEG, ACR with different BA and MMA content, KM365 and Paralloid BPM 500 from Rohm and Haas, Biomax strong from DuPont dynamic vulcanization: EBAGMA, EMAA based ionomers, PUEP, NR, ENR, UPE random aliphatic copolyesters, polyurethanes, and other flexible polymers: P(CL-co-LA), P(CL-co-VL), TPU, CPU, EVA

Charpy: 46.1−72 kJ/m2 Izod: 40−80 kJ/m2, 450−650 J/m Izod: nonbreak Charpy: 16−40 kJ/m2

reference 21−27 20, 28, 29 31−41

notched Izod: 35−120 kJ/m2 540 J/m 42−56 notched Izod: 480−800 J/m (nonbreak) 38−60 kJ/m2 57−66 notched Izod: 40−83 kJ/m2 450−550 J/m

weight PLA containing 20% EGMA was found to have a drastic effect on elongation,21 as the value went up from 26% to >200%. However, a super toughened PLA blend with 72 kJ/m2 of impact strength was achieved only after annealing the processed samples at 90 °C for 2.5 h. Another parameter appearing to have a significant effect on the resulting toughening is GMA grafting content. Polyethylene octene (POE), a metallocene catalyzed thermoplastic polyolefin elastomer grafted with different percentages of GMA (1.8 and 0.8%), was used to toughen PLA.22,23 To achieve an impact strength of 55 kJ/m2, 45 wt % of POE-g-GMA (1.8%) had to be blended with PLA,22 whereas in another study,23 20 wt % of POE-g-GMA (0.8%) was sufficient to attain super toughness of over 80 kJ/m2. These super toughened blend systems were proved to be efficient in absorbing external energy through a combination of crazing and shear yielding mechanisms. Poly(ether-block-amide), PEBA, a commercial class of thermoplastic copolyester elastomer from Arkema, is seen as an efficient impact modifier for brittle polymers as it is highly resistant to sudden impact even at very low temperatures (−40 °C). In spite of such favorable properties,24 30 wt % PEBA was required to improve the impact strength of PLA to 60 kJ/m2. Zhang et al.25 used EMAGMA as a reactive interfacial compatibilizer for blends of PLA/PEBA and achieved impact strength up to 500 J/m, while maintaining tensile strength at 50 MPa (Figure 1). Performance improvements in these ternary

mendations for future work. Range of impact properties thus far obtained in super toughened PLA blends are summarized in Table 1. Most of the articles in this section were focused on achieving super toughness and have not investigated the effect on crystallinity or heat resistance. Reactive Compatibilization with Functional Monomers. Successful application of a reactive compatibilization technique has provided enormous opportunities to compatibilize otherwise immiscible and incompatible blends. Reactive compatibilization is therefore seen as a powerful technique to enhance effectively the compatibility of PLA with other tough polymers. Melt blending PLA with other suitable polymers in the presence of a reactive monomer forms a graft copolymer at the interphase, decreases the interfacial tension of the immiscible polymer components and promotes interfacial adhesion. A finer phase morphology developed in the blends facilitates stress transfer between the two phases, thereby improving the properties of the blends. Maleic anhydride, glycidyl methacrylate, isocyanate and epoxy are some of the widely investigated reactive monomers proving to be successful in compatibilizing the blends of PLA with other bio- and petroleum based polymers. In the work of Harada et al.,20 0.5% lysine triisocyanate (LTI) was found to increase the impact strength of PLA/PBS (90/10) blend from 18 kJ/m2 to 50−70 kJ/m2. These improvements were attributed to effective interfacial reactions accomplished between the isocyante functionalities of LTI and carboxyl, hydroxyl end groups of the blending polymers. Glycidyl methacrylate (GMA) is one of the versatile functional monomers tailored to meet a variety of applications. A great number of PLA super toughening studies report use of GMA in one or other forms to facilitate compatibility by reacting with functional end groups of PLA. Effectiveness of GMA in improving the toughness of PLA is explored mainly through these three routes: (i) addition of GMA monomers or copolymers such as ethylene glycidyl methacrylate (EGMA), ethylene methyl acrylate glycidyl methacrylate (EMAGMA) and ethylene butyl acrylate glycidyl methacrylate (EBAGMA), (ii) addition of tough polymers grafted with GMA to facilitate compatibility between the blending components (two-step process of grafting followed by reactive compatibilization) and (iii) addition of tough thermoplastic elastomers in combination with GMA copolymers in one-step reactive extrusion. Factors drastically affecting the toughening behavior of PLA blends containing GMA are the reactive extrusion screw rpm and residence time, which in turn affects important morphological aspects such as dispersed phase size and interparticle distance. Increasing the screw rpm from 30 to 200 in low molecular

Figure 1. Components, morphology and impact strength of supertoughened PLA blends. [Reprinted with permission from ACS Applied Materials and Interfaces, Vol. 6, K. Zhang, V. Nagarajan, M. Misra, A. K. Mohanty. Supertoughened renewable PLA multiphase blends system: Phase morphology and performance, 12436−12448, Copyright 2014, American Chemical Society.] 2901

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however a challenge in simple binary blends of PLA with such polymers.31−33 Researchers have looked into core−shell acrylic copolymers such as methyl methacrylate−butadiene styrene, acrylonitrile−butadiene styrene and methyl methacrylate−butyl acrylate copolymers to super toughen PLA.34−40 The rubbery core provides impact resistance whereas the glassy shell imparts rigidity. Outer shell can be designed specifically to be compatible with the PLA matrix. Core−shell composition, particle diameter and its distribution, grafting percentage and cross-linking degree are all important factors to achieve the necessary toughening and they have all been investigated in detail in PLA matrix. Acrylic impact modifiers (ACRs) containing different ratios of methyl methacrylate, MMA (hard/shell monomer) and butyl acrylate, BA (soft/core monomer) were used to super toughen PLA.36,37 Impact strength and elongation at break gradually increased with increase in the amount of soft monomer in the ACR. In PLA/ ACR (90/10) containing BA/MMA in the ratio of 90/10, the unnotched impact strength was significantly increased to 68 kJ/ m2 compared to 17 kJ/m2 for neat PLA. Tensile and flexural properties were not drastically reduced as the ACR content was only 10%.36 As the concentration of MMA hard shell monomer increased, the impact strength initially increased and then decreased, signifying the presence of a critical concentration of MMA. The highest notched Izod impact strength of 77.1 kJ/m2 was achieved when the ACR core−shell ratio was optimized at 79.2/20.8 for the 80/20 PLA/ACR blend.37 With increase in BA content, the interactions between ACR and PLA were postulated to get stronger and the interface between these phases was indistinct. Internal and debonding cavities in the ACR domains induced crazes and shear bands in the PLA making the matrix around the ACR particles to deform easily to achieve shear yielding as shown in the schematic,36 Figure 3.

blends were attributed to their unique morphology of partial encapsulation of PEBA by EMAGMA in PLA matrix. Interfacial cavitation and good adhesion between phases resulted in massive shear yielding of PLA matrix. Vachon et al.26 used EMAGMA and poly(maleic anhydridealt-octadecene) (PMAOD) to compatibilize PLA and thermoplastic poly(ether ester) elastomer, PEE. A sharp transition in impact strength values to 650 J/m was noticed in PLA ternary blends containing 12% of PEE and 12% of EMAGMA, with EMAGMA being more efficient compared to PMAOD. Recently, Zhou et al.27 investigated the effect of adding GMA grafted PEBA (PEBA-g-GMA) as an impact modifier for PLA and thermoplastic starch acetate (TPSA). This work showed a notched Izod impact strength of ∼60 kJ/m2 could be achieved for PLA/TPSA/PEBA-g-GMA (70/15/15) blend. A TPSA esterification degree of 0.04% was needed to improve the compatibility between TPSA and PLA/PEBA-g-GMA. Properties of polymers are influenced to a greater extent by the length of the macromolecule. When the macromolecular chain is longer, the molar mass and entanglement degree is higher, which increases the melt temperature and viscosity. Adding a chain extender (CE) to PLA increases the molar mass of PLA by connecting the short and long polymer chains via a reactive functional end groups present in the CE. When a multifunctional epoxy based chain extender, Joncryl was used for in situ reactive compatibilization of PLA and poly(butylene succinateco-adipate), PBSA, the alteration of blend structure from linear to long branched chains enhanced the impact strength of PLA/ PBSA (60/40) blend28,29 as shown in Figure 2.

Figure 2. Notched impact strength as a function of PBSA content and Joncryl weight fraction. The schematic depicts the modification of the PLA/PBSA blend interface by Joncryl through the formation of nonlinear copolymer. [Reprinted from Polymer, Vol. 80, V. Ojijo, S. S. Ray. Supertoughned biodegradable polylactide blends with nonlinear copolymer interfacial architecture obtained via facile in situ reactive compatibilization, 1−17, Copyright 2015, with permission from Elsevier, License number: 3794351260132.]

The particle size of the PBSA dispersed phase was reduced by 74% with the addition of 0.6% Joncryl, and further reduction was noticed with increase in Joncryl content owing to effective compatibilization. Dong et al.30 have also reported Joncryl is effective in increasing the ductility and percentage elongation of PLA/PBAT blends, to a maximum of 500%. As previous studies29 have established the presence of induction time for reactivity of Joncryl, the effect of increasing the temperature to increase the reactivity of Joncryl could be an interesting aspect of future investigations. Acrylic Copolymers and Core−Shell Impact Modifiers. Acrylic polymers such as poly(methyl methacrylate), PMMA, and poly(butyl acrylate), PBA, have been found to be partially miscible with PLA; therefore, they have been used to toughen PLA.31,32 Achieving significant increase in impact strength is

Figure 3. A simple schematic of a possible mechanism by which ACR toughens PLA. [Adapted from BioResources, Vol. 9, X. Song, Y. Chen, Y. Xu, C. Wang. Study of tough blends of polylactide and acrylic impact modifier, 1939−1952, 2014.]

Poly(ether glycol) methyl ether acrylate, abbreviated as AcrylPEG, has been most effective in imparting super toughness to PLA thus far. Two different approaches were investigated by Kfoury et al.:38 (i) polymerization of AcrylPEG to poly(AcrylPEG) using free radical initiator, Luperox and (ii) direct one step reactive extrusion with PLA, where in situ grafting of AcrylPEG onto PLA backbone was achieved. Substantial improvement of notched Izod impact strength to 102 kJ/m2 was achieved for PLA with 20 wt % AcrylPEG, and 35 kJ/m2 for PLA with poly(AcrylPEG). Commercial non2902

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ACS Sustainable Chemistry & Engineering biodegradable acrylic impact modifiers available under the tradename Paraloid BPM-50039 and KM-36540 from Rohm and Haas, and Biomax Strong41 from DuPont are also available to toughen PLA. PLA blends with Paraloid BPM-50039 possessed good flexibility compared to neat PLA, impact strength however did not improve beyond 40 J/m. PLA has been reported to show brittle to ductile transitions when KM-365 and Biomax Strong are added beyond 20 wt %. In some cases, impact modifiers were observed to hinder the crystallization of PLA and decrease the tensile properties of the blends. Dynamic Vulcanization. Dynamic vulcanization is one of the most versatile areas of polymer modification. It is a process in which selective vulcanization of elastomer with nonvulcanizing thermoplastic is achieved during shearing in melt mixing, leading to the formation of a two-phase material where particulate cross-linked elastomeric phases are dispersed in the plastic matrix.42 Zhang et al.43 introduced a super toughened PLA ternary blends with moderate tensile strength and modulus by melt blending PLA with ethylene n-butyl acrylate GMA (EBAGMA) and ethylene methacrylic acid based zinc ionomer (EMAA-Zn). Unlike other blends, in addition to reactive compatibilization between PLA and EBAGMA, dynamic vulcanization of EBAGMA was also achieved. Ternary blends containing EMAGMA/ionomer weight ratio ≥ 1, Zn metal ion, higher percentage of MMA functionality and increased degree of neutrality were found to have enhanced interfacial compatibility and hence higher impact strength.44−46 Morphological analysis based on SEM images demonstrated that with the increase in EMAA-Zn content, the occluded subinclusion phase of EMAA-Zn turned to continuous phase within the “salami”-like dispersed domains. This morphology was not dependent on reactive blending temperature; however, higher reactive extrusion temperatures resulted in an unfavorably higher degree of cross-linking in EBAGMA that was resisting internal cavitation. Polyurethane elastomer prepolymer (PUEP) with isocyanate (−NCO) terminal groups vulcanized to a rubber phase has been shown to toughen PLA.47 The −NCO groups reacted with hydroxyl, carboxyl end groups of PLA to form urethane linkages in addition to vulcanization reaction of the PUEP. These reaction products bridged the PLA phase with vulcanized rubber phase of PUEP. Predominant internal cavitation in dynamic vulcanized blends imparted major toughening effect to PLA/PEUP (70/30) blends with impact strength of 55 kJ/m2 and elongation values reaching over 400%. In another recent work, researchers have developed super tough PLA materials through in situ reactive blending with polyethylene glycol based diacrylate (PEGDA) monomers.48 The cross-linking of acrylate groups resulted in phase separated morphology with PEGDA as the dispersed phase. Sea-island morphology had been the typical, predominant morphology of thermoplastic vulcanizates (TPVs) but Chen49,50 and Yuan et al.51,52 discovered it is possible to achieve continuous cross-linked rubber phase in peroxide induced dynamic vulcanization of PLA with natural rubber (NR) and epoxidized natural rubber (ENR). Impact strength results and SEM morphology of dynamic vulcanized PLA/NR (65/35)49 are shown in Figure 4. After cryofracture and etching of PLA phase, formation of continuous honeycomb-like network structure by the NR phase was clearly visible. Extensive plastic deformation of the surrounding PLA deformed the rubber domains due to heterogeneous stress fields and enhanced the toughness. A brittle ductile transition was observed at PLA/ENR (60/40) blend ratio with notched

Figure 4. (a) Notched Izod impact strength of neat and dynamically vulcanized PLA/NR blends, (b) SEM Images of dynamically vulcanized PLA/NR (65/35). [Reprinted with permission from ACS Applied Materials and Interfaces, Vol. 6, Y. Chen, D. Yuan, C. Xu. Dynamically vulcanized biobased polylactide/natural rubber blend material with continuous cross-linked rubber phase, 3811−3816, Copyright 2014, American Chemical Society.]

Izod impact strength of 47 kJ/m2, which was 15 times higher compared to 3 kJ/m2 for neat PLA.50 At dicumyl peroxide (DCP) content beyond 0.03 phr, interfacial adhesion between phases were enhanced and a higher degree of cross-linking was achieved in ENR. “Fully biobased and super tough PLA TPV” displaying a quasi-co-continuous morphology with vulcanized unsaturated polyester elastomer (UPE) is yet another successful effort to super toughen PLA using dynamic vulcanization.53 Tensile and impact strength of PLA/UPE TPVs improved from 3.2 MJ/m3 and 16.6 J/m to 99.3 MJ/m3 and 586 J/m, respectively. Other researchers have also experimented with the dynamic vulcanization technique on PLA blends of biobased polyester elastomers (BPE),54 ethylene covinyl acetate (EVA)55 and ultrafine fully vulcanized powder rubber (UFPR).56 They have been successful in achieving tremendous improvements in elongation at break (>400%); however, the impact strength is either not reported or very low in the case of UFPR. Melt Blending with Random Aliphatic Copolyesters, and Other Toughening Polymers. In a series of studies, Joziasse57 and Odent et al.58−60 synthesized random biodegradable copolyester: CL with D,L-lactide, (P[CL-co-LA]) and CL with δ-valarectone (VL), (P[CL-co-VL]) to be used as impact modifiers for PLA. When silica nanoparticles (10%) were added to PLA blends containing these copolyesters, spherically dispersed domains converted to cocontinuous morphology, increasing the impact strength to 39.7 kJ/m2 vs 2.7 kJ/m2 for neat PLA.60 Li et al.61 prepared sliding graft copolymer (SGC) where PCL side chains are bound to polyrotaxane (PR) cyclodextrin rings and used them to toughen PLA. Methylene diphenyl diisocyanate (MDI) was used as the reactive compatibilizer. Blends of PLA/SGC/MDI displayed super toughening with impact strength values as high as 48.6 kJ/m2. Unfortunately, preparing such copolymers is not currently economically viable to be adopted by the industry for wide scale production. Unique combination of toughness, durability and flexibility makes thermoplastic polyurethane elastomers (TPU) a suitable material to blend with PLA. Addition of 30% TPU to PLA resulted in blends with impact strength of 315 J/m and elongation at break of 363%.62 Liu et al.63 noticed that toughening PLA by in situ polymerization of PEG and PMDI to form cross-linked polyurethance (CPU) was successful, where the impact strength of PLA with 30% CPU increased from 16 to 546 J/m. Liu et al.64 introduced PDLA into PLLA/TPU blends to form stereocomplex crystals that can dramatically improve the melt viscosity and change the sea2903

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Figure 5. Morphology of the PLA/EVA50 (80/20) blends after impact testing: (a) initial impact bars, (b) optical images, and TEM images of (c) undeformed part, (d−d″) the stress whitening zone at different magnifications. [Reprinted from European Polymer Journal, Vol. 48, P. Ma, D. G. Hristoca-Bogaerds, J. G. P. Goossens, A. B. Spoelstra, Y. Zhang, P. J. Lemstra. Toughening of poly(lactic acid) by ethylene-co-vinyl acetate copolymer with different vinyl acetate contents, 146−154, Copyright 2012, with permission from Elsevier, License number: 3794310181202.]

the inability of the second phase to act as stress concentrator does not favor multiple crazing or shear yielding resulting in blends with only moderate toughness. Similarly, low toughness was reported for PLA with in situ formed PU containing noncross-linked product that acted as a plasticizer.63 In the case of PLA/EVA blends, toughness improvements were marginal because of the formation of homogeneous morphology below 20 wt % EVA.65 Toughness improvement is also the highest at an optimum rubber particle size. When the dispersed phase is incompatible with the matrix, it would exist as spherical particles to reduce surface tension. If the components in the blend have good compatibility, uniform dispersion of the rubbery toughening agent with relatively small particle size can be expected. With an overlap in stress fields around the well dispersed particles, plastic deformation can propagate through the entire matrix giving rise to effective energy dissipation. Reactive compatibilization has been found to reduce the particle size of the thermoplastic elastomer or rubbery copolymer considerably20,23,29,63 and in some cases their shape evolves from spherical to distinct cocontinuous morphology.29,61 The shape and size of the dispersed particles are dependent on the dynamic viscosity, the shear rate of melt blending, and the interfacial tension. The dispersed particles will have the smallest average size when the viscosity ratio of the two phases is closer to unity and when the interfacial tension is lower.17 Higher shear rate generated by increasing the screw rotation speed in an extruder can drastically reduce the particle size of the rubber. For example, increasing the screw rpm from 30 to 200 significantly decreased the particle size of EGMA in high molecular weight PLA (PLA-H) compared to low molecular weight PLA (PLA-L).21 Proximity of viscosity ratio to unity in the case of PLA-H reduced the particle size to 50−100 nm whereas in PLA-L it was reduced to 100−300 nm.21 However, very small particle size may not be beneficial for achieving super toughness as small particles may not effectively absorb the energy of the external force. Other researchers who quantified the particle size of dispersed phase in super toughened PLA also have established the fact that having optimum particle size had resulted in superior toughening effect.20,23,63 On the basis of the theories of Wu,15,16 the entanglement density, νe is recognized to be one of the main factors governing the

island morphology of PLLA/TPU to a unique network-like structure. High levels of crystallinity in these blends were achieved by injecting the samples into a preheated mold at 130 °C and postannealing. This resulted in PLLA/TPU/PDLA (70/15/15) blends with remarkable improvement in impact strength up to 63.2 kJ/m2. Ethylene-co-vinyl acetate (EVA) with different vinyl contents and ethylene acrylic elastomer (EAE) has also been found to impart super toughness to PLA at 20 wt %.65,66 Formation of shear bands initiated by the internal cavitation of EVA resulted in shear yielding type of fracture behavior in the blend, no crazing or interfacial debonding occurred. As a consequence of numerous internal cavitations, stress whitening was noticed on a macroscopic scale as shown in the Figure 5.

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EVALUATION OF FACTORS AFFECTING TOUGHNESS: INSIGHTS FOR FUTURE WORK In any rubber toughened polymers, factors such as rubber content, type, particle size, particle size distribution and interparticle distance are closely interrelated and greatly affect the resulting toughening effect. The majority of all investigations concerning super toughened PLA have reported the existence of an optimum loading level of the toughening agent, and beyond this level fracture toughness ceases to improve or in some cases starts to decline. This might be due to several intrinsic factors related to the microstructure and the efficiency of rubber to support any kind of toughening mechanism at high rubber contents when there is not much matrix material to undergo plastic deformation. Toughness improvements can be expected only in a certain rubber content range, in which the rubber is dispersed in desired particle sizes and size distribution to cavitate effectively or fibrillate for maintaining a substantial degree of structural integrity in response to impact. The experimental evidence reported for such a limit is 20−30 wt % of rubber content; therefore, modeling and theoretical work can be developed to predict and explain this limit in future. The rubbery phase added as a toughening agent is generally preferred to be compatible with PLA to such an extent that there is satisfactory dispersion and wetting but not completely miscible to result in a single homogeneous phase morphology. When the two phases are miscible as in PEG-plasticized PLA, the elongation ratio (percentage) is improved tremendously but 2904

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observations reporting very small particle size but no substantial toughening effect do not follow this theory on L and Lc. Further studies are needed to establish any possible relationships.

deformation mechanism. For brittle polymers in general super toughening is predicted to occur at an optimum νe of 0.1 mmol/cc, as massive crazing and yielding of the matrix occurs at this level of νe.16 Depending on composition, PLA is predicted to have νe in the range of 0.12−0.14 mmol/cc.57,67 Using Wu’s relationship16 between optimum rubber particle size, do and νe, log d o = 1.19 − 14.1ve

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HEAT RESISTANCE: CRITICAL ASPECT CONFERRING DURABILITY Heat resistance can be defined as the ability of a material to maintain properties of interest at a desired level at the maximum service temperature for a prolonged period of time. Having a certain level of heat resistance is one of the principal criteria for material selection. The heat resistance of PLA depends on its level of crystallinity and crystallization behavior. The crystallization model suggests the chain segments of semicrystalline PLA coexist in three different forms: (i) crystalline fraction, (ii) rigid amorphous fraction (RAF) and (iii) mobile amorphous fraction (MAF).70 Crystalline fraction is where the chain segments are all in ordered crystalline state. Random long molecular chains of amorphous fraction coexist with the crystalline chains.70,71 When a polymer approaches its glass transition temperature, Tg, molecular chains of the crystalline region are unlikely to move due to strong intermolecular interactions, but chains of the amorphous phase move freely. Within the amorphous region, there are some chain segments that are rigid, consequently hindering free movement of the entire long chain. This fraction is referred to as rigid amorphous fraction (RAF). The remaining long molecular chains in the amorphous region are known as MAF.70−72 PLA with very low degree of crystallinity has a great proportion of its chains in the MAF, which has high mobility near its Tg and therefore exhibits very low heat resistance, with distortion temperatures often occurring close to its Tg. When the crystallization of PLA is facilitated with external aids such as nucleating agents, the proportion of the crystalline and rigid amorphous fraction is increased, which impedes chain mobility and resists heat induced distortions, resulting in enhanced heat resistance.71,72 A schematic of the CF, RAF and MAF is shown in the graphical abstract. Heat resistance is often quantified by the detection of a softening point under a certain load. The two most commonly adopted techniques measure: heat deflection or distortion temperature (HDT) and Vicat softening temperature (VST). HDT is defined as the temperature at which a specimen deflects 250 μm, under a specified load and thickness at a heating rate of 2 °C per min.72 The two common loads used are 0.46 MPa (66 psi) and 1.8 MPa (264 psi). VST is defined as the temperature at which the specimen is penetrated to a depth of 1 mm by a flat-ended needle with a 1 mm2 cross-sectional area.72 Common loads are 10 and 50N with heating rates of either 50 or 120 °C per hour depending on the standards followed.73,74 It is generally understood VST is the temperature at which a material loses its form-stability and HDT is the temperature at which material loses its load bearing capacity. However, the difference in assessing the softening point by HDT or VST is mainly a matter of defining the “end point”.74 VST values are usually higher than the HDT values, and the difference is quite modest in the case of PLA, which shows HDT of ca. 55 °C and VST of 65 °C. Various techniques and methods have been explored to improve the crystallinity and heat resistance of PLA. This section reviews the state-of-the art technologies for improving the heat resistance of PLA by (i) addition of nucleating agents and stereocomplex; (ii) adopting different processing strategies; (iii) blending with heat resistant

(1)

the do for PLA can be calculated to be in the range of 0.16−0.31 μm. On the basis of theoretical investigations, this range can be expected to be the guiding value of particle size in future PLA work aiming at achieving successful super toughening effect. However, if the dispersed rubber phase contains rigid subinclusions as in the case of core−shell or salami-like morphology in ternary blends, the inclusion phase can anchor the load bearing fibrils to the matrix, which can effectively reduce premature cavitation. Therefore, in an alternative view, particle size range required to achieve optimum toughness also depends on other factors such as strain rate, morphology of the dispersed particles, rubber content and the rubber shear modulus. Unfortunately, PLA super toughening studies have not delved into the effect of particle size distribution. Bimodal particle size distribution was observed when P[CL-co-VA] with high molar mass was used to toughen PLA samples prepared by compression molding.59 Although a super toughening effect was not achieved, compression molded samples containing P[CL-co-VA] in bimodal particle size distribution attained higher impact strength compared to their injection molded counterparts. Such improvements in compression molded samples were thought to be because of the relatively larger size of the microdomains in them compared to the morphology of injection molded samples. The authors did not provide further explanation behind this experimental observation. Smaller particles can toughen the localized shear bands formed in between the large particles.68 This makes the crack tip region sustain higher fracture load by maintaining a higher critical stress level. If this critical stress level generates greater triaxial stress ahead of the crack tip, it causes higher degree of cavitation in the larger particles; consequently, the adjacent matrix undergoes shear yielding before fracture.68 Optimum size and biomodal distribution would be necessary to achieve synergistic super toughening. There is great scope for interesting future work on examining the effects of such biomodal particle size distribution. One way to achieve such distribution in PLA matrix would be to use small fine rubber particles in combination with large coarse particles. Synergistic toughening with a combination of 1−2 and 70 μm rubber particles from recycled tires has been observed in epoxy resin.69 Another factor to consider for efficient rubber toughening is the average interparticle distance, L. According to toughening theories,18 L should be below a value, Lc, in order for the rubbery particles to effectively initiate plastic deformation in the surrounding matrix, despite L being directly related to rubber particle size and content. In PLA toughened with POE-gGMA,23 when the rubber content and particle size were increased, interparticle distance was reduced. The critical value, Lc for effective toughening of the blend was found to be 0.5 μm.23 However, there is no unique agreement between researchers whether Lc is more important than content and size. If so, the ultimate goal of manipulating the content and size will be to drive the L below the Lc. On the other hand, 2905

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adopted to achieve substantial improvements (results are in Table 2). In spite of the successful enhancement of the crystallization rate of PLA through the addition of nucleating agents, obtaining injection molded articles of PLA with high crystallinity remains difficult with a fast mold cooling rate. Nucleated PLA molded in room temperature molds with fast cooling (>100 °C/min) does not show substantial improvement in HDT. Therefore, the effect of performing annealing postprocessing on the mechanical and thermal properties, and the fracture behavior of PLA has been studied. The crystallinity of PLA has been found to increase consistently through annealing in most of the studies and the increase lead to an improvement in its heat resistance and overall mechanical performance. Park et al.90 and Nascimento et al.91 performed annealing of PLA under various conditions to obtain microstructures with different spherulite sizes and densities. The heat resistance of PLA was markedly improved when its crystallinity was increased by annealing. PLA with 1% EBH molded at room temperature and then annealed for 1, 2, 4, 10 and 20 min at 105 °C showed increasing HDT with increasing annealing time. A sharp step change in HDT was noticed when the crystallinity went 25%, indicating a threshold for crystallinity content.77 However, annealing adds a postprocessing step, which may not be economical or industrially feasible. As an alternative to annealing, researchers84,92,93 have looked at increasing the mold temperature during the injection molding process. This technique can be called as an in-mold annealing process, where the cooling time is increased to facilitate effective demolding of the samples. Harris and Lee92 increased the injection mold temperature to 110 °C and were successful in obtaining PLA molded articles with high percentage of crystallinity and high HDT. However, the problem with this step is molding cycle time of ∼2 min is required due to higher cooling time; demolding of the processed components would be difficult with short cooling cycle. Li and Huneault93 also observed similar effect of mold temperature on crystallinity as shown in Figure 6. At mold

polymers; and (iv) fabrication of biocomposites with natural fibers and nanoreinforcements. A summary of PLA blends with improved HDT/VST is presented in Table 2. Table 2. PLA Blends with Improved Heat Resistance: Summary of Results PLA blends with improved heat resistance nucleating agents, stereocomplex TMC-328 (0.6%) OMBH (1%) EBH/talc mixture (1%) PLLA/PDLA (50/50) blend PLLA/hPLLA (95/5) processing strategies PLA/1% EBH, 10 min annealing at 105 °C PLA with NA annealing at 80 °C for 15 min PLA/2% EBS PLA/2% talc PLA/talc/PEG (80/10/10) 23 °C epoxy mold 90 °C steel mold blending PLA with heat resistant polymers and nanofillers PLA/POM (60/40) and (50/ 50) PLA/PHBV/PBS (30/60/ 10) and (10/60/30) PLA/organically modified MMT (93/7) PLA/ OMSFM (96/4) and (90/10) PLA/5% DCPD capsules

softening point from HDT and VST

reference

134.3 °C (VST, 10N) 124 °C (HDT, 0.45 MPa) 110 °C (HDT, 0.45 MPa) 150 °C (HDT, 0.45 MPa) no deformation at 70 °C, 50 g for 5 min

72 84 84 103 105

93 °C (HDT, load not mentioned) HDT, 0.45 MPa

77

70 °C 77 °C VST, 10N 123.6 °C 117.9 °C

92

∼72 and 135 °C (HDT, 0.45 MPa) 72.2 and 87.5 °C (HDT, 0.45 MPa) 112 °C (HDT, 0.98 MPa)

99

92 and 117 °C (HDT, 0.98 MPa) 78.05 °C (HDT, load not mentioned)

109

95

100 108

110

Nucleating Agents and Processing Strategies. Nucleating agents can effectively promote crystallization by providing nucleation sites around which the polymer chains can crystallize. Shorter crystallization half time achieved with the addition of nucleating agents can help to increase the crystallinity and shorten the molding cycle time. Nucleating agents for PLA include, but are not limited to, talc,75,76 N,N′ethylene bis-stearamide (EBS),77 carbon nanotubes,78 metal salts of phenylphosphonic acid,79 multiamide and hydrazide compounds,80−85 barium sulfate,86 titanium dioxide,86 calcium carbonate (CaCO3),86 nano-CaCO387 and orotic acid.88 Numerous investigations have been conducted on improving crystallization of PLA with the help of nucleating agents. However, only a handful of them corelate the increase in crystallinity due to nucleation to increase in heat resistance measured through HDT/VST. Recently, TMC-328, a commercial heterogeneous multiamide nucleating agent, has been found to enhance greatly the heat resistance of PLA at a very small concentration (0.2%).72 Benoylhydrazide (BH) compounds, in particular octamethylenedicarboxylic dibenzoylhydrazide (OMBH) and decamethylenedicarboxylic dibenzoylhydrazide (DMBH), are known to impart enhancement in the crystallization of PLA.84,89 The nucleation ability (Tc and ΔHc) of OMBH was found to be higher than that of DMBH, and ethylenebis (12-hydroxystearylamide), EBH/talc mixture at 1 wt % loading in PLA.84 In addition to using hydrazide nucleating agent, a high molding temperature of 110 °C was

Figure 6. Effect of molding temperatures on crystallinity (Xc) developed for PLA with 5% acetyl triethyl citrate (ATC) and 1% talc. [Reprinted from Polymer, Vol. 48, H. Li, M. A. Huneualt. Effect of nucleation and plasticization on the crystallization of poly(lactic acid), 6855−6866, Copyright 2007, with permission from Elsevier, License number: 3794330203855.] 2906

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ACS Sustainable Chemistry & Engineering temperatures below 50 °C, low crystallinity level was observed, and crystallinity reached maximum level at 80 °C mold temperature with a combination of 5% plasticizer and 1% talc. In most of the above reviewed works,72,77,84,89 addition of nucleating agent in combination with annealing or high temperature molding was helpful in increasing the mechanical properties. Increase in crystallinity increased the tensile and flexural modulus. In some cases, increased number of spherulite structures with low spherulite size was believed to consume more energy and thus increase the impact strength of PLA samples containing nucleating agents. On the contrary, Vadori et al.94 have showed increasing the mold temperature of PLA decreases the impact toughness and percentage elongation of high impact PLA. Unique approach of using epoxy based PolyJet mold instead of steel mold for conventional injection molding to produce PLA parts with high crystallinity has been proposed to offer promising results (Table 2).95 Because of low thermal conductivity of epoxy based PolyJet mold, PLA parts containing nucleating agents produced from this mold had a significantly higher level of crystallinity, thermal and mechanical properties compared to PLA samples molded from steel molds. When PLA is injected into 23 °C steel mold, it is cooled below its Tg in 15 s due to high thermal conductivity of the steel mold, whereas, in PolyJet mold, PLA material stays above Tg for around 66 s, allowing the material to crystallize. As a result, VST of PLA molded in 23 °C PolyJet mold increased to 118−124 °C compared to VST of 60−65 °C for PLA samples obtained from 23 °C steel mold. Use of such PolyJet molds show promise in achieving higher levels of crystallinity for PLA at room temperature molding conditions. Blending with Heat Resistant Polymers, Stereocomplexation, and Use of Nanofillers. Blending PLA with heat resistant engineering polymers such as polycarbonate,96 poly(acrylonitrile−butadiene−styrene),97 nylon,98 polyoxymethylene99 can improve the HDT of PLA when there is good compatibility between the blending polymers. Biodegradable ternary blends of PLA, PHBV and PBS with balanced stiffness and toughness attained HDT of ∼72 °C with 30 wt % PLA in the blend.100 Polyoxymethylene, POM, has a high HDT of 160 °C and it crystallizes fast with 70−80% crystallinity content.99 Nonetheless, having POM as a dispersed phase in PLA did not help in improving the HDT; to achieve desired improvements in HDT, POM should be the major phase in the blend as significant improvements were observed with phase inversion, beyond 40%.99 Two different monomers, D-lactide and L-lactide, exist due to chirality of PLA. Homopolymers of Dand L-lactide (PDLA and PLLA) have faster crystallization and higher melting points compared to common PLA, which has a small percentage of D-lactide with atactic stereoregularity in a majority of L-lactide. A stereocomplex of two polymers with same structure but different configuration has a melting temperature between 190 and 230 °C. Stereocomplex (SC) PLA can work as a nucleating agent promoting the formation of ordered structures. Various mixtures of PLLA and PDLA have been investigated101−104 and 50−50 blend with stereocomplex crystalline structure improved the HDT to 150 °C. Nevertheless, the high cost of PDLA is a bottleneck to stereocomplexation due to difficulty in production of D-lactide and hence PDLA. In a recent publication, Yin et al.105 used high melting point PLLA homocrystallites (hPLLA) as a nucleating agent to improve the thermomechanical properties of PLA. About a 20 °C difference in melting point between PLA

(4032D, Ingeo NatureWorks) and hPLLA helped to keep hPLLA crystallites unmelted at the processing temperature of 170 °C. Presence of 5% hPLLA accelerated PLLA crystallization at a remarkable rate compared to PLA containing the same amount of talc and SC PDLA. PLLAs, with and without talc and PDLA, were noticed to deform in less than 2 min when placed in an oven at 70 °C for 5 min under a constant load of 50 g, whereas PLLA with hPLLA crystallites showed no visible deformation for 5 min, HDT/VST needs to be measured for any practical comparisons. This work, however, has contributed to new ways of tailoring the crystallization of PLLA without involving any post processing techniques and more importantly without compromising the biodegradable nature of the polymer. Incorporation of nanoparticles into PLA is a relatively new strategy that researchers are exploring to improve the heat resistance of PLA. Although addition of 2 wt % talc to PLA resulted in 3 °C HDT improvement,106 addition of 8 wt % montmorillonite (MMT) to PLA increased its HDT by 28 °C.107 Layered silicate nanocomposites offer desired improvement in HDT only when the silicate layers of the clay are intercalated, stacked and well distributed in PLA matrix.108,109 Organomodified montmorillonite (OMMT) containing trimethyl octadecyl ammonium cation at 7 wt % increased the HDT of PLA to ∼112 °C,109 10 wt % of organically modified synthetic fluorine mica (OMSFM) increased the HDT of PLA to ∼117 °C,108 under a deflection load of 0.98 MPa. Dicyclopentadiene (DCPD) filled urea formaldehyde microcapsules added to arrest the crack propagation and promote self-healing in PLA was observed to act as a nucleating agent.110 Formation of stable cocontinuous morphologies of heat resistant polymer with the aid of well intercalated nanoparticle is a recently explored promising strategy to increase the crystallinity. PLA phase interpenetrated with a continuous framework of nylon (30 wt %) and 3 phr OMMT showed resistance to temperature up to ∼160 °C (Figure 7); however, the HDT at 0.25 mm was the same as that of neat PLA.98

Figure 7. (a) Sample deflection recorded during creep tests for the sample PLA (squares), PA11 (diamonds), PLA70 (circles) and PLA70-C3 (triangles). The pictures show the samples PLA70 (b) and PLA70-C3 (c) at the end of the test, which is after the temperature had reached ≈160 °C. [Reprinted from Macromolecular Materials and Engineering, Vol. 299, A. Nuzzo, S. Coiai, S. C. Carroccio, N. Dintcheva, C. Gambarotti, G. Flippone. Heat resistant fully biobased nanocomposite blends based on poly(lactic acid), 31− 40, Copyright 2013, with permission from Elsevier, License number: 3794371367676.] 2907

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PLA BLENDS WITH CONCURRENT IMPROVEMENTS IN TOUGHNESS AND HEAT RESISTANCE Having higher crystallinity in a semicrystalline polymer sometimes negatively affects the impact strength, hence the inverse relationship between HDT and impact strength. Crystallites in the polymer can act as stress concentrators, thereby causing the stress acting on a small volume of the material to grow much higher than the average stress applied to the entire sample.11 As a result, material breaks at a stress value typically less than the expected critical value.11 Shear yielding and multiple crazing are also observed to decrease due to the presence of crystallites. Unfortunately, little attention has been paid to achieving PLA blends with balanced toughness and heat resistance because it is still a challenge to control simultaneously phase structure and matrix crystallization of blends. Perhaps the most useful work toward the search for such PLA blends are confined to using nonbiodegradable engineering polymers such as polycarbonate (PC) having high impact strength and heat resistance. Several commercial PLA/PC blends111−114 have been developed, which are seen as environmentally benign materials containing over 50% biodegradable and renewably sourced polymer, PLA. Addition of over 40% PC to PLA has shown some promise in increasing the impact strength of PLA; however, increasing the heat resistance of this blend has remained a challenge without the use of compatibilizers. Hashima et al.96 developed a fourcomponent super toughened blend containing PLA/PC/ EGMA/SEBS (40/40/15/5) where SEBS toughened PLA in the presence of EGMA and a further improvement in toughness and heat resistance was achieved through the incorporation of PC in the blend. Wang et al.115 investigated the effect of compatibilizers, epoxy (EP) resin and poly(butylene succinate-co-lactate), PBSL for PLA/PC binary blends. Combination of PBSL (10%) and EP (10%) in the presence of catalyst, tetrabutyl ammonium bromide (TBAB, 1%) in 50/50 blend of PLA/PC resulted in significant and concurrent improvement in impact strength and heat resistance, the values are listed in Table 3. Chain extenders such as Joncryl and tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) in combination with small percentage of acrylic impact modifiers (BPM-520) have been used to improve the interfacial interactions in PLA/PC blends.116,117 Although PLA/PC blends showed phase separated morphology and there were no sign of PLA−PC chain entanglements, interfacial connection was established between the chain extender and blending polymers that increased the impact strength and heat resistance upon annealing.

Table 3. PLA Blends with Concurrent Improvement in Impact Strength and Heat Resistance PLA blend formulations PLA/PC/EGMA/SEBS (40/ 40/15/5) 40 °C mold temperature 80 °C mold temperature PLA/PC/PBSL/EP/TBAB (50/50/5/0/0) (50/50/10/0/0) (50/50/10/10/0) (50/50/10/10/0.1) PLA/PC with Joncryl or TGDDM (70/30/0.3phr) room temperature molding followed by annealing at 120 °C for 6h PLA/PC/BPM/Joncryl (85/10/ 5/0.3phr) sample molded at room temperature sample annealed at 120 °C for 6h

notched Izod impact strength

65.9 kJ/m2 63.3 kJ/m2

36.6 65.1 25.4 34.0

kJ/m2 kJ/m2 kJ/m2 kJ/m2

∼30 kJ/m2 (Joncryl) ∼13 kJ/m2 (TGDDM)

∼10 kJ/m2 ∼40 kJ/m2

HDT at specified load and deflection 0.45 MPa, 0.36 mm 88.6 °C 94.5 °C 0.45 MPa, 0.25 mm 94.8 °C 76.8 °C 82.5 °C 94.2 °C 1.82 MPa, 0.32 mm ∼86 °C (Joncryl) ∼81 °C (TGDDM) 0.45 MPa, 0.32 mm ∼57 °C ∼135 °C

reference 96

115

116

117

biocomposites that used tough PLA blends as the matrix for incorporation of fibers and fillers. The increase in fracture toughness observed for PLA biocomposite is not as high as in the case of neat PLA. For instance, improving the toughness of neat PLA by 20-fold might increase the fracture toughness of the composite by 3−6-fold only. Such poor translation of matrix toughness into the composite is due to the presence of fiber, which is a constraint that suppresses elastic deformation of the matrix at the crack front. However, having a toughened PLA blend as a starting material to incorporate fibers can be a good way to achieve a balanced performance. Furthermore, cost of developing such blends can be offset to a certain extent by adding less expensive lignocellulosic fibers. PLA blended with tough biopolymers such as PBAT and PCL have been explored as a matrix system to incorporate natural fibers.126−128 In most cases, surface treatment has proved to be effective in promoting interfacial interactions between the relatively hydrophobic matrix and hydrophilic filler. Having 30 wt % PBAT in PLA−PBAT/alkali treated saw dust (70/30) composites improved the unnotched Izod impact strength by 50%.126 The surface of Kenaf treated with 2% silane coupling agent was observed to become hydrophobic with the ability to bind active groups of the polymer.127 Chemical interactions formed between hydroxyl, silanyl and alkoxy groups increased the impact strength of the PLA−PBAT biocomposites by 22%.127 By treating ramie fiber with silane coupling agent (KH550) for in situ polymerized PLLA−PCL matrix, tensile and impact strength increased from 12.14 MPa, 30.0 J/m to 23.45 MPa, and 88.9 J/m, respectively.128 Incorporation of Cordenka fiber at 25 wt % has been shown to triple the impact strength of PLA without any tough component being present; however, more research is needed toward the effect of this fiber on HDT.129 Although addition of 5 wt % lignin resulted in toughness improvement in PLLA130 from 8.2 to 12.5 kJ/m2, addition of 5 wt % of lignin-g-rubber-gPDLA to PLLA exhibited a 7-fold enhancement in toughness (from stress−strain curves) compared with neat PLLA. This

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PLA BIOCOMPOSITES: THE QUEST CONTINUES FOR HIGH PERFORMANCE A biocomposite is a multiphase system, where plant-derived fiber or mineral/synthetic filler is dispersed in the biopolymer matrix; either the matrix or the reinforcement phase is biobased.118,119 Toughened PLA biocomposites have a far greater potential for minimizing the limitations of PLA, hence major research efforts are being taken to develop and commercialize them. Numerous research works have been conducted in the field of PLA composites; however, most of the works report only marginal improvements in impact strength and HDT.120−125 The scope of this section has been limited to reviewing the research progress in injection molded PLA 2908

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Stiffness and HDT of the PLA were improved with the addition of newspaper fibers/talc hybrid with drastic reduction to impact strength.153,154 HDT of the PP−PLA composites could be increased to 120 °C with 30% Oat hull but in a major phase of PP, with a drastic reduction in impact strength.155 Incorporating 30 wt % agricultural residues like soy stalk, corn stalk, wheat straw and their hybrids in PLA matrix did not provide a desired increase in HDT. Only the modulus of the composites increased while impact strength remained essentially the same as virgin or neat PLA.156 In such cases, addition of fibers alone would not be sufficient to increase the HDT, a combinatorial approach of adding fibers, and use of high mold temperature could be beneficial. By taking super toughened PLA blend developed based on PEBA and EMAGMA 25 as the matrix material, such combination of approaches have proved to be successful in achieving concurrent improvement in impact strength and HDT of PLA biocomposites.157 Although the impact strength reduced as expected with addition of 10 wt % miscanthus, it was still considerably higher than the neat PLA matrix, exhibiting 120 J/m (Figure 9). A high mold temperature of

improvement is significant considering the copolymer contains only 3.8 wt % of rubber.131 In the case of PLA/pine wood floor composites, notched Charpy impact strength was found to increase gradually with increase in addition of wood floor and further increment in impact strength was achieved by toughening the PLA matrix with styrene−butadiene−styrene (SBS) block copolymer.132 Use of reactive impact modifiers can form ductile interface between PLA and fiber, thus increasing the resultant properties. With this hypothesis, ethylene acrylate copolymer (Biomax) was used as an impact modifier (IM) for PLA/kenaf fiber (KF) composites.133 Impact strength and elongation at break increased, but only at a high loading level of 40 wt % coupled with substantial reduction in tensile strength and modulus. Liu et al.134 compared the toughening effect of three different reactive elastomers: polyoxyethylene grafted with maleic anhydride (POE-g-MAH), ethylene−propylene− diene rubber grafted with maleic anhydride (EPDM-g-MAH) and ethylene−acrylate−glycidyl methacrylate copolymer (EAGMA) on PLA/basalt fiber composites. EAGMA at 20 wt % imparted the most toughening effect by recording a value of 33.7 KJ/m2 for unnotched charpy impact strength.134 Other mineral fillers such as barium sulfate135 and calcium sulfate136,137 have also been reported to increase the toughness of the PLA composites. PLA based nanocomposites prepared by incorporation of nanofillers such as cellulose nanofibers and nanowhikers,138 nanocalcium carbonate,139−141 nano- and mesoporous silica,142−145 halloysite nanotubes,146,147 nanoclay147−150 and titanium oxide nanoparticles151 exhibited good improvement in toughness, mechanical and barrier properties. However, none of these studies have reported the heat resistance of the developed materials. Although the hybridization of PLA with impact modifier and nanoparticle can offer a toughened composite material, challenges exist in achieving good level of dispersion and distribution of the nanoparticles, its compatibility with the matrix and ease of processing. On the flip side, a considerable number of research investigations have shown the heat resistance of injection molded PLA biocomposites to increase with fiber/filler incorporation in spite of affecting impact strength negatively. Crushed Kenaf fiber152 has been reported to significantly increase the HDT of injection molded PLA composites when added beyond 10 wt %, as shown in Figure 8.

Figure 9. Impact strength and HDT of PLA biocomposites with and without nucleating agent (NA) molded at different mold temperatures and injection cycle times. PLA blend/MS (90/10) at (A) 30 °C, 30 s; (B) 110 °C, 60 s. PLA blend/MS/NA (89/10/1) at (C) 30 °C, 30 s; (D) 60 °C, 60 s; (E) 90 °C, 60 s; (F) 110 °C, 60 s; (G) 120 °C, 60 s. [Reprinted with permission from ACS Applied Materials and Interfaces, Vol. 7, V. Nagarajan, K. Zhang, M. Misra, A. K. Mohanty. Overcoming the Fundamental Challenges in Improving the Impact Strength and Crystallinity of PLA Biocomposites: Influence of Nucleating Agent and Mold Temperature, 11203−11214, Copyright 2015, American Chemical Society.]

110 °C was required to improve the HDT to 85 °C. A high level of crystallinity developed in the composites facilitated easier demolding of the samples and the total cycle time was limited to 1 min including cooling, making it an industrially feasible technique.157 Promise of further significant improvement in properties and possibilities of cost reduction with use of specialty additives and processing strategies continues to excite areas of composite material research.

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CONCLUSIONS: QUO VADIS PLA RESEARCH? Biobased content is an important driver in development of durable biopolymer blends and composites. Many major industries and business operations are moving toward sustainable sourcing and use of renewable materials. Principles

Figure 8. Distortion temperature under load (DTUL) of PLA/crushed Kenaf fiber. [Reprinted from Journal of Applied Polymer Science, Vol. 100, S. Serizawa, K. Inoue, M. Iji. Kenaf fiber reinforced poly(lactic acid) used for electronic products, 618−624, Copyright 2006, with permission from John Wiley and Sons, License number: 3794390263152.] 2909

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ACS Sustainable Chemistry & Engineering Table 4. Commercial Toughened and/or Heat Resistant PLA Formulations for Durable End Use Applications company RTP Co.

grade RTP 2099 X Series

PolyOne reSound FR Corporation Kingfa Sci & Tech Ecopond AFR-97 Co., Ltd. UGM ABS Ltd. ECO PELLET LA Series Interfacial Solutions Teknor Apex Company Inc. Corbion Sukano Polymers Toray Industries Inc. Unitika Ltd. NatureWorks LLC Supla Co., Ltd. SK Chemicals

deTerra XP698 Terraloy 3D-40040 Series Carbion Purac (development grades) Sukano Bioloy 003, 004 NC001 ECODEAR V751X53, V751X52 Terramac TE 7000, 7307, 7300, 8210, 8300 Ingeo 3100HP, 3260HP SUPLA 135 Ecoplan-Dura

FKuR Plastics

Bio-Flex F 6513

NaturePlast

PLI 013, PLE 013, high temperature KEBACOMP FE 120204

Barlog Plastics EcolBiotech Co.,Ltd. GEHR Plastics WinGram Industrial Co Ltd. Teijin Ltd.

EcolGreen EGP Series ECOGEHR PLA-L

impact strength 694−854 J/m (notched Izod) 43−187 J/m (notched Izod) 620 J/m (notched Izod) 55 kJ/m2 (notched Izod) 12−27 kJ/m2 (notched Charpy) 880 J/m (notched Charpy) 267 J/m (unnotched Izod) 5−23 kJ/m2 (notched Charpy) 60−70 kJ/m2 (unnotched Charpy) 21−24 kJ/m2 (notched Charpy) 2.0−4.0 kJ/m2 (notched Charpy) 16−32 J/m (notched Izod) 40 J/m (notched Izod) 3 kJ/m2 (notched Charpy) 89 kJ/m2 (unnotched Charpy) 5 kJ/m2 (notched Izod) 382−477 J/m (unnotched Izod) 59.8 kJ/m2 (notched Izod)

HDT at 0.45 MPa (°C)

tensile strength (MPa)

comment

96−124

48−52

PLA−PC blends

91−160

38−114

PLA with glass fibers or talc

112 84 (1.82 MPa)

52

78−92

48−58 38

75

reference 111

PLA−engineering plastic blend

112, 158

PLA−PC and PLA−ABS blends, >40% biobased PLA−PC and PLA−ABS blends

113

PLA blend, compostable, >85% biobased PLA blend, extrusion filament for use in 3D printers PLLA/PDLA blends with and without talc PLA blend, compostable (EN13432), 35−97% biobased

114 159 160

85−120

30−45

161

50−90

35−50

81

49−52

PLA blend

163

110−140

50−70

compostable (ISO 14855)

164

149−151

63−65

165

150 100

42

PLA with nucleating agent, mold temperature of 120 °C PLA blend (90% PLA) compostable, 80−100% biobased

68−130

32

162

166 167 168

123−133

PLA blend, HDT of up to 130 °C by appropriate processing injection and extrusion grades

100

compostable (ISO 13432)

170

nanocomposite with 12 different additives, compostable PLA blend with lignin and fatty acid, compostable, >80% biobased compostable (ISO 14855)

171

62−72

31.7−45.5

58.4 (VST)

49.5

Ecoplant HRS

heat resistance up to 120 °C

Biofront grade J20, J201, L201

highly heat resistant

of green chemistry, sustainability and engineering are being integrated in the R&D to achieve a good balance of product performance and environmental friendliness. Extensive research effort has been devoted to developing PLA blends and biocomposites with desirable morphology and crystallinity for durable applications. However, achieving feasible and economical manufacturing processes for mass production of such materials has been quite a challenge. Enhancing matrix crystallization has been reported to be an effective strategy toward creating heat resistant PLA blends. Both thermal annealing and nucleating agent induced matrix crystallization could significantly enhance heat resistance of the blends, while maintaining or further increasing the toughening efficiency. However, increasing matrix crystallinity alone cannot guarantee toughness improvement in most cases because suitable morphology must be obtained for PLA matrix to undergo plastic deformation. Specifically, optimum elastomer content, particle size and interparticle distance are identified to be the most important deciding factors for toughening PLA. Reactive compatibilization along with dynamic vulcanization techniques have been shown to tailor successfully the morphology of the

stereocomplex PLA melting point of 210 °C

169

172 173 174

blends. Recent explorations have revealed that a unique network-like or cocontinuous morphology unevenly distributed in the matrix to exhibits much better super toughening compared to the common sea-island morphology containing well dispersed spherical elastomer particles in a polymer matrix. The network-like distribution of the elastomer particles can facilitate the percolation of the stress field as the plastic deformation of the matrix around them at lower content. Adding inorganic nanoparticles with strong self-networking capability in polymer melts has been shown to assist in the transition of morphology from immiscible sea-island structure to the network-like, cocontinuous structure. Approach of adding nucleating agents and natural fiber in combination with a high molding temperature to a super toughened PLA blend has resulted in composites with concurrent improvements in both the impact strength and HDT. Epoxy based mold with low thermal conductivity has demonstrated significant advantages over conventional steel molds. Future work is needed to shed light on the effect of applying an intense shear flow field through oscillation shear injection molding (OSIM) to trigger dramatic enhancement of 2910

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ACS Sustainable Chemistry & Engineering Biographies

crystallization kinetics of PLA. Durable blends of PLA/PC have shown promising properties with simultaneous improvements in impact and heat resistance in the presence of compatibilizer and chain extenders. Thermally stable fillers like biochar from different fiber sources can be added to such highly toughened engineering plastic based blends. Properties of these composites can be tailored to have enhanced performance with affordable cost to performance ratios for industrial applications. Much research is needed in the direction of developing such high performance PLA composites. Future technological development may focus on the emergence and exploitation of such renewable carbon based fillers for PLA materials to serve the need of the era for lightweight, carbon neutral durable materials. Many PLA formulations with improved toughness and/or HDT are available in the market for durable applications, as summarized in Table 4. Most of the impact modifiers used are high molecular weight polymeric materials with a flexible component such as acrylic rubber, and hence the problem of migration is not a concern. However, these impact modifiers are typically nonbiodegradable. Minimal use of even 5% may prevent the products from being certified compostable due to the stringent requirements of the American and European compostability standards. ASTM D6400-12 describes that organic constituents present at concentrations of less than 1% do not need to demonstrate biodegradability. However, the sum of such unproven constituents should not exceed 5%.175 Finally, one might ask when and where durable PLA materials may find application. Before answering, we should consider the evolution of bioplastics industry, which has had multiple shifts in direction. The first phase was focused on biodegradable and/ or compostable characteristics, primarily intended for single use packaging applications. The second phase offered compostable and renewable resource based alternative for nondegradable petroleum based commodity plastics. The current trend is the development of durable bioplastics. Commercialization arguably marks the success of research and development efforts, but the timeline should not be compared to that of mature technologies. Although PLA based materials are aimed for high volume applications in interior automotive parts and other structural and semistructural applications, they will initially find application in consumer goods such as cell phone casings, personal and home care products.

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Ms. Vidhya Nagarajan is currently a Ph.D. candidate in Biological Engineering, Bioproducts Discovery & Development Centre (BDDC) at the University of Guelph, ON, Canada. Vidhya graduated with a Master’s degree from University of Guelph in 2012. She is a recipient of highly qualified personnel (HQP) scholarship from Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). She is also a HQP of the AUTO21 Network of Centers of Excellence program, a national research initiative supported by the Government of Canada. She holds a Bachelor’s degree in Polymer Technology from Crescent Engineering College, Anna University, India. The primary focus of her research is processing and characterization of biopolymer blends and composites for sustainable industrial applications. She has published 6 peer reviewed journal articles, 1 patent application (filed) and coauthored 2 book chapters.

Dr. Amar Mohanty, Professor and Premier's Research Chair in Biomaterials and Transportation, is the Director of the Bioproducts Discovery & Development Centre (BDDC) at the University of Guelph, ON, Canada. Dr. Mohanty's research interests include natural fiber composites, biobased and biodegradable polymers, biorefinery, biocarbon reinforcement, reactive extrusion and utilization of biofuel and biomass coproducts. He has more than 600 publications to his credit, including 274 peer-reviewed journal articles (including accepted manuscripts), four edited books, 20 book chapters, and 40 patents awarded/applied. He has received distinguished awards for his work, including the “Andrew Chase Forest Product Award” from the American Institute of Chemical Engineers and most recently the “Lifetime Achievement Award”, from the BioEnvironmental Polymer Society (BEPS) in the year 2015. Dr. Mohanty holds the Alexander von Humboldt Fellowship at the Technical University, Berlin. His ResearchGate score is 44.43, higher than 97.5% of the 7 million ResearchGate members. His research impact resulted in 16 962 citations with h-index of 62 (Google Scholar, as of April 2016).

AUTHOR INFORMATION

Corresponding Author

*A. K. Mohanty. E-mail address: [email protected]. Tel.: +1-519-824-4120 ext. 56664. Fax: +1-519-763-8933 Notes

The authors declare no competing financial interest. 2911

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(8) Krishnan, S.; Pandey, P.; Mohanty, S.; Nayak, S. K. Toughening of Polylactic Acid: An Overview of Research Progress. Polym.-Plast. Technol. Eng. 2015, DOI: 10.1080/03602559.2015.1098698. (9) Odent, J.; Raquez, J.; Dubois, P. Highly Toughened PolylactideBased Materials through Melt-Blending Techniques. In Biodegradable Polyesters; Fakirov, S., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Berlin, 2015. (10) Zeng, J.; Li, K.; Du, A. Compatibilization strategies in poly(lactic acid)-based blends. RSC Adv. 2015, 5 (41), 32546−32565. (11) Kfoury, G.; Raquez, J.; Hassouna, F.; Odent, J.; Toniazzo, V.; Ruch, D.; Dubois, P. Recent advances in high performance poly(lactide): From “green” plasticization to super-tough materials via (reactive) compounding. Front. Chem. 2013, 1 (32), 1−46. (12) Liu, H.; Zhang, J. Research progress in toughening modification of poly(lactic acid). J. Polym. Sci., Part B: Polym. Phys. 2011, 49 (15), 1051−1083. (13) Anderson, K. S.; Schreck, K. M.; Hillmyer, M. A. Toughening polylactide. Polym. Rev. 2008, 48 (1), 85−108. (14) Rasal, R. M.; Janorkar, A. V.; Hirt, D. E. Poly(lactic acid) modifications. Prog. Polym. Sci. 2010, 35 (3), 338−356. (15) Wu, S. Chain structure, phase morphology, and toughness relationships in polymers and blends. Polym. Eng. Sci. 1990, 30 (13), 753−761. (16) Wu, S. Control of intrinsic brittleness and toughness of polymers and blends by chemical structure: a review. Polym. Int. 1992, 29 (3), 229−247. (17) Wu, S. Formation of dispersed phase in incompatible polymer blends: Interfacial and rheological effects. Polym. Eng. Sci. 1987, 27 (5), 335−343. (18) Wu, S. Phase structure and adhesion in polymer blends: a criterion for rubber toughening. Polymer 1985, 26 (12), 1855−1863. (19) Perkins, W. G. Polymer toughness and impact resistance. Polym. Eng. Sci. 1999, 39 (12), 2445. (20) Harada, M.; Ohya, T.; Iida, K.; Hayashi, H.; Hirano, K.; Fukuda, H. Increased impact strength of biodegradable poly(lactic acid)/ poly(butylene succinate) blend composites by using isocyanate as a reactive processing agent. J. Appl. Polym. Sci. 2007, 106 (3), 1813− 1820. (21) Oyama, H. T. Super-tough poly(lactic acid) materials: Reactive blending with ethylene copolymer. Polymer 2009, 50 (3), 747−751. (22) Su, Z.; Li, Q.; Liu, Y.; Hu, G.; Wu, C. Compatibility and phase structure of binary blends of poly(lactic acid) and glycidyl methacrylate grafted poly(ethylene octane). Eur. Polym. J. 2009, 45 (8), 2428−2433. (23) Feng, Y.; Hu, Y.; Yin, J.; Zhao, G.; Jiang, W. High impact poly(lactic acid)/poly(ethylene octene) blends prepared by reactive blending. Polym. Eng. Sci. 2013, 53 (2), 389−396. (24) Han, L.; Han, C.; Dong, L. Morphology and properties of the biosourced poly(lactic acid)/poly(ethylene oxide-b-amide-12) blends. Polym. Compos. 2013, 34 (1), 122−130. (25) Zhang, K.; Nagarajan, V.; Misra, M.; Mohanty, A. K. Supertoughened Renewable PLA Reactive Multiphase Blends System: Phase Morphology and Performance. ACS Appl. Mater. Interfaces 2014, 6 (15), 12436−12448. (26) Vachon, A.; Pépin, K.; Béland, O.; Monfette, W. G.; Rochette, A.; Vuillaume, P. Y. Thermal, Mechanical and Morphological Properties of Binary and Ternary PLA Blends Containing a Poly(ether ester) Elastomer. J. Biobased Mater. Bioenergy 2015, 9 (2), 205−217. (27) Zhou, L.; Zhao, G.; Feng, Y.; Yin, J.; Jiang, W. Toughening polylactide with polyether-block-amide and thermoplastic starch acetate: Influence of starch esterification degree. Carbohydr. Polym. 2015, 127 (2015), 79−85. (28) Ojijo, V.; Ray, S. S.; Sadiku, R. Toughening of biodegradable polylactide/poly(butylene succinate-co-adipate) blends via in situ reactive compatibilization. ACS Appl. Mater. Interfaces 2013, 5 (10), 4266−4276. (29) Ojijo, V.; Ray, S. S. Super toughened biodegradable polylactide blends with non-linear copolymer interfacial architecture obtained via facile in-situ reactive compatibilization. Polymer 2015, 80, 1−17.

Dr. Manjusri Misra is a Professor in the School of Engineering and holds a joint appointment in the Department of Plant Agriculture at the University of Guelph, ON, Canada. Dr. Misra’s current research is primarily focused on novel biobased polymers, and composite materials from agricultural and forestry resources for the sustainable bioeconomy; and application of nanotechnology in materials uses. She has coauthored more than 450 publications, including 250+ peerreviewed journal papers, 24 book chapters, and 15 granted patents. She was an editor of the CRC Press volume, “Natural Fibers, Biopolymers and Biocomposites,” Taylor & Francis Group, Boca Raton, FL (2005); American Scientific Publishers volume “Packaging Nanotechnology”, Valencia, California (2009), and “Polymer Nanocomposites”, Springer (2014). She was the chief editor of “Biocomposites: Design and Mechanical Performance” Woodhead Publishing (2015). She was the 2009 President of the BioEnvironmental Polymer Society (BEPS). She is one of the Associate Editors of the journal “Advanced Science Letters”. Dr. Misra received the prestigious “Jim Hammer Memorial Award” from the BioEnvironmental Polymer Society in 2012.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from (1) the Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA)- University of Guelph Bioeconomy-Industrial Uses Theme (Project # 200425); (2) the Ontario Ministry of Economic Development and Innovation (MEDI), Ontario Research Fund, Research Excellence Round 4 program (ORFRE04) (Project # 050231 and 050289); and (3) the Natural Sciences and Engineering Research Council (NSERC) Canada Discovery Grants (Project # 400322) and Networks of Centres of Excellence (NCE) AUTO21 Program (Project # 460372).

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