Starch blends: A review of toughening strategies

Accepted Manuscript Title: Fully Biodegradable Poly(lactic acid)/Starch Blends: A Review of Toughening Strategies Authors: J.Justin Koh, Xiwen Zhang, Chaobin He PII: DOI: Reference:

S0141-8130(17)34302-7 https://doi.org/10.1016/j.ijbiomac.2017.12.048 BIOMAC 8709

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

1-11-2017 27-11-2017 7-12-2017

Please cite this article as: J.Justin Koh, Xiwen Zhang, Chaobin He, Fully Biodegradable Poly(lactic acid)/Starch Blends: A Review of Toughening Strategies, International Journal of Biological Macromolecules https://doi.org/10.1016/j.ijbiomac.2017.12.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fully Biodegradable Poly(lactic acid)/Starch Blends:

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A Review of Toughening Strategies J. Justin Koh†§, Xiwen Zhang§ and Chaobin He*†‡

Department of Materials Science and Engineering, National University of Singapore, 9

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Singapore Institute of Manufacturing Technology, Agency for Science, Technology and

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§

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Engineering Drive 1, Singapore 117576

Institute of Materials Research and Engineering, Agency for Science, Technology and Research

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‡

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Research (A*STAR), 73 Nanyang Drive, Singapore 637662

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(A*STAR), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634 Corresponding author*

Email: [email protected]

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Keywords: Poly(lactic acid), Starch, Sustainability, Biodegradable, Bio-based, Toughening, Compatibilization.

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Abstract

Polylactic acid (PLA) and Starch are both bio-based biodegradable polymers that have properties that are complementary to each other. PLA/starch blend exploits the good mechanical property of

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PLA and the low cost of Starch. However, PLA/Starch blend is intrinsically brittle. This paper reviews the current state of arts in toughening of PLA/Starch blend, which are categorized as: Additive

Plasticization,

Mixture

Softening,

Elastomer

Toughening

and

Interphase

Compatibilization. These strategies are not mutually exclusive and can be applied jointly in a

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single blend, opening up a wide range of toughening techniques that can be employed in

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PLA/Starch blend. Even though significant progress has been made in this area, there is still much

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room for research, in order to achieve easy to process, fully bio-based and completely

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biodegradable PLA/Starch blends that have mechanical properties suitable for a wide range of

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applications.

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1. Introduction

The increasing global population and increasing consumption has resulted in increasing

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environmental pollution and waste accumulation. Plastic waste now poses a major challenge not only to our environment, but also to the marine ecosystem. These problems are predominantly

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contributed by plastics employed in short-term applications such as packaging and disposable products. A large portion of these issues can be resolved by employing biodegradable materials,

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and hence they are receiving considerable attention in recent years. Among the biodegradable materials, polylactic acid (PLA), also known as polylactide, is one of

the most promising polymers. PLA is an aliphatic polyester that is capable of breaking down into smaller molecular weight species and subsequently into carbon dioxide, water and small organic

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molecules through hydrolysis in the presence of certain microorganisms distributed in the soil environment [1]. Besides being biodegradable, PLA is also bio-based (bio-derived), hence renewable. Its monomers, lactic acids, are obtained from the fermentation of carbohydrates by

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bacteria of the genus Lactobacillus [2]. This further supports the environmental aspect of PLA as a sustainable alternative. The conventional method of synthesizing high molecular weight PLA starts from lactic acids, to intermediate oligomeric lactic acid and lactides, and eventually high molecular weight PLA [3]. Firstly, lactic acids undergo condensation polymerization into low molecular weight oligomeric lactic acids. Subsequently, depolymerisation of the low molecular

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weight PLA into lactides and eventually the ring-opening polymerization of lactides into high

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molecular weight PLA. Due to a chiral centre, lactic acids exists in two enantiomeric forms, L-

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(+)-lactic acid and D-(-)-lactic acid, allowing the formation of L-lactide, D-lactide and DL-lactide

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(meso-lactide). This in turn provides the freedom to adjust the tacticity of PLA from isotactic PLLA and PDLA, to syndiotactic and atactic PDLLA. Chart 1 illustrates the various chemical

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structures of the stereoisomers of lactic acid, lactide and PLA. Another interesting feature of the

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PLA’s structure is the ability to form PLLA and PDLA stereocomplex [4]. This stereocomplex has

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higher inter-chain interactions due to unconventional hydrogen bonding resulting in better thermal stability and mechanical properties. These tacticity variations allow the tuning of PLA’s

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microstructure and hence its physical properties. In general, PLA possess outstanding mechanical properties. Its stiffness and strength are comparable to conventional petroleum-based synthetic

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polymers such as polyethylene terephthalate (PET) and polystyrene (PS), making PLA a promising However, PLA has its shortcomings. Firstly, it has a higher cost of production as compared to

those petroleum-derived, non-biodegradable counterparts. Secondly, PLA is inherently brittle

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despite its tuneable tacticity, microstructure and mechanical properties. These drawbacks have impeded PLA’s path to be employed in a wide range of short-term applications In order for PLA to be a more attractive replacement to conventional petroleum-based polymers,

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it can be blended with another component that is much more economical. Starch appears to be to an appealing choice because of its low cost. Furthermore, Starch is completely biodegradable [2] and renewable from annual harvest. The composition of Starch comprises of amylose and amylopectin which are made up of glucose units. The ratio of amylose and amylopectin in Starch varies from different Starch sources, which can result in slightly different physical properties of

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the material.

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In spite of the advantages of Starch, Starch is also inherently brittle with a high glass transition

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and melting temperature. Starch and PLA are also different in their interaction characteristics,

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Starch hydrophilic while PLA is hydrophobic. This means that Starch-based materials are susceptible to water. However, the Starch component can be shielded from contact with water if

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the dispersed Starch phase is embedded within the hydrophobic PLA matrix in the PLA/Starch

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blend. Moreover, it would be spontaneous for the hydrophobic (less polar) polyester to form the

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outer layer in order to lower the surface tension of the material, hence, improving the water resistance aspect of the blend [5–7]. More importantly, the thermodynamically incompatible

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hydrophobic PLA and hydrophilic Starch components results in phase separation and poor interfacial adhesion between the phases. As such, mechanical stress cannot be properly distributed

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from the PLA matrix to the dispersed Starch phase, creating defects in the system that results in decreased strength, ductility, and ultimately toughness of the binary blend. In fact, PLA/Starch blend results in weaker and even more brittle material than that of the pure PLA.

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Unfortunately, many of the short-term applications such as packaging and disposable tableware, are primarily structural applications that require the material to possess some form of mechanical robustness. For example, for the case of most packaging applications, such as food packaging,

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certain amount of ductility is required so that the material can withstand the storage and distribution process without failing catastrophically. Therefore, toughening of PLA/Starch blends by increasing its ductility without sacrificing too much of its strength is crucial for PLA/Starch blend to be employed in a wide range of short-term applications.

In the recent decades, many researchers have come up with different methods to toughen the

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PLA/Starch polymer blend to achieve balanced mechanical properties suitable for a wide range of

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short-term applications. In this paper, key strategies employed in the toughening of the polymer

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blend will be reviewed, organised and discussed. These strategies can be categorised into 4 main

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categories: Additive Plasticization, Mixture Softening, Elastomer Toughening and Interphase Compatibilization; and 11 sub-categories: Plasticization of Starch, Plasticization of PLA, Ductile

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Immiscible Component, Ductile Miscible Component, Elastomer Compatible to PLA and Starch,

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Elastomer Compatible to PLA, Elastomer Compatible to Starch, Chemical Crosslinking,

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Amphiphilic Bridging, Componential Modification and Interfacial Transitioning. Lastly, the complication of the combinations of these strategies will also be examined. This paper focuses on

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the PLA/Starch binary blend where PLA is the matrix and Starch as the dispersed phase. 2. Toughening Strategies

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2.1. Additive Plasticization Plasticization is a common technique to increase the flexibility of the polymers as well as to

improve their processability. As a result of increased flexibility, the toughness of the material can be increased. However, the increase in flexibility through plasticization strategy usually come with

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the price of decrease in strength due to the reduction in polymer inter-chain interaction. Nevertheless, this allow mechanical properties to be tunable by manipulating the polymer/plasticizer ratio. The additive plasticization strategy for the PLA/Starch blend can be

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targeted at the dispersed starch phase, PLA continuous phase or both. 2.1.1. Plasticization of Starch. The primary purpose of plasticizing Starch is to improve the processability of Starch for the blend. For pure dry Starch, which are semi-crystalline, thermal degradation occurs before the melting [8]. This is largely due to the strong intra- and inter-chain hydrogen bonds by the hydroxyl groups of the glucose units [9]. With the presence of plasticizer,

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heat and mechanical shear, intermolecular hydrogen bonds between Starch molecules can be

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weakened, while hydrogen bonds can be formed between the plasticizer and the Starch molecules.

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This process is known as gelatinisation, which transforms Starch into a more amorphous material,

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resulting in the depression of melting temperature (Tm) and glass transition temperature (Tg) such that Starch can be process by conventional methods such as moulding and extrusion. This

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depression of Tm and Tg by plasticizer can be attributed to the translational entropic effect. The

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level of gelatinisation and hence the Tm and Tg of the Thermoplastic Starch (TPS) is dependent on

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the amount and type of plasticizer used. Chart 2 shows the chemical structure of various small molecules that have been successfully

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used to plasticize Starch. Water can act as a plasticizer for starch but is not preferred due to its volatility [8,10–14]. Various less volatile polyols such as glycerol [8,11,13–18], xylitol [11],

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sorbitol [13,15,19,20], ethylene glycol [19], propylene glycol [19], diethylene glycol [19] has successfully been used to plasticize Starch. Many other bio-derived plasticizers such as glucose [9, 11], fructose [14], sucrose [14] and citric acid [18] has been reported to be able to plasticize Starch as well. Amongst these hydroxyl groups-containing molecules, glycerol remains as the most

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conventional small molecule plasticizer for Starch. However, these small molecules plasticizer tends to migrate during aging, leading to recrystallization (retrogradation) of Starch, causing the embrittlement of TPS [5, 18]. To tackle this issue, Ma and co-workers discovered that a series of

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amides and amines, such as urea [22–24], ethanolamine [22] formamide [23,24] and acetamide [24] can form stronger hydrogen bond with Starch as compared to those hydroxyl-groupscontaining molecules. The stronger hydrogen bond between the plasticizer and Starch serves to supress the retrogradation process, and hence improving material stability.

However, as mentioned previously, the poor interfacial adhesion between Starch and PLA

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hinders the stress transfer between the matrix and the dispersed phase. Therefore, the effect of

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plasticizing Starch on the toughness of the PLA/Starch is limited without proper compatibilization

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between the two phases [25,26]. However, if Starch has been sufficiently plasticized, it can induce

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elastomer toughening effect (see 2.3.3. Elastomer Compatible to Starch) or even improve interphase compatibility through interfacial transitioning (see 2.4.4. Interfacial Transitioning).

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Indeed, with suitable interphase compatibilization, a brittle-to-ductile transition can be observed

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from the PLA/Starch blend if the Starch is sufficiently plasticized, in other words, when plasticizer

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content exceeds a critical threshold [25,26]. 2.1.2. Plasticization of PLA. The plasticization strategy can be employed on the PLA matrix.

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Chart 3 illustrates the chemical structures of molecules that has plasticizing effect on PLA. Many small molecule esters such as triethyl citrate [25,27,28], tributyl citrate [27,29–31], acetyl triethyl

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citrate [27,31], acetyl tributyl citrate (Tributyl O-acetylcitrate) [27,31,32], Diethyl adipate [31], dioctyl adipate (Bis(2-ethylhexyl) adipate (DEHA))[32,33], diisodecyl adipate [31], glycerin triacetate (triacetin) [30,32,34], cardanol acetate [35], diethyl bishydroxymethyl malonate [29], glyceryl tribenzoate [34], dipropylene glycol dibenzoate [34] has successfully been used to

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plasticized PLA and consequently increase its ductility and toughness. Small building blocks of PLA, such as lactides [36] and oligomeric lactic acid [25], are also known to be effective plasticizers of PLA. Glucose monoester and partial fatty acid ester had been explored but their

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effect on PLA’s toughness is limited [36]. Similarly, PLA plasticized by small molecules also face aging issues. The low molecular mass of these plasticizers facilitated their migration ability from their original position after processing [30], which will hasten the cold crystallization process of PLA leading to the embrittlement of the material [29]. One possible solution to impede migration of plasticizers is to increase their

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molecular weight hence reduce their mobility. Some large molecules such as polyethylene glycol

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(PEG) [25,36–40], polypropylene glycol (PPG) [25,41], block copolymer of PEG and PPG

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(Pluronic) [42], as well as polymeric adipate [33] have been employed successfully as plasticizers

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for PLA. Efforts have also been made to polymerize small molecules plasticizers of PLA, such as

their molecular weight.

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oligomeric tributyl citrate [29] and oligomeric diethyl bishydroxymethyl malonate [29], to increase

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The increase in molecular weight of the plasticizers comes at a price of compatibility and

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functionality. As a result of increased molecular weight, the miscibility [39,42,43] and efficiency [25,29,39] of the PLA plasticizer may decrease. For example, in the case of PEG, phase separation

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occurs at a lower plasticizer content for higher molecular weight PEG [39]. In addition, larger molecular weight plasticizer is less effective, which can be gauged from the smaller magnitude of

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Tg depression of PLA at the same plasticizer content [25,29,39]. This phenomenon can be easily predicted by the Flory-Fox equation and Fox equation.

According to the Flory-Fox equation, the increased molecular weight of the plasticizer will lead to an increase in its own Tg. With the increase in Tg of the plasticizer, the Tg final polymer blend

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(plasticized PLA) will be higher, as described by the Fox equation. Hence higher molecular weight molecules are less effective plasticizers. Indeed, the effectiveness of the plasticizer seems to compete with stability of the material

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through molecular weight alteration. However, Ljungberg and Wesslén [29] reported that by introducing an amide group into the oligomeric diethyl bishydroxymethyl malonate, hydrogen bonding can form between the plasticizer and PLA. This increase in intermolecular interaction decrease the plasticizers’ migration and cold crystallization rates without decreasing the compatibility and functionality of the plasticizer.

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Nevertheless, by trading some strength for flexibility through plasticization of PLA, impact

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strength of the PLA/Starch blend can be greatly enhanced; even higher than that of neat PLA [37,

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41]. It can be seen that employing plasticization strategy on the PLA matrix is a more effective

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method to toughen the PLA/Starch polymer blend compared to plasticizing the dispersed Starch

2.2. Mixture Softening

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phase, as mechanical stress rest mainly on the PLA matrix for the uncompatibilized blend.

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The gist of this strategy is to tune the mechanical properties of the PLA/Starch blend by

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incorporating a 3rd component that possess complementary properties such as ductility and flexibility. This additional component should be compatible to PLA in order to impart ductility to

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the matrix, which can either miscible or immiscible with PLA. This section will illustrate this strategy using 3 different biodegradable polyesters, namely, Polybutylene succinate (PBS),

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Polycaprolactone (PCL) and Atactic Polyhydroxybutyrate (a-PHB). 2.2.1. Ductile Immiscible Component. One successful example of this strategy is the addition

of Polybutylene succinate (PBS). PBS is a semi-crystalline polymer that has a melting temperature (Tm) of about 114oC and glass transition temperature (Tg) of about -32oC [45]. Therefore, it is a

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ductile material at ambient temperature and can achieve elongation at break of >300% depending on molecular weight [45]. The morphology and mechanical properties of PLA/PBS blend has been documented by Deng

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and Thomas [46]. At a much lower mass fraction of PBS, between 8.4wt% to 42wt%, PBS is able to form a co-continuous phase with PLA. This is because at the processing temperature, the viscosity of PBS is significantly lower than that of PLA, which facilitates the dispersed PBS to deform and coalesce into continuous fibres [46,47]. This phenomenon has been reported in other polymer blends with huge disparity in viscosity as well [48]. Beyond 42wt%, phase inversion takes

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place and PLA becomes the dispersed phase in PBS matrix.

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Despite being immiscible, the good interfacial adhesion between the PLA and PBS phase results

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in reasonably accurate predictions of the PLA/PBS mechanical properties using mixture rules.

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Young’s modulus of the blend fall between the upper and lower bound of the mixture rules. However, tensile strength is slightly lower than lower bound of the mixture rules; an indication

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that the interfacial adhesion is not ideal. Nonetheless, it is quite close to the lower bound of the

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theoretical value, suggesting reasonable compatibility.

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Indeed, given the reasonable compatibility between PLA and PBS, high elongation at break ~270-340%, is also obtained when the co-continuous phase is formed. In addition, scanning

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electron micrograph of the fracture surface with this composition range (8.4 to 42 wt% PBS) showed drawn tiny fibrils; another indication of ductility. Zhong and coworkers [49] employed

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this strategy on the PLA/Starch blend to form the PLA/Starch/PBS ternary blend and has successfully increased the ductility of the material by increasing and decreasing the PBS and PLA content respectively, trading off some strength and stiffness.

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Another example of a complementary polymer is Polycaprolactone (PCL), whose Tm and Tg is about 60oC and -60oC respectively [50,51], is also a ductile material at ambient temperature. PCL is employed in a similar fashion as PBS due to its complementary properties to PLA/Starch blends.

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It is known that PLA and PCL are immiscible and hence, a much smaller proportion of PCL in the PLA/PCL blend will form a dispersed PCL phase in PLA matrix [50,52]. Unmodified PLA/PCL blends have shown improved the ductility and toughness as compared to neat PLA [50], However, after compatibilization, PLA/PCL blend of showed better mechanical properties that are closer to that predicted by the rules of mixture [50,52–55], an indication that interfacial adhesion of the

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unmodified PLA/PCL blend is non-ideal but reasonable. Nonetheless, even without PLA and PCL

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compatibilization, Sarazin and co-workers reported improved ductility and toughness of

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PLA/Starch blend by incorporating PCL [51]. However, this improvement in toughness may be

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partially contributed by another a compatibilizing mechanism between the three components, which will be elaborated in 2.4.4. Interfacial Transitioning sub- section.

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2.2.2. Ductile Miscible Component. Atactic Polyhydroxybutyrate (a-PHB), a type of

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Polyhydroxyalkanoate (PHA), is an amorphous polymer that has a Tg of about 0oC, and hence

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behaves like an elastomer at ambient temperature [56]. It can be synthesized via ring-opening polymerization of racemic β-butyrolactone using KOH/18-crown-6 complex as the initiator [57].

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Focarete and co-workers [56,58] reported that a-PHB is fully miscible with PDLLA and PLLA over a full range of compositions. However, Ohkoshi et al. [59] suggest that the miscibility

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between PLA and a-PHB is dependent on their molecular weight. Low molecular weight a-PHB (Mw = 9 400) is miscible with PLLA (Mw = 680 000) up to 50wt% a-PHB content, whereas high molecular weight a-PHB (Mw = 140 000) is immiscible with PLLA. Nonetheless, this strategy has allow 50/50wt% of PDLLA/a-PHB blend to achieve the elongation at break of >300%. Therefore,

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a mixture of PLA/a-PHB can serve as a more ductile matrix to the blend with Starch instead of pure PLA. Unfortunately, to the best knowledge of the authors of this review paper, there are no studies done on PLA/Starch/a-PHB ternary blend [56]

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2.3. Elastomer Toughening Elastomer toughening has been a common strategy used for toughening of brittle polymers. Classical examples of elastomer toughened engineering plastics are high impact polystyrene (HIPS) and the copolymer of acrylonitrile butadiene styrene (ABS). Phase separation usually occurs, leading to the formation of rubbery dispersed phase embedded within the brittle polymer

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matrix. The dispersed rubbery phase can act as the location of plastic deformation, as well as craze

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or cavitation initiating centres. These mechanisms serves to absorb energy upon stress, resulting

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in the toughening of the material. There are several factors that affects the final result of elastomer

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toughening. Firstly, it depends on the mechanical properties of the elastomer such as elasticity, ductility, strength etc. Secondly, it also depends on the concentration of the rubbery phase. Lastly,

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it also depends on the interaction between the rubber phase and the matrix phase, which will

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determine the miscibility and dispersion of the rubber phase in the matrix, as well as the interfacial

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adhesion between them.

Nonetheless, it is difficult to find a compatible elastomer for a given matrix, let alone in the case

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of two different phases: PLA and Starch. Therefore, in order to elastomer-toughen the PLA/Starch blend, one would have 3 options: 1) Use an elastomer that is compatible to PLA. 2) Use an

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elastomer that is compatible to Starch or 3) Use an elastomer that is compatible to PLA and Starch. 2.3.1. Elastomer Compatible to PLA and Starch. Polyethylene octene (POE) elastomer has

been successfully used to toughened PLA/Starch blend by Shi and co-workers [60]. However, POE is neither compatible to PLA nor Starch given its non-polar characteristic. In the study, POE was

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compatibilized with both PLA and Starch via grafting of glycidyl methacrylate (GMA) onto POE (POE-g-GMA or GPOE). GMA can react with both terminal carboxyl and hydroxyl groups of PLA and Starch respectively (see 2.4.1. Chemical Crosslinking).

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With a fixed 20wt% of TPS and varying proportion of PLA/GPOE, the ductility of the PLA/GPOE/TPS blend peak at ~400% elongation at break impact strength of ~11.5kJ/m 2 [60] with 15wt% of GPOE. The subsequent decline in ductility and impact toughness can be attributed to excess elastomer in the blend. This phenomenon is common in elastomer-toughening material [61,62]. One possible reason is that the high concentration of the elastomer leading to a high

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concentration of craze nucleation centres. Hence, crazes impinge on one another resulting in the

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failure of the material before the crazes can be fully developed, causing the decrease in ductility

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and toughness of the material.

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2.3.2. Elastomer Compatible to PLA. As for the PLA matrix, many different elastomers has been used to successfully toughen PLA, such as polyolefin elastomer [63], polyamide elastomers

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[64], poly(ether-block-amide) [65], Poly(n-butyl acrylate) (PBA) elastomer [66] and Poly(lactide-

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random-caprolactone) [62]. In order to compatibilize these elastomers with PLA, they are usually

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copolymerized (block or graft) with PLA or chemically bonded to PLA matrix through reactive coupling agents. These techniques should also be applicable to toughen the PLA matrix of the

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PLA/Starch blend as well.

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2.3.3. Elastomer Compatible to Starch. No deliberate attempt was made to toughen

PLA/Starch blend by elastomer toughening the Starch phase as mechanical stress cannot be properly distributed to the dispersed Starch phase due to poor interfacial adhesion between PLA and Starch. However, if Starch has been sufficiently plasticized by small molecule plasticizers, its

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Tg can depressed to a level below ambient temperature making it rubbery, as described by the Fox equation. Many researchers have also suggested the formation of a glycerol-rich β-TPS phase on the outer

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region of the dispersed Starch phase above a certain plasticizer content [51,60,67–72]. This phenomenon can be explained by the Harkins spreading theory [72–74], that describes the morphology of a blend when two different immiscible polymers are dispersed within a third. The Harkins’s equation is as follows: 𝜆31 = 𝛾12 − 𝛾32 − 𝛾13

(1)

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Where 𝜆31 is the spreading coefficient for situation when component 1 is the core being

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encapsulated by component 3, while component 2 is the matrix. 𝜆31 must be positive in order for

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this situation to be true. 𝛾 is the interfacial tension of the respective components pair. The following

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equation (2) is Harkins’ equation for the case where for the case where starch-rich α-TPS phase being encapsulated by plasticizer-rich β-TPS phase:

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𝜆𝛽/𝛼 = 𝛾𝛼/𝑃𝐿𝐴 − 𝛾𝛽/𝑃𝐿𝐴 − 𝛾𝛼/𝛽

(2)

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Where 𝛾𝛼/𝑃𝐿𝐴 is known to be large [75], which can also be estimated from the poor interfacial

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adhesion that led to the poor mechanical properties. 𝛾𝛽/𝑃𝐿𝐴 can be assumed to much smaller than 𝛾𝛼/𝑃𝐿𝐴 due to entropic effects, given that Starch (macromolecules) has a much larger molecular

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weight than that of the plasticizer (small molecules e.g. glycerol) [76]. Lastly, 𝛾𝛼/𝛽 can also be

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estimated as very small since Starch and glycerol are largely miscible components. Hence, a plasticizer-rich β-TPS phase encapsulating a starch-rich α-TPS phase being dispersed in the PLA matrix illustrated in Figure 1. is formed. The strongest evidence of this model is the appearance of two relaxation temperature of the TPS phase in DMA analyses, which has been assigned to as the Tg of the α-TPS phase and β-TPS phase

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respectively [51,77]. This plasticizer-rich β-TPS phase naturally has a much lower Tg than α-TPS phase given the higher plasticizer content. With 24wt% of glycerol in the TPS, Tg of β-TPS phase can reach ~ -45 to – 47oC [51]. Nevertheless, it has been reported that both α- and β-TPS Tg can

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be depressed to under ambient temperature though sufficient plasticization, forming a fully rubbery dispersed phase [51].

However, Müller et al. [78] suggest that the plasticizer-rich phase encapsulating the starch-rich phase lack substantial evidence. They propose that the β relaxation temperature was due to the movement of the smaller structural units in the within the Starch molecules. Indeed, it is difficult

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to characterize this thin layer, or to come up with substantial evidence on the thickness of the layer

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[72], hence, more studies are required to prove the glycerol-rich model.

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Despite the controversial model, TPS can perform the role of elastomers in PLA matrix when

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sufficiently plasticized, i.e. Tg of α-TPS phase is depressed to under ambient temperature. The effect of elastomeric TPS can be observed from the increase in ductility as TPS increases in various

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studies [26,60]. This is followed by a subsequent decline in ductility observed when TPS content

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is in excess, similar to the case of usual elastomer toughening cases. However, its effect are limited

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due to poor interfacial adhesion with the PLA matrix. With proper compatibilization, the elastomeric TPS can aid to improve the ductility of the PLA/TPS blend tremendously even when

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used at large amount. Indeed, Huneault and Li [26] observes a brittle-to-ductile transition of the compatibilized PLA/TPS blend when plasticizer content exceed a critical threshold.

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2.4. Interphase Compatibilization Compatibilization strategies are techniques that sought to increase the interfacial adhesion

between the PLA and Starch phase, such that defects can be reduced and mechanical stress can be properly transferred from the matrix to the dispersed phase. From literature, compatibilization

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strategies can be classified into four categories: 1) chemical crosslinking 2) amphiphilic bridging 3) componential modification and 4) interfacial transitioning. 2.4.1. Chemical Crosslinking. This is the most commonly employed strategy to compatibilize

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PLA and Starch, which uses a coupling agent to chemically bond PLA and Starch molecules together.

Maleic Anhydride (MA) has been widely used as compatibilizer for binary immiscible polymer blends [79–84]. Peroxide initiators such as BPO or L101 are used for hydrogen abstraction of PLA chains to generate radicals, which lead to either recombination with another PLA radical that

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results in a crosslink between PLA chains or grafting of MA onto PLA [26,85–87]. If the latter is

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true, the second step is the reaction between the anhydride group and the hydroxyl group of Starch

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(amylose or amylopectin), which results in the opening of the five-member ring and the formation

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of ester linkages [79]. In addition, if TPS is used instead of native starch, the anhydride group can also react with the hydroxyl group of Starch’s plasticizer (e.g. glycerol) [26,85]. It is also possible

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that the anhydride group reacts with the terminal carboxyl group of PLA, which will not aid in

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compatibilization between PLA and Starch. Nonetheless, the crosslinking mechanism between

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PLA and Starch (or TPS) that provides the compatibilization effect is illustrated in Scheme 1. The advantages of MA includes its low toxicity and that it does not homopolymerize under normal

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melt grafting conditions.

Scheme 1. Chemical reaction of the PLA-Starch (TPS) crosslinking process using MA (AA) as

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the coupling agent. Acrylic Acid (AA) can be used as a coupling agent in similar fashion [88], which is also illustrated in Scheme 1 in brackets. Instead of having a highly reactive anhydride group, the

16

carboxylic acid of AA is proposed to undergo condensation reaction with hydroxyl group of Starch at 190oC. Similar to MA and AA, the grafting of Glycidyl Methacrylate (GMA) [89] onto PLA can be

SC RI PT

done via radical generation by peroxides like BPO. The difference between MA and GMA is that GMA is known to be able to homopolymerize [90]. Regardless of its ability to homopolymerize, the reactive epoxide group can react with the hydroxyl groups of Starch or the plasticizer to form ether linkages. Similar to the MA case, the epoxide can also react with the terminal carboxyl group of PLA, which does not contribute to compatibilization efforts [60,91]. The crosslinking

U

mechanism between PLA and Starch (or TPS) via GMA is illustrated in Scheme 2.

N

The effects of using various Diisocyanates (DI) to crosslink PLA and Starch molecules have

A

been explored by many researchers. They include aromatic diisocyanates such as toluene

M

diisocyanates (TDI) [42] , phenylene diisocyanates (PDI) [92] and methylene diphenyl diisocyanates (MDI) [28,42,93,94], as well as aliphatic one such as lysine diisocyanates (LDI) [95]

D

and hexamethylene diisocyanates (HDI) [42,95].

TE

The key difference between DI as compared to previously discussed coupling agents is that its

EP

has two functional groups (isocyanate groups) that can react directly with PLA or Starch, therefore, additional grafting process is not required. The isocyanate group can react with the hydroxyl group

CC

of Starch or carboxyl terminal group of PLA to form urethane linkages. Indeed, the reaction can also results in PLA-crosslinked-PLA or Starch-crosslinked-Starch that does not aid in

A

compatibilization of PLA and Starch. Nonetheless, the crosslinking mechanism between PLA and Starch (or TPS) via DI is illustrated in Scheme 3. 2.4.2. Amphiphilic Bridging. Another way to compatibilize hydrophobic PLA and hydrophilic Starch is to include some amphiphilic molecules to act as physical bridges at the interface of the

17

two incompatible components. This bridging molecule should constitute two different segments; one compatible to PLA and one compatible to Starch. The mechanism is illustrated using a diblock amphiphilic copolymer in Figure 2, however, this strategy is not just restricted to

SC RI PT

amphiphilic block copolymers Unquestionably, one of the polymers that are compatible with any polymer is itself. Therefore, copolymers of PLA and Starch, namely, PLA grafted Starch (PLA-g-Starch) or Starch grafted PLA (Starch-g-PLA) is definitely a suitable and favoured candidate to carry out this compatibilization strategy.

U

Starch-g-PLA can be synthesized via polymerization of PLA onto Starch, using the hydroxyl

N

groups as the initiator instead of the usual alcohol [96,97]. One key challenge of this synthesis

A

method is the removal water content from the hydrophilic Starch, as polymerization of PLA is

M

known to be sensitive to water.

On the other hand, Starch can also be grafted onto PLA. Wootthikanokkhan et al. reversed the

D

chemical crosslinking procedure to produce PLA-g-TPS by reversing the MA crosslinking process

TE

described in the 2.4.1. Chemical Crosslinking sub-section [98]; TPS is first maleated with MA

EP

before being grafted onto TPS using peroxide initiators to generate PLA radical. In this case, plasticizer (e.g. glycerol) might be grafted onto PLA as well, which also act as the amphiphilic can

CC

bridge in PLA/Starch blends. Similarly, Amylose, the linear minor component in Starch, can also be use as the hydrophilic

A

component in the amphiphilic bridge. PLA is also polymerize on Amylose using its hydroxyl group as the initiator [93,99]. Ke and Sun [100] suggested that the copolymer of Polyvinyl acetate (PVA) and Polyvinyl alcohol (PVOH) can act as the physical amphiphilic bridge to compatibilize PLA and Starch. Due

18

to vinyl alcohol’s instability, PVOH is usually synthesized through hydrolysis of the PVA ester bonds. Therefore, partial or incomplete hydrolysis can result in the formation of PVA-PVOH random copolymer (PVA-r-PVOH). PVA has been reported to be miscible with PLA in previous

SC RI PT

studies [100]. Likewise, studies have shown that PVOH and Starch blends demonstrate partial miscibility and compatibility due to formation of intermolecular hydrogen bonding between the two polymers [101–104].

2.4.3. Componential Modification. Another way to compatibilize two incompatible components is to modify of the components such that the modified component has a better affinity

U

with the other. In the case of PLA/Starch blend, this strategy is usually applied on Starch such that

N

it becomes compatible to PLA. Firstly, Starch is usually the minor component as PLA’s superior

A

mechanical properties are preferred in the final material as the matrix. Hence, it is more economical

M

to modify Starch than PLA. Secondly, Starch’s hydrophilicity is one of its greatest Achilles’ heel that hindered its application prospect. This drawback jeopardizes the material’s structural integrity

D

under moist or wet conditions. Therefore, to preserve a certain level of water resistance in the final

TE

material, Starch is usually modified into a more hydrophobic component rather than the reverse

EP

modification on PLA. In order to modify Starch into a more compatible component to PLA, the hydroxyl groups of Starch can be substituted with hydrophobic groups instead.

CC

A classic example of this is acetylation, which has been commonly performed on cellulose to increase its hydrophobicity, forming cellulose acetate. Similarly, as a polysaccharide, Starch can

A

undergo acetylation to form Starch acetate as well. Starch acetate can be synthesized via hydroxyls’ reaction with acetic anhydrides [91,105], as illustrated in Scheme 4. Indeed, Zhou et al. [91] reported that acetylation has proved to be effective in compatibilizing PLA/Starch blend that resulted in increased impact strength depending on the degree of substitution of the hydroxyl

19

groups with acetyl groups. This improved compatibility can be largely be attributed the formation of ester side group on Starch that has better affinity with PLA, a polyester. A series of plant oil derivatives, namely soybean oil [106], castor oil [107], tung oil [108] and

SC RI PT

cardanol [109] were also grafted onto Starch molecules via various coupling agents, such as MA and DI mentioned in the 2.4.1. Chemical Crosslinking sub-section. These plants oils are generally hydrophobic [109] and indeed, the final blends show improved mechanical properties. However, more studies are required to show the decrease in interfacial energy between PLA and the modified Starch that leads to an increased interfacial adhesion.

U

Besides acetylation and grafting of plant oil derivatives, PEG has also been grafted onto Starch

N

[40]. As mentioned previously (see 2.1.1. Plasticization of PLA), PEG has a partial miscibility

A

with PLA and can act as a plasticizer for PLA. Even though PEG is a generally known as a

M

hydrophilic polymer because of its solubility in water, it has occasional hydrophobic behaviour as well [110]. In the study, PEG was first reacted with MA to form dicarboxylic acid. The carboxyl

D

groups can then react with the hydroxyl group of Starch to complete the grafting. The chemical

TE

reaction and proposed morphology is illustrated in Scheme 5. An increased impact strength of

EP

2.37kJ/m2 (more than 3x of unmodified PLA/Starch blend) was achieved with 20wt% of Starchg-PEG in the blend. Elongation for that blend was also increased to ~190%, while tensile strength

CC

remains at a reasonable ~32.2MPa. Instead of chemically substituting the hydroxyl groups of Starch with a PLA-compatible side

A

group, surface coating of a compatible substance on the surface of Starch particles may work as well. Hemvichian et al. [111] employed the admicellar technique to coat a thin film of Polymethyl methacrylate (PMMA) onto Starch particles. Having an ester group as a side group, hydrophobic PMMA has previously demonstrated good compatibility with PLA. In an earlier study, the same

20

admicellar polymerization technique has been used to coat PMMA onto graphene nanosheets to improve the interfacial adhesion between the graphene filler and the PLA matrix [111]. This method of modifying Starch consists of 4 main steps in Scheme 6: 1) Formation of

SC RI PT

admicelle on the Starch surface 2) Adsolubilization of MMA (monomers) within the admicelle 3) Polymerization of the monomers and 4) Removal of the top layer surfactant to expose coated PMMA.

Indeed, the surface modified Starch demonstrated increased hydrophobicity from contact angle tests. Mechanical properties of the blends of PLA and modified Starch were studied with up to

U

30wt% of modified Starch. All the blends demonstrated higher tensile strength and ductility than

N

PLA blends with unmodified Starch, signifying improved compatibility between components.

A

2.4.4. Interfacial Transitioning. Another technique that is not usually applied in deliberate

M

attempt to improve the compatibility of the PLA/Starch blend is to provide a transitioning phase in between PLA phase and Starch phase. This transitioning phase should have better compatibility

D

and interfacial adhesion to both the PLA and Starch phase as compared to the compatibility

TE

between PLA and Starch phases. In order for this strategy to work, the sum of interfacial tension

EP

between PLA and the transitioning phase (γPLA/trans), and the interfacial tension between the transition phase and Starch (γtrans/Starch), should be smaller than the interfacial tension of PLA and

CC

Starch (γPLA/Starch). Then according to Harkins’ theory (see 2.3.3. Elastomer Compatible to Starch) , a spontaneous mechanism will occur in order to lower the free energy of the system, resulting in

A

the transitioning phase encapsulating the Starch phase as illustrated in Figure 3. One possible example of the transitioning phase is the PCL. Previous studies have shown that

PCL has rather good compatibility with Starch [112]. In addition, PCL is also more compatible to PLA than Starch, from interfacial energy estimations [53,75] and microscopic characterization

21

[113]. More importantly, blends of PLA and PCL have demonstrated some form of compatibility through the mechanical properties of their blends (see 2.2.1. Ductile Immiscible Component). For example, Finotti and co-workers [50] reported an increase in elongation at break to ~10% with

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5wt% of PCL and elongation at break of up to 30% with 20wt% of PCL in PLA/PCL binary blends. This mechanical synergy indicates some form of compatibility and interfacial adhesion between PLA and PCL. However, as mentioned previously (see 2.2.1. Ductile Immiscible Component) interfacial adhesion of the unmodified PLA/PCL blend is less than ideal [50,53] but nonetheless, it is still better than PLA/Starch.

U

Indeed, studies have shown that ternary blends of PLA/PCL/TPS results in a better dispersion

N

of the TPS phase as compared to binary blends of PLA/TPS [51,113], indicating improved

A

compatibility within the blend. Furthermore, TPS phases are characterized to be located within the

M

PCL phase, demonstrating the spontaneous mechanism for the phases to lower their interfacial energy through morphological rearrangement to ultimately lower the free energy of the entire

D

system. More importantly, Sarazin et al. [51] reported an increase in ductility of the ternary blend

TE

with increasing PCL content, up to 10wt%. The increase in ductility has also lead to the increase

EP

in impact toughness, where PLA/PCL/TPS 40/10/50 blend achieve an impact strength of ~70J/m (three times that of neat PLA).

CC

However, this transitioning layer is difficult to characterize. Furthermore, interfacial tension between the components may be affected by several factors such as impurities, plasticizer content,

A

molecular weight and polydispersity [76]. Therefore, more investigation regarding the interfacial tension between PLA, PCL and Starch is required in order for PCL compatibilizing effect to be more conclusive.

22

Similarly, the adequate plasticization of Starch that led to the formation of the β-TPS phase (see 2.3.3. Elastomer Compatible to Starch), serving as a transitioning phase to lower the interfacial tension between PLA and Starch. Be it a glycerol-rich or smaller units of amylose and amylopectin

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[51,60,67–72,78,7] , these smaller molecules aggregate at the interface by expelling large chains to lower the interfacial tension through configurational entropic effects [76]. Indeed, Huneault and Li [26] reported decreasing TPS disperse phase size with increasing content of plasticizer. Although they attributed this effect to the lower viscosity of the more plasticized TPS [26], this morphology can also be seen as the result of lower interfacial tension between the matrix and the

U

dispersed phase. However, relying on the entropic effect alone, β-TPS phase may not be sufficient

N

to compatiblize PLA/Starch, which can be observed from their rather poor mechanical properties.

A

Therefore, additional interphase compatibilization may be required to enhance the interfacial

M

adhesion. 3. Disscussion Section

D

3.1. Combination of Strategies

TE

All the toughening strategies described in this review has been summarised in Table 1. It is

EP

important to note that these strategies not mutually exclusive and are able to function jointly in a single blend.

CC

In some of the studies, several strategies are applied collectively on a single blend in search for the optimal mechanical properties. For example, Wootthikanokkhan et al. [98] utilized

A

Plasticization of PLA, Plasticization of Starch and Amphiphilic Bridging strategies concurrently in their study. Glycerol acetate and glycerol was used as the plasticizer for PLA and Starch respectively, while PLA-g-TPS was used as the amphiphilic bridge.

23

However, some of these strategies can also contradict one another when applied simultaneously in a single blend. One common example is the competition for the limited hydroxyl groups in Starch. For example, in a study conducted by Zhou [91], the Chemical Crosslinking (GMA)

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mechanism competes with the Componential Modification (acetylation) mechanism for Starch’s hydroxyl group. This produces in a toughness peak at an esterification degree of 0.04 given a fixed amount of GMA.

In some toughening techniques, several strategies are being applied concurrently in a single operation. For example, the plasticization of Starch can lead to the formation of elastomeric TPS

U

that can provide elastomer toughening effect. In addition, the formation of β-TPS phase layer also

N

improves interphase compatibility through the interfacial transitioning mechanism [26,51,60,67–

A

72,76,78,7]. Similarly, the addition ductile PCL to form a ternary PLA/PCL/Starch blend may

M

provide toughening through mixture softening and interfacial transitioning strategy [50,51,53,75,112,113].

D

In contrast, some of the toughening techniques requires several strategies to work. One example

TE

is the Elastomer Toughening using Elastomer Compatible to PLA and Starch. Since it is difficult

EP

to find an elastomer that is compatible to both PLA and Starch, compatibilization strategies are required to execute this technique. In the study by Shi, the elastomer is chemically crosslinked

CC

with both Starch and PLA, hence acting as a compatibilizer as well as an elastomeric toughening agent [60]. In another example, where the Componential Modification strategy that utilizes

A

adimicellar polymerization technique to coat a thin layer of PMMA onto Starch particles’ surfaces, the surfactant act as an Amphiphilic Bridge compatibilizer between the Starch and PMMA [111]. Without this surfactant as the amphiphilic compatibilizer, the interfacial adhesion between hydrophobic PMMA and hydrophilic Starch will be poorer, hindering stress transfer.

24

3.2. Preserving the Novelty Many of the strategies mentioned in this review employed non-bio-based or non-biodegradable substances. For example, non-biodegradable components such as PEG and PMMA, as well as non-

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bio-based components such as petrol-derived plasticizers, PVA, PVOH, PCL, PBS and PEBA. Their inclusion diminishes the novelty of the final material as a fully bio-based and completely biodegradable material.

Furthermore, some of the techniques involve complicated and excessive processes. For example, the admicellar polymerization to surface modify Starch and chemical synthesis of elastomer

U

compatible to both PLA and Starch can prove to be very tedious. One key advantage of employing

N

Starch in the blend is to utilize its economical aspect. However, these complicated and excessive

A

processes would reduce the commercialization aspect of the PLA/Starch blend to compete with

M

the commodity plastics.

A good strategy should be economically viable. For example, if a readily available material that

D

have strong interactions with both the PLA and Starch phase can be identified, then it can serve as

TE

the interphase compatibilizer either via amiphiphilic bridging or interfacial transitioning. Hence,

EP

mere compounding is required and without the need for tedious chemical synthetic processes. In addition, if the identified compatibilizer is a ductile or elastomeric material, it can perform a second

CC

role of mixture softening or elastomer toughening for the blend. Otherwise, plasticizing Starch to form elastomeric TPS may be sufficient. Preferably, the identified compatibilizing material is both

A

bio-based and biodegradable. This will be the ideal scenario to achieve a toughened PLA/Starch blend that has its bio-based, biodegradable and economical novelty preserved. 4. Conclusion

25

PLA/Starch polymer blend possess a great prospect as a green composite that is both bio-based and biodegradable. Although many of PLA’s and Starch’s qualities complement each other, their incompatibility ultimately result in poor mechanical properties. This paper reviews the toughening

Additive

Plasticization,

Mixture

Softening,

Elastomer

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strategies for PLA/Starch blend conducted by researchers over years and categorizes them into: Toughening

and

Interphase

Compatibilization; which can be further sub-categorized into: Plasticization of Starch, Plasticization of PLA, Ductile Immiscible Component, Ductile Miscible Component, Elastomer Compatible to PLA and Starch, Elastomer Compatible to PLA, Elastomer Compatible to Starch,

U

Chemical Crosslinking, Amphiphilic Bridging, Componential Modification and Interfacial

N

Transitioning. These strategies are also not mutually exclusive and the combinations of these

A

strategies provides a wide range of toughening methodology for the PLA/Starch blend. However,

M

many of the strategies mentioned in this review employed non-bio-based substances, nonbiodegradable components or complicated processing that diminishes the novelty of the eventual

D

polymer blend. Therefore, there is still much room for research on the toughening of PLA/Starch

TE

blend in order to come up with a final material that is fully bio-based, completely biodegradable

A

CC

EP

and easy to process, while keeping these toughening strategies in mind.

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26

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SC RI PT

Figures:

Figure 1. Schematic morphology of TPS with a phase separation of plasticizer-rich β-TPS phase

U

and Starch-rich α-TPS phase. a) α-phase remains glassy for TPS with lesser plasticizer content. b)

TE

D

M

A

N

α-phase becomes rubbery for TPS with higher plasticizer content.

Figure 2. Schematic morphology of amphiphilic bridging using amphiphilic di-block copolymer

A

CC

EP

as an example.

Figure 3. Schematic morphology of Interfacial Transitioning mechanism.

43

CC

EP

TE

D

M

A

N

U

SC RI PT

Schemes:

Scheme 2. Chemical reaction of the PLA-Starch (TPS) crosslinking process using MA (AA) as

A

the coupling agent.

44

SC RI PT U N A M D TE EP CC A Scheme 3. Chemical reaction of the PLA-Starch (TPS) crosslinking process using GMA as the coupling agent.

45

SC RI PT U N

A

Scheme 4. Chemical reactions of the PLA-Starch (TPS) crosslinking process using DI as the

A

CC

EP

TE

D

M

coupling agent.

46

SC RI PT

CC

EP

TE

D

M

A

N

U

Scheme 4. Acetylation of Starch using Acetic Anhydride.

Scheme 5. Chemical reaction for the synthesis of Starch-g-PEG and the proposed morphology of

A

PLA/Starch-g-PEG blend. Adapted from [40].

47

SC RI PT U N A M D

TE

Scheme 6. Process of surface modifying Starch with a thin PMMA coat using admicellar

A

CC

EP

polymerization technique. Adapted from [111].

48

D

M

A

N

U

SC RI PT

Chart:

TE

Chart 1. Conventional synthesis of high molecular weight PLA (top). Stereoisomers of lactic acid,

A

CC

EP

lactide and PLA (bottom).

49

SC RI PT U N

A

CC

EP

TE

D

M

A

Chart 2. Chemical structures of plasticizers for Starch.

50

SC RI PT U N A M D TE EP

A

CC

Chart 3. Chemical structures of plasticizers for PLA.

51

Table 1. Summary of the strategies for toughening PLA/Starch blend. Main Strategies

Sub-Strategies

Additive Plasticization

Plasticization of Starch Plasticization of PLA Ductile Immiscible Component Ductile Miscible Component Elastomer Compatible to PLA and Starch Elastomer Compatible to PLA Elastomer Compatible to Starch Chemical Crosslinking Amphiphilic Bridging Componential Modification Interfacial Transitioning

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Mixture Softening Elastomer Toughening

A

CC

EP

TE

D

M

A

N

U

Interphase Compatibilization

52