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Biopolyesters

Polyesters are formed by the monomers bonded via ester groups. Compared with other types of plastics, polyesters have greater biogradability as ester bonds are generally easy to be hydrolyzed [127]. Degradation of polyesters consists of breaking certain bonds such as ester, ether, and amide by hydrolysis which could be catalyzed by either an acid or a base, or by oxidation via reactions with free radicals

Scheme 8.3 Preparation of aliphatic polyesters with reactive groups [129].

Source: Tian H, et al. Biodegradable synthetic polymers: preparation, functionalization and biomedical application. Progr Polym Sci 2012;37(2):237—80.

formed by the action of ultraviolet radiation from the sun, heat, and mechanical deformation [128].

Aliphatic polyesters can be prepared by the copolymerization or homopolymerization of cyclic monomers (Scheme 8.3).

Biopolyesters, such as poly(L-lactic acid) (PLA), poly(hydroxybutyrate) (PHB), and other poly-(hydroxalkanoates), can be produced by bacteria and are fully biodegradable to produce water, carbon dioxide, and humus. Stiff, soft, and elastomeric polyesters may be tailored by tuning their molecular architecture. Bacteria and transgenic plants have been successfully used to produce biopolyesters on a large- scale [130]. PLA and PHB are linear aliphatic polyesters with short chain branching, which are highly biodegradable. Aliphatic polyesters have been receiving special attention because they are sensitive to hydrolytic degradation while some of them are not enzymatically degradable [131]. It shall be noted, however, from a life cycle analysis point of view, biopolymers are not necessarily superior in terms of low carbon footprint, resource-, eco- and energy-efficiency, when compared to the same polyesters prepared from fossil feedstocks [132].

PLA is a hydrophobic and semicrystalline polyester that is currently mainly produced from lactic acid derived from renewable sources (mainly corn) [133—135]. PLA can be easily processed into a high strength and high modulus thermoplastic by various techniques. The superb performances of PLA make it attractive as a biodegradable polymer not only for commodity products, but also for specific applications in medical and agricultural areas. For instance, there are quite a large number of publications on PLA and its copolymers for biomedical applications, such as drug releasing microparticles, bone fixation devices, surgical implant material, and porous scaffolds for the growth of neotissue [136,137].

PLA can be synthesized in four methods: direct polycondensation; ring-opening polymerization; solid-state polymerization; and azeotropic condensation polymerization (Fig. 8.9) [138,139].

PLA has been the most attractive polyester over the last several decades because of its renewable origins, controlled synthesis, degradability to nontoxic byproducts, good mechanical properties, and biocompatibility [138]. Depending on the stereoisomer composition, PLA can be amorphous or crystalline, with a melting

Synthesis methods for PLA

Figure 8.9 Synthesis methods for PLA.

Source: Lasprilla AJ. et al. Poly-lactic acid synthesis for application in biomedical devices— a review. Biotechnol Adv 2012; 30(1): 321—328.

temperature of up to 185°C, while it is relatively less thermally stable, decomposing at below 230°C.

However, applications of PLA are currently limited by its high price due to the complicated synthesis process. One strategy to lower its cost while enhancing its mechanical properties is to composite PLA with other materials or fillers. Nanofillers with at least one dimension less than 100 nm have been most commonly chosen, such as silicates, CNC, metals, metal oxide ceramics, and metal nonoxide ceramics in preparation of PLA-based bionanocomposites by solid-state mixing, solvent casting, in situ polymerization, melt-blending, or melt-extrusion. The major challenge is to find the right chemistry to achieve increased interfacial area at the nanolevel [140].

An emerging area that should be highlighted here is the manufacture of natural fiber-reinforced biocomposites because of their excellent mechanical properties and light weight [141]. Natural fiber-reinforced PLA biocomposites can be prepared by compression molding using the film-stacking method with up to 40 wt% fiber. As an example, Kenaf fibers were pretreated with 3-aminopropyltriethoxysilane (APS) and sodium hydroxide coupling agent in order to improve the degree of cross-linkage in the interface area. The resulting PLA/kenaf biocomposites have mechanical and thermomechanical properties significantly higher than the neat PLA matrix [142].

Jonoobi et al. prepared carbon nanofiber (CNF)-reinforced PLA nanocomposites via twin screw extrusion and studied their mechanical properties [143]. For nanocomposites with 5 wt% CNF the tensile strength and modulus increased from 58 MPa (neat PLA) to 71 MPa and from 2.9 GPa (neat PLA) to 3.6 GPa, respectively. The morphology, mechanical, and dynamic properties of the composites were found to depend on the CNF concentration. Similarly, biocomposites of PLA and microfibrillated cellulose (MFC) were produced. The addition of MFC increased tensile strength and Young’s modulus of PLA by 25% and 40%, respectively [144].

Dispersing functional nanoparticles into PLA matrix can yield a new class of hybrid materials, PLA bionanocomposites, with unique properties for uses in green plastics [145] and biomedical materials [146]. Ren et al. prepared binary and ternary blends using thermoplastic starch (TPS) nanoparticles, PLA, and poly(butylene adipate-co-terephthalate) (PBAT) in a one-step extrusion process [147]. According to the morphology analysis of the blends, most of the TPS particles were well dispersed in the polyester matrix of the blends. Similar to PLA, PHB is another commonly used thermoplastic biopolyester, can be produced by fermentation of glucose, acetic acid, and other biofeedstocks by a number of bacteria [148,149].

Biocomposites based on poly(butylene adipate-co-terephthalate) (PBAT), an aliphatic-aromatic and biodegradable copolyester, were also produced by blending with fillers such as Nisin [150] and clay [151].

 
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