Polymer blends PHAs

Blending PHAs and in particular PHB with other polymers, or with plasticizers, may offer opportunities to improve processability by lowering the processing temperature and reducing the brittleness of PHA-based plastics. So far, many blends containing PHB/PHAs have been studied and also many types of plasticizers have been proposed [26, 27]. 

Several PHAs were also blended with P(3HB). For example, Satoh et al. 1994 [71] describes the possibility of regulating P(3HB) biodegradability by blending this material with P(3HB)-co(3HV). They have concluded that the rate of degradation of the polymer blend was accelerated with respect to P(3HB) alone. In addition, they have proven that there is a linear correlation between crystallinity and degradation rate. 

A special attempt to blend PHA (hard and brittle) with natural latex rubber (soft and high elasticity) was carried out by Kaewkannetra and Promkotra [9]. The blends were prepared using different ratios of each polymer. The morphological and thermal characteristics of the blends were observed and evaluated. It was found that the porosity of the blends was increased upon increasing the PHA content and decreasing the latex content. As expected, the crystallinity was found to be reduced upon increasing the flexible latex content. 

Polymer Blends Incorporating PHA. The mechanical properties, morphology, biodegradability, and thermal and crystallization behavior of PHAs melt-blended or solvent-cast with nonbiodegradable pol5uners [such as poly(vinyl acetate)] and with biodegradable materials [such as wood cellulose fibers (21) and starch] have been reviewed (22). PHB blends with poly(ethylene oxide), poly(vinyl alcohol), poly (L-lactide), poly(D,L-lactide), poly( -caprolactone), poly(3-butyrolactone), P(HB-co-HV), and cellulose and starch derivatives have been... 

Figure 2, Break-up mechanism of threads in polymer blends with a low viscous dispersed pha e.

Miscibility in polymer blends is controlled by thermodynamic factors such as the polymer-polymer interaction parameter [8,9], the combinatorial entropy [10,11], polymer-solvent interactions [12,13] and the "free volume effect [14,15] in addition to kinetic factors such as the blending protocol, including the evaporation rate of the solvent and the drying conditions of the samples. If the blends appear to be miscible under the given preparation conditions, as is the ca.se for the blends dcscibcd here, it is important to investigate the reversibility of phase separation since the apparent one-phase state may be only metastable. To obtain reliable information about miscibility in these blends, the miscibility behavior was studied in the presence and absence of solvents under conditions which included a reversibility of pha.se separation. An equilibrium phase boundary was then obtained for the binary blend systems by extrapolating to zero solvent concentration. 

Other blends such as polyhydroxyalkanoates (PHA) with cellulose acetate (208), PHA with polycaprolactone (209), poly(lactic acid) with poly(ethylene glycol) (210), chitosan and cellulose (211), poly(lactic acid) with inorganic fillers (212), and PHA and aUphatic polyesters with inorganics (213) are receiving attention. The different blending compositions seem to be limited only by the number of polymers available and the compatibiUty of the components. The latter blends, with all natural or biodegradable components, appear to afford the best approach for future research as property balance and biodegradabihty is attempted. Starch and additives have been evaluated ia detail from the perspective of stmcture and compatibiUty with starch (214). 

Blends of poly(3-hydroxyalkanoic acid)s (PHAs) with various natural and synthetic polymers have been reported as reviewed in Refs. [21,22]. By blending with synthetic polymers it is expected to control the biodegradability, to improve several properties, and to reduce the production cost of bacterially synthesized PHAs. The polymers investigated as the blending partners of PHAs include poly(ethylene oxide) [92, 93], poly(vinyl acetate) [94], poly(vinylidene fluoride) [95], ethylene propylene rubber [94, 96], po-ly(epichlorohydrin) [97, 98], poly(e-caprolactone) [99], aliphatic copolyesters of adipic acid/ethylene glycole/lactic acid [100] and of e-caprolactone/lactide... 

However, in addition to their thermoplasticity, representatives of PHAs have optical activity, increase induction period of oxidation, exhibit the piezoelectric effect and, what is most important, they are characterized as being biodegradable and biocompatible. At the same time, the PHAs have disadvantages (high cost, brittleness), which can be partially or completely compensated by using composite materials based on blends with other polymers, with dispersed fillers or plasticizers. Taking into account all the above, we have suggested to create a mixed polymer composite based on poly-3-hydroxybutyrate (PHB) and polyisobutylene (PIB). 

Some synthetic polymers like, polyurethanes, specifically polyether-polyurethanes, are likely to be degraded by microbes but not completely. However, several polymers such as, polyamides, polyfluorocarbons, polyethylene, polypropylene, and polycarbonate are highly resistant to microbial degradation. Natural polymers are generally more biodegradable than synthetic polymers specifically, polymers with ester groups like aliphatic polyesters [1]. Therefore, several natural polymers such as cellulose, starch, blends of those with synthetic polymers, polylactate, polyester-amide, and polyhydroxyalkanoates (PHAs) have been the focus of attention in the recent years [3]. 

Varieties of blends with different types of PHAs have been produced. P(3HB) is the most common, lowest cost and commercially available member of the PHAs. Although P(3HB) can be considered as a polymer of high potential for numerous applications, its low crystallization rate and brittle nature is a disadvantage in some cases [69]. Several smdies were carried out to enhance P(3HB) mechanical and... 

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