PHA polymers

Holmes PA (1988) Biologically produced PHA polymers and copolymers. In Bassett (ed) Developments in crystalline polymers. Elsevier, London, p 1... 

An alternative to the extraction of intact PHA polymer is the isolation of PHA monomers, oligomers, or various derivatives such as esters [74]. PH As are composed of stereo-chemically pure P-3-hydroxyacids, and therefore can be used as a source of optically pure organic substrates for the chemical and pharmaceutical industry [79]. In this protocol, the defatted cake containing PHA polymer would be chemically treated to obtain the PHA derivatives. For example, transesterification of the meal with methanol would give rise to methyl esters of 3-hydroxyalkanoic acids. The PHA derivatives would then be separated from the meal with appropriate solvents. One potential disadvantage of this method is the potential alteration of the quality of the residual meal if the harsh chemical treatments required for the production of PHA derivatives lead to protein or amino acid breakdown. 

Table 3. Mechanical Properties of Various PHAs Polymers Compared to Conventional Industrial ...

The physical properties claimed for Metabolix PHA polymers are described below. 

Gas barrier properties - Metabolix PHA polymers have lower moisture vapour transmission rates than other biodegradable polymers. The oxygen transmission rates for unoriented PHA films are 25-30 cc-mil/(100 in2-day) at 77 °C, 0% relative humidity. 

Another terpolymer poly(3-hydroxybutyrate-co-4-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-4HB-co-3HHx)] was found to have better thermal stability due to the introduction of 4HB and 3HHx monomers into P(3HB) (Xie and Chen 2008). It has lower crystalUnity and better flexibility compared to P(3HB) homopolymer and copolymers. Poly(3-hydroxybutyrate-C( -3-hydroxyvalerate-co-4-hydroxybutyrate) [P(3HB-co-3HV-co-4HB)] also showed superior properties over those of 3HB and its copolymer (Madden et al. 2000 Chanprateep and Kulpreecha 2006). Table 2.3 shows the comparison of different types of PHA polymer. 

P(3HB-co-4HB) copolymers are known to be biocompatible material. The biocompatible nature of these copolymers allows it to be utilized for various medical applications. These PHA polymers have been tested in tissue engineering applications as surgical sutures, bone plates, implants, gauzes, osteosynthetic materials, and also as matrix material assisting slow release of drugs and hormones (Zinn et al. 2001 Williams and Martin 2002 Sudesh 2004 Chen and Wu 2005 Freier 2006). Recently, electrospun nanofibers of P(3HB-co-4HB) have been evaluated as scaffolds in vivo and in vitro (Ying et al. 2008). 

Naylor LA, Wood JC (1999) Process for the microbiological production of pha-polymers. US Patent 5871980... 

Among all PHA polymers existing, PHB, its copolymers, and PHBV are today the most widely commercially available. The first commercialization of PHB is found back in the 1950s by W.R. Grace Co., USA, but the trial soon failed [27], Subsequently in the 1970s, Imperial Chemical Industries [ICI], United Kingdom, developed... 

A remarkable characteristic of PHAs is their biodegradability in various environments. The PHA-degrading microorganisms were first isolated by Chowdhury in 1963. Subsequently, however, only a few studies on isolation and characterization of PHA-degrading microorganisms were reported for three decades. With the increased demand to use microbial PHA polymers as environmentally degradable thermoplastic, the... 

The development of new PHA polymers can be achieved by genetic engineering. In addition, the elucidation of metabolic pathways could also aid in the development of new metabolic routes. 

Graft copolymers are polymers produced when molecular fragments belonging to others polymers have been inserted into their side chains (Hazer and Steinbiichel 2007). The condensation reaction between the carboxy-terminal groups of certain PHAs poly[(/ )-3-hydroxybutyrate] poly[(/ )-3-hydroxybutyrate-co-(l )-3-hy-droxyvalerate] or PHO and the amine function of chitosan resulted in grafting of PHA polymers onto chitosan (Fig. 2). These grafted polymers formed opaque, viscous solutions in water and, once dried, afforded strong elastic films (Yalpani et al. 1991 Arslan et al. 2007). 

The functional architecture of this class of PHA copolymers is substantially different from that of more familiar types of PHAs, such as poly[(/ )-3-hydroxybutyrate] (PHB) homopolymer or poly[(/ )-3-hydroxybutyrate-c<9-(/ )-3-hydroxy valerate] (PHBV) copolymer. The size of the side groups in the conventional PHAs is limited only to short-side-chain types with no more than two carbon atoms. Although PHAs with only one or two carbon side groups may be viewed essentially as linear polymers, PHA copolymers with mcl side groups belong to a broad class of moderately branched polymers. The inclusion of a small amount of mcl-3HA units into the PHA polymer backbone leads to some profound changes in important physical properties of this class of copolymers. 

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