Although aliphatic polyesters have been studied extensively but their remarkable properties such as sufficiently high molecular mass coupled with polymerization characteristics were realized only recently which can be exploited in order to replace conventional plastics such as polypropylene. Naturally occurring PHAs are optically active linear polyesters with each repeating unit in the stereochemical R-configuration. Monomeric compositions, chemical structure as well as the molecular weight are the key factors influencing the physical properties of the polymer. The molecular mass of PHAs varies per PHA producer but is generally in the order of 50,000 to 1,000,000 Dalton
Scl-PHAs such as P(3HB) are crystalline polymers which are quite brittle and rigid, with high melting points and low glass transition temperatures. These unique characteristics of this biologically synthesized P(3HB) arises due to its exceptional purity. The brittleness is due to the formation of large crystalline domains in the form of spherulites. On the other hand, P(4HB), P(3HB-co-4HB) P(HB-co- HV), scl-PHAs are strong and malleable thermoplastic polyesters, especially P(HB-co-HV). It is created by incorporating PV into PHB which is less stiff and brittle than PHB, as a result it can be used to prepare films with excellent water and gas barrier properties similar to polypropylene. It can also be processed at lower temperature along with preserving most of mechanical properties of PHB. Moreover, Mcl-PHAs are the thermoplastic elastomers with low crystallinity and tensile strength but high elasticity. They also have a lower melting point and glass transition temperatures in contrast to scl-PHAs and polypropylene.
Last date updated on July, 2014