Polyhydroxyalkanoates crystallinity

Biodegradable polymers can be mainly classified as agro-polymers (starch, protein, etc.) and biodegradable polyesters (polyhydroxyalkanoates, poly(lactic acid), etc.). These latter, also called biopolyesters, can be synthesized from fossil resources but main productions can be obtained from renewable resources (Bordes et al. 2009). However for certain applications, biopolyesters cannot be fully competitive with conventional thermoplastics since some of their properties are too weak. Therefore, to extend their applications, these biopolymers have been formulated and associated with nano-sized fillers, which could bring a large range of improved properties (stiffness, permeability, crystallinity, thermal stability). The resulting nano-biocomposites have been the subject of many recent publications. Bordes etal. (2009) analyzed this novel class of materials based on clays, which are nowadays the main nanoflllers used in nanocomposite systems. 

K. Sudesh and H. Abe, Practical Guide to Polyhydroxyalhanoates-Crystalline and Solid State Structures of polyhydroxyalkanoates, ed. 

The family of polyhydroxyalkanoates (PHA) exhibits a wide variety of mechanical properties from hard crystalline to elastic, depending on the composition of monomer units [12]. Solid-state poly(3-hydroxybutyrate) (P(3HB)) is a compact right-handed helix with a two-fold screw axis (i.e. two monomer units complete one turn of the helix) and a fibre repeat of 0.596 nm [13]. The stereoregularity of P(3HB) makes it a highty crystalline material. Its melting point is around 177°C close to that of polypropylene, with which it has other similar properties, although the biopolymer is stiffer and more brittle. 

Polyhydroxyalkanoates (PHAs) are polyesters synthesized by many bacteria. These polymers are accumulated intracellularly imder nutrient stress and act as a carbon and energy reserve. PHAs are non-toxic, biodegratkble and biocompatible. They are produced from renewable sources. They have high degree of polymerization, are highly crystalline, isotactic and insoluble in water. Because of these properties, PHAs have a real potential in medical and pharmaceutical applications, such as drug delivery systems and tissue engineering. 

The simplest of the family of polyhydroxyalkanoate (PHA) biopolymers is poly-R-3-hydroxybutyrate or PHB. This polymer was first discovered in 1925 by Lemoigne and was initially described as a lipid inclusion in the bacterium Bacillus megaterium. The technical challenges for PHA are its narrow processing window and high brittleness. To overcome those problems, it is usual to make the copolymer with valerate resulting in PHBV that exhibit reduced crystallinity. 

Moving from the Cj and C2 sidechain polyhydroxyalkanoates to longer sidechains leads to a progressive change in the crystallinity and mechanical properties of the resulting copolymers. 

Polyhydroxyalkanoates and polyhydroxybutyrate in particular may be considered as rather controversial polymers. PHB is sometimes referred to as a material that appeared too early, before its time has came. To summarize, PHAs are prepared from renewable resources and are fully biodegradable. They outperform most of the other biodegradable polymers and many synthetic plastics in properties, notably mechanical strength and modulus, resistance to water and moisture due to high hydrophobicity, high crystallinity and several other physical properties such as barrier behavior or piezoelectricity. 

Polyhydroxyalkanoate is a polyester identified in 1925 by the microbiologist Maurice Lemoigne. It can be synthesised by various bacteria (Alcaligenes Eutrophus, cyanobacteria). Lower concentrations of carbon, nitrogen and phosphorus sources increase the yield and the quality of the polymers produced (Steinbiichel, 2002). There are numerous potential applications for PHA (cosmetics containers, disposable articles, medical implants, paper coatings). Moreover, PHA can be formulated in many grades, from elastic products to crystalline ones, it is a good candidate for blends and easy to process with traditional equipment (Whitehouse, 2000). 

Table 9.1 Physical characteristics of polyhydroxyalkanoate polymers and petroleum-based plastics, including melting temperature (T ,), glass transition temperature (Tg), and degree of crystallinity (% crystall.)...

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