Polyhydroxyalkanoate-based Multiphase Materials - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 126

separation in the blend and was found to offset UV-induced yellowing of PVC
providing colour hold throughout the entire UV exposure. 18

6.1.5 Crystalline Structures of Polyhydroxyalkanoates
PHAs are optically active biological polyesters with the hydroxyalkanoic acid
monomers having an R configuration due to the stereospecificity of the
polymerizing enzyme. Once extracted from the cell, PHB presents high
crystallinity. In contrast, in vivo PHB exists as an amorphous polymer. 14 It
was initially thought that the amorphous state of the PHAs in vivo may be
due to the presence of plasticizers or nucleation inhibitors within the cell
inclusion. However, amorphous PHA was also produced in vitro showing that
the process of crystallization of PHA upon its extraction may be the result of
coalescing of the polymer chains that in turn accelerates the crystallization
process. 19
Though crystallinity of PHAs has been well documented in the literature
along with their X-ray diffraction analysis, here we shall only briefly discuss
certain salient features worth mentioning of the crystalline forms of PHB
and some other PHAs. PHB can result in lamellar crystalline structures when
crystallized from dilute solutions or into spherulites when prepared from
melt processing. Typically, the crystals of P(3HB) are lath-shaped of 5-10 mm
length with a thickness of 4-10 nm. 20
PHB can take up the helix conformation (called the a-form) or a planar zig-
zag conformation (called the b-form). The a-conformation results in lamellar
crystals while the less common b-form is usually formed by extending the
uniaxially oriented films of the PHB further, and occupies the amorphous
regions between the lamellar crystals of the a-form. 21
Zhang et al. tried to schematically represent the crystallization of PHA into
the a-form and the b-form with respect to the different elaboration con-
ditions (reproduced in Figure 6.3). As in the representation in Figure 6.3, and
as suggested in various literature reports, the b-form occupies the amorph-
ous region between the lamellar a-form. 22 It should be noted here that while
both the b-form and the a-form can be found in the PHB films, it is the
a-form that is more stable. 20
The mCL-PHAs obtained from mixed cultures have also been reported to
result in helix conformations forming an orthorhombic lattice with two
molecules per unit cell. The side chains in the comonomers are believed to
form ordered sheets, such that the longer the side chain, the thicker the
lamellar sheet. Longer side chains are believed to form stable ordered
sheets.
The crystallinity of copolymers is also affected by the mol% of the
comonomer. The group of Ozaki evaluated the crystal structures in PHBV
with regard to the intermolecular interactions and reported that for HV
content less that 30%, the strength of the H-bonds between the methyl group
and the carbonyl group of the HB part was similar to that of PHB. As the HV
content in the copolymer increases, H-bonding begins to take place between

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.