4.2. The ˇ-form crystals of PHA
The -form of the P(3HB) crystal is a strain-inducedparacrystalline structure of highly extended chains witha fiber period of 0.46 nm corresponding to a twisted pla-nar zigzag conformation [178]. It has an orthorhombiccrystal system with unit cell parameters of a = 0.576 nm,b = 1.320 nm, and c (fiber axis) = 0.598 nm and space groupP212121. It can be obtained under such conditions as furtherdrawing of uniaxially stretched films, and is metastable – itcan be annealed back to the -form at 130◦C, with a result-ing increase in crystallinity. The -form was first observedin a stretched P(3HB) film by Yokouchi et al. [163], and alsoobtained in uniaxially cold-stretched films of P(3HB) andP(3HB-co-3HV) [179]. From the calculated value of the fiberrepeat length for fully extended P(3HB), the -form has anear completely extended chain conformation. The shift tothe -form did not require prior alignment of the -formlamellae, but followed orientation of the free chains in theamorphous regions between the crystals [180]. In partic-ular, the tie molecules between the lamellar crystals werestrongly extended, which leads to high mechanical proper-ties. Techniques such as melt spinning and drawing of PHAfibers or one- or two-stage cold drawing of films followedby annealing have been shown to dramatically increase thetensile strength, Young’s modulus and elongation to breakof PHA materials and are associated with the introduc-tion of a -crystalline form as at least part of the lamellarstructure [180]. These properties remain unchanged afterseveral months suggesting that secondary crystallizationis suppressed when materials are highly orientated andcrystalline [181].