The crystal structure of PHBV polymers

As a general rule, copolymers capable of crystallizing do so with rejection of the minor comonomer component. 

In special cases where the crystalline lattice requirements of the comonomers are very similar, the two components can crystallize with a single crystal structure across the complete range of copolymer composition such systems are called isomorphic. The PHBV polymers, however, display the phenomenon of isodimorphism . The lattice requirements of the HB and larger HV units are such that the copolymers cannot crystallize into single crystal structure across the entire copolymer composition instead, there are two lattice types, the PHB lattice and the PHV lattice. Below an HV content of approximately 40% [21, 22, 31-33] the copolymers crystallize with a PHB-type lattice, above this the copolymers adopt a PHV-type lattice. At the transition point the two lattice types can coexist. 

PHB crystallizes with an orthorhombic unit cell, P2 P2 P2 with a = 5.76 Kb - 13.20 A, c = 5.96 A [34-37]. Two chains, each with 2 left-handed screw symmetry pass through the unit cell, with the chains packed in an antiparallel fashion. The PHV unit cell is very similar and has a = 9,32 A, b = 10.02 A and c = 5.56 A [40, 41]. As one moves across the HV range from the homopolymer up to the 40% HV copolymer there is an increase in the a spacing as the PHB lattice expands to incorporate the bulkier HV units. An expansion in the b parameter has also been observed but the c spacing (the fibre repeat) remains constant. Increasing the HB content in the HV lattice leads to a slight decrease in b as the lattice contracts (Table 5.5) [35]..  

Although wide-angle X-ray diffraction (WAXD) gives the most direct indication of isodimorphism, other measurements also reflect this aspect of the crystalline morphology. The crystalline melting temperature versus HV content curve shows a minimum at approximately 40%HV [21, 22] and the spherulite growth rate curve likewise has a minimum in this region [35]. 

A consequence of this isodimorphism is that the crystallinity of the HB-H V copolymers remains consistently high across the entire range of HV content. Crystallinity values of greater than 60% are typical [33,42]. 

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