By using compositionally fractionated and labeledcopolymers, Yoshie et al. [208] also showed there is a structural transition in the crystal state at around 10 mol% HV. It was confirmed that above 10 mol% HV the d(1 1 0) spacingincreased with an increase in HV, and a small increase ind(0 2 0) was also observed. The d(0 0 2) spacing remainedconstant, confirming the expansion in the ab plane. Below10 mol% HV, the crystal was the same as for P(3HB). Twomodels for P(3HB-co-3HV) crystals were introduced: (1)the sandwich model, which applies below 10 mol% HV: in this case the core of the lamella is pure HB units, and the HV units are at the crystal edges and in the amorphousregions; and (2) the uniform lamella model in which the HV units are uniformly distributed through the crystal. It was of note that crystallization rates dropped rapidly above10 mol% HV as well.