Cocrystallization in random copolymers of poly(β-hydroxybutyrate-co-β-hydroxyvalerate) and its effect on crystalline morphology

Abstract

Small-angle X-ray scattering (SAXS) and solid state CPMAS l3C NMR were used to describe the crystalline morphology of a series of bacterially produced poly(β-hydroxybutyrate-co-β-hydroxyvalerate) copolymers containing random distributions of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV). Modeling of SAXS results showed that the morphology of this isodimorphic system is better described by two phases, crystalline and amorphous, having defects within each domain. This is in contrast to a model with a large interfacial region between phases. For the 3HV composition range 0–27 mol%, the polymer crystallizes in a poly(β-hydroxybutyrate)-type crystalline lattice. Solid state NMR results showed that there is significant incorporation of the 3HV minor component into the poly(β-hydroxybutyrate) crystalline phase over this composition range. The ratio of the 3HV content in the crystalline phase relative to the overall 3HV content is not linear, but increases with increasing 3HV. For the 21 and 27% 3HV samples, the 3HV content in the crystalline phase is as much as 2/3 of the overall 3HV content. Inclusion of 3HV is correlated to an increase in crystalline disorder (as measured by SAXS), implying that it is easier to accommodate the bulkier 3HV comonomer into a crystalline region that already contains defects. Keywords: bacterial polyesters, poly(3-hydroxyalkanates), small-angle X-ray scattering, solid state NMR, cocrystallization.