Dl. Vanderhart et al., C-13 NMR DETERMINATION OF THE DEGREE OF COCRYSTALLIZATION IN RANDOM COPOLYMERS OF POLY(BETA-HYDROXYBUTYRATE-CO-BETA-HYDROXYVALERATE), Macromolecules, 28(19), 1995, pp. 6394-6400
A series of bacterially produced poly(beta-hydroxybutyrate-co-beta-hyd
roxyvalerate) copolymers containing random distributions of 3-hydroxyb
utyrate (3HB) and 3-hydroxyvalerate (3HV) were characterized by solid-
state CPMAS C-13 NMR to determine the degree of cocrystallization of t
he two components. The 3HV minor component was shown to be incorporate
d into the poly(beta-hydroxybutyrate)-type crystalline phase over the
3HV composition range of 0-27 mol %. The ratio of the 3HV content in t
he crystalline phase to the overall 3HV composition increases with inc
reasing 3HV content, implying that it is easier to accommodate the lar
ger 3HV residue in a crystalline region which already contains lattice
imperfections. For samples with 3HV compositions of 21 and 27% the 3H
V content in the crystalline phase was roughly 2/3 of the overall 3HV
content. Asymmetric line broadening in the C-13 MMR spectra showed tha
t the 3HV ethyl side branches assume multiple configurations when incl
uded in the crystalline phase. For the 27% 3HV sample, two different c
rystalline structures were detected-the poly(beta-hydroxybutyrate) (PH
B) and poly(beta-hydroxyvalerate) (PHV) type lattice structures. The l
atter crystal structure is present despite the fact that the overall 3
HV composition is below 40%, the composition at which the PHV-type cry
stal structure is generally formed. Both crystal types are formed beca
use the 3HV content in the noncrystalline phase is above the changeove
r composition. The coexistence of PHV and PHB lattices suggests that t
here may also be a change in crystal composition as a function of crys
tallization time.