Studies on comonomer compositional distribution of the bacterial poly(3-hydroxybutyric acid-co-3-hydroxypropionic acid)s and crystal and thermal characteristics of their fractionated component copolyesters

Abstract

Two different kinds of natural poly(3-hydroxybutyric acid-co-3-hydroxypropionic acid)s [P(3HB-co-3HP)] with individual 3HP comonomer contents of 36.5 and 68.1 mol. % were biosynthesized by the bacteria Alcaligenes latus, fed on the cosubstrates of ( R)-3-hydroxybutyric acid (3HBA) and 3-hydroxypropionic acid (3HPA). Employing the mixed chloroform/ n-heptane solvent, the bacterial products were found to be fractionated into a number of fractions, and the presence of broad comonomer compositional distributions was unambiguously demonstrated. A deep insight into fractionation provided by means of g.p.c. and n.m.r. spectrometry confirmed that the fractionations were predominantly governed by the factor of comonomer composition, while the effect of molecular weight was not so significant. 13C n.m.r. investigation of carbonyl diad sequence distributions for the fractionated copolyesters along with the original bacterial product clearly revealed that the copolymerization occurring in the bacterial cell bodies was virtually in accordance with the random copolymerization model, and this feature was, however, obscured due to the presence of comonomer compositional distribution. Further, the crystal structures and thermal behaviors were investigated via WAXD and d.s.c. for the fractionated copolyesters with 3HP comonomer content spanning the whole comonomer composition. These investigations showed that both 3HB- and 3HP-rich copolyesters formed different 2 1 helix crystal structures, respectively, corresponding to the P(3HB) and P(3HP) types of lattice structures with distinctive fiber repeats, while those bearing intermediate 3HP content (about 48–75 mol.%) appeared as the amorphous state. The existence of the minor comonomer drastically retarded the crystallizability of either 3HB- or 3HP-rich copolyester chains; however, the glass transition behaviors reflected that the chain mobility in the amorphous state was enhanced linearly with the increase in the 3HP unit content. Moreover, the growth rates and morphologies of spherulites for the fractionated copolyesters revealed that the growth rates were markedly suppressed by the incorporated minor comonomer, and the uncrystallizable chains were likely trapped in the interfibrillar regions.