Physicochemical properties of multicomponent polyhydroxyalkanoates: Novel aspects

The physicochemical properties such as the degree of crystallinity and temperature and molecularmass characteristics of a number of polyhydroxyalkanoates of various chemical composition synthesized on a complex carbon substrate by bacteria Cupriavidus eutrophus В10646 have been investigated. Two-, three-, and four-component copolymer samples have different sets and ratios of monomers with various lengths of carbon chains: 3-hydroxybutyrate (3HB), 4-hydroxybutyrate (4HB), 3-hydroxyvalerate (3HV), 3-hydroxyhexanoate (3HH), 3-hydroxy-4-methyl valerate (3H4MV), and diethylene glycol (DEG). It has been shown that weight-average molar mass М _w and polydispersity vary in a wide range with no correlation existing with the composition of copolymer polyhydroxyalkanoates and that thermal stability is preserved in the temperature interval between the melting temperature and the thermal degradation temperature from 100 to 120–140°С. The composition and ratio of monomers most notably affect the degree of crystallinity of polyhydroxyalkanoates. Significant differences between the degrees of crystallinity of three- and four-component polyhydroxyalkanoates have been found for the first time. The degree of crystallinity for copolymers P(3HB/3HV/4HB) is 9–22%, and the degree of crystallinity for copolymers P(3HB/3HV/3HH) and P(3HB/3GV/3H4MV) is 41–63%; this value is close to the degree of crystallinity for diblock copolymers P(3HB)/DEG, which is 56–69%. For the four-component copolymers P(3HB/3GV/4HB/3HH), the degree of crystallinity is 30–41%. The values of М _w for the copolymers P(3HB/DEG) are inhomogeneous and the polymers contain fractions uneven with respect to molecular mass: a high-molecular-mass polymer (М _w from 2700 to 4900 kDa) and a low-molecular-mass polymer (М _w = 46–167 kDa). For the copolymers P(3HB)/DEG and P(3HB/3HV/3H4MV), two peaks are observed in the region of melting with the gap between these peaks being 4–20°С. All of the types of copolymer samples, regardless of the monomer ratio, show an increase in elongation at break against the background of a decrease in tensile stress and Young’s modulus, with these effects being pronounced to different extents. On the whole, the properties of multicomponent polyhydroxyalkanoates differ appreciably.