Melt viscoelasticity of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymers

The rheological properties of a series of microbially synthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)s (PHB-HHxs), with varying comonomer (HHx) content, were systematically investigated. Shear viscosities show dependence on the rate of deformation, temperature, molecular weight, and copolymer compositions. The zero-shear viscosity η0 follows the classical Mw3.4 power-law relationship with the weight average molecular weight Mw. The characteristic relaxation time λ, which indicates the onset of shear thinning, ranges from 0.02 to 0.2 s for different PHB-HHxs and is roughly linearly related to η0. The temperature dependence of rheological properties follows an Arrhenius form. Activation energies for flow Ea are obtained from the slope of the natural logarithm of the shift factor αT plotted against the inverse of temperature curve, and the values for PHB-HHxs are found to be in the range of 27-36 kJ/mol Ea decreases with HHx content in the copolymer, a trend that can be related to the difference in chemical structure between HHx and HB, according to the method of Vankrevelen and Hoftyzer. A Generalized Maxwell model models the viscoelastic behavior of the PHB-HHx melt well. The value of the plateau modulus GN0 obtained suggests a highly entangled configuration. The molecular weight between entanglements Me decreases from 11,600 to 9400 as HHx content increases from 3.8 to 10.0 mol%. Our results suggest that the presence of propyl groups in HHx increases the steric hindrance of the PHB-HHx chains, thus resulting in increased segmental friction and entanglement density. As a result, viscoelastic parameters for PHB-HHx copolymers, such as η0 and GN0, are readily tunable by varying the HHx content, making them attractive as “green” substitutes for non-degradable thermoplastics.