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

The theological properties of a series of microbially synthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)s (PHB-HHm), 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 eta(0) follows the classical M-w(3.4) power-law relationship with the weight average molecular weight M-w. The characteristic relaxation time lambda, 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 eta(0). The temperature dependence of rheological properties follows an Arrhenius form. Activation energies for flow E-a are obtained from the slope of the natural logarithm of the shift factor alpha(T) 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 E-a 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 G(N)(0) obtained suggests a highly entangled configuration. The molecular weight between entanglements M-e 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 eta(0) and G(N)(0) are readily tunable by varying the HHx content, making them attractive as green substitutes for non-degradable thermoplastics. (C) 2009 Elsevier Ltd. All rights reserved.