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

Abstract

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 η 0 follows the classical M w 3.4 power-law relationship with the weight average molecular weight M w. 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 η 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 α 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 η 0 and G N 0 , are readily tunable by varying the HHx content, making them attractive as “green” substitutes for non-degradable thermoplastics.