Rheology-determined critical conditions for shear-induced crystallization of biosynthesized polyhydroxyalkanoates

Shear-induced crystallization plays a crucial role in the manufacturing process of polymers. In this work, crystallization kinetics of biosynthesized polyhydroxyalkanoates (PHA) under different shear conditions were systematically investigated by rheometers. First, rheological properties of PHA melts were performed at different temperature to obtain mastercurves via the time-temperature superposition principle at 170 °C as a reference temperature. Then the stretch relaxation time and corresponding critical shear rate at different temperatures for the flow regime transition were calculated via the discrete Maxwell relaxation time spectrum and Arrhenius equation. Finally, the influence of shear temperature (T_s), shear time (t_s) and shear rate (γ̇) on the crystallization process of PHA were discussed. The results showed the crystallization rate of PHA was improved significantly under high shear rate and long shear time. Interestingly, the t_1/2 reached the minimum value when the γ̇ or the t_s was large enough, which reached around 450 s at isothermal crystallization condition of 100 °C. Moreover, the nucleation density for PHA increased by appx.100 times than that under quiescent conditions. Therefore, this work may provide a useful theoretical guidance on the shear-induced crystallization of PHA.