Shear-Induced Nucleation and Morphological Evolution for Bimodal Long Chain Branched Polylactide

The effects of long chain branching on the nucleation density enhancements and morphological evolution for polylactide (PLA) materials during shear-induced isothermal crystallization process were thoroughly investigated by using rotational rheometer and polarized optical microscopy (POM). Shear-induced nucleation density enhancements for the long chain branched PLA (LCB PLA) were studied on the basis of the determination of the critical shear rate, for which the stretch of the longest chains of the linear component is expected. The results of shear-induced isothermal crystallization kinetics show that the crystallization process under shear is greatly enhanced compared to the quiescent conditions and the crystallization kinetics is accelerated with the increases in shear rate and/or shear time. LCB PLA crystallizes much faster than linear PLA under the same shear condition. A saturation effect of shear time on crystallization kinetics is observed for both linear PLA and LCB PLA. In-situ POM observations demonstrate that LCB PLA not only possesses higher nucleation density under the identical shear time and a constant lower value of spherulitic growth rate compared with that of linear PLA but also forms the shish-kebab structure after sheared for sufficient time. The quantitative evaluation of the shear-induced nucleation densities from rheological measurements is based on the space-filling model by using the Avrami equation, and the obtained nucleation density values are well consistent with that estimated from POM observations. A saturation of nucleation density under shear can be reached for both linear PLA and LCB PLA. The saturated nucleation density values are higher than that under the quiescent condition by a factor of over 3 orders of magnitude, and the saturated nucleation density value for LCB PLA is more than that for linear PLA by a factor of 1 order of magnitude under the same shear condition. The enhancement of nucleation ability and the morphological evolution from the spherulitic to shish-kebab structures induced by shear flow can be ascribed to the broadened and complex relaxation behaviors of LCB PLA.