Rheologically Determined Critical Shear Rates for Shear-Induced Nucleation Rate Enhancements of Poly(lactic acid)

Abstract

Both the nonisothermal and isothermal crystallization kinetics under the influences of different shear conditions for poly(lactic acid) (PLA) were investigated by rheometry. The nucleation and growth of PLA spherulites during isothermal crystallization with different shear conditions were observed by polarized optical microscopy (POM). Shear-induced nucleation rate enhancements of PLA were studied on the basis of the prerequisite determination of the critical shear rates, for which the stretch of the longest chains (high molecular mass tails) of PLA would be expected. The transitions between different shear flow regimes for shear-induced crystallization of PLA at the temperature of 135 °C were determined by two characteristic Weissenberg numbers on the basis of reptation time and Rouse time for the high molecular mass tails, which were determined through combination of the discrete Maxwell relaxation time spectra of PLA at the reference temperature of 190 °C and the Arrhenius type of temperature dependence for the horizontal shift factor, aT. It was then found that the crystallization process of PLA was greatly enhanced by shear compared to the quiescent condition, and the crystallization kinetics could be accelerated by the increased shear rate and/or shear time. It was more interesting to find that there existed a critical shear time under a certain shear rate, and a further increase in the shear time did not lead to further acceleration of the crystallization kinetics. POM observation indicated that the acceleration of crystallization kinetics was obviously brought about by the enhanced nucleus density under the application of shear and the subsequent spherulitic growth rates kept about constant. Thus, a kinetic model based on directly relating the extra number of activated nuclei promoted by shear to the shear rate was further applied to well predict the effects of shear time on the shear-induced isothermal crystallization kinetics of PLA.