Effect of molecular weight on the shear-induced crystallization of poly(lactic acid)

Effect of weak and strong shear flow on the crystallization of poly(lactic acid), PLA, with different molecular weights, (Mw), was investigated by rheological measurements. To study the effect of the weak flow, PLA samples were heated up to 190 °C to erase thermal history, then rapidly cooled down to 130 °C and finally subjected to a shear flow with shear rates ranging between 0.01 and 0.1 s−1. Effect of shear rate and strain on the shear-induced crystallization of PLA was examined. A remarkable effect of Mw on the shear-induced crystallization of PLA with increasing MW was observed. It was also found that higher levels of strain were required to initiate crystallization with increasing deformation rates. The effect of the strong shear flow on the crystallization kinetics of PLA was also examined. For this, different molecular weight samples were pre-sheared and then crystallized at the same temperature for a specific period of time. Evolution of crystallization was monitored by the variation of storage modulus, G′, versus time, t. Thermal behavior and the microstructure of the sheared samples were studied by Differential Scanning Calorimetry, DSC, and Wide-Angle X-ray Diffraction, WAXD. [Display omitted] •Shear-induced crystallization rate was increased with Mw.•A novel double crystallization peak upon cooling was observed for the sheared samples.•Shear flow promotes the formation of PLA's stable α phase.