Quiescent and shear-induced crystallization of linear and branched polylactides

The quiescent and shear-induced isothermal crystallization behavior of linear and long chain branched (LCB) polylactides (PLAs) was investigated at a temperature of 130oC. LCB-PLAs were produced by the reaction with a multi-functional chain extender, Joncryl©. In quiescent crystallization the presence of the LCB structure accelerated the nucleation process and reduced the induction time, depending on the level of branching. The impact of shear strain, and shear rate on crystallization was also examined. The shear-induced crystallization of the linear and LCB-PLAs was affected by both the total shear strain and shear rate. The crystallization kinetics of the LCB-PLAs was more affected by shear than that of the linear PLA. The crystalline morphology of the linear and LCB- PLAs under quiescent and step shear conditions was examined using a Linkam optical shearing system. An increase in the spherulite density was observed in the strained melt of both linear (33 %) and LCB-PLAs (15 %), in comparison with those of unstrained counterparts. Optical micrographs confirmed that the crystal nucleation was affected by the shear flow. . Long chain branching significantly promoted the nucleation density (6.7 times), although it diminished the crystal growth rate from 4.4 to 2.0 µm/min.