Studies on crystallization kinetics of bimodal long chain branched polylactides

To investigate the effects of long chain branching (LCB) on the crystallization kinetics of polylactides (PLA), a series of long chain branched PLA samples have been prepared, which have been proven to be bimodal of linear PLA and long chain branched PLA with different branching degrees. The crystallization kinetics of these LCB PLA samples have been investigated using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The POM results show that the spherulitic growth rates of the LCB PLA samples are lower than that of the linear PLA precursor and the PLA spherulitic growth rate decreases with increasing branching degree at each isothermal crystallization temperature. In contrast, the nucleation density increases with increasing branching degree. The crystallization kinetics from the POM observation are analyzed using the Hoffman-Lauritzen theory. In the studied temperature range, both linear PLA and LCB PLA samples crystallize according to the Regime II mechanism. The nucleation constant (Kg) and fold surface free energy (se) decrease with increasing branching degree, suggesting that the LCB PLA samples have lower free energy barriers for nucleation than the linear PLA precursor. Analysis of the DSC data by the Avrami equation indicates that the crystallization of both the linear PLA and LCB PLA samples follows a three-dimensional crystal growth. The half crystallization time reduces with increasing branching degree and the overall crystallization rates for the LCB PLA samples are higher than that of linear PLA.