Studies On Crystallization Kinetics Of Long Chain Branched Polylactide System

Polylactide (PLA) is a kind of renewable energy materials, which has been intensively studied because it is biodegradable, biocompatible, and nontoxic to environment. However, relatively poor processing ability limits its application in several processing procedures. On one hand, the melt strength of PLA is weak, which hinders its processing. On the other hand, PLA is a slowly crystallizing polymer with low crystallinity, which requires a long processing cycle and restricts its practical application. In this article, we used several methods to study the crystallization kinetics of PLA system to improve the crystallization behavior of PLA. The mainly aspects as follows:1. A series of long chain branched PLA samples have been prepared, which have been proved to be bimodal of linear PLA and long chain branched PLA with different branching degrees. The crystallization kinetics of these LCB PLA samples are investigated by using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The POM results show that the spherulitic growth rates of LCB PLA samples are lower than that of linear PLA precursor and the PLA spherulitic growth rate decreases with increasing branching degree at each isothermal crystallization temperature. On contrast, the nucleation density increases with increasing branching degree. The crystallization kinetics from POM observation are analyzed by using the Hoffman-Lauritzen theory. In the studied temperature range, both linear PLA and LCB PLA samples crystallize according to the Regime Ⅱ mechanism. Nucleation constant (Kg) and fold surface free energy (σe) decrease with increasing branching degree, suggesting that LCB PLA samples have lower free energy barriers for nucleation than linear PLA precursor. Analysis of DSC data by the Avrami equation indicates that crystallization of both 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 LCB PLA samples are higher than that of linear PLA.2. A series of asymmetric biodegradable poly(L-lactide) (PLLA)/poly(D-lactide) (PDLA) blends with low PDLA compositions are prepared by using solution blending method. Isothermal crystallization kinetics under different shear conditions at the specific high temperature of 160℃ for the PLLA/PDLA blends are investigated by using polarized optical microscopy (POM) and rheometry. It is found that the crystallization behaviors of PLLA/PDLA blends are greatly accelerated due to the existence of SC crystallites and the crystallization kinetics are promoted with increasing shear rate or shear time. The crystalline morphology remain spherulitic with the spherulitic growth rates unaltered at the applied shear conditions, and the accelerated crystallization kinetics could be attributed to the significantly enhanced nucleation density. The discrete Maxwell relaxation time spectra at the applied isothermal crystallization temperature of 160℃ are used to obtain the reptation and Rouse times of PLLA chains with high molecular masses. Even though the PLLA chains might be orientated under the applied shear, the relaxation time of the blends is still too short to induce any orientated crystal nuclei.3. The LCB-PLA and PDLA blends are prepared by using solution blending method. The crystallization kinetics under different shear conditions are studied by using POM and rheology. It is found that the nucleation density increases with increasing PDLA content, after applied an appropriate shear condition, many orientated row-like crystals appear under POM. And with increasing shear time or shear rate, the orientated crystals increase by sight. The rheology shows that the crystallization rate of LCB-PLA and LCB-PLA/PDLA blends increase dramatically with increasing shear time and shear rate, which is mainly due to the enhancement of nucleation density. The result also shows that the crystallization process for LCB-PLA/PDLA blends with 5 wt% and 10 wt% PDLA content present two platforms, the first is the crystallization process of stereocomplex crystal and the second is homo-crystal of PLA, which is result from different induction times.4. The nucleation and growth of spherulites for the beneath polylactide (PLA) layer in poly(ethylene glycol)/polylactide (PEG/PLA) double-layer films with different thicknesses of PEG films during isothermal crystallization at various temperatures are studied by using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). It is found that the top covering molten PEG layer can obviously accelerate the spherulitic growth rate (G) of the beneath PLA layer and the growth rate of PLA spherulites in PEG/PLA (2 μm/7 μm) double-layer films is the fastest of all which is about 46 times of that in PLA film. In addition, the PLA spherulities in PEG/PLA double-layer films show ring-band texture. Another significant result is that the temperature for the measurable nucleation and spherulitic growth of PLA in the double layer films can be eventually pushed down close to the glass transition temperature of neat PLA. The glass transition temperature reduces with increasing of thickness of PEG film. Compared with PEG/PLA double-layer films system, the nucleation and growth of PLA spherulites in PLA/PEG blend films with different proportion of PLA and PEG are investigated as well. The growth rate of PLA spherulite in PLA/PEG blend system accelerates obviously as well, however, the crystallization morphology and growth rate are different from the PLA spherulites in PEG/PLA double-layer films at the same temperature.