Chiral Effect on Crystallization Behavior of Crystalline Polylactides

掌性效應於結晶性聚乳酸高分子結晶行為之影響

林穎慧(Ying-Huei Lin)

指導教授 ： 何榮銘

國立清華大學/工學院/化學工程學系/碩士(2008年)

https://doi.org/10.6843/NTHU.2008.00315

摘要

Typical living ring-opening polymerization has been used to prepare enantiomeric polylactides, PLLA and PDLA homopolymers, so as to examine the chiral effect on the crystallization of the polylactides. As evidenced by differential scanning calorimetry, the crystallization rate of polylactides was strongly dependent upon the maximum annealing temperature (Tmax) above the suggested equilibrium melting (Tmo). The crystallization rate decreased with the increase of the Tmax. It is reasonable to suggest that the dependence might be attributed to the long-life crystalline nucleus and referred to nucleus memory effect for polymeric crystallization. However, the observed change on crystallization rate was independent upon annealing time at the Tmax; revealing that it is an equilibrium behavior instead of kinetic cause. We conjecture that the dependence might be attributed to the formation of partially ordered phase (i.e., mesophase) which exists above the suggested equilibrium melting. Furthermore, we speculate that the formation of mesophase is a signature of chiral interaction for chiral crystalline polymers. To further examine the chiral effect on crystallization, blends of the enantiomeric polylactides were prepared so as to clarify the formation of crystalline stereocomplex based on the concerns of chiral interaction. Consistent to previous studies by different groups, our results realized that the crystallization can be significantly increased by introducing the enantiomeric polymers for crystallization. The change on crystallization rate is a character of crystalline stereocomplex formation and referred to the significant interaction resulting from enantiomeric chiral interaction. According to the experimental results, significant increase in the crystallization rate can be simply achieved by adding small amount of enantiomeric polylactide that may play as a nucleation agent for the promotion of crystallization. Also, two crystallization processes can be recognized in the blends with 0.1 volume fraction of enantiomeric polylactide whereas single crystallization process was found for blends with higher fraction of enantiomeric polylactide. To further investigate the mechanism during crystallization (namely, the self-assembly process for ordering), the optical activity of the single-chain conformation in dilute solution and corresponding change on crystallization process were conducted so as to examine the self-assembly evolution for the occurrence of crystalline stereocomplex. The variation of intensity and corresponding shifting in the spectrum of the blends of enantiomeric polylactides can be identified by time-resolved circular dichroism measurements. It reflects that the change should correspond to the significant chiral interaction resulting from the blends of enantiomeric polylactides during self-assembly in dilute solution. Consequently, the crystalline stereocomplex can be formed so as to give significant high melting as compared to polylactide homopolymers.

關鍵字

掌性效應 ； 聚乳酸

並列摘要

HASH(0x1ae2e6d0)

並列關鍵字

Chiral effect ； polylactides

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