剪應力對左旋聚乳酸結晶行為之影響

Abstract

本研究探討在剪應力作用下，左旋聚乳酸 (PLLA) 分子鏈之方向性(orientation) 對結晶動力學的影響。在非晶質狀態下的PLLA分子鏈受到剪應力作用後，產生沿剪切方向排列之鏈構型，使接近平面鋸齒狀的ttt構型存在比例增加。PLLA分子鏈構型分布的變化與方向性的增加，影響材料的亂度，進而改變熱力學平衡狀態。PLLA在結晶時，會在較高的結晶溫度形成排列較緊密的α晶體結構；在較低的結晶溫度形成排列相對鬆散的α’ 晶體結構；而在這之間的溫度，則為兩種晶體結構同時存在的共存區。我們以POM、DSC、XRD觀察帶有分子鏈方向性非晶質狀態PLLA的後續結晶行為，找出結晶動力學模式與熱力學參數間的關係後，發現結晶前方向性的存在提升了分子鏈的成核能力，進而影響PLLA的α與α’ 結構在共存區的結晶動力學。

Influence of chain orientation on crystallization kinetics after applying shear stress on Poly L-lactide was studied in the research. After the shear treatment, the chain conformation of amorphous PLLA was aligned along the shear direction, and the population of ttt conformer, which is the most extended conformer of PLLA and the conformation type is nearly planar zigzag. The increase of chain orientation and the variation of population of chain conformers alter the entropy which changes the equilibrium state on thermodynamic. PLLA is totally ordered α structure which has more tightly packing at a higher crystallization temperature, and relatively loosely packing in another structure α' at a lower crystallization temperature. These structures coexist between the higher and lower temperature. Using polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction to observe the crystallization behavior of oriented amorphous PLLA, we connected the relation between crystallization kinetics model and thermodynamic parameters. We found that the occurrence of chain orientation before crystallization improves the nucleation ability of PLLA and finally changes the crystallization kinetics at the temperature α and α' coexist.