Shouyu Chen, Yaqiong Zhang, Huagao Fang, Yunsheng Ding and Zhigang Wang
CrystEngComm, 2013, 2013, 15, 5464–5475.
本文通過偏光顯微鏡法（POM）研究了聚乳酸（PLLA）樣品在110-140oC溫度范圍內的等溫結晶過程。PLLA樣品分別通過兩種方法制備：熔融熱壓法制備表面受限樣品；融壓后揭去一個玻片或溶液滴膜法制備非表面受限樣品。通過仔細測試PLLA結晶過程中球晶半徑隨時間的變化關系，驚奇地發現，PLLA在某些溫度下等溫結晶后期存在球晶生長加速的現象，此時過冷非晶區PLLA樣品同時存在表面受限（玻片覆蓋）和平面受限（相鄰球晶相互碰撞形成一定的“lake-like”區域），但是非表面受限樣品以及只存在表面受限的樣品在結晶過程中球晶生長速率則保持恒定。通過從多方面對實驗數據進行解析后，發現是等溫結晶過程中受限非晶區過冷熔體玻璃化溫度降低導致了結晶后期球晶生長速率加速現象的出現。
Abstract: Isothermal crystallization processes in the temperature range from 110 to 140 °C were studied by using polarizing optical microscopy (POM) for poly(L-lactide) (PLLA) film samples with and without confinement conditions. The PLLA film samples were prepared by melt pressing between two cover glasses to provide the surface confinements and by removing the top cover glasses or by solution cast to keep the top surface free of confinements, respectively. Through carefully following the changes of spherulite radius with crystallization time, it is surprisingly found that there exists an acceleration of spherulitic growth rates at the late stage following the normal spherulitic growth rates at the early stage of isothermal crystallization for PLLA film samples with both surface confinement (by cover glasses) and planar confinement conditions (by aggregation of adjacent spherulites to form “lake-like” regions) at certain temperatures, while the spherulitic growth rates remain constant at each isothermal crystallization temperature for PLLA film samples without the confinement conditions. On the basis of data analyses on the spherulitic growth rates under various conditions from several different aspects, it is finally concluded that the decreases of glass transition temperature induced by crystallization under confinement conditions during isothermal crystallization of PLLA film samples cause the peculiar spherulitic growth rate acceleration at the late stage.