聚乳酸的非等温结晶动力学研究

Study on Non-isothermal Crystallization Kinetics of Polylactic Acid

采用差示扫描量热仪研究了聚乳酸(PLA)样品在5种不同升温速率下的非等温结晶行为,采用Jeziorny法、Ozawa法、Mo法等分析了PLA的非等温结晶动力学。结果表明:随着升温速率的增加,PLA的熔融结晶峰向高温端移动,结晶峰变宽,结晶时间变短,同时结晶也变差。利用Jeziorny法和Mo法求得Avrami指数n和Ozawa指数m,在不同升温速率下,结晶初期的Avrami指数n和Ozawa指数m接近,均处于2~3,说明PLA的均相成核以一维方式为主、二维方式为辅;在升温速率为5~20 K/min时,结晶中期的PLA Avrami指数n>5、Ozawa指数m>3.5,均高于结晶初期的Avrami指数n与Ozawa指数m,表明PLA除了呈现均相成核的三维生长方式之外,还出现了结晶堆积的现象;当升温速率为30~40 K/min时,结晶中期的Avrami指数n和Ozawa指数m均与初期表现一致,Avrami指数n在3.5左右,Ozawa指数m为2.5~3.5,表明该条件下PLA以均相一维和二维2种方式共存结晶。应用Mo法得到的升温速率F(T)随着相对结晶度的增加而增加,表明随着结晶度增大,结晶变难。而应用Ozawa法计算时,PLA的动力学参数的准确性往往容易受限于数据量,因而在实际应用上存在一定的局限性。

The non-isothermal crystallization behavior of polylactic acid(PLA)samples at five different heating rates was studied by differential scanning calorimetry.The non-isothermal crystallization kinetics of PLA was analyzed by Jeziorny method,Ozawa method,Mo method,etc.The results show that with the increase of heating rate,the melting crystallization peak of PLA shifts towards the high-temperature end,the crystallization peak widens,the crystallization time shortens,and the crystallization also deteriorates.The Avrami index n and Ozawa index m are calculated by Jeziorny method and Mo method.At different heating rates,the Avrami index n and Ozawa index m at the initial stage of crystallization are close,both ranging from 2 to 3,which indicates that the homogeneous nucleation of PLA is mainly in one-dimensional mode and supplements by two-dimensional mode.When the heating rate is 5-20 K/min,the Avrami index n>5 and Ozawa index m>3.5 of PLA in the middle crystallization stage,which are higher than the Avrami index n and Ozawa index m in the initial crystallization stage,indicating that PLA not only exhibits a three-dimensional growth pattern of homogeneous nucleation,but also exhibits crystal accumulation phenomenon.When the heating rate is 30-40 K/min,the Avrami index n and Ozawa index m in the middle crystallization stage are consistent with those in the initial stage.The Avrami index n is around 3.5,and the Ozawa index m is 2.5-3.5,indicating that PLA coexists in both homogeneous and two-dimensional crystallization under this condition.Temperature rising rate as the F(T)values obtained by the Mo method increases with the increase of relative crystallinity,indicating that crystallization becomes difficult as crystallinity increases.When using the Ozawa method to calculate,the accuracy of PLA dynamic parameters is often limited by the amount of data,thus there are certain limitations in practical applications.