聚乳酸的非等温结晶行为

用差示扫描量热法(DSC)研究聚乳酸(PLA)从熔体及玻璃态为初始条件下的非等温结晶行为,采用Ozawa方程、Mo法、Khanna法和Kissinger法对结晶动力学参数进行计算处理。 实验结果表明,这几种方法均适合处理PLA的非等温结晶过程,而Khanna法提出的结晶速率系数(CRC)能够方便地评价PLA相对结晶速率的大小。 PLA从玻璃态升温结晶比从熔体降温结晶容易得多,升温过程有利于晶核生成,而降温有利于晶体生长。 升温结晶时,升温速率2.0 ℃/min时,结晶焓(ΔH_c)达到最大为27.1 J/g。 从熔体等速降温过程中,随着冷却速率的降低ΔH_c单调增加,冷却速率为0.25 ℃/min时ΔH_c增加到28.3 J/g。 在较低温度下从玻璃态结晶,主要表现为异相成核的二维生长方式。 在较高的温度下从玻璃态升温结晶及从熔体冷却结晶时,以均相成核的三维生长方式结晶为主。 与升温过程相比,冷却不利于晶核的生成,所以导致冷却过程总体ΔH_c偏低,扩散活化能偏大。

Non-Isothermal Crystallization Behavior of Polylactide

The non-isothermal crystallization behavior of polylactide(PLA) was investigated by differential scanning calorimetry(DSC), including heating from glass state and cooling from melted state for the PLA polymer. Crystallization kinetic parameters were calculated using Ozawa equation, Mo method, Khanna law and Kissinger method. These methods are all suitable for handling non-isothermal crystallization process of PLA.The crystallization rate coefficient(CRC) proposed by Khanna can easily evaluate the relative crystallization rate of PLA. In contrast to the cooling process from melted state, the heating process of PLA from glass state stimulates crystal nucleus generation, while cooling process is conducive to the growth of crystals. When the heating rate is at 2.0 ℃/min, crystallization enthalpy(ΔH_c) reaches the maximum of 27.1 J/g. When the crystallization starts from the melted state, ΔH_c increases with the decreasing cooling rate. The cooling rate is lowered to 0.25 ℃/min, and ΔH_c is increased to 28.3 J/g. At a lower temperature, the process of crystallization from the glass state, is mainly heterogeneous nucleation of two-dimensional crystal growth pattern. At higher temperature, crystallization from the glass state and melted polymer largely attributes to the homogeneous nucleation for three-dimensional growth pattern. Compared with the heating process, the cooling process is not conducive to the formation of crystal nucleus, so that in the cooling process ΔH_c is much lower, and the diffusion activation energy is a little higher than that in the heating process.