Isothermal Crystallization Kinetics of Poly(lactic acid) Stereocomplex/Graphene Nanocomposites

The isothermal crystallization kinetics of PLA stereocomplex (scPLA) and scPLA/graphene oxide nanocomposites was investigated. scPLA was prepared by symmetric blending the two isomers, PLLA and PDLA and the scPLA nanocomposite by polymerization of the respective lactides in presence of commercial graphene oxide nanoplatelets (GNPO+) followed of blending the two composites. The effect of the filler on the crystallization behavior and thermal properties of scPLA was investigated by using differential scanning calorimetry (DSC). Avrami equation was used to describe the isothermal crystallization kinetics and melting behavior. Addition of 0.5 wt% GNPO+ to PLA matrix increases the crystallization rate, lowering the half-time of crystallization t1/2 to around 5.5 min for scPLA nanocomposites, which is about 2.7 min below that found for scPLA when isothermally crystallized at 165ºC. During the isothermal crystallization, homochiral and stereocomplex crystals are formed, depending on the crystallization temperature. As crystallization temperature reaches 160 and 165ºC, the crystallization of homocrystals become prevented and only stereocomplex crystals are formed.本研究针对聚乳酸立体络合物（PLA stereocomplex, scPLA）及其氧化石墨烯纳米复合材料的等温结晶动力学展开了探究。其中，scPLA通过将两种异构体左旋聚乳酸（PLLA）与右旋聚乳酸（PDLA）对称共混制备得到；scPLA/氧化石墨烯纳米复合材料则先在商用氧化石墨烯纳米片（GNPO+）存在下聚合相应丙交酯，随后将两种复合物共混制得。本研究采用差示扫描量热法（DSC）探究了该填料对scPLA结晶行为与热性能的影响。实验采用阿夫拉米方程（Avrami equation）描述其等温结晶动力学与熔融行为。向聚乳酸基体中添加0.5 wt%的GNPO+可提升结晶速率，使scPLA纳米复合材料的结晶半寿期t₁/₂降至约5.5 min，相较于165℃下等温结晶的纯scPLA，其结晶半寿期缩短了约2.7 min。在等温结晶过程中，晶体的生成类型取决于结晶温度，可形成均手性晶体与立体络合晶体。当结晶温度达到160℃与165℃时，均手性晶体的结晶过程会被抑制，仅生成立体络合晶体。