长支链聚乳酸在挤出发泡过程中的晶致异相成核机理研究

With good biodegradability and biocompatibility, polylactide foams avoid environmental pollution and ecological crisis caused by traditional foamed plastics. During extrusion foaming of long-chain-branched polylactide when using CO2 as a blowing agent, the cell nucleation is particularly important because it will directly influence the physical and mechanical properties of final foamed products. The crystallization rate of long-chain-branched polylactide will increase dramatically due to the coupling effect of CO2 plasticization and shear during extrusion foaming and the produced crystals will significantly affect cell nucleation and properties of final products. In this project, the shear-induced crystallization mechanism for long-chain-branched polylactide will be studied by using a self-designed batch-foaming equipment with shear function. The crystal growth model under coupling effect in the multiple fields (temperature, pressure and shear, etc.) will be made and the dynamic impact factors will be introduced into the classic cell nucleation theory. In addition, the crystal-induced heterogeneous nucleation mechanism will be investigated and the relationships of processing conditions, crystal morphologies, cell structures and foam product properties will be clarified. The study results of crystal-induced heterogeneous nucleation mechanism will not only provide scientific basis to facilitate to make processing conditions for foaming extrusion of long-chain-branched polylactide, but also offer valuable reference to study foaming extrusion process of other semicrystalline polymers.

聚乳酸泡沫塑料具有良好的生物降解性和兼容性，避免了传统泡沫塑料引起的环境污染和生态危机。在以CO2为发泡剂的长支链聚乳酸挤出发泡过程中，气泡成核是非常关键的一步，直接影响最终制品的物理和力学性能。在挤出发泡过程中因受CO2塑化和流场剪切等因素耦合作用，长支链聚乳酸的结晶速率得到显著提高，产生的晶体将对气泡成核产生重要影响。本项目拟采用带剪切功能的间歇发泡装置研究长支链聚乳酸在高压CO2下的剪切诱导结晶机制，建立多场（温度场、压力场和剪切场）耦合机制作用下的晶体生长模型，并将动态影响参数引入经典成核理论，建立晶体诱导气泡成核的晶致异相成核机理和工艺条件-晶体形态-泡孔结构-宏观性能的关系模型。长支链聚乳酸在挤出发泡过程中的晶致异相成核机理研究，不仅可以为长支链聚乳酸挤出发泡生产工艺的制定提供科学依据，对其他半结晶型聚合物的挤出发泡生产的研究也有重要借鉴意义。

以CO2为发泡剂制备的聚乳酸（PLA）泡沫塑料，具有良好的生物降解性和生物兼容性，避免了传统泡沫塑料引起的环境污染和生态危机。长支链PLA因熔体强度较高可用挤出法制备泡沫制品，极大地拓宽了其应用领域。长支链PLA结晶缓慢，但其结晶行为在高压CO2作用下的挤出流场中会产生显著变化，产生的晶体又会对发泡过程产生重大影响。本项目研究了长支链PLA在高压CO2或剪切作用下的结晶行为，总结了高压CO2或剪切作用对晶体形态和数量的影响规律，并研究了挤出发泡过程中晶体形态和数量对泡沫制品微观结构的影响，探讨了挤出发泡过程中的晶致异相成核机理，建立了工艺条件——微观结构——宏观性能的关系模型，实现了通过调整工艺控制制品性能的目的。长支链PLA在高压CO2下的剪切诱导结晶及其对发泡的影响研究，不仅可以为长支链PLA挤出发泡生产工艺的制定提供科学依据，对其他半结晶型聚合物的挤出发泡研究也有重大借鉴意义。