Study On The Fabrication And Crystallization Kinetics Of PLLA With Different Microstructures

Poly(L-lactide)(PLLA) is a crystalline polymer. PLLA spherulites are readily formed in materials from its molten and concentrated solutions. Owing to its easy crystallization, this study aims to prepare PLLA materials with various patterned morphologies by thermally induced phase separation (TIPS) from its concentrated solutions. Through the tuning of experimental parameters, patterned PLLA materials are obtained because of different thermodynamic properties of polymer solution and polymer crystallization kinetics. This thesis includes the following aspects:(1) Porous PLLA materials were fabricated from PLLA/N,N-Dimethylformide solutions by TIPS. The morphology, thermal properties and crystal structure of PLLA were measured by SEM, DSC and XRD. The effect of polymer concentration, quenching temperature and quenching time on the morphology, thermal performance and crystal form of PLLA was investigated. The results showed that PLLA with nanofibrous network structure was obtained from low concentration solution, while porous PLLA microspheres was obtained from high concentration solution, the crystal form was α’and α-form, respectively. The Tm of PLLA increased slightly for specimens from solutions with higher PLLA concentration. For PLLA samples from the PLLA/DMF solution with same concentration, porous PLLA microspheres were obtained when quenched at higher temperature, while bundle-like PLLA was obtained when quenched at lower temperature. In addition, the Tm of PLLA decreased slightly and the stability of the crystal became poor with decreasing quenching temperature.(2) PLLA materials with various patterns were successfully prepared from PLLA/ethyl acetate solutions by TIPS. The PLLA morphology transited from leaf-like structure to sheaves to microspheres in response to the experiment conditions. The effect of solution concentration, quenching temperature, quenching time, extractant, and solvent composition of DMF/EA on the morphology, thermal performance and crystal form of PLLA was systematically studied. The results showed that immature PLLA spherulites were obtained when quenched at-10℃, and high concentration and long quenching time were in favor of the formation of PLLA microspheres with a-form crystal and high degree of crystallinity. The results also showed that extractants had no great influence on the morphology and thermal performance of PLLA materials. With increasing ratio of DMF in DMF/EA solvent, α’PLLA was prone to form and the crystallinity of PLLA materials decreased.(3) To simulate the crystallization in the thermally induced phase separation of PLLA solutions, DSC tests were used to investigate the isothermal and non-isothermal crystallization behavior of PLLA/DMF solutions. In addition, to study the influence of EA on the crystallization of PLLA, the crystallization behavior of PLLA/DMF/EA solution with different ratio of DMF/EA was also studied. The non-isothermal crystallization behavior was analyzed by Jeziorny and Mo methods, while the isothermal crystallization behavior of PLLA was investigated by Avrami method. The results showed that PLLA crystallization mechanism was heterogeneous nucleation and three dimensional growth. In addition, with decreasing PLLA concentration and increasing ratio of EA in the PLLA/DMF-EA solution, the crystallization of PLLA appeared at lower temperature.