Free poly(l-lactic acid) spherulites grown from thermally induced phase separation and crystallization kinetics

• Published in: Journal of polymer science. Part B, Polymer physics Polymer physics, 2014-11, Vol.52 (22), p.1476-1489
• Main Authors: Liu, Ruilai, Li, Kaina, Liu, Min, Liu, Yingying, Liu, Haiqing
• Format: Article
• Language: English
• Subjects: Applied sciences; Crystal structure; Crystallization; Exact sciences and technology; fibers; Formations; Morphology; Nanofibers; Organic polymers; Phase separation; Physicochemistry of polymers; poly(L-lactic acid); Polylactic acid; Properties and characterization; Quenching; Sheaves; solution crystallization; Spherulites; Tetrahydrofuran; thermally induced phase separation
• ISSN: 0887-6266; 1099-0488
• DOI: 10.1002/polb.23587

ABSTRACT Free poly(L‐lactic acid) (PLLA) sheaves and spherulites were prepared by thermally induced phase separation method from its tetrahydrofuran solution without the assistance of other additives. The effects of variables such as polymer concentration, quenching temperature and time on the morphology of PLLA spherulites were studied. The morphology, size, degree of crystallinity, and crystal structure of spherulites were characterized by SEM, DSC and XRD, and so forth. No obvious sheaves or spherulites were observed at quenching temperature of 8 and 0 °C, whereas sheaves composed of fluffy nanofibers with diameter of about 250 nm were formed at quenching temperature range of −10 to −40 °C. With increasing quenching time, the PLLA morphology changed from small sheaves to big sheaves (cauliflower‐like) to spherulites. Low concentration (3 and 5 wt %) solutions were favorable for the formation of sheaves, whereas high concentration (7 wt %) solution as good for the formation of spherulites. The mechanism for the formation of PLLA sheaves or spherulites was examined by the isothermal and nonisothermal crystallization of PLLA/tetrahydrofuran solutions using DSC. The Avrami equation was used to analyze the data and good linear double‐logarithmic plots were obtained. The Avrami exponent n and rate constant K indicated the crystal growth mechanism was intermediate between completely instantaneous and completely sporadic types of nucleation and growth, and the spherulites were there dimensional. Compared to the spherulites embedded in the bulky film obtained from the melt processing, this study provided a feasible technique for the fabrication of free PLLA spherulites. The PLLA spherulites composed of fluffy nanofibers with a high porosity (≥90%) may be potentially applied as functional materials such as catalyst support, adsorption and biomedical materials, and so forth. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1476–1489 Free poly(l‐lactic acid) sheaves and spherulites were prepared by thermally induced phase separation from a tetrahydrofuran solution without the assistance of other additives. Sheaf and spherulites morphologies were readily tuned by adjusting the processing parameters including solution concentration, quenching temperature, and time. The formation of sheaves and spherulites resulted from nucleation and crystal growth. The poly(L‐lactic acid) spherulites were composed of fluffy nanofibers with a high porosit