标题：Poly(L-lactic acid) Crystallization in Pressurized CO2: An In Situ Microscopic Study and a New Model for Secondary Nucleation in Supercritical CO2
作者：Zhang, Lei; Zhao, Guoqun
作者机构：[Zhang, Lei; Zhao, Guoqun] Shandong Univ, Minist Educ, Key Lab Liquid Solid Struct Evolut & Proc Mat, Jinan 250061, Shandong, Peoples R China.
通讯作者地址：Zhao, GQ (corresponding author), Shandong Univ, Minist Educ, Key Lab Liquid Solid Struct Evolut & Proc Mat, Jinan 250061, Shandong, Peoples R China.
来源：JOURNAL OF PHYSICAL CHEMISTRY C
摘要：Due to its biocompatible and pollution-free features, CO2 has been utilized to tune polymer crystallization, and the crystallization dynamics of the polymer in a CO2 environment has been investigated. However, some different viewpoints still exist in theoretical explanations of the influence of CO2 on polymer crystallization. In this work, the crystal growth behavior of poly(L-lactic acid) (PLLA) in pressurized CO2 was studied using a self-established in situ high-pressure microscopic system, and a new pressurerising crystallization experiment was conducted by stepwise increasing CO2 pressure in an isothermal process. Based on an experimental study, we proposed a new secondary nucleation model to quantitatively calculate the secondary nucleation rate of PLLA crystals in supercritical CO2 based on the two-step mode that includes a preordering stage and nanocluster deposition. Phase separation energy, translational free energy, and absorption free energy caused by CO2 were introduced into the secondary nucleation model. The calculated results of the secondary nucleation model agree well with the experimental results of PLLA crystallization. Finally, we found that the CO2 absorption free energy plays a dominating role in the nucleation-limited effect on PLLA crystals, and the entropy-driven crystallization in the highly concentrated PLLA solution is more pronounced at low crystallization temperatures.