Poly(l‑lactic acid) Crystallization in Pressurized CO₂: An In Situ Microscopic Study and a New Model for Secondary Nucleation in Supercritical CO₂

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 pressure-rising 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.