jp0c00049_si_002.avi (27.41 MB)
Poly(l‑lactic acid) Crystallization in Pressurized CO2: An In Situ Microscopic Study and a New Model for Secondary Nucleation in Supercritical CO2
media
posted on 2020-04-09, 14:43 authored by Lei Zhang, Guoqun ZhaoDue 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.