CO2-diffusion controlled mesophase formation in poly(L-lactic acid) and its effect on cold-crystallization

A mild CO2-treatment of a poly(L-lactic acid) PLLA disk with tailored geometry allowed to compare cold crystallization of PLLA containing ?"-mesophase, with coldcrystallization of fully amorphous PLLA, both coexisting within a single sample. The use of PLLA disks with 0.5 mm thickness, low CO2 pressure (3.0 MPa) and short treatment time (5 min) at 25 oC, led to only partial diffusion of the gas into the polymer. CO2-induced mesophase developed in surface-near regions of the disks, where CO2 penetrated, whereas the core of the samples was not reached by the gas and remained amorphous. Cold-crystallization of CO2-treated PLLA was analyzed after full desorption of the gas, using different heating rates. Slow heating allowed to identify two cold-crystallization events: low-temperature cold-crystallization in the outer parts of the sample treated with CO2, promoted by prior formation of the ?"-mesophase, and high-temperature cold-crystallization, occurring at similar temperatures as in case of non-CO2-treated amorphous PLLA. In both cases, cold-crystallization led to formation of conformationally disordered ??-crystals, as revealed by temperature-resolved wide-angle X-ray scattering. This study confirms that formation of ??-crystals is mainly controlled by the crystallization temperature, also when crystallization is promoted by the prior presence of a mesophase.