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 α"-mesophase, with cold- crystallization 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 ◦C, 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 α"-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 α′-crystals, as revealed by temperature-resolved wide-angle X-ray scattering. This study confirms that formation of α′-crystals is mainly controlled by the crystallization temper- ature, also when crystallization is promoted by the prior presence of a mesophase.