Morphological changes of polycaprolactone with high-pressure CO 2 treatment

Abstract

Morphological changes of semicrystalline polycaprolactone (PCL), induced by melting under high-pressure CO 2 and recrystallization during depressurization, were investigated by DSC and SAXS. Isothermal CO 2 condition at 35 °C and nonisothermal condition at a cooling rate of 0.5 °C/min from 90 to 30 °C were both studied at pressures of 36, 84, and 304 atm. At 35 °C, the semicrystalline PCL having melting temperature of about 60 °C was found to melt under CO 2 at 84 and 304 atm, except at 36 atm. The CO 2-assisted melting of PCL recrystallized during depressurization of CO 2, resulting in a varied thickness of crystal layers. The thickness of the formed crystal layers decreased with increasing CO 2 pressures. Moreover, heterogeneity, with a size larger than the thickness of the crystalline and amorphous layers, was found to form in the PCL sample after CO 2 treatments as observed by SAXS and supported by DSC data. This heterogeneous morphology of PCL formed during CO 2 depressurization might arise from the segregated amorphous domains that were located between bundles of the lamellar stacks and/or arise from lamellar stacks that had considerably different sizes, possibly as a result of the molecular dragging and/or molecular fractionation on PCL during depressurization of the PCL-interacted CO 2.