Morphological changes of polycaprolactone with high-pressure CO2 treatment

Morphological changes of semicrystalline polycaprolactone (PCL), induced by melting under high-pressure CO2 and recrystallization during depressurization, were investigated by DSC and SAXS. Isothermal CO2 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 CO2 at 84 and 304 atm, except at 36 atm. The CO2-assisted melting of PCL recrystallized during depressurization of CO2, resulting in a varied thickness of crystal layers. The thickness of the formed crystal layers decreased with increasing CO2 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 CO2 treatments as observed by SAXS and supported by DSC data. This heterogeneous morphology of PCL formed during CO2 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 CO2.