Carbon dioxide-induced melting point depression of biodegradable semicrystalline polymers

The melting point depression of semicrystalline polymers exposed to a compressed fluid allows certain biodegradable polymers to be processed in the molten state using carbon dioxide at modest temperatures. Experimental melting temperature (Tm) data are presented for the aliphatic polyesters, polycaprolactone (PCL) and poly(butylene succinate) (PBS), as a function of the applied pressure of carbon dioxide. The characteristic features of the Tm versus P curve – a linear regime at modest P, followed by an abrupt transition to a second regime where Tm changes little with P – are interpreted using the Clapeyron equation for two-component, three-phase equilibrium. For PCL and PBS, as well as for other polymers, the slope of the lower pressure linear regime can be quantitatively estimated from the normal Tm, the enthalpy of fusion of the polymer, and the Henry's law coefficient for CO2 dissolved in the melt. The location of the abrupt change in slope is correlated with equivalence of the molar volumes of the compressed fluid and the Kuhn monomer unit of the polymer crystal. The method is also applicable to non-polymeric compounds which have high critical points and low solubility in the compressed fluid.