On the unexpected non-monotonic profile of specific volume observed in PCL/CO2 solutions

Recently, the specific volume of molten solutions of polycaprolactone and carbon dioxide (PCL/CO2) was measured by coupling gravimetric measurements with the observation of the swelling of a pendant drop. Quite interestingly, this approach highlighted a non-monotonic dependence of the specific volume of the solution on CO2 pressure. Herein, in order to get a molecular insight into this experimental finding, Raman spectroscopy has been adopted for the investigation into PCL/CO2 solutions at several pressures. This investigation has shown that – due to the interactions of CO2 and PCL - polymer-associated and non-associated CO2 species can be found in the solution. Furthermore, the dependence of CO2 association on gas pressure has been analyzed and invoked for the discussion of the non-monotonic evolution of the specific volume with increasing CO2 pressure.