Effect of Unexpected CO2's Phase Transition on the High-Pressure Differential Scanning Calorimetry Performance of Various Polymers

摘要

We used a high-pressure differential scanning calorimeter (HP- DSC) to study polymer plasticization by compressed gases at pressures of up to 30 MPa for polylactide (PLA), polycarbonate (PC), isotactic polypropylene (iPP), and polystyrene (PS). The pressure reached values twice as high as the previously published data. We found that the polymer/carbon dioxide (CO2) system's heating curves have an unidentified endothermic peak above 5 MPa, which turns out to be from CO2's phase transition. The HP-DSC could accurately determine the depression of the glass transition temperature (Tg), crystallization temperature (Tc), and melting temperature (Tm) of various polymers at low pressures by simply starting at a higher temperature to avoid CO2's phase transition; however, the increased plasticization effect of the dissolved CO2 lowered the Tg to the level of overlapping with CO2's phase transition phenomena at elevated pressures, and therefore, the depressed Tg could not be measured above 6 MPa for PLA, PC, or PS. On the other hand, the Tc of iPP decreased with an increase in pressure, whereas Tm values of PLA and iPP decreased slightly with an increase in pressure and then remained almost unchanged above a certain pressure, which may indicate an increased hydrostatic pressure effect at elevated pressures.