Quantifying Plasticization and Melting Behavior of Poly(vinylidene fluoride) in Supercritical CO₂ Utilizing a Linear Variable Differential Transformer

Abstract

Supercritical CO2 plasticization of semicrystalline poly(vinylidene fluoride) (PVDF) was investigated by measuring linear dilation as a function of temperature (75−130 °C) and pressure (138−670 bar) using a linear variable differential transformer (LVDT). The robustness of the LVDT technique allows the study of CO2 plasticization at high temperatures (to Tm) and pressures. Experiments at constant temperature (varying pressure) and constant pressure (varying temperature) were performed. With constant temperature experiments below 100 °C, dilation increased up to 414 bar, while at higher pressures, the change of dilation with pressure attenuated. At higher temperatures (≥117 °C), dilation increased almost linearly with pressure through the experimental range. Constant pressure experiments were carried out to assess the effect of CO2 pressure on Tm. With increasing pressure, Tm decreased to a minimum of 135 °C at 483 bar (ΔTm = 23 °C). Above 483 bar, hydrostatic effects override plasticization and Tm increases. By comparing Tm under N2, a gas with minimal interaction with the polymer, competition between plasticization and hydrostatic pressure on Tm is clarified.