Quantifying supercritical CO2 dilation of poly(vinylidene fluoride) and poly(vinylidene fluoride-co-hexafluoro-propylene) utilizing a linear variable differential transducer: plasticization and melting behavior

Melting behavior of semicrystalline poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-hexafluoropropylene) is investigated as a function of supercritical CO₂ pressure using a Linear Variable Displacement Transformer (LVDT). The melting temperature (T_m) of both polymers is lowered due to supercritical CO₂ plasticization. For PVDF, the maximum lowering of T_m (ΔT_m/n=23°C) occurs between 483 and 552 bar. The corresponding value for the copolymer is Δ T_m = 26°C at 552 bar. At higher pressures, hydrostatic effects override plasticization and T_m increases for both polymers. By comparing T_m in N₂, a noninteracting gas, the opposing effects of plasticization and hydrostatic pressure on T_m are explored.