Pressure-volume-temperature relationships of solid and molten poly(ethylene terephthalate)

The pressure-volume-temperature (PVT) properties of poly(ethylene terephthalate) (PET) have been studied experimentally and theoretically in the temperature range 30–342 °C and at pressures to 2000 kg/cm2. In the semicrystalline solid state (ρ = 1.409 g/ cm3 at 20°C), two transitions were resolved. One is the glass transition at Tg = 70°C (P = 0), increasing with pressure according to dTg/dP = 0.025 °C/(kg/cm2). A second transition was observed as a break in the zero-pressure isobar and also as a break in the zero-pressure compressibility versus temperature curve at 160°C. It is interpreted as a premelting (αc) transition. The melt data were fitted successfully to both the empirical Tait equation and the Simha-Somcynsky hole theory (with T∗ = 11710°K, V∗ = 0.7408 cm3/g, and P∗ = 11570 kg/cm2). The Simha-Somcynsky theory was also used to establish a semi-empirical equation of state for amorphous PET in the temperature range Tg < T < Tm and to derive a theoretical pressure dependence of Tg in completely amorphous PET of about 0.036°C/(kg/cm2). This pressure dependence is discussed in relation to the smaller experimental dependence of Tg found in the semicrystalline sample of this study.