Thermodynamic Properties of Poly(ethylene glycol) and Poly(tetrahydrofuran). I. P—V—T Relations and Internal Pressure

Abstract

P—V—T relations are measured for poly(ethylene glycol) (PEG) with different molecular weights over the temperature range from −40° to 210°C and for poly(tetrahydrofuran) (PTHF) from −40° to 170°C. The thermal expansion coefficient α, the compressibility β, and the internal pressure P_i are obtained and their temperature dependence is related to the molecular weight. The α and β of liquid PEG do not depend strongly on the molecular weight. On the other hand, the α of solid PEG becomes larger as the molecular weight increases, whereas the β is almost independent of molecular weight. These facts manifest themselves in the behavior of the internal pressure. In the liquid state the internal pressures of higher-molecular-weight PEG are approximately proportional to the second power of the reciprocal volume, but those of low-molecular-weight PEG and PTHF deviate from this rule. The results of internal pressure measurements are analyzed in terms of the theorem of corresponding states.