Network structure dependence of volume and glass transition temperature

Abstract

A series of polyurethanes was used to determine the molar contributions of chain ends (CE) and branch points (BP) to free volume and glass transition temperature Tg. The polyurethanes were copolymers of diphenylmethane diisocyanate and poly(propylene oxide) (PPO) with hydroxyl functionalities of one, two, and three. The equivalent weights of all the PPOs were equal, such that the chemical composition of the chain segments was essentially identical. Therefore, the only distinctions among polymers were differences in CE and BP concentration. Theory of branching processes computer simulations were used to determine the concentration of CE due to imperfect network formation. Other CE contributions were from the monofunctional PPO. Polymer volumes and Tgs were correlated to CE and BP concentrations, and the contributions of these species were determined from least squares fits. The molar volume and Tg contributions were then used to determine free volume thermal expansion coefficients. These values were compared to thermal expansion coefficients obtained from WLF parameters (c1,c2) obtained from the measurement of dynamic moduli as a function of temperature.