Network structure dependence of volume and glass transition temperature

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 T-g. The polyurethanes were copolymers of diphenylmethane diiso cyanate and poly(propylene oxide) (PPO) with hydroxyl functionalities of on e, two, and three. The equivalent weights of all the PPOs were equal, such that the chemical composition of the chain segments was essentially identic al. Therefore, the only distinctions among polymers were differences in CE and BP concentration. Theory of branching processes computer simulations we re used to determine the concentration of CE due to imperfect network forma tion. Other CE contributions were from the monofunctional PPO. Polymer volu mes and T(g)s were correlated to CE and BP concentrations, and the contribu tions of these species were determined from least squares fits. The molar v olume and T-g contributions were then used to determine free volume thermal expansion coefficients. These values were compared to thermal expansion co efficients obtained from WLF parameters (c(1),c(2)) obtained from the measu rement of dynamic moduli as a function of temperature. (C) 2000 The Society of Rheology. [S0148-6055(00)01204-9].