Effect of physical ageing on mechanical behaviour of an elastomeric glass under combined pressure and temperature

Abstract

Abstract The relaxation or physical ageing behaviour of a polyurethane elastomeric glass (Solithane 113, Tg = −20°C at atmospheric pressure) has been studied at various states of pressure and temperature (P, T), near and far away from the glass transition. Those (P, T) states were reached via either isobaric or isothermal paths. The relaxation behaviour of these glasses has been found to be a function of P, T and the path. The Young's modulus (E) has been determined as a function of ageing time. When all the experimental data, obtained at various (P, T) states for a specific path, were replotted in the form of log E vs. 1 P for an isothermal glass and log E vs. 1 T for an isobaric glass, all the curves appear to converge to a specific value of E. The value was chosen as E∞, the ’near-equilibrium’ modulus. The relaxation behaviour was analysed by a parameter δ E = E ∞ −E(P,T,t) E ∞ When δ E ⋍ 1 is used to define a glass transition, the pressure-dependent glass transition lines for isobaric glasses are found to be entirely different from those for isothermal glasses. This may be the result of the pressure rate and temperature rate used. The non-equilibrium behaviour of E of the glass was analysed by an equation analogous to Kovac's equation for the kinetics of relaxation, namely − d δ E (P,T,t) d t = δ E (P,T,t) τ(P,T,δ E ) where δE(P,T) is the relative departure of E(P,T,t) from E∞. In general, the isobaric glasses at a specific state (P,T) relaxed faster, relatively, than isothermal glasses for the experimental conditions of heating rate or pressurization rate.