Effect of simple stress on the glass transition of polymers at high pressures

Experimental studies, which have been carried out in this laboratory, showed the yield strength in tension, compression, and shear in the rubbery and the glassy states increased with increasing hydrostatic pressure. Moreover, the Young's modulus also increased with pressure and the amount of the increase across the glass transition temperature (T _g) at a given pressure can be as large as three orders of magnitude in the case of elastomers. An extension of the Gibbs-Dimarzio theory is proposed to account for the effect of applied stress on the glass transition temeprature of glass-forming polymers. When a simple stress, such as tensile, compressive or shear stress, is applied to a polymer, the T _g will decrease, compared to a polymer without applied stress. A glass-forming polymer in the vicinity of the transition would behave differently from that predicted by rubber elasticity. The partition function taking into account the effect of stress is suggested to be $$Gamma = Sigma W(f, n_0 ){text{ }}exp {text{ }}[ - beta (PV + U - sigma Vvarepsilon )]$$ where the strain ?=σ (f ? f ₀) in which f and f ₀ are the fraction of flexed bonds with and without stress, respectively. Furthermore, by this model, the Young's modulus across the transition, E _L and E _G, can be evaluated. The Young's modulus increases with increasing pressure at lower and moderate pressure range but the increase is rather small at very high pressure range.