Breakdown of time-temperature superposition in a bead-spring polymer melt near the glass transition temperature.

The breakdown of the time−temperature superposition (TTS) near its glass transition temperature (T(g)) in simple bead−spring polymer melts with and without the chain angle potential was numerically investigated. The stress relaxation modulus at different temperatures G(t,T) was calculated by the Green−Kubo relation. The TTS of G (t,T) of bead−spring polymer melts worked well at temperatures sufficiently higher than its T(g). However, when the system temperature is approaching the glass transition regime, the breakdown of TTS is observed. At temperatures near the Tg,the temperature dependence of the shift factor (a(TB)), which is defined on the time scale between the bond relaxation and the chain relaxation regimes of a G(t) function, is significantly stronger than ones (a(TA)) defined by the time scale of the chain relaxation modes. The analysis of the van Hove function G(s)(r,t) and non-Gaussian parameter, α(2)(t), of the bead motions strongly suggests that the TTS breakdown is concerned with the dynamic heterogeneity. The effect of the chain stiffness on the temperature dependence of the shift factors was also investigated in this study. The stiffer chains melt has a stronger temperature dependence of the shift factors than the ones of the flexible chains melt. However, regardless of the chain stiffness, the stress relaxation modulus functions of the bead−spring polymer melts will begin to break down the TTS at a similar T(g)-normalized temperature (T/T(g)).