  The breakdown of the time-temperature superposition (TTS) near its glass
transition temperature (Tg) 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 Tg. 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 aTB, which is defined at the time scale between the bond relaxation and
the chain relaxation regimes of a G(t)-function, is significantly stronger than
ones aTA defined by the time scale of the chain relaxation modes. In direct
relation to the breakdown of TTS of G(t,T), the decoupling of Stokes-Einstein
law of diffusion-viscosity relation also appears in the glass transition
regime. The analysis of the van Hove function Gs(r,t) and non-gaussian
parameter, a2(t), of the bead motions strongly suggest 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 breakdown the TTS at a similar
Tg-normalized temperature around T/Tg ~ 1.2.
