Influence of molecular weight on dynamic crossover temperature in linear polymers

Dielectric and light scattering spectra of two linear polymers, polyisoprene (PIP) and polystyrene (PS), were analyzed in broad temperature and frequency range above the glass transition temperature, Tg. The crossover temperature, TC, was estimated using two approaches: (i) derivative analysis of relaxation times proposed by Stickel and (ii) Mode-Coupling Theory approach. Both estimates provide consistent values. TC varies with molecular weight (MW) in both polymers, while the ratio TC/Tg changes significantly with MW in PS only. It appears that the segmental relaxation time at TC has value τ(TC) ∼ 10−7 s for both polymers independent of MW and similar to the value reported for many non-polymeric glass-forming systems. No sign of the dynamic crossover has been observed in the chain relaxation around TC of the segmental dynamics.