title: Dynamic crossover of alpha' relaxation in poly(vinyl acetate) above glass transition via mechanical spectroscopy.
abstract: The molecular relaxation dynamics of poly(vinyl acetate) (PVAc) has been studied by mechanical spectroscopy above the glass transition temperature (T(g)) within a frequency range from 1 mHz to 100 Hz. The temperature-dependent mechanical spectra reveal the existence of two relaxation modes: alpha, ascribed to the glass transition, and alpha', which may be related to the softening dispersion, composed of the sub-Rouse modes and the Rouse modes. The alpha' mode has a weaker temperature dependence than the alpha mode. Furthermore, the alpha' mode from the frequency-domain spectra exhibits a similar dynamic crossover at temperature T(B) approximately 387 K as the alpha mode through the temperature dependence of relaxation time, relaxation strength, and shape parameter. However, the crossover of the alpha' mode occurs at a time of about 0.08 s, longer than 10(-6)-10(-7) s for the alpha mode by dielectric spectroscopy. According to the coupling model, the crossover is suggested to be caused by the strong increase of intermolecular cooperativity below T(B).
authors.affiliation: Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui, PR China. xbwu@issp.ac.cn
authors.forename: Xuebang
authors.initials: X
authors.lastname: Wu
authors.forename: Zhengang
authors.initials: Z
authors.lastname: Zhu