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The temperature dependence of the viscoelastic softening and terminal dispersions of linear amorphous polymers
Authors:Donald J Plazek
Abstract:Thermorheological simplicity is shown to hold for poly(vinyl acetate) in the temperature range extending from Tg + 25°C to Tg + 80°C. Between Tg and Tg + 25°C the softening (glass to rubberlike) viscoelastic dispersion exhibits time-scale shift factors aT different from those of the terminal (rubberlike to steady-state) dispersion. The aT values calculated from zero-shear viscosities coincide with those from the terminal dispersion in the temperature range 60–154°C (Tg ? 35°C). The aT shifts obtained from the response in the terminal dispersion can be fitted to the Williams, Landel, and Ferry equation over the entire temperature range 42–154°C. The aT obtained from the softening dispersion is shown to exhibit a different functionality. An empirical modification of the Doolittle equation yields a very flexible relation which can be fitted to some aTs which cannot be represented by the usual Doolittle free-volume expression.
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