The temperature dependence of the viscoelastic softening and terminal dispersions of linear amorphous polymers

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.