Enthalpy and dielectric relaxation in the glass transition region of polypropylene glycol

We investigated enthalpy and dielectric relaxation behaviors in the glass transition region of polypropylene glycol of molecular weight 1000 and 4000 by measuring both dynamic heat capacity and dielectric susceptibility as a function of frequency and temperature. The data at each temperature were fitted to the Kohlrausch–Williams–Watt (KWW) relaxation function, and the relaxation time, the shape parameter β, and the relaxation strength were extracted. The relaxation times follow the same Vogel–Tammann–Fulcher (VTF) equation for both enthaply and dielectric responses, while β of the KWW relaxation function showed different temperature dependences. β of the enthalpy relaxation shows a linear temperature dependence, whereas it stays constant in the dielectric relaxation. Significant molecular weight dependence occurs only in the relaxation strength. We discuss the temperature dependence of the shape parameter in terms of broadening of the relaxation time distribution associated with cooperative molecular rearrangements.