WLF (Williams-Landel-Ferry) Dependence of the Dielectric Properties of DGEBA (Diglycidyl Ether of Bisphenol-A) Epoxy Resins

The frequency and temperature dependence of the complex dielectric constant Epsilon of seven diglycidyl ether of bisphenol-A DGEBA epoxy resins having epoxide equivalent weights EEW in the range 175 to 1880 have been measured from T subscript -30 C to T subscript 70 C at frequencies between 0.1 and 10,000 Hz. In the vicinity of T subscript g, Epsilon is dominated by dipole relaxation, while at higher temperatures ionic conductivity dominates. For all resins, the temperature dependences of the frequency of maximum dipole loss, fmax, and of the conductivity, sigma, obey the Williams-Landel-Ferry WLF equation. The WLF constants C subscript 1 and C subscript 2 were determined for both the fmax and sigma data for each of the resins. In a given material, the WLF constants for sigma and fmax differed, indicating that the temperature dependences of the mobilities of ionic impurities and permanent dipole moments differ quantitatively. As the EEW of the material increased, the C subscript 1 values determined for the conductivity at high EEWs, while the corresponding C subscript 2 constants decreased slightly. Free volume and entropic theories of the glass transition are used to interpret these results in terms of the underlying conduction and dipole relaxation processes.