Relationship between melt viscosity and dielectric relaxation time for a series of epoxide oligomers

Melt viscosity has been investigated for a series of bisphenol‐A type epoxide oligomers with different weight‐average mol wts (M̄w), ranging from 388 to 2640. The temperature dependence of the melt viscosity is described by the Williams–Landel–Ferry (WLF) equation. The melt viscosity η is correlated with both the direct current (dc) conductivity σ and the dielectric relaxation time τ. The two relationships between these three properties, σ·ηκ = const (0.63 ≦ κ ≦ 1.12) and η/τℓ = const (0.73 ≦ ℓ ≦ 1.06), are experimentally derived. Both exponents, κ and ℓ, depend on the M̄w of the oligomer. The lower M̄w oligomer has the larger value of κ. The κ value is close to unity for the low M̄w oligomer, which agrees with Walden's rule, σ·.η = const, applicable to most low mol wt liquids. The ℓ value is near unity for the epoxide oligomer with higher M̄w than 2000, which means that the melt viscosity is proportional to the dielectric relaxation time. The low M̄w oligomer (M̄w < 2000), on the other hand, has a smaller value of ℓ below unity. The result indicates that the melt viscosity is not proportional to the dielectric relaxation time for the low M̄w epoxide oligomer, whose dielectric α‐relaxation is not governed by the Debye equation. © 1993 John Wiley & Sons, Inc.