Thermophysical Properties and Molecular Relaxations in Cured Epoxy Resin + PEO Blends: Observations on Factors Controlling Miscibility

Abstract

AbstractSummary: Thermophysical properties and molecular relaxations in aromatic amine‐cured diglycidyl ether of bisphenol‐A (DGEBA) epoxy oligomer and poly(ethylene oxide) (PEO) mixtures were determined by DSC and dielectric techniques (TSC, DRS). The binary blends were judged to be fully miscible in the amorphous state (wPEO < 40 wt.‐%), as evidenced by the single composition‐dependent glass transition temperatures Tgs. In the amorphous blends, negative deviations of dielectric/thermal Tg‐estimates from the linear mixing rule or the behavior predicted by the Fox equation reveal weaker intermolecular interactions, compared to strong self‐association of hydroxyls in the cured thermoset. Morphological changes in PEO‐rich blends (wPEO ≥ 40 wt.‐%) are in accordance with their complicated interface structure, previously reported to consist of amorphous PEO regions, branched epoxy resin chains and an imperfect epoxy resin network located between PEO lamellae. In these blends, PEO crystallites exert steric hindrances in the amorphous regions, causing strong Tg upshifts. Changes in the relaxation dynamics of glyceryl segments (e.g., in the activation energy barrier and relaxation strength) are in accordance with the idea that the close matching between the molecular polarities of PEO, epoxy resin and the cure agent significantly contributes to the observed miscibility.Plots of transition temperatures as function of blend composition.magnified imagePlots of transition temperatures as function of blend composition.