Thermally Induced Structural Transitions in Epoxy Thermoset Polymer Networks and Their Spectroscopic Responses

The polymer network structure of epoxy thermosets playsa significantrole in its final material properties. However, the effects of mildthermal exposure on these network structures are poorly studied. Inthis work, wide-angle X-ray scattering was used to investigate thepolymer network structure of two epoxy thermosets: homopolymerizedbisphenol A (BPA) epoxy resin and BPA epoxy resin cured with a polyetheramine hardener (BPA/T-403). Using density functional theory and wide-angleX-ray scattering, insights into the polymer network structure wereobtained. Diffraction features were determined to originate from hardener-to-hardenermolecular distance, perpendicular & pi;-& pi; stackingof aromatic p-phenylene rings, and the average carbon-carbondistance in the polymer. Thermal exposure was found to permanentlyalter these structural features for both thermosets, with an increasein the & pi;-& pi; stacking distance. Homopolymerized BPAhad an additional decrease in the hardener-to-hardener distance. Thesestructural alterations were found to be detectable using Fourier transforminfrared spectroscopy and Raman spectroscopy, with changes in thehardener-to-hardener distance having the largest variations in theresulting spectra specifically at the aromatic and ether frequencies.