Magnetic Resonance Studies of Epoxy Resins.

Electron paramagnetic resonance EPR and hydrogen nuclear magnetic resonance NMR were used to investigate main-chain molecular motions in the epoxy resins tetraglycidyldiaminodiphenylme thane TGDDM and diglycidyl ether of bisphenol A DGEBA cured with dimethyldiaminohexane DDH and diethylenetriamine DETA. The cured resins were studied close to the glass transition as a function of temperature and as a function of solvent content methylene chloride and water. In the EPR approach, stable nitroxide free radicals were used either as spin probes or spin labels to probe their dynamic environment in the polymer. The uncured TGDDM was spin labeled at the epoxy group with nitroxide amines to form either an end label or a quaternary base. The latter was found to undergo a Hofmann elimination reaction to release a stable nitroxide spin probe. In the dry cured epoxy resins at each temperature, the EPR lineshapes of the spin labels and spin probes could be characterized by one value of the motional correlation time. The temperature dependence of this motional correlation time obeys the WLF equation. On the other hand, at some solvent contents, the EPR lineshapes could be resolved into two spectra associated with a slow phase and a fast phase indicating a bimodal distribution of motional correlation times. The solvent dependence of the motional correlation times for the fast phases obeyed a modified form of the Fujita-Doolittle equation. The two phases were identified with nitroxides located in regions with different microstructures, viz., regions with high crosslink density the slow phase embedded in and bound to regions of low crosslink density the fast phase.