Crosslinked epoxies: Network structure characterization and physical–mechanical properties

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Abstract
A homologous series of epoxy resins, based on the digylcidyl ether of bisphenol‐A, were reacted with stoichiometric quantities of m‐phenylenediamine (mPDA) to form networks with varying crosslink densities. Infrared spectroscopic analyses revealed that the networks were formed predominantly by the epoxy—amine addition reactions with little or no OH‐etherification. The ultimate glass transition temperatures were inversely proportional to the epoxy chain mol wt, as predicted by a model based on the assumptions of additivity and redistributivity of free volume. The average mol wt between crosslinks, M̄c, determined from the dynamic mechanical shear moduli, agreed to within 15 to 18% with the values predicted by the reaction stoichiometry. The glassy‐state modulus did not exhibit any M̄c dependence above room temperature, however, the modulus—temperature plot for all the networks exhibited a discontinuity near −50°C with a steeper slope below −50°C than above it. The change in slope occurred over the same temperature interval as the β‐transition temperature of the networks. The discontinuity in the modulus—temperature plot can be attributed to the freezing of the localized motions of the molecular groups responsible for the β‐transition. © 1992 John Wiley & Sons, Inc.