Structure and Properties Relationships of Epoxy Resins Part 1: Crosslink Density of Cured Resin: (II) Model Networks Properties

Carefully designed resin precursors of high purity, viz. N,N-bis-(2,3-epoxypropyl)-N',N-dimethyl-4,4′-diaminodiphenylenemethane (G2A) and N,N-bis-(2,3-epoxypropyl)-N,N-dimethyl-4,4′-diamino-diphenylenemethane (G2S) were used in combination with N,N,N',N-tetrakis-(2,3-epoxypropyl)-4,4′-diaminodiphenylene methane, TGDDM, and cured with stoichiometric amounts of 4,4′-diamino-diphenylene methane (DDM) to produce networks with a range of controlled crosslink density. The tensile moduli E of the networks in the rubbery state, at Tg+30°C, Tg+45°C and Tg+60°C, were measured using a thermal mechanical analyser. Using the statistical theory of rubber elasticity and the observed values of E, the number average molecular weights between crosslink points Mc for the cured resins were deduced. The experimental Mc values were then compared with those derived by calculations based on a probabilistic model of the network proposed by Chu and Seferis.1 The experimental Mc values were 2.5 to 5.5 times larger than the calculated ones. The differences were attributable to a consumption of only 40% of the available secondary amino hydrogen via epoxy-amine reaction. A direct relationship was established between the glass transition temperature and the crosslink density 1/Mc for the resins, and the dynamic mechanical properties were studied. The thermal stability of cured resins studied by thermo-gravimetric analysis indicated an enhancement of stability as 1/Mc was reduced. The amount of water absorbed by cured resin was directly proportional to 1/Mc.