Six epoxy networks with various structures built up from a diepoxy prepolymer, DGEBA, and three different diamines or mixtures of a monoamine and a diamine were studied by dynamic mechanical analysis in the glass transition region. The systems were designed in order to investigate the dependence of glass transition Tg on both crosslink density and network chain flexibility. The time (frequency)—temperature superposition principle (WLF equation) was used to determine the viscoelastic coefficients Cg1 and C 2italicg which are related to some free volume characteristics on the molecular scale. Cg1, related to the free volume fraction available at Tg depends mainly on crosslink density, even though the product Cg1C 2italicg, related to the free volume expansion coefficient, is dependent on both chain flexibility and crosslink density. Thus, viscoelastic properties determined over large temperature and frequency ranges are shown to yield more precise information on epoxy network structure than the simple analysis of glass transition temperature.

The dynamic mechanical properties of model epoxy-amine networks are investigated in the glassy state over a wide range of frequencies, at temperatures between 123 K and 350 K. The effects of crosslink density and network chain flexibility on the p relaxation are examined. Motions responsible for the p process begin to develop at the same temperature, whatever the crosslink density. However, an increase in crosslink density is accompanied by an increase in amplitude and a broadening towards high temperatures of both damping tan6 and loss modulus E". This effect is responsible for the decrease of elastic modulus E' at room temperature with increasing crosslink density.

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Relationships existing between chemical structure and the dynamic mechanical properties of epoxide networks are reviewed. This paper is focused on different model networks, prepared by the reaction between amines and DGEBA or DGEBD diepoxide monomers in stoichiometric proportions. The limit glass transition temperature, Tg, , is shown to depend mainly on molecular flexibility of network chains and on crosslink density. Frequency dependence of Tg, obeys the time-temperature superposition principle and can be described by a WLF-type equation. However, the coefficients C1 and C2 in this equation depend on the molecular characteristics of the network, in agreement with free volume concepts. Values of elastic modulus well above Tg reflect mostly the influence of crosslink density. Surprisingly to some extent, application of rubber elasticity theory to these densely crosslinked systems yields very reasonable average network chain molecular weights Mc. In the glassy region, the mechanical relaxationsp and y are shown to originate from motions of -CH2-CHOH-CH2-0-epoxide groups and of -(CH2)4-groups, respectively. Motions responsible for the @-relaxation appear at the same temperature, whatever the network characteristics may be. On the other hand, both amplitude and broadening of the p-relaxation loss peak are functions of crosslink density, From a practical viewpoint, these two characteristics influence markedly the plastic behavior of the networks and their elastic modulus at room temperature as well.