Relations Entre Structure Chimique Et Propriétés Mécaniques Dans Les Réseaux Époxydes

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Abstract
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 relaxations β and γ are shown to originate from motions of ‐CH2‐CHOH‐CH2‐O‐ epoxide groups and of ‐(CH2)4‐ groups, respectively. Motions responsible for the β‐relaxation appear at the same temperature, whatever the network characteristics may be. On the other hand, both amplitude and broadening of the β‐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.