DYNAMIC-MECHANICAL AND C-13 NMR INVESTIGATIONS OF MOLECULAR MOTIONS INVOLVED IN THE BETA-RELAXATION OF EPOXY NETWORKS BASED ON DGEBA AND ALIPHATIC-AMINES

As shown by a precise analysis of the dynamic mechanical data in terms of activation energies and entropies, the extent of the beta transiti on strongly depends on the role played by the crosslinks. In a densely crosslinked network, localized motions at the spatial scale of an epo xy-amine repeat unit occur at low temperatures, whereas cooperative mo des implying more than six units are responsible for the high temperat ure part of the relaxation. In the quasi-linear systems, only modes wi th a low spatial extent exist. Measurements of the strength of the C-1 3-H-1 dipolar couplings, together with other n.m.r. techniques, have s hown that the motions of the hydroxypropylether units take part in the beta transition. The DGEBA ring flips also take part in the beta tran sition. The comparison of dynamic mechanical and C-13 n.m.r. results t ends to indicate that the sensitivity of the n.m.r. experiments allows it to probe the cooperative motions, which induce a reorientation of quite a large amount of C-13-H-1 bonds, rather than the isolated ones, whose effect on the C-13-H-1 dipolar coupling is rather weak. In addi tion, it is shown that the beta transition is broader and more intense in networks prepared in the presence of secondary diamines than in sy stems with the same crosslink density obtained by using primary monoam ines. This result is interpreted in terms of motions that have a highe r intramolecular cooperativity than the cooperativity developed in net works incorporating pending hexamethylene units, and a lower spatial e xtent than the one observed in the densely crosslinked network. (C) 19 97 Elsevier Science Ltd.