HIGH-RESOLUTION SOLID-STATE C-13 NUCLEAR-MAGNETIC-RESONANCE INVESTIGATION OF LOCAL MOTIONS IN MODEL EPOXY-RESINS

The local dynamics of some 'model' epoxy networks based on various die poxides and diamines was investigated in the bulk state using high-res olution solid-state C-13 nuclear magnetic resonance. The strength of t he C-13-H-1 dipolar coupling was determined from the rises of C-13 mag netization in cross-polarization experiments using very short contact times. A restricted motional averaging of the C-13-H-1 dipolar couplin g was observed below the glass transition temperature for the hydroxyp ropyl ether groups and methylene carbons adjacent to the crosslinks, i ndicating the existence of local motions of these units in the tens of kilohertz range. The extent and temperature dependence of the motiona l averaging depend on the chemical structure of the epoxy network cons idered. Comparison with results derived from dynamic mechanical experi ments leads to the conclusion that the motions of the hydroxypropyl et her groups are likely to participate in the phenomena responsible for the beta secondary relaxation. Moreover, the relatively high mobility of the central methylene units in the aliphatic sequences of the digly cidyl ether of the butanediol-based network, and of the hexamethylened iamine-based network, can be associated with the gamma relaxation of e poxy networks.