SOLID-STATE N-15 NMR-STUDY OF N-15 ENRICHED MELAMINE-FORMALDEHYDE RESINS

Molecular dynamics and structure of uncured and cured melamine-formaldehyde resins isotopically N-15 enriched at amine sites were studied by solid-state N-15 nuclear magnetic resonance (NMR). Spectra recorded with direct (DP) and cross-polarization (CP) pulse sequences reflect two motionally different regions arising from similar chemical structures. DP spectra of uncured resins at higher temperatures have narrow lines and the detection of slightly different structural units is possible. With increasing crosslinking resonances broaden and overlap and the direct detection of individual signals in cured resins is not possible. On the basis of variable contact time, variable spinning speed, and interrupted decoupling experiments three protonated and one nonprotonated group of signals are identified in the CP spectra for all samples. Short polarization-transfer rates, T-NH, for nonprotonated nitrogen in uncured and lightly cured samples reveal more effective hydrogen bonding in Viscous and rubber-like resins compared to the highly cured rigid resins. The rigid portions of the resins exhibit longer T-1 and short T-1 rho relaxation times, while the shorter T-1 times and longer T-1 rho times are associated with the more mobile portion of samples. (C) 1995 John Wiley & Sons, Inc.