SOLID-STATE C-13 NMR-STUDY OF HEXA(METHOXY METHYL)MELAMINE AND MELAMINE-FORMALDEHYDE SELFCONDENSATES

The structure and molecular dynamics of self-condensation products of three different melamine resins based on hexa(methoxy methyl)melamine are studied by C-13 NMR in the solid state. The application of direct (DP) and cross-polarization (CP) pulse sequences shows that uncured an d cured melamine resins are motionally heterogeneous systems, with the mobile and rigid parts consisting of the same basic structural units. Viscous lightly cured samples based on commercial melamine-formaldehy de resins contain low-molecular weight species which act as plasticize rs. In these resins, three motionally different methyl groups are obse rved. Their existence is confirmed either by spectral deconvolution or by fitting the experimental signal intensities to the cross-polarizat ion dynamics with the assumption of two cross-polarization-transfer ra tes. Fast cross-polarizing rigid methyl groups are accompanied with th e spinning sidebands. On the basis of DP and CP relaxation measurement s, quantitative results for three major structural units are calculate d. Problems concerning the N-14-C-13 quadrupolar interactions and quan titative analysis are discussed. (C) 1995 John Wiley & Sons, Inc.