Abstract: The average spectral density functions under MAS condition were derived from various models of jump motion and models of freely diffusive motion. The contents of crosslinking agent DVB were evaluated by using CP/MAS spectra. The rotational correlation times and diffusion coefficients of α-CH₃ in a series of poly (methyl methacrylates) crosslinked with DVB were calculated according to the measured T₁ values. The results showed that:(1) the correlation times increase and the diffusion coefficients decrease as the contents of DVB increase, (2) the rotation activation energy of the α-CH₃ is independent of the contents of DVB and less than that from dielectric and mechanical relaxations.

Key words: Spin-Lattice relaxation, Average spectral density function, Crosslinked poly (methyl methacrylates)