H-1 NMR relaxation study of the gelation of sydiotactic poly(methyl methacrylate) in toluene

The temperature-induced helix-to-coil conformational transition accompanyin g the thermoreversible gelation of syndiotactic poly(methyl methacrylate) i n toluene was studied by rheology and FTIR and H-1 NMR spectroscopy. Both r heological and FTIR measurements, carried out dynamically as a function of temperature, revealed a marked hysteresis between the heating and the cooli ng cycles. It was not possible to assess a quantitative difference between the values of the temperature of onset of the physical variations between t he rheological and the spectroscopic experiments. It is then possible to co nclude that a conformational change of the backbone (from the evolution of the band at 860 cm(-1)) is the molecular basis of the macroscopic formation of the gel. Spin-spin relaxation time, or T-2, measurements were used in t he H-1 NMR study. The data obtained from the relaxation behavior of the a-m ethyl group, indicated a fraction of 0.28 for the chain repeating units inc orporated in highly cooperative helical stretches, which are the junctions of the physical gel. The analysis of the relaxation behavior of the methoxy group led to a fraction of 0.24 for the same species. From the study of th e H-1-relaxation behavior of the latter group, one also can conclude that a n equilibrium exists between two populations of chain repeating units, diff erent as to their mobility. The former group is characterized by a low mobi lity, which is ascribed to polymer-polymer interactions. Specific interacti ons with the solvent are proposed as the cause of the high mobility of the latter group of ester residues.