1H-NMR relaxation study of the gelation of syndiotactic poly(methylmethacrylate) in toluene

Abstract

The temperature-induced helix-to-coil conformational transition accompanying the thermoreversible gelation of syndiotactic poly(methyl methacrylate) in toluene was studied by rheology and FTIR and H-1 NMR spectroscopy. Both rheological and FTIR measurements, carried out dynamically as a function of temperature, revealed a marked hysteresis between the heating and the cooling cycles. It was not possible to assess a quantitative difference between the values of the temperature of onset of the physical variations between the rheological and the spectroscopic experiments. It is then possible to conclude 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 the H-1 NMR study. The data obtained from the relaxation behavior of the a-methyl group, indicated a fraction of 0.28 for the chain repeating units incorporated 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 the H-1-relaxation behavior of the latter group, one also can conclude that an equilibrium exists between two populations of chain repeating units, different as to their mobility. The former group is characterized by a low mobility, which is ascribed to polymer-polymer interactions. Specific interactions with the solvent are proposed as the cause of the high mobility of the latter group of ester residues.