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INTERPRETATION OF COMPLEX MOLECULAR MOTIONS IN SOLUTION. A VARIABLE FREQUENCY CARBON-13 RELAXATION STUDY OF CHAIN SEGMENTAL MOTIONS IN POLY(N-ALKYL METHACRYLATES)
Citation:
Levy, G., D. Axelson, R. Schwartz, AND J. Hochmann. INTERPRETATION OF COMPLEX MOLECULAR MOTIONS IN SOLUTION. A VARIABLE FREQUENCY CARBON-13 RELAXATION STUDY OF CHAIN SEGMENTAL MOTIONS IN POLY(N-ALKYL METHACRYLATES). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/J-78/194.
Description:
An extensive variable temperature study of poly(n-butyl methacrylate) and poly(n-hexyl methacrylate) at two widely separated frequencies (67.9 and 22.6 MHz) has revealed that a model requiring a nonexponential autocorrelation function, or, its mathematical equivalent, a distribution of correlation times, describes the NMR parameters obtained for the backbone carbons. However, frequency-dependent spin-lattice relaxation time (T(1)) and nuclear Overhauser effect (NOE) behavior observed for all side-chain carbons, including the terminal methyls, with NT(1)s of the order of 20s, could not be described in terms of present theoretical approaches. A new model developed retains the distribution of correlation times for the backbone carbons and incorporates the effects of multiple internal rotations about the carbon-carbon single bonds for the side-chain carbons. This model predicts a substantial frequency dependence for broad distribution widths which can quantitatively reproduce almost all of the observed data. For the highest temperatures attained (about 110C) the observed T(1) frequency dependence is quite large and only semiquantitatively accounted for using this modified theory. The ramifications of multifrequency experiments with respect to the proper interpretation of complex motions are explored.