An expression is derived for the motionally averaged NMR fourth moment of a partially oriented polymer. This result, taken together with expressions previously obtained for the second moment, makes it possible to study quantitatively the effects of different types of molecular motion. The case of drawn polyoxymethylene is used to show how the theory may be applied. Detailed comparisons are made with the previous experimental results of Olf and Peterlin. For this system it is found that the experimental data at room temperature may be accounted for by a model in which the chains are oscillating about the helical axis with an amplitude of 42°. At a higher temperature (100°C) it is necessary to assume, in addition, that the chains are undergoing rapid translational motion which has the effect of greatly reducing the intermolecular contributions to the moments.
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McBrierty et al. (1973) studied this question.
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