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A new rotor-synchronized carbon-13 three-dimensional magic angle spinning NMR experiment for the extraction of orientational distribution functions from partially ordered solid materials is presented. Problems with overlap of two-dimensional spinning sideband patterns inherent in earlier methods are overcome by extension to three dimensions, allowing the degree of order for each resolvable carbon site to be measured. The new technique is demonstrated on a frozen smectic liquid-crystalline side group polymer which shows an order gradient from the aligned mesogen to the disordered polymer main chain.

The variation in the degree of orientational order as a function of molecular site is investigated in a homologous series of liquid-crystalline side group polymers based on a polysiloxane backbone using multidimensional carbon-13 and silicon-29 NMR spectroscopy. Significant differences in ordering behavior are observed amongst the members of the series as the length of the flexible spacer group is increased. In particular the degree of order on the polymer backbone and the size of the order gradient away from the aligned mesogen are correlated with the parity of the spacer chain.

Two new solid-state N M R methods for measuring orientation distributions in partially ordered solids are applied to high tensile-strength fibre polymers. Both methods use the anisotropy ofthe carbon-13 chemical shift tensor as a probe for local molecular directions, but they differ in the manner in which the information is extracted and complement one other. The spectral resolution is higher in the ORDER (ORientation Distributions with Enhanced Resolution) experiment, which employs rotor-synchronized three-dimensional magic angle spinning (MAS) NMR, while the angular precision with which the orientation distribution can be determined is higher in the DECODER(Direction Exchange with Correlation ofOrientation Distribution Evaluation and Reconstruction) experiment. The utility ofthe two techniques is demonstrated by their applicationto the study of industrially produced poly(p-phenyleneterephthalamide) fibres. Both give the same result for the width (x = 13 ") and the order parameters ( = 0.85, = 0.57, = 0.34, = 0.14) of the orientation distribution.

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