Theory and applications of a new 2D-NMR technique for characterizing the molecular structure and order of partially oriented solids are described. After deriving a general expression for the signal intensities in two dimensions in cases of perfect order, the influence of disorder on the spectra is discussed. It is demonstrated that an expansion of the orientational distribution function in terms of Wigner rotation matrices allows spectra from residues with an arbitrary degree of order and arbitrary chemical shielding tensor orientation to be fully characterized. The technique is illustrated by obtaining the orientational distribution function for the crystalline and amorphous regions in semicrystalline polyethylene terephthalate, showing that the molecular orientation is quite different in the two phases.

Two-dimensional magic-angle-spinning (2D-MAS) NMR has been used to measure the orientation parameter, P2, of the diphenylene propane unit in bisphenol-A polycarbonate oriented by stretching to various extension ratios, ~, at T = 295 K and T = 403 K. P2 is proportional to the birefringence An, with a maximum birefringence Ano = 0.189. There is some evidence that the order parameter P~ of the DPP units with respect to the chain axis deviates from unity. Ano is therefore expected to be different from the birefringence, An'o, of perfectly aligned chains of polycarbonate. The experimental resuks obtained for P2(~) are compared to those predicted by the aggregate model.