Conformation and dynamics of atactic poly(acrylonitrile). 3. Characterization of local structure by two-dimensional H-2-C-13 solid-state NMR

The detailed conformational structure of a disordered crystalline polymer, atactic poly(acrylonitrile) (aPAN), has been characterized by two-dimension al (2D) NMR experiments. A solid-state H-2-C-13 heteronuclear multiple-quan tum correlation (HMQC) NMR experiment has been applied to an aPAN sample co ntaining both alpha -hydrogen- deuterated and C-13 dropN-carbon-labeled rep eat units. The 2D spectrum yields information in particular on the two succ essive torsion angles in racemo trans-trans dyads. Their average values are found as (180 +/- 5 degrees, 180 +/- 5 degrees), with distributions of sta ndard deviation sigma = 10 +/- 5 degrees for both torsion angles. The torsi on angles and their distributions in racemo trans-trans dyads thus obtained are closer to the ideal trans conformation than are those in meso trans-tr ans dyads determined by 2D double-quantum solid-state NMR spectroscopy on a C-13 dropN-carbon-labeled aPAN sample. The differences are considered to o riginate from larger steric hindrance and larger electric dipole interactio n between the C dropN groups in meso trans-trans dyads. The experimental sp ectra are also compared with those calculated for three models for PAN prop osed in the literature. Further detailed analyses of the 2D spectra and the ir projections have also yielded information on intermolecular side group a lignments. There are indications that the C-13 dropN side groups are orient ed preferentially along the crystalline lattice directions a, b, and -(a b) in the hexagonal lattice. Intermolecular antiparallel alignments of C dr opN groups, often found in organic nitrile compounds, have not been observe d in the 2D spectra.