Conformation and Dynamics of Atactic Poly(acrylonitrile). 3. Characterization of Local Structure by Two-Dimensional 2H−13C Solid-State NMR

The detailed conformational structure of a disordered crystalline polymer, atactic poly(acrylonitrile) (aPAN), has been characterized by two-dimensional (2D) NMR experiments. A solid-state 2 H− 13 C heteronuclear multiple-quantum correlation (HMQC) NMR experiment has been applied to an aPAN sample containing both α-hydrogen-deuterated and 13 C⋮N-carbon-labeled repeat units. The 2D spectrum yields information in particular on the two successive torsion angles in racemo trans−trans dyads. Their average values are found as (180 ± 5°, 180 ± 5°), with distributions of standard deviation σ = 10 ± 5° for both torsion angles. The torsion angles and their distributions in racemo trans−trans dyads thus obtained are closer to the ideal trans conformation than are those in meso trans−trans dyads determined by 2D double-quantum solid-state NMR spectroscopy on a 13 C⋮N-carbon-labeled aPAN sample. The differences are considered to originate from larger steric hindrance and larger electric dipole interaction between the C⋮N groups in meso trans−trans dyads. The experimental spectra are also compared with those calculated for three models for PAN proposed in the literature. Further detailed analyses of the 2D spectra and their projections have also yielded information on intermolecular side group alignments. There are indications that the 13 C⋮N side groups are oriented preferentially along the crystalline lattice directions a, b, and −( a + b ) in the hexagonal lattice. Intermolecular antiparallel alignments of C⋮N groups, often found in organic nitrile compounds, have not been observed in the 2D spectra.