Exploring the local structure of amorphous polymers by solid -state NMR

The conformational statistics of isotopically labeled amorphous atactic polystyrene (aPS) and amorphous poly(ethylene terephthalate) (PET) were examined by solid-state NMR. Double-quantum pulse sequences were used to select 13C–13C spin pairs and correlate their chemical shift anisotropies, providing torsion angle information. A trans:gauche ratio of 68:32 (±10%) was determined for 13CH2–CH(Ph)– 13CH2 labeled segments in aPS while a cis:trans ratio of 60:40 was determined for O=13C–(Ph)–13C=O labeled segments in amorphous PET. The position and width (ψt = 185° ± 15°) of the trans peak in C2H(Ph)– 13CH2–13CH(Ph) labeled segments of aPS was determined by correlating well defined 2H quadrupolar and 13C–13C dipolar interactions. Auxiliary NMR experiments were performed on suitably labeled aPS to determine the 13CH2 chemical shift tensor orientations, the backbone bond angles (&thetas;α = 117° ± 3° and &thetas; β = 114° ± 3°), and separate the 13C MAS signals of 13CH2 and 13CH(Ph) units. An average angular amplitude of 5°–8° was determined for the backbone motion of aPS by examining the C–2H motion in C2H(Ph)–CH2 labeled aPS by 2H NMR with 1H decoupling. A distribution of motional amplitudes was determined for 13C=O groups in amorphous PET by CSA lineshape changes at different temperatures