Molecular mechanism of the ring-flip process in polycarbonate

Calculations of the conformational preferences of isolated single chains of poly(2,6-di- methylphenylene oxide) (PPO) and bisphenol-A polycarbonate (PC) predict that the rings of both chains are nearly free rotors at room temperature. However, experimental dipolar rotational spin-echo 13C NMR shows that in the glass, the rings of PPO execute only small-amplitude motions while those of PC undergo primarily 180' ring flips (a hindered rotation) superimposed on some wiggles. Geometrical considerations of the dense packing of chains in the glass suggest that the rings of adjacent chains block rotational freedom. We propose that the mobility of the PC main chain results in lattice distortions which allow ring flips not permitted by the stiffer PPO main chain.