A study of molecular motion in polycarbonate by broad line nuclear magnetic resonance

Abstract

The nuclear magnetic resonance (n.m.r.) second moment has been measured for polycarbonate of bisphenol A over the temperature range 8 to 414K. An extensive study of the saturation behaviour has shown that the maximum previously reported for the second moment at low temperatures is incorrect and arises from selective saturation of the resonance. We have shown that at low radio-frequency power levels the second moment and line shape remain sensibly unchanged over the temperature range 8 to 80K. The rigid solid second moment has been considered theoretically from the intramonomer and interchain interactions with reference to the structures proposed by Prietzschk and by Bonart. We find that the structure of Bonart agrees best with a chain arrangement satisfying Van der Waals contact distances and that the rigid lattice second moment deduced correlates well with that measured for the rigid solid amorphous polymer. The temperature variation of the second moment shows two regions of relaxation processes, the low temperature one corresponding to the onset of methyl group rotations, followed by a region between 160K and the glass transition temperature in which the part of the monomer containing the phenyl and methyl groups acquires considerable mobility. These findings are in agreement with, and complementary to, previous suggestions that the toughness of amorphous polycarbonate of bisphenol A arises from extensive motion of the chains below the glass transition temperature.