C-13 NMR-STUDY OF THE CONFORMATION AND DYNAMICS OF BISPHENOL-A POLYCARBONATE

Local chain dynamics in solid Bisphenol A polycarbonate (BPAPC) was studied through its effect on the line width of the C-13 NMR signals of samples spinning at the magic angle. The homogeneous line widths, which are associated in part with motion, were separated from the inhomogeneous line widths, which have causes such as a distribution of local packing environments, through generation of a spin-echo train. The line widths of the protonated aromatic carbons depended strongly on temperature. A motion that could cause such temperature-dependent line widths is ring flipping. Ring flipping would not contribute to the widths of carbons lying on the flipping axis, and indeed the widths of the resonances from the nonprotonated carbons were temperature independent. The temperature of maximum line broadening of the signals from the proton-bearing carbons is consistent with an activation energy of 8-9 kcal/mol. Three-fold rotation of the methyl groups is adequate to explain the width of the signals from the methyl carbon at low temperature. However, isotropic motion of the methyls with a temperature-dependent amplitude, in addition to methyl rotation, is not ruled out.