Local order in polycarbonate glasses by 13C{19F} rotational-echo double-resonance NMR

Nearest-neighbor chain packing in a homogeneous blend of carbonate C-13-labeled bisphenol A polycarbonate and CF3-labeled bisphenol A polycarbonate has been characterized using a shifted-pulse version of magic-angle spinning C-13{F-19} rotational-echo double-resonance (REDOR) NMR. Complementary NMR experiments have also been performed on a polycarbonate homopolymer containing the same C-13 and F-19 labels. In the blend, the C-13 observed spin was at high concentration, and the F-19 dephasing or probe spin was at low concentration. In this situation, an analysis in terms of a distribution of isolated heteronuclear pairs of spins is valid. A comparison of the results for the blend and homopolymer defines the NMR conditions under which higher concentrations of probe labels can be used and a simple analysis of the REDOR results is still valid. The nearest neighbors of a CF3 on one chain generally include a carbonate group on an adjacent chain. A direct interpretation of the REDOR total dephasing for the polycarbonate blend indicates that at least 75% of carbonate-carbon (CF3)-C-13-F-... nearest neighbors are separated by a narrow distribution of distances 4.7 +/- 0.3 angstrom. In addition, analysis of the variations in REDOR spinning-sideband dephasing shows that most of the (CF3)-C-13-F-... dipolar vectors have a preferred orientation relative to the polycarbonate mainchain axis. This combination of distance and orientational constraints is interpreted in terms of local order in the packing of the carbonate group of one polycarbonate chain relative to the isopropylidene moiety in a neighboring chain. (c) 2006 Wiley Periodicals, Inc.