Local order in polycarbonate glasses by C-13{F-19} rotational-echo double-resonance NMR
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abstract
Nearest-neighbor chain packing in a homogeneous blend of carbonate 13C-labeled bisphenol A polycarbonate and CF 3-labeled bisphenol A polycarbonate has been characterized using a shifted-pulse version of magic-angle spinning 13C{ 19F} rotational-echo double-resonance (REDOR) NMR. Complementary NMR experiments have also been performed on a polycarbonate homopolymer containing the same 13C and 19F labels. In the blend, the 13C observed spin was at high concentration, and the 19F 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 CF 3 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 13CF 3 nearest neighbors are separated by a narrow distribution of distances 4.7 0.3 . In addition, analysis of the variations in REDOR spinning-sideband dephasing shows that most of the 13CF 3 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. 2006 Wiley Periodicals, Inc.
