Orientational order of locally parallel chain segments in glassy polycarbonate from C-13-C-13 dipolar couplings

Interchain C-13-C-13 distances between C-13 labels and between C-13 labels and natural-abundance C-13 have been determined using dipolar restoration at the magic angle (DRAMA) with XY8 phase alternation and controlled simple excitation for dephasing rotational amplitudes (CEDRA) for two samples: (i) [carbonyl-C-13]polycarbonate and (ii) a homogeneous blend of 95% [carbonyl-C-13]polycarbonate-d(14) and 5% [methyl-C-13] polycarbonate. DRAMA is suitable for dipolar-coupled C-13-C-13 pairs having the same isotropic chemical shift, whereas CEDRA is used for pairs with dissimilar isotropic shifts. Pairs with dissimilar shifts were also examined using selective pi pulses and a homonuclear version of rotational-echo double resonance called DANTE-Selective REDOR(DSR). The combination of results from DRAMA, CEDRA, and DSR experiments shows that the glass consists on average of densely packed chains. This result is interpreted in terms of the orientational order of locally parallel chain segments (bundles). The bundles are structurally irregular and are characterized by a distribution of interchain spacings, which is consistent with the homonuclear NMR results from this work and with the results of previous heteronuclear distance measurements.