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 la bels and natural-abundance C-13 have been determined using dipolar res toration at the magic angle (DRAMA) with XY8 phase alternation and con trolled simple excitation for dephasing rotational amplitudes (CEDRA) for two samples: (i) [carbonyl-C-13]polycarbonate and (ii) a homogeneo us 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 p airs with dissimilar isotropic shifts. Pairs with dissimilar shifts we re also examined using selective pi pulses and a homonuclear version o f rotational-echo double resonance called DANTE-Selective REDOR(DSR). The combination of results from DRAMA, CEDRA, and DSR experiments show s that the glass consists on average of densely packed chains. This re sult is interpreted in terms of the orientational order of locally par allel 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 wit h the results of previous heteronuclear distance measurements.