Main-Chain Reorientation in Polycarbonates

The amplitudes of main-chain reorientations have been determined by dipolar rotational spin-echo 13C NMR for bisphenol A polycarbonate, for the phenoxy resin made from bisphenol A and formaldehyde, and for the phenoxy resin made from bisphenol A and epichlorohydrin. All three polymers display prominent low-temperature mechanical-loss peaks, and all three polymers have main-chain rings undergoing rapid 180 flips at room temperature. The present results show that the amplitude of the main-chain rotational reorientation at the isopropylidene position for bisphenol A polycarbonate, and the isopropylidene and carbonate positions for the other two polymers, is less than 20 (root-mean-square). Small-amplitude motion at the carbonate position in bisphenol A polycarbonate has been observed before in measurements of carbonyl carbon chemical-shift tensors. Similar small-amplitude motion at the carbonate position has also been inferred from measurements of the volumes of activation for the ring-flip processes in bisphenol A polycarbonate and the phenoxy resin made from bisphenol A and epichlorohydrin. The combination of all of these results leads to the conclusion that the same kind of small-amplitude lattice reorganization controls ring flips for all three of these polycarbonates.