In order to obtain the mechanism for the infrequent phenyl ring π-flips in glassy polycarbonate, a generalized Langevin dynamics simulation was performed on a reduced model consisting of a flipping ring and its keeper ring. The frequency of π-flips and activation energy for π-flips obtained from the simulation are in agreement with experiment. A phenyl ring π-flip occurs when there is an increase in the separation distance between the ring and its nearest neighbor ring on another chain, accompanied by, and in synchrony with, an increase in its rotational kinetic energy.

Carbon-13 rotational-echo double-resonance (REDOR) NMR with 15 N or 2 H dephasing, combined with 15 N REDOR NMR with 13 C dephasing, has been obtained for a fully cross-linked epoxy resin prepared from a nominally uniform mixture of two parts of diglycidyl ether of Bisphenol A, one part of hexamethylenediamine, and 19% (by weight) antiplasticizer made from a carbonyl-13 C-labeled aromatic acetamide. The antiplasiticizer contains the hydroxypropyl ether moiety of the epoxy repeat unit. A partially cross-linked resin was formed from five parts of epoxide, one part of hexamethylenediamine, three parts of hexylamine, and 19% (by weight) antiplasticizer. Labels were introduced into the resin by replacing both methyl groups of the isopropylidene moiety of Bisphenol A with CD3 groups and by using [ 15 N2]hexamethylenediamine and [ 15 N]hexylamine (and their unlabeled counterparts) in various combinations. The antiplasticizer 13 C-carbonyl carbon is 4.9 ( 0.5 Å from an amine 15 N (with no preference for free or cross-linked sites) and 6.7 ( 1 Å from a quaternary carbon of the isopropylidene moiety.