Ajoy K. Roy

A lattice model for dynamics in a mixed polymer-diluent glass

Abstract A lattice model is developed to interpret NMR lineshape and relaxation data on polymer-diluent systems. The lattice is used to count nearest-neighbor contacts which are assumed to influence local dynamics. When applied to the polymer chain, the presence of one diluent-polymer repeat unit contact is considered to suppress sub-glass transition motion through improved packing. Polymer repeat units in contact with more than one diluent molecule on the lattice are presumed to have enhanced mobility. Polymer repeat units surrounded by other polymer repeat units are thought to have no change in their sub-glass transition dynamics. When applied to the diluent, isotropic rotational motion of diluent surrounded by polymer is considered to commence at the apparent thermal glass transition. For those diluent molecules in contact with other diluent molecules on the lattice, sub-glass transition rotational motion occurs at a temperature determined by the intrinsic mobility of the diluent. The motions of the chain and the diluent are reflected in the modulus and are traditionally discussed in terms of plasticization and antiplasticization.

The application of a simultaneous model for multisite exchange to solid-state NMR lineshapes

Abstract A multisite exchange treatment is developed to simulate NMR powder lineshapes in cases where motional processes of more than one type occur simultaneously. An example is presented where one type involves rotational diffusion, libration, or oscillation over a limited range whereas the other includes jumps of larger amplitudes between potential minima, the axis of motion being the same for both types. Restricted rotation of lower angular amplitude is conceived in terms of jumps between sites within one well with the amplitude of this restricted rotation being the key parameter which can be the same or different for each potential minimum. The second motional process of larger amplitude involves a series of jumps between sites in one well to sites in the other well. In the cases treated, the first process is always considered too fast to exhibit rate-dependent effects. One rate parameter is used for all jumps. This rate is always kept sufficiently large to keep the first process in the rapid exchange limit. At very large rates, both motions exhibit rate-independent effects. At slower rates, the rate dependence of the second process leads to conspicuous changes in the lineshape. The model sets an equal probability for all jumps and represents an alternative to the conventional treatment where one deals with the processes sequentially. The method is illustrated for the case of bisphenol-A polycarbonate (BPAPC), where results show an improvement over the previously published treatment based on a sequential approach.