MOLECULAR-ORIENTATION IN DEFORMED BIMODAL NETWORKS .1. THEORY

Expressions for the molecular orientation of long and short chains in bimodal networks, SL and Ss, are derived based on the theory developed by Kloczkowski et al. (Macromolecules 1991, 24, 3266) for regular bim odal phantom networks and compared with those (S-L degrees and S-S deg rees) occurring in,corresponding unimodal networks. The changes in seg mental orientation of the respective chains in bimodal networks relati ve to their unimodal counterparts, expressed as the ratio S-L/S-L degr ees or S-S/S-S degrees, are shown to be independent of macroscopic str ain and intrinsic chain configurational characteristics but depend ess entially on network topology and composition. The latter are accounted for by two variables: the ratio xi of the molecular weights of short chains to that of long chains, and the number phi(S) and phi(L) of sho rt and long chains at each junction. Results of the formulation show t hat the long (short) chains in the bimodal network orient more (less) than those in the corresponding unimodal network. These differences in the orientation behavior of the two chains arise from differences in fluctuations of chain dimensions that, in turn, affect the microscopic strain of each component. Predictions of the theory are compared in t he following paper with results of Fourier transform infrared measurem ents on well-defined poly(dimethylsiloxane) networks.