SEGMENTAL ORIENTATION AND INFRARED DICHROISM OF MODEL BIMODAL ELASTOMERIC NETWORKS

Model bimodal networks of polytetrahydrofuran (PTHF) were prepared by end-linking reactions. They consist of two different molecular-weight species designated L (long) and S (short). With the S, the hydrogen atoms were replaced by deuterium, so that C-H and C-D infrared bands could be associated with the L and S species, respectively. Infrared dichroism could then be used to determine the second-order orientation function, P2, for each species. Kuhn and Grun's (K-G) rubber elasticity theory permits a calculation of P2 as a function of the elongation ratio, lambda. For the unimodal system, the P2 was found to become greater with decreasing molecular weight between crosslinks, M(c), and is consistent with the theory. For the bimodal network system, the P2 for the L species was found to be almost equal to the P2 for the S species, suggesting a heterogeneous (or non-affine) deformation of the S and L chain segments at molecular level. In another way, the equality of (P2)L and (P2)S allows us to assume that the bimodal network can be treated as the "unimodal" network consisting of the chain segments with the weight-averaged length between the long and short chains. Under the assumption of affine deformation for such an "averaged unimodal" system, the number of statistical segments, N, was evaluated using the K-G theory; the value of N is linearly proportional to the averaged molecular weight between the L and S species and one statistical segment was found to consist of three tetrahydrofuran (THF) monomeric units.