Self-concentrations and effective glass transition temperatures in polymerblends

In a miscible polymer blend the local environment of a monomer of type A wi ll, on average, be rich in A compared to the bulk composition, phi, and sim ilarly for B; this is a direct consequence of chain connectivity. As a resu lt, the local dynamics of the two chains may exhibit different dependences on temperature and overall composition. By assigning a length scale (or vol ume) to particular dynamic mode, the relevant "self-concentration" phi(s) c an be estimated. For example, we associate the Kuhn length of the chain, l( K), with the monomeric friction factor, zeta, and thus the composition and temperature dependences of zeta should be influenced by phi(s) calculated f or a volume V similar to l(K)(3). An effective local composition, phi(eff), can then be calculated from phi(s) and phi. As lower T-g polymers are gene rally more flexible, the associated phi(s) is larger, and the local dynamic s in the mixture may be quite similar to the pure material. The higher T-g component, on the other hand, may have a smaller phi(s), and thus its dynam ics in the mixture would be more representative of the average blend compos ition. An effective glass transition temperature for each component, T-g(ef f), can be estimated from the composition-dependent bulk average T-g as T-g (phi(eff)). This analysis provides a direct estimate of the difference in t he apparent T-g's for the two components in miscible blends, in reasonable agreement with those reported in the literature for four different systems. Furthermore, this approach can reconcile other features of miscible blend dynamics, including the asymmetric broadening of the calorimetric T-g, the differing effects of blending on the segmental relaxation times of the two components, and the failure of time-temperature superposition.