No abstract

Miscible blends of 1,4-polyisoprene (PIP) and polyvinylethylene (PVE ) are studied using rheo-optical methods to extract the dynamics of each component in the blend. Since orientational coupling contributes to the birefringence, but not the stress, simultaneous analysis of these observables for bidisperse blends is used to determine the coupling coefficient, ε. No systematic dependence of ε on composition or temperature was detected. The dynamics of the blend components extracted using the mean value of ε are in good agreement with those observed previously for equally entangled blends:  blending causes only small changes in the component entanglement molecular weights but dramatically alters the species' friction coefficients, with the dynamics of the high T g species (PVE ) being more sensitive to both temperature and composition.

The apparent thermorheological simplicity of disordered diblocks is reconciled with the failure of time−temperature superposition of corresponding miscible blends by examining the relaxation of the constituent blocks using rheo-optical techniques. Diblocks of 1,4-polyisoprene (PIP) and polyvinylethylene (PVE) are examined over a range of temperatures for two compositions (φPIP = 0.25 and 0.75). Unlike blends of PIP and PVE, the block copolymers appear to obey time−temperature superposition on the basis of their viscoelastic properties. However, departure from thermorheological simplicity is exposed in their stress−optical behavior. In particular, the copolymer rich in the high T g component (φPIP = 0.25) shows distinct temperature dependencies for the individual blocks, in accord with the behavior of PIP/PVE blends. The block copolymer rich in the low T g component (φPIP = 0.75) is thermorheologically simple because both blocks have similar monomeric friction coefficients ζo, PVE ≈ ζo,PIP, again in accord with prior results on PIP/PVE blends. The failure of time−temperature superposition in these diblocks was not previously observed because the change in ζo,PVE/ζo,PIP with temperature produces subtle changes in the overall relaxation spectrum relative to a linear chain of uniform friction.