Effect of pressure on the segmental and chain dynamics of polyisoprene. Molecular weight dependence

We report on the effect of pressure on the relaxation modes of the type-A p olymer polyisoprene as a function of the chain length for M/M-e ratio's (M- e is the entanglement molecular weight): 1/5, 1/2, 2/3, 2, and 5. We find t hat the spectral shape of the segmental and normal modes is invariant under temperature (T) and pressure (P) variations, but the time-pressure superpo sition fails as a consequence of the stronger P dependence of the segmental mode. The segmental mode activation volume for the different molecular wei ghts was found to scale with the temperature difference from the respective glass transition temperature. Pressure is found to slow down both modes in a quantitatively similar way, irrespective of the chain length, implying t hat M-e is largely independent of pressure. (C) 1999 American Institute of Physics. [S0021-9606(99)51845-X].