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 polymer 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 that the spectral shape of the segmental and normal modes is invariant under temperature (T) and pressure (P) variations, but the time-pressure superposition fails as a consequence of the stronger P dependence of the segmental mode. The segmental mode activation volume for the different molecular weights 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 that M-e is largely independent of pressure. (C) 1999 American Institute of Physics. [S0021-9606(99)51845-X].