(173c) Viscoelastic Bulk Modulus of Polystyrene and Nanocomposites

The relationship between the viscoelastic bulk and shear moduli has been debated for the last fifty years since Leaderman's suggestion that the two responses arise from different molecular mechanisms.  Using a custom-built pressurizable dilatometer, we have examined the pressure-volume-temperature (PVT) behavior and the time-dependent bulk modulus of several polymers, including polystyrene and star polystyrene in our laboratory.  Our results indicate that although there is quantitative agreement between the slopes of the retardation spectra for the bulk and shear responses at short times, the long-time chain mechanisms available to the shear response are not available to the bulk response.  This latter point, that chain modes are unavailable to the bulk response, was also recognized by Markovitz and by Berry and Plazek.  In fact, much of the apparent controversy concerning the origin of the bulk and shear responses may arise from this detail.  However, the short-time retardation spectra for the shear response is significantly higher than that for the bulk, a finding that is, as yet, unexplained.  The PVT and pressure relaxation experiments are currently being extended to polystyrene nanocomposites containing silica nanoparticles.