Bulk and shear rheology of a symmetric three‐arm star polystyrene

The bulk and shear rheological properties of a sym- metric three-arm star polystyrene were measured using a self- built pressurizable dilatometer and a commercial rheometer, respectively. The bulk properties investigated include the pressure-volume-temperature behavior, the pressure-depend- ent glass transition temperature (Tg), and the viscoelastic bulk modulus and Poisson's ratio. Comparison with data for a linear polystyrene indicates that the star behaves similarly but with slightly higher Tgs at elevated pressures and slightly higher limiting bulk moduli in glass and rubbery states. The Poisson's ratio shows a minimum at short times similar to what is observed for the linear chain. The horizontal shift factors above Tg obtained from reducing the bulk and shear viscoelastic responses are found to have similar temperature dependence when plotted using TTg scaling; in addition, the shift factors also exhibit a similar temperature dependence to linear poly- styrene. The retardation spectra for the bulk and shear responses are compared and show that the long time molecu- lar mechanisms available to the shear response are unavailable to the bulk. At short times, the two spectra have similar slopes, but the short-time retardation spectrum for the shear response is significantly higher than that for the bulk, a finding that is, as yet, unexplained. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 1233-1244, 2012