Measurement of the rheological properties of molten polystyrenes

Abstract

To examine the processability of four commercial grades of the domestic polystyrenes (GP125, GP150, HI425 and HI425E) in the vicinity of the actual processing conditions, the steady shear flow properties such as viscosity($\eta$), first normal stress difference($N_1$) and second normal stress difference($N_2$) were measured by use of a capillary type instrument. For the study, a capillary rheometer equipped with a computer system was constructed, and the rheological properties were measured and discussed in terms of viscosity, two normal stress differences($N_1$ and $N_2$) and the dynamic properties by a parallel plate type instrument. All grades of polystyrene showed a non-Newtonian flow properties over the range of shear rates investigated. In general, HI grades showed slightly higher melt viscosities than GP grades. The viscosity data also suggested that the use of a melt index as a parameter for the polymer fluidity turned out errorneous in actual polymer processing because the difference in pseudoplasticity could give very different behavior in other range of shear rates and different temperatures. An inspection of $N_1$ and $N_2$ revealed that GP grades showed higher melt elasticity than HI grades and the ratio of $-N_2/N_1$ lay between 0.14 and 0.72 for the four grades of polystyrene observed. Comparison of capillary rheometer data with the dynamic data yielded a close analogy. That is, the shapes of viscosity curves and the dynamic viscosity curves were similar and in the same range. However, the shapes of $N_1$ curves and the dynamic storage modulus curves showed some discrepancy at the high range of shear rates or frequencies. This difference should be studied further in the future.