Abstract
The shear stress in capillary flow of polymer melts at the wall is discussed. The theoretical result suggests that Bagley's formula holds even if the exit pressure is not negligible in capillary flow of polymer melts. The importance is emphasized of distinguishing the pressure in tne axial direction (driving pressure) from that in the radial direction in cases of flow where primary and secondary normal stress differences N1 and N2, respectively, exist.
Experimentally, N1 and N2 are obtained from the data of mean normal stress difference and the pressure difference between inner and outer walls of the annulus. The magnitude of N2 reletive to N1 is 20~40% for high-density polyethylenes we used. The N2 increases with the square of shear stress at low shear stress, while it does at a rate slightly less when shear stress is above 0.08 MPa.
A simple model of a polymer melt is proposed to explain the normal stress effect and the Barus effect in cases of a steady shear flow and a converging flow. Assuming that the polymer melt consists of numerous flowing units which in turn are aggregates of Voigt elements with balls at their outer ends, we calculate the primary normal [stress difference and swelling ratio. In sofar as our experiments are concerned, the theoretical results show reasonable agreement with the experimental results in both cases of the normal stress effect and the Barus effect.
