Abstract
It is well known that normal stresses appear as well as shear stress when polymer melts are subjected to shear.
Various manifestations of this effect have been described in literature, but comparatively few experiments have been made to measure this effect quantitatively.
The measurement of the flow birefringence in the slit-like duct is considered to give facilities to get normal stress effect of polymer melts in shear flow.
The slit-like ducts which have side walls made of glass were made for this purpose. The photographs of isochromatic patterns were taken and the pressure gradient along with the flow were measured on the flow of polyethylene melts. From these experiments the normal stress difference S11-S22 were obtained through the following equation.
S11-S22=√(Δn12/c)2-4S122=√(nλ/cw)2-4(yΔp/Δx)2
where c is stress optical coefficient, Δn12 is flow birefringence in the“1-2”plane and S12 is shear stress.
Using the values of S11-S22 the elastic energy in capillary flow that is imparted to the melt at the entrance of the capillary were calculated, and these results were compared with the values of total entrance pressure loss subtracted Couette loss which were obtained from the capillary experiment.
As the results of these experiments it has been ascertained that the former is far less than the latter.
This result is considered to show that the greater part of the total entrance pressure loss in capillary flow has been consumed as the viscous energy loss at the entrance region, and that the elastic energy imparted to the melt and therefore normal stress difference S11-S22 or recoverable strain SR in shear flow cannot be obtained by Philippoff18)-Bagley's19) method in a capillary experiment.
