Chemistry Reference
In-Depth Information
Figure 6.18. The first normal stress difference
vs.
the shear rate. (From Kiss and
Porter, 1980a.)
plastic products becomes convenient and the molded shape is sharper and
better.
A more detailed study verified that PBLG/m-cresol solution and
thermotropic copolyester under shear rate exhibits negative first normal
stress difference,
i.e.
, the difference between normal stresses parallel and
perpendicular to the flow direction, as shown in Figure 6.18.
The negative first normal stress difference under a medium shear rate,
characterized by liquid crystalline polymers, makes the material avoid the
Barus effect—a typical property of conventional polymer melt or concen-
trated solution,
i.e.
, when a polymer spins out from a hole, or capillary, or
slit, their diameter or thickness will be greater than the mold size. The liq-
uid crystalline polymers with the spin expansion effect have an advantage
in material processing. This phenomenon is verified by the Ericksen-Leslie
theory. On the contrary, the first normal stress difference for the normal
polymers is always positive.
