Negative first normal stress differences

Steady-state shear viscosity versus shear rate for PBLG solutions (molecular weight = 238,000) in m-cresol for several concentrations. The 38 wt% and 40 wt% samples show Region I behavior. (From Walker et al. 1995, with permission of the Journal of Rheology.) 

OQO(phenylsulfonyl)10. For this polymeric nematic, tends to level off at high shear rates, but remains positive even at shear rates well beyond that at which r shows significant shear thinning. 

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Thermotropic LCPs have high melt elasticity, but exhibit little extrudate swell. The latter has been attributed to a yield stress and to long relaxation times (60). The relaxation times for LCPs are normally much longer than for conventional polymers. Anomalous behavior such as negative first normal stress differences, shear-thickening behavior and time-dependent effects have also been observed in the. rheology of LCPs (56). Several of these phenomena are discussed for poly(benzylglutamate) solutions in the chapter by Moldenaers et al. 

The most striking phenomenon in the steady flow of LCPs is the occurrence of a negative first normal stress difference (Ni)—two sign changes in Ni as a function of shear rate. In contrast, isotropic solutions only exhibit positive Ni at all shear rates. Negative Ni has been reported in HPC/H20 and HPC/m-cresol systems. ... 

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 concentrated 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 liquid 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. 

Driscoll, P., Takigawa, T., Nakamachi, N. and Masuda, T. (1990) On negative first normal stress difference and a dip in steady shear viscosity in PLCs, unpublished manuscript obtained from A. Metzner. 

LCP s often display a range of phenomena not often seen, if at all, with Isotropic polymers. These Include the existence of negative first normal stress difference, regions of shear thickening viscosity, and secondary maxima In transient shear flow [2,19]. 

On the other hand, Marrucci et al (Marrucci G. and Maffettone 1989) have solved a two dimensional version of the Doi model for nematics (Doi M. and Edwards S.. F. 1986), in which the molecules are assumed to lie in the plane perpendicular to the vorticity axis, that is, in the plane parallel to both, the direction of the velocity and the direction of the velocity gradient. Despite this simplification, the predicted range of shear rates over which Ni is negative, is in excellent agreement with observations. This result opens up the possibility that negative first normal stress differences may be predicted in a two dimensional flow. 

The negative first normal stress difference (Ni) could not be predicted by Leonov s nematodynamics model. The values of Ni for filled LCPs are shown as a trend beyond 10 s . ... 

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