Textured fluids

Abstract

The rheology and development morphology of textured fluids have been investigated. The first fluid considered in this work was a liquid crystalline polymer consisting of isotropic and anisotropic solutions of poly-p-phenyleneterephthalamide (PPT) in sulfuric acid. The second textured fluid considered in this work was an immiscible polymer blend consisting of poly(ethylene terephthalate) (PET) and nylon 6,6.

The role played by liquid crystalline order (LCO) and a polydomain texture on the rheology of PPT solutions was investigated. It was found that several of the rheological phenomena commonly attributed to liquid crystalline order in polymers (e.g., three region flow curve, negative steady state first normal stress difference, and oscillatory behavior at the start up of shear flow) were not observed in the solution in its anisotropic state. The solution in both its anisotropic and isotropic state exhibited a two region flow curve (Newtonian plateau and shear thinning region at rates ranging from 10-4 to 10 2 sec-I), a positive steady state first normal stress difference which increased with shear rate, and a transient shear stress which displayed a single overshoot before reaching a steady state value.

The rheology of PET/nylon 6,6 blends was found to be a function of both polymer degradation and the two phase texture. An accelerated degradation rate was found for the blends relative to the neat polymers, and as a consequence, the values of the steady shear viscosity (Î ), magnitude of the complex viscosity |Î *|, storage modulus (G') and steady state first normal stress difference (N 1) for samples melt blended in an extruder were lower than those of the neat polymers. Blends prepared by dry blending followed by mixing in a cone and plate device where the degradation occurring during extrusion was avoided were found to have a higher value of |Î *| and G' and enhanced transient behavior relative to those of the neat polymers. Scaling of the transient stress indicated there was no intrinsic time constant for these blends at shear rates lower than the longest relaxation time of the neat polymers

The theory developed by Doi and Ohta which describes the additional stresses arising as a consequence of interfacial tension in two phase systems was evaluated for its ability to model the rheology of the 2575 w/w PET/nylon 6,6 blend. The Doi-Ohta theory was found to be capable of qualitatively predicting the extra stresses arising as a result of the interfacial tension as observed in the steady state viscosity and steady state first normal stress difference and the transient stresses at the start up of steady shear flow. While the overshoot and undershoot of the stresses observed during stepwise changes of shear rate were not predicted, the scaling relation for the transient stresses predicted by the theory were found to hold for the blend using stepwise changes of shear rate at a constant step ratio.