SHEAR FLOWS OF LIQUID-CRYSTAL POLYMERS - MEASUREMENTS OF THE 2ND NORMAL STRESS DIFFERENCE AND THE DOI MOLECULAR THEORY

A novel cone-and-plate rheometer has been used to measure the shear rate dependence of the second normal stress difference (N2) for various solutions of a rodlike polymer that exhibits liquid crystallinity. Remarkable differences are observed between measurements on isotropic and liquid crystalline phases of the same rodlike polymer. The rheology of the isotropic phase is more or less similar to that of a typical concentrated polymer solution. By contrast, the liquid crystalline phase exhibits a number of distinctive rheological features. Thermodynamic pressure is below atmospheric at most locations within the flowing liquid crystal, with the minimum value occurring near the tip of the cone. N2 is an oscillatory function of shear rate and is often comparable in magnitude to the primary normal stress difference (N1). Most surprisingly of all, the measured value of N2 is positive within certain narrow shear rate ranges. Although most of these experimental observations are without precedent in the rheological literature, a recently published version of the Doi molecular theory successfully describes the essential features of liquid crystal behavior in steady shear flow.