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Marques Mendes Almeida da Rosa Leal, Catarina
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- Aging effects on the rheology of LC solutions of hydroxypropylcellulosePublication . Leal, Catarina R.; Godinho, M. H.; Martins, A. F.; Fried, F.We apply the recently developed continuum theory of liquid crystalline polymers1 to interpret the behavior of the shear viscosity η(γ.) and the first normal-stress difference N1(γ.) measured for liquid crystalline solutions (c = 37%, c > c*) of HPC (Aldrich, Mw = 100,000) in acetic acid (AA) with different ages: 1 day, 1 month and 2 years. η(γ.) and N1(γ.) were measured over four decades in γ. N1(γ.) is observed to change from positive to negative and again to positive, as the shear rate γ. increases. η(γ.) shows a small newtonian plateau at low shear rates and a strong shear-thinning at higher values of γ. The rate of decrease of η(γ.) in this region is not monotonous, as usual, but shows an “hesitation” similar to one previously observed in a different system2. The aging effect promotes a “depolymerization”3 of HPC. This, in turn, should have a strong influence on the behavior of Nl(γ.) and η(γ.) which is indeed observed4. All these observations can be rationalized within the framework of the theory1.
- The influence of polymer molecular weight on the first normal-stress difference and shear-viscosity of LC solutions of hydroxypropylcellulosePublication . Martins, A. F.; Leal, Catarina R.; Godinho, M. H.; Fried, F.first normal-stress difference Ni(+) and the shear viscosity V(?) have been measured for liquid crystalline solutions of HFC in acetic acid (AA) as functions of the shear rate i. and the molecu lar mass of HPC. The measurements were done over four decades in +, for two samples of HPC with M, =6O,OOO and 100,OOO (Klucel E and L, respectively) and solution concentration c = 37% (c > c*). N1(+) is observed to change from positive to negative and again to positive, as the shear rate + increases. The 7 values at which N, changes sign depend on M,. The viscosity q(+) shows a small Newtonian plateau at low shear rates and a strong shear-thinning at higher values of +, includ ing an “hesitation” similar to one previously observed in LC solutions of PBUj [2]. All these obser vations are rationalized within the framework of the constitutive equations for liquid crystalline polymers recently proposed by one of us [l]. Expressions for q(+) and Nl(+) derived from this theory fit very well (quantitatively) to the experimental data and some fundamental viscoelastic parameters of the system are thereby obtained for the first time.