Browsing by Author "Martins, A. F."
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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.
- Rheological properties of acetoxypropylcellulose in the thermotropic chiral nematic phasePublication . Cidade, M. T.; Leal, Catarina R.; Godinho, M. H.; Martins, A. F.; Navard, P.Experimental data for the rheological behavior of two thermotropic liquid crystalline (LC) samples of acetoxypropylcellulose (APC) with different molecular weights, at 120°C, and in shear rates between 0.01 and 10s −1, are presented and analyzed in the framework of the continuum theory for LC polymers recently proposed by Martins1. The viscosity η(γ.)shows a strong shear thinning in the range of shear rates γ. studied, with an hesitation at shear rates of about 0.1-0.2 s−1, depending on the molecular weight, and the first normal stress difference Nl(γ.) shows only positive values, increasing with shear rate γ., with an hesitation at shear rates of an order of magnitude higher, i.e. about 1-2 s−l, also depending on the molecular weight. The hesitation points of the flow functions are displaced towards lower values of the shear rate, with increasing molecular weight. For small and intermediate γ. the shear viscosity of the higher molecular weight sample is greater than the corresponding viscosity for the lower molecular weight sample, but this pattern is reversed at higher γ., the crossover point being at γ. = 1.5 s−1. The molecular weight dependence of the first normal stress difference follows a similar pattern. All these observations can be interpreted by Martins' theory. The expressions for η(γ.) and N1(γ.) derived from this theory fit very well to the experimental data, therefore allowing for some fundamental viscoelastic parameters to be estimated.
- The first normal stress difference and viscosity in shear of liquid crystalline solutions of hydroxypropylcellulose: new experimental data and theoryPublication . Fried, F.; Leal, Catarina R.; Godinho, M. H.; Martins, A. F.The constitutive equations for liquid crystalline polymers recently proposed by one of us [1] are applied here to interpret the behaviour of the shear viscosity η equation image and the first normal stress difference N1($ \dot \gamma $equation image) measured for liquid crystalline (LC) solutions of hydroxypropylcellulose in acetic acid. N1(equation image) is observed to change from positive to negative and again to positive, as the shear rate $ \dot \gamma $equation image increases, at lower concentrations, in the LC phase. The $ \dot \gamma $equation image-values at which N1 changes sign depend on the molecular mass (degree of polymerization) and on the concentration. η $ \dot \gamma $equation image shows a small Newtonian plateau at low shear rates and a strong shear-thinning at higher values of $ \dot \gamma $equation image. The rate of decrease of η $ \dot \gamma $equation image in this region shows an “hesitation” similar to one previously observed in LC solutions of poly-γ-benzyl-L-glutamate PBLG. All these observations can be rationalized within the frame-work of Martins' theory. The expressions for N1($ \dot \gamma $equation image) and η $ \dot \gamma $equation image derived from this theory fit very well (quantitatively) to the experimental data and some fundamental viscoelastic parameters of the system under study are thereby obtained for the first time.
- 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.