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Marques Mendes Almeida da Rosa Leal, Catarina
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- Rheo-NMR velocimetry characterisation of PBLG/m-cresolPublication . Leal, Catarina R.; Feio, Gabriel; Almeida, Pedro L.NMR spectroscopy was used to characterise the velocity profile in nematic solutions of poly(gamma-benzyl-L-glutamate) (PBLG) in m-cresol during the application of a simple shear-flow at constant shear-rate values. In this study, the rheo-NMR technique was used that allows an insight over the fluid dynamics during the application of a shear flow in the presence of an external magnetic field, in a perpendicular direction to the shear gradient. Using this Magnetic Resonance Imaging (MRI) based technique, the velocity profile developed inside the system is accessed.
- NMR study of flow and viscoelastic properties of PBLG/m-Cresol lyotropic liquid crystalPublication . Veron, Alain; Gomes, Alexandre E.; Leal, Catarina R.; Klink, Jacques Van Der; Martins, Assis F.Director distortion patterns and backflow effects in transient flows of nematic solutions of PBLG/m-cresol are studied by deuteron NMR. The simulation of the observed NMR spectra as a function of time allows to characterise the director field pattern and to evaluate most of the viscoelastic properties of the system. A good agreement with classical rheometry is observed.
- Experimental results on electrorheology of liquid crystalline polymer solutionsPublication . Neves, S.; Leal, Catarina R.; Cidade, M. T.The electrorheological (ER) effect is known as the enhancement of the apparent viscosity upon apphcation of an external electric field. Suspensions of polarizable particles in non-conducting solvents are the most studied electrorheological fluids, however, liquid crystalline materials may also present ER effect as long as their dielectric anisotropy is positive. In the liquid crystalline state of a positive dielectric anisotropy, the application of the electric field makes the director align perpendicular to the fiow direction, thus increasing the apparent viscosity. In this work results of two liquid crystalline polymer solutions, acetoxypropylcellulose (APC) in dimethylacetamide (DMAc) and poly-y-benzyl-L-glutamate (PBLG) in 1,4-dioxane, presenting opposite behavior upon application of the electric field, will be presented. APC/DMAc (negative dielectric anisotropy) presents a decrease of the apparent viscosity upon application of the electric field, as expected, while PBLG/l,4-dioxane (positive dielectric anisotropy) presents the opposite behavior. For this last solution we will present the shear fiow curves for different electric fields in function of polymer molecular weight and solution concentration.
- Some Aspects of the Rheo-Nmr Behavior of the Lyotropic Liquid Crystal Poly(γ-BENZYL-L GLUTAMATE) in m-CresoPublication . Leal, Catarina R.; Klink, Jacques; Martins, AssisWe report the use of 2 H NMR spectroscopy to study the director orientation in two nematic solutions of poly(c-benzyl-L-glutamate) (PBLG) in m-cresol during simple shear flow and after its sudden cessation. In the range of applied shear rates ðc_ ¼ 5 ... 150 s 1Þ, a steady-state NMR line shape is reached after a deformation of a few hundred units. Two different line shapes occur that can be related to distinct shear responses in mechanical rheology: the tumbling=wagging and the flow-aligning regimes. The relaxation after cessation of the shear is similar for both cases and essentially follows the ‘‘inhomogeneous reorientation’’ regime known from sudden-rotation NMR experiments. A fit of the relaxation curves indicates that the tumbling parameter k is (slightly) less than one, as expected for a tumbling system.