Browsing by Author "Leal, Catarina R."
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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.
- An electro-rheological study of the nematic liquid crystal 4-n-heptyl-4'-cyanobiphenylPublication . Cidade, M. T.; Leal, Catarina R.; Patricio, PedroAn experimental and theoretical study of the electro-rheological effects observed in the nematic phase of 4-n-heptyl-4'-cyanobiphenyl has been conducted. This liquid crystal appears to be a model system, in which the observed rheological behaviour can be interpreted by the Leslie-Ericksen continuum theory for low molecular weight liquid crystals. Flow curves are illustrated at different temperatures and under the influence of an external electric field ranging from 0 to 3 kV mm-1, applied perpendicular to the direction of flow. Also presented is the apparent viscosity as a function of temperature, over similar values of electric field, obtained at different shear rates. A master flow curve has been constructed for each temperature by dividing the shear rate by the square of the electric field and multiplying by the square of a reference value of electric field. In a log-log plot, two Newtonian plateaux are found to appear at low and high shear rates, connected by a shear-thinning region. We have applied the Leslie-Ericksen continuum theory, in which the director alignment angle is a function of the electric field and the flow field boundary conditions are neglected, to determine viscoelastic parameters and the dielectric anisotropy.
- Antibiotic activity screened by the rheology of S. aureus culturesPublication . Portela, Raquel; Valcovo, Filipe; Almeida, Pedro L.; Sobral, Rita; Leal, Catarina R.Multidrug resistant bacteria are one of the most serious public health threats nowadays. How bacteria, as a population, react to the presence of antibiotics is of major importance to the outcome of the chosen treatment. In this study we addressed the impact of oxacillin, a β-lactam, the most clinically relevant class of antibiotics, in the viscosity profile of the methicillin resistant Staphylococcus aureus (MRSA) strain COL. In the first approach, the antibiotic was added, at concentrations under the minimum inhibitory concentration (sub-MIC), to the culture of S. aureus and steady-state shear flow curves were obtained for discrete time points during the bacterial growth, with and without the presence of the antibiotic, showing distinct viscosity progress over time. The different behaviors obtained led us to test the impact of the sub-inhibitory concentration and a concentration that inhibited growth. In the second approach, the viscosity growth curves were measured at a constant shear rate of 10 s−1, over time. The obtained rheological behaviors revealed distinctive characteristics associated to the presence of each concentration of the tested antibiotic. These results bring new insights to the bacteria response to a well-known bacteriolytic antibiotic.
- Crosslinked bacterial cellulose hydrogels for biomedical applicationsPublication . Almeida, Ana; Saraiva, João N.; Cavaco, Gonçalo; Portela, Raquel; Leal, Catarina R.; Sobral, Rita; Almeida, Pedro L.The skin, fundamental barrier that protects internal tissues, prevents pathogen invasion, and maintains the body fluid equilibrium, may be compromised upon traumas, such as incisions and burns. The healing process of such wounds is costly and usually hindered by the patient’s physiological conditions, associated diseases, inflammation and external factors, namely bacterial infections. Recently, increasing attention has been given to bacterial cellulose-based membranes to be applied as dressings for healing purposes. Bacterial cellulose is an attractive biomaterial due to its unique structural characteristics such as high porosity, high water retention capacity, high mechanical strength, low density, and biodegradability. One drawback of bacterial cellulose hydrogels is that, after the first dehydration, the water retention capacity is hindered. In this work we produced, modified, and characterized hydrated and de-hydrated BC membranes. Two crosslinking methods were adopted (using citric acid and epichlorohydrin as crosslinking agents), and the results obtained from the characterizations such as water retention capacity, mechanical properties or contact angle were compared to those of unmodified bacterial cellulose. We demonstrate that the cross-linked bacterial cellulose membranes present physical properties suitable to be used as surgical and burn wound dressings when hydrated, or as exuding wound dressings, diapers dressing or sanitary pads when dehydrated.
- Electro-rheology study of a series of liquid crystal cyanobiphenyls: experimental and theoretical treatmentPublication . Patricio, Pedro; Leal, Catarina R.; Pinto, L. F. V.; Boto, A.; Cidade, M. T.In this work we study the electro-rheological behaviour of a series of four liquid crystal (LC) cyanobiphenyls with a number of carbon atoms in the alkyl group, ranging from five to eight (5CB–8CB). We present the flow curves for different temperatures and under the influence of an external electric field, ranging from 0 to 3 kV mm−1, and the viscosity as a function of the temperature, for the same values of electric field, obtained for different shear rates. Theoretical interpretation of the observed behaviours is proposed in the framework of the continuum theory of Leslie–Ericksen for low molecular weight nematic LCs. In our analysis, the director alignment angle is only a function of the ratio between the shear rate and the square of the electric field – boundary conditions are neglected. By fitting the theoretical model to the experimental data, we are able to determine some viscosity coefficients and the dielectric anisotropy as a function of temperature. To interpret the behaviour of the flow curves near the nematic–isotropic transitions, we apply the continuum theory of Olmsted–Goldbart, which extends the theory of Leslie–Ericksen to the case where the degree of alignment of the LC molecules can also vary.
- Electrorheological characterization of dispersions in silicone oil of encapsulated liquid crystal 4-n-penthyl-4 '-cyanobiphenyl in polyvinyl alcohol and in silicaPublication . Brehm, Tiago; Pereira, Gonçalo; Leal, Catarina R.; Gonçalves, Clara; Borges, João P.; Cidade, Maria TeresaThe electrorheological (ER) effect is known as the change in the apparent viscosity upon the application of an external electric field perpendicular to the flow direction. In this work we present the electrorheological behaviour of suspensions in silicone oil of two different dispersed phases: foams of liquid crystal 4-n-penthyl-4'-cyanobiphenyl (5CB) encapsulated in polyvinyl alcohol (PVA) and nano/microspheres of 5CB encapsulated in silica. We will present the viscosity curves under the application of an electric field ranging between 0 and 3 kV mm(-1). The ER effect was observed for the suspensions of 5CB/PVA but not in the case of 5CB/silica. For the case of the suspensions of 5CB/PVA, the effect of the viscosity of the continuum phase and the concentration of the dispersed phase was analysed, showing that the enhancement of the viscosity of the suspension increases with the concentration, as expected, however the continuum phase viscosity has no significant effect, at least in the investigated viscosity range.
- Experimental Results for the Rheological and Rheo-Optical Behavior of Poly(ethylene terephthalate)/ Liquid-Crystalline Polymer BlendsPublication . Cidade, M. T.; Menon, A. R.; Leal, Catarina R.; Pillai, C. K. S.The use of thermoplastic/liquid-crystalline polymer (LCP) blends is recognized as a good strategy for reducing viscosity and improving mechanical properties relative to pure thermoplastics. This improvement, how ever, is only noticeable if the LCP fibrillates, in situ, during processing and the fibrils are kept in the solid state. In this article, we report a morphological, rheological, and rheo optics study performed with two blends of poly(ethylene terephthalate) with a LCP, Rodrun LC3000 (10 and 25 wt % LCP content), and we show that the obtained droplet shape relaxation time (the time the deformed droplet took to regain its spherical form after the cessation of flow) allowed for the explanation of the morphological observa tions. In fact, the droplet-shape relaxation time was higher for the blend with higher LCP content, for the higher experimentally accessible shear rates, and still increased at the highest shear rate, which explained the fibrils of the LCP dispersed phase observed in this blend, whereas for the lower LCP content blend, the droplet-shape relaxation time reached a low-value plateau for higher shear rates, which explained the absence of fibrillation in this blend.
- 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.
- Influence of processing conditions on the morphological and mechanical properties of compatibilized PP/LCP blendsPublication . Filipe, Susana; Maia, João Manuel; Leal, Catarina R.; Cidade, Maria TeresaThe main aim of this work is to study theinfluence of the application of different processing conditionson the morphological and mechanical properties of thermo-plastic/LCP blends, in which the viscosity ratios are inferiorto unity and decrease with increasing temperature. The waythe microstructure evolves along the extruder determines thefinal morphology and thus, the mechanical performance ofthe systems. In the present case, the mechanical properties arerelated with the degree of fibrillation in the final composites.The best degree of fibrillation was obtained for low screwspeeds and temperatures and for intermediate outputs. Theuse of high screw speeds and processing temperatures resultsin a decrease of the viscosity ratio, in the former case via anincrease in the viscous dissipation, at the regions of highershear rates (kneading-elements). The application of a lowerprocessing temperature is advantageous for deformation,break-up, and fibrillar formation because of the higher viscos-ity ratios and higher shear stresses involved.
- Influence of type of compatibilizer on the rheological and mechanical behavior of LCP/TP blends under different stationary and nonstationary shear conditionsPublication . Filipe, Susana; Maia, João M.; Duarte, Ana; Leal, Catarina R.; Cidade, Maria T.The addition of small amounts of liquid-crystalline polymers to thermoplastics leads to the formation of in situ–reinforced materials, with improved processability and mechanical properties. Nevertheless, the lack of adhesion between the thermoplastic and the liquid-crystalline polymer often occurs, thus requiring the use of compatibilizers. In this case, the results of several previous works show that there is an improvement of strength, usually accompanied by a decrease of toughness and, thus, the interest of LCP/TP blends for industrial applications will certainly increase if both strength and toughness are obtained. Additionally, the emphasis of previous studies has been on the evaluation of the properties of the blend under stationary conditions and not under non-stationary ones, which are, in fact, those most relevant to processing sequences. Thus, the present work focuses on the influence of type of compatibilizer on the mechanical and rheological properties of polypropylene/LCP blends under nonstationary conditions. In terms of mechanical properties, the traditional increase of tensile strength was obtained for all compatibilizers, which was essentially due to the formation, during processing, of thinner and longer fibrils of LCP dispersed in the matrix than those observed for the noncompatibilized blends. Additionally, an improvement of the impact strength and flexural modulus was also observed for the blend in which a compatibilizer with an elastomeric nature was used. Rheologically, the experiments most sensitive to the structure were those performed in transient shear, with an increase of the transient stress (in the form of an overshoot) of different magnitudes being observed for the different compatibilizers.
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