Browsing by Author "Olsson, U."
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- Metastability of Multi-Lamellar Vesicles in a Nonionic SystemPublication . Filippelli, L.; Rossi, C. O.; Miguel, Maria Graça; Olsson, U.Nuclear magnetic resonance spectroscopy and rheological analysis have been used to investigate the stability of mechanically induced tri-ethylene-glycol-mono-n-decyl-ether (C(10)E(3))/deuterium oxide (D(2)O) multi-lamellar vesicles (MLVs) and the transition from MLVs to planar lamellae. It was found that MLVs prepared by vortex stirring, relax back to the lamellar phase in a few hours while the relaxation of the shear induced MLVs takes days. Pulsed gradient spin echo and water self-diffusion coefficient experiments, revealed that the MLVs texture, obtained by vortex stirring, is composed of large size structures. These data indicate that the kinetics of lamellar re-formation depend on the MLVs number density.
- Metastability of multi-lamellar vesicles in a nonionic systemPublication . Filippelli, L.; Medronho, B.; Rossi, C. O.; Miguel, M. G.; Olsson, U.Nuclear magnetic resonance spectroscopy and rheological analysis have been used to investigate the stability of mechanically induced tri-ethylene-glycol-mono-n-decyl-ether (C(10)E(3))/deuterium oxide (D(2)O) multi-lamellar vesicles (MLVs) and the transition from MLVs to planar lamellae. It was found that MLVs prepared by vortex stirring, relax back to the lamellar phase in a few hours while the relaxation of the shear induced MLVs takes days. Pulsed gradient spin echo and water self-diffusion coefficient experiments, revealed that the MLVs texture, obtained by vortex stirring, is composed of large size structures. These data indicate that the kinetics of lamellar re-formation depend on the MLVs number density.
- Planar lamellae and onions: a spatially resolved rheo-NMR approach to the shear-induced structural transformations in a surfactant model systemPublication . Medronho, B.; Brown, J.; Miguel, M. G.; Schmidt, C.; Olsson, U.; Galvosas, P.The shear-induced transformations between oriented planar lamellae and a state of closely packed multilamellar vesicles (MLVs) in a lyotropic nonionic surfactant model system were studied by the combination of nuclear magnetic resonance (NMR) spectroscopy and diffusometry with magnetic resonance imaging (MRI). (2)H NMR imaging confirmed the discontinuous nature of the transition from onions to planar lamellae, revealing the spatial coexistence of both states within the gap of the cylindrical Couette geometry. On the other hand, NMR diffusion measurements in three principal directions and at various values of strain strongly suggest that a multi-lamellar cylindrical or undulated intermediate structure exists during the continuous and spatially homogeneous transition from planar lamellae to MLVs.
- Probing cellulose–solvent interactions with self-diffusion NMR: Onium hydroxide concentration and co-solvent effectsPublication . Medronho, Bruno; Pereira, Ana; Duarte, Hugo; Gentile, L.; Rosa Da Costa, Ana; Romano, A; Olsson, U.The molecular self-diffusion coefficients were accessed, for the first time, in solutions of microcrystalline cel-lulose, dissolved in 30 wt% and 55 wt% aqueous tetrabutylammonium hydroxide, TBAH (aq), and in mixtures of 40 wt% TBAH (aq) with an organic co-solvent, dimethylsulfoxide (DMSO), through pulsed field gradient stim-ulated echo NMR measurements. A two-state model was applied to estimate alpha (i.e., average number of ions that "bind" to each anhydroglucose unit) and Pb (i.e., fraction of "bound" molecules of DMSO, TBAH or H2O to cellulose) parameters. The alpha values suggest that TBA+ ions can bind to cellulose within 0.5 TBA+ to 2.3 TBA+/ AGU. On the other hand, the Pb parameter increases when raising cellulose concentration for TBA+, DMSO and water in all solvent systems. Data suggests that TBAH interacts with the ionized OH groups from cellulose forming a sheath of bulky TBA+ counterions which consequently leads to steric hindrance between cellulose chains.
- Reversible size of shear-induced multi-lamellar vesiclesPublication . Medronho, B.; Fujii, S.; Richtering, W.; Miguel, M. G.; Olsson, U.We have investigated the reversibility in the shear-induced multi-lamellar vesicle (MLV) size during stepwise cycling of the shear rate by employing common rheometry, polarized light microscopy and rheo-optic techniques. We thus address the question whether there is a true MLV steady state, irrespective of history. The system studied, was the nonionic surfactant triethylene glycol decyl ether (C(10)E(3)) with a concentration of 40 wt.% in D(2)O and a constant temperature of 25 degrees C. It was found that the MLV size varies reversibly with varying shear rate, and hence there exists a true steady state in the presence of shear flow. The experimental observations of reversibility are however restricted to higher shear rates. Because the transformation of the size results from the shear strain, the process is very slow at lower shear rates, where the steady state cannot be reached within a reasonable experimental time.
- Shear-induced defect formation in a nonionic lamellar phasePublication . Medronho, B.; Rodrigues, M.; Miguel, M. G.; Olsson, U.; Schmidt, C.(2)H NMR experiments on a nonionic oriented lamellar phase demonstrate that shear flow induces structural defects in the lamellar structure. These substantial structural changes give rise to a transition from a viscous to a solidlike behavior; the elastic modulus of presheared samples was found to increase, reversibly, with the applied preshear rate. A similar behavior was found when step-cycling the temperature toward the layer-to-multilamellar-vesicle transition and back at constant shear rate. However, while shear rate controls the defect density, the temperature is found to control the defect rigidity. The lamellar phase exhibits a shear-thinning behavior under steady shear conditions, following the power law eta similar to gamma(n), with n approximate to -0.4. Both the shear thinning and the elastic behavior are in agreement with the available theoretical models. The observed shear-induced structural defects are reversible and can be regarded as a pretransition prior to the shear-induced formation of multilamellar vesicles.
- Shear-induced transitions between a planar lamellar phase and multilamellar vesicles: continuous versus discontinuous transformationPublication . Medronho, B.; Shafaei, S.; Szopko, R.; Miguel, M. G.; Olsson, U.; Schmidt, C.The shear-induced transitions between an oriented lamellar phase and shear-induced multilamellar vesicles (MLVs) in a nonionic surfactant system were studied by deuterium rheo-NMR spectroscopy as a function of time in start-up experiments at several temperatures and shear rates. By starting from an initial state of oriented lamellae and observing the transformation to the final steady state of MLVs and vice-versa, two different mechanisms were found, depending on the direction of the transition and the initial state. The transition is continuous when MLVs are formed, starting from the oriented lamellar phase. On the other hand, a discontinuous nucleation-and-growth process with a coexistence region is observed when transforming MLVs into an oriented lamellar phase.
- Size determination of shear-induced multilamellar vesicles by Rheo-NMR spectroscopyPublication . Medronho, B.; Schmidt, C.; Olsson, U.; Miguel, M. G.A model for analyzing the deuterium ((2)H) NMR line shapes of D(2)O ill surfactant multilamellar vesicle (MLV, "onion") systems is proposed. The assumption of the slow exchange of water molecules between adjacent layers implies that the (2)H NMR line shape is simply given by a SLIM of Lorentzians if the condition of motional narrowing is also fulfilled. Using the classical two-step model for the NMR relaxation in structure fluids allows Lis to calculate how the NMR line shape depends oil the MLV size. The model is tested oil two different MLV systems for which the NMR line shapes are measured as a function of the applied shear rate using rheo-NMR. The MLV sizes obtained are in good agreement with previous data from rheo-small-angle light scattering.
- Transient and steady-state shear banding in a lamellar phase as studied by Rheo-NMRPublication . Medronho, B.; Olsson, U.; Schmidt, C.; Galvosas, P.Flow fields and shear-induced structures in the lamellar (L-alpha) phase of the system triethylene glycol mono n-decyl ether (C10E3)/water were investigated by NMR velocimetry, diffusometry, and H-2 NMR spectroscopy. The transformation from multilamellar vesicles (MLVs) to aligned planar lamellae is accompanied by a transient gradient shear banding. A high-shear-rate band of aligned lamellae forms next to the moving inner wall of the cylindrical Couette shear cell while a low-shear-rate band of the initial MLV structure remains close to the outer stationary wall. The band of layers grows at the expense of the band of MLVs until the transformation is completed. This process scales with the applied strain. Wall slip is a characteristic of the MLV state, while aligned layers show no deviation from Newtonian flow. The homogeneous nature of the opposite transformation from well aligned layers to MLVs via an intermediate structure resembling undulated multilamellar cylinders is confirmed. The strain dependence of this transformation appears to be independent of temperature. The shear diagram, which represents the shear-induced structures as a function of temperature and shear rate, contains a transition region between stable layers and stable MLVs. The steady-state structures in the transition region show a continuous change from layer-like at high temperature to MLV-like at lower temperature. These structures are homogeneous on a length scale above a few micrometers.
- Viscoelasticity of a nonionic lamellar phasePublication . Medronho, B.; Miguel, M. G.; Olsson, U.The linear viscoelastic properties of a nonionic lamellar phase in C-orientation were studied as a function of temperature by small-amplitude oscillatory measurements in the frequency range 0.5-5 Hz. An almost solidlike elastic response was observed at all studied temperatures, from 42 to 20 degrees C. In this range, the elastic modulus was found to increase strongly with decreasing temperature. The elasticity is attributed to screw dislocations connecting layers in the stack, and the data thus suggest that the density of screw dislocations decreases with increasing temperature. The lamellar phase forms an "onion" texture when continuously sheared at lower temperatures. It is argued that a possible origin for the shear-induced "onion" texture is the instability of the screw dislocations in shear flow. By H-2 NMR experimentation, we also find the formation of a random mesh phase at lower temperatures. The presence of equilibrium bilayer perforations, however, does not correlate with the "onion" stability.