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| 2. |
- Kihlman, Martin, et al.
(författare)
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Cellulose Dissolution in an Alkali Based Solvent : Influence of Additives and Pretreatments
- 2013
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Ingår i: Journal of the Brazilian Chemical Society. - : SOC BRASILEIRA QUIMICA, Brazil. - 0103-5053 .- 1678-4790. ; 24:2, s. 295-303
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Tidskriftsartikel (refereegranskat)abstract
- The distinction between thermodynamic and kinetics in cellulose dissolution is seldom considered in the literature. Therefore, herein an attempt to discuss this topic and illustrate our hypotheses on the basis of simple experiments was made. It is well-known that cellulose can be dissolved in a aqueous sodium hydroxide (NaOH/H2O) solvent at low temperature but it is here shown that such an alkaline solvent can be considerably improved regarding solubility, stability and rheological properties as a whole if different additives (salts and amphiphilic molecules) are used in the dissolution stage. This work probes new aqueous routes to dissolve cellulose, thereby improving the potential to commercially dissolve cellulose in an inexpensive and environmentally friendly manner.
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| 5. |
- Medronho, Bruno, et al.
(författare)
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Planar lamellae and onions: a spatially resolved rheo-NMR approach to the shear-induced structural transformations in a surfactant model system
- 2011
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Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-6848 .- 1744-683X. ; 7:10, s. 4938-4947
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Tidskriftsartikel (refereegranskat)abstract
- 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). H-2 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.
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| 6. |
- Medronho, Bruno, et al.
(författare)
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Rationalizing cellulose (in)solubility: reviewing basic physicochemical aspects and role of hydrophobic interactions
- 2012
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Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 19:3, s. 581-587
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Forskningsöversikt (refereegranskat)abstract
- Despite being the world's most abundant natural polymer and one of the most studied, cellulose is still challenging researchers. Cellulose is known to be insoluble in water and in many organic solvents, but can be dissolved in a number of solvents of intermediate properties, like N-methylmorpholine N-oxide and ionic liquids which, apparently, are not related. It can also be dissolved in water at extreme pHs, in particular if a cosolute of intermediate polarity is added. The insolubility in water is often referred to strong intermolecular hydrogen bonding between cellulose molecules. Revisiting some fundamental polymer physicochemical aspects (i.e. intermolecular interactions) a different picture is now revealed: cellulose is significantly amphiphilic and hydrophobic interactions are important to understand its solubility pattern. In this paper we try to provide a basis for developing novel solvents for cellulose based on a critical analysis of the intermolecular interactions involved and mechanisms of dissolution.
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| 7. |
- Medronho, Bruno, et al.
(författare)
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Shear-Induced Defect Formation in a Nonionic Lamellar Phase
- 2010
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 26:13, s. 11304-11313
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Tidskriftsartikel (refereegranskat)abstract
- H-2 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.
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| 8. |
- Medronho, Bruno, et al.
(författare)
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Size Determination of Shear-Induced Multilamellar Vesicles by Rheo-NMR Spectroscopy
- 2010
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 26:3, s. 1477-1481
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Tidskriftsartikel (refereegranskat)abstract
- A model for analyzing the deuterium (H-2) NMR line shapes of D2O ill surfactant multilamellar vesicle (MLV, "onion") systems is proposed. The assumption of the slow exchange of water molecules between adjacent layers implies that the H-2 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.
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| 9. |
- Medronho, Bruno, et al.
(författare)
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Transient and Steady-State Shear Banding in a Lamellar Phase as Studied by Rheo-NMR
- 2012
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Ingår i: Zeitschrift für Physikalische Chemie. - : Walter de Gruyter GmbH. - 0942-9352 .- 2196-7156. ; 226:11-12, s. 1293-1313
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Santos, Salome, et al.
(författare)
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Phase behavior and rheological properties of DNA-cationic polysaccharide mixtures.
- 2012
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Ingår i: Journal of Colloid and Interface Science. - : Elsevier BV. - 1095-7103 .- 0021-9797. ; 383:1, s. 63-74
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Tidskriftsartikel (refereegranskat)abstract
- Associative aqueous mixtures over a range of concentrations of double- (ds) or single- (ss) stranded DNA with dilute or semidilute solutions of two cationic derivatives of hydroxyethyl cellulose (cat-HEC and cat-HMHEC,(1) the latter carrying grafted hydrophobic groups), were studied. The phase behavior showed an interesting asymmetry: Phase separation occurred immediately when small (sub-stoichiometric) amounts of cationic polyelectrolyte were added to the DNA solution, but redissolution into a single cat-(HM)HEC/DNA/H(2)O phase occurred already with a modest charge excess of the cationic polyelectrolyte, at a charge ratio approximately independent of the overall polyelectrolyte concentration. Cat-HEC/dsDNA/H(2)O and cat-HEC/ssDNA/H(2)O systems presented a considerable difference in the extension of the phase separation region. The one-phase samples with excess cationic polyelectrolyte were studied by rheology. The presence of DNA strengthened the viscoelastic behavior of the solutions of the cationic polyelectrolytes, reflected in an increase in storage modulus and viscosity. Differences in phase behavior and rheology were observed, particularly between systems containing cat-HEC or cat-HMHEC, but also between dsDNA and ssDNA. Thus, these systems allow for the preparation of DNA formulations with widely variable rheology and water uptake.
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