Rheological properties of ionic and nonionic polyacrylamide solutions

Non-Newtonian shear viscosities were measured over six decades of strain rate k for 13 solutions of both the ionic and nonionic forms of polyacrylamide. By using the Weissenberg rheogoniometer with both the cone-and-plate and the parallel-plate attachments, the normal stress functions σ₁ (k²) and σ₂(k²) were obtained for four of the solutions. From the measurements of the shear viscosity and the normal stresses at low rates of strain, characteristic times τ and τ_N, respectively, were determined for each solution. The quantity τ was then used to nondimensionalize the strain rate τk, and when plotted versus the reduced shear viscosity, found successfully to correlate the experimental data for all the polyelectrolyte solutions over the entire range of τk and the data for the concentrated solutions of the nonionic polymer over a smaller range of τk. However, in order to correlate the normal stress data for the polyelectrolyte solutions, a second reduced strain rate (τ_Nk) was used. Thus, two different times were required to correlate all the observed data. The shear viscosity data for the dilute solutions of the nonionic polymer were well represented by the two-parameter, non-Newtonian intrinsic viscosity function that has been computed by Fixman.     

Rheological properties of water-soluble cross-linked xanthan gum

Water-soluble crossslinked xanthan gum (CXG) was prepared from xanthan gum (XG) and epichlorohydrin under alkaline condition by ethanol solvent method. Rheological properties and heat resistance performance of different concentrations of aqueous XG and CXG solutions were investigated. The results showed that the apparent viscosity of 4 g · L^?1 CXG solution was 2.57 times that of 4 g · L^?1 XG solution. The storage modulus G′ and the loss modulus G″ of CXG solutions were greater than those of XG solutions, and viscoelastic and thixotropic properties were more significant in CXG solutions. At 80°C, these two solutions were sheared at 170 s^?1 for 90 minutes, the reserved viscosity was 32.30 and 62.15 mPa · s for XG and CXG solutions, respectively. The heat resistance performance of CXG solution was better than that of XG solution. Nonlinear co-rotational Jeffreys model could be applied to describe the flow curves of XG and CXG solutions correctly, and the calculated values were in good agreement with the experimental data.     

Rheological behavior of cellulose/silk fibroin blend solutions with ionic liquid as solvent

The rheological behavior of cellulose and silk fibroin blend in 1-butyl-3-methylimidazolium chloride was studied. The data from the rheological results was analyzed to understand the microstructure of the blend solutions. The viscosity and dynamic modulus of the blend solution decreased with increasing ratio of silk fibroin. While comparing the experimental results with the calculated data from the log-additivity rule, it is revealed that zero-shear viscosity, dynamic modulus show positive–negative deviations and a typical continuous–discrete type of morphology could be imaged. At lower shear rate, the change of phase morphology took place at the ratio of about 0.5 volume fraction of cellulose. However, the blend solution showed positive deviations for all cellulose/silk fibroin blend ratios at high shear rate, which indicates that the dispersion of cellulose and silk fibroin was improved under shear stress. The properties of cellulose/silk fibroin blends observed by Fourier transform infrared spectroscopy and scanning electron microcopy agreed with the result from rheology.     

Surface tension of ionic liquids and ionic liquid solutions

Some of the most active scientific research fronts of the past decade are centered on ionic liquids. These fluids present characteristic surface behavior and distinctive trends of their surface tension versus temperature. One way to explore and understand their unique nature is to study their surface properties. This critical review analyses most of the surface tension data reported between 2001 and 2010 (187 references).     

Thermophysical properties of aqueous solutions of the 1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid

Thermophysical behavior of the binary system [water + 1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid (IL)] was thoroughly characterized through systematic measurements of (vapor + liquid) equilibria (water activity a_w), mixing enthalpy, density, viscosity, and refractive index. The measurements were performed in the entire composition range and/or specifically in the highly dilute IL region, at T = 298.15 K or as a function of temperature in the range from (288.15 to 318.15) K. Effective experimental methods minimizing IL sample consumption, using flow arrangements, instrument couplings and high degree of automation were preferably employed. In particular, the a_w determination based on the chilled-mirror dew point technique and implemented by an AquaLab 4TE instrument was identified as a generally superior approach to study VLE of (water + IL) systems. Excess thermodynamic properties (Gibbs free energy, enthalpy, heat capacity, and volume) and property deviations from the linear mixing rule (viscosity, refractive index) were evaluated, Padé approximants being used to correlate adequately their complex composition dependences. The extensive a_w data were processed by a two-step procedure fitting first the temperature dependence at each isopleth and subsequently the composition dependence at each isotherm. Good estimates could be thus obtained for derivative thermal properties (enthalpy, heat capacity). Alternatively, the water activity and excess enthalpy data were correlated simultaneously by a NRTL-type model, providing their compact, thermodynamically consistent and adequate representation. Despite small absolute values of excess Gibbs free energy (G^E), the system is revealed to be highly nonideal, the small G^E resulting from close compensation of its large enthalpy and entropy contributions. Large endothermic effects and an enhanced increase of entropy upon mixing found for this system indicate relative weakness of interactions between unlike molecules and a massive structure breakage in the solution. Positive values of excess volume and negative values of viscosity and refractive index deviations found in the major part of the composition range corroborate this general energetic and structural pattern, although the situation appears to be more complicated in the highly dilute IL region, where these properties congruently exhibit a sign inversion.     

Rheological properties of lyotropic liquid crystals encapsulating curcumin

This study constructed new curcumin-loaded lyotropic liquid crystals containing pharmaceutically accepted oil, and ethyl oleate (EtOL). Three liquid crystalline phases including lamellar, hexagonal, and cubic phases were identified by means of the polarized optical microscopy and rheology method. By analyzing the shear viscosity (η_0.1), the viscosity of curcumin-liquid crystals is smaller than those without curcumin. Dynamic rheological results show that: Dissolved curcumin in EtOL can make the elastic modulus of hexagonal and cubic phase increase compared with that without curcumin, while the elastic modulus of lamellar phase decreases. Dissolved curcumin in Brij 97 can lead to the decreasing of the elastic modulus for cubic and lamellar phases, whereas it has little influence on hexagonal phase. When the curcumin is solubilized in both EtOL and Brij 97, the elastic modus of hexagonal phase increase, the elastic modus of lamellar and cubic phases decrease compared with that without curcumin. Furthermore, three temperature turning points were identified by the change in the slope of tanδ (G″/G′) for curcumin-hexagonal liquid crystal. These studies might be a help to study the storage of drug carrier and in vitro release properties of lyotropic liquid crystals containing curcumin.     

Physical properties of xanthan,galactomannan and their mixtures in aqueous solutions

Some new information on the conformation of xanthan in aqueous solutions is given. A single helical chain conformation characterizes xanthan in the native state. When heated over the critical temperature for conformational change ( helixŕ coil), the xanthan is denatured and renatured when it is cooled down in a localy double helical structure. A galactomannan was also characterized and its persistence length was obtained (Lp ≈︁ 90A°). Then mixtures of the galactomannan and xanthan were investigated to propose a mechanism for the specific gelation. From the results of microcalorimetry and circular dichroism, it is concluded that a complex is formed between one disordered xanthan chain and one galactomannan chain and that an ordered conformation is stabilized at temperatures lower than 25°C when the galactomannan has a M/G ratio of ≈︁ 3. This temperature corresponds to the sol-gel transition. This is the first time that a structure of the crosslink points is demonstrated.     

Rheological properties of anisotropic poly(para-benzamide) solutions

A study has been made of the viscous properties of poly(para-benzamide) (PBA) solutions in dimethyl acetamide, which undergo a transition from an isotopic to an anisotropic (liquid-crystal) state at a definite concentration C^*. The polymer solutions behave in many respects (as regards the concentration and temperature dependence of viscosity, etc.) like solutions of low molecular weight compounds forming a liquid crystal phase, although the transitions are less pronounced in the polymer solutions owing to their polydispersity. It is shown that the viscometric method, being extremely sensitive to C^*, is convenient for determining phase diagrams of anisotropic polymer solutions. The values of C^* as related to the molecular weight of PBA have been determined, and a general criterion for transition from isotropic to anisotropic solutions established; the latter has the form (CM?)^* ≈ 1.3 × 10⁵ at 20°C. This criterion is in line with the condition for the formation of the liquid-crystal structure in a dispersion of rodlike particles as proposed by Flory. Generalized concentration dependences of viscosity have been plotted by reducing concentration to C^* and viscosity, to the maximum viscosity at the phase transition point. In investigating the flow properties of PBA solutions we revealed the existence of a yield point in the range of low shear stresses, and an intersection of the flow curves of solutions of different concentration at high shear stresses, which excludes a generalized representation of the flow curves in reduced ordinary-type coordinates.     

Rheological properties of carboxymethyl cellulose (CMC) solutions

In this study, we investigated the way of predicting two critical concentrations of sodium carboxymethyl cellulose (CMC) solutions using simple experimental procedures with a rotational rheometer. It was found that, above a critical shear rate, all CMC solutions (0.2 to 7 wt.%) exhibit shear-thinning behavior and the flow curves could be described by the Cross model. A first critical CMC concentration c*, transition to semidilute network solution, was determined using the following methods (1) study of the flow curve shapes, (2) Cross model parameters, (3) plot of the specific viscosity vs the overlap parameter, and (4) empirical structure–properties relationships. Furthermore, both creep and frequency-sweep measurements showed that the solutions behaved as viscoelastic materials above a second critical CMC concentration c** (transition to concentrated solution). The characterization of CMC solutions was completed with a time-dependent viscosity study that showed that the CMC solutions exhibited strong thixotropic behavior, especially at the highest CMC concentrations.     

Physicochemical properties of an N-decylpyridinium tetrachloroferrate ionic liquid

A low-temperature ionic liquid N-decylpyridinium tetrachloroferrate was synthesized. The temperature dependences of density and viscosity of the synthesized ionic liquid, as well as its thermal stability and magnetic properties were studied. The response of N-decylpyridinium tetrachloroferrate to magnetic field was visualized, and its structure was calculated at the DFT/B3LYB/6-31G(d,p) level of theory.     

Thermal and viscoelastic properties of xanthan gum/chitosan complexes in aqueous solutions

The viscoelasticity and thermal properties of aqueous solutions of blended xanthan gum (XA) with chitosan were investigated in order to study the electrostatic interaction established between two polysaccharides. Storage modulus G’, loss modulus G” and zero shear rate viscosity η_O attain a maximum at a chitosan concentration C _max. The above results indicate that the junction between XA and chitosan is formed in a concentration range lower than C _max and the viscoelasticity of systems increases with increasing concentration. In a concentration range higher than C _max, junction formation may not occur effectively since the excess amount of chitosan completely screens anions of XA. The chain rigidity of XA decreases by the screening of the repulsive interaction between anions on XA chains. The ineffective junction formation and the decrease of XA chain rigidity may cause the decrease of viscoelasticity of systems with increasing concentration. The value of C _max decreases with increasing molecular mass of chitosan. From melting enthalpy of the above system measured by DSC, the amount of non-freezing water (W _nf) was evaluated. W _nf shows a minimum at the concentration C _max. This fact suggests that hydrophobic fields increased by junction structure formation through ion-complexation between XA and chitosan molecules.     

十二烷基聚氧乙烯硫酸钠胶束体系的流变性质研究

用流变学方法研究了无机电解质KBr存在时,阴离子表面活性剂十二烷基聚氧乙烯(3)硫酸钠(SDES)水溶液中胶束的生长和结构。通过测量体系的稳态剪切粘度(η)和应力(σ)关系,得到零剪切粘度(η0)、复合粘度(|η^*|)、动态模量[储能模量(G')和损耗模量(G")、平台模量(G0)、结构松驰时间(τ)等流变学参数,并应用Cox-Merz规则和Cole-Cole图,发现在SDES/KBr体系中可以形成蠕虫状胶束网络结构,体系为假塑性流体,偏离Maxwell模型,具有非线性粘弹性,没有单一的结构松驰时间。     

Rheological properties of polyacrylonitrile solutions containing highly dispersed carbon nanotubes

Rheological properties of dispersions of carbon nanotubes in dimethylsulfoxide solutions of polyacrylonitrile are studied at different concentrations of the components. The viscosity values of all the studied dispersions are substantially dependent on shear rate. For a number of systems, the energies of activation of viscous flow are determined. The relationship of viscosity and the energy of activation of flow to the compositions and possible structural features of dispersions is discussed. The concentration regime where a thermally reversible mixed network exists with crosslinks formed by physical contacts of the polymer with nano-tubes is characterized.     

Rheological properties of multisticker associative polyelectrolytes in semidilute aqueous solutions

Multisticker associative polyelectrolytes of acrylamide (≈86 mol %) and sodium 2‐acrylamido‐2‐methylpropanesulfonate (≈12 mol %), hydrophobically modified with N,N‐dihexylacrylamide groups (≈2 mol %), were prepared with a micellar radical polymerization technique. This process led to multiblock polymers in which the length of the hydrophobic blocks could be controlled through variations in the surfactant‐to‐hydrophobe molar ratio, that is, the number of hydrophobes per micelle (N_H). The rheological behavior of aqueous solutions of polymers with the same molecular weight and the same composition but with two different hydrophobic block lengths (N_H = 7 or 3 monomer units per block) was investigated as a function of the polymer concentration with steady‐flow, creep, and oscillatory experiments. The critical concentration at the onset of the viscosity enhancement decreased as the length of the hydrophobic segments in the polymers increased. Also, an increase in the N_H value significantly enhanced the thickening ability of the polymers and affected the structure of the transient network. In the semidilute unentangled regime, the behavior of the polymer with long hydrophobic segments (N_H = 7) was studied in detail. The results were well explained by the sticky Rouse theory of associative polymer dynamics. Finally, the viscosity decreased with an increase in the temperature, mainly because of a lowering of the sample relaxation time. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1640–1655, 2004     

Rheological and thermal properties of agarose aqueous solutions and hydrogels

In this article, we report on the rheological properties of agarose aqueous solutions and gels. Viscosity curves were determined for homogeneous agarose aqueous solutions at different temperatures (from 68 to 38 °C) to study the viscosity behavior as the systems undergo gelation. The gelation phenomenon of agarose solutions was also investigated by shear oscillation experiments and differential scanning calorimetry. The gelation and melting temperature as a function of agarose concentration were determined together with the gelation and melting enthalpies. The results obtained were interpreted using the two‐step model describing the gelation of agarose in water. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 322–328, 2008     

Evidence for lower critical solution behavior in ionic liquid solutions

Lower critical solution temperatures (LCST)-type of phase diagrams, including the presence of closed loops, have been encountered for the first time in binary and quasi-binary liquid solutions of ionic liquids. Furthermore, the results constitute the first experimental support for the existence of a theoretically postulated, but never encountered, special kind of type VII phase diagram. Two distinct mechanisms are involved in the appearance of demixing upon temperature increase. These findings underlie the presence of specific, oriented interactions between the ionic liquid, 1-alkyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide, [Cnmim][NTf2], and trichloromethane, as well as aggregation phenomena.     

Acrylate functionalized tetraalkylammonium salts with ionic liquid properties

Acrylate functionalized ionic liquids based on tetraalkylammonium salts with terminal acrylates- and methylacrylates were synthesized. Melting points and ionic conductivity of twenty compounds in six groups were determined. Within one group the effect of three different counterions was investigated and discussed. The groups differ in cationic structure elements because of their functional groups such as acrylate and methacrylate, alkyl residues at the nitrogen and number of quaternary ammonium atoms within the organic cation. The effect of these cationic structure elements has been examined concerning the compiled parameters with a view to qualifying them as components for solid state electrolytes. The newly synthesized ionic liquids were characterized by NMR and FTIR analysis. The exchange of halide ions like bromide as counter ions to weakly coordinating [PF?]?, [OTf]? or [TFSI]? reduces the melting points significantly and leads to an ion conductivity of about 10?? S/cm at room temperature. In the case of the dicationic ionic liquid, an ion conductivity of about 10?3 S/cm was observed.     

Rheological and microrheological characterization of a novel amphoteric xanthan gum

A novel amphoteric xanthan gum (AXG) containing anionic carboxyl groups and cationic quaternary ammonium substituents was prepared from xanthan gum (XG) and 3-chloro-2-hydroxypropyltriethanolammonium acetate. The rheological and microrheological properties of AXG and XG solutions were studied in this work. The rheological results showed that the apparent viscosity of the 0.4% AXG solution was 5.26 times that of the 0.4% XG solution, and the AXG solution exhibited more obvious thixotropy and much stronger viscoelasticity than the XG solution. Both experiment and numerical simulations were adopted to investigate the gel-breaking process of the AXG solution, and the rheokinetics equations can well describe the evolution of complex viscosity. Moreover, the optical microrheology method was also adopted to investigate the microrheological behavior of AXG and XG solutions. The microrheological results showed that the AXG solution exhibited higher viscosity index at zero shear and a more obvious solid-like behavior than the XG solution. Good agreement is found when comparing the results obtained from microrheology to classical rheology, which verifies the feasibility of the microrheology method for measuring the solution properties, especially some properties not well described by the current rheological approach.     

Vibrational spectroscopy and dynamics of small anions in ionic liquid solutions

Fourier-transform infrared (FTIR) and time-resolved IR spectroscopies have been used to study vibrational band positions, vibrational energy relaxation (VER) rates, and reorientation times of anions in several ionic liquid (IL) solutions. The ILs primarily investigated are based on the 1-butyl-2,3-dimethylimidazolium ([BM(2)IM]) cation with thiocyanate (NCS-), dicyanamide (N(CN)2-), and tetrafluoroborate (BF4-) anions. Spectroscopic studies are carried out near 2000 cm-1 for the C[Triple Bond]N stretching bands of NCS- and N(CN)2- as the IL anion as well as for NCS-, N(CN)2-, and azide (N3-) anions dissolved in [BM2IM][BF4]. The VER studies of N(CN)2- are reported for the first time. VER of N3-, NCS-, and N(CN)2- is measured in normal solvents, such as N-methylformamide, to compare with the IL solutions. The spectral shifts and VER rates of the anions in IL solution are quite similar to those in polar aprotic, conventional organic solvents, i.e., dimethylsulfoxide, and significantly different than those in methanol, in which there is hydrogen bonding. Similar studies were also carried out for the anions in another IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), in which the C2 hydrogen is present. The results for the anions are similar to those in the [BM2IM] containing ILs, in which the C2 hydrogen is methyl substituted. This suggests that substituting this hydrogen has, at most, a minor effect on the degree of hydrogen bonding in the anion-IL solvation interaction based on the infrared spectra and dynamics.     

Synthesis and mesomorphic properties of rigid-core ionic liquid crystals

Ionic liquid crystals combine the unique solvent properties of ionic liquids with self-organization found for liquid crystals. We report a detailed analysis of the structure-property relationship of a series of new imidazolium-based liquid crystals with an extended aromatic core. Investigated parameters include length and nature of the tails, the length of the rigid core, the lateral substitution pattern, and the nature of the counterion. Depending on the molecular structure, two mesophases were observed: a bilayered SmA2 phase and the more common monolayered SmA phase, both strongly interdigitated. Most materials show mesophases stable to high temperatures. For some cases, crystallization could be suppressed, and room-temperature liquid crystalline phases were obtained. The mesomorphic properties of several mixtures of ionic liquid crystals were investigated. Many mixtures showed full miscibility and ideal mixing behavior; however, in some instances we observed, surprisingly, complete demixing of the component SmA phases. The ionic liquid crystals and mixtures presented have potential applications, due to their low melting temperatures, wide temperature ranges, and stability with extra ion-doping.