Moez Guettari

A correspondence between Grunberg–Nissan constant d′ and complex varieties in water/methanol mixture

The aim of this study was to analyse the change in the complex structure formed in water/methanol mixture under temperature effect and/or mixture composition by a simple method. First, the dynamic viscosity of mixture was measured by rheoviscosimetry, and then the variation of the total activation energy versus methanol molar fraction was determined and discussed. The Grunberg–Nissan constant d′ was calculated in a large mixture composition and over the 20–40°C temperature range. By comparing the complex formulas in water/methanol mixture (H2O) m (CH3OH) n reported in literature at 20°C and the Grunberg–Nissan constant d′ variation versus methanol molar fraction X A, a correspondence between Grunberg–Nissan constant d′ and the complex varieties was established. According to this hypothesis, the change in the complex varieties for different temperatures was determined by showing a reduction of the complex varieties of water/methanol complex at high temperatures (35–40°C) in the mixture.

Effect of Temperature on Cononsolvency of Polyvinylpyrrolidone in Water/Methanol Mixture

The behavior of polyvinylpyrrolidone in mixed water/methanol solvents was studied by rheoviscosimetry over a temperature range of 20°C–40°C. For the lower temperatures of this range, the intrinsic viscosity variation of the polymer vs. methanol molar fraction shows structural transitions (coil–globule–coil). This transition, which is usually attributed to the cononsolvency phenomenon, agrees with our previously published results obtained by dynamic light scattering. For higher temperatures, near 40°C, the intrinsic viscosity increase shows an expansion of the polymer over the alcohol molar fraction range 0.2 < X A < 0.5. This last result can be attributed to the water/alcohol complex destruction under temperature increase. The “excess viscosity” of the polymer-mixed solvents vanishes with increasing temperature and becomes positive at 40°C. So, the polymer chain tends to transit from a globular to an ideal chain in the middle composition range of the mixed solvents.

A Model to Study the Behavior of a Polar Polymer in the Mixture of Polar Solvents

The aim of the present work was to purpose a simple model to study the behavior of a polar polymer in a mixture of two polar solvents where the mixed solvents are considered as an effective solvent. This effective solvent interacts with the polymer. The ternary system polymer/solvent1/solvent2 is described by an effective solvent interacting with polymer model (E.S.I.P. model). We propose to calculate the polymer–mixed-solvents interaction parameter on the basis of the solubility parameter concept. New expressions for the second virial coefficient and the preferential adsorption coefficient are given. The proposed model is used to explain the structural transition observed in the system of polyvinylpyrrolidone (PVP)/water/methanol mixture at 25°C and to calculate the preferential adsorption parameter. The enthalpy contribution to the PVP/water interaction parameter was found to dominate the PVP/water interaction parameter. Thus, we consider the dominance of the enthalpy contribution to the polymer–solvent interaction parameter as a fundamental hypothesis when polymer and solvent interact. In addition, according to this hypothesis, the calculated PVP/methanol interaction parameter shows that methanol is a good solvent for PVP at 25°C, as reported in the literature. The E.S.I.P model and the dominance of the enthalpy contribution to the polymer-interaction parameter are used to calculate the polymer–mixed-solvents interaction parameter. Then, the second virial coefficient of the PVP in mixed solvents is deduced, showing a change in the solvent quality in agreement with our previous experimental studies. Finally, the variation of the preferential adsorption parameter versus methanol molar fraction X A is calculated showing different behavior of the polymer versus the mixture composition. The obtained results are in agreement with previous studies by light scattering and rheoviscosimetry of the ternary system PVP/water/methanol at 25°C.

Simple-Complex Fluid Transition in Microemulsions

The aim of this work was to study the rheological behavior of water in-oil microemulsion formulated with AOT (sodium bis(2-ethylhexyl) sulfoccinate) in isooctane over a temperature range from 25°C to 55°C for the oil/AOT volume fraction φ m =0.1 and various AOT and H2O molar ratios, W0, ranging from 0 to 45. The apparent viscosity was measured vs. shear rate by a rotational concentric cylinder viscometer. It was shown that above s−1, the solutions utilized behaved as Newtonian liquids. The absolute viscosities were deduced from the shear stress- shear rate rheograms. For W0 < 17, the solutions behaved as simple fluids, where the viscosities decrease as the temperature increases. However, an opposite behavior was observed for W0 > 17; the viscosity remained constant and then increased above a critical temperature, Tc, depending on W0. For the molar ratio W0,c = 17; the studied viscosity remained constant, η(W0,c) = (1.17 ± 0.04) cP, for the temperature range studied. Thus, a simple–complex fluid transition was evident in the studied system. For the simple fluids, W0 < W0,c, the total activation energy vs. W0 showed three linear parts. The different slopes were attributed to the change of the microscopic structure of the formed micelles. For the critical molar ratio W0,c = 17, the total activation energy vanished. So, according to the Eyring lattice model, the jump of the formed micelles from one site to another was not permitted and the entire molecular groups were in vibration and rotation motions. For W0 > W0,c, where the formed micelles were fully hydrated, the relative viscosity of the studied solutions vs. temperature was fitted according a polynomial law, where the exponent was found to be dependent on W0.

Determination of the Flory Exponent by Study of Steady Shear Viscosity

The aim of the present work was to determine first the behavior of a neutral polymer, polyvinylpyrrolidone (Mw = 3,6 105 g/mol), in the dilute and semidilute regimes in water by a rotational concentric cylinder viscometer and then to deduce the Flory exponent. The absolute viscosity of the samples was directly determined from the shear stress–shear rate rheograms over a temperature range of 20°C–40°C and a polymer concentration range of 0.114–1.438 g/dl. The rheological nature of the polymer solutions was systematically analyzed, showing Newtonian behavior. Then, the intrinsic viscosity [η] was determined according to the Schultz–Blaschke equation. The overlapping concentration C* was determined by plotting the double logarithmic plots of reduced viscosity ηr versus polymer concentration C, yielding . The Flory exponents, ν, were deduced from viscosity measurements in the semidilute regime (ν = 0.544). In addition, the hydrodynamic radius, RH, was measured for different polymer molar mass (Mw (g/mol) = 10,000, 40,000, 50,000, and 360,000) by a dynamic light scattering technique at 25°C, yielding a Flory exponent at this temperature of 0.521. The obtained value was thus in agreement with that obtained by the rheological method in the semidilute regime with a relative deviation close to 4.5%.

Study by 1H NMR of the Influence of the Porosity and Solvent on the Conformations of Macromolecules at an Interface: Poly(Ethylene Oxide) Grafted on Silica

ABSTRACT The 1H NMR technique was used to study the behaviour of poly(ethylene oxide) chains grafted chemically on silica. A noticeable influence of the nature of the surface and solvent on the conformation of the grafted macromolecules was observed. For precipitated silica the chains adopted a disordered conformation whereas for pyrogenic silica they formed a more uniform layer. The grafting ratio and the solvent can both modify the conformations of the chains. The dependence of the relaxation times (T1 and T2) on the grafting ratios is discussed qualitatively from a phenomenological point of view. Different models are proposed for the two extreme grafting ratio levels in both the absence and the presence of solvent. Without solvent, for both grafting ratios, the macromolecules lie very flat on the silica and the layer is more organized, especially at high grafting ratio. In the presence of solvent the chains spread out in the solvent, adopt a more extended conformation and the local concentration of monomers at the surface decreases.

Effect of Temperature and a Non-Solvent Addition on the Specific Volume of Poly (Sodium-4-Styrene Sulfonate) in Water

ABSTRACT The effect of temperature and a non-solvent, ethanol, addition on the specific volume of poly (sodium 4-styrene sulfonate) in water was investigated. The refractive indices of the studied solutions were measured over the 298.15–323.15 K temperature range. Then, the specific volumes of the polyelectrolyte were calculated according to the Lorentz–Lorenz and Gladstone–Dale equations. The relative deviations of the specific volumes between measured and calculated according to these equations did not exceed 2.5%. Several empirical equations were deduced to describe the variations of the refractive index increment and the specific volume versus the temperature and the mixture composition.

A Novel Interpretation of Polymer Viscosity in Mixed Solvents

A novel interpretation of the viscosity of a polymer in mixed solvents in the dilute concentration regime is presented. The ternary system polymer/solvent 1/solvent 2 was considered as a quasi-binary system and the presence of the polymer acts as a perturbation on the mixed solvents where the interaction between unlike molecules was quantified by the Grunberg–Nissan constant, . Thus, a pseudo-Grunberg–Nissan constant, , was introduced to quantify the interaction between the unlike molecules of the mixed solvents in the presence of the polymer. The effect of the polymer addition on the interaction between unlike molecules was quantified by introducing the deviation of the Grunberg–Nissan constant, , concept which is connected to both the intrinsic polymer viscosity and the Huggins constant in pure and mixed solvents. This novel interpretation of the viscosity data was used to study the effect of polystyrene dissolution on the interaction between benzene and ethyl acetate at 20°C.