Structure and dynamics of dextran in binary mixtures of a good and a bad solvent

The structure and dynamics of the common polysaccharide dextran have been investigated in mixed solvents at two different temperatures using small-angle X-ray scattering (SAXS) and viscosity measurements. More specifically, binary mixtures of a good solvent (water, formamide, dimethylsulfoxide, ethanolamine) and the bad solvent ethanol as the minority component have been considered. The experimentally observed effects on the polymer conformation (intrinsic viscosity, coil radius, and radius of gyration) of the bad solvent addition are discussed in terms of hydrogen bonding density and are correlated with the Hansen solubility parameters and the surface tension of the solvent mixtures. Hydrogen bonding appears to be an important contributor to the solubility of dextran but is not sufficient to capture the dextran coil contraction in the mixtures of good+bad solvents.     

Adsorption thin-layer chromatography and viscometry of polystyrenes in solvent mixtures

The effect of the thermodynamic quality of solvent mixtures on the adsorption behavior of macromolecules under dynamic conditions was investigated. The chromatographic behavior of polystyrenes (PSs) in various mixed eluents was studied under conditions of adsorption and size-exclusion thin-layer chromatography as well as at the exclusion-adsorption transition point with silica gel KSKG adsorbent. The thermodynamic quality of the solvent mixtures used in the chromatographic experiments was determined viscometrically. The dependences of the intrinsic viscosity of PSs on solvent composition for solvent-non-solvent, two solvents, and theta-solvent-solvent mixtures were obtained. A correlation was found between Snyder's polarity indices for the solvent mixtures used in polymer chromatography under "critical conditions" and the intrinsic viscosity values of PSs in the same mixtures.     

Bimolecular photoinduced electron transfer in imidazolium-based room-temperature ionic liquids is not faster than in conventional solvents

The fluorescence quenching of 3-cyanoperylene upon electron transfer from N,N-dimethylaniline in three room-temperature ionic liquids (RTILs) and in binary solvent mixtures of identical viscosity has been investigated using steady-state and time-resolved fluorescence spectroscopy. This study was stimulated by previous reports of bimolecular electron transfer reactions faster by one or several orders of magnitude in RTILs than in conventional polar solvents. These conclusions were usually based on a comparison with data obtained in low-viscous organic solvents and extrapolated to higher viscosities and not by performing experiments at similar viscosities as those of the RTILs, which we show to be essential. Our results reveal that (i) the diffusive motion of solutes in both types of solvents is comparable, (ii) the intrinsic electron transfer step is controlled by the solvent dynamics in both cases, being slower in the RTILs than in the conventional organic solvent of similar viscosity, and (iii) the previously reported reaction rates much larger than the diffusion limit at low quencher concentration in RTILs originate from a neglect of the static and transient stages of the quenching, which are dominant in solvents as viscous as RTILs.     

Conformation of poly(methacrylic acid) in alcohol–water mixtures. I. Effect of concentration,temperature, pH,and ionic strength

The reduced specific viscosity of poly(methacrylic acid) has been studied in ethanol–0.002N HCI solvent mixtures as a function of polymer concentration, alcohol concentration, and temperature. In addition, experiments were performed at different HCI concentrations and with KCI instead of HCI. Both intrinsic viscosity and Huggins coefficient were shown to undergo unusually strong variations. Two minima and two maxima could be demonstrated in intrinsic viscosity. The Huggins coefficient seems to show corresponding variations. The first minimum in intrinsic viscosity indicates that the coil volume has collapsed almost to an Einstein sphere. In this region the Huggins coefficient is extremely large (of order 10²) and is controlled by coil association. It was shown that several forms of intramolecular interaction must be assumed to be competing to account for this behavior. The presence of HCI, particularly in the preponderantly aqueous phase, is required to suppress the polyelectrolyte effect. It is found, however, that the behavior of the solutions at relatively high ethanol concentrations is more sensitive to HCI content than is that of highly aqueous solutions. KCI can be used to replace HCI over most of the range. Increase in temperature shifts the turning points of the curves to lower alcohol concentrations. Some evidence has been found that the association constant giving rise to dimers increase with rate of shear. The importance of poly(methacrylic acid) as a chemically simple model substance for various biopolymer effects is stressed.     

Viscosity effect on the CIDEP of laser photolysis of duroquinone in hydrogen-donor solvents

Chemically induced dynamic electron polarization (CIDEP) is one of the important research fields of dynamic spin chemistry. In the present work, with ethylene glycol, 1,2-propanol and their mixtures with different ratios in volume as solvents and duroquinone as photosensitive molecule, the influence of solvent viscosity on the CIDEP of photolyzed duroquinone radicals has been studied experimentally. In each solvent, duroquinone neutral radicals are observed. Duroquinone neutral radicals are generated through hydrogen transfer reaction from hydrogen-donor solvent molecules to excited duroquinone triplet. The CIDEP mechanism is mainly triplet mechanism. The CIDEP intensity of duroquinone neutral radicals decreases with the increase in the solvent viscosity, and the variation was rapid in low-viscosity range and slow in high-viscosity range.     

Thermodynamics and viscosities of dilute polymer solutions in binary solvents

Experiments aimed at comparison of calculated and experimental intrinsic viscosities of PS and PMMA in some binary solvents used as eluents in chromatography are performed. The intrinsic viscosities are determined in mixed solvents of different thermodynamic qualities, including a θ solvent, in a wide composition range. The analysis of the thermodynamic and rheological theoretical equations for ternary systems allowed the interpretation of anomalies observed on the experimental curves of intrinsic viscosity versus solvent composition.     

Limits of the dilute regime for the solution viscosity of PMMA in good and in poor solvents

The concentration (c′) marking the first deviation from linearity in the Huggins plot of specific viscosity η_sp/c vs c) has been determined for PMMA in chloroform, benzene (good solvents), acetonitrile, chlorobutane (poor solvents) and acetonitrile/chlorobutane mixtures (cosolvent). The dependence of c′ on polymer chain length and on solvent quality is given. The results are analysed in terms of the influence on c′ of incipient coil overlap, peripheral entanglements and other interactions, such as polymer association.     

Viscometric study of poly(vinyl chloride)/poly(vinyl acetate) blends in various solvents

The intermolecular interactions between poly(vinyl chloride) (PVC) and poly(vinyl acetate) (PVAc) in tetrahydrofuran (THF), methyl ethyl ketone (MEK) and N,N^′-dimethylformamide (DMF) were thoroughly investigated by the viscosity measurement. It has been found that the solvent selected has a great influence upon the polymer-polymer interactions in solution. If using PVAc and THF, or PVAc and DMF to form polymer solvent, the intrinsic viscosity of PVC in polymer solvent of (PVAc+THF) or (PVAc+DMF) is less than in corresponding pure solvent of THF or DMF. On the contrary, if using PVAc and MEK to form polymer solvent, the intrinsic viscosity of PVC in polymer solvent of (PVAc+MEK) is larger than in pure solvent of MEK. The influence of solvent upon the polymer-polymer interactions also comes from the interaction parameter term Δb, developed from modified Krigbaum and Wall theory. If PVC/PVAc blends with the weight ratio of 1/1 was dissolved in THF or DMF, Δb<0. On the contrary, if PVC/PVAc blends with the same weight ratio was dissolved in MEK, Δb>0. These experimental results show that the compatibility of PVC/PVAc blends is greatly associated with the solvent from which polymer mixtures were cast. The agreement of these results with differential scanning calorimetry measurements of PVC/PVAc blends casting from different solvents is good.     

Polyamides in solution. III. Viscometry of linear polycaprolactam

The viscometric behavior of linear polycaprolactam has been studied in the absence of the electroviscous effect in aqueous solvents containing 85% and 64.5% formic acid and in trifluoroethanol, as a function of temperature and also under unperturbed conditions. Results are discussed in terms of the existing theories; in particular, the negative temperature coefficient of the intrinsic viscosity arises only from the variation of the expansion coefficient, the molecule in the unperturbed state being a normal random coil. The absence of aggregation and draining effects in the above solvents has been varified.     

Influence of preferential solvation on the interpolymer complex formation between methacrylic acid–methacrylamide copolymer and poly(vinyl pyrrolidone)

Interpolymer complex formation has been studied between methacrylic acid–methacrylamide copolymer and poly(vinyl pyrrolidone) in acetone–water mixtures of different compositions. At a specific composition of the solvent (e.g., 70% water + 30% acetone by volume), the intrinsic viscosity of the copolymer and the reduced viscosity of 1:1 interpolymer complex had minimum values. Preferential solvation coefficients (λ/C) have been calculated for the various compositions of the solvent mixtures. An excellent coincidence could be obtained between the maximum (λ/C) and the minimum in the viscosity of the polymer solutions. Interpretations have been sought in terms of conformational change of the copolymer at the specific composition of the solvent mixture.     

Determination of effective hydrodynamic diameters of biopolymer molecules in high-viscosity mixtures by photon-correlation spectroscopy

Using model high-viscosity single-component and mixed systems based on biopolymers with different molecular sizes (poly(ethylene glycol), dextran, and polysucrose) as examples, it is shown by photon-correlation spectroscopy combined with monoand polymodal analysis that solvent viscosity should be used, when calculating the hydrodynamic diameter of molecules in single-component aqueous solutions and mixed solutions of dextran and polysucrose, which have close molecule sizes, by the Stokes–Einstein equation. For mixtures of dextran and polysucrose with polyethylene glycol, the viscosity of the medium, the role of which is played by the poly(ethylene glycol) solution, should be used.     

Solution properties of cellulose triacetate. II. Solubility and viscosity studies

The solubility of cellulose triacetate in a range of solvents was measured, and the results for tetrachloroethane, chloroform, and acetic acid were compared with those from initial phase separation in solvent–nonsolvent mixtures and viscosity–concentration studies. The correlation found between solubilities, precipitation values, and values of the Huggins viscosity constant is discussed with reference to the type of polymer–solvent interaction proposed previously to explain fractionation behavior. A qualitative comparison of solubility–swelling behaviour was also made for a very low molecular weight cellulose triacetate sample in a wide range of solvents. Results are compared with those for higher molecular weight samples and discussed with regard to the cohesiveenergy densities of solvent and polymer. Some attempt has been made to predict suitable solvents for cellulose triacetate, based on consideration of their molecular structures.     

Dilute solution behavior of polyelectrolytes. Intrinsic viscosity and light scattering studies

The conformational character of a random copolymer of ethyl acrylate and acrylic acid (mole ratio 3:1) has been examined by intrinsic viscosity and light scattering in organic and in aqueous media. The unperturbed dimensions of this copolymer in its un-ionized state in an organic theta solvent are 1.3 to 1.4 times those obtained for the fully ionized polymer in an aqueous theta solvent. The data also suggest that a change in conformation from a swollen random coil to a compact random coil occurs in aqueous media as a function of ionic strength. These results are interpreted in terms of the hydrophobic interaction of the ester groups on the chain. An application of the wormlike chain model shows that viscosity data can be used to predict the light scattering results well with in experimental error.     

Temperature dependence of the viscosity of dilute poly(β-naphthyl methacrylate) solutions

The temperature dependence of the actual viscosity of dilute poly(β-naphthyl methacrylate) solutions was described by an Arrhenius expression according to Moore's treatment. The apparent activation energy of flow was found to be lower for solutions in benzene, toluene and tetralin than for solvent. In dioxane, a good solvent, the activation energy was nearly constant and close to that for the solvent. The pre-exponential terms were, in all cases, higher for solutions than for the solvents. The decrease in activation energy and increase in pre-exponential term are largely dependent on solvent power: the maximum effect is found in the poorest solvent. This behaviour is discussed in connection with the temperature dependence of the Mark-Houwink and of the Fox-Flory constants obtained from intrinsic viscosity data: the differences in the activation energy of flow and in the pre-exponential term between the solvent and the solutions are related to variation of coil expansion with temperature.     

Conformational behavior of styrene–dymethylsiloxane block copolymers in dilute solution

To determine the behavior of a copolymer is dilute solution, a viscosity study has been performed on a polystyrene–polydimethylsiloxane block copolymer in three solvents presenting different thermodynamic conditions. The results are discussed in relation to a mixture of homopolymers and a segregated model. The unperturbed dimensions, obtained by the Stock–mayer–Fixman method, are intermediate between those of the parent homopolymers. The intrinsic viscosity measured in a good solvent, toluene, was close to the weighted averages of those of the corresponding homopolymers of equal molecular weight, but higher in decalin and in butanone, θ solvents for PS and PDMS, respectively. According to the low value obtained for the interaction parameter, the chain is slightly expanded as a result of the interactions between the unlike monomer units. Both segregation and random conformation would probably occur, depending on the quality of the solvent.     

Dilute solution properties of anionic polystyrene in ternary mixture toluene-2-butanone-2-methyl-1-propanol

Several polystyrene samples with relatively narrow molecular weight distributions have been prepared by anionic polymerization. Dilute solution properties of these samples in binary and ternary liquid mixtures were investigated by light scattering and viscometry, using toluene and 2-butanone as solvents and 2-methyl-1-propanol as non-solvent. At 25° experiments were performed to find a set of compositions of the mixed solvent where the second virial coefficient of the osmotic pressure expansion equals zero for the polymer solution, and a set of compositions of the mixed solvent where the intrinsic viscosity of the polymer solution equals the value for polystyrene a theta solvent, such as cyclohexane at 34.5°. The results suggest that these sets of compositions are not identical. The specific effects of sorption and preferential sorption which give rise to the difference between the sets of compoisitions are qualitatively discussed in terms of the Flory-Huggins theory.     

High-temperature liquid chromatography. Part II: Determination of the viscosities of binary solvent mixtures—Implications for liquid chromatographic separations

This paper is the second in a series of consecutive publications, explaining the concept of high temperature liquid chromatography under various important aspects. The second publication deals with the determination of the viscosity of binary solvent mixtures used in reversed phase liquid chromatography in a temperature range between 25 and 250 °C. In literature, only limited data of the temperature dependent viscosities of liquid solvents or binary solvent mixtures can be found. Therefore, the viscosities of the pure solvents as well as the binary mixtures had to be determined experimentally up to 250 °C. The viscosity data were used to estimate the pressure drop in a capillary connecting a high-temperature HPLC system with a mass spectrometer. The solvent perturbation could be avoided by adjusting the diameter of the transfer capillary to the viscosity and vapour pressure of the mobile phase. The viscosity data were also used to show that a significant gain in analysis speed is theoretically feasible. This factor clearly depends on the nature of the solvent system, because for mixtures with a large viscosity maximum at ambient temperature, this effect is most pronounced.     

Ionic liquid–water mixtures as solvents for poly(N-vinylimidazole)

Ionic liquids extend the Hofmeister series and create a wide range of new possibilities for processes involving salt effects on both soluble and crosslinked systems. This work reports on some mixtures of water with NaCl or an ionic liquid, either 1-ethyl-3-methylimidazolium tosylate or 1-hexyl-3-methylimidazolium chloride, which are better solvents for linear poly(N-vinylimidazole) (L-PVI) than water, i.e., that exhibit a salting-in effect. The intensity of the salt effects was measured on the basis of the polymer solubility, the decrease in polymer–solvent interaction parameter (measured by light scattering), and the increase of coil size (measured through the intrinsic viscosity). It was thus found that the intensity of the salting-in effect of either NaCl or 1-hexyl-3-methylimidazolium chloride on L-PVI is different (larger for the ionic liquid), which denotes that salt effects are not under anion control, and the mechanisms operating in the linear and crosslinked polymers are different. These results are discussed after accounting for the role of ion–polymer interactions.     

VISCOMETRIC INVESTIGATIONS OF POLYVINYLPYRROLIDONE IN MIXED SOLVENTS AND WITH VARYING TEMPERATURE

The viscosity behavior of polyvinylpyrrolidone (PVP) has been determined at 25℃ in mixed solvents comprising water/dimethylformamide (DMF) and water /methanol (MeOH). Analysis of the data has considered the PVP as being both host and guest polymer in solution. The intrinsic viscosity of PVP in DMF is higher than in water and in MeOH, but also increases in a mixed solvent with high water content because of the effect of polymer-solvent interactions. It was also found that the intrinsic viscosity of PVP at finite concentration, [ηpvp]c decreases with an increase in the concentration of PVP in solution. The viscosity behavior of PVP in a mixed solvent is affected by the concentration-dependent intermolecular excluded volume effect, which can be quantitatively expressed by the parameter, bY, which reflects the shrinkage of PVP chain coils, resulting in a decrease of [ηpvp]c. The effect of temperature on the viscosity behavior of PVP in MeOH shows that the interaction parameter increases up to a maximum value, and then decreases after a certain temperature.     

Viscosity of dilute solutions of styrene copolymers in the course of polymer-analogous transformations

The properties of dilute solutions of styrene copolymers and their porphyrin derivatives prepared via the functional group transformations have been studied in DMF at 20–35°C. The effect of the copolymer composition, on the intrinsic viscosity, the Huggins constant, the macromolecule dimensions, and thus on the interaction with solvent has been elucidated. The nature and content of functional groups in the polymer influence the behavior of macromolecule coil in the solution. Introduction of the porphyrin fragment loosens the macromolecular coil, but does not qualitatively change the solution behavior.