Thermodynamic interaction parameters in polymer solutions and gels

Polymer-solvent interaction parameters are reported for poly(vinyl acetate)-acetone, poly(vinyl acetate)-toluene, and poly(dimethyl siloxane)-toluene systems using different techniques. Results obtained by osmotic deswelling are compared with those from quasi-elastic light scattering and small-angle neutron scattering (SANS). In gels, the latter techniques involve separation of the time-dependent from the static component of the scattered radiation+ Separation is achieved in quasi-elastic light scattering through the heterodyning properties of the light, and in SANS by subtracting an appropriate static structure factor. The interaction parameters obtained by different separation procedures are consistent with measurements using the osmotic method. © 1995 John Wiley & Sons, Inc.     

Thermodynamic parameters of dilute solutions of a polymer in benzene at varying temperatures and concentrations

Dilatometric studies of solutions of four different mole fractions of poly(ethylene glycol) of average molecular mass 400 g mol^?1 dissolved in benzene have been performed to estimate molar volumes (V) and volume expansion coefficients (α) at different temperatures. From these, thermodynamic parameters appropriate to these solutions have been evaluated at different temperatures ranging from 299 to 328 K. These parameters have been employed to collect and discuss the information on molecular structure, molecular order, molecular packing, polymer segment extension, and molecular interactions.     

Polymer-solvent interaction in physical networks

The theoretical model of thermodynamical properties of swollen polymer network with physical junctions is proposed. Temperature transitions in network depending on solvent volume fraction and strain are investigated. As is shown, phase transition and collapse — decollapse processes are obtained in dependence on thermodynamical parameters and concentration of physical junctions.     

Studies on the viscosity behavior of polymer solutions at low concentrations

Viscosity measurements had been made on poly(vinyl alcohol) (PVA), poly(N-vinyl-2-pyrrolidone) (PVP) and poly(ethylene oxide) (PEO) solutions down to low concentrations. It was found that defined as the flow time of the pure solvent in ideal conditions and obtained practically by extrapolating the flow time of polymer solution t to zero concentration, was not equal to the flow time of the pure solvent t₀ measured. The reduced viscosity η_sp/C determined by (t/t₀-1)/C exhibited either a drastic increase or a significant decrease with dilution, depending upon the polymer solution investigated. On the other hand, η_sp/C determined by was proportional to C even at low concentrations. The anomalous viscosity behavior of neutral polymer solutions at low concentrations, therefore, was due to the incorrect method by which η_sp/C was determined. The detailed experiments indicated that the effective diameter of the viscometer capillary, the surface property of the capillary wall and the additional pressure corresponding to the measurement of t and t₀ for PVA, PVP and PEO solutions were not the same. Taking into account the contact anger and the surface tension of the liquid, together with the geometric parameter of the viscometer, the influence of the additional pressure upon the flow time measurement could be studied quantitatively. The calculation was in a good agreement with the experimental result. According to the method presented in this paper, the thickness of the adsorbed polymer layers on the capillary walls could be determined. It was noted that the thickness of the adsorbed polymer layers on the capillary walls was closely related to the solvent in which the polymer molecules were dissolved. The polymer molecular weight, however, had little or no effect on the thickness of the adsorbed polymer layers on the walls of the viscometer capillary.     

Determination of microscopic interaction parameters of ternary polymer solutions with light scattering data

Using a renormalized form of the Random Phase Approximation (or Tree Graph Approximation) we analysed data on the zero angle scattering intensity and the apparent radius of gyration for the system polystyrene-poly(methyl methacrylate)-bromobenzene. Since the microscopic parameters of this system are known from the analysis of the zero-angle scattering data, the theory yields a parameterfree prediction of the apparent radius of gyration which fits the data very well. To show the influence of renormalization we also analyse the data by an unrenormalized version of RPA.     

Ultracentrifugal equilibrium measurement of high polymer solutions at elevated temperatures

To investigate the solution properties of polyethylene, which has the simplest structure of the vinyl polymers, experiments were made with a magnetically suspended equilibrium ultracentrifuge. Preliminary studies were carried out with a polystyrene–chloroform system at 25°C. and a polystyrene–methylcyclohexane system at 68°C. (which is close to the theta temperature) in order to check the difficulties involved in the flotation equilibrium in the former case and the high temperature measurement in the latter. However, no trouble was encountered in either system, and the results were discussed and compared with earlier results for polystyrene solutions. It was found that chloroform is a good solvent for polystyrene, and the measured weight-average molecular weight is somewhat smaller than the value obtained in a theta solvent. After overcoming some technical difficulties involved in studies at higher temperatures, we carried out experiments on polyethylene in α-chloronaphthalene at 130°C. The results are considered reasonable by comparison with results obtained by other methods. The sample employed, Marlex 50 of melt index 0.7, has a wide molecular weight distribution: i.e., M_z/M_w = 5.2 and M_z+1/M_z = 2.4.     

Stress overshoot of polymer solutions at high rates of shear

Overshoot of shear stress, σ, and the first normal stress difference, N₁, in shear flow were investigated for polystyrene solutions. The magnitudes of shear corresponding to these stresses, γ_σm and γ_Nm, for entangled as well as nonentangled solutions were universal functions of γ˙τ_eq, respectively, and γ_Nm was approximately equal to 2γ_σm at any rate of shear, γ˙. Here τ_eq = τ_R for nonentangled systems and τ_eq = 2τ_R for entangled systems, where τ_R is the longest Rouse relaxation time evaluated from the dynamic viscoelasticity at high frequencies. Only concentrated solutions exhibited stress overshoot at low reduced rates of shear, γ˙τ_eq < 1. The behavior at very low rates, γ˙τ_eq < 0.2, was consistent with the Doi–Edwards tube model theory for entangled polymers. At high rates, γ˙τ_eq > 1, γ_σm and γ_Nm were approximately proportional to γ˙τ_eq. At very high rates of shear, the peak of σ is located at t = τ_R, possibly indicating that the polymer chain shrinks with a characteristic time τ_R in dilute solutions. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1917–1925, 2000     

Polymer-polymer interaction parameters in solvent/polymer/polymer ternary systems

Empirical equations defining the relationships between b₂₃ and χ₂₃ interaction parameters for the ternary systems toluene (1)/poly(dimethylsiloxane), PDMS, (2)/polystyrene, PS, (3) and benzene (1)/PDMS(2)/PS(3) have been determined. The dependences of both interaction parameters on the mean molecular weights of the macromolecular components have also been studied. A single dependence of b₂₃ (or χ₂₃) upon the mean molecular weight is found when both polymeric components have intermediate to high molecular weights. However, that single dependence is not found when the molecular size of polymer (2) is low.     

Polymer-solvent effects in cellulose urethane and methyl cellulose urethane solutions

The polymer-solvent interaction was studied for two similar cellulose derivatives in the semi-dilute concentration range by static and dynamic light scattering. The trisubstituted 3-chlorophenyl carbamate (3Cl-CTC) and a mixed trisubstituted derivative with methyl groups (degree of methyl substituents: DS _Me = 1.6–1.7)combined with the abovementioned 3-chlorophenyl carbamate groups filling the still open positions at the cellulose backbone were synthesized, fractionated and characterized according to standard methods. Different kinds of associations, entangled clusters with a rod-like shape on one side and entanglement networks on the other side, exist in semi-dilute dioxane solutions caused by different polymer-solvent interactions. These quite different associations lead to either a liquid crystalline or a gel-like state upon increase of concentration.     

Polymer-solvent co-crystallization during thermoreversible gelation in solutions of the helical polypeptide PBLG

Macromolecular aggregation during thermoreversible gelation in solutions of the helical polypeptide poly(γ-benzyl-L-glutamate) [PBLG] in benzyl alcohol [BA] were studied by small angle neutron and small angle X-ray scattering. Gelation is apparent as a large increase in the intensity scattered at low angles, signifying formation of a microfibrillar PBLG network. The aggregated phase in isotropic gels from semidilute solutions contains about 28% solvent. A periodic structure is observed when gelation is induced by rapid cooling to a low temperature, but not by slow cooling or gelation at a higher temperature. In gels from concentrated liquid crystal solutions, two crystalline structures are observed, depending on whether the solution is rapidly quenched and then annealed or slowly gelled at an elevated temperature. A phase diagram for the PBLG/BA system is presented and the observed microstructural transitions are rationalized in terms of a gelation mechanism involving a combination of liquid-liquid phase separation and crystallization in the form of polymer-solvent co-crystals.     

Determination of the critical parameters of polymer solutions

The contribution of short-range orientation order to the chemical potentials of components in a polymer-solvent system was determined. The temperature dependence of the degree of orientation of solvent molecules relative to polymer chains in a polyisobutylene-benzene system was obtained. It was shown that the short-range order in the polymer-solvent system fails with temperature to a much lesser extent than in the pure solvent. The expressions for the upper and lower critical solution temperatures with allowance for the contribution of short-range order to the entropy and the free energy of solution, as well as to the chemical potentials of components, were found. It was demonstrated that, in the polymer-solvent system, two upper and two lower critical solution temperatures can exist.     

Polymer–polymer interaction parameters of polystyrene and polybutadiene from studies in solutions of toluene or tetrahydrofuran

Equations used for the determination of polymer–polymer interaction parameters for narrow molecular weight distribution polymers which have undergone phase separation in solutions are discussed. Binodals with tie lines and plait points are presented for ternary systems polystyrene-polybutadiene-toluene and polystyrene-polybutadiene-tetrahydrofuran. Typical interaction parameters determined from the experimental data for the polystyrene-polybutadiene systems are presented.     

On the nature of phase separation of polymer solutions at high extension rates

Experiments with stretching moderately concentrated polymer solutions have been carried out. Model experiments were carried out for poly(acrylonitrile) solutions in dimethyl siloxane. Just the choice of concentrated solutions allowed for a clear demonstration of a demixing effect with the formation of two separate phases—an oriented polymer fiber and solvent drops sitting on its surface. An original experimental device for following all subsequent stages in the demixing process was built. It combined two light beams, one transverse to the fiber and a second directed along (inside) the fiber, the latter played the role of an optical line. This gives a unique opportunity to observe processes occurring inside a fiber. The process of demixing starts from the volume phase separation across the whole cross section of a fiber at some critical deformation and the propagation of the front of demixing along the fiber. Then a solvent cylindrical skin appears which transforms into a system of separate droplets. New experimental data are discussed based on a comparison of the current different points of view on the phenomenon of deformation‐induced phase separation: thermodynamic shift of the equilibrium phase transition temperature, growth of stress‐induced concentration fluctuations in two‐component fluids, and mechanically pressing a solvent out from a polymer network. The general belief is that a rather specific (so‐called “beads‐on‐a‐string”) structure of a filament is realized in stretching dilute solutions: beads of a polymer solution connected by oriented polymer bridges forming a single object. The situation in stretching moderately concentrated solutions appears quite different: real phase separation was observed. So, the alternative phenomenon to the formation of the “beads‐on‐a‐string” structure has been experimentally proven. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 559–565     

Hydrodynamic interaction in polymer solutions simulated with dissipative particle dynamics

The authors analyzed extensively the dynamics of polymer chains in solutions simulated with dissipative particle dynamics (DPD), with a special focus on the potential influence of a low Schmidt number of a typical DPD fluid on the simulated polymer dynamics. It has been argued that a low Schmidt number in a DPD fluid can lead to underdevelopment of the hydrodynamic interaction in polymer solutions. The authors' analyses reveal that equilibrium polymer dynamics in dilute solution, under typical DPD simulation conditions, obey the Zimm [J. Chem. Phys. 24, 269 (1956)] model very well. With a further reduction in the Schmidt number, a deviation from the Zimm model to the Rouse model is observed. This implies that the hydrodynamic interaction between monomers is reasonably developed under typical conditions of a DPD simulation. Only when the Schmidt number is further reduced, the hydrodynamic interaction within the chains becomes underdeveloped. The screening of the hydrodynamic interaction and the excluded volume interaction as the polymer volume fraction is increased are well reproduced by the DPD simulations. The use of soft interaction between polymer beads and a low Schmidt number do not produce noticeable problems for the simulated dynamics at high concentrations, except for the entanglement effect which is not captured in the simulations.     

Determination of molecular parameters from polymer solutions in shear flow

A light scattering photometer with a double cylinder shear cell has been developed which allows detection not only in one plane as usual, but in a half-spherical range around the scattering volume. The anisotropic scattering behavior of oriented and deformed polymer molecules in dilute solution was investigated. All measurements were performed on polystyrene in several viscous solvents. From measurements in the plane of flow the average orientation angle was determined. By variation of detector position and wavelength of the primary beam, the determination of all three axes of the molecular gyration space of polymer coils was possible. Compared to predictions of the well-known dynamic theories by Kuhn, Rouse and Zimm, corresponding orientation data were found while the molecular deformation ratio shows much lower values than expected.     

Determination of pair interaction parameters for multicomponent polymer blends

Pair interaction parameters for multicomponent polymer blends were found to be determined by analyzing the sorption isotherms of common solvent.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2494–2496, November, 2004.     

Vibrational spectroscopy of aqueous solutions at high concentrations: Raman and inelastic neutron scattering

We review work performed by our group on vibrational spectroscopy of water and aqueous solutions of strong II-I electrolytes at high concentrations. Evidence shall be presented for the existence of intermediate range, solute-connected, ordered patches, whose collective vibrational modes yield a continuous vibrational density of states, as in amorphous solids. We shall review Raman and inelastic neutron scattering results, obtained for electrolytes such as ZnCl₂, NiCl₂, CuBr₂, CdCl₂. Other results, such as EXAFS, viscosity and ultrasonic attenuation will also be discussed.