Rheology of viscoelastic solutions of cationic surfactant. Effect of added associating polymer

Rheological studies were performed with aqueous salt solutions of viscoelastic cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) and its mixtures with hydrophobically modified polyacrylamide. The solutions of surfactant itself above the concentration of crossover of wormlike micelles exhibit two regions of rheological response. In the first region, they behave like polymer solutions in semidilute regime characterized by viscoelastic behavior with a spectrum of relaxation times. In the second region, unlike polymer solutions their relaxation after shear is dominated by a single relaxation time. Being composed of "living" micelles, the EHAC solutions easily lose their viscosity at the variation of the external conditions. For instance, heating from 20 to 60 degrees C reduces viscosity by up to 2 orders of magnitude, while added hydrocarbons induce a sudden drop of viscosity by 3-6 orders of magnitude. Polymer profoundly affects the rheological properties of EHAC solutions. The polymer/surfactant system demonstrates a 10,000-fold increase in viscosity as compared to pure-component solutions, the effect being more pronounced for polymer with less blocky distribution of hydrophobic units. A synergistic enhancement of viscosity was attributed to the formation of common network, in which some subchains are made up of elongated surfactant micelles, while others are composed of polymer. At cross-links the hydrophobic side groups of polymer anchor EHAC micelles. In contrast to surfactant itself, the polymer/surfactant system retains high viscosity at elevated temperature; at the same time it keeps a high responsiveness to hydrocarbon medium inherent to EHAC.     

Mixed solutions of an associating polymer with a cleavable surfactant

Mixtures of hydrophobically modified hydroxyethyl cellulose (HMHEC) and alkali-sensitive cleavable betaine ester surfactants have been studied by viscometry, 1H NMR, absorbance measurements, and birefringence determinations. Before the hydrolysis, the surfactants behaved as conventional nondegradable surfactants in terms of the effect on the viscosity of increasing surfactant concentration. As the surfactants were hydrolyzed, systems with time-dependent viscosity were obtained. The viscosity either decreased monotonically or went through a maximum as a function of time, depending on the initial surfactant concentration. Different surfactant chain lengths gave rise to different viscosity profiles. The rate of hydrolysis, and thus the time-dependency of the surfactant concentration, could be controlled by changing the pH of the solution.     

Stress relaxation in polymer networks: equilibrium behavior and dynamics

The elastic and relaxational properties of a polymer network have been calculated using a stress based formulation based on the Rouse mode expansion [W. L. Vandoolaeghe and E. M. Terentjev, J. Chem. Phys. 123, 34902 (2005)]. In this article, we propose an improved Rouse mode expansion incorporating appropriate boundary conditions. In contrast to the previous work, this improved formulation provides a smooth crossover from the classical equilibrium result of rubber elasticity to the shorter-time-scale Rouse relaxation of a polymer melt. Our results are compared with the classical phantom network approach in equilibrium, as well as both equilibrium and dynamic elongation experiments. The model captures the qualitative features of the data well and some of the quantitative aspects, such as the exponents seen in the dynamic modulus G(omega).     

Microwave dielectric relaxation in some hydrogen-bonded solutions

Dielectric constant and loss values have been obtained between 1 and 35 GHz for several:polyhydroxyl alcohols in p-dioxane solution at 25°C. The data have been analyzed in terms of a Cole-Cole distribution. The relaxation times are sensitive to the proximity of the hydroxyl groups and the degree of intramolecular hydrogen bonding.     

Rheological behavior of silica suspensions in aqueous solutions of associating polymer

Associating polymers are hydrophilic long-chain molecules containing a small amount of hydrophobic groups. The aqueous solutions show viscoelastic responses above some critical concentrations because a three-dimensional structure is formed by association of hydrophobic groups. When the associating polymers are added to silica suspensions at low concentrations, the flocculation is induced by bridging mechanisms, and the flow of suspensions become shear-thinning. For suspensions prepared with polymer solutions in which the associating network is developed, the viscosity decreases, shows a minimum, and then increases with increasing particle concentration. The viscosity decrease may arise from the breakdown of associating network due to adsorption of polymer chains onto the silica surfaces. As the particle concentration is increased, the polymer concentration in solution is decreased, and finally, all polymer chains are adsorbed on the surfaces. Beyond this point, the partial coverage of particle surfaces takes place and strong interactions are generated between particles by polymer bridging. Since the stable suspensions are converted to highly flocculated systems, the viscosity is increased and the flow becomes shear-thinning. The concentration effect of silica particles on the viscosity behavior of suspensions can be explained by a combination of viscosity decrease in solution due to polymer adsorption and viscosity increase due to flocculation.     

Modified interfacial statistical associating fluid theory: application to tethered polymer chains

Modified interfacial statistical associating fluid theory density functional theory is extended to tethered polymer chains in the absence or presence of free polymer chains. The structures of the "dry" and "wet" polymer brushes have been calculated and compared with simulation results available in the literature. The comparisons show that the theory accurately predicts the structure of the tethered polymer brush. The average brush heights calculated from the theory agree with well-established scaling theories for tethered polymers. However, these scaling theories cannot predict the detailed structure, accurately. The effects of the segment-segment interactions of the tethered polymer and the free polymer have been effectively captured by the theory.     

Effects of added surfactants on thermoreversible gelation of associating polymer solutions

The influence of added surfactants on physical properties of associating polymer solutions was examined by a new statistical‐mechanical theory of associating polymer solutions with multiple junctions and by computer simulation. The sol–gel transition line, the spinodal line, and the number of elastically effective chains in the mixed networks were calculated as functions of the concentration of added surfactants. All of them exhibited nonmonotonic behavior as a result of the following two competing mechanisms. One was the formation of new mixed micelles by binding surfactants onto the polymer associative groups. These micelles serve as crosslink junctions and promote gelation. The other was the replacement of polymer associative groups in the already formed network junctions by added surfactants. Such replacement lowers connectivity of junctions and destroys networks. The critical micelle concentration was also calculated. The results are compared with the reported experimental data on poly(ethylene oxide)‐based associating polymers and hydrophobically modified cellulose derivatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 733–751, 2004     

Correlation between the equilibrium and relaxation dielectric properties of 1,2-ethanediol

The data on the dispersion of the permittivity ?*(ω) of 1,2-ethanediol over the temperature range 161–453 K and the frequency range 0.1 Hz–150 GHz were analyzed using the Dissado-Hill cluster model. The relaxation frequency ω_p = τ _DH ^?1 and intra-(n _DH) and intercluster (m _DH) correlation parameters were calculated. The energy barrier to the libration of molecular axes in clusters was found to be B _DH = 2.96 kJ/mol. The apparent enthalpy of activation was determined; it increased from ΔH _{DH exp} ^# = 22.18 kJ/mol to ΔH _{DH exp} ^# = 129.19 kJ/mol close to the glass transition temperature. The mean dipole moments $ \bar \mu _c The data on the dispersion of the permittivity ɛ*(ω) of 1,2-ethanediol over the temperature range 161–453 K and the frequency range 0.1 Hz–150 GHz were analyzed using the Dissado-Hill cluster model. The relaxation frequency ω_p = τ_DH⁻¹ and intra-(n _DH) and intercluster (m _DH) correlation parameters were calculated. The energy barrier to the libration of molecular axes in clusters was found to be B _DH = 2.96 kJ/mol. The apparent enthalpy of activation was determined; it increased from ΔH _{DH exp}^# = 22.18 kJ/mol to ΔH _{DH exp}^# = 129.19 kJ/mol close to the glass transition temperature. The mean dipole moments of 1,2-ethanediol clusters were calculated; they decreased from 162920 to 18.08 D as the temperature increased from 161 to 453 K. According to approximate estimates, the number of 1,2-ethanediol molecules in a cluster /μ_v decreased from 72405 at 161 K to 8.04 at 453 K (μ_v is the dipole moment of the molecule in the vacuum), which substantiated the suggestion of the existence of a spatial structure close to the boiling point. Original Russian Text ? N.V. Lifanova, T.M. Usacheva, V.I. Zhuravlev, V.K. Matveev, 2008, published in Zhurnal Fizicheskoi Khimii, 2008, Vol. 82, No. 10, pp. 1973–1981.     

Proton spin-lattice relaxation study of polymer solutions

Proton spin-lattice relaxation times of solvent molecules were measured on ternary mixtures of a polymer and two solvents by the adiabatic rapid-passage method. The selective adsorption of a good solvent was verified by this experimental technique for the systems benzene—cyclohexane—polystyrene(PS), benzene—carbon tetrachloride—poly(methyl methacrylate)(PMMA), and chloroform—carbon tetrachloride—PMMA. The number of molecules of adsorbed benzene per monomeric unit of PS was estimated to be about four, which is almost identical with that determined previously by thermodynamic measurements. The number of molecules of benzene and chloroform adsorbed on PMMA were also determined to be about five and four, respectively. It was found that the interaction between chloroform and PMMA has the greatest effect on the molecular motion of the solvent, whereas the benzene—PS interaction is weak.     

Anisotropy of the dielectric relaxation of a crystalline polymer

A theory which relates the change in the strength of absorption to the change in crystal orientation function is presented for the anisotropy of dielectric relaxation for the dipolar orientation change in an oriented crystalline polymer. Experimental measurements are presented for the α dielectric absorption of a stretched ethylene–carbon monoxide copolymer and compared with the orientation of crystals of this copolymer as determined by x-ray diffraction.     

Length-scale dependent relaxation shear modulus and viscoelastic hydrodynamic interactions in polymer liquids

A quantitative theory of hydrodynamic interactions in unentangled polymer melts and concentrated solutions is presented. The study is focussed on the pre-Rouse transient time regimes (t < τ(R), the Rouse relaxation time) where the hydrodynamic response is governed mainly by the viscoelastic effects. It is shown that transient viscoelastic hydrodynamic interactions are not suppressed (screened) at large distances and are virtually independent of polymer molecular mass. A number of transient regimes of unusual and qualitatively different behavior of isotropic and anisotropic hydrodynamic response functions are elucidated. The regimes are characterized in terms of two main length-scale dependent characteristic times: momentum spreading time τ(i) ∝ r(4∕3) and viscoelastic time τ(?) ∝ r(4). It is shown that for t > τ(i) the viscoelastic hydrodynamic interactions can be described in terms of the time or length scale dependent effective viscosity which, for t < τ(R) and/or for r < R(coil), turns out to be much lower than the macroscopic "polymer" viscosity η(m). The theory also involves a quantitative analysis of the length-scale dependent stress relaxation in polymer melts. The general predictions for hydrodynamic interactions in thermostated systems with Langevin friction are obtained as well.     

Hydrogen network fluctuations of associating liquids: dielectric relaxation of ethylene glycol oligomers and their mixtures with water

Complex dielectric spectra of ethylene glycol and of various derivatives as well as of mixtures of water with an ethylene glycol oligomer and with a poly(ethylene glycol) dimethyl ether oligomer have been measured. The spectra can be well represented by a Cole-Cole [Cole and Cole, J. Chem. Phys. 9, 341 (1941)] spectral function. The extrapolated low frequency (static) permittivity of this function has been evaluated to yield the effective dipole orientation correlation factor of the liquids. The relaxation time of the ethylene glycols displays a characteristic dependence upon the ratio of concentrations of hydrogen bond donating and accepting groups, indicating two opposing effects. With increasing availability of hydrogen bonding sites effects of association and also of dynamical destabilization increase. Both effects exist also in the mixture of water with the oligomers. They are discussed in terms of a wait-and-switch model of dipole reorientation in associating liquids. Another feature in the dependence of the dielectric relaxation time of poly(ethylene glycol)/water mixtures upon mixture composition has been tentatively assigned to precritical demixing behavior of the binary liquids in some temperature range.     

Microwave dielectric permittivity and relaxation of aqueous potassium trimethylacetate solutions

The MW dielectric properties of aqueous potassium trimethylacetate (pivalate) solutions have been measured at six frequencies (10–25 GHz) at 288, 298, and 308 K. The static dielectric constants and dielectric relaxation times and activation parameters have been calculated. Trimethylacetate ion leads to the decrease in the mobility of water molecules and strengthening of their hydrogen bonds. These changes of water are similar to those in solutions of other carboxylates with a large number of nonpolar groups. The hydrophobic hydration of the trimethylacetate ion is maximal in this series.     

一种超支化疏水缔合聚合物的制备与性能评价

首先制备了疏水单体2-丙烯酰胺基十四烷磺酸,在此基础上又以改性Si O2功能单体为反应核制备了超支化疏水缔合聚合物(HBPAM),结构经红外光谱(FTIR)和核磁共振氢谱(1H NMR)表征证实。HBPAM在低浓度时主要是分子内缔合,表观黏度低,随着浓度的增加,分子内缔合逐渐变为分子间缔合,又因其独特的三维立体网状结构,溶液黏度显著增加。与梳形KYPAM相比,HBPAM在耐温抗盐及抗剪切方面有较高的优势:升温到85℃时黏度保留率为62.2%;100000mg·L-1Na Cl、150000 mg·L-1Na Cl、500 mg·L-1Mg Cl2、1000 mg·L-1Mg Cl2、500 mg·L-1Ca Cl2、1000 mg·L-1Ca Cl2时的黏度保留率分别为233.0%、132.9%、64.4%、26.1%、66.2%、15.7%;3400rpm/min剪切30s后HBPAM的黏度保留率为69.8%,比KYPAM高10.9%。在60℃烘箱中的30d老化实验证明,HBPAM比KYPAM有明显抗老化能力,尤其是在高矿化度条件下优势更明显。     

Theory of relaxation spectra and dielectric relaxation of rigid rodlike particles incorporated in a polymer network

The theory of molecular mobility and relaxation spectra is developed for rodlike particles embedded in a polymer network with allowance for the involvement of the particles in collective network dynamics through topological entanglements with network fragments. A regular cubic coarse-grained network model is used, where the motion of junctions describes the mobility of large fragments (domains) of the initial network with a size equal to the distance between adjacent rodlike particles. The involvement of the rods in collective network dynamics is taken into account by introducing an effective quasi-elastic potential acting between the rods and junctions of the coarse-grained network and preventing long-distance diffusion of the embedded particles. The viscoelastic parameters of the coarse-grained (“renormalized”) network are functions of the viscoelastic characteristics of the initial network. The relaxation time spectra are calculated as well as the frequency dependences of the dielectric loss factor of the embedded particles that possess a permanent dipole moment directed along the major axis of each rod. Depending on the ratio between the viscoelastic characteristics of the rods and the network, the frequency dependence of the dielectric loss factor may have two maxima. The high-frequency maximum corresponds to local orientational movements of particles at fixed junctions of the coarse-grained network, which correspond to the position of the domain centers in the initial network. The low-frequency maximum corresponds to movements of particles involved in large-scale dynamics of network fragments. The dependence of the dielectric loss factor on the ratio between the viscoelastic parameters of the rods and the network is studied.     

Analysis of uncertainties in polymer viscoelastic properties obtained from equilibrium computer simulations

We critically evaluate the uncertainties in the stress autocorrelation function obtained from equilibrium molecular dynamics simulation of model polymer melts. This quantity is central to evaluating transport properties, e.g., the complex modulus and the viscosity. In contrast to the intuitive expectation that simulations have to be run five to six orders of magnitude longer than the chain relaxation time to reduce uncertainties to acceptable levels, our analysis shows that the majority of the uncertainty is associated with rapidly oscillating bonded interactions. These fluctuations occur on time scales which are approximately 10(4) times shorter than the relaxation time of a chain of length 80. Consequently, the effects of these oscillations on the stress autocorrelation function can be dramatically reduced by (i) conducting long simulations (typically 10(6) times longer than the bond relaxation times or only 10(2) chain relaxation times) and (ii) by performing running averages with time windows whose time scales are much longer than these oscillations. Conducting such long simulations also allows for the accurate determination of the melt viscosity and the low-frequency complex modulus, but performing running averages do not impact these quantities since they are time integrals of the stress autocorrelation function.