单分散、大粒径聚苯乙烯微球的制备

以聚乙烯基吡咯烷酮为分散剂、偶氮二异丁腈为引发剂、醇／水混合物为分散介质进行了苯乙烯的分散聚合，讨论了初始单体浓度、分散剂用量、引发剂浓度、分散介质组成和反应温度等反应条件对所得聚合物颗粒直径和直径分布的影响．通过大量的试验，筛选出了较为理想的分散聚合的条件及配方，制备出了粒径为４８μｍ的单分散聚苯乙烯微球．然后，以分散聚合所制得的聚合物颗粒为种子，用动力学溶胀法制成了粒径增大近四倍的单分散、大粒径聚苯乙烯微球，并讨论了滴水速度和补加分散剂对溶胀的影响     

Interaction of cetyl-trimethylammonium bromide with swollen and collapsed poly(N-isopropylacrylamide) nanogel particles

The interaction of cetyl-trimethylammonium bromide (CTAB) with swollen and collapsed poly(N-isopropylacrylamide) (pNIPAM) monodisperse nanogel particles was investigated by electrophoretic mobility, dynamic light scattering, and potentiometric surfactant activity measurements. The surfactant binds to the nanogel particles as monomers in the whole CTAB concentration range and binds in the form of surfactant aggregates as well above a critical concentration (cac) in both the swollen and collapsed state of the pNIPAM. The swollen particle system is a thermodynamically stable solution. The collapsed nanogel particle system is an electrically stabilized colloid dispersion, which coagulates when the particles are near the electrically neutral state. An analytically undetectably small amount of surfactant binding (5 × 10(-7) mol/g of pNIPAM) leads to a dramatic effect on the stability of the pNIPAM nanogel system. The electrokinetic potential versus surfactant concentration functions unexpectedly strongly depend upon the temperature around the lower critical solution temperature (LCST) of the polymer, which was interpreted by the change of the polymer segment density in the surface layer of the collapsing nanogel particles.     

Investigation on the Stability of the Dispersion System of Cross-Linked Polyacrylamide Microspheres

The salt resistance, temperature resistance, and shear stability of a cross-linked polyacrylamide microsphere system are studied by microfiltration, light diffraction analysis, and optical microscopy. The results show that other conditions being equal, the particle diameter of cross-linked polyacrylamide microspheres decreases with increased NaCl concentration. When NaCl concentration is lower than 10,000 mg/L, its effect on the plugging performance of a cross-linked polyacrylamide microsphere system in regard to the nuclear pore membrane is weak in comparison with a linked polymer solution, but the former system has better salt tolerance. Particle diameter decreases with increased swelling temperature. When the swelling temperature is below 90°C, its effect on the plugging performance of the cross-linked polyacrylamide microsphere system in regard to nuclear pore membrane is weak in comparison with the linked polymer solution, but has better temperature tolerance. Particle size shows little change, with shearing rate being increased, while the shape remains about the same and the effect of shearing on the plugging performance of the cross-linked polyacrylamide microsphere dispersion system in regard to the nuclear pore membrane is weak in comparison with the linked polymer solution, but has better shear stability. The salt tolerance, temperature tolerance, and shear stability of microspheres are associated with a particular cross-linked structure.     

Effect of pH and Salt on Rheological Properties of Aerosil Suspensions

Ionic strength and pH will influence the zeta potential of suspended particles, and consequently particle interactions and rheological properties as well. In this study the rheological properties and aggregation behaviour of Aerosil particles dispersed in aqueous solutions with various pH and salt concentration were studied. The potential energy was estimated by the DLVO theory and short range hydration forces and compared to the experimentally determined zeta potential. The strongest attraction between particles occurs at the isoelectric point (pH 4) and resulted in large aggregates, which gave relatively higher values of viscosity, yield stress, moduli, and shear thinning effects. The relative viscosity as a function of volume fraction was fitted to the Krieger and Dougherty model for all the suspensions. Oscillation measurements showed that the suspensions display elastic behaviour at low pH and viscous behavior at high pH. Furthermore, suspensions with high salt content had higher storage moduli. A power law dependency of storage moduli with volume fraction could be used to indicate the interaction strength between particles.     

Swelling kinetics for a pH-induced latex-to-microgel transition

The kinetics of swelling of a series of six near-monodisperse, lightly cross-linked poly(2-vinylpyridine) latexes with mean diameters ranging from 380 to 1010 nm has been investigated by the pH jump method using a commercial stopped-flow instrument. These pH-responsive particles become substantially protonated at around pH 4.1, which leads to a rapid latex-to-microgel transition within a time scale of tens of milliseconds. The characteristic swelling time correlates linearly with the mean particle diameter, as predicted by the Tanaka equation. However, faster swelling is observed in the presence of added salt. This is contrary to the theory developed by Tanaka, which assumes that the relaxation of the polymer chains is the rate-limiting step. An alternative viewpoint, in which infusion of solvent determines the characteristic swelling time, satisfactorily explains the experimental observations and collapses most of the data, except for the largest microgels. This discrepancy is suggested to be due to the inaccurate sizing of these micrometer-sized swollen microgels by dynamic light scattering.     

Thickening effect of dispersions of ethyl acrylate-methacrylic acid copolymer prepared by different polymerization routes

Thickening of latices (particle diameters 105, 157 and 221 nm) by model dispersions of ethyl acrylate-methacrylic acid copolymer (15 wt.% of the acid) prepared by both non-seeded and seeded semicontinuous emulsion copolymerizations was investigated. Using viscometry and dynamic and steady shear measurements, we found that the thickening effect of the dispersions strongly depends on their particle structure and the intensity of interactions between the components in the system. In weakly interacting systems (lower latex concentrations, large latex particles) the thickening effect of the dispersions is controlled by effective volume fraction of swollen particles. This leads to a higher viscosity of systems thickened by more swollen (less crosslinked) particles obtained by the non-seeded process. On the other hand, in strongly interacting systems (high latex concentrations, small latex particles) lower deformability of more crosslinked particles prepared by the seeded process causes a higher flow resistance of the systems thickened by this dispersion.     

Molecular-dynamics simulation of model polymer nanocomposite rheology and comparison with experiment

The shear-rate dependence of viscosity is studied for model polymer melts containing various concentrations of spherical filler particles by molecular-dynamics simulations, and the results are compared with the experimental results for calcium-carbonate-filled polypropylene. Although there are some significant differences in scale between the simulated model polymer composite and the system used in the experiments, some important qualitative similarities in shear behavior are observed. The trends in the steady-state shear viscosities of the simulated polymer-filler system agree with those seen in the experimental results; shear viscosities, zero-shear viscosities, and the rate of shear thinning are all seen to increase with filler content in both the experimental and simulated systems. We observe a significant difference between the filler volume fraction dependence of the zero-shear viscosity of the simulated system and that of the experimental system that can be attributed to a large difference in the ratio of the filler particle radius to the radius of gyration of the polymer molecules. In the simulated system, the filler particles are so small that they only have a weak effect on the viscosity of the composite at low filler volume fraction, but in the experimental system, the viscosity of the composite increases rapidly with increasing filler volume fraction. Our results indicate that there exists a value of the ratio of the filler particle radius to the polymer radius of gyration such that the zero-shear-rate viscosity of the composite becomes approximately independent of the filler particle volume fraction.     

Mesoscale constitutive modeling of magnetic dispersions

A constitutive model for dispersions of acicular magnetic particles has been developed by modeling the particles as rigid dumbbells dispersed in a solvent. The effects of Brownian motion, anisotropic hydrodynamic drag, a steric force in the form of the Maier-Saupe potential, and, most importantly, a mean-field magnetic potential are included in the model. The development is similar to previous models for liquid-crystalline polymers. The model predicts multiple orientational states for the dispersion, and this phase behavior is described in terms of an orientational order parameter S and an average alignment parameter J; the latter is introduced because the magnetic particles have distinguishable direction due to polarity. A transition from isotropic to nematic phases at equilibrium is predicted. Multiple nematic phases-both prolate and oblate-are predicted in the presence of steady shear flow and external magnetic field parallel to the flow. The effect of increasing magnetic interparticle interactions and particle concentration is also presented. Comparisons with experimental data for the steady shear viscosity show very good agreement.     

Effect of aggregation on viscosity of colloidal suslension

This experimental study of viscosity of colloidal suspensions was performed using monodisperse polystyrene latex with particle diameter of 1.15 μm and a pH dependent negative zeta potential of up to 120 mV in aqueous solutions. The range of electrostatic repulsion between the particles was controlled by varying the concentration of potassium chloride. Suspensions under investigation were either in a stable, coagulated, or gelated, state depending on the salt concentration. Shear thinning behaviour was observed for all the samples studied. The dependence of viscosity on shear rate imposed was found to depend substantially on the salt concentration.     

pH-induced deswelling kinetics of sterically stabilized poly(2-vinylpyridine) microgels probed by stopped-flow light scattering

Near-monodisperse, sterically stabilized poly(2-vinylpyridine) (P2VP) microgels were synthesized by emulsion polymerization. These particles exhibited completely reversible pH-responsive swelling/deswelling behavior in aqueous solution. Stopped-flow light scattering was employed to investigate the kinetics of pH-induced deswelling in highly dilute dispersions. Upon a pH jump from 2 to various final solution pH values (>or=5.4), the scattered light intensity of an aqueous dispersion of a 1,960 nm microgel exhibited an abrupt initial increase, followed by a gradual decrease to the final equilibrium value. The whole microgel-to-latex deswelling process occurred over time scales of approximately 0.5-1.0 s, which is much slower than the kinetics for latex-to-microgel swelling. The microgel deswelling kinetics depends on the final pH, with a higher final pH leading to a faster rate of shrinkage. Close inspection of the deswelling kinetics during the early stages (<0.2 s) revealed that initial microgel collapse occurred within approximately 50 ms, with more rapid transitions being observed when higher final pH values were targeted. Addition of external salt significantly accelerates the kinetics of deswelling. Systematic studies of the microgel-to-latex transition for a series of six near-monodisperse P2VP particles (with swollen microgel diameters ranging from 1270 to 4230 nm) has also been investigated. The characteristic deswelling time for initial microgel collapse, tau deswell, correlated fairly well with the initial swollen microgel radius, R, in agreement with the Tanaka equation. Moreover, the collective diffusion coefficient of the gel network, D, calculated from the slope of the tau deswell- R (2) curve, was of the order of 10 (-7) cm (2) s (-1).     

Self-assembly of hydrophobic associating polyacrylamide prepared by aqueous dispersion polymerization

Abstract

Hydrophobic associating polyacrylamide (PDH) was prepared via aqueous dispersion polymerization in the presence of ammonium sulfate. In order to dissolve the polymer at high salt solution concentration, a hydrophobic precursor monomer with two cationic heads was synthesized. Upon polymerization, spherical PDH polymer particles were obtained with an average size of about 9.1?µm. The PDH was fully dispersed in water in twenty seconds, leading to a transparent gel. PDH demonstrated superior properties compared to the partially hydrolyzed polyacrylamide, for instance, regarding salt tolerance and shear resistance. Addition of a small amount of an anionic surfactant significantly affected the PDH dispersion viscosity due to the formation of mixed micelles. The viscosity slightly decreased after strong shearing (>500?s^?1), showing excellent shear resistance. These features indicate that the synthesized PDH offers a great potential for application in hydraulic fracturing and enhanced oil recovery.     

Characterization of thermo-reversible gels by means of sedimentation equilibria

This paper describes the deformation of gels in a centrifugal field leading to a continuous equilibrium. A gel is considered to be a binary mixture of cross-linked polymer and solvent and is assumed to remain isotropic during the deformation. From the equation for the osmotically effective pressure, called swelling pressure, the thermodynamic properties of a gel can be calculated. For highly swollen gels the expression of Svedberg and Pedersen is obtained. It is shown that the complete concentration dependence of the swelling pressure in the concentration range of the maximally swollen gel up to that at the cell bottom can be measured in a single equilibrium experiment. The homogeneity of weakly cross-linked gels can be examined by means of the method described. Soluble parts which are not incorporated into the polymer network can also be detected if they are present. From the swelling pressure-concentration curves it is possible to derive the thermodynamic properties of the physically crosslinked gelatin/water gels that were investigated. These gels can be described by means of a slightly modified Flory-Huggins equation with an interaction parameter χw in the weight fraction scale, which depends linearly on concentration. The interaction parameters show a dependence on concentration which is explained by an increased branching and crosslinking of the polymer with increasing initial polymer concentration of the gels. At low initial polymer concentration, the primary chains have to aggregate to build up relatively long chains between the network junctions. The static shear modulus G which can be calculated from the network term has the same order of magnitude as the real part of the complex shear modulus which is measured at low frequency.     

Solution properties of thermothickening copolymers bearing hydrocarbon end‐capped oxyethylene units

Novel thermothickening copolymers composed of acrylamide and a macromer bearing hydrocarbon end‐capped oxyethylene units were synthesized. Influences of polymer concentration, salt content, shear rate, and temperature on the solution behavior were investigated. The polymer solution exhibited shear‐thickening behavior at low‐to‐moderate shear rates (<50 s^?1), and the shear‐thickening behavior was dependent on polymer concentration, NaCl content, and temperature. With the increase of salinity, apparent viscosity of polymer solution increased dramatically (especially at low shear rates). At higher NaCl content (>20 wt %), polymer solutions became physical gel, and the apparent viscosity increased by several orders of magnitude. The polymer solutions exhibited excellent thermothickening behavior, even at the low concentration of 0.15 wt %. The results of rheological measurements showed that the storage and loss modulus were successfully fitted to a single Maxwell element at low temperature (<60 °C). © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1799–1808, 2010     

Kinetics and equilibria associated with the absorption and desorption of water and lithium chloride in an ethylene—vinyl alcohol copolymer

Sorption and desorption equilibria and kinetics for LiCl and H₂O in an ethylene—vinyl alcohol copolymer film containing 70 mole percent vinyl alcohol were investigated at 25°C. The swelling behavior of water in the polymer was characterized by vapor and liquid sorption experiments over a range of water activities. p]The effects of LiCl content on the water sorption kinetics and equilibria in the films are presented and discussed. The kinetics and mechanism of LiCl sorption have also been studied. The amount of salt sorbed into the polymeric films increases linearly with the salt concentration in the external aqueous solutions. Both the rate and the amount of sorbed water increase significantly as the LiCl content increases. p]The desorption of LiCl, previously sorbed into the polymer, was characterized for different salt loadings. The rate of fractional salt release is independent of LiCl concentration in the film. Initially, the salt release is controlled by the nearly constant-rate absorption of water. The salt release, at long times, lags behind the swelling-controlled water uptake, indicating that the salt release is not completely controlled by the water sorption and that diffusion in the swollen polymer matrix contributes significantly to the long term elution of LiCl. Independent thermal analysis experiments suggest the formation of a metal salt—poly(ethylene—vinyl alcohol) complex.     

Effect of polyethylene oxide on the viscosity of dispersions of charged silica particles: interplay between rheology, adsorption, and surface charge

In this study a systematic investigation on the adsorption of polyethylene oxide (PEO) onto the surface of silica particles and the viscosity behavior of concentrated dispersions of silica particles with adsorbed PEO has been performed. The variation of shear viscosity with the adsorbed layer density, concentration of free polymer in the solution (depletion forces), polymer molecular weight, and adsorbed layer thickness at different salt concentrations (range of the electrostatic repulsion between particles) is presented and discussed. Adsorption and rheological studies were performed on suspensions of silica particles dispersed in solutions of 10⁻² M and 10⁻⁴ M NaNO₃ containing PEO of molecular weights 7,500 and 18,500 of different concentrations. Adsorption measurements gave evidence of a primary plateau in the adsorption density of 7,500 MW PEO at an electrolyte concentration of 10⁻² M NaNO₃. Results indicate that the range of the electrostatic repulsion between the suspended particles affects both adsorption density of the polymer onto the surface of the particles and the viscosity behavior of the system. The adsorbed layer thickness was estimated from the values of zeta potential in the presence and absence of the polymer and was found to decrease with decreasing the range of the electrostatic repulsive forces between the particles. Experimental results show that even though there is a direct relation between the viscosity of the suspension and the adsorption density of the polymer onto the surface of the particles, variation of viscosity with adsorption density, equilibrium concentration of the polymer, and range of the electrostatic repulsion cannot be explained just in term of the effective volume fraction of the particles and needs to be further investigated. Received: 15 February 2000/Accepted: 26 June 2000     

Temperature influence of nonionic polyethylene oxide and anionic polyacrylamide on flocculation and dewatering behavior of kaolinite dispersions

Nonionic polyethylene oxide (PEO) and anionic polyacrylamide (PAM) flocculation of kaolinite dispersions has been investigated at pH 7.5 in the temperature range 20-60 degrees C. The surface chemistry (zeta potential), particle interactions (shear yield stress), and dewatering behavior were also examined. An increase in the magnitude of zeta potential of kaolinite particles, in the absence of flocculant and at a fixed PEO and PAM concentration, with increasing temperature was observed. The zeta potential behavior of the flocculated particles indicated a decrease in the adsorbed polymer layer thickness, while at the same time, however, the adsorbed polymer density showed a significant increase with increasing temperature. These results suggest that polymer adsorption was accompanied by temperature-influenced conformation changes. The hydrodynamic diameter and supernatant solution viscosity of both polymers decreased with increasing temperature, consistent with a change in polymer-solvent interactions and conformation, prior to adsorption. The analysis of the free energy (DeltaG(ads)) of adsorption showed a strong temperature dependence and the adsorption process to be more entropically than enthalpically driven. The polymer conformation change and increased negative charge at the kaolinite particle surface with increasing temperature resulted in decreased polymer bridging and flocculation performance. Consequently, the shear yield stress and the rate and the extent of dewatering (consolidation) of the pulp decreased significantly at higher temperatures (>40 degrees C). The temperature effect was more pronounced in the presence of PEO than PAM, with 40 and 20 degrees C indicated as the optima for enhanced performance of the latter and former flocculants, respectively. The results demonstrate that a temperature-induced conformation change, together with polymer structure type, plays an important role in flocculation and dewatering behavior of kaolinite dispersions.     

液化膜对胶体悬浮液粘度的影响

Viscosity is one of the most important properties of colloids in mixing, transportation, stabilization, energy consumption, and so on. According to Einstein‘s viscosity equation, the viscosity of a colloidal dispersion increases with the increase of particle concentration. And the equation can be applicable to all micro-particle dispersions, because the effect of solvation films coated on particles can be neglectable in that case. But with the decrease of particle size to nano-scale, the formation of solvation films on nano-particles can greatly affect the viscosity of a dispersion, and Einstein‘s equation may not be applicable to this case. In this work, one kind of micro-size silica particle and two kinds of nano-size silica particles were used to investigate the effect of solvation films on dispersion viscosity, dispersed in water and ethyl alcohol solvents, respectively. The results of theoretical calculation and experimental investigation show that the increase of viscosity is contributed from solvation films by more than 95 percent for nano-particle dispersions, while less than 10 percent for micro-particle dispersions.     

The synthesis of silica and silica-ceria, core-shell nanoparticles in a water-in-oil (W/O) microemulsion composed of heptane and water with the binary surfactants AOT and NP-5

In a previous work [1] we showed that the swelling behavior of carboxylated core-shell particles (PS-PC) can be modified using a specific sample preparation route or favoring the hydrophobic attractive interaction by other way, i.e. controlling the temperature. In that paper, we found that the swelling was promoted in those particles which were initially in a highly swollen state (pH?10) while it was hindered for those particles which were not previously in this trigger pH. In this work, we present a discussion of the salt-induced swelling of the same carboxylated core-shell system (PS-PC) with two tuned swelling behaviors: the former, called A-2, exhibits promoted swelling while in the latter, called B-1, the swelling is greatly suppressed because of a compact conformation of the polymer shell is induced [1]. Good agreement between experimental, numerical and theoretical results at all pH values is obtained for promoted particles (A-2). On the other hand, the salt-induced swelling behaviors shown by hindered particles (B-1) corroborate that polymer restructuring includes assembly among ionic groups which affect their ionization degree and also the electrosteric interaction between particles. Finally, the salt-induced swelling behavior shown by the B-1 system at pH 8.6 resembles the Pincus regime predicted by scaling theory.     

Cationic and anionic poly(N-isopropylacrylamide) based submicron gel particles: electrokinetic properties and colloidal stability

A cationic and an anionic poly(N-isopropylacrylamide) (poly(NIPAM)) microgel latex were synthesized via batch radical polymerization under emulsifier-free conditions. The hydrodynamic properties, colloidal stability, and electrokinetic characteristics of these two samples were studied. The hydrodynamic particle size variation was discussed by considering the effect of salinity and temperature on the shrinkage of the thermally sensitive polymer domains. The colloidal stability also depended on temperature and electrolyte concentration. A stability diagram with two well-defined domains (stable and unstable) was obtained. The flow from one domain to the other was fully reversible due to the peculiar (de)hydration properties of the polymer. The electrokinetic behavior, which depends on electrical and frictional properties of the particles, was analyzed via electrophoretic mobility measurements. Results were discussed by considering both the particle structure dependence on temperature and salinity, and the electric double layer compression. In addition, the electrophoretic mobility data were analyzed using Ohshima's equations for particles covered by an ion-penetrable surface charged layer, as well as using another simpler equation for charges located on a hydrodynamic equivalent hard sphere. Differences between the properties of both latexes were justified by the presence of a hydrophilic comonomer, aminoethyl methacrylate hydrochloride (AEMH), in the cationic microgel.