Small-angle neutron scattering study of adsorbed pluronic tri-block copolymers on laponite

The adsorption of selected poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) tri-block copolymers on synthetic clay particles (laponite) has been investigated. The adsorbed amount and distribution of polymer was determined as a function of relative block composition and size, using the technique of contrast variation small-angle neutron scattering. The pluronic molecules appear to adsorb via a preferential segregation of hydrophobic PPO segments at the surface, with hydrophilic PEO segments dangling into solution. The effect of the PPO segments is substantial with large increases in adsorbed amount and layer thickness as the anchor fraction decreases/PEO chain length increases. This is in direct contrast to the behavior observed for PEO homopolymer adsorption (of much higher molecular weights) where the adsorbed amount and layer thickness are smaller and change little with molecular weight.     

Dynamic light scattering studies of poly(ethylene oxide) adsorbed on Laponite: layer conformation and its effect on particle stability

Dynamic light scattering has been used to determine the hydrodynamic thickness of poly(ethylene oxide) (PEO) adsorbed on synthetic anisotropic clay particles (Laponite) as a function of molecular weight. The layer thicknesses, and their increase with molecular weight, indicate that the conformation of the adsorbed layer is very compact and is much smaller than those normally observed for polymer adsorption on flat interfaces. The aggregation kinetics of the polymer coated particles in 5 mM NaCl was analyzed in a quantitative manner, revealing that the potential barrier to aggregation is strongly enhanced when polymer is present.     

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     

Adsorption of non-ionic surfactants on hydrophobic silica particles and the stability of the corresponding aqueous dispersions

The temperature stability of aqueous dispersions of hydrophobic monodisperse silica particles stabilized with nonionic surfactants has been investigated. Adsorption isotherms in conjunction with surface tension measurements showed that the surfactant formed a monolayer on the surface of the particles, where the adsorbed amount depended on the molecular weight of the ethylene oxide headgroup. The temperature stability of these dispersions has been measured by a standard turbidimetric technique and visual observations in terms of their critical flocculation temperature (CFT). Parameters controlling the CFT of the individual dispersions stabilized with a monolayer of surfactant include the thickness of the steric layer, the particle size, and the volume fraction of the particles. Calculations show that the van der Waals attraction between the particles with adsorbed polymer layers increases as the temperature of the dispersion increases, and this largely accounts for the observed CFT behavior.     

Intercalation of solvent and polymer in galleries of mobile clay platelets by a Monte Carlo simulation

The intercalation of solvent particles and polymer chains of concentration C_w = 0.2 and C_p = 0.2, respectively, in a layer of (4) clay platelets is studied by a Monte Carlo simulation on a cubic lattice. Polymer chains and platelets are modeled by bond fluctuations. Besides the excluded volume, a set of polymer-clay (cs) and solvent-clay (ws) interactions with (i) cs = 1, ws = −2, (ii) cs = −2, ws = 1 and (iii) cs = ws = −2 are considered. The global dynamics of platelets is constrained due to the presence of three components, i.e., solvent, polymer, and platelets, which retain their interstitial spacing with well-defined galleries. Intercalation of solvent particles and polymer chains (low molecular weight) occurs with their attractive interaction with the platelets, which further reinforces the layered clay morphology. The density profiles of the solvent particles are similar to previous studies with platelets in a mobile solvent. The density profile of polymer chains differs considerably from the platelets in a polymer matrix alone, particularly with its attractive interaction (ii). For the same attractive interaction of solvent and polymer chains with the clay platelets (iii), the solvent particles (the smallest constituents) intercalate the fastest in the clay galleries, whereas the intercalation of polymer chains decreases with their molecular weight. The polymer density profiles, both longitudinal (x) and transverse (y), show maxima peaks around outer platelets (surface) of the layer and decay sharply both in the adjacent galleries and in the bulk. The amplitude of oscillation in the transverse density profiles, a measure of the degree of intercalation, decreases with increasing molecular weight of the polymer. The intercalation of the polymer is driven by its attractive interaction at the low molecular weight, but reduces considerably at high molecular weight because of both entanglement and larger radius of gyration. Variations of the gyration radius of the diffusing polymer chains with molecular weight and interaction with the clay are consistent with the results of their corresponding density profiles. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2487–2500, 2009     

Effect of amount of monomer on radiation-induced polymerization of styrene adsorbed on silica gel

The effect of amount of monomer on radiation-induced polymerization of styrene adsorbed on silica gel was investigated with the monomer amounting from less than monolayer adsorption to more than the equilibrium adsorption. The rate of graft polymerization and the molecular weight of the polymer changed with the amount of monomer adsorbed on silica gel. Maximum grafting efficiency was obtained at monolayer adsorption. The molecular weight of graft polymer was higher than that of homopolymer in both radical and cationic polymerizations, and the ratio in molecular weight of graft polymer to that of homopolymer tends to be unity with increasing amount of adsorbed monomer. These results can mainly be explained in terms of the number of initiating species (radical and cation) that change in relation to the amount of adsorbed monomer. Propagation and termination change with amount of adsorbed monomer in relation to the molecular mobility of adsorbed monomer. A very high-molecular-weight graft polymer is formed only with a small amount of adsorbed monomer in the initial stage. The grafting percent with a large amount of adsorbed monomer increased after most of the monomer has been polymerized. Secondary effect of radiation on the graft and homopolymers due to energy transfer from silica gel is suggested from the complicated phenomena in the later stage of the reaction.     

The effect of the amphiprotic nature of human hair keratin on the adsorption of high charge density cationic polyelectrolytes

Adsorption of the cationic polymers poly(methacrylamidopropyltrimethyl ammonium chloride) (PMAPTAC) and poly(1,1-dimethylpiperidinium-3,5-diallylmethylene chloride) (PDMPDAMC) on human hair was studied by measurements of the amount of polymer adsorbed and by the streaming potential method. Results reflect the amphoteric nature of the keratin surface and show that the excess of anionic sites at pH values above 4 is the main driving force for the adsorption of cationic polyelectrolytes. Lowering the pH below 4 or addition of neutral salt (KCl) reduces the amount of adsorbed polymer. It was shown that the adsorption of cationic polymer in the concentration range 0.01 to 0.1 % and at neutral pH reverses the overall character of the surface from anionic to cationic. Keratin fibers modified in this manner do not exhibit amphoteric character and bear excess positive charge in the pH range 2–9.5. The value of the amount of the polymer adsorbed at saturation concentration (2 mg/g) as well as the lack of molecular weight effect in the range (5 · 10⁴ – 10⁶) on the amount of polymer adsorbed suggest that polymer chains adopt a rather extended conformation on the fiber surface. Some data concerning the formation of a complex between adsorbed cationic polymer and anionic detergents or polyelectrolytes are also presented.     

Stabilization of magnetorheological suspensions by polyacrylic acid polymers

This work is devoted to the synthesis and stabilization of magnetorheological suspensions constituted by monodisperse micrometer-sized magnetite spheres in aqueous media. The electrical double-layer characteristics of the solid/liquid interface were studied in the absence and presence of adsorbed layers of high molecular weight polyacrylic acids (PAA; Carbopol). Since the Carbopol-covered particles can be thought of as "soft" colloids, Ohshima's theory was used to gain information of the surface potential and the charge density of the polymer layer. The effect of the pH of the solution on the double-layer characteristics is related to the different conformations of the adsorbed molecules provoked by the dissociation of the acrylic groups present in polymer molecules. The stability of the suspensions was experimentally studied for different pH and polymer concentrations, and in the absence or presence of a weak magnetic field applied. The stability of the suspensions was explained using the classical DLVO theory of colloidal stability extended to account for hydration, steric, and magnetic interactions between particles. Diagrams of potential energy vs interparticle distance show the predominant effect of steric, hydrophilic/hydrophobic, and magnetic interactions on the whole stability of the system. The best conditions to obtain stable suspensions were found when strong steric and hydrophilic repulsions hinder the coagulation between polymer-covered particles, simultaneously avoiding sedimentation by the thickening effect of the polymer solution. When a not too high molecular weight PAA was employed in a low concentration, the task of a long-time antisettling effect compatible with the desired magnetic response of the fluid was achieved.     

Polymeric stabilized emulsions: steric effects and deformation in soft systems

Polymeric stabilizers are used in a broad range of processes and products, from pharmaceuticals and engine lubricants to formulated foods and shampoos. In rigid particulate systems, the stabilization mechanism is attributed to the repulsive force that arises from the compression of the polymer coating or "steric brush" on the interacting particles. This mechanism has dictated polymer design and selection for more than thirty years. Here we show, through direct measurement of the repulsive interactions between immobilized drops with adsorbed polymers layers in aqueous electrolyte solutions, that the interaction is a result of both steric stabilization and drop deformation. Drops driven together at slow collision speeds, where hydrodynamic drainage effects are negligible, show a strong dependence on drop deformation instead of brush compression. When drops are driven together at higher collision speeds where hydrodynamic drainage affects the interaction force, simple continuum modeling suggests that the film drainage is sensitive to flow through the polymer brush. These data suggest, for drop sizes where drop deformation is appreciable, that the stability of emulsion drops is less sensitive to the molecular weight or size of the adsorbed polymer layer than for rigid particulate systems.     

Coupling between polymer adsorption and colloidal particle aggregation

Studies of the adsorption of high molecular weight polymers on colloidal latex and silica particles and their subsequent flocculation were carried out. Neutral polyethylene oxide samples with both a narrow and a broad molecular weight distribution were used together with low charged cationic copolymers. The influence of the particle concentration and polymer dose on the flocculation were systematically investigated under quiescent conditions.Equilibrium bridging only occurred with polyelectrolyte, even in very dilute suspensions, at high particle coverage. In contrast to this, non-equilibrium bridging occurred with both neutral polymer and polyelectrolytes but only for more concentrated suspensions and small amounts of adsorbed polymer. Polymer adsorption in dilute suspensions, which did not show particle aggregation was measured an electrophoretic technique. In more concentrated suspensions, where flocculation takes place, we found that aggregation prevents further polymer adsorption and induces both an excluded volume and a surface effect. The consequences on the shape of the isotherms differ according to the aggregation mechanism.A significant decrease of the amount, , of adsorbed polymer is observed with non-equilibrium bridging. When both mechanisms simultaneously contribute to the aggregation, the value of depends on their relative importance. In the intermediate range of copolymer dose their respective contributions are critically sensitive to the details of the mixing step and stirring, leading to non reproducible experimental results.     

Charging and aggregation of positively charged latex particles in the presence of anionic polyelectrolytes

Charging behavior and colloidal stability of amidine latex particles are studied in the presence of poly(sodium styrene sulfonate) (PSS) and KCl. Detailed measurements of electrophoretic mobility, adsorbed layer thickness, and aggregation (or coagulation) rate constant on varying the polymer dose, molecular mass of the polymer, and ionic strength are reported. Polyelectrolyte adsorption leads to the characteristic charge reversal (or overcharging) of the colloidal particles at the isoelectric point (IEP). In accordance with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, uncharged particles tend to aggregate because of van der Waals attraction, whereas charged particles are stabilized by electrical double layer repulsion. Attractive patch-charge interactions originating from the laterally inhomogeneous structure of the adsorbed polymer substantially decrease the suspension stability or even accelerate the aggregation rate beyond diffusion control. These electrostatic non-DLVO forces become progressively important with increasing molecular mass of the polymer and the ionic strength of the solution. At higher polymer dose of typically 10 times the IEP, one observes the formation of a saturated layer of the adsorbed polymer with a thickness of several nanometers. Its thickness increases with increasing molecular mass, whereby the layer becomes increasingly porous. This layer does not seem to be involved in the suspension stabilization, since at such high polymer doses the double layer repulsion has attained sufficient strength to stabilize the suspension.     

Polymer adsorption and its effect on the stability of hydrophobic colloids

Summary As a first step in a study on the interaction between polymers and hydrophobic colloids we investigated in detail the adsorption of polyvinyl alcohol (PVA) on aqueous silver iodide sols. The adsorption is irreversible. Adsorption isotherms are of the highaffinity type. The amount adsorbed increases with molecular weight and with the fraction of acetate groups in the PVA chain. The effective thickness of the adsorbed layer was determined viscosimetrically and independently checked by an electrophoretic method. Double layer studies enabled the determination of the occupancy of the first layer on the surface by polymer segments. It was found that even at maximal coverage with polymer this layer is still about 30% void. The combination of these data enabled the assessment of the polymer segment distribution. It was found that with not too low coverages the distribution isHoeve- like. The distribution, thus obtained reflects itself in the flocculation of AgI sols by PVA.Presented at the 25th Colloid-Meeting in Munich, October 13–15, 1971     

Evaluation of the Dispersing Properties of Polycarboxylate-Type Superplasticizers with Different Molecular Weight in Cement Systems

In order to evaluate the dispersing properties of polycarboxylate-type superplasticizers (PCs) with different molecular weight in cement pastes, PCs with different molecular weight (low, medium, high) were synthesized, and used as superplasticizer for cement suspensions. The effect of molecular weight of PC on the zeta potential, adsorption, fluidity of the corresponding cement suspensions was investigated systematically, and total interparticle potential energy between particles was calculated. The results show that, the higher molecular weight of PC, the larger adsorption amount of PC. PC with medium molecular weight presents a better dispersing property than PC with high molecular weight, while PC with low molecular weight falls in between. The maintaining dispersing ability of PC for cement particles is weakened gradually with the increasing of molecular weight. The changing rule of total interparticle potential energy produced by PC is in accordance with the changing rule of dispersing property of PC in cement pastes. This indicates that the calculated results of total interparticle potential energy further support the explanation of dispersing property difference of PC in theory. This article uses the sum of electrostatic energy and steric hindrance potential as the total interparticle potential energy to evaluate the dispersing property of PC for the first time, which is meaningful for evaluation of dispersing property of polymer dispersant adsorbed on particle surfaces.     

pH dependent stability of aqueous suspensions of graphene with adsorbed weakly ionisable cationic polyelectrolyte

Stable graphene suspensions were prepared through ultrasonic exfoliation followed by surface modification with the cationic polyelectrolyte poly(ethyleneimine) (PEI). The stability of the suspensions was found to be dependent upon the pH of the solution and the molecular weight of the PEI adsorbed. For the graphene sheets with adsorbed PEI with a molecular weigh of 600 Da, the particles were stabilised through an increased electrostatic repulsion at low pH inferred from in an increase in the measured zeta potential of the particles. However, the graphene with higher molecular weight PEI (70 kDa) was stable over a comparatively larger pH range through a combination of electrostatic repulsion at low pH and steric repulsion at elevated pH. Thus, solution conditions allowing the control of the colloidal sized graphene particles can be easily tuned through judicious management of solution conditions as well as polymer layer properties.     

Effect of polycarboxylate ether comb-type polymer on viscosity and interfacial properties of kaolinite clay suspensions

The interactions between kaolinite clay particles and a comb-type polymer (polycarboxylate ether or PCE), so-called PCE super-plasticizer, were investigated through viscosity and surface forces measurements by a rheometer and a Surface Forces Apparatus (SFA). The addition of PCE shows a strong impact on the viscosity of concentrated kaolinite suspensions in alkaline solutions (pH=8.3) but a weak effect under acidic conditions (pH=3.4). In acidic solutions, the high viscosity measured is attributed to the strong electrostatic interaction between negatively charged basal planes and positively charged edge surfaces of clay particles. Under the alkaline condition, the suspension viscosity was found to first increase significantly and then decrease with increasing PCE dosages. The results from surface forces measurement show that PCE molecules at low dosages can bridge the kaolinite particles in the concentrated suspensions via hydrogen bonding, leading to the formation of a kaolinite-PCE "network" and hence an increased suspension viscosity. At high PCE dosages, clay particles are fully covered by PCE molecules, leading to a more dispersed kaolinite suspensions and hence lower suspension viscosity due to steric repulsion between the adsorbed PCE molecules. The insights derived from measuring viscosity and interfacial properties of kaolinite suspensions containing varying amount of comb-type super-plasticizer PCE at different pH provide the foundation for many engineering applications and optimizing industrial processes.     

Functional colloidal particles stabilized by layered silicate with hydrophilic face and hydrophobic polymer brushes

In this study, we describe a new strategy for producing narrowly dispersed functional colloidal particles stabilized by a nanocomposite with hydrophilic clay faces and hydrophobic polystyrene (PS) brushes on the edges. This method involves preparation of polymer brushes on the edges of clay layers and Pickering suspension polymerization of styrene in the presence of the nanocomposites. PS brushes on the edges of clay layers were prepared by atom transfer radical polymerization. X‐ray diffraction and thermogravimetric analysis results indicated that PS chains were grafted to the edges of clay platelets. Transmission electron microscope results showed that different morphologies of clay‐PS particles could be obtained in different solvents. In water, clay‐PS particles aggregated together, in which PS chains collapsed forming nanosized hydrophobic domains and hydrophilic clay faces stayed in aqueous phase. In toluene, clay‐PS particles formed face‐to‐face structure. Narrowly dispersed PS colloidal particles stabilized by clay‐PS were prepared by suspension polymerization. Because of the negatively charged clay particles on the surface, the zeta potential of the PS colloidal particles was negative. Positively charged poly(2‐vinyl pyridine) (P2VP) chains were adsorbed to the surface of PS colloidal particles in aqueous solution at a low pH value, and gold nanoparticles were prepared in P2VP brushes. Such colloidal particles may find important applications in a variety of fields including waterborne adhesives, paints, catalysis of chemical reactions, and protein separation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1535–1543, 2009     

Self-assembled monothiol-terminated hyperbranched polyglycerols on a gold surface: a comparative study on the structure, morphology, and protein adsorption characteristics with linear poly(ethylene glycol)s

Monothiol-terminated hyperbranched polyglycerols (HPGs) were synthesized by ring-opening polymerization of glycidol from partially deprotonated 2,2'-dihydroxyethane disulfide as the initiator and subsequent reduction of the disulfide group. Two molecular weights of HPG thiols were synthesized. The molecular weights of the polymers were determined by MALDI-TOF analysis, and the presence of thiol was verified by Ellman's assay. The self-assembly of HPG thiols on gold was studied and compared with that of linear poly(ethylene glycol) (PEG) thiols utilizing various surface analysis techniques. Monothiol-functionalized HPGs readily adsorbed to a gold surface and formed highly uniform thin films on the surface. The graft density of the HPG layer decreased with an increase in the molecular weight of the polymer. The amount of polymer on the surface increased with increasing incubation concentration and saturated above 6 g/L polymer concentration. Generally, HPG thiols gave lower graft density compared to linear PEG thiols of similar molecular weight. AFM morphological studies showed that HPG thiols form more uniform and smooth surface films compared to PEG thiols. Incubation of a polymer-coated surface (HPG thiols and PEG thiols) with bovine serum albumin and immunoglobulin showed that the high molecular weight hyperbranched polyglycerol was more resistant to protein adsorption than linear PEG of similar molecular weight or lower molecular weight HPG. The protein adsorption decreased with increasing graft density of the HPG chains on the surface. Our results show that HPG could be a good alternative to PEG in the development of nonfouling functional surfaces.     

Adsorption of modified dextrins on talc: effect of surface coverage and hydration water on hydrophobicity reduction

The adsorption of three modified dextrins on the basal plane of talc has been studied using in situ tapping mode atomic force microscopy (TMAFM). The images have been used to determine the layer thickness and coverage of the adsorbed polymers. Adsorption isotherms of the polymers on talc particles were also determined using the depletion technique. Values of the adsorbed amount at equilibrium were compared with the volume of adsorbed material as determined using in situ TMAFM, revealing the presence of significant amounts of hydration water in the adsorbed layer structure. This deduction was confirmed by comparing in and ex situ TMAFM images of the adsorbed dextrins. The effect of layer thickness, coverage, and hydration water content on the contact angle of talc particles treated with polymer was investigated using the Washburn method and the equilibrium capillary pressure (ECP) method. Distinct correlations were observed between adsorbed layer properties and the measured contact angles, with the ECP measurements especially highlighting the effect of the adsorbed polymer layer hydration water. The implications for the performance of the modified dextrins in flotation are discussed.     

聚丙烯酸在纳米TiO2表面吸附行为的研究

讨论了聚丙烯酸在纳米TiO₂水悬浮体系中的吸附行为.红外光谱分析和吸附实验结果表明,纳米TiO₂通过氢键吸附PAA.PAA吸附量随着浓度的升高而增大直至饱和吸附量,且分子量越大,饱和吸附量越大.pH值增大,则饱和吸附量减小.在相同条件下,表面吸附层的厚度随PAA分子量、浓度和pH值增大而增大.这是由PAA在颗粒表面构型的变化所致.吸附PAA后的纳米TiO₂的表面电荷密度和ζ电位发生变化,pHiep值向低值方向移动.表面吸附自由能的计算结果说明,PAA在纳米TiO₂表面的吸附是自发过程.     

Hydrogel composites of neutral and slightly charged poly (acrylamide) gels with incorporated bentonite. Interaction with salt,linear polyelectrolytes and ionic surfactants

The swelling behavior in the solutions of sodium chloride, linear polyelectrolytes and ionic surfactants of the composites based on clay mineral bentonite (BENT) embedded in neutral and slightly charged poly(acrylamide) (PAAm) gels is studied. Negatively charged flat clay particles incorporated into polymer gel adsorb oppositely charged surfactant and linear polyelectrolyte and attract the charged chains of cationic polymer matrix. The results of SAXS study manifest the formation of lamella structure of the cationic surfactant adsorbed by the clay plates. The gels loaded with the clay show a strong response to changes in the nature and the composition of the ionic environment.