A light scattering study of concentrated dispersions in nonaqueous media

Interparticle interactions between colloidal poly(methylmethacrylate) particles stabilised by poly(12-hydroxystearic acid) in non-aqueous media have been investigated using time-average light scattering. The problem of multiple scattering was avoided by using a binary mixture of solvents such that the colloidal particles were optically matched. This enabled the static structure factor to be measured and from the small scattering vector expansion the osmotic pressure to be determined. The softness of the pairwise interaction potential has been exposed using the Chandler-Weeks-Anderson perturbation theory. However, it is concluded that dispersions of the type studied can be reasonably well approximated by a hard sphere fluid model.     

Significance of glass transition temperature to polymer latex stabilisation by nonionic surfactants

Equilibrium colloid stability measurements with nonionic surfactant (C₁₂E₈) stabilised polybutyl methacrylate (PBMA) latex dispersions indicate a sudden temperature induced destabilisation coinciding with the glass transition temperature,Tg, of the polymer. In control experiments with polystyrene latex particles of similar size, for whichTg was not approached, the flocculation temperature was significantly higher. The effect is interpreted in terms of a reduced adsorbed layer thickness aboveTg caused by mixing of part of the surfactant molecule with the polymer. This interpretation is supported by DSC, elastic modulus and mechanical damping measurements on films made from dispersions of the same latex containing commercial nonionic surfactants. These measurements indicate a shift inTg in the presence of surfactant consistent with partial penetration of the polymer surface by the surfactant. In addition, C₁₂E₈ adsorption measurements show increased adsorption (or absorption) onto PBMA aboveTg which is irreversible on both dilution and temperature reduction.     

Microgel particles containing methacrylic acid: pH-triggered swelling behaviour and potential for biomaterial application

pH-responsive microgels are crosslinked polymer particles that swell when the pH approaches the pK(a) of the ionic monomer incorporated within the particles. In recent work from our group it was demonstrated that the mechanical properties of degenerated intervertebral discs (IVDs) could be restored to normal values by injection of poly(EA/MAA/BDDA) (ethylacrylate, methacrylic acid and butanediol diacrylate) microgel dispersions [J.M. Saunders, T. Tong, C.L. Le Maitre, T.J. Freemont, B.R. Saunders, Soft Matter 3 (2007) 486]. In this work we report the pH dependent swelling and rheological properties of poly(MMA/MAA/EGDMA) (methylmethacrylate and ethyleneglycol dimethacrylate) microgel dispersions. This system was investigated because it contains monomers that are already used as biomaterials. The poly(MMA/MAA/EGDMA) particles exhibit pH-triggered volume swelling ratios of up to ca. 250. The swelling onset for these particles occurs at pH values greater than ca. 6.0. A pK(a) for these particles of ca. 6.7 is consistent with titration and swelling data. Fluid-to-gel phase diagrams for concentrated poly(MMA/MAA/EGDMA) dispersions were determined as a function of polymer volume fraction and pH using tube-inversion measurements. The rheological properties for the gelled microgel dispersions were investigated using dynamic rheology measurements. The elastic modulus data for the poly(MMA/MAA/EGDMA) gelled dispersions were compared to data for poly(EA/MAA/BDDA) microgels. A similar pH-dependence for the elastic modulus was apparent. The maximum elastic modulus was achieved at a pH of about 7.0. The elastic modulus is an exponentially increasing function of polymer volume fraction at pH 7.0. Preliminary cell challenge experimental data are reported that indicate that gelled poly(MMA/MAA/EGDMA) microgel dispersions are biocompatible with cells from human intervertebral discs. However, the duration over which these experiments could be performed was limited by gradual redispersion of the gelled microgel dispersions. Based on the results presented it is suggested that poly(MMA/MAA/EGDMA) microgel would be a good candidate as a biomaterial for structural support of soft connective tissues.     

Dynamic light scattering from spherical particles

The dynamic light scattering behavior of poly(butylmethacrylate) PBMA microgels and of kappa-casein micelles is compared with that from hard sphere latex particles. The latex particles and the kappa-casein micelles exhibited a single exponential decay of the time correlation function (TCF). For the microgels, progessively stronger deviations from a single exponential were observed as the scattering angle was made larger. These deviations are interpreted as being the result of internal modes of motion. From measurement of the first cumulant of the TCF, extrapolated towards zero angle, the translational diffusion coefficients D were determined, and the hydrodynamically effective radii were calculated via the Stokes-Einstein relationship. The ratio of the radius of gyration to the hydrodynamic radius was found to be?=0.775+0.012 for the latex particles, in good agreement with theory. The microgels, however, exhibit much lower?-parameters of 0.49 to 0.58, while the kappa-casein micelles showed the opposite behavior with values between 1.1 and 2.5. The results are interpreted on the basis of the DebyeBueche and Deutsch-Felderhof theory for porous spheres.     

Temperature-Triggered Modification of Polymer- Solvent Interactions: From Fluid-to-Gel Transitions to Particle Capture

Temperature-responsive polymers are of considerable interest in the literature. In this work the ability to combine temperature-responsive polymer-solvent interactions with architectural control to achieve a range of macroscopic effects is considered. The first part of the work considers poly(DEA) (N,N-dieth ylacrylamide) microgel particles. The particles exhibit temperature-triggered particle collapse at temperatures more than ca. 27 °C. As a consequence concentrated temperature-responsive microgel dispersions change from gels to fluids when heated. The opposite effect is observed when dispersions or emulsions are stabilised by temperature-responsive polymer surfactants. Recent results involving a gel-forming castor oil-in-water emulsion are considered. The gelled emulsion releases a model drug (lidocaine) according to first-order kinetics. We extend the principle of temperature-triggered control of particle-surface interactions to test a new approach for immobilising particles on surfaces. The method consists of electrodepositing Laponite particles onto a carbon surface, grafting of poly(NIPAM) (N-isopropylacrylamide) onto the deposited particles and then increasing the temperature of the modified surface to trigger capture of dispersed polystyrene particles. This new approach uses chemistry that is potentially applicable to any conductive surface.     

Hydrocarbon dispersions of acrylic microspheres with polar surface functionality

Submicron and micron sized particles containing a crosslinked core and a polar shell were prepared by 3-stage nonaqueous dispersion [NAD] polymerization in an aliphatic hydrocarbon medium. When a poly (12-hydroxystearic acid) [PHS] comb stabilizer was used in all three stages, the particles produced were spherical, submicron in size, and had a relatively broad size distribution. If the monomer mixture in the third stage contained relatively large amounts of butyl acrylate, stable dispersions of sponge-like aggregates, 3–5 μm in size, were obtained. When butyl rubber was employed as a stabilizer precursor in the seed formation step, the shape of the final particles depended upon whether a crosslinker was employed in the second stage polymerization. When a crosslinker was present, the second-stage particles and the subsequent third-stage particles had a shrunken, raisin-like appearance. When no crosslinker was present, spherical particles were obtained. In both cases, the resulting third-stage particles were easily dispersed in water containing a small amount of amine base, indicating that ? COOH and ? OH groups are located at the surface (or in a shell at the surface) in these NAD particles. © 1994 John Wiley & Sons, Inc.     

Reversible Thermal Flocculation of Aqueous α-Fe2O3Dispersions Stabilized with Novel Poly(Vinyl Ether) Block Copolymers

The colloidal stability with respect to temperature of aqueous α-Fe₂O₃dispersions stabilized with novel poly(vinylmethylether)-block-poly(vinyloxy-4-butyric acid) diblock copolymers was studied by rheological and turbidimetric measurements. Adsorption of the block copolymers provides the particles with a steric barrier due to the nonadsorbing poly(vinylmethylether) (PVME) blocks. Rheological measurements on concentrated (15 vol %) dispersions showed that flocculation occurred near the θ temperature of PVME in water. For the turbidimetric analysis, the fraction of small particles was used at a very low concentration. With these dispersions, flocculation was found at higher temperatures, corresponding to the lower critical solution temperature of the block copolymer used. The particles spontaneously redispersed when a heated and flocculated dispersion was cooled to below the flocculation temperature.     

Fluorescence quenching studies on poly(methyl methacrylate) particles : Matching of the refraction index of the stabilizer phase to that of the solvent with added carbon disulphide

Detailed information about the interior morphology of sterically stabilized colloid particles ought to be accessible from light-scattering measurements when the index of refraction of the stabilizer layer is matched to that of the diluent. When carbon disulphide is added to dispersions of poly(methyl methacrylate) (PMMA) latex particlesstabilized with poly(12-hydroxystearic acid) (PHSA) is dodecane, the scattered light intensity varies continuously with time; a permanent match of indices of refraction in this system is not achieved. Fluorescence quenching experiments with CS₂ as quencher were performed on similar particles containing 0.02 mol% anthracene groups covalently bound to the PMMA chains in order to obtain information about CS₂ swelling of these particles. Time-dependent quenching experiments indicate rapid panetration of CS₂ (< 10 min) into the 150-nm diameter particles. Stern-Volmer analysis of steady-state CS₂ quenching data yielded results typical for polymer systems containing fluorophores in a distribution of environments; the particle-bound anthracenes are not all equally accessible to quencher. Nevertheless, all the anthracene labels are quenched to some extent. It is concluded that within 10 min after exposure, CS₂ penetrates throughout the entire volume of the particles.     

Preparation and characterization of cationic pH-sensitive SiO2/polymer core-shell nanoparticles with amino groups in the shell

Silica/poly(styrene-N,N′-dimethylaminoethyl methacrylate) (SiO₂/P(St-DMAEMA)) cationic pH-responsive core-shell particles with a narrow size distribution and diameter of less than 200 nm were synthesized by emulsion polymerization. The effects of the St/DMAEMA molar ratio, SiO₂ core size, monomer amount, and cross-linking degree on the morphology and pH-responsiveness of the core-shell particles were investigated by transmission electron microscopy, dynamic light scattering, and conductometric titration. The results showed that core-shell particles with only one SiO₂ core could be obtained when the cross-linker divinyl benzene (DVB) was used, and the diameter of the core-shell particles increased with the size of the SiO₂ core and the total amount of monomer. It was observed that the amount of surface amino groups, zeta potential, and volume swelling ratio of the core-shell particles were significantly affected by the St/DMAEMA molar ratio, and a high volume swelling ratio was achieved at pH 4 and a DVB content of 3 mol%. The zeta potential was observed to be a function of pH, and the particles were positively charged when the pH value was below approximately 7.2.     

The Properties of Polymer-Containing BeO Dispersions in an Acetone–Chloroform Binary Medium

The properties of Newtonian and concentrated (non-Newtonian) BeO dispersions in solutions of acidic poly(vinyl acetate-co-vinyl chloride) in binary mixtures of chloroform (a Lewis acid) and acetone (a Lewis base) of different compositions were studied. In contrast to systems characterized by strong acid–base interactions at the particle–medium interface, the role of the steric stabilization factor was found to be extremely low in the studied dispersions. Polymer-containing BeO dispersions in acetone had the highest kinetic stability due to the specific interactions of this solvent with BeO particles and polymer macromolecules contributing to the redispersion of particles and the formation of protective solvation shells. The obtained data on the effect of the dispersion medium composition on the rheological properties of concentrated BeO dispersions can be used for the selection of optimal regimes for preparation of pastes for high-temperature ceramics.     

Osmotic compression of mixtures of polymers and particles

Aqueous dispersions of nanometric ceria particles have been concentrated through osmotic stress. Mixed dispersions of ceria with small adsorbing macromolecules of poly (vinylpyrrolidone) have been prepared by the same method. The osmotic pressure of pure ceria dispersions results from electrostatic repulsions between particles. The osmotic pressure of dispersions containing a non-saturating amount of PVP is the same as that of pure dispersions, and the colloidal stability is depressed with respect to the pure dispersions. The osmotic pressure of dispersions containing an excess of PVP is increased by the free macromolecules, and the colloidal stability is enhanced. The organization of particles in these dispersions has been examined by small-angle x-ray scattering and cryotransmission electron microscopy. In pure ceria dispersions and in saturated dispersions, a liquid-like short-range order was found; when the concentration is increased, this short-range order follows a three-dimensional swelling law. In dispersions containing a non-saturating amount of PVP, the structure shows an alternance of clusters and voids, and the separations of clusters follow an unusual one-dimensional swelling law.     

Application of TXRF for burn leach test of TRISO coated UO2 particles

A Total Reflection X-Ray Fluorescence (TXRF) methodology, employing burn-leach test, was developed to assess the quality of SiC layer in tristructural isotropic (TRISO) coated UO₂ particles prepared in BARC. The coated particles were heated at 1,123 K for about 96 h to constant weight. The calcined particles, thus obtained, were then heated in suprapure concentrated nitric acid. The supernatant was analysed for trace elements by TXRF. No uranium was detected in this supernatant solution. The study demonstrates the good quality of SiC layer and applicability of TXRF for burn leach test of TRISO coated UO₂ particles prepared.     

Preparation of monodisperse, fluorescent PMMA-latex colloids by dispersion polymerization

We report a single step procedure to prepare monodisperse colloidal poly(methyl)methacrylate (PMMA) particles where fluorescent dyes are incorporated into the polymer network. The particles are sterically stabilized against flocculation by poly(12-hydroxystearic acid). The fluorescent dyes are RITC (rhodamine isothiocyanate)-aminostyrene (RAS) and 4-methylaminoethylmethacrylate-7-nitrobenzo-2-oxa-1,3-diazol (NBD-MAEM), which are prepared from (commercially available) RITC and 4-chloro-7-nitrobenzo-2-oxa-1,3-diazol (NBD-Cl), respectively, as starting materials. The particles can be grown larger with nonfluorescent PMMA. Examples of the usefulness of these model particles in colloid science are given.     

Effect of a low-molecular-weight salt on colloidal dispersions of interpolyelectrolyte complexes

Colloidal dispersions of an interpolyelectrolyte complex were prepared by mixing dilute aqueous solutions of poly(dimethyldiallylammonium chloride) and the sodium salt of the alternating copolymer of maleic acid propene in amounts providing about a threefold excess of the charged groups of the cationic polyelectrolyte over those of the anionic polyelectrolyte. These dispersions were examined by means of analytical sedimentation, quasielastic light scattering, and laser Doppler microelectrophoresis. The experimental results obtained suggest that the particles of the interpolyelectrolyte complex are multicomplex aggregates bearing cationic charge. Such aggregates were assumed to consist of a hydrophobic core formed by coupled oppositely charged macromolecules and a hydrophilic shell formed by cationic macromolecules. Hydrodynamic and electrophoretic properties of these aggregates were found to be rather sensitive to variations in the ionic strength of the surrounding medium: with rising salt concentration, their sedimentation coefficient and hydrodynamic size increase, these increases becoming more strongly pronounced at higher salt concentrations, whereas their electrophoretic mobility gradually decreases. The salt effects revealed suggest that the aggregation level of the particles of the interpolyelectrolyte complex rises in response to an increase in the ionic strength of the surrounding medium. This phenomenon was associated with the salt-induced decrease of the stabilizing effect of the hydrophilic shells that protect such particles from progressive aggregation. Received: 15 May 1998 Accepted in revised form: 28 August 1998     

Debundling of multiwalled carbon nanotubes in N,N-dimethylacetamide by polymers

Structure and properties of the dispersions of multiwalled carbon nanotubes (MWCNTs) in N,N-dimethylacetamide (DMAc) with different dispersing polymers: polyvinylpyrrolidone (PVP), poly(ethyleneoxide), triblock copolymers poly(ethyleneoxide)-b-poly(propyleneoxide)-b-poly(ethyleneoxide) (Pluronic F127 and Pluronic F108), ethylenediamine tetrakis(ethoxylate-b-propoxylate) tetrol, and ethylenediamine tetrakis(propoxylate-b-ethoxylate) tetrol (Tetronic) of different molecular weights were studied. All studied polymers were shown to be able to disperse MWCNT in DMAc, and MWCNT dispersions appear free of aggregates by visual inspection even after 3 months of keeping at room temperature. Dispersions were characterized by UV–VIS absorption spectroscopy and dynamic light scattering measurements. PVP was found to be the best dispersing polymer for MWCNT in DMAc. It was shown that the yield of the dispersed MWCNT and the average particle size of the MWCNT in DMAc depend on the chemical nature, molecular weight of the dispersing polymer, and solvent quality. The difference in dispersive capacity of the studied polymers is attributed to different dispersion mechanisms for PVP (“polymer wrapping” model) and for other studied dispersing polymers (“loose adsorption” model), which have different efficiencies in DMAc. It was revealed that an increase of dispersing polymer (PVP) concentration at the range of 4.7–37.6 g l^?1 results in an average particle size enlargement and MWCNT final concentration reduction.     

The interface between immiscible polymers in composite latexes: a small-angle x-ray scattering study employing contrast variation

An investigation of the radial structure of composite latex particles by small-angle x-ray scattering (SAXS) is given. Measurements at different contrasts were done by addition of sucrose to the dispersion medium water. The latex particles investigated here consist of a poly(styrene) core and a shell of poly(methylmethacrylate) and were prepared by seeded emulsion polymerization. Since the electron density of both polymers can be easily matched by concentrated sucrose solution, a full analysis of the radial electron density by contrast variation can be given. Depending on the mode of monomer addition during the second polymerization step a very sharp or a diffuse interface between the two incompatible polymers may result.Respectfully dedicated to Prof. E. W. Fischer on the occasion of his 65th birthday     

Weak acid–base interaction induced assembly for the formation of rambutan-like poly(styrene-alt-maleic anhydride)/silica composite microspheres

To investigate the effect of surface functionality on the morphology of polymer/silica composite, poly(styrene-alt-maleic anhydride) (SMA) spheres prepared via precipitation polymerization method was employed. In water/ethanol solution, diethanolamine (DEA) was used to catalyze the hydrolysis reaction of tetraethoxysilane (TEOS), and rambutan-like poly(styrene-alt-maleic anhydride)/silica (SMA/SiO₂) microspheres were synthesized through in situ sol–gel process. The obtained structure and morphology were characterized by FTIR, NMR, TEM, SEM, and TGA. The results showed that the hydrolyzed SMA chains on the surface was crucial to the nucleation and growth of silica, and the morphologies of SMA/SiO₂ composite microspheres can be controlled by the amount of DEA and the ratio of SMA/TEOS. In addition, the SMA/SiO₂ microspheres were used to prepare hierarchical structure of SMA/SiO₂/Ag particles, which were utilized for the construction of surface-enhanced Raman scattering substrate (SERS).     

Hollow polymer particles that are pH-responsive and redox sensitive: two simple steps to triggered particle swelling, gelation and disassembly

A new, simple, two-step method is introduced for preparing hollow particles that are both pH-responsive and redox sensitive. Hollow poly(methyl methacrylate-co-methacrylic acid) particles swell at moderate pH values, form gels in concentrated dispersions and can be disassembled by adding reducing agents.     

Characterisation by X-ray photoelectron spectroscopy of surface layers of poly(vinyl alcohol) on particles formed by dispersion polymerisation of divinylbenzene

Well-defined dispersions of polydivinylbenzene in methanol and a binary liquid mixture of methanol and water were stabilised by samples of partially hydrolysed poly(vinyl alcohol) (PVOH). Mean particle diameters in the range 0.3–1.0 m were produced in dispersion polymerisations. In order to assess surface coverage by PVOH, alcohol groups in the stabilising surface layer were derivatised with trifluoroacetic anhydride. Characterisation of trifluoroacetate groups by X-ray photoelectron spectroscopy permits the quantitative determination of atomic fluorine. The relative concentrations of surface fluorine are in agreement with expectation based on the degrees of hydrolysis of 35 mol% and 72.5 mol% of the PVOH stabilisers. Treatments based on broadscan and high-resolution spectra permit reasonable estimates of the layer thickness for PVOH chains collapsed onto the surface of dry particles. Calculations of the area occupied per PVOH chain are consistent with estimates for other stabilising chains in hydrocarbon media.     

Preparation of polymethylmethacrylate latices in non-polar media

The dispersion polymerization of methylmethacrylate stabilized by poly(12-hydroxy-stearic acid) in hydrocarbon media has been investigated. Unlike earlier results [7] it was found that stable latex particles can be prepared in the initial monomer concentration range 8.5 % to 34 %. To obtain stable particles varying amounts of stabilizer were used.