Disk Centrifuge Photosedimentometry Research Articles

Avoiding the Spherical Particle Assumption: Fractal Particle Density, Size, and Structure Characterization through Combined Sedimentation and Viscometry Measurements.

Physically meaningful characterization of irregularly shaped particles continues to present substantial challenges to the experimentalist. "Equivalent diameters" based on experimental techniques such as static and dynamic light scattering or sedimentation have proliferated to the point that they are often no longer recognized as equivalent. This study demonstrates the use of dual-fluid disk centrifuge photosedimentometry coupled with rheological measurements of viscosity to provide direct insights into both the average mass of a structured particle size distribution and the average hydrodynamic diameter.

Adsorption of Small Cationic Nanoparticles onto Large Anionic Particles from Aqueous Solution: A Model System for Understanding Pigment Dispersion and the Problem of Effective Particle Density.

The present study focuses on the use of copolymer nanoparticles as a dispersant for a model pigment (silica). Reversible addition–fragmentation chain transfer (RAFT) alcoholic dispersion polymerization was used to synthesize sterically stabilized diblock copolymer nanoparticles. The steric stabilizer block was poly(2-(dimethylamino)ethyl methacrylate) (PDMA) and the core-forming block was poly(benzyl methacrylate) (PBzMA). The mean degrees of polymerization for the PDMA and PBzMA blocks were 71 and 100, respectively. Transmission electron microscopy (TEM) studies confirmed a near-monodisperse spherical morphology, while dynamic light scattering (DLS) studies indicated an intensity-average diameter of 30 nm. Small-angle X-ray scattering (SAXS) reported a volume-average diameter of 29 ± 0.5 nm and a mean aggregation number of 154. Aqueous electrophoresis measurements confirmed that these PDMA71–PBzMA100 nanoparticles acquired cationic character when transferred from ethanol to water as a result of protonation of the weakly basic PDMA chains. Electrostatic adsorption of these nanoparticles from aqueous solution onto 470 nm silica particles led to either flocculation at submonolayer coverage or steric stabilization at or above monolayer coverage, as judged by DLS. This technique indicated that saturation coverage was achieved on addition of approximately 465 copolymer nanoparticles per silica particle, which corresponds to a fractional surface coverage of around 0.42. These adsorption data were corroborated using thermogravimetry, UV spectroscopy and X-ray photoelectron spectroscopy. TEM studies indicated that the cationic nanoparticles remained intact on the silica surface after electrostatic adsorption, while aqueous electrophoresis confirmed that surface charge reversal occurred below pH 7. The relatively thick layer of adsorbed nanoparticles led to a significant reduction in the effective particle density of the silica particles from 1.99 g cm–3 to approximately 1.74 g cm–3, as judged by disk centrifuge photosedimentometry (DCP). Combining the DCP and SAXS data suggests that essentially no deformation of the PBzMA cores occurs during nanoparticle adsorption onto the silica particles.

Determining the Effective Density and Stabilizer Layer Thickness of Sterically Stabilized Nanoparticles.

A series of model sterically stabilized diblock copolymer nanoparticles has been designed to aid the development of analytical protocols in order to determine two key parameters: the effective particle density and the steric stabilizer layer thickness. The former parameter is essential for high resolution particle size analysis based on analytical (ultra)centrifugation techniques (e.g., disk centrifuge photosedimentometry, DCP), whereas the latter parameter is of fundamental importance in determining the effectiveness of steric stabilization as a colloid stability mechanism. The diblock copolymer nanoparticles were prepared via polymerization-induced self-assembly (PISA) using RAFT aqueous emulsion polymerization: this approach affords relatively narrow particle size distributions and enables the mean particle diameter and the stabilizer layer thickness to be adjusted independently via systematic variation of the mean degree of polymerization of the hydrophobic and hydrophilic blocks, respectively. The hydrophobic core-forming block was poly(2,2,2-trifluoroethyl methacrylate) [PTFEMA], which was selected for its relatively high density. The hydrophilic stabilizer block was poly(glycerol monomethacrylate) [PGMA], which is a well-known non-ionic polymer that remains water-soluble over a wide range of temperatures. Four series of PGMAx–PTFEMAy nanoparticles were prepared (x = 28, 43, 63, and 98, y = 100–1400) and characterized via transmission electron microscopy (TEM), dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS). It was found that the degree of polymerization of both the PGMA stabilizer and core-forming PTFEMA had a strong influence on the mean particle diameter, which ranged from 20 to 250 nm. Furthermore, SAXS was used to determine radii of gyration of 1.46 to 2.69 nm for the solvated PGMA stabilizer blocks. Thus, the mean effective density of these sterically stabilized particles was calculated and determined to lie between 1.19 g cm–3 for the smaller particles and 1.41 g cm–3 for the larger particles; these values are significantly lower than the solid-state density of PTFEMA (1.47 g cm–3). Since analytical centrifugation requires the density difference between the particles and the aqueous phase, determining the effective particle density is clearly vital for obtaining reliable particle size distributions. Furthermore, selected DCP data were recalculated by taking into account the inherent density distribution superimposed on the particle size distribution. Consequently, the true particle size distributions were found to be somewhat narrower than those calculated using an erroneous single density value, with smaller particles being particularly sensitive to this artifact.

Loading of Silica Nanoparticles in Block Copolymer Vesicles during Polymerization-Induced Self-Assembly: Encapsulation Efficiency and Thermally Triggered Release.

Poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer vesicles can be prepared in the form of concentrated aqueous dispersions via polymerization-induced self-assembly (PISA). In the present study, these syntheses are conducted in the presence of varying amounts of silica nanoparticles of approximately 18 nm diameter. This approach leads to encapsulation of up to hundreds of silica nanoparticles per vesicle. Silica has high electron contrast compared to the copolymer which facilitates TEM analysis, and its thermal stability enables quantification of the loading efficiency via thermogravimetric analysis. Encapsulation efficiencies can be calculated using disk centrifuge photosedimentometry, since the vesicle density increases at higher silica loadings while the mean vesicle diameter remains essentially unchanged. Small angle X-ray scattering (SAXS) is used to confirm silica encapsulation, since a structure factor is observed at q ≈ 0.25 nm–1. A new two-population model provides satisfactory data fits to the SAXS patterns and allows the mean silica volume fraction within the vesicles to be determined. Finally, the thermoresponsive nature of the diblock copolymer vesicles enables thermally triggered release of the encapsulated silica nanoparticles simply by cooling to 0–10 °C, which induces a morphological transition. These silica-loaded vesicles constitute a useful model system for understanding the encapsulation of globular proteins, enzymes, or antibodies for potential biomedical applications. They may also serve as an active payload for self-healing hydrogels or repair of biological tissue. Finally, we also encapsulate a model globular protein, bovine serum albumin, and calculate its loading efficiency using fluorescence spectroscopy.

One‐Pot Preparation of Conducting Polymer‐Coated Silica Particles: Model Highly Absorbing Aerosols

Near‐monodisperse 0.50 μm and 1.0 μm silica particles are surface‐modified using 3‐(trimethoxysilyl)propyl methacrylate (MPS) and subsequently coated by aqueous deposition of an ultrathin polypyrrole (PPy) overlayer to produce PPy‐coated silica particles. The targeted degree of MPS modification and PPy mass loading are systematically varied to optimize the colloidal stability and PPy coating uniformity. MPS surface modification is characterized by contact angle goniometry and the PPy overlayer uniformity is assessed by scanning electron microscopy. HF etching of the silica cores produces hollow PPy shells, thus confirming the contiguous nature of the PPy overlayer and the core–shell morphology of the original particles. Four‐point probe measurements and XPS studies indicate that the electrical conductivity of pressed pellets of PPy‐coated silica particles increases with PPy surface coverage. Colloidal stabilities of the bare, MPS‐modified, and PPy‐coated silica particles in aqueous solution are assessed using disk centrifuge photosedimentometry. MPS surface modification results in weak flocculation, with subsequent PPy deposition causing further aggregation. In contrast, white light aerosol spectrometry indicates a relatively high degree of dispersion for PPy‐coated silica particles in the gas phase. Such PPy‐coated silica particles are expected to be useful mimics for silica‐rich micrometeorites and may also serve as a model highly absorbing aerosol.

Heterocoagulation as a Facile Route To Prepare Stable Serum Albumin-Nanoparticle Conjugates for Biomedical Applications: Synthetic Protocols and Mechanistic Insights

There is increasing interest in using serum albumin, the most abundant plasma protein, as a stabilizing agent in the context of nanomedicine. Using poly(vinyl amine)-stabilized polypyrrole nanoparticles as an example, we report a facile generic route to prepare serum albumin-nanoparticle conjugates via heterocoagulation. Time-resolved dynamic light scattering (DLS), disk centrifuge photosedimentometry (DCP), and circular dichroism (CD) spectroscopy studies confirm that bovine serum albumin (BSA) adsorbs rapidly onto the cationic poly(vinyl amine)-stabilized polypyrrole nanoparticles and suggest that the initial well-defined protein coronal is subsequently cross-linked via thiol-disulfide exchange. These BSA-nanoparticle conjugates were further characterized by X-ray photoelectron spectroscopy (XPS), aqueous electrophoresis, field emission scanning electron microscopy (FE SEM), and transmission electron microscopy (TEM). They exhibit excellent long-term colloidal stability under physiological conditions without further purification, suggesting strong irreversible adsorption by the BSA. Protein adsorption appears to be co-operative and both thermodynamic and mechanistic aspects were examined via aqueous electrophoresis, DCP, and DLS studies.

Synthesis and Characterization of Novel Polyacid-Stabilized Latexes

A series of novel polyacid macromonomers based on 2-hydroxypropyl methacrylate (HPMA) were prepared by atom transfer radical polymerization (ATRP) via a two-step route. First, a range of well-defined PHPMA homopolymer precursors were synthesized by ATRP using a tertiary amine-functionalized initiator, 2-(dimethylamino)ethyl-2-bromoisobutyrylamide, and a CuCl/2, 2'-bipyridine (bpy) catalyst in alcoholic media at 50 °C. ATRP polymerizations were relatively slow and poorly controlled in pure isopropanol (IPA), especially when targeting higher degrees of polymerization (DP > 30). Improved control was achieved by addition of water: low polydispersity (M(w)/M(n) < 1.25) PHPMA homopolymers of DP = 30, 40, 50, 60, or 70 were successfully prepared using a 9:1 w/w % IPA/water mixture at 50 °C. These PHPMA homopolymer precursors were then derivatized to produce the corresponding poly(2-(succinyloxy)propyl methacrylate) (PSPMA) macromonomers by quaternizing the tertiary amine end-group with excess 4-vinylbenzyl chloride, followed by esterification of the pendent hydroxyl groups using excess succinic anhydride at 20 °C. These polyacid macromonomers were evaluated as reactive steric stabilizers for polystyrene latex synthesis under either aqueous emulsion polymerization or alcoholic dispersion polymerization conditions. Near-monodisperse polystyrene latexes were obtained via aqueous emulsion polymerization using 10 wt % PSPMA macromonomer (with respect to styrene monomer) with various initiators as evidenced by scanning electron microscopy, disk centrifuge photosedimentometry and light scattering studies. PSPMA macromomer concentrations as low as 1.0 wt % also produced near-monodisperse latexes, suggesting that these PSPMA macromonomers are highly effective stabilizers. Alcoholic dispersion polymerization of styrene conducted in various ethanol/water mixtures with 10 wt % PSPMA(50) macromonomer produced relatively large near-monodisperse latexes. Increasing the water content in such formulations led to smaller latexes, as expected. Control experiments conducted with 10 wt % PSPMA(50) homopolymer produced relatively large polydisperse latexes via emulsion polymerization and only macroscopic precipitates via alcoholic dispersion polymerization. Thus the terminal styrene group on the macromonomer chains is essential for the formation of well-defined latexes. FT-IR spectroscopy indicated that these latexes contained PSPMA macromonomer, whereas (1)H NMR spectroscopy studies of dissolved latexes allowed stabilizer contents to be determined. Aqueous electrophoresis and X-ray photoelectron spectroscopy studies confirmed that the PSPMA macromonomer chains were located at the latex surface, as expected. Finally, these polyacid-stabilized polystyrene latexes exhibited excellent freeze-thaw stability and remained colloidally stable in the presence of electrolyte.

Correcting for a Density Distribution: Particle Size Analysis of Core–Shell Nanocomposite Particles Using Disk Centrifuge Photosedimentometry

Many types of colloidal particles possess a core-shell morphology. In this Article, we show that, if the core and shell densities differ, this morphology leads to an inherent density distribution for particles of finite polydispersity. If the shell is denser than the core, this density distribution implies an artificial narrowing of the particle size distribution as determined by disk centrifuge photosedimentometry (DCP). In the specific case of polystyrene/silica nanocomposite particles, which consist of a polystyrene core coated with a monolayer shell of silica nanoparticles, we demonstrate that the particle density distribution can be determined by analytical ultracentrifugation and introduce a mathematical method to account for this density distribution by reanalyzing the raw DCP data. Using the mean silica packing density calculated from small-angle X-ray scattering, the real particle density can be calculated for each data point. The corrected DCP particle size distribution is both broader and more consistent with particle size distributions reported for the same polystyrene/silica nanocomposite sample using other sizing techniques, such as electron microscopy, laser light diffraction, and dynamic light scattering. Artifactual narrowing of the size distribution is also likely to occur for many other polymer/inorganic nanocomposite particles comprising a low-density core of variable dimensions coated with a high-density shell of constant thickness, or for core-shell latexes where the shell is continuous rather than particulate in nature.

When Does Silica Exchange Occur between Vinyl Polymer−Silica Nanocomposite Particles and Sterically Stabilized Latexes?

The redistribution of silica nanoparticles between "core-shell" polymer-silica nanocomposites and sterically stabilized latexes is investigated using a combination of electron microscopy, disk centrifuge photosedimentometry (DCP), and X-ray photoelectron spectroscopy (XPS). Facile exchange of silica nanoparticles occurs on addition of sterically-stabilized polystyrene (or poly(2-vinylpyridine)) latex to polystyrene-silica (or poly(2-vinylpyridine)-silica) nanocomposite particles previously prepared by heteroflocculation. In contrast, no silica exchange occurs after such a latex "challenge" if similar polymer/silica nanocomposite particles are prepared via in situ polymerization. Silica redistribution can be confirmed by post mortem electron microscopy studies, which are facilitated if the original nanocomposite and latex particles differ sufficiently in their mean diameters. Ideally, XPS requires a unique elemental marker for the nanocomposite particle cores, which become progressively more exposed if silica exchange occurs. DCP is a particularly convenient in situ technique for assessing whether or not silica exchange has occurred. If no silica exchange occurs, there is little or no change in the nanocomposite and latex size distributions. On the other hand, silica redistribution always results in a larger mean particle diameter for the (partially) silica-coated latex particles relative to the original bare latex. In addition, incipient flocculation is typically observed after silica exchange. Like electron microscopy, DCP studies are aided if there is a significant difference in particle diameter between the original polymer-silica nanocomposite particles and the added latex. Moreover, silica redistribution can be prevented for heteroflocculated polymer-silica nanocomposite particles under certain conditions. For example, although silica exchange is observed at pH 10 when adding sterically-stabilized polystyrene (or poly(2-vinylpyridine)) latex to heteroflocculated poly(2-vinylpyridine)-silica particles, it does not occur at pH 5. Presumably, this is due to greater electrostatic attraction between the cationic P2VP cores and the anionic silica nanoparticles at this lower pH.

Polystyrene−Silica Nanocomposite Particles via Alcoholic Dispersion Polymerization Using a Cationic Azo Initiator

Submicrometer-sized polystyrene-silica nanocomposite particles have been prepared by alcoholic dispersion polymerization of styrene using commercial alcoholic silica sols of 13 or 22 nm diameter as the sole stabilizing agent. The key to the formation of colloidally stable nanocomposite particles is the selection of a cationic azo initiator (use of nonionic or anionic initiators leads either to the formation of silica-stabilized polystyrene latex particles with very low silica contents or to the precipitation of polystyrene, respectively). Neither surface modification of the silica sol nor the addition of surfactant or polymeric stabilizers is required for successful nanocomposite syntheses. The purified polystyrene-silica nanocomposite particles have relatively narrow particle size distributions, with mean diameters ranging from 331 to 464 nm as judged by disk centrifuge photosedimentometry. Thermogravimetric analyses indicated mean silica contents of 13-26 wt. %, depending on the synthesis conditions. Calcination of the polystyrene-silica nanocomposite particles leads to the formation of hollow silica shells, which indicates a well-defined core-shell morphology for the original nanocomposite particles.

Efficient Synthesis of Sterically Stabilized pH-Responsive Microgels of Controllable Particle Diameter by Emulsion Polymerization

Emulsion polymerization of 2-vinylpyridine (2VP) in the presence of divinylbenzene (DVB) cross-linker, a cationic surfactant, and a hydrophilic macromonomer, monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA), at around neutral pH and 60 degrees C afforded near-monodisperse, sterically stabilized latexes at approximately 10% solids. Judicious selection of the synthesis parameters enabled the mean latex diameter to be varied over an unusually wide range for one-shot batch syntheses. Scanning electron microscopy studies confirmed near-monodisperse spherical morphologies, with mean weight-average particle diameters ranging from 370 to 970 nm depending on the initiator, polymeric stabilizer, and surfactant concentrations. Particle sizing studies were also conducted using disk centrifuge photosedimentometry and dynamic light scattering and gave similar data. These lightly cross-linked latexes acquired cationic microgel character at low pH, as expected. The critical pH for this latex-to-microgel transition was around pH 4.1 at 1.0 wt % DVB, which is significantly lower than the pKa of 4.92 estimated for linear P2VP homopolymer by acid titration. 1H NMR and aqueous electrophoresis studies indicated that substantial swelling occurred at low pH due to protonation of the 2VP groups, while dynamic light scattering (DLS) studies indicated volumetric swelling ratios of up to 3 orders of magnitude, depending on the initial latex diameter. Systematic variation of the degree of cross-linking led to a monotonic decrease in the pKa values of the P2VP latexes (as judged by acid titration) and also the critical swelling pH (as judged by visual inspection). This was attributed to the increasingly branched nature of the P2VP chains in their swollen microgel form. Preliminary studies of the kinetics of acid-induced swelling were also conducted using the pH jump method in conjunction with a stopped-flow apparatus. These P2VP latexes swell significantly faster than P2VP latexes described in the literature and the characteristic time scales observed in the present study are much closer to those predicted by the Tanaka equation.

Synthesis of Micrometer-Sized Silica-Stabilized Polystyrene Latex Particles

Micrometer-sized silica-stabilized polystyrene latexes have been readily prepared by alcoholic dispersion polymerization using a 13 or 22 nm commercial alcoholic silica sol as the sole stabilizing agent. These resulting surfactant-free polystyrene particles have relatively narrow particle size distributions and contain surprisingly low levels of silica (</=1.1% by mass), as judged by disk centrifuge photosedimentometry and thermogravimetry, respectively. Transmission electron microscopy studies indicate that the silica sol is located exclusively at the surface of the polystyrene latex particles at submonolayer coverage. The polymerization of styrene in the absence of the silica sol leads to macroscopic precipitation of the polystyrene. Hence, the presence of silica sol is essential for the colloid stability of these latexes, which presumably involves a charge stabilization mechanism.

Synthesis of Vinyl Polymer−Silica Colloidal Nanocomposites Prepared Using Commercial Alcoholic Silica Sols

The surfactant-free synthesis of vinyl polymer-silica nanocomposite particles has been achieved in aqueous alcoholic media at ambient temperature in the absence of auxiliary comonomers. Styrene, methyl methacrylate, methyl acrylate, n-butyl acrylate, and 2-hydroxypropyl methacrylate were homopolymerized in turn in the presence of three commercially available ultrafine alcoholic silica sols. Stable colloidal dispersions with reasonably narrow size distributions were obtained, with silica contents of up to 58% by mass indicated by thermogravimetric analysis. Particle size distributions were assessed using both dynamic light scattering and disk centrifuge photosedimentometry. The former technique indicated that the particle size increased for the first 1-2 h at 25 degrees C and thereafter remained constant. Particle morphologies were studied using electron microscopy. Most of the colloidal nanocomposites comprised approximately spherical particles with relatively narrow size distributions, but in some cases more polydisperse or nonspherical particles were obtained. Selected acrylate-based nanocomposites were examined in terms of their film formation behavior. Scanning electron microscopy studies indicated relatively smooth films were obtained on drying at 20 degrees C, with complete loss of the original particle morphology. The optical clarity of solution-cast 10 microm nanocomposite films was assessed using visible absorption spectrophotometry, with 93-98% transmission being obtained from 400 to 800 nm; the effect of long-term immersion of such films in aqueous solutions was also examined. X-ray photoelectron spectroscopy studies indicated that the surface compositions of these nanocomposite particles are invariably silica-rich, which is consistent with their long-term colloidal stability and also with aqueous electrophoresis measurements. FT-IR studies suggested that in the case of the poly(methyl methacrylate)-silica nanocomposite particles, the carbonyl ester groups in the polymer are hydrogen-bonded to the surface silanol groups. According to differential scanning calorimetry studies, the glass transition temperatures of several poly(methyl methacrylate)-silica and polystyrene-silica nanocomposites can be either higher or lower than those of the corresponding homopolymers, depending on the nature of the silica sol.

Synthesis of Vinyl Polymer−Silica Colloidal Nanocomposites via Aqueous Dispersion Polymerization

The surfactant-free synthesis of colloidal dispersions of vinyl polymer−silica nanocomposite particles in aqueous media using a batch emulsion polymerization protocol has been previously described [Percy, M. J.; et al. Langmuir 2000, 16, 6913]. In the present work 2-hydroxypropyl methacrylate [HPMA] was copolymerized with 4-vinylpyridine [4VP] using ammonium persulfate in the presence of an ultrafine silica sol. 4VP is used as an auxiliary in these syntheses; the strong interaction of this basic monomer with the acidic surface of the silica particles is essential for successful nanocomposite particle formation. HPMA monomer was selected since it has appreciable water solubility (up to 13% at 20 °C), but HPMA homopolymer is water-insoluble. This unusual solubility behavior ensured that these nanocomposite syntheses were conducted under true dispersion polymerization conditions. In view of the success of these syntheses, we conclude that emulsion monomer droplets and micelles are not a prerequisite for the formation of nanocomposite particles. Both thermogravimetric analysis and elemental microanalyses were used to determine the silica contents of the nanocomposite particles, which ranged from 5% to 42% by mass and depended on the proportion of 4VP in the comonomer feed. Under the conditions investigated the minimum amount of 4VP auxiliary required was around 15%. Depending on the synthesis conditions, the mean particle diameter of the HPMA−4VP/SiO2 particles varied from 205 to 330 nm, as judged by disk centrifuge photosedimentometry. Scanning electron microscopy studies of selected nanocomposites indicated that discrete particles were obtained on freeze-drying, partial particle coalescence occurred on drying at ambient temperature, and complete particle coalescence occurred on annealing at 50 °C. The transmittance of the annealed films was greater than 80% over the entire visible wavelength range, indicating a high degree of dispersion for the ultrafine silica sol within the film. Aqueous electrophoresis measurements combined with X-ray photoelectron spectroscopy studies indicated that the surface compositions of these HPMA−4VP/SiO2 particles are silica-rich.

Effect of Carbon Black Morphology on Dynamic Mechanical Properties of Rubber.

Carbon black samples with almost all the same properties (i.e., nitrogen surface area, CTAB surface area, DBP absorption, compressed DBP absorption and tint strength) except aggregate size distribution were tested by disk centrifuge photosedimentometry. The effect of the aggregate size distribution on rubber vulcanizates was evaluated by the dynamic mechanical modulus as a probe. The values of c storage modulus (ΔE') decreased when the aggregate size distribution became broader. This occurred because the ΔE' levels were determined by the aggregate-aggregate interaction, rather than by the carbon black-polymer interaction, when the aggregate size distribution was broader, where ΔE' is the difference between the largest and smallest values of dynamic storage modulus E' observed under the dynamic strain amplitude dependence test.

Polypyrrole Latex: Surface Analysis by XPS

Polypyrrole colloids are synthesized by an oxidative polymerization of pyrrole by ferric chloride in aqueous media containing a steric stabilizer. Polyvinyl alcohol (125,000 to 186,000 mol. wt., 99% hydrolyzed) was used as the stabilizer to prepare the particles for this study. Depending on the synthesis conditions, polypyrrole colloids typically have diameters between 100 and 200 nm, as evidenced by transmission electron microscopy and disk centrifuge photosedimentometry. The conjugated pyrrole polymer imparts intense black coloration to the colloid. Molecules of biological interest, such as antibodies, can be immobilized in active form on the particle surface. Thus these nanoparticles can be used as colored “markers” for immunodiagnostic assays.

Carboxylated Polypyrrole-Silica Nanocomposite: Surface Analysis by XPS

Carboxylated polypyrrole-silica colloids are synthesized in aqueous media by the oxidative copolymenzation of molar equivalents of pyrrole and 1-(2-carboxyethyl) pyrrole in the presence of ultrafine (20 nm) silica particles [see Maeda et al., Macromolecules 28, 2901 (1995)]. The resulting nanoparticles have a diameter of ∼100 nm as evidenced by transmission electron microscopy and disk centrifuge photosedimentometry and obtain 24–43 wt. % of silica. The conducting polymer component imparts intense coloration to the nanoparticle and the silica component ensures good colloidal stability. The surface carboxylic groups allow the covalent attachment of biological ligands of interest, such as antibodies. Thus these functional nanoparticles can be used as colored “markers” for immunodiagnostic assays.

Carboxylated Polypyrrole Latex: Surface Analysis by XPS

Carboxylated polypyrrole colloids are synthesized by post-modification of polypvrrole latex in organic media. The modification begins with solvent exchange of the aqueous colloid into N-methyl pyrrolidone, followed by reaction with bromoacetyl bromide. The last reaction is a nulceophilic substitution on the bromoacetyl pendant group with mercaptoacetic acid. After reaction the particles are solvent-exchanged into water. Depending on the synthesis conditions of the base polypvrrole colloid, the resulting nanoparticles can have diameters between 100 and 200 nm, as evidenced by transmission electron microscopy and disk centrifuge photosedimentometry. The conjugated pyrrole polymer imparts intense, intrinsic coloration to the colloid. The surface carboxylic groups allow the covalent attachment of biological ligands of interest such as antibodies. Thus these functional nanoparticles can be used as colored “markers” for immunodiagnostic assays.

Controlled structure copolymers for the dispersion of high-performance ceramics in aqueous media

Statistical copolymers of a methoxy oligo(ethylene glycol) methacrylate (OEGMA) and either 2-(dimethylamino))ethyl methacrylate (DMA) or methacrylic acid (MAA) have been examined as dispersants for barium titanate powder in water and water isopropyl alcohol mixtures. The dispersing efficiency of the polymers was assessed using gravitational sedimentation and by measuring the particle size distributions of the dispersed powders by disk centrifuge photosedimentometry (DCP). OEGMA copolymers with MAA were very efficient, causing the mean particle size of the dispersed powder to decrease by an order of magnitude in the presence of dispersant. In contrast, OEGMA-DMA copolymers were completely ineffective. The best MAA copolymers gave higher dispersion efficiency over a wider pH range (pH 1 to 10) compared to a typical commercial dispersant Darvan-C, which was efficient only above pH 7. X-Ray photoelectron spectroscopy, zeta potential measurements, infra red spectroscopy and DCP studies all show that the acid-containing copolymers bind strongly to the barium titanate surface via the carboxylic acid groups resulting in good quality dispersions. There were no changes in the dispersion quality due to desorption of the dispersant even after extensive washing and drying of the copolymer-coated particles. The dried particles showed complete redispersion in water even after washing and drying, again indicating strong binding of the copolymer to the oxide surface.

Particle Size Distribution Measurements of PolymericDispersions: A Comparative Study

Average particle sizes and particle size distributions (PSD) were measured using four different commercial techniques for standard polystyrene latexes in the submicron range, dynamic light scattering (DLS), capillary hydrodynamic fractionation (CHDF), disk centrifuge photosedimentometry (DCP) and transmission electron microscope (TEM). Both monodisperse latexes and broad particle size distribution latexes (obtained by mixing two or three monodisperse standards) were used in each equipment. It was found that for monodisperse latexes, DLS, CHDF and DCP gave similar measurements of particle size, which were also comparable to the particle sizes measured by TEM and reported by the supplier of the standard latexes. However, with the lowest particle size standard (39 nm), both DLS and CHDF gave larger particle size values, whereas with DCP, reasonable agreement with the measured size was obtained only when extremely long centrifugation times (at least 6 h) were used. For latexes having bimodal and trimodal distributions, both CHDF and DCP were able to reproduce the entire distribution, providing relatively accurate values for the average size of each mode and also the relative masses of each population of particles. However, the analysis time required by CHDF was shorter than by DCP, but the broadness of the peak was better captured by DCP than CHDF, which always provided broader peaks. DLS failed in most cases when measuring the broad size populations when the size ratio was greater than 2 : 1.