Neal Williams

Unexpected Facile Redistribution of Adsorbed Silica Nanoparticles Between Latexes

Addition of excess sterically stabilized P2VP latex to a colloidal dispersion of P2VP-silica nanocomposite particles (with silica shells at full monolayer coverage) leads to the facile redistribution of the silica nanoparticles such that partial coverage of all the P2VP latex particles is achieved. This silica exchange, which is complete within 1 h at 20 degrees C as judged by small-angle x-ray scattering, is observed for nanocomposite particles prepared by heteroflocculation, but not for nanocomposite particles prepared by in situ copolymerization. These observations are expected to have important implications for the optimization of nanocomposite formulations in the coatings industry.

Styrenic surfmer in emulsion copolymerization of acrylic monomers. II. Copolymerization and film properties

Polymerizable styrenic surfactants (surfmers) and nonreactive analogs, have been applied in emulsion copolymerization of acrylic monomers in a seeded semibatch process. Stable core-shell latexes with low levels of coagulum and controlled particle size have been obtained; some of them, with either steric or electrosteric stabilization, display excellent stability to electrolytes, freeze–thaw cycles, and shear flocculation. In addition, the reactive surfactants lead to films with superior performance due to reduced migration of surfactant to the surface (contact angle measurements) and dimensional stability when the films are dipped in water, as well as less water uptake. Some differences also appear in particle morphologies.

Time-Resolved Small-Angle X-ray Scattering Studies of Polymer-Silica Nanocomposite Particles: Initial Formation and Subsequent Silica Redistribution

Small angle X-ray scattering (SAXS) is a powerful characterization technique for the analysis of polymer-silica nanocomposite particles due to their relatively narrow particle size distributions and high electron density contrast between the polymer core and the silica shell. Time-resolved SAXS is used to follow the kinetics of both nanocomposite particle formation (via silica nanoparticle adsorption onto sterically stabilized poly(2-vinylpyridine) (P2VP) latex in dilute aqueous solution) and also the spontaneous redistribution of silica that occurs when such P2VP-silica nanocomposite particles are challenged by the addition of sterically stabilized P2VP latex. Silica adsorption is complete within a few seconds at 20 °C and the rate of adsorption strongly dependent on the extent of silica surface coverage. Similar very short time scales for silica redistribution are consistent with facile silica exchange occurring as a result of rapid interparticle collisions due to Brownian motion; this interpretation is consistent with a zeroth-order Smoluchowski-type calculation.

Effect of Monomer Solubility on the Evolution of Copolymer Morphology during Polymerization-Induced Self-Assembly in Aqueous Solution

Polymerization-induced self-assembly (PISA) has become a widely used technique for the rational design of diblock copolymer nano-objects in concentrated aqueous solution. Depending on the specific PISA formulation, reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization typically provides straightforward access to either spheres, worms, or vesicles. In contrast, RAFT aqueous emulsion polymerization formulations often lead to just kinetically-trapped spheres. This limitation is currently not understood, and only a few empirical exceptions have been reported in the literature. In the present work, the effect of monomer solubility on copolymer morphology is explored for an aqueous PISA formulation. Using 2-hydroxybutyl methacrylate (aqueous solubility = 20 g dm–3 at 70 °C) instead of benzyl methacrylate (0.40 g dm–3 at 70 °C) for the core-forming block allows access to an unusual “monkey nut” copolymer morphology over a relatively narrow range of target degrees of polymerization when using a poly(methacrylic acid) RAFT agent at pH 5. These new anisotropic nanoparticles have been characterized by transmission electron microscopy, dynamic light scattering, aqueous electrophoresis, shear-induced polarized light imaging (SIPLI), and small-angle X-ray scattering.

The application of distance distribution functions to structural analysis of core–shell particles

The structure of core–shell latex particles of polymethylmethacrylate (the core) and polyurethane (the shell) have been investigated by methods of small-angle X-ray scattering (SAXS) and atom-force microscopy. A set of SAXS patterns has been obtained using contrast variation method. Indirect methods have been used to follow the evolution of distance distribution functions from SAXS for lattices in various sucrose solutions over a range of solution density, yielding structural parameters of the particles such as core size, shell thickness and density of the polymers including density deviations within the particles core and shell. A model for an ensemble of core–shell particles with a normal distribution of average electron density of both the core and the shell has been developed to fit the distance distribution functions using a random search algorithm. The effects of nanophase separation in the polyurethane is estimated using Monte Carlo simulations of the distance distribution functions where the phase-separated polyurethane is represented by spherical truncated cones in a shell simulating the location of hard and soft polyurethane blocks, respectively.

A comprehensive population balance model of emulsion polymerisation for PSD & MWD: Comparison to experimental data

Abstract A population balance model for emulsion polymerisation has been developed. This model captures PSD and MWD, both of which are key performance indicators for the end latex product. The model employs purely mechanistic kernels and is aimed at maximising predictive capacity. The model is validated against a multi-objective experimental target. The aim is to predict data for PSD, solids, particle number as well as global molecular weight. The experimental system investigated is a vinyl acetate/butyl acrylate copolymerisation with ionic emulsifier and thermal initiator. The predictive capacity is tested by tuning the model to one set of experimental data, then trying to predict results from a further perturbed experiment, with no further tuning. The results of this study indicate that the model is able to capture the main process trends as well as providing an accurate representation of quantitative data.

Optimization of the high-throughput synthesis of multiblock copolymer nanoparticles in aqueous media via polymerization-induced self-assembly

Over the past fifteen years or so, polymerization-induced self-assembly (PISA) has become widely recognized as a powerful and versatile platform technology for the synthesis of a wide range of block copolymer nanoparticles of controlled size, shape and surface chemistry. In the present study, we report that PISA formulations are sufficiently robust to enable high-throughput experiments using a commercial synthesis robot (Chemspeed Autoplant A100). More specifically, we use reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization of either n-butyl methacrylate and/or benzyl methacrylate to prepare various examples of methacrylic multiblock copolymer nanoparticles using a poly(methacrylic acid) stabilizer block. Adequate stirring is essential to generate sufficiently small monomer droplets for such heterogeneous polymerizations to proceed efficiently. Good reproducibility can be achieved under such conditions, with well-defined spherical morphologies being obtained at up to 45% w/w solids. GPC studies indicate high blocking efficiencies but relatively broad molecular weight distributions (Mw/Mn = 1.36–1.85), suggesting well-defined (albeit rather polydisperse) block copolymer chains. These preliminary studies provide a sound basis for high-throughput screening of RAFT-mediated PISA formulations, which is likely to be required for commercialization of this technology. Our results indicate that methacrylic PISA formulations enable the synthesis of diblock and triblock copolymer nanoparticles in high overall yield (94–99%) within 1–3 h at 70 °C. However, tetrablocks suffer from incomplete conversions (87–96% within 5 h) and hence most likely represent the upper limit for this approach.