Martin W. Murray

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.

Autostratification in drying colloidal dispersions: experimental investigations

In films cast from a colloidal dispersion comprising two particle sizes, we experimentally examine the distribution of particles normal to the substrate. The particle concentrations at various positions in the film are determined through atomic force microscopy and NMR profiling. The results are compared to a previously derived diffusional model. Evidence for diffusional driven stratification is found, but the importance of other flows is also highlighted. The conditions that enhance particle stratification are found to be a colloidally stable dispersion, low initial volume fractions, a low concentration of the stratifying particle, and for the Peclet numbers of the two components to straddle unity.

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.

Auto-stratification in drying colloidal dispersions: A diffusive model

The mechanism by which the particles in a drying film come into close packing during solvent evaporation has an important role to play in the final film morphology. During drying the particles can develop non-uniform concentrations across the vertical height of the film, depending on their diffusion rate. By applying the principles of classical diffusion mechanics to a hard sphere system, a theory for this novel method of stratification during drying of a two component film has been derived. The model is dependent on the particle Peclet numbers and when one is above unity and the other below, maximum stratification is observed.

Cracking of drying latex films: an ESEM experiment.

In this study environmental scanning electron microscopy was used to observe the cracking of drying latex films below their glass-transition temperature. By controlling the relative humidity so that it decreases linearly with time, a critical level of humidity at which cracking occurs can be determined and this is measured as a function of film thickness. It was found that the cracking humidity decreases with increases in film thickness for thicknesses in the range of 30 to 100 mum and then remains almost unchanged. A scaling argument can be used to fit the data very well and indicates that cracking occurs as soon as the entire film is consolidated into close packing.

Competition between Polymers for Adsorption on Silica: A Solvent Relaxation NMR and Small-Angle Neutron Scattering Study

The competition between poly(vinylpyrrolidone) and poly(ethylene oxide) for adsorption at the silica surface was studied by solvent relaxation nuclear magnetic resonance and small-angle neutron scattering. The additive nature of the NMR relaxation rate enhancement was used to observe changes in the train layer when the two polymers were in direct competition for an increasing weight percentage of silica. PVP is shown to displace preadsorbed PEO from the particle surface, and this was observed for a range of PVP molecular weights. SANS measurements were found to give detailed information on the adsorption of the polymers in the separate systems; however, only qualitative information on the adsorption of the polymers could be obtained from the mixed samples. At a total polymer concentration of 0.4% w/v with 1.1% w/v silica, the SANS data were consistent with PVP adsorbing at the surface and dPEO remaining in solution, in agreement with the NMR data.

Bespoke periodic topography in hard polymer films by infrared radiation-assisted evaporative lithography

Polymer coatings with periodic topographic patterns, repeating over millimetre length scales, are created from lateral flows in an aqueous dispersion of colloidal particles. The flow is driven by differences in evaporation rate across the wet film surface created by IR radiative heating through a shadow mask. This new process, which we call IR radiation-assisted evaporative lithography (IRAEL), combines IR particle sintering with the concept of evaporative lithography. We show that the height of the surface features increases with an increase in several key parameters: the initial thickness of the film, the volume fraction of particles, and the pitch of the pattern. The results are interpreted by using models of geometry and particle transport. The patterned coatings can function as “paintable” microlens arrays, applicable to nearly any surface. Compared with existing methods for creating textured coatings, IRAEL is simpler, inexpensive, able to create a wide variety of bespoke surfaces, and applicable to nearly any substrate without prior preparation.

The use of solvent relaxation NMR to study colloidal suspensions

Solvent relaxation nuclear magnetic resonance has been widely used to study the interactions of polymers and surfactants with nanoparticles, an important area of research for use in a range of industrial formulations, especially with regards to competition effects between components. The ability of the solvent relaxation technique to distinguish between solvent molecules at the surface and those in the bulk solution has been used to obtain valuable information on the interfacial interactions and structure. We focus on systems containing combinations of polymer, surfactant and colloidal particles and illustrate how solvent relaxation measurements have addressed problems of stabilisation, flocculation and depletion in both academically and industrially relevant systems.

Resolving the Film Formation Dilemma with Infrared Radiation-Assisted Sintering

The film formation of an acrylate latex with a glass-transition temperature of 38 °C has been achieved through the use of near-infrared (NIR) radiative heating. A hard, crack-free coating was obtained without the addition of plasticizers. Sintering of acrylate particles was confirmed through measurements using atomic force microscopy. The addition of an NIR-absorbing polymer increased the rate of particle deformation such that it was significantly greater than obtained in a convection oven at 60 °C. The results are consistent with a lower polymer viscosity under infrared radiation, according to a simple analysis using a standard model of sintering.

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.