For a wide range of applications, films are deposited from colloidal particles suspended in a volatile liquid. There is burgeoning interest in stratifying colloidal particles into separate layers within the final dry film to impart properties at the surface different to the interior. Here, we outline the mechanisms by which colloidal mixtures can stratify during the drying process. The problem is considered here as a three-way competition between evaporation of the continuous liquid, sedimentation of particles, and their Brownian diffusion. In particle mixtures, the sedimentation of larger or denser particles offers one means of stratification. When the rate of evaporation is fast relative to diffusion, binary mixtures of large and small particles can stratify with small particles on the top, according to physical models and computer simulations. We compare experimental results found in the scientific literature to the predictions of several recent models in a quantitative way. Although there is not perfect agreement between them, some general trends emerge in the experiments, simulations and models. The stratification of small particles on the top of a film is favoured when the colloidal suspension is dilute but when both the concentration of the small particles and the solvent evaporation rate are sufficiently high. A higher particle size ratio also favours stratification by size. This review points to ways that microstructures can be designed and controlled in colloidal materials to achieve desired properties.

A major drawback of conventional emulsion polymers arises from the presence of migrating low molecular weight surfactants that contribute to poor water barrier properties and low adhesion to substrates. In this paper, we demonstrate how living polymer chains obtained by reversible addition-fragmentation chain transfer (RAFT) can be used as an efficient stabilizer in emulsion polymerization, leading to the production of surfactant-free latexes, which then form crosslinked films with beneficial properties. Hydrophilic poly(methacrylic acid) (PMAA) chains obtained by RAFT performed in water are used to mediate emulsion polymerization and produce film-forming latex particles from mixtures of methyl methacrylate, n-butyl acrylate and styrene. Stable dispersions of particles with sizes between 100 and 200 nm are obtained, with very low amounts of coagulum (< 0.5 wt.%). The particles are stabilized by the PMAA segment of amphiphilic block copolymers formed during the polymerization. Remarkably, low amounts of PMAA chains (from 1.5 wt.% down to 0.75 wt.%) are enough to ensure particle stabilization. Only traces of residual PMAA macroRAFT agents are detected in the final latexes, showing that most of them are successfully chain extended and anchored on the particle surface. The Tg of the final material is adjusted by the composition of the hydrophobic monomer mixture so that film formation occurs at room temperature. Conventional crosslinking strategies using additional hydrophobic comonomers, such as 1,3-butanediol diacrylate (BuDA), diacetone acrylamide (DAAm), and (2acetoacetoxy)ethyl methacrylate, are successfully applied to these formulations as attested by gel fractions of 100%. When particles are internally crosslinked with BuDA, chain interdiffusion between particles is restricted, and a weak and brittle film is formed. In contrast, when DAAm undergoes crosslinking during film formation, full coalescence is achieved along with the creation of a crosslinked network. The resulting film has a higher Young's modulus and tensile strength as a result of crosslinking. This synthetic strategy advantageously yields a surfactant-free latex that can be formed into a film at room temperature with mechanical properties that can be tuned via the crosslinking density.

The presence of low-molar-mass surfactants in latex films results in detrimental effects on their water permeability, gloss, and adhesion. For applications such as coatings, there is a need to develop formulations that do not contain surfactants and have better water barrier properties. Having previously reported the synthesis of surfactant-free latex particles in water using low amounts (<2 wt %) of chains synthesized by controlled radical polymerization (Lesage de la Haye et al. Macromolecules 2017, 50, 9315-9328), here we study the water barrier properties of films made from these particles and their application in anticorrosion coatings. When films cast from aqueous dispersions of acrylate copolymer particles stabilized with poly(sodium 4-styrenesulfonate) (PSSNa) were immersed in water for 3 days, they sorbed only 4 wt % water. This uptake is only slightly higher than the value predicted for the pure copolymer, indicating that the negative effects of any particle boundaries and hydrophilic-stabilizing molecules are minimal. This sorption of liquid water is 5 times lower than what is found in films cast from particles stabilized with the same proportion of poly(methacrylic acid) (PMAA), which is more hydrophilic than PSSNa. In water vapor with 90% relative humidity, the PSSNa-based film had an equilibrium sorption of only 4 wt %. A small increase in the PMAA content has a strong and negative impact on the barrier properties. Nuclear magnetic resonance relaxometry on polymer films after immersion in water shows that water clusters have the smallest size in the films containing PSSNa. Furthermore, these films retain their optical clarity during immersion in liquid water for up to 90 min, whereas all other compositions quickly develop opacity ("water whitening") as a result of light scattering from sorbed water. This implies a remarkably complete coalescence and a very small density of defects, which yields properties matching those of some solvent-borne films. The latex stabilized with PSSNa is implemented as the binder in a paint formulation for application as an anticorrosive barrier coating on steel substrates and evaluated in accelerated weathering and corrosion tests. Our results demonstrate the potential of self-stabilized latex particles for the development of different applications, such as waterborne protective coatings and pressure-sensitive adhesives.

The molecular composition of polymer blend surfaces defines properties such as adhesion, wetting, gloss, and biocompatibility. The surface composition often differs from the bulk because of thermodynamic effects or modification. Mixtures of colloids and linear polymers in a common solvent are often used to deposit films for use in encapsulants, inks, coatings, and adhesives. However, means to control the nonequilibrium surface composition are lacking for these systems. Here we show how the surface composition and hydrophilicity of a film deposited from a bimodal mixture of linear polymers and colloids in water can be adjusted simply by varying the evaporation rate. Ion beam analysis was used to quantify the extent of stratification of the linear polymers near the surface, and the results are in agreement with a recent diffusiophoretic model. Because our approach to stratification relies solely on diffusiophoresis, it is widely applicable to any system deposited from colloids and nonadsorbing polymers in solution as a means to tailor surface properties.

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