Evidence of Stratification in Binary Colloidal Films from Microbeam X-ray Scattering: Toward Optimizing the Evaporative Assembly Processes for Coatings

Abstract

Colloidal films have many important applications where a layered configuration is desirable, including flexible electronics, anti-reflective coatings, and anti-microbial paints. We report stratification during evaporative film formation in binary colloidal dispersion, probed using a novel microbeam small-angle X-ray scattering (SAXS) technique. To our knowledge, SAXS approaches have not been used to experimentally obtain quantitative data of concentration profiles in multicomponent colloidal films. We measured the local scattering of a film at different vertical locations using a microfocused X-ray beam and determined particle concentrations at different film depths using a linear combination analysis of the mixed film and pure film scattering data. Using small particle size ratios ranging from 2.55 to 1.25, we experimentally observed and quantify three distinct stratification configurations: inverted smallon-top, large-on-top, and no stratification. Our results show some agreement with a previously proposed stratification state diagram, although there are some limitations. Experimental verification of these stratification phenomena is critical to fully understanding the physics of particle movement and structure development during film formation, which is crucial for optimizing evaporative assembly processes for coatings.

Evaporation-driven self-assembly of binary and ternary colloidal polymer nanocomposites for abrasion resistant applications

“…However, experimental data shows that this does not occur, and the surface is made up predominantly of large latex particles. These results are similar to those obtained by Carr et al which suggest that the ZJD model overpredicts small-on-top stratification 51,52 .…”

Self-stratification of amphiphilic Janus particles at coating surfaces

“…Previous studies demonstrated that self-stratification will only happen for polymers of specific chemistry or particle mixtures of certain size ratios when a coating suspension dries. [13][14][15][16][17][18] These constraints severely limit the application of self-stratification in common coating materials. In order to achieve effective stratification, current coating formulation and binder chemistry need to be comprehensively altered, which demands significant efforts.…”

On the effect of particle surface chemistry in film stratification and morphology regulation

“…This signifies that the film formation process involving particle migration is an intricate process, controlled by several factors including the particle dimensions, interactions, solvent system, processing parameters and others. 28 Most of these studies focused on binary mixtures of latex dispersions with film forming temperature above room temperature which enabled to retain their original particle dimensions during the film forming process. In real scenario, most latex systems have a film forming temperature below room temperature to enable film formation without application of external heat.…”