Self-stratification of model blends of colloidal spheres
has recently
been demonstrated as a method to form multifunctional coatings in
a single pass. However, practical coating formulations are complex
fluids with upward of 15 components. Here, we investigate the influence
of three different rheology modifiers (RMs) on the stratification
of a 10 wt % 7:3 w:w blend of 270 and 96 nm anionic latex particles
that do not stratify without RM. However, addition of a high molar
mass polysaccharide thickener, xanthan gum, raises the viscosity and
corresponding Péclet number enough to achieve small-on-top
stratification as demonstrated by atomic force microscopy (AFM) measurements.
Importantly, this was possible due to minimal particle–rheology
modifier interactions, as demonstrated by the bulk rheology. In contrast,
Carbopol 940, a microgel-based RM, was unable to achieve small-on-top
stratification despite a comparable increase in viscosity. Instead,
pH-dependent interactions with latex particles lead to either laterally
segregated structures at pH 3 or a surface enrichment of large particles
at pH 8. Strong RM–particle interactions are also observed
when the triblock associative RM HEUR10kC12 is used. Here, small-on-top,
large-enhanced, and randomly mixed structures were observed at respectively
0.01, 0.1, and 1 wt % HEUR10kC12. Combining rheology, dynamic light
scattering, and AFM results allows the mechanisms behind the nonmonotonic
stratification in the presence of associative RMs to be elucidated.
Our results highlight that stratification can be predicted and controlled
for RMs with weak particle interactions, while a strong RM–particle
interaction may afford a wider range of stratified structures. This
takes a step toward successfully harnessing stratification in coatings
formulations.