Latexes (polymer colloids) are frequently used in household paints & varnishes, textile & carpet backings, paper coatings, inks & gloves etc. While the structure & morphology evolution of a homogenous latex film are well understood, polymer nanocomposite films made from, e.g., mixture of latexes or multiphase latexes are still a topic of intensive research. Although a composite film reflects the properties of constituent polymers, the optimized design and composition for enhanced properties & self-stratification are not known, due to a competition between kinetic & thermodynamic effects, and various transport regimes.

In this work, we study composite latexes to optimize them for self-stratified films, which provide strong adhesion with the substrate & water repellency (or any other desired property) at the air-film interface simultaneously. We tested various systems including, 1) a multiphase latex with a weakly entangled acrylic component & a strongly entangled acrylic-silicone copolymer, and 2) a blend of very low M_w silicone oil emulsion with a homogenous acrylic latex. A confident assessment of the morphology of composite films & the underlying mechanisms required complementary data from different techniques at various stages of drying: Cryo-TEM, rheology, microscopy, SIMS, SAXS & XPS etc.

We observe that the morphology of multiphase latex film is kinetically controlled, & at room temperature (~T_g +25 C) the large M_w acrylic-silicone cores are embedded in the lower M_w acrylic matrix. Drying at elevated temperatures (~T_g +75 C) significantly changes the properties of film, resulting from the unraveling of cores to form interchain entanglements. However, the thermodynamic equilibrium state of two polymeric layers on top of each other is not achievable on the timescale of observations. The blend of oil emulsion with latex performs better in terms of self-stratification of silicone at air-film interface, but it requires very low concentrations of emulsion. Future directions for this work include studying blends of more than two latexes, and mixing anisotropic particles (nonspherical shape, Janus particles etc) with latex. Our work provides a robust toolbox and strategies for studying complex latexes, and fundamental insights into the development of new smart coating formulations.

Yoo Kyung Go

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