Aromatic polyesters are one of the most widely used coating polymers with superior mechanical, optical, and processing properties. Such specialty polyesters were rendered water-dispersible by functionalizing the polymers with ionic monomers in order to process them in solutions without volatile organic solvents. As a result of being partially water-soluble, these polyesters form self-assembled nanoscale (20 ~ 50 nm) particles in water which are composed of hundreds of polymer molecules. Despite their widespread commercial use, fundamental understanding of their film formation properties is still limited. We attempt to fill this gap by investigating water droplet interactions with polyester nanofilms of brilliant structural colors deposited by convective assembly. We correlated the microscale thickness of the thin films to their macroscale optical properties via the theory of thin film interference. To test the structure and properties of the dried films, we deposited sessile water drops on the films which led to partial dissolution and evaporation-induced coffee ring formation phenomena. We found that the nanoparticles spontaneously get partially re-dissolved in the sessile water droplets and driven by the capillary evaporation flows, become redistributed forming multiple colorful ring patterns which correspond to the different thickness of the films. By using these structural colors as a means to understand this polymer redistribution in detail, we characterized further the coffee ring patterns as a function of the polymer composition and molecular weight. We found that the generated patterns are a strict function of polymer composition and not governed by molecular weight. As expected by colloidal theory, this phenomenon was suppressed in presence of electrolytes. Furthermore, we show that the integrity of these thin polyester films can be significantly improved without the need for any further chemical curing. This research enables the understanding of the fundamental mechanism of film formation by such waterborne polymer nanoparticles. Moreover, the findings of our study suggest how such environmentally-friendly waterborne polymers can be used in in paintings, photonic paper, and optical displays.

Mechanisms and interactions revealed by post-deposition coffee ring formation in droplets on thin polyester films

256th National Meeting (2018)

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