Effects of Solvent Blends on the Microstructure of Polymer Coatings Produced by Self-Limiting Electrospray Deposition (SLED)*

We investigate the controlled engineering of microstructures in polymer films using self-limiting electrospray deposition (SLED), with implications for advanced coating applications. Functional coatings play a pivotal role in a wide range of sectors, including technical textiles, microfluidic devices, and microelectronics, where precise control of surface properties is essential. In this study, we utilize polystyrene (PS) as a representative glassy material to elucidate the ability of SLED to generate tunable porous polymer composite films. These coatings are formed of regular microstructures, characterized by intercalated hollow spheres typically measuring between a few hundred nm and up 10 μm in diameter. Common electrospray parameters such as flow rate, nozzle distance, and voltage have been previously investigated, but their ability to control the microstructure of the coatings is limited. By varying the ratios of 1% wt. PS solutions in solvents with different boiling points, namely 2-Butanone (MEK) with a boiling point of ~80°C and propylene glycol methyl ether acetate (PGMEA) with a higher boiling point (~146°C), we systematically explore the influence of solvent properties on film morphology. Specifically, we are able to modify the porosity, and thereby, degree of fusion of the powder structure in the same way as if we were to increase the temperature of the spray. Consequently, the charge retention properties that lead to the self-limiting effect are also reduced, requiring the use of additional processing approaches, such as substrate biasing, to recover the same conformal effects.