Determining the Self-Limiting Electrospray Deposition Compositional Limits for Mechanically Tunable Polymer Composites

Electrospray deposition (ESD) is a versatile micro-/nanocoating technology that utilizes the competition between the surface charge of a droplet and its surface tension to create monodisperse generations of micro-/nanodroplets. ESD can deposit uniform thin films by including dilute solutes in these droplets. One mode of ESD, self-limiting ESD (SLED), has been shown to exist when glassy polymers are sprayed in a volatile solvent below the polymer glass-transition temperature (Tg). This leads to charge accumulation on the coating surface that slows the growth of the film thickness. Since solutes can be easily blended in dilute ESD solutions, we investigate the SLED limits of self-limiting and non-self-limiting solute blends. As a motivating application, we focus on the mechanical properties of the films. Specifically, we blend self-limiting polystyrene and SU-8 epoxy resin with different non-self-limiting mechanical modifiers, such as plasticizers and curing agents. To characterize the resulting morphologies and mechanical properties, we employ scanning electron microscopy and nanoindentation of the as-received porous and thermally smoothed films. The results illustrate the formation of composited polymers that exhibit self-limiting ability by SLED, depending on the interaction between the two components. Furthermore, mechanical properties could be effectively fine-tuned within these compositional ranges. This signifies that the 3D coating capabilities available through SLED can be enhanced by incorporating additional functionalities and properties beyond the base matrix.