Mechanical Tunability of Hierarchical Porous Polymer Thin Films*

This work explores the tunability of the mechanical properties of porous polymeric thin films produced via self-limiting electrospray deposition (SLED). In this SLED regime, charged monodispersed droplets of glassy insulating materials are sprayed onto conductive substrates. As the particles are deposited, charge accumulates and repels additional droplets from being deposited, resulting in a thickness limiting effect. This leads to uniform conformal coatings that are able to coat complex 2D and 3D geometries. In this study, we examine a model glassy material, polystyrene (PS), a model plasticized material, PS blended with styrene-butadiene-styrene block copolymer (Kraton), and a model crosslinked plastic, SU-8 epoxy resin crosslinked with Versamid 125 crosslinking agent. Key SLED parameters were then selectively altered, in which changes in particle morphology, film density, and film thickness were observed using both optical microscopy and SEM. To measure the effects of these modifications on the mechanical properties, a combination of nanoindentation and nanoballistic analysis was conducted across a wide range of strain-rates, revealing the importance of both the composition and the form on the final mechanical properties.