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

Abstract

Electrospray deposition (ESD) is a versatile micro/nano coating technology that utilizes the competition between surface charge of a droplet and its surface tension to create monodisperse generations of micro/nano droplets. ESD can deposit uniform thin films by including dilute solutes in these droplets. One mode of ESD, self-limiting electrospray deposition (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 mechanical properties of the film. Specifically, we blend self-limiting polystyrene (PS) 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 as received and 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. Further, mechanical properties could be effectively fine-tuned within these compositional ranges. This signifies the 3D coating capabilities through SLED can be implemented incorporating additional functionalities and properties beyond the base matrix.

Viscoelastic properties of electrospray-deposited polymer shells via quartz crystal microbalance with dissipation (QCM-D)

“…In addition, LIPIT has been used to assess the mechanical performance of ESD films subject to high strain rates (10 1 -10 6 s -1 ). 17 The observed dissipation under these conditions was attributed to the collapse of the porous film and thermo-mechanical melting. For some samples under high-speed conditions ( >700 m/s), substrate interactions had significant influence on the material and structural behavior of the films, and infrequently led to complete disintegration of the impacting particle.…”

“…For some samples under high-speed conditions ( >700 m/s), substrate interactions had significant influence on the material and structural behavior of the films, and infrequently led to complete disintegration of the impacting particle. 17 Both LIPIT and nanoindentation exhibit experimental limitations due to nanoparticle confinement at the substrate interface, resulting from the geometric constraints of the impacting particle and indenter tip, respectively. QCM-D facilitates characterization of the complex moduli and dissipative response of polymeric materials yet is not subject to the geometric limitations observed in LIPIT and nanoindentation due to its loading configuration.…”

“…compared LIPIT and sphero-conical nanoindentation results from PS by evaluating volumetric energy dissipation. 17 They show that the primary source of dissipation during nanoindentation is due to the collapse of the hollow particles. In contrast, thermo-mechanical annealing and crazing were significant contributors to energy dissipation under high-speed impact, in the range of 200-Figure 1.…”

“…17 However, it was found that at depths greater than 50% of the original film thickness, substrate rigidity and particle confinement effects also contribute to the elastic behavior of SLED films under both loading conditions. 5,17 Since both nanoindentation and LIPIT suffer from effects of confinement, film-substrate interactions, and loading configuration such as indenter tip size for nanoindentation and projectile size for LIPIT, alternative methods to evaluate the mechanical behavior of ESD films are necessary to fully characterize them across a wide spectrum of strain rates.…”