(485c) Geometrically Patterned Interfaces on Neat and Composite Polymeric Films By Electrohydrodynamic Film Patterning

We demonstrate PARC’s electrohydrodynamic film patterning (EHD-FP) capability, enabling rapid fabrication of multi-scale hierarchical features on neat and composite polymeric films through the application of an electric field in a contactless manner. Typical implementation of EHP-FP involves application of high electric fields across a thin gap ~O(µm) between two conductive substrates that are used as electrodes. Often the wet polymeric film is spin-coated on the bottom substrate, which is electrically grounded, and the top electrically conductive substrate, which can involve a two-dimensional or a three-dimensional pattern, is the high voltage electrode. A wide range of different electrode materials can be used in this process, but we present patterned films using doped silicon wafers etched using conventional cleanroom techniques as electrodes unless stated otherwise. We present results produced using our batch setup allowing fundamental studies of the core physics. The process can be extended to a continuous process, and yet PARC is working on scaling EHD-FP to large area continuous roll-to-roll processing.

We had demonstrated this non-contact patterning method as part of a contract with the Defense Advanced Research Projects Agency (DARPA) under contract HR0011-14-C-0036 to create 25mm X 75 mm patterns on UV curable materials to evaluate the improvements in mechanical properties. In this study, we expand our exploration to thermoplastic films and study different polymers with varying viscosity, surface tension, thermal and electrical properties in molten form. It is important to understand the unique processing capabilities and limitations of different polymers for obtaining sharp and precise patterns. Our system is designed to operate at high temperatures with a precise gap control and can accommodate high temperature thermoplastics. Prior to patterning process, we prepare uniform spun coat films from polymer solutions. We prepare thin to thick polymeric films (in the range of ~5 to 100 µm) from polyvinylbutyral, polycaprolactone, polystyrene and also from conductive materials. We disperse inorganic filler micro-particles and study the alignment of microparticles in polymeric films under an external electric field with an emphasis on particle aspect ratio and particle loading. We systematically study the effect of physical parameters; electric field strength, spatially and temporally varying electric fields, initial polymer film thickness, and operating temperature. We map the process parameters influencing the pattern height and present several key trends for neat polymer films. While the strength and distribution of electric field has a direct impact on pattern height, controlling the interplay of electric field and molten polymer properties is essential in fine tuning the height and resolution of the obtained patterns on the polymer film surface. Our findings show higher operating temperature leads to taller patterns which is attributed to lower viscosity of the polymer at higher temperatures. For a fixed electric field strength, we observe an increase in feature height in three folds (from ~6µm to ~18µm) on ~50µm thick polyvinylbutyral films when the operating temperature is increased from 155^oC to 175^oC, which are both well beyond the onset of softening temperature of polyvinylbutyral. We also observe differences in pattern resolution for different polymers with different viscosities under similar processing conditions. In addition to geometrically patterning neat polymer films, we study the alignment and ordering of micron size particles in a polymer matrix under an external electric field. We investigate the behavior of particles with different electrical properties and aspect ratios. Ordering and aligning particles in a polymer matrix enables enhanced interfaces with better transport properties that is of paramount importance in a wide span of applications from membrane separations, chemical reaction engineering to electrochemical energy conversion and storage.