Polymer self-assembly and thin film deposition in supercritical fluids

Patterning of flexible electronic devices using large-area printing techniques is the focus of intense research due to their promise of producing low-cost, light-weight, and flexible devices. The successful integration of advanced materials like semiconductor nanocrystals, carbon nanotubes and polymer semiconductors into microscale electronic devices requires deposition techniques that are robust, scalable, and enable fine patterning. To this end, we have established a deposition technique that leverages the unique solubility properties of supercritical fluids. The technique is the solution-phase analog of physical vapour deposition and allows thin films of a semiconducting polymer to be grown without the need for in-situ chemical reactions. To demonstrate the flexibility of the technique, we demonstrated precise control over the location of material deposition using a combination of photolithography and resistive heating. The versatility of the technique is demonstrated by creating a patterned film on the concave interior of a silicone hemisphere, a substrate that cannot be patterned via any other technique. More generally, the ability to control the deposition of solution processed materials with lithographic accuracy provides the long sought-after bridge between top-down and bottom-up self-assembly. In addition, we investigated the self-assembly of polymers in supercritical fluids by depositing thin films and studying their morphology using polarized optical microscopy and grazing incidence wide angle x-ray scattering. We summarized our observations with a two-step model for film formation. The first step is pre-aggregation in solution whereby the local crystalline order is established, and the solution turbulence can easily disrupt the solution-phase self-assembly. The second step to film formation is the longer length scale organization that is influenced by the chain mobility on the surface. We identified pressure and solvent additive as two powerful tools to facilitate the local crystalline order and longer length scale organization. The work demonstrated key insights necessary to optimizing thin-film morphologies and principles for understanding self-assembly in supercritical fluids that could be applied to self-assembly of materials in other contexts. Finally, we developed a simple empirical model based on classical thermodynamics that highlights the interplay of intermolecular interactions and solvent entropy and describes both the temperature and pressure dependence of polymer solubility in supercritical fluids.