Polymers exhibit an inherently poor tendency to mix with other components (such as additives, particles or other polymers) due to the low entropy of mixing that derives from their macromolecular nature and typically weak intermolecular forces. The propensity for polymers to phase separate can be detrimental, for example, in polymer recycling, where incomplete sorting of waste plastics can yield recycled products with poor mechanical properties. But immiscibility can also be exploited, such as in block copolymers, chemically dissimilar polymers covalently joined that phase separate into nanoscale periodic structures of wide technological interest. Our research seeks to develop models that predict the phase behavior and morphologies of polymer blends and block copolymers in bulk and thin film geometries, and to apply that understanding towards the design of new materials and structures.