Bottlebrush copolymers can be used as surface-active additives for commodity homopolymers. When thin films of bottlebrush copolymers and linear homopolymers are cast from solution, the bottlebrush copolymers tend to enrich one or both surfaces if the linear homopolymer is much longer than bottlebrush side chains. However, the conditions that favor surface enrichment without lateral phase separation are not entirely clear, as there is a broad parameter space for bottlebrush copolymer design and film processing. In this work, we examine the thin film structure of model blends based on linear polystyrene and bottlebrush polystyrene-co-polytertbutylacrylate or bottlebrush polystyrene-co-polyacrylic acid. The variables for this study include bottlebrush architecture and composition, bottlebrush loading, linear polystyrene molecular weight, and solvent type (for casting). When using bottlebrush polystyrene-co-polytertbutylacrylate additive, surface enrichment is usually achieved without lateral phase separation. The exceptions are when the bottlebrush additive has a long backbone and/or long side chains, and when the films are cast from a high boiling solvent. When using bottlebrush polystyrene-co-polyacrylic acid, the surfaces are enriched but more susceptible to roughening with lateral phase separation. This is likely a consequence of the higher surface energy of polyacrylic acid relative to polytertbutylacrylate as well as its greater enthalpic incompatibility with polystyrene. These studies show how to design bottlebrush copolymer additives that tailor the energy and polarity of a polymer surface, offering a simple route to tune surface functions of commodity thermoplastics.

Design rules for surface-active bottlebrush copolymer additives

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