Design of surface-active "high energy" bottlebrush copolymer additives for polymer films

When blends of chemically distinct polymers are processed in air, the surface is usually enriched by the polymer with the lowest surface energy, making it difficult to tailor certain surface properties by incorporating functional polymer additives. We report the development of surface-active "high energy" bottlebrush copolymer additives for commodity polymers, an advance that leverages the strong entropic attraction of branched polymers towards surfaces. The model surface-active additive is a bottlebrush copolymer with polystyrene (PS) and poly(2-vinyl pyridine) (P2VP) side chains, and the commodity polymer host is linear polystyrene (L-PS). The surface energy of P2VP is approximately 20-25% higher than that of L-PS across a broad temperature range. The bottlebrush copolymer additives enrich the surface of solution-cast films when L-PS is at least twice as long as the PS side chains, raising the surface energy relative to L-PS by 6 mN/m. The "high energy" surface is stable through thermal annealing when the relative lengths of PS side chains, P2VP side chains, and L-PS are optimized such that the entropic preference for bottlebrush copolymer at the surface is stronger than unfavorable bulk interactions that drive restructuring.