Defects, solvent quality, and photonic response in lamellar block copolymer gels

Stimuli-responsive photonic gels are made from the lamellar block copolymer (BCP) poly(styrene-b-2-vinylpyridine) (PS-P2VP), where the photonic responses are triggered by swelling/deswelling of the P2VP block with a selective solvent. When compared to isotropic swelling in chemically cross-linked homopolymer gels, the P2VP block in the lamellar BCP shows significantly lower degrees of swelling in alcohol-water cosolvents. The glassy PS layers completely constrain the lateral expansion of the P2VP gel layers and the dislocation defect network that develops during BCP self-assembly provides a counter force to vertical swelling. A model based on Flory-Huggins mixing and dislocation network strain energy is proposed to capture the swelling behavior of the BCP and is then used to estimate the dislocation network density in the lamellar BCP. This work establishes the quantitative relationship between the reflective color of the photonic gel, the effective χ parameter of the swellable block and the solvent, and the defect density of the BCP film and demonstrates the potential utility of these photonic materials as a quick means to measure solvent quality or defect density.