Effect of confinement on the mesoscale and macroscopic swelling of thin block copolymer films

We report on the swelling behavior and the corresponding morphological behavior of cylinder-forming polystyrene-b-polybutadiene diblock copolymers, which are confined to several layers of structures. The equilibration of thin films has been done under controlled atmosphere of a nonselective solvent. In situ spectroscopic ellipsometry measurements revealed more than 10% increase of the solvent uptake with decreasing film thickness. With scanning force microscopy of the microphase separated patterns in quenched films, the correlation between the degree of the long-range order of cylinder domains and the degree of the macroscopic swelling has been established. In the case of spontaneously formed micrometer-sized topographic features with discreet film thickness (terraces), the increased solvent uptake by thinner films holds true even for isolated terraces on the mesoscale. The observation of nonhomogeneous swelling of the films on the micrometer scale brings novel insights into the properties of confined soft matter, and suggests new approaches toward the fabrication of polymer-based nanostructured responsive materials.