Target Patterns Induced by Fixed Nanoparticles in Block Copolymer Films
It is well-known that thin films of cylinder-forming block copolymers (BCP) can exhibit a transition from a perpendicular to a parallel cylinder orientation with respect to the supporting solid substrate upon varying film thickness. We show that wave-like oscillations between these morphologies can be induced through the introduction of nanoparticles (NP) into flow-coated and annealed BCP films where the particles span the film thickness and are fixed by irreversible adsorption to the supporting substrate. We hypothesize that these novel "target" patterns arise from residual stresses that build up in the film while undergoing thermal treatment and film formation, and we support this hypothesis by showing the suppression of this type of pattern formation in films that are first thermally annealed near their glass transition T(g) to relax residual stress. Similar undulating height patterns are also observed in heated homopolymer films with nanoparticles, consistent with our thermally induced stress hypothesis of the target pattern formation in BCP films and pointing to the general nature of wave-like thermally induced height variations in heated heterogeneous polymer films. Similar wave patterns should be induced by lithographically etched substrate patterns arising in device fabrication using BCP materials, which makes the phenomena of technological interest. These target patterns also potentially provide valuable information about the presence of residual stresses in cast films that arise during their processing.