Extended-Chain Induced Bulk Morphologies Occur at Surfaces of Thin Co-Oligomer Films

One approach to create novel amphiphilic surface nanostructures with typical lateral pattern dimensions in the order of 10 nm is to employ double crystalline co-oligomers with domain sizes largely determined by the block length of extended molecular chains. The aim of the study was to test the hypothesis that the extended chain bulk morphology of an asymmetric polyethylene-block-poly(ethylene oxide) (PE-b-PEO) co-oligomer can be induced at its thin film surface. Furthermore, we explored the central role of the extended oligomer orientation and the surface affinity to achieve an amphiphilic surface nanostructure. The co-oligomer was drop-cast from dilute solution onto hydrophobic, neutral and hydrophilic substrates. Atomic-force microscopy revealed that independent of the substrate chemistry, the film thickness was quantized in integral multiples of the calculated extended chain length. At the same time, the terraces exhibited lateral domains that coarsen with increasing substrate hydrophobicity. Although subsequent annealing tends to induce dewetting, on the neutral substrate a residual polymer layer with perpendicular lamellar surface morphology remained. Grazing incidence X-ray diffraction suggested the latter to be induced by crystallization. We propose that simultaneous formation of lateral domains and discrete terraces during drop-casting are facilitated by surface-diffusion and due to a dense-packing and crystallization of vertically aligned, extended oligomer chains. Annealing permits the polymer film on the neutral surface to overcome the energy barrier for chain rotation from vertical to parallel to create amphiphilic surface nanostructures. To our best knowledge, we demonstrated for the first time that double crystalline co-oligomers can be used to pattern surfaces laterally with asymmetric lamellae with dimensions in the order of 10 nm.