Probing Buried Interfaces in Polymers with Soft X-ray Reflectivity

Studies on lamellar forming block copolymers (BCPs) organized parallel to a substrate are important for understanding assembly in confined conditions and the impact of surface effects. These have typically been conducted using either hard X-rays or neutron reflectivity where the contrast mechanism makes it difficult to distinguish depth dependent structural variations. Using soft X-rays the optical constants of a material can be controlled by varying the energy near an atomic absorption edge. As you approach the edge, the real and imaginary components can change dramatically as a function of functional group type, concentration and orientation. Additionally, unlike hard X-rays or neutrons, the absorption is no longer negligible and shifting energies can significantly change the depth profile of the electromagnetic field, providing additional control over sensitivity as a function of film depth. This technique is applied to two systems of BCP multilayers with different affinities for a surface. Strong interactions with the substrate lead to interface broadening due to the distribution of adsorbed positions along the chain. Near the free surface the interface width is reduced due to the suppression of capillary wave fluctuations.