Using Neutron Reflectometry to Investigate Interactions across Polymer Thin Films

David Uhrig, Jamie Messman, Jimmy Mays, and Phil Britt, CNMS
J. F. Ankner, X. Tao, C. Halbert, Spallation Neutron Source, ORNL
S. M. Kilbey, II, Clemson University, Clemson, SC, and CNMS Visiting Scientist

Understanding how ultrathin layers of polymers organize at the solid-fluid interface and adjust their structure in response to species in the solution environment is relevant to a broad array of technological applications for polymer-modified surfaces, ranging from biomaterial coatings to gates on microfluidic devices or membranes. Researchers in the Macromolecular Complex Systems group of the Center for Nanophase Materials Science are collaborating with scientists at the Spallation Neutron Source and using neutron reflectometry to investigate the molecular-level structure of polymer nanolayers and their response to complementary polymers in solution. Neutron reflectometry not only enables the nanoscale structure to be examined, but by using isotopic substitution (e.g., deuterium for hydrogen), it becomes possible to distinguish the polymers of the surface layer from those in solution, thereby allowing the absorption and penetration of chains into the layer to be investigated.

Results emerging from studies of isotopically-substituted polystyrene nanolayers in contact with dilute polystyrene solutions are providing experimental verification of theories that predict compaction of the layer due to repulsive osmotic interactions, and also suggest absorption of low molecular weight species. The resources and tools available at CNMS and coming online at SNS provide unprecedented opportunities for understanding the links between the design, assembly, structure and dynamics of “soft” (polymeric) materials, engendering “bottom-up” strategies for the rational design of polymer thin films for numerous technologies.

This work was carried out within Partner User Project CNMS2003-041, including contributions from researchers sponsored by the BES Division of Scientific User facilities. Partner users were supported by the BES Division of Chemical Sciences, Geosciences and Biosciences under FWP ERKCC49.

Reflectivity measurements of surface-tethered polymer nanolayers (left, cartoon in middle) reveal the structure (right) of the interfacial layer and structural rearrangement upon exposure to and interaction with ultra-dilute solutions of polymers. Experiments were carried out on the SNS Liquids Reflectometer at a beam power of 30 kW and source frequency of 15 Hz.