Isotopic selectivity in ultrathin Langmuir-Blodgett membranes of a cross-linked cellulose derivative

Cross-linked Langmuir-Blodgett films of isopentylcellulose cinnamate (IPCC) sandwiched between a polymeric diffusion couple provide an ideal system fo r probing fundamental transport processes across ultrathin (approximate to6 nm) membranes. Neutron reflectivity experiments for polystyrene/IPCC/polys tyrene trilayers (with one perdeuterated polystyrene layer) reveal that the isotopically labeled (perdeuterated) species crosses the membrane faster t han normal polystyrene of the same size resulting in displacement of the fr ee-standing membrane. Mismatched polystyrene molecular masses confirm isoto pic selectivity and also indicate size discrimination by the membrane. The membrane displacement is reminiscent of but different from Kirkendall effec t experiments in metallic systems and marker displacement studies in polyme rs.