Chain Conformation in Ultrathin Polymer Films Using Small-Angle Neutron Scattering

Abstract

Small-angle neutron scattering (SANS) experiments were conducted on isotopic polystyrene blend films spin-coated onto hydrogen passivated silicon wafers over a wide range of wavevectors, q (0.56 ≤ qRG,bulk ≤ 50, where RG,bulk is the unperturbed radius of gyration of the chains). The films thickness varied from “bulk” (D/RG,bulk ≈ 50, where D is the film thickness) to smaller than RG,bulk (≈10 nm, i.e., D/RG,bulk ≈ 0.5). Samples were annealed for long times at temperatures well above the glass transition temperature to ensure thermal equilibrium and then quenched to room temperature. Scattering experiments were then performed in a transmission geomtery. Analysis of the data using the RPA model (most appropriate for qRG,bulk ≤ 1) and the Kratky approach (4 ≤ qRG,bulk ≤ 50) independently confirms that the chains retain their unperturbed Gaussian conformations in the direction parallel to the surfaces, i.e., normal to the confinement direction. These results lend significant credence to computer simulations and past analytical theories, which suggest that chain conformation in these thin films can be viewed as reflected random walks.