Following the Kinetics of a Chemical Reaction in Ultrathin Supported Polymer Films by Reliable Mass Density Determination with X-ray Reflectivity

摘要

Ultrathin supported films of polymers are frequently used fornapplications such as transistors,n1nsensors,n2nintelligent materials,n3nand model systems,n4nto name but a few. Many of these function-nalities involve chemical reactions, but analysis of such films isnchallenging because of the minute amount of film material inncontrast to the copious amount of substrate. Often the surfacenmorphology is probed by atomic force microscopy and the filmnthickness with ellipsometry or profilometry, while the chemicalncomposition is determined by X-ray photoelectron spectroscopyn(XPS) or various surface-sensitive applications of IR or Ramannspectroscopy. Mass densities of ultrathin polymer films, however,nhave rarely been reported. In this communication, we report thatnreliable determination of the mass density of an ultrathin polymernfilm by X-ray reflectivity (XRR) is feasible within limited thicknessnregimes. Furthermore, we demonstrate how the density analysisncan be applied to follow the kinetics of a chemical reactionnoccurring within the film. Previous efforts to measure the densitynof organic films have imposed limitations on the instruments ornmaterials, requiring a special (multichannel) detectorn5nor deuteratednsamples.n6nThis also includes the recently introduced resonant softnX-ray reflectivity technique designed to enhance the contrast withinnpolymer bilayers, which utilizes a synchrotron beamline for datancollection.n7nA more effortless method to follow organic reactionnkinetics on solid supports by magnetic levitation was publishednrecently by Mirica et al.,n8nbut this method is not applicable to thinnfilms with lateral dimensions of macroscopic scale.