Spin-polarized xenon-129 NMR study of a polymer surface

Abstract

A method for NMR studies of low surface area materials using optically pumped xenon gas is described. The method has been used to investigate spin-polarized xenon adsorbed onto poly(acry1ic acid). The temperature dependence of the xenon-surface interaction, as measured by the xenon chemical shifts extrapolated to zero pressure, is shown to be consistent with a simple model of chemical exchange between the gas and adsorbed phases. The magnitude of the surface contribution to the 129Xe chemical shift indicates a relatively strong interaction between xenon and poly(acry1ic acid), possibly due to the polar carboxylic acid functional groups at the polymer surface. From the pressure dependence of the It9Xe shift, the diffusion coefficient of xenon on poly(acry1ic acid) is estimated to be 3.3 X lo-' cm2/s. I. Iatroduction The surface and interfacial characteristics of solid systems, such as inorganic solids, polymers, and thin films, are important to many of their useful material properties. The task of relating the microscopic structure to their macroscopic physical properties, however, has been hindered by the lack of suitable techniques capableof probing these materials. Due to their instability under ultrahigh-vacuum conditions, organic solids and catalytic materials such as metal oxides are difficult to study by the surface science techniques that have been successfully employed for singlecrystal studies. As a nonintrusive experimental method capable of giving dynamic and structural information at an atomic level, nuclear magnetic resonance spectroscopy should be an attractive probeof heterogeneous surfaces. Indeed, through thedevelopment of high-resolution techniques, NMR has contributed greatly toward the chemical and physical understanding of bulk so1ids.l The main obstacle to the successful application of NMR to surface studies is, of course, its low sensitivity compared to other spectroscopic methods as well as the problem of distinguishing between the surface and bulk spins. Various approaches to increasing the NMR signal have been pursued, such as isotopic enrichment, high applied magnetic fields, millikelvin NMR? and cross polarization.3 These experiments, which are based on increasing the quilibrium magnetization, do not, in general, give the signal enhancements rquired to detect the surface spins of materials with surface areas lower than - 10 m2/g. Another approach toward enhancing the magnetic resonance signal is to produce a large nonquilibrium magnetization by optical p~mping.~ Large nuclear spin polarization of xenon and other noble gases can be created via spin exchange collisions with Optically pumped alkali metal vapor. These experiments have long been employed to measure fundamental physical quantities and to study spin relaxation processes in the gas phase.5 Due to the formation of relatively long lived van der Waals molecules, xenon can be efficiently spin polarized through collisions with optically pumped rubidium vapor: allowing the investigation of surface interactions between i3lXe gas and glass