‘Marker’ grazing-incidence X-ray photon correlation spectroscopy: a new tool to peer into the interfaces of nanoconfined polymer thin films

Abstract

It is well known that physical and mechanical properties of polymeric materials confined at the nanoscale differ substantially from those of the bulk, known as the so-called ‘nanoconfinement’ effects. There is now growing evidence that the air/polymer interface and polymer/substrate interface have crucial roles in these nanoconfinement effects. Although most previous work has revealed deviations in the ‘average’ properties of entire nanometer films from the bulk, it is expected that the local properties within nanoconfined polymer thin films are different from the average ones, depending on the interplay between these interfaces. Here, we review our recent experimental results to measure local properties by utilizing the newly developed ‘marker’ grazing-incidence X-ray photon correlation spectroscopy (marker GIXPCS). The results provide new insight into the nanoconfinement effects on the local viscosity distributions within single nanoconfined polymer thin films at temperatures much greater than the bulk glass transition temperature. Various properties of ultrathin polymer films differ substantially from their bulk values. However, the critical question is whether the unusual properties are uniform throughout the films. To explore the presence of heterogeneous viscosity distributions, we have established grazing-incidence X-ray photon correlation spectroscopy for single polymer films with embedded metal nanoparticles that act as markers. In addition, the combined use of resonance enhanced X-ray scattering enables us to intensify the probing electrical field in the regions of interest within a single polymer film.