Dynamics of Buried Polymer -polymer Interfaces in Thin Films

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Dynamics of Buried Polymer -polymer Interfaces in Thin Films L. B. Lurio1, Xuesong Hu2, Suresh Narayanan3, Xuesong Jiao1 and Jyotsana Lal2 1 Department of Physics, Northern Illinois University, DeKalb IL 60115 2 IPNS, Argonne National Laboratory, Argonne IL, 60439 3 APS, Argonne National Laboratory, Argonne IL, 60439 ABSTRACT: We have performed x-ray photon correlation spectroscopy (XPCS) measurements on a polymer-bilayer system comprised of 100 nm polystyrene film on top of an 80 nm polybromostyrene film, supported on a Si substrate. In order to distinguish the dynamics at the top interface from that at the polymer-polymer interface we have performed the measurement at grazing incidence. In this geometry, a standing wave is set up in the film. We derive a relation for the intensity of the standing wave and the resulting diffuse scattering. This model is compared with the measured diffuse scattering from which we extract a value of 0.7±0.4 dyne/cm for the surface tension between PS and PBrS at 180C. XPCS was then measured in each of two standing wave conditions, first where diffuse scattering only occurs at the polymer-vacuum interface and then where it only occurs at the interior polymer-polymer interface. The measured time correlation functions for each of the two interfaces show clear differences, with the polymer-polymer interface exhibiting much slower dynamics. INTRODUCTION: The static surface structure of polymers can be probed using scanning probe techniques such as AFM or via scattering with x-rays and neutrons. Measurement of the dynamics at an interface is more difficult to obtain. Nevertheless, dynamical information is critical to understanding the properties of soft materials. In particular, the modification to the dynamical properties of polymers in restricted geometries remains an open question. Measurements of dynamics at the free surface of polymers have been made using AFM(1), and dynamic light scattering(2), and more recently via XPCS(3). However, it has not been possible, up to now, to measure dynamics at a buried interface such as the polymer-polymer interface within a bilayer. In this paper, we report a new technique for measuring dynamics at a buried interface using x-ray photon correlation spectroscopy (XPCS). XPCS is an x-ray analog of dynamic light scattering and can probe the length scale dependent dynamics within a sample by measuring the time correlation function of speckles in the scattering pattern. In order to measure XPCS at a buried interface it is crucial to measure scattering from that interface alone, excluding possible scattering from the free surface or polymersubstrate interface. We have solved this problem by directing a coherent x-ray beam onto a polymer bilayer at grazing incidence so that a standing wave is set up in the sample. The phase of this standing wave can be adjusted to have a high intensity at the polymerpolymer interface and simultaneously a node at the polymer-air interface. In addition, at sufficiently small angles of grazing incidence, the x-rays do