Fluctuation Dynamics of Soft Matter Studied via Multispeckle X-Ray Photon Correlation Spectroscopy
S. G. J. Mochrie
Department of Physics, Yale University, New Haven, CT
X-ray photon correlation spectroscopy (XPCS) promises insights into the dynamics of materials at shorter length scales that can be reached with light scattering, and on longer timescales and in smaller samples that can be achieved with neutron spin echo (NSE). Unfortunately, photon correlation experiments with x-rays are currently far more challenging than those with visible light: even today’s most brilliant x-ray sources produce many times fewer coherent photons than does an ordinary laser, and x-ray scattering cross-sections are generally much smaller than light scattering cross-sections. As a result, noise in XPCS measurements is generally dominated by photon counting statistics, and the feasibility of an XPCS experiment hinges on whether it is possible to collect data with a sufficient signal-to-noise ratio (SNR). A further challenge for XPCS experiments, on soft matter systems especially, is the possibility of x-ray sample damage, and how to avoid it. In spite of these obstacles, XPCS is now coming into its own, especially in studies of soft matter.
In this talk, multispeckle XPCS (http://xpcs.physics.yale.edu/xpcs.mpeg) -- using a multi-pixel x-ray area detector to acquire data simultaneously into up to 106 channels, increasing the SNR correspondingly -- will be reviewed in the context of recent experiments carried out at beamline 8-ID-I at the Advanced Photon Source to characterize the dynamics of spontaneous thermal fluctuations within polymer-based membrane phases, including vesicle (L4) and sponge (L3) phases, which occur in blends of a symmetric poly (styrene ethylene/ butylene-styrene) triblock copolymer with polystyrene homopolymer. XPCS measurements of the intermediate scattering function reveal stretched or compressed exponential behavior versus time, with a wavevector- and temperature-dependent stretching/compression exponent. This work was carried out in collaboration with Peter Falus, Matt Borthwick, Suresh Narayanan and Alec Sandy.
Cryo
TEM image of the PSEBS-in-PS sponge (L3) phase.
Narayanan and Alec Sandy XPCS is strictly a brilliance-limit technique with a SNR that scales linearly with brilliance. (The pulse-to-pulse separation also provides a natural limit on the shortest time that can be probed.) Therefore, the 104 increase in brilliance at the ERL promises a genuine revolution in XPCS. A brief discussion will be given of the sorts of XPCS experiments that will become possible with such an increase in brilliance. In addition, the desirable format of a dedicated XPCS beamline will be sketched in light of what has been learned at 8-ID-I over the last several years.
