Applications of X-Ray Photon Correlation Spectroscopy in Soft Matter

Yuriy Chushkin¹, Chiara Caronna¹, Anders Madsen¹, Antonio Cupane²

¹European Synchrotron Radiation Facility

ID10A beamline, 6, rue Jules Horowitz, B.P. 220, 38043 Grenoble Cedex 09

²Department of Physical and Astronomical Sciences, Palermo, Italy

X-Ray Photon Correlation Spectroscopy (XPCS) is a new technique that takes full advantage of the high intensity of partially coherent X-ray beams at third generation synchrotron sources. XPCS is based on the same principles as Dynamic Light Scattering (DLS) employing visible laser light. Thanks to the short wavelength of X-rays, XPCS can probe dynamics on small length scales and turbid samples may be investigated.

The interaction of a coherent beam with a disordered system produces a “speckle” scattering pattern. The speckle pattern reflects the exact, instantaneous distribution of the scatterers in the system. If the scatterers change their position with time the corresponding speckle pattern also changes and hence the intensity fluctuations of the speckles can provide a measure of underlying dynamics. The XPCS technique is used to study meso-scale dynamics of soft matter systems like polymers, gels, colloidal glasses, liquid crystals and supercooled liquids. The latter is of high scientific interest and the object of the study presented in the following.

The glass transition of supercooled liquids is an interesting phenomenon which continues to attract a lot of interest. Supercooling of a glass former leads to a dramatically slowing down of the molecular motion and finally a vitrification below the glass transition temperature Tg. The complex molecular processes that take place during such transition are still poorly understood. It is believed that thermally activated motion between different minima of the energy landscape gets important in the supercooled state. The transition to landscape dominated dynamics was found to happen at temperatures higher than the glass transition temperature Tg. In this work we studied the dynamical properties of two supercooled liquids propanediol and polypropylene glycol (PPG) with molecular weight 4000 (PPG-4000) by means of XPCS.

The dynamics of probe nano-particles suspended in propanediol was studied above the glass transition temperatures i.e. in the supercooled state. At high temperatures (>230 K) the particles undergo Brownian motion and the measured correlation functions are simple exponential decays. Upon cooling further and approaching the glass transition temperature (Tg=170K) the dynamics of the particles change. This can be readily seen as the correlation functions suddenly decay faster that expected from a simple exponential behavior. To analyze the data we applied the continuous time random walk model where the motion of a particle during the time interval t is described by N discrete steps. The low-temperatures data indicate that the particles exhibit hyper-diffusive, cooperative behavior. Our measurements show that the transition to this behavior occurs about 40K above Tg but the detailed connection with the glass transition remains an open question.

Another study was devoted to the investigation of surface capillary wave fluctuations on supercooled PPG-4000. All liquid surfaces undergo constant fluctuations due to the presence of thermally activated capillary waves. The dynamics of the waves is determined mainly by the surface tension and the viscosity which is a bulk characteristic of the liquid. We investigated the capillary wave dynamics of supercooled PPG-4000 by grazing-incidence XPCS. Our results show a deviation of the dispersion relations from classical theory. We present a novel viscoelastic model that can successfully describe the experimental data. It combines the well known Maxwell-Debye and Voigt-Kelvin viscoelastic models pointing out to the presence of a low-frequency elastic plateau. This solid-like behavior of the supercooled low molecular polymer liquid is intriguing and its possible origin is discussed.

[1] G. Grübel, F. Zontone, J. Alloys Comp. 362, 3, (2004)

[2] C. Caronna, Y. Chushkin, A. Madsen and A. Cupane, Phys. Rev. Lett. 100, 055702 (2008)

[3] Y. Chushkin, C. Caronna, A. Madsen, Europhys. Lett. (submitted)