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44858-AC7
X-ray Photon Correlation Spectroscopy Studies of Nanoscale Particle Motion Within Heterogeneous Complex Fluids
Microrheology is an emerging technique that employs the motion of small probe particles suspended in complex fluids to characterize local mechanical properties of the fluids. We are pursuing a research program that applies x-ray photon correlation spectroscopy (XPCS) to microrheology by investigating the dynamics of nanometer-scale particles within a set of model heterogeneous complex fluids. Due to its access to dynamics at short lengths and long times, XPCS has strong potential as a microrheological technique, expanding greatly the range of studies feasible with established techniques that employ visible light [1]. Our experiments to date have focused on nanometer-scale gold particles within short-chain polystyrene (PS) melts of molecular weight between 2K and 36K g/mol. The Au particles have a radius of approximately 2 nm, are functionalized with PS chains to stabilize them against aggregation, and are in a highly dilute concentration within the melt (Au volume fraction 0.0004). Such a dilute concentration of particles and their stability places the PS melts outside the regime of polymer nanocomposites, in which nanoparticle concentrations of few percent can change melt viscosity significantly, and leads us to consider the Au particles as isolated tracers.
At temperatures above the glass transition of the PS, the dynamic structure factors, f(q,t), that we extract from the XPCS measurements indicate diffusive motion of the nanoparticles. Specifically, f(q,t) has an exponential lineshape with a characteristic decay time that depends on wave vector as tau = 1/(Dq^2). The diffusion coefficients D obey a Vogel-Fulcher temperature dependence, as expected for polymer melts near the glass transition. We are currently performing rheometry studies to correlate the observed diffusivity of the nanoparticles with the macroscopic melt viscosity.
The nature of the observed dynamics changes when the melts are quenched through the glass transition. Specifically, at these low temperatures the XPCS results indicate non-diffusive motion that can be modeled as strain in the melt resulting from localized stress relaxation. These dynamics evolve with time following the quench, indicating that they are coupled to the aging behavior of the polymer glass. The signature features of these dynamics are a compressed-exponential lineshape for f(q,t) with a decay time that varies inversely with wave vector, tau ~ 1/q [2]. Such non-diffusive motion has been observed in numerous disordered soft solids, such as colloidal gels [2,3], clay suspensions [4], and concentrated emulsions [2,5], as well as in a number of polymeric materials, including block copolymers [6], blends [7], nanocomposites [8], and filled elastomers [9]. However, in each of these previous polymer systems, complicating factors such as mesophase ordering and particle-particle interactions have been identified as likely playing a role in creating the internal stress that drives the strain-like motion. By focusing on a simpler system, specifically low molecular-weight PS melts containing a dilute concentration of stable nanoparticle tracers, our work has provided evidence that this motion is intrinsic to the quenched melts. Thus, we believe these results are significant in demonstrating that this non-diffusive motion can be inherent to quenched molecular and polymer glasses, and therefore in showing that this phenomena is a general feature not only of soft but also hard disordered materials.
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[2] L. Cipelletti, L. Ramos, S. Manley, E. Pitard, D. A. Weitz, E. E. Pashkovski, and M. Johansson, Faraday Discuss. 123, 237 (2003).
[3] B. Chung, S. Ramakrishnan, R. Bandyopadhyay, D. Liang, C. F. Zukoski, J. L. Harden, and R. L. Leheny, Phys. Rev. Lett. 96, 228301 (2006).
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[5] H. Guo, J. N. Wilking, D. Liang, T. G. Mason, J. L. Harden, and R. L. Leheny, Phys. Rev. E 75, 041401 (2007).
[6] P. Falus, M. A. Borthwick, S. Narayanan, A. R. Sandy, and S. G. J. Mochrie Phys. Rev. Lett. 97, 066102 (2006).
[7] M. L. Ruegg, A. J. Patel, S. Narayanan, A. R. Sandy, S. G. J. Mochrie, H. Watanabe, and N. P. Balsara, Macromol. 39, 8822 (2006).
[8] R. A. Narayanan, P. Thiyagarajan, S. Lewis, A. Bansal, L. S. Schadler, and L. B. Lurio, Phys. Rev. Lett. 97, 075505 (2006).
[9] F. Livet, F. Bley, F. Ehrburger-Dolle, I. Morfin, E. Geissler, and M. Sutton, J. Synchro. Rad. 13, 453 (2006).
