Microscopic dynamics in nanocomposite photosensitive films studied by X-ray photon correlation spectroscopy
Résumé
We report X-ray Photon Correlation Spectroscopy (XPCS) measurements of microscale dynamics in Langmuir-Schaeffer deposited multilayers of a photosensitive azopolymer with a low concentration of gold nanoparticles embedded. Correlation functions were measured as a function of exchanged momentum and illumination conditions (dark and UV light) and fitted with the Kohlrausch-Williams-Watts (KWW) exponential form. The microscopic dynamic of the nanoparticles was quantified, evidencing a non-Brownian superdiffusive behavior with relaxation times tau ≈ q^-1, a result analogous to what previously had been observed in the pure azopolymer [D. Orsi et al., submitted to Phys. Rev. E (2010)]. Such behavior has been related to intermittent rearrangements [L. Cipelletti et al., Phys. Rev. Lett. 84, 2275-2278 (2000)] or random dipolar interactions within an elastic medium [J.-P. Bouchaud and E. Pitard, Eur. Phys. J. E 6, 231-236 (2001)]. Photoperturbation with UV light makes the dynamics faster, in accordance with the reduction of the viscosity of the polymer found by shear rheology, but the KWW form of the correlation functions persists. At constant temperature, the dynamics of the nanoparticles embedded in the polymeric matrix is sensibly faster than the slow microscopic dynamic of the polymer. At the same time, the Vogel-Fulcher-Tammann (VFT) law for the relaxation times indicates a less pronounced temperature dependence than for the pure polymer, resulting in a slightly lower activation temperature T_A.
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