  We report the results of X-ray photon correlation spectroscopy (XPCS)
experiments on Langmuir Blodgett multilayers of a photosensitive azo-polymer.
Time correlation functions have been measured at different temperatures and
momentum transfers (q) and under different illumination conditions (darkness,
UV or visible). The correlation functions are well described by the
Kohlrausch-Williams-Watts (KWW) form with relaxation times that are
proportional to q^-1, which in other systems have been explained in terms of
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)]. The
characteristic relaxation times follow the well known Vogel-Fulcher-Tammann law
describing the temperature dependence of the bulk viscosity of this polymer. UV
photoperturbation accelerates the relaxation dynamics, in qualitative agreement
with the fluidification effect of UV photo-perturbation previously observed by
surface rheometry, and is used to drive the system out of equilibrium.
Transient dynamics is characterized, by the variance chi of the two-times
correlation functions. A clear peak in chi appears at a well defined time tau_C
which scales with q^-1 and with the ageing time, in a similar fashion as
previously reported in colloidal suspensions [O. Dauchot et al., Phys. Rev.
Lett. 95, 265701 (2005)]. From an accurate analysis of the correlation
functions we could demonstrate a temperature dependent cross-over from KWW
compressed to simple exponential behaviour, which is modified by the
fluidification due to the optical pumping of the cis-trans isomerisation of the
side-chain azobenzene group.
