Relaxation dynamics and evidence of scaling behaviours in aqueous polymer solutions

Some Brillouin scattering experiments on aqueous solution of Polyethylene glycol and Polyethylene glycol dimethyl ether are presented. In all the systems non-Debye relaxation processes have been detected, occurring on the picosecond time scale. The average values of the relaxation time distributions fail to follow simple Arrhenius behaviours. The temperature evolution of the relaxation time is adequately fitted after the adoption of a phenomenological Vogel-Fulcher-Tamman model. In spite of the different temperature and concentration dependences observed for each system, a unique scaling behaviour has been evidenced, at relatively low water contents, when the real and imaginary parts of the loss modulus are plotted as a function of a reduced reverse temperature, T0/T, being T0 the values of the arrest temperature as obtained by the Vogel- Fulcher-Tamman fits. The observation of a unique scaling law, for aqueous solutions of polymers characterized by the same average molecular weight but with different end groups, suggests for the establishment of similar hydrogen bonded local structures where water plays a main role, acting as a stabilizer bridging neighbouring polymer chains. The possible physical meanings of the phenomenological fitting parameters are discussed in the framework of recent approaches and the results are compared with literature reports on similar systems.