Relaxation processes in mixtures of liquid crystals and polymers near phase boundaries and during phase separation

A multiscale time-Laplace method to extract relaxation times from non-stationary dynamic light scattering signals

Dynamic Light Scattering (DLS) is a well-known technique to study the relaxation times of systems at equilibrium. In many soft matter systems, we actually have to consider non-equilibrium or non-stationary situations. We discuss here the principles, the signal processing techniques we developed, based on regularized inverse Laplace transform, sliding with time, and the light scattering signal acquisition, which enable us to use DLS experiments in this general situation. In this article, we show how to obtain such a time-Laplace analysis. We claim that this method can be adapted to numerous DLS experiments dealing with non-equilibrium systems so as to extract the non-stationary distribution of relaxation times. To prove that, we test this time-Laplace method on three different non-equilibrium processes or systems investigated by means of the DLS technique: the cooling kinetics of a colloidal particle solution, the sol-gel transition and the internal dynamics of a living cell nucleus.

Relaxation Processes in Semidilute Solutions of Polymers in Liquid Crystal Solvents

We investigate the relaxation phenomena in a polymer (polystyrene)/liquid crystal (4-cyano-4'-n-octyl-biphenyl) system, in its homogeneous isotropic phase near the isotropic-isotropic, isotropic-nematic, and isotropic-smectic coexistence curve, using both polarized and depolarized photon correlation spectroscopy (PCS). We study this system for different polystyrene molecular weights (4750, 12 500, and 65 000 g/mol), different compositions (50, 40, 30, and 10% polystyrene (PS) by weight), and different temperatures close to phase boundaries. First of all, we determine the phase diagrams of this system for the different molecular weights. The shape of the phase diagrams strongly depends on the molecular weight. However, in all cases, at low temperatures, these systems separate into an almost pure liquid crystalline (LC) phase and polystyrene-rich phase. PCS measurements show that the relaxation processes in the homogeneous phase are not affected by the proximity of the nematic, or smectic, boundaries (even at a temperature of 0.1 degrees C above the phase separation in two phases). In polarized PCS experiments, we always see three relaxation processes well separated in time: one, very fast, with a relaxation time of the order of 10(-5) s; a second one with a relaxation time within the range 10(-2)-10(-3) s; and a last one, very slow, with a relaxation time of the order of 1 s. Both the fast and slow modes are independent of the wave vector magnitude, while the intermediate relaxation process is diffusive. In depolarized PCS experiments, the intermediate mode disappears and only the fast and slow relaxation processes remain, and they are independent of the magnitude of the wave vector. The diffusive mode is the classical diffusive mode, which is associated with the diffusion of polymer chains in all polymer solutions. The fast mode is due to the rotational diffusion of 4-cyano-4'-n-octyl-biphenyl (8CB) molecules close to polystyrene chains (transient network). Finally, we assign the slowest mode to reorientational processes of small aggregates of PS chains that are not dissolved in 8CB.