Structure and dynamics of cylindrical micelles at equilibrium and under shear flow.

The dynamics and rheology of semidilute unentangled micellar solutions are investigated by Langevin dynamics mesoscopic simulations coupled to a microreversible kinetic model for scissions and recombinations. Two equilibrium state points, differing by the scission energy and therefore by the corresponding average micelle length, have been examined. The kinetic rates are tuned by an independent parameter of the model, whose range is chosen in such a way that the kinetics always strongly couple to the chain dynamics. Our results confirm, as predicted by Faivre and Gardissat, that the stress relaxation, as well as the monomer diffusion, is characterized by a time tauLambda, defined by the lifetime of a segment Lambda, whose Rouse relaxation time is equal to its lifetime. Moreover, the power-law dependence of the zero-shear viscosity versus tauLambda was evidenced. Under stationary shear, the chains are deformed and their average bond length is increased, which enhances the overall scission frequency. In turn, this induces an overall shortening of the chains in order to increase the overall corresponding chain-end recombination frequency, as required by the stationary conditions. Nonequilibrium simulations show that the chain deformation and orientation, as well as the rheology of the system, can be expressed as universal functions of a single reduced shear rate betaLambda=gammatauLambda (with gamma the bare shear rate). Furthermore, local analysis of the kinetics under stationary shear gives insights on the variation of the average length with shear rate.

Kinetics and dynamic properties of equilibrium polymers.

The statistical mechanics and scission-recombination mechanism of self-assembling linear micelles are investigated by Brownian dynamics using a newly proposed mesoscopic model representing the micelles as equilibrium polymer chains. A semidilute concentration regime, yet dynamically unentangled, is considered over a wide range of scission/recombination rates. We focus on the analysis of short and long time behaviors of the scission and recombination mechanisms. Our results show that at time scales larger than the life time of the average chain length, the kinetics is in agreement with the mean-field kinetic model proposed by Cates and Candau [J. Phys.: Condens. Matter 2, 6869 (1990)] provided the kinetic constants are estimated as effective ones. These values do take into account through a transmission coefficient that a fraction of scission/recombination events is correlated over a short time (diffusion controlled mechanism) and thus turn out to be ineffective reactive events by annihilation effects. By studying macroscopic relaxation phenomena such as the average micelle length evolution after a T jump, the monomer diffusion, and the zero shear stress relaxation function, we confirm that the effective kinetic constants found are indeed the relevant parameters when macroscopic relaxation is coupled to the kinetics of micelles.

Surface ordering of diskotic liquid crystals.

We present Monte Carlo simulations of diskotic molecules using the Gay-Berne potential in a slab geometry. The disk-wall interaction is described by two different functions according to whether or not the equilibrium distance is dependent on the relative orientation of the disk to the wall. Furthermore, by changing the parameters of these potentials, we model either homeotropic (face-on) or planar (edge-on) anchoring of the disks. We have found that the isotropic-nematic transition does not change in comparison with the bulk situation. The temperature of the nematic-columnar transition, on the contrary, is found to increase for homeotropic anchoring, and decrease for planar anchoring, independently of the details of the potential. We explain the decrease of the transition temperature in the planar anchoring situation as the result of an induced frustration, due to the competition between the two orientations induced independently by the upper and lower walls.

Influence of shape and energy anisotropies on the phase diagram of discotic molecules.

We present Monte Carlo simulations of discotic molecules using the Gay-Berne potential with shape (kappa) and energy (kappa(')) anisotropies. Following the previous work of Bates and Luckhurst [J. Chem. Phys. 104, 6696 (1996)] at kappa=0.345, kappa(')=0.2 when we determine the sequence of different phases at the same reduced pressure P(*)=50, we find an additional phase at low temperatures corresponding to an orthorhombic crystalline phase and we characterize it. Keeping the shape anisotropy fixed at kappa=0.2, we determine the evolution of the phase diagram with varying energy anisotropy. At high kappa('), low anisotropy, the system is not able to build columns while at low kappa('), the system exhibits both orthorhombic crystal as well as hexagonal liquid crystal phases over a wide range of pressures and temperatures. The domain of stability of the nematic phase is found to systematically shift towards higher pressures as kappa(') decreases.