Mesoscopic Simulations on Colloidal Dispersions (Star Polymers)

Dr. Anastasia Rissanou, Marianna Yiannourakou,

Dr. Ioannis G. Economou, Dr. Ioannis Bitsanis

Experimental studies on dense suspensions of multi-arm star polymers in marginal solvents have reported a reversible densification of these systems upon increasing temperature. This feature has been attributed to the similarities between multi-arm star polymers and colloidal particles interacting via soft, long ranged potentials. Furthermore, Molecular Dynamics simulations, have shown a transition of these solutions towards a "glassy" state, due to the arm expansion at high temperature which causes jamming. This "glass" formation has been studied through both static properties (i.e. structural changes in the pair correlation function, g(r)) and dynamic properties (i.e. qualitative changes in the diffusion process). Moreover, the features of the transition are consistent with those of ideal glass transitions, as described by ideal Mode Coupling Theory. Although there is no explicit experimental evidence for the crystallization of dense suspensions of multi-arm star polymers, theoretically predictions, as well as recent experiments, support this scenario. Following our study on these systems regarding their jamming transition upon increasing temperature, we explore the possibility of the existence of a crystalline phase under certain conditions by means of Molecular Dynamics simulations. Above a critical temperature, the system abandons the supercooled liquid state after some time and falls into a crystalline phase with the characteristics of an FCC crystal. Large scale motion is totally frozen in this state. We study the stability of this structure and the degree of crystallinity through a structural analysis technique. We observe that crystallization slows down further with increasing temperature. Polydispersity in star polymer size, which is always present in experimental systems, keeps the system in the meta-stable "glassy" state and retards the crystal formation. The critical temperature and the characteristic time which is required for the crystallization are studied as a function of the degree of polydispersity.