2015-02-18
10:15 at HCI J 574

Dynamics in polymers blends close to and below the glass transition temperature: aging and rejuvenation.

Grégoire Julien

Laboratoire Polymère et Matériaux Avancés, UMR 5268 CNRS/Solvay-Rhodia, France

In this work, we propose a model for describing the dynamics in polymer blends where at least one of the species corresponds to molecular weight below the entanglement threshold. The model incorporates the he- terogeneous nature of dynamics close to Tg on a scale of dynamical heterogeneities of size ξ of order 3-5 nm. We assume that spatial distributions of relaxation times are the consequence of concentration fluctuations. A Gibbs free energy model (which is an extension of the Flory-Huggins model) for compressible blends is proposed for calculating driving forces. The spatial dynamics follows then from an Onsager like description and the evolution of concentrations is calculated by Langevin equations on the scale of dynamical heteroge- neities. The dynamics takes also into account a ”facilitation mechanism” which describes the relaxation of slow dynamic heterogeneities when surrounded by faster subunits as due to free volume diffusion or diffusion of different components. This model is applied to study phase separation close to and below Tg upon co- oling, and rejuvenation upon heating. In the course of a phase separation, slow structures are observed to be forming in coexistence with faster ones. In the same time, the global dynamics of the system slows down and domains grow like the logarithm of the time. During rejuvenation, the temperature is increased again in a totally miscible range of temperatures far above Tg. In particular, we observe that glassy morphologies melt much faster the elapsed time required to build them during aging : there is a temporal asymmetry between the aging and the rejuvenation process. We also show that the facilitation mechanism is a key for describing the rejuvenation of such a material. Finally, this work can be used for designing long-lived nanocomposite polymer materials with a desired shape of morphologies.