Dynamical heterogeneities in glass-forming materials

Cooperative dynamics around the glass transition leads to complex motional behavior of the individual molecules, resulting in non-exponential relaxation. The nature of this non-exponentiality is clarified theoretically as well as experimentally. The non-exponentiality may be due to heterogeneous relaxation (superposition of exponential processes with different rate constants) or homogeneous relaxation (identical intrinsically non-exponential processes). A precise definition of both limits is given. It is shown that the type of relaxation, i.e. to which degree heterogeneous and homogeneous contributions are present, reflects geometrical properties of the dynamics. The heterogeneous contribution can be further classified according to the timescale of fluctuations within the underlying distribution of relaxation rates, thereby introducing the concept of the rate memory. Determination of the type of relaxation and the rate memory essentially boils down to analysing the properties of multi-time correlation functions. Experimentally they are accessible by multidimensional NMR methods. Alternatively, they can be directly obtained from computer simulations. In order to clarify these concepts, the dynamics of polymers above the glass transition is analysed experimentally as well as numerically. The rotational dynamics of polymer segments turns out to be mainly heterogeneous with only small homogeneous contributions. Interestingly, the fluctuations within the heterogeneous distribution occur on the same timescale as the reorientation process itself.