In physics, the term “glass” is used to denote materials that solidify without crystallising. Besides common window glass, many materials such as certain polymers, organic compounds and metallic glasses fall into this category. Despite the long history of glass research, the molecular nature of glass transition and the associated relaxation processes is still not well understood. We use inelastic and quasielastic neutron scattering to study these dynamics on a microscopic length scale.
The most prominent glass specific dynamics is the α relaxation. It cannot be described by a simple exponential function and does not follow the Arrhenius law. Nevertheless, it is universally observable by methods ranging from rheology to quasielastic neutron scattering. Until now, there is no complete theory describing all features of the α relaxation over the whole temperature range.
The first quasielastic neutron scattering experiments were done at single wave vectors q and proved that the behaviour of the α relaxation is the same for microscopic and macroscopic length scales (universality). Recent experiments concentrate on the q-dependence of the α relaxation that is expected to reveal its spatial structure, e.g. the question whether it is of homogeneous or heterogeneous nature. In this respect a crossover was found from the validity of the Gaussian approximation at low q to a range where it is violated. This q-dependence can be consistently explained by a model involving discrete jumps.
- The β relaxation is a process that separates from the α relaxation at lower temperatures. The relation of both and the spatial nature of this process could be clarified by neutron scattering.
- The ‘fast process’ in the picosecond range corresponds in many aspects to the (fast) β relaxation predicted by mode coupling theory or the elementary relaxation in Ngai’s coupling model.
- The boson peak is an anomaly in the vibrational spectrum of amorphous materials whose origin and relation to the relaxation phenomena could not be clarified until today.
A largely unsettled question related to the glass transition is that of a possible existence of a length scale (cooperativity length, ξ). In some theories such a length plays a key role in explaining the properties of the α relaxation. An (indirect) access is given by reducing the system size to the of that length scale.
For this purpose glass forming liquids, polymers, and liquid crystals are put into controlled pore glasses with pore sizes of 2.5–20 nm. Dielectric spectroscopy has shown that this confinement leads to a broadening of the distribution of times for the α relaxation. This could be confirmed by quasielastic neutron scattering on the molecular level.
A new effect found in these experiments was a cut-off in the boson peak density of states. This effect could be explained qualitatively by a model of the boson peak as modified sound waves. In more recent experiments a direct access to the cooperativity length is looked for by identifying the length-dependent time scale of neutron scattering to that of thermal relaxation.