Structural Dynamics of Glass-Forming Liquids

Doctoral thesis, 1997

Neutron and light scattering experiments on glass-forming liquids, an oxide network, a polymeric system and a polymeric electrolyte, were performed. The network glass-former B2O3 was investigated from 60 - 1300 K. The experimental results were interpreted using mode coupling theory and the soft potential model. It was found that the mode coupling theory, a theory which starts out from the liquid state, can qualitatively explain the behaviour when the glass transition is approached from high temperatures, although deviations from the theoretical predictions were found in the energy region of the Boson peak (~ 2 meV). The soft potential model, originally developed to explain properties of the glassy state, describes the behaviour of the system well when approaching the glass transition from low temperatures. The density of states, as obtained from neutron scattering, in the region of the Boson peak, was found to contain a contributions from sound waves and some additional motions. It was shown, by comparison with light scattering data, that only the density of additional states contributes to the Raman scattering, in accordance with the soft potential model. The polymer system poly(propylene glycol) (PPG) was investigated for different numbers of repeat units of the chains and for polar (OH) and non-polar (CH3) end-groups. The dependence of the dynamics on chain lengths and end-groups was shown to be large for short chains. Finally, a polymer electrolyte Mg(ClO4)2-PPG was investigated. Although it demonstrates efficient salt solvation abilities, the system is an extremely poor ion conductor. This is explained by the cations and anions of the solvated salt that provide ionic links between the polymer chains, and the links are stable enough to produce a stiff ion-linked network which hinders ionic transport.