Relaxation dynamics in molecular glass-formers with systematic structure modifications

Glasses are amorphous materials which do not exhibit the long-range order or periodicity found in crystalline solids. A glass is formed by cooling a liquid at a sufficient rate such that crystallisation can be avoided. The structural disorder of glasses give them unique properties which make them suitable for a wide range of industrial, pharmaceutical or biological applications. Glass-forming materials generally exhibit several characteristic mechanisms of molecular motion. The physical origins and interrelation between these mechanisms are not well understood. In order to address this, detailed investigations of how glass-transition related dynamics are affected by systematic modification of the molecular structure are needed. This thesis concerns the measurement of the glass-forming properties of three series of molecular glass-formers. These series are comprised of samples which vary systematically in their structure: an alkylbenzene series involving the systematic variation of the length of an alkyl-tail attached to a phenyl-ring and two series involving the successive oligomerisation of styrene and alpha-methylstyrene. The glass forming properties of these series were analysed using Broadband Dielectric Spectroscopy (BDS) and Differential Scanning Calorimetry (DSC). Thermogravimetric Analysis was also employed in order to optimise the sample preparation procedure. The work in this thesis identifies and characterises the detailed molecular weight dependent behaviour of several key relaxation mechanisms in the glassy and supercooled state of three different glass-forming systems. Strong similarities between the relaxation behaviour of the two polymeric and the alkyl chain modified benzene series were found. This demonstrates that much of the observed phenomenology is remarkably general and the work forms a basis for developments of models to address the glass-transition and glassy behaviour. Moreover, it is demonstrated that the glass transition in the three different series of samples behave in a highly similar manner with regards to the system molecular weight and strong support is found for a link between the primary structural relaxation that exists in the supercooled state and the secondary relaxation mechanisms that persist within the glassy state.