Glass Transition Dynamics in Soft Condensed Matter

Doctoral thesis, 2002

Materials of very different character, ranging from polymers to metals can be made into glasses. The structural and dynamical properties of glass-forming materials are fundamentally different from those found in crystals. Hence, glassy materials exhibit unique characteristics, which make them suitable for a wide range of applications. This thesis concerns experimental investigations of the glass transition dynamics for a range of different glass formers, from molecular glasses to ultrathin polymer films. By the use of dielectric relaxation spectroscopy, a number of oligomer chain-length series have been investigated in the supercooled regime. The chosen oligomer systems had the same basic building block, but the chain-length, and the end-capping were varied. Using the same technique, glass-formers confined within a clay have also been investigated. Results are presented on: (i) the connection between molecular architecture and the dynamics in the supercooled state, (ii) the role of secondary relaxations and, (iii) the origin of the currently much debated excess wing. An understanding, on the microscopic level, of polymers at surfaces and in confined geometries is becoming increasingly important. A defining parameter for any amorphous polymer is its glass transition temperature, Tg. By the use of Brillouin light scattering spectroscopy, Tg was studied in ultrathin (>~200AA) free-standing polystyrene films for a wide range of film thicknesses and molecular weights, M. Tg decreases strongly when the film thickness is reduced for all M. The data provide evidence for two competing physical mechanisms, one due to a finite size effect and the other due to a chain confinement effect. A model quantifying the effect of a characteristic length scale for glass transition dynamics in thin films is introduced. A quartz crystal microbalance (QCM) techique was used to study the adhesion of small particles to the surface of PS films. An anomalous behaviour was observed, and interpreted in terms of segregation of chain-ends to the free surface. Collective vibrational excitations in an archetypal glass former have been studied with inelastic X-ray scattering spectroscopy. In the mesoscopic regime, the acoustic-like excitations display a linear dispersion up to intermediate momentum transfers, where a cross-over to a momentum transfer independent excitation energy and width is observed.