Surface structure and dynamics of supercooled polymer melts and metal-polymer nanocomposites

Abstract

The focus of this work is the investigation of the surface properties of supercooled liquids and metal-polymer nanocomposites around the glass transition temperature TG at nanometer length scales. This thesis provides two major contributions in the field of surface sensitive x-ray scattering techniques employed to characterize the behavior of surface fluctuations in polymeric liquids around TG and the nanoparticle dynamics in out-of-equilibrium systems. The research topic of the first part of this thesis consists in the determination of the cooling rate dependent freezing of capillary waves on the glass forming liquid poly(propy- lene glycol) by means of in-situ cooling x-ray reflectivity measurements. The freezing in of density fluctuations while approaching the glass transition has been known in the bulk for a long time, but it has not been determined until now whether an analogous effect also occurs at the free surface. Above TG the surface roughness o can be described by the standard capillary wave model for simple liquids, whereas the surface fluctuations are frozen in at temperatures below TG. As the state of a glass forming liquid strongly depends on its thermal history, for fast cooling rates this effect occurs at higher temper- atures than for slow cooling. Large shifts of TG to 240 K compared to the bulk value of 196 K have been observed. This represents a significant difference to the bulk where the dependence of TG on the cooling rate is typically weak. The temporal evolution of gold nanoparticles moving on the surface of thin polystyrene films is investigated in the second part of this work. As the measurements are performed around the glass transition of the polymer, the metal-polymer system is out of equilib- rium and therefore exhibits interesting dynamics. The dynamic structure factor f(qjj) of the gold clusters with the lateral wavevector transfer qjj and the relaxation time is measured with two-dimensional x-ray photon correlation spectroscopy. There is cur- rently extensive interest in bulk soft matter systems where a similar behavior of the dynamic structure factor and aging phenomena are observed. Above the glass transition of the polymer the peculiar modified exponential form f(qjj)..] is found with 0:7 < alpha < 1:9, depending on sample age and temperature. The relaxation rates scale linearly with qjj, excluding a simple Brownian diffusive motion. This type of behavior, already observed in aging bulk soft matter systems, is explained by a power law distribution of particle velocities due to ballistic motion.