Sekundäre Relaxationen in amorphen Festkörpern

Abstract

The mechanical properties of a solid are determined by the character of the active relaxation modes. They govern how a sample relaxes if an external stress is applied and are dependent on the temperature as well as on the frequency of the excitation. This thesis is focused on the experimental investigation of the relaxations that participate in the glass transition using mechanical spectroscopy. Fragile and strong metallic glasses serve as a model system of hard spheres. In polymers, the segmental mode is dominant in the softening process. A manipulation of the monomer distribution in copolymers results in an additional relaxation mode, denoted as "Chemical Confinement".Particularly, the existence of a secondary relaxation mode, the slow β-relaxation, is shown in the fragile metallic glass Pd77,5Cu6Si16,5 and reproduced in the stronger Zr65Al7,5Cu27,5 which has been previously found on higher frequencies. The used frequencies are in the range of some Hz. The existence in both, strong and fragile metallic glasses, gives rise to the assumption that the slow β-relaxation is a universal feature of the glass transition. Moreover, a direct connection between the slow β-relaxation and the physical aging process, observed upon heating an as-quenched sample, is revealed.A selective manipulation of the segmental mobility can be achieved in poly(ethylene-co-methacrylic acid) (EMAA). The sample preparation is performed in high pressure free radical polymerization at approx. 540 K under steady state conditions. A high synthesis pressure of up to 2300 bar results in a random monomer distribution, while a non-random distribution stems from a lower synthesis pressure down to 1300 bar. In between methacrylic acid blocks, interacting via Hydrogen bonds, an additional relaxation mode of the ethylene blocks can occur if the distribution is non-random. The effect is absent in high pressure samples with a random distribution. This "Chemical Confinement" is shifted to lower temperatures or higher frequencies respectively if the methacrylic acid content is increased due to shorter ethylene blocks.