No abstract

Dielectric spectroscopy in the frequency range from 10-2 to 106 Hz and in the temperature range from 190 to 430 K is employed to analyze the temperature dependence of the relaxation rates of the α and δ relaxation of liquid-crystalline polymethacrylates having derivatives of (p-alkoxyphenyl)benzoate as mesogenic units in the side group. Different mesophases were achieved by variation of the spacer length. When a temperature derivative method was applied, it was found that the temperature dependence of the relaxation rate of the dielectric α relaxation in these systems displays two different regions:  an Arrhenius-like behavior at high temperatures and a behavior according to the Vogel−Fulcher−Tammann law at low temperatures which is characteristic for the dynamic glass transition in general. The crossover between these two dependencies characterized by a temperature T dc is shifted to lower temperatures with increasing spacer length, but the ratio of T dc and the glass transtion temperature is nearly independent of the spacer length. The result is discussed in the framework of the cooperativity of the glass transtion. The ratio of the relaxation rates of the α and δ processes increases with decreasing temperature in the liquid-crystalline mesophase range. This is explained by an increase of local order with decreasing temperature. Close to the glass transiton temperature, the ratio of the relaxation rates of the α and δ processes decreases. This indicates that the dielectric α and δ processes freeze at least together and cannot be regarded further as independent relaxation processes in that temperature regime.

Determining a suitable and reliable end-of-lifetime criterion for O-ring seals is an important issue for long-term seal applications. Therefore, seal failure of ethylene propylene diene rubber (EPDM) and hydrogenated nitrile butadiene rubber (HNBR) O-rings aged in the compressed state at 125 °C and at 150 °C for up to 1.5 years was analyzed and investigated under static conditions, using both non-lubricated and lubricated seals. Changes of the material properties were analyzed with dynamic-mechanical analysis and permeability experiments. Indenter modulus measurements were used to investigate DLO effects. It became clear that O-rings can remain leak-tight under static conditions even when material properties have already degraded considerably, especially when adhesion effects are encountered. As a feasible and reliable end-of-lifetime criterion for O-ring seals under static conditions should include a safety margin for slight dimensional changes, a modified leakage test involving a small and rapid partial decompression of the seal was introduced that enabled determining a more realistic but still conservative end-of-lifetime criterion for an EPDM seal.

Elastomers are susceptible to chemical ageing, i.e., scission and cross-linking, at high temperatures. This thermally driven ageing process affects their mechanical properties and leads to limited operating time. Continuous and intermittent stress relaxation measurements were conducted on ethylene propylene diene rubber (EPDM) and hydrogenated nitrile butadiene rubber (HNBR) samples for different ageing times and an ageing temperature of 125 °C. The contributions of chain scission and cross-linking were analysed for both materials at different ageing states, elucidating the respective ageing mechanisms. Furthermore, compression set experiments were performed under various test conditions. Adopting the two-network model, compression set values were calculated and compared to the measured data. The additional effect of physical processes to scission and cross-linking during a long-term thermal exposure is quantified through the compression set analysis. The characteristic times relative to the degradation processes are also determined.