• Mikkelsen, Mathias

Abstract

This thesis describes the development, validation, and use of a piezoelectric high-frequency rheometer for measuring the shear modulus of rubbers and viscous liquids. The rheometer is an improvement of a previously developed device called the piezoelectric shear gauge (PSG), which has been in use for several decades at Roskilde University for fundamental research into the physics of viscous liquids.<br/><br/>In this project, the data analysis has been improved based on a new algorithm for inverting raw data, which makes it possible to determine the shear modulus at higher frequencies than ever before. A version of the PSG has been developed that can be routinely and reliably used for measurements on rubber, as well as measurement protocols and methods for these measurements. And these innovations have been validated by comparison with previous methods and devices. It is now possible to use the PSG to measure shear moduli between 1 MPa and 10 GPa at frequencies up to 80 kHz.<br/><br/>This has been utilized for studies of the mechanical dynamics of phenylalcohols, which show signs of polymer-like structures formed by hydrogen bonding between the alcohol molecules. The data also show a generic form of the mechanical α-relaxation and a strong correlation between the characteristic times of the dielectric and shear mechanical relaxations.<br/><br/>New and simpler versions of the PSG have been developed and validated by comparison with previous versions. One of the new PSGs has been used for measurements on carbon black-filled, vulcanized natural rubbers. The results of these broad-spectrum measurements challenge the widespread assumption of time-temperature superposition; they also demonstrate the limitations of using the Williams-Landel-Ferry equation for the construction of broadband master curves from narrowband data.

Topics

• impedance spectroscopy
• Carbon
• Hydrogen
• rubber
• alcohol
• rheometry