| Linear viscoelastic analysis is an essential tool for any rational design of a polymer-based structure or device. In order to carry out such an analysis, properties such as stress relaxation moduli, or creep compliance functions, or complex dynamic moduli over a wide range of time or frequency are needed. A master curve of any modulus is determined by shifting the experimental data over a wide range of temperature to that at the reference temperature based on the assumption that the material follows the time-temperature superposition principle. In practice, the construction of a master curve for a soft elastomer is somewhat arbitrary and subject to a significant degree of uncertainty. The objective of this thesis is to conduct a thorough study on the validity of the time-temperature superposition principle as applied to soft gum (or uncrosslinked) elastomers.;Our first objective is to establish a protocol with which reliable and reproducible data can be obtained. It requires an application of sufficiently large compressive stress. Consistency among new specimens with different diameter and thickness, which were cut from the same sheet, was established by the agreement of their limiting damping factor values over a frequency range from 0.2 rad/s to 200 rad/s. And most of the repeated experiments of the same specimen were reproducible in damping factor values, except those had been subjected to a severe thermal-mechanical history.;Effect of the gap level (distance between the two plates), which measures the extent of compression, on the superimposed shear storage modulus, after a correction to the effect of the sample diameter, was studied at a fixed frequency such as 10.4 rad/s. As the gap decreased the corrected shear storage modulus G' value first increased, reached a maximum and then decreased rapidly. Both the peak value and its corresponding compression ratio, which is defined as final gap/initial thickness, decreased with increasing value of the initial shape factor, which is defined as Diameter/(4xHeight). We postulate that the measured corrected G' depends on the following factors: the contact condition between the specimen surface and the corresponding plate, the shear storage modulus of the material at zero (infinitesimal) strain, and the state of the internal strain in the specimen. As the internal strain increases, the corrected G' decreases.;In the earlier stage of compression, the interfacial contact increases by forcing the elastomer to conform to the surface of the measurement plate, and the main body of the specimen is subjected to very little strain. After the best contact is reached the lateral surface of the specimen is forced to bulge and the shape of the lateral surface becomes parabolic, which can be calculated based on stress analysis with a non-slippage boundary condition. However, upon further compression, the tangential stress becomes large enough such that the slippage occurs, which causes the strain state in the specimens to change completely, leading to a rapid decrease in the corrected G'. Specimens with different shape factor have different tendency to slip and hence exhibit different behavior.;A very thorough and systematic research over four years on the effect of different thermo-mechanical histories on the superimposed linear viscoelastic properties was carried out. This set of data will be extremely valuable to us as well as to any future researchers to further study the dependence of the corrected G' on the internal strain in the material. One type of the tests is the relaxation experiment. With the normal stress reduced at the constant gap, damping factor decreased slightly, whereas corrected G' increased, supporting the hypothesis that the corrected G' increases as the internal strain decreases. On the other hand, a rest without the application of an external load stresses caused an opposite change that G' decreased and damping factor increased. A series of experiments on introducing large internal strain in the specimens by applying severe deformation clearly showed the significant effect of the internal strain. (Abstract shortened by UMI.). |
