Investigation of the influence of the relaxation spectrum on the vulcanization conditions.
The samples are described in Section 4.2.2.
Measuring cell: DMA/SDTA861e with shear sample holder
Sample preparation: Cylinders of 5-mm diameter were punched out from a 1-mm thick film and mounted in the shear sample holder with 10% predeformation.
DMA measurement: The measurements were performed at 10 °C in the frequency range 10-3 Hz to 103 Hz. Maximum force amplitude 10 N, maximum displacement amplitude 10 µm, offset control zero.
The figure displays the tan g curves in log-log presentation. For comparison purposes, an unvulcanized sample was also measured. With the unvulcanized sample, it can be seen in the log-log presentation that the high and low frequency sides of the peaks of tan g are straight lines. This behavior is typical for relaxation processes
With increasing degree of vulcanization, the relaxation region is shifted to lower frequencies. The degree of cross-linking also has an influence on the width of the tan g peak. The broadening is an indication that there is a wider distribution of relaxation times, which is a result of greater inhomogeneity. With greater degrees of vulcanization, the slope of the high frequency side of the peaks hardly changes. The slope of the low frequency side, however, becomes smaller. If one assumes that the high frequencies concern rearrangements over short characteristic lengths, then these are hardly influenced by cross-linking. Cooperative rearrangements over long characteristic lengths, however, determine the behavior on the low frequency side of the tan g peak. Theses cooperative rearrangements are therefore more strongly influenced by crosslinking. Cross-linking increases the relative proportion of slow rearrangements.
Frequency-dependent measurements on relaxation behavior allow information to be obtained on the change of cooperative rearrangements due to differences in chemical structure (degree of cross-linking). Information on the type and composition of the material is also obtained. The temperature has to be held extremely constant for such frequency-dependent measurements because the relaxation spectrum exhibits a frequency dependence of about 0.2 decades/K.
Frequency Sweep DMA Measurements of Differently Vulcanized SBR Samples | Thermal Analysis Application No. HB447 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2