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Polydimethylsiloxane
Polymers have a wide range of commercial applications but they are complex materials and there is not one uniform model for the behavior of their structure. Therefore, comprehension of the characteristics of polymers involves determining their mechanical properties in order to compare with theoretical models. The model we are interested in validating is based on atomistic molecular dynamics simulations of polydimethylsiloxane (PDMS) molecules interacting with filler particles.[1] Ideally, microscopic observations of these interactions are required, but useful initial information can be obtained with various tensile testing methods.
We have carried out a series of tensile tests on several forms of PDMS that include fillers such as silica. All of the tensile testing was done in the instrument shown below.
During testing we observed the surfaces with high-resolution optical microscopy and captured the video output to a movie file. Four types of polymers, all PDMS/silica, received from different companies, were tested. The samples were labeled as follows: DC745U/white silastic, green silastic/Silastic J, cab-o-sil, and PS443. Pictures of each sample after failure are shown in the figure below.
The white silastic and green silastic underwent eighteen trials each in which they were stretched until broken. The resulting graphs for the white silastic and green silastic were obviously different in shape. Examples are shown in the next figure.
Videos of the surfaces taken during the tests were used to track fiducal marks. The location coordinates of particles on the sample were followed and their trajectories were plotted. The trajectories generally behaved as expected, but there were some features that suggested possible sample non-uniformity.
Several attempts to test and gather data similar to the white and green silastic were applied to the cab-o-sil and PS443. Unfortunately the cab-o-sil proved to be too weak even to mount in the tensile stage; post-curing did not make it any stronger. Trials with the PS443 were faulty because breakage continually occurred at the ends where clamped, suggesting that a better method must be used to mount the sample, and the material was too weak to withstand the force of the clamping.
Cyclic loading and unloading was carried out on the white silastic. The behavior of the curves after the first loading cycle showed a lower elastic modulus and a different structure, indicating a Mullins effect. This is illustrated in the figure below.
References
[1] D. E. Hanson, Journal of Chemical Physics, 113, 7656 (2000).
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Related Links
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Structure/Property Relations, MST-8