Reports: AC7
42540-AC7 Broadband Viscoelasticity of Polymers with Initial Stress: Applicability of Time-Temperature Superposition
Apply a steady force to a polymer and it will continue to deform, or "creep" with time. This creep occurs more rapidly at high temperature than at low temperature. Creep is objectionable if it results in warp of a plastic part, or loosening of a fastener over its service life. Creep at short time is beneficial for the purpose of absorbing noise and vibration, since it is linked with viscoelastic dissipation of vibration energy. It is, however, difficult to do a test which covers more than a multiple of about 1,000 in time or frequency. For example a test may be done from 10 seconds to 10,000 seconds (a bit less than three hours). That will not suffice to evaluate a material with an expected service life of years under load. Also the test does not reveal how well the material absorbs sound, corresponding to a time scale of from about five hundredths of a second to fifty millionths of a second. One can get around this experimental limitation and also predict behavior for short and long time periods. To do this, one assumes that a temperature change is equivalent to a multiplication of the time scale by some number. Then, several experiments are done at different temperatures. The results are shifted on a logarithmic time scale to obtain a prediction for an extended time scale. This time-temperature shift approach will work if all the molecules in the polymer have the same temperature sensitivity.
