Detailing the visco-elastic origin of thermo-mechanical training of twisted and coiled polymer fiber artificial muscles

Artificial muscles made be twisting and coiling polymer fibers provide outstanding performance. However, these materials show inconsistency in their non-loaded length that depends on their thermo-mechanical history. Typically, this behavior has been treated by "training" the samples before any actuation testing. A change in sample length occurs during training but remains consistent during subsequent heat/cool cycles at the same applied load. In this study, the training effect is investigated for a twisted and coiled nylon yarn heated over two temperature ranges: 25-50 degrees C and 50-75 degrees C. The training effect was most obvious in the lower temperature range, but nearly absent in the higher temperature range. When loaded below the glass transition temperature (T-g similar to 40 degrees C) the viscoelastic strain occurs slowly but is rapidly released when the sample is first heated above T-g. The net effect of the first heating through T-g after loading is a small length change because the contraction due to actuation is offset by the expansion due to the release of the viscoelastic strain. A simple spring and dashpot model was developed and by changing only two relaxation times it was possible to simulate the observed training phenomena.