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

Abstract

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°C and 50–75°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 (Tg ~ 40°C) the viscoelastic strain occurs slowly but is rapidly released when the sample is first heated above Tg. The net effect of the first heating through Tg 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.