A thermally-electrically double-responsive polycaprolactone – thermoplastic polyurethane/multi-walled carbon nanotube fiber assisted with highly effective shape memory and strain sensing performance

A kind of thermally-electrically responsive shape memory fibers made of polycaprolactone (PCL) and thermoplastic polyurethane (TPU) with electric heating and strain sensing properties was prepared by wet-spinning and dip-coating of multiwalled carbon nanotube (MWCNT) on the fiber surface. Aggregate structure investigation on the PCL-TPU fibers and those coated with MWCNT (PCL-TPU/MWCNT fibers) showed that no obvious change of long period, lamellar thickness or orientation after the fibers were recovered from the deformed states with the strain ranging from 100% to 400%. The melting and crystallization of PCL are the switches of shape memory function. After 9 dip-coating cycles of MWCNT, the content of MWCNT is 1.12 wt%, the steady-state maximum temperature (Tmax) of the as spun PCL-TPU/9WMCNT fiber and the one stretched to a strain of 50% could reach 59.7 °C and 54.4 °C, which were on the melting range of PCL. As thus, the shape memory effects of the PCL-TPU/9MWCNT fiber with a strain of less than 50% could be realized in form of both thermal and electrical stimuli. Meanwhile, the PCL-TPU/9WMCNT fiber showed highly gauge factor (21.11), fast response speed (187 ms), and excellent cycle stability (2000 cycles) at a strain of 20%. A linear relation between strain and resistance change was found for this type of MWCNT coated PCL-TPU fiber on all the three different strain regions from 0% to 300%, which was an important criterion for their application on strain sensors. As a whole, the PCL-TPU/MWCNT fiber prepared in this work could serve as an excellent sensing unit for wearable devices.