This paper presents the melt spinning and characterisation of polymer actuator fibres; fibres that reversibly contract along the fibre axis in response to heat. Recently, Haines et al (1) showed that low-cost filaments, e.g. fishing lines, can be relevant precursors for artificial muscles. They demonstrated a reversible fibre-direction thermal contraction, which was significantly amplified when the fibres were twisted and coiled. The effect was explained to result from an increase in the conformational entropy of the amorphous phase. In earlier studies on negative thermal expansion in anisotropic polymer structures, it has been shown that the negative thermal expansion in oriented highly crystalline polymers approaches values typical of polymer crystals (2).

To further investigate the mechanisms behind these seemingly simple artificial muscles, we have melt spun fibres from poly(vinylidene fluoride) (PVDF) – Solef  1006 and 1008 kindly provided by Solvay (Milan, Italy) – and compared their properties to a commercially available PVDF-fishing line. The fibres were characterised with respect to their thermal actuation properties, crystal morphology and degree of orientation along the spin-line axis.

We have further done modelling on the molecular and macroscopic levels examining the possible mechanisms of negative thermal expansion in semi-crystalline PVDF. We believe that tie molecules (a polymer chain linking two crystalline regions) are the predominant factor influencing actuation. Two mechanisms are considered: an entropic effect and a conformational change effect. The entropic effect causes an increase in the elastic stiffness with an increase in temperature, effectively resulting in a contraction of a strained fibre. The conformational change effect is also expected to contribute to contraction as tie molecules, under strain, revert to their unloaded preferred conformation when heated.

1. C. S. Haines et al., Artificial Muscles from Fishing Line and Sewing Thread. Science 343, 868-872 (2014).
2. C. L. Choy et al., Negative Thermal Expansion in Oriented Crystalline Polymers. Journal of Polymer Science: Polymer Physics Edition 19, 335-352 (1981).