Supersoft elasticity and slow dynamics of isotropic-genesis polydomain liquid crystal elastomers investigated by loading- and strain-rate-controlled tests

The supersoft elasticity and slow dynamics of isotropic-genesis polydomain nematic elastomers are investigated by loading- and strain-rate-controlled tests. Loading-controlled tests reveal the stretching-driven polydomain-to-monodomain (PM) transition under true equilibrium condition without viscoelastic (time) effect. The equilibrium PM transition is observed as a discontinuous dimensional change at a threshold stress with extremely small magnitude (sigma(infinity)(PM) approximate to 1 kPa). The mechanical work required for 80% elongation of the elastomer accompanying the PM transition is only 2% of that required in the high-temperature isotropic state, reflecting the supersoft elasticity effect. The dimensional growth rate (R) under constant loading becomes low as the imposed stress (sigma(0)) approaches sigma(infinity)(PM). The dependency of the dimension on the reduced time (Rt) is, however, independent of sigma(0). In the strain-rate ((epsilon) over dot) controlled tests, the stress-stretch curves show a plateau region characteristic of the PM transition in a finite range of stretch, which is equivalent to the discontinuous stretch in the loading-controlled tests. The plateau stress sigma(pl) significantly decreases with decreasing (epsilon) over dot, whereas the sigma(pl) at the practically accessible low strain rate (on the order of 10(-4) s(-1)) is still significantly higher than sigma(infinity)(PM). The dependency of sigma(pl) on (epsilon) over dot is almost similar to the dependency of sigma(0) on R in the loading-controlled tests. This similarity signifies that the two types of tests with different controlled stimuli are governed by the same dynamics of the local director.