Stress Relaxation in the Strain Fixing Process for Shape Memory Liquid Crystalline Elastomers

Shape memory behavior can be found in liquid crystalline elastomers (LCEs) for which the origin of shape memory is directly related to the monodomain structure of the LCE.  We wish to report a study of the stress relaxation (and mechanical hysteresis) of a smectic C LCE, a siloxane-based main chain liquid crystalline polymer which is covalently crosslinked.  This elastomer can retain significant levels of strain at room temperature when stretched well above its Tg, but at temperatures well below the isotropization (clearing) temperature.  For these LC elastomers it was found that, after uniaxial extension to a predetermined strain level followed by immediate release of load, there is initially a fast recovery of strain within the first minute followed by a much slower recovery.  This latter strain recovery is sufficiently slow such that significant strain retention can occur over long periods of time (years).  It was found that the stress relaxation time (t) of our LCE, C11(MeHQ)Si8XL10, with different initial strains ranges from 228 seconds to 306 seconds and the parameter (b) is in the range of 0.52 - 0.55.  No apparent dependence of the stress relaxation time on the initial strain was observed.  We will discuss the result above in light of a nanoscale macromolecular mechanism.  Additionally, we will present data on the thermally activated strain recovery process for these LCE materials.