Mechanism of strain retention and shape memory in main chain liquid crystalline networks

Abstract

Liquid crystalline networks (LCNs) are described in which there is anelasticity in strain recovery response (under zero load) after uniaxial tensile loading. This strain retention is shown as a function of time after release of load and is further characterized by thermal, X‐ray, and stress/strain experiments. It was found that, at temperatures in the smectic phase far below the isotropization temperature, this LCN film retains significant levels of strain when in the monodomain state. On free recovery (zero load) of the LCN film there is a rapid elastic response followed by a slow anelastic response for those films that had undergone a polydomain‐to‐monodomain transition during the initial imposed strain regimen. It is postulated that the mechanism leading to the strain retention involves nanosegregation‐driven pinning of unfolded hairpins in shallow energy wells and that this effect is responsible for the thermally activated recovery of strain (shape memory) at elevated temperatures. Stress–strain curves of pre‐strained C11(MeHQ)Si8XL10 LC networks having various initial strains: (a) 350%, (b) 300%, (c) 250%, (d) 200%, (e) 150%, (f) 100%, and (g) 50%.

Soft Elasticity in Main Chain Liquid Crystal Elastomers

“…A specimen of ~25 mm length was elongated at a rate of 0.125 mm/s. These elastomers usually show significant strain retention and, after the formation of monodomains, nearly half of the strain is retained [20] as shown in Figure 6. The residual strain increases with the magnitude of the applied strain above 50%, i.e., λ = 1.5.…”

“…Polydomain smectic MCLCEs have been extensively studied [20,[60][61][62][63][64][65][66][67]. Sánchez-Ferrer et al [64] reported the polydomain to monodomain (P-M) transition caused by uniaxial strain in these materials.…”

“…In the second and third regions, the system is only partially reversible, as shown in Figure 6, and can only be brought back to the original pre-stretched state by raising the temperature above T I or putting the sample in a solvent. The second and third elastic regions are termed, respectively, as anelastic and plastic in earlier reports [20,50,65,67] but to avoid any ambiguity we shall use the terms "non-elastic reversible" and "non-elastic irreversible" following Ricco and Pegoretti [69]. Here, "non-elastic" means that stress and strain are not linearly proportional and "non-reversible" implies that the specimen does not instantaneously return to strain-free state upon removal of stress.…”

“…The ring corresponding to ~7.2 Å arises from the flexible siloxane segments. This includes contributions from the randomly placed siloxane-based crosslinkers and the siloxane spacers attached to the main chains, which appear to nano-segregate [20] and become parallel to the stretch direction. Its intensity begins to concentrate in arcs along the vertical direction as the sample is elongated.…”

“…Some very good reviews [17,19] and monographs [15,17] on this topic have discussed the mechanical, chemical, and structural properties of LCEs in general. In this article, we shall give an overview of some of these exciting effects in the main chain LCEs [20] which exhibit the shape memory effect.…”