Necking Instability during Polydomain−Monodomain Transition in a Smectic Main-Chain Elastomer

Abstract

The mechanical response and the evolution of director orientation are characterized in a smectic, main-chain liquid crystalline elastomer (LCE) as it undergoes the familiar polydomain−monodomain (P−M) transition. Under uniaxial tension, the LCE behaves like an ordinary rubber-like network at low strains, and local director rotations are shown to slightly favor the perpendicular (“anomalous”) orientation of chain axes with respect to the draw direction. As strain increases, a well-defined yield stress is observed due to the onset of a necking instability. Macroscopic elongation proceeds by growth of the necked monodomain region, which appears to consume the non-necked polydomain region(s) at its boundaries. Within the necked region, the parallel (“normal”) orientation of chain axes with respect to the draw direction is strongly favored. The P−M transition is attributed to a change in the conformation of the elastic polymer backbones from hairpinned coils to extended chains. Under the conditions of temperature and strain rate studied, buckling of the smectic layers is observed as the fully necked monodomain state is approached.

Polydomain–Monodomain Orientational Process in Smectic‐C Main‐Chain Liquid‐Crystalline Elastomers

“…Several previous communications have reported the soft plateau region [5,12,13] and necking [10,11] during PM transformations, but have only used the mechanical response from the nominal stress (s n ¼ F/A 0 ) in nonequilibrium conditions. The present results use true stress (s t ¼ s n Á l) versus strain (l), and are thus closer to reality and to the existent theories.…”

“…[10] Other studies showed the mechanical behaviour of SmC MCLCEs as function of the crosslinking density, where the necking effect appeared at high degrees of crosslinking, while a soft plateau region was observed for low crosslinked elastomers. [11,12] Recently, the PM transition in a smectic-C A (SmC A ) MCLCE was attributed to the change in the polymer backbone conformation from hairpinned to fully extended.…”

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

“…There are, however, recent reports of slow, anelastic strain recovery in liquid crystalline networks (LCNs) [7][8][9][10][11][12][13][14][15] leading to what has been described by some as shape memory behavior [7-9, 12, 13]. Since it is anelasticity, rather than fast reversing elasticity, of these lightly crosslinked networks, that is, essential for strain retention in these materials; we prefer to refer to them as LCNs rather than as LCEs in this context [15].…”

Liquid crystalline elastomers based on cyclic cyclosiloxane and mesogenic crosslinking units

“…After cooling to room temperature, the mixture was poured into 1000 mL cold water and acidified with 6 N sulfuric acid. The precipitates were recrystallized in alcohol, isolated by filtration, and dried in a vacuum oven to obtain white crystals of 3-(4-(undec-10-enoyloxy)phenyl)acrylic acid (1) The intermediate 1 (33.0 g, 0.10 mol), 120 mL thionyl chloride, and 1.0 mL of pyridine were added into a round flask. The mixture was stirred at room temperature for 3 h, then heated to 60°C, and kept for 6 h in a water bath to ensure that the reaction finished.…”

“…They have been developed as promising functional materials due to their remarkable properties such as interesting mechanical, electrical, and optical properties originating from rubber elasticity [1][2][3]. In addition, the orientation of the mesogens can be controlled through mechanical stress as well as by electric and magnetic fields.…”