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

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 its the fully necked monodomain state is approached.