Liquid crystal (LC) elastomers encompass a unique combination of the anisotropic order of liquid crystals and the tendency of the polymeric network to disorder and maximize entropy by adopting isotropic configuration. As a result, they exhibit unconventional elastic behavior, namely, soft-elasticity and the shape memory effect, which is otherwise absent in a classical elastomer network. In this work, two main-chain smectic-C liquid crystal elastomers have been investigated with synchrotron x-ray diffraction to understand the evolution of liquid crystal microstructure under applied strain and its relationship to soft-elasticity and the shape memory effect. The elastomers were subjected to uniaxial strain, allowed to relax at constant strains, allowed to recover after removal of the external stress, and finally annealed by heating above their clearing temperatures while changes in their molecular organization were measured and analyzed.
The experiments reveal the presence of two different relaxation mechanisms in these systems at low and high strains. At low strains, the system's behavior is elastic and the smectic layers are reoriented with layer-normals distributed in a plane perpendicular to the stretch direction. The system relaxes relatively slowly (time constant ~ 45 minutes) which is attributed to the flow properties of the liquid crystal layers embedded in the elastomer network. A different relaxation mechanism dominates at high strains and appears to have its origin in the polymer component of the system. The equilibration time (~ 4 - 8 minutes) conforms to an order of magnitude faster relaxation.
Due to misaligned microdomains at small strains, the value of global orientational order parameter S for the mesogenic parts is initially small (~ 0.15). With increasing strain, the local domain-directors, the mesogens, and the polymer chains, all tend to align parallel to the stretch direction giving rise to a higher measured value of S ~ 0.83 at a strain of ~ 4.0. The siloxane segments remain less ordered, attaining a value of only 0.4 for S for a strain of ~ 4.0. The layers gradually become oblique to the stretch direction conforming to the structural property of the smectic-C phase and the system finally assumes a chevron-like optically monodomain structure. This monodomain structure is enhanced at high strains and both elastomers are "locked-in" this state even after removal of the external stress giving rise to strain-retention and the shape memory effect. The presence of a transverse component in the main-chain leads to higher strain-retention in the second elastomer. A preference for the orientation of the smectic layer-normals toward the stretch direction persists after removal of external stress. Upon thermal annealing, the chevron-like microstructure gradually melts via a different path to the initial polydomain structure.
The results provided by these investigations into the relationship between stress-strain behavior and changes in LC elastomer's microstructure, the dominant role of the LC and elastomer components in low and high strain limits, respectively, and better shape-memory arising from incorporation of a transverse rigid group in molecular structure should lead to better theoretical understanding and design of LC elastomers with desired physical properties for use in specific applications.