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Structural studies by synchrotron x-ray diffraction on two main-chainsmectic-C elastomers reveal the presence of two different relaxation mechanismsin these systems at low and high strains. At low strains, the smectic layersare reoriented with layer-normals distributed in a plane perpendicular to thestretch direction. The system relaxes relatively slowly (time-constant ~ 45minutes) which is attributed to the flow properties of the LC layers embeddedin the elastomer network. At high strains, the equilibration time (~ 4 - 8minutes) conforms to a faster relaxation and appears to have its origin in thepolymer components of the system. Due to misaligned microdomains at smallstrains, the value of global orientational order parameter S for the mesogenicparts is initially small (~ 0.15). With increasing strain, the localdomain-directors, the mesogens, and the polymer chains, all tend to alignparallel 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 avalue of S ~ 0.4 for a strain of 4.0. The layers gradually become oblique tothe stretch direction conforming to the structural property of the smectic-Cphase. The system finally assumes a chevron-like optically monodomainstructure. Both elastomers are locked-in this state even after removal of theexternal stress giving rise to strain retention and the shape memory effect.The presence of a transverse component in the main-chain leads to higher strainretention in the second elastomer. A preference for the orientation of thesmectic layer-normals toward the stretch direction persists after removal ofexternal stress. Upon thermal annealing, the chevron-like microstructuregradually melts via a different path to the initial polydomain structure.

Evolution of liquid crystal microstructure during shape memory sequence in two main-chain polydomain smectic-C elastomers