  Structural studies by synchrotron x-ray diffraction on two main-chain
smectic-C elastomers reveal the presence of two different relaxation mechanisms
in these systems at low and high strains. At low strains, 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 LC layers embedded
in the elastomer network. At high strains, the equilibration time (~ 4 - 8
minutes) conforms to a faster relaxation and appears to have its origin in the
polymer components of the system. 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 S ~ 0.4 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. The system finally assumes a chevron-like optically monodomain
structure. 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.
