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

TL;DR

This study investigates the microstructural evolution of main-chain smectic-C elastomers during shape memory cycles, revealing strain-dependent relaxation mechanisms, microdomain alignment, and the formation of a chevron-like monodomain structure that persists after stress removal.

Contribution

It provides detailed insights into the relaxation dynamics, microstructure reorientation, and shape memory behavior of smectic-C elastomers under mechanical strain, using synchrotron x-ray diffraction.

Findings

Two relaxation mechanisms identified at low and high strains.

Microdomains tend to align parallel to the stretch direction with increasing strain.

The elastomers form a chevron-like monodomain structure that is retained after stress removal.

Abstract

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.