(303b) Photoplasiticity in Crosslinked Liquid Crystalline Networks: A Route to Reconfigurable Shape-Changing Materials

Reversible
addition fragmentation chain transfer (RAFT) presents a powerful approach to
control the bond connectivity, molecular orientation, and three-dimensional
shape of crosslinked networks postpolymerization
with spatial and temporal control. Here, we demonstrated its implementation
in crosslinked liquid crystalline networks (LCNs) and used RAFTÕs light
responsive bond exchange process coupled with a mechanical or thermal bias to
permanently alter the alignment of the LCN. The radical mediated mechanism of
RAFT bond exchange was initiated through photo-driven cleavage of radical
photoinitiators, molecules that fragment into radical species upon excitation
with the appropriate wavelength. Because of this light initiated process, the
bond exchange process was decoupled from the thermotropic phase behavior of the
LCN, unlike other covalent adaptable LCNÕs, allowing unprecedented control over
the alignment and stress relaxation. The RAFT bond exchange process and
thermoresponsive LCN behavior gives rise to a multitude of controllable
variables that were tuned to alter birefringence, develop surface topographies,
and program thermoreversible shape changes. We programmed thermoreversible
optical images, which are written in the birefringence, by using temperature to
drive disruption of pre-aligned LCNs coupled with varying light intensity and
exposure time. The programmed LCN disruption occurred in a continuous manner
allowing more complex grayscale images to stored in
the birefringence. Extending this approach to surface coating, we found that
mismatches in alignment, programmed with RAFT bond exchange, lead to
protrusions and wrinkling that was used to develop ordered surface topography
on an other flat surface. In freestanding RAFT-LCNs, complex and hierarchical
shape change was programmed and erased repeatable demonstrating the power of
light responsive processes to program shapes independent of the bias being
applied to the LCN. By decoupling the thermal behavior and the bond exchange
process in RAFT-LCNs, we have demonstrated broad applications and powerful
control over thermoreversible shape changes in elastomeric LCNs.