Siloxane crosslinks with dynamic bond exchange enable shape programming in liquid-crystalline elastomers

Abstract

Liquid crystalline elastomers (LCEs) undergo reversible shape changes in response to stimuli may enable a wide range of smart applications, such as soft robot, adhesive systems or implantable medical devices. In this manuscript, we introduce new dynamic covalent chemistry based on siloxane equilibrium exchange into the LCEs to enable processing (director alignment, remolding, and welding). Unlike, the traditional siloxane based LCEs, which are produced by a other reaction schemes with irreversible bonds (e.g. hydrosilylation), here, we use a much more robust reaction (thiolacrylate/thiol-ene 'double-click' chemistry) to obtain highly uniform dynamically crosslinked networks. Combining the siloxane crosslinker with click chemistry produces LCEs with tunable properties, low glass transition (-30˚C), controllable nematic to isotropic transition (32 to 75˚C), and a very high vitrification temperature (250˚C). Accordingly, this class of dynamically crosslinked LCEs shows unprecedented thermal stability within the working temperature range (-50 to 140 ˚C), over many thermal actuation cycles without any creep. Finally, multiple LCEs sharing the same siloxane exchangeable bonds can be welded into single structures to allow for materials that sequentially and reversibly undergo multiple phase transformations in different sections of the sample.

Continuous spinning aligned liquid crystal elastomer fibers with a 3D printer setup

“…Next, after the fiber spinning process, the dithiol nematic oligomers were radically crosslinked with either vinyl siloxane crosslinker, (TVCS) and vinyl siloxane spacer, (DVS) by photo-initiated UV reaction to produce LCE fiber with dynamic polymer network topology (xLCE) or with regular vinyl crosslinker, (TATATO) to obtained LCE fiber with static polymer network topology. The overall reaction scheme is similar a previously reported method 18,24 .…”

“…Informed by our recent work on correlating the rheological properties and the mesogenic behavior (e.g. nematic to isotropic phase transition temperature) to the degree of polymerization and molecular weight of the oligomer, 24 we have designed oligomers with molar ratio of acrylate : thiol = 1 : 1.2 to have the average molecular weight between 7000 to 10000 g/mol for T-oligomer and S-oligomer, respectively. Oligomers with molecular weight in this range can form extrude-able materials, and are readily pulled into fibers.…”

“…The strip is attached to a free-hanging weight while being heated to 200 • C for at least 2h under tension to initiate the bond exchange and plastic flow. 24 Bulk LCE based on T-oligomer (TM): Mix the prescribed ratio of all chemicals used when making T-oligomer in a vial and allow the reaction to proceed without addition of solvent. The viscous oligomer is taken out after 30 minutes of reaction and exposed to weak UV light ( 0.5mW/cm ) to establish the initial crosslinking.…”

“…One fiber is formulated with LCE crosslinked by conventional permanent covalent bonds, 18,23 while the other is crosslinked by dynamic covalent bonds, capable of equilibrium bond-exchange. 24 We then characterize the fibers and successfully obtain reversible actuation upon repeated heat-cooling cycles. These fibers also show a controllable variation of elastic modulus and strain-to-failure as the spinning conditions vary, and a high stress to break of several MPa.…”

Toward Application of Liquid Crystalline Elastomer for Smart Robotics: State of the Art and Challenges

“…The exchange reaction leads to network rearrangement of the materials, providing new functions such as self-healing, welding, reprocessing, and some specific stimulus responsiveness for polymeric material. To address the adaptability and recyclability of LCEs, a number of novel chemical transformations, such as boronic ester exchange reaction [ 46 ], disulfide metathesis reaction [ 47 , 48 , 49 ], reversible addition fragmentation chain-transfer reactions [ 50 , 51 , 52 ], and siloxane exchange reactions [ 53 , 54 ], have been introduced into LCEs. The general reaction scheme and conditions of dynamic chemistries are given in Table 1 (g–o).…”