skip to main content


Title: Body‐temperature s hape‐shifting liquid crystal elastomers
Abstract

Nematic monodomain liquid crystal elastomers (LCEs) undergo efficient temperature‐induced reversible shape‐shifting around the nematic‐isotropic transition temperature (Tni) due to the presence of the liquid‐crystalline order of mesogens. Usually, theTniof nematic LCEs is much higher than the human body temperature, and therefore LCEs are not often considered for biomedical applications. This study describes an LCE system where theTniis tuned by substitution of the rigid mesogens RM257 with a flexible backbone PEGDA250. By systematically substituting the RM257 with PEGDA250, theTniof LCEs was observed to decrease from 66°C to 23°C. A rate‐optimized LCE material was fabricated with 10 mol % rigid mesogens substituted with a flexible backbone that demonstratedTniat 32°C, in‐between the room temperature of 20°C and the body temperature of 37°C. TheTniallowed the programmed shape at room temperature, quick shape‐shifting upon exposure to body temperature, and before‐programmed shape when kept at body temperature. This LCE material displayed reversible length change of 23%, opacity change, and shape change between room temperature and body temperature.

 
more » « less
Award ID(s):
1827288
PAR ID:
10454257
Author(s) / Creator(s):
 ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Journal of Applied Polymer Science
Volume:
138
Issue:
14
ISSN:
0021-8995
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Liquid crystal elastomers (LCEs) with intrinsic anisotropic strains are reversible shape‐memory polymers of interest in sensor, actuator, and soft robotics applications. Rapid gelation of LCEs is required to fix molecular ordering within the elastomer network, which is essential for directed shape transformation. A highly efficient photo‐cross‐linking chemistry, based on two‐step oxygen‐mediated thiol–acrylate click reactions, allows for nearly instant gelation of the main‐chain LCE network upon exposure to UV light. Molecular orientation from the pre‐aligned liquid crystal oligomers can be faithfully transferred to the LCE films, allowing for preprogrammed shape morphing from two to three dimensions by origami‐ (folding‐only) and kirigami‐like (folding with cutting) mechanisms. The new LCE chemistry also enables widely tunable physical properties, including nematic‐to‐ isotropic phase‐transition temperatures (TN‐I), glassy transition temperatures (Tg), and mechanical strains, without disrupting the LC ordering.

     
    more » « less
  2. Abstract

    Liquid crystal elastomers (LCEs) with intrinsic anisotropic strains are reversible shape‐memory polymers of interest in sensor, actuator, and soft robotics applications. Rapid gelation of LCEs is required to fix molecular ordering within the elastomer network, which is essential for directed shape transformation. A highly efficient photo‐cross‐linking chemistry, based on two‐step oxygen‐mediated thiol–acrylate click reactions, allows for nearly instant gelation of the main‐chain LCE network upon exposure to UV light. Molecular orientation from the pre‐aligned liquid crystal oligomers can be faithfully transferred to the LCE films, allowing for preprogrammed shape morphing from two to three dimensions by origami‐ (folding‐only) and kirigami‐like (folding with cutting) mechanisms. The new LCE chemistry also enables widely tunable physical properties, including nematic‐to‐ isotropic phase‐transition temperatures (TN‐I), glassy transition temperatures (Tg), and mechanical strains, without disrupting the LC ordering.

     
    more » « less
  3. Abstract

    Diarylethene‐functionalized liquid‐crystalline elastomers (DAE‐LCEs) containing thiol‐anhydride bonds were prepared and shown to undergo reversible, reprogrammable photoinduced actuation. Upon exposure to UV light, a monodomain DAE‐LCE generated 5.5 % strain. This photogenerated strain was demonstrated to be optically reversible over five cycles of alternating UV/Visible light exposure with minimal photochrome fatigue. The incorporation of thiol‐anhydride dynamic bonds allowed for retention of actuated states. Further, re‐programming of the nematic director was achieved by heating above the temperature for bond exchange to occur (70 °C) yet below the nematic‐to‐isotropic transition temperature (100 °C) such that order was maintained between mesogens. The observed thermal stability of each of the diarylethene isomers of over 72 h allowed for decoupling of photo‐induced processes and polymer network effects, showing that both polymer relaxation and back‐isomerization of the diarylethene contributed to LCE relaxation over a period of 12 hours after actuation unless bond exchange occurred.

     
    more » « less
  4. Abstract

    Diarylethene‐functionalized liquid‐crystalline elastomers (DAE‐LCEs) containing thiol‐anhydride bonds were prepared and shown to undergo reversible, reprogrammable photoinduced actuation. Upon exposure to UV light, a monodomain DAE‐LCE generated 5.5 % strain. This photogenerated strain was demonstrated to be optically reversible over five cycles of alternating UV/Visible light exposure with minimal photochrome fatigue. The incorporation of thiol‐anhydride dynamic bonds allowed for retention of actuated states. Further, re‐programming of the nematic director was achieved by heating above the temperature for bond exchange to occur (70 °C) yet below the nematic‐to‐isotropic transition temperature (100 °C) such that order was maintained between mesogens. The observed thermal stability of each of the diarylethene isomers of over 72 h allowed for decoupling of photo‐induced processes and polymer network effects, showing that both polymer relaxation and back‐isomerization of the diarylethene contributed to LCE relaxation over a period of 12 hours after actuation unless bond exchange occurred.

     
    more » « less
  5. Liquid crystal elastomers (LCEs) are composed of rod-like liquid crystal (LC) molecules (mesogens) linked into elastomeric polymer networks. They present a nematic phase with directionally ordered mesogens at room temperature and an isotropic phase with no order at high temperatures, enabling large thermal-induced deformation. As a result, LCEs have become promising candidates for new applications in soft robotics and shape morphing. LCEs are being actively studied in both experiment and theory in recent years. However, the fundamental relationship among synthesis, processing, and thermomechanical behaviors of modern LCEs are still largely unclear. This knowledge gap is further complicated by the various LCE types, including polydomain, monodomain, nematic-genesis, and isotropic-genesis, each fabricated and used under different experimental conditions and applications. Here we explore synthesis-processing-property relationships in thermomechanics of various LCEs, by combining fabrication, characterization, and theoretical modeling. We adapt the widely used two-stage method to fabricate isotropic-genesis polydomain LCEs and nematic-genesis LCEs with varying pre-stretches during polymerization. We characterize the thermal-induced spontaneous deformation and the temperature-dependent uniaxial stress-stretch responses of the LCEs. We identify a new relationship among the soft elasticity, the thermal-induced spontaneous deformation, and the pre-stretch during polymerization, in the LCEs under study. Building on classical theories and our experimental results, we develop a constitutive model to describe the uniaxial behaviors of various LCEs. The theoretical predictions agree well with the experimental results on uniaxial stress-stretch responses at different temperatures. Finally, we discuss the remaining challenges and future opportunities in synthesis-processing-property relationships of LCEs. 
    more » « less