Liquid
crystalline elastomers (LCEs) are functional materials whose
stimuli response is strongly influenced by the composition and structure
of the polymer network. LCEs are commonly fabricated by copolymerizing
acrylate monomers with thiols. This work explores the formation of
LCE polymer networks via photopolymerization reactions of diacrylate
liquid crystalline monomers with dithiols. Detailed analysis of model
systems based on monoacrylate–dithiol reactions indicates that
the polymer network architecture of LCEs prepared by this reaction
contains extensive unreacted thiol content as dangling ends. Further,
kinetic analysis indicates that polymerization in a liquid crystalline
phase strongly hinders the mobility of reactive species during the
formation of the LCE polymer network, which is evident in the substantial
difference in viscosity between liquid crystalline monomers (0.0195
Pa s) and non-liquid crystalline monomers (0.0025 Pa s). In copolymerization
with the diacrylate liquid crystal monomers, the dithiol comonomer
significantly decreases the elastic modulus and T
g. The residual thiol in the LCE polymer network can be
postfunctionalized with reactive additives.
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 thiolanhydride 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.
The macroscopic alignment of the
nematic director within liquid
crystalline elastomers (LCEs) amplifies the
magnitude of the directional strain response. Theory predicts that
the stimuli response of LCEs will be affected by the orientational
genesis of cross-linking. We examine compositionally identical LCEs
aligned by either mechanical or surface techniques. The two-step procedure
to prepare LCEs by mechanical alignment first forms cross-links in
a disordered state (e.g., isotropic orientational genesis), which
thereafter are aligned by deformation during which polymerization
(e.g., cross-linking) is completed. Comparatively, cross-linking within
surface-aligned LCEs exclusively occurs within the ordered state (e.g.,
nematic orientational genesis). The orientational genesis of cross-linking
in aligned LCEs, in compositionally identical samples, significantly
affects the thermotropic and phototropic deformation of these materials.
LCEs prepared by mechanical alignment (e.g., isotropic orientational
genesis) have actuation temperatures that are as much as 75 °C
lower than the analogous LCEs prepared by surface alignment (e.g.,
nematic orientational genesis). Likewise, the magnitude and rate of
photoinduced strain generation in azobenzene-functionalized LCEs prepared
by mechanical and surface alignment differ by a factor of 2 or greater.
Snap-through mechanisms are pervasive in everyday life in biological systems, engineered devices, and consumer products. Snap-through transitions can be realized in responsive materials via stimuli-induced mechanical instability. Here, we demonstrate a rapid and powerful snap-through response in liquid crystalline elastomers (LCEs). While LCEs have been extensively examined as material actuators, their deformation rate is limited by the second-order character of their phase transition. In this work, we locally pattern the director orientation of LCEs and fabricate mechanical elements with through-thickness (functionally graded) modulus gradients to realize stimuli-induced responses as fast as 6 ms. The rapid acceleration and associated force output of the LCE elements cause the elements to leap to heights over 200 times the material thickness. The experimental examination in functionally graded LCE elements is complemented with computational evaluation of the underlying mechanics. The experimentally validated model is then exercised as a design tool to guide functional implementation, visualized as directional leaping.
Liquid crystalline elastomers (LCEs) are stimuli-responsive materials capable of undergoing large deformations. The thermomechanical response of LCEs is attributable to the coupling of polymer network properties and disruption of order between liquid crystalline mesogens. Complex deformations have been realized in LCEs by either programming the nematic director via surface-enforced alignment or localized mechanical deformation in materials incorporating dynamic covalent chemistries. Here, the preparation of LCEs via thiol-Michael addition reaction is reported that are amenable to surface-enforced alignment. Afforded by the thiol-Michael addition reaction, dynamic covalent bonds are uniquely incorporated in chemistries subject to surface-enforce alignment. Accordingly, LCEs prepared with complex director profiles are able to be programmed and reprogrammed by (re)activating the dynamic covalent chemistry to realize distinctive shape transformations.
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