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Elastomers Grow into Actuators

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Huan Liang
Huan Liang
Huan Liang
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Yahe Wu
Yahe Wu
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Yubai Zhang
Yubai Zhang
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Erqiang Chen
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Abstract and Figures

It is common knowledge that when an elastomer (rubber) is stretched, its length will maintain if its two ends are fixed. Here, we serendipitously find that an elastomer slowly elongated further to achieve buckling under such conditions, whose final length is much longer than the pre‐stretched length. This allows to design untethered autonomous synthetic material‐based soft robots that do not need any other chemical or electrical energy sources or external stimuli after the pre‐strain is installed. Once the growth starts, the elongation continues to proceed even when the applied force is removed. Moreover, the elastomer after growing eventually forms a robust soft actuator which can be reshaped for different purposes. Few synthetic materials can grow like this so far. Our investigation shows that the material has an uncommon liquid crystal phase. Contrary to normal liquid crystals, it becomes birefringent only at high temperatures. The formation and the reshaping of the resulting soft actuators relate to a usually unnoticed reversible reaction. The work is promised to promote further understanding of dynamic covalent chemistry and liquid crystal elastomers, as well as to foster new designs and high‐impact applications of bio‐inspired sustainable soft actuators in areas other than soft robots. This article is protected by copyright. All rights reserved
Demonstrations of the growth. a) Schematic illustration of typical elastic deformation and plastic deformation. b) Images of the self‐elongation process of our elastomer (growing beyond the fixed pre‐strain). Scale bar: 5 mm.
Demonstrations of the growth. a) Schematic illustration of typical elastic deformation and plastic deformation. b) Images of the self‐elongation process of our elastomer (growing beyond the fixed pre‐strain). Scale bar: 5 mm.
… 
The LCE capable of “self‐elongation” in the ambient environment. a) Chemical compositions for the synthesis of the elastomer that can grow. b) Curves of net elongation strain (εg) versus pre‐strain (εs) of LCEs with different crosslinking densities. c) Curves of final strain after growth (εf) versus pre‐strain of LCEs (εs) with different crosslinking densities. εs=(Ls−L0)/L0\[{\varepsilon _{\rm{s}}} = ({L_{\rm{s}}} - {L_0})/{L_0}\]; εf=(Lg−L0)/L0\[{\varepsilon _{\rm{f}}} = ({L_g} - {L_0})/{L_0}\]; εg=(Lg−Ls)/Ls\[{\varepsilon _{\rm{g}}} = ({L_{\rm{g}}} - {L_{\rm{s}}})/{L_{\rm{s}}}\]; L0: the original length; Ls: the length after pre‐stretching; Lg: the length after growth. d) A “lifter” made of the elastomer closed the circuit automatically without energy input. Scale bars: 1 cm. e) Actuation stability test of the obtained actuator with reversible contraction motion. f) X‐ray diffraction analysis of the actuator formed after growth. The arrow indicated the uniaxially stretching direction to obtain the alignment. g) POM images of the film actuator formed after growth under the crossed polarizers and analyzers. The arrows denoted the pre‐stretching direction.
The LCE capable of “self‐elongation” in the ambient environment. a) Chemical compositions for the synthesis of the elastomer that can grow. b) Curves of net elongation strain (εg) versus pre‐strain (εs) of LCEs with different crosslinking densities. c) Curves of final strain after growth (εf) versus pre‐strain of LCEs (εs) with different crosslinking densities. εs=(Ls−L0)/L0\[{\varepsilon _{\rm{s}}} = ({L_{\rm{s}}} - {L_0})/{L_0}\]; εf=(Lg−L0)/L0\[{\varepsilon _{\rm{f}}} = ({L_g} - {L_0})/{L_0}\]; εg=(Lg−Ls)/Ls\[{\varepsilon _{\rm{g}}} = ({L_{\rm{g}}} - {L_{\rm{s}}})/{L_{\rm{s}}}\]; L0: the original length; Ls: the length after pre‐stretching; Lg: the length after growth. d) A “lifter” made of the elastomer closed the circuit automatically without energy input. Scale bars: 1 cm. e) Actuation stability test of the obtained actuator with reversible contraction motion. f) X‐ray diffraction analysis of the actuator formed after growth. The arrow indicated the uniaxially stretching direction to obtain the alignment. g) POM images of the film actuator formed after growth under the crossed polarizers and analyzers. The arrows denoted the pre‐stretching direction.
… 
Characterizations of the abnormal phase. a) Chemical compositions of LCEs that could not grow. b) POM images of the freshly prepared sample under crossed polarizer and analyzer in the first heating–cooling cycle. Scale bars: 500 µm. c) WAXS results of the elastomer in different states. d) POM images of the annealed white sample under crossed polarizer and analyzer upon heating. The birefringence change was contrary to that of normal liquid crystal behavior. Scale bars: 200 µm.
Characterizations of the abnormal phase. a) Chemical compositions of LCEs that could not grow. b) POM images of the freshly prepared sample under crossed polarizer and analyzer in the first heating–cooling cycle. Scale bars: 500 µm. c) WAXS results of the elastomer in different states. d) POM images of the annealed white sample under crossed polarizer and analyzer upon heating. The birefringence change was contrary to that of normal liquid crystal behavior. Scale bars: 200 µm.
… 
WAXS and mechanical tests on the LCE with an abnormal phase. a–c) WAXS curves of our annealed LCE (a), non‐LC elastomer (b) and normal side‐chain LCE (LCE 6) (c). d) Comparison of the intensity of our annealed LCE, non‐LC elastomer, and normal side‐chain LCE. e) Comparison of d‐spacing of our annealed LCE, non‐LC elastomer, and normal side‐chain LCE. f) Comparison of FWHM of our annealed LCE, non‐LC elastomer, and normal side‐chain LCE. g) Tensile loading and unloading curves of the fresh and annealed sample. h) Stress–strain curves at different ramp force rates of the fresh and annealed sample.
WAXS and mechanical tests on the LCE with an abnormal phase. a–c) WAXS curves of our annealed LCE (a), non‐LC elastomer (b) and normal side‐chain LCE (LCE 6) (c). d) Comparison of the intensity of our annealed LCE, non‐LC elastomer, and normal side‐chain LCE. e) Comparison of d‐spacing of our annealed LCE, non‐LC elastomer, and normal side‐chain LCE. f) Comparison of FWHM of our annealed LCE, non‐LC elastomer, and normal side‐chain LCE. g) Tensile loading and unloading curves of the fresh and annealed sample. h) Stress–strain curves at different ramp force rates of the fresh and annealed sample.
… 
Investigation on the self‐elongation process. a) POM tracking on the self‐elongation process with time. b) WAXS tracking on the self‐elongation process. c) Order parameter S profile as a function of time of the fresh elastomer after stretching (pre‐strain: 30%). Scale bars: 500 µm.
+2
Investigation on the self‐elongation process. a) POM tracking on the self‐elongation process with time. b) WAXS tracking on the self‐elongation process. c) Order parameter S profile as a function of time of the fresh elastomer after stretching (pre‐strain: 30%). Scale bars: 500 µm.
… 
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2209853 (1 of 11) © 2023 Wiley-VCH GmbH
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Elastomers Grow into Actuators
Huan Liang, Yahe Wu, Yubai Zhang, Erqiang Chen,* Yen Wei,* and Yan Ji*
H. Liang, Y. Wu, Y. Zhang, Y. Wei, Y. Ji
The Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical
Biology (Ministry of Education)
Department of Chemistry
Tsinghua University
Beijing 100084, China
E-mail: weiyen@tsinghua.edu.cn; jiyan@mail.tsinghua.edu.cn
E. Chen
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Polymer Chemistry and Physics of Ministry of
Education
Center for Soft Matter Science and Engineering
College of Chemistry and Molecular Engineering
Peking University
Beijing 100871, China
E-mail: eqchen@pku.edu.cn
Y. Wei
Department of Chemistry
Center for Nanotechnology and Institute of Biomedical Technology
Chung-Yuan Christian University
Chung-Li, Taiwan 32023, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202209853.
DOI: 10.1002/adma.202209853
external forces, elastomers show two
kinds of typical deformation, plastic and
elastic (Figure 1a).[1] In elastic deforma-
tion, the stretched elastomer restores to
its original state when the external force is
removed, like the common shape recovery
of a rubber band. In plastic deforma-
tion (also known as irreversible deforma-
tion), after the external force is removed,
the elastomer becomes longer than the
original length but it will not exceed the
length reached during deformation, which
resembles stretching plasticine. Distinct
from ordinary elastomers, the elastomer
reported here “grows” by itself to a signifi-
cant extra length in ambient conditions
without any extra manipulation when
it is stretched and its two ends are fixed.
As shown in Figure 1b, the pre-stretched
elastomer elongated spontaneously and
arched upwards gradually. Biological
systems are capable of spontaneously
growing longer or bigger as time goes
by. However, few synthetic materials can
mimic this and “grow” longer by themselves without external
stimuli or other assistance.
The implication of the spontaneous growth after a simple
pre-stretching without other external energy input and stimuli
is far-reaching. For example, such growth oers a good oppor-
tunity to tackle the long-lasting hurdle of power and control for
untethered autonomous devices, such as soft robots.[2] Autono-
mous soft robots for exploration and beyond are in enormous
demands. This field is blooming as soft robots can perform a
variety of operations that rigid robots cannot. Compared to the
rigid counterparts widely available nowadays, soft robots stand
out for their higher flexibility and safety with human interac-
tion. Soft robots take inspiration from biological systems. While
many biological systems can grow longer to accomplish the
specific movement by themselves, elastomers, as a major class
of structural materials for soft robots, cannot do so without
energy supply. Great eorts have been paid to integrate the
power supply and control unit within the soft robot’s body so
as to make it autonomous, which is still extremely challenging
despite the recent exciting progress.[3] If a synthetic soft mate-
rial can elongate spontaneously without external stimuli and
energy, then, making good use of this intelligent matter,[3b]
engineers can easily design and make advanced autonomous
soft robots without consideration of the challenge posed by
energy supply and external stimuli. The growth we reported
here does not require further external energy or intervention
once the stretched state is fixed. As the growth speed and mode
can be “encoded” during material preparation, the “operation
It is common knowledge that when an elastomer (rubber) is stretched, its
length will bemaintained if its two ends are fixed. Here, it is serendipitously
found that whenan elastomer isslowly elongated further to achieve buckling
under such conditions, the final length is much longer than the pre-stretched
length. This allows the designof untethered autonomous synthetic-material-
based soft robots that do not need any other chemical or electrical energy
sources or external stimuli after the pre-strain is installed. Once the growth
starts, the elongation continues to proceed even when the applied force is
removed. Moreover, the elastomer, after growing, eventually forms a robust
soft actuator that can be reshaped for dierent purposes. Few synthetic
materials can grow like this, so far. This investigation shows that the material
has an uncommon liquid crystal phase. Contrary to normal liquid crystals,
it becomes birefringent only at high temperatures. The formation and the
reshaping of the resulting soft actuators relate to a usually unnoticed revers-
ible reaction. The work is promising to promote further understanding of
dynamic covalent chemistry and liquid crystal elastomers, as well as to foster
new designs and high-impact applications of bioinspired sustainable soft
actuators in areas other than soft robots.
ReseaRch aRticle
1. Introduction
Elastomers are everywhere in our daily life and industry from
toys and clothes to automobiles and sports. Once subject to
Adv. Mater. 2023, 35, 2209853
... Even until recently, there is only one shape-shifting polymer that has realized spontaneous self-growth 11 . The material belongs to liquid crystal elastomers (LCEs), which are outstanding shape-shifting polymers due to their large actuation strain and muscle-like energy density 12,13 . ...
... Normal LCE actuation is based on the liquid crystal-isotropic phase transition, which generally requires external stimuli 14 . Different from LCE actuations, the self-growth is a rare phenomenon that is observed only in the specific LCE system in our previous work 11 . The self-growing LCEs can independently and spontaneously grow to an extended length beyond the original length at room temperature without external stimuli or energy input, based on the transformation from an unstable state to a stable state 11 . ...
... Different from LCE actuations, the self-growth is a rare phenomenon that is observed only in the specific LCE system in our previous work 11 . The self-growing LCEs can independently and spontaneously grow to an extended length beyond the original length at room temperature without external stimuli or energy input, based on the transformation from an unstable state to a stable state 11 . Despite all these advantages, the self-growth has strict requirements for LCE samples. ...
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