Non-Uniform Optical Inscription of Actuation Domains in a Liquid Crystal Polymer of Uniaxial Orientation: An Approach to Complex and Programmable Shape Changes

Article abstract of DOI:10.1002/anie.201709528

Achieving complex shape change of liquid-crystal polymer networks (LCNs) under stimulation generally requires spatial configuration of the orientation direction, that is, patterned directors, of liquid crystal monomers prior to polymerization by means of treated surfaces. A strategy is demonstrated that needs only the simple uniaxial orientation of mesogens (monodomain) induced by mechanical stretching of LCNs. Using a rationally designed liquid crystal polymer, photocrosslinking is utilized to pattern or spatially organize the actuating monodomains in order to generate a differential contractile and/or extensional force field required for targeted shape change. Moreover, the approach enables versatile actuation modes and allows multiple shape changes to be programmed on a single piece of the polymer. This important feature is demonstrated with a specimen cut to have eight strips that, upon thermal stimulation, simultaneously display eight types of shape morphing.

Full text of DOI:10.1002/anie.201709528

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Accepted Article
Title: Non-uniform Optical Inscription of Actuation Domains in Liquid
Crystal Polymer of Uniaxial Orientation: An Approach to Complex
and Programmable Shape Changes
Authors: Rong Yang and Yue Zhao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709528
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10.1002/anie.201709528
Angewandte Chemie International Edition
COMMUNICATION
Non-uniform Optical Inscription of Actuation Domains in Liquid
Crystal Polymer of Uniaxial Orientation: An Approach to Complex
and Programmable Shape Changes
Rong Yang[a,b] Yue Zhao*[a]
Abstract: Achieving complex shape change of liquid crystal polymer
networks (LCNs) under stimulation generally requires spatial
configuration of the orientation direction, i.e., patterned directors, of
liquid crystal monomers prior to polymerization by means of treated
surfaces. Herein we demonstrate a strategy that needs only the
simple uniaxial orientation of mesogens (monodomain) induced by
mechanical stretching of LCNs. Using a rationally designed liquid
crystal polymer, photocrosslinking is utilized to pattern or spatially
curling and helical formation, to be obtained from a flat LCN
film.[4] It consists in polymerizing and crosslinking LC monomers
(reactive mesogens) after spatially configuring their orientation
state or patterning the directors of mesogens using generally
treated surfaces. Moreover, using azobenzene derivatives as
mesogenic moieties, LCN actuators prepared from both
methods can be activated by light as a result of the trans-cis
photoisomerization and related order-disorder phase transition.[5]
In the present study, we demonstrate how to achieve complex
shape morphing using only stretched LCN specimen with
uniaxial LC orientation. We show that photocrosslinking can be
applied to a monodomain LCN purposely in a non-uniform
manner, i.e., only in spatially selected regions, to separate the
actuation and non-actuation domains. Under thermal stimulation,
a differential contraction and/or extension force field across the
specimen can be generated, leading to targeted shape change.
It should be emphasized that photocrosslinking monodomain of
LCNs is well known,[2, 4] what is new in our study is the concept
of spatially configuring or patterning the monodomain through
photocrosslinking. As demonstrated below, this strategy makes
complex shape morphing possible with easily processable,
stretched LCNs.
Fig. 1 shows the characteristics of our designed LCN and its
actuation behavior (synthesis details in Supporting Information).
The polymer structure contains a biphenyl unit as mesogenic
moiety, a pendent phenyl group for reducing phase transition
temperatures and enhancing deformability,[6] and a cinnamyl
group for crosslinking through photodimerization[7] (Fig. 1A). It
has a low Tg near room temperature, a smectic and a nematic
phase prior to the isotropic state (Fig. 1B). The efficient
photodimerization of cinnamyl units in the polymer film exposed
to UV light (320 nm) is visible from the decreasing absorption
peak at 325 nm over irradiation time (Fig. 1C). This polymer has
an appealing property that facilitates its preparation as LCN
actuator. Compression-molded films (200 µm in thickness) can
be readily stretched in the LC phases to induce the monodomain
of uniaxial LC orientation without any pre-crosslinking; after
removing external force the film relaxes only slightly and the
monodomain retains without any load. The film can then be
photocrosslinked to lock in the state of oriented mesogens for
the reversible actuation. For a film exposed to UV light on both
sides to have crosslinking throughout the sample, it contracts
upon heating into the isotropic phase and extends after cooling
to the LC phase (Fig. 1D), with a large actuation amplitude
defined as the reversible actuation degree (RAD). The loss and
recovery of the monodomain associated with the actuation can
be noticed from the change in order parameter, S, calculated
from X-ray diffraction (Figs. S4, S5). We further investigated the
effect of film draw ratio λ (length after stretching over initial
length) on the actuation performance. Fig. 1E summarizes the
organize the actuating monodomains in order to generate
a
differential contractile and/or extensional force field required for
targeted shape change. Moreover, the approach enables versatile
actuation modes and allows multiple shape changes to be
“programmed” on a single piece of the polymer. This important
feature is demonstrated with a specimen cut to have eight strips that,
upon thermal stimulation, display simultaneously eight types of
shape morphing.
Future device applications in soft robotics, medicine, energy
and aerospace require the use of advanced polymer actuators
that can perform reversible, complex and programmable shape
change under stimulation. Liquid crystal polymer networks
including elastomers (LCNs) have been extensively studied and
emerged as the material of choice.[1] Through the same
actuation mechanism relying on the reversible LC–isotropic
phase transition, there are two main preparation methods for
LCN actuators. The first starts with a LCN film or sheet obtained
from solution-casting or molding; after inducing the uniaxial
orientation of mesogens (monodomain) by elongation, the
specimen is chemically crosslinked.[2] Upon heating above and
cooling below the clearing temperature, the actuator undergoes
contraction and extension along the elongation direction,
respectively. A recent study showed that by loading carbon
nanotubes, light-triggered bending/unbending, resulting from
photothermally induced superficial phase transition, could add
up to the thermally activated contraction/extension and thus
achieve versatile shape morphing.[3] The second approach
enables more complex and precise shape changes, such as
[a]
[b]
Prof. Y. Zhao
Département de chimie, Université de Sherbrooke, Sherbrooke,
Québec, J1K 2R1,
E-mail: yue.zhao@usherbrooke.ca
Dr. R. Yang
Jiangsu Key Laboratory of Environmentally Friendly Polymeric
Materials, School of Materials Science and Engineering, Changzhou
University, Changzhou, 213164, P. R. China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201709528
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processed like a thermoplastic to generate the monodomain in
free specimen that can then be photocrosslinked to give rise to
LCN actuator with large reversible shape change. In what
follows, the used LCN actuators were prepared using molded
films with 300-400% elongation (60-80 µm-thick after stretching).
The presence of cynnamyl units in the polymer structure is for
optically crosslinking a monodomain specimen only in spatially
selected regions. Because only crosslinked monodoamin
regions can permanently “memorize” the shape change
associated with the LC-isotropic phase transition, they are the
actuation domains, while uncrosslinked or crosslinked
polydomain regions are non-actuation domains. It is easy to
picture that varying the separation and organization of the
actuation and non-actuation domains can change the differential
contraction/extension force field that acts across the specimen
and dictates its shape evolution upon actuation. Before showing
the achievable complex shape changes, it is worth highlighting
an interesting feature of our polymer due to the easy
processability: a specimen with the same optically inscribed
actuation domains can be processed to exhibit the same shape
change from a flat state either by cooling (isotropic-LC) or
heating (LC-isotropic). In the illustration in Fig. 2A, only the
central area close to the side exposed to UV light is crosslinked
and thus can act as actuation domain, while the remainder of the
sample is non-actuation domain. The actuator film can then be
treated in two ways. In one (left side of Fig. 2A), it is allowed to
contract for stress release in the isotropic phase while being
kept flat; on subsequent cooling into the LC phase, the actuation
domain undergoes a superficial extension to result in a “bump”
in the center of the film. Taking the second way, the film under
strain is heated in the isotropic phase for stress release, then by
cooling to the LC phase, the monodomain is recovered in the
actuation domain while the film remains flat after removing the
external force. When this film is heated to the isotropic phase,
the actuation domain leads to a spatially localized contraction
that also results in a “bump”. Either way, the treatment of the
LCN in the isotropic phase to release stress in non-actuation
domains is important for the actuation to be repeated over
numerous cycles. After this step, non-actuation domains
(polydomain in the LC phases) can also be crosslinked by
“flood” exposure of the specimen to UV light, which may adjust
the rigidity of the entire actuator but have no effect on the
actuation domains.
Figure 1. (A) Chemical structure of the liquid crystal polymer. (B) DSC heating
and cooling curves of the polymer. (C) Absorption spectral change of a thin
polymer film under UV irradiation (320 nm) for different times (from 0 to 90
min), the inset shows increase in photodimerization degree of cinnamyl groups
calculated from decrease in absorbance at 325 nm. (D) Left side: photos
showing the reversible contraction and extension along the stretching direction
of a photocrosslinked polymer film upon heating to the isotropic state and
cooling to the LC phase, respectively; right side: change in order parameter (S)
of the mesogenic moieties upon heating to the isotropic phase, calculated from
the 2D-XRD patterns. (E) 2D-XRD patterns, azimuthal diffraction profiles
(black lines: 2θ=1.7-3.5° for smectic layers; red lines: 2θ=17.6-20.9° for
nematic order), order parameters, and reversible actuation degrees for
polymer films stretched to different draw rations (λ).
Given the spatiotemporal control of photocrosslinking that can
be done in many ways, it is easy to imagine that actuation
domains can be inscribed in a 3D and on-demand manner to
give many possibilities of shape morphing as a result of an
imbalanced contraction and/or extension force field, which is
determined by the size, location and distribution of actuation
domains within the LCN actuator. Fig. 2B shows examples of
shape change, each of which can be achieved from a flat film
either by heating or cooling. By increasing the proportion of
actuation domain on one side of the film, i.e., deepening the
photocrosslinked region along the thickness direction, actuation
can lead to simple bending, buckling and rolling into multiple
rings, respectively. The bending curvature increases with
increasing the photocrosslinking time on one side (Fig. S6). If
the one-side actuation domain is confined in a diamond-shape-
region, the film deforms into a “dumpling” upon actuation.
results by showing the 2D-XRD pattern, azimuthal diffraction
profiles at the small-angle reflection of smectic order and wide-
angle reflection of nematic order, calculated
S for both
mesogenic moieties and smectic layers as well as the resulting
RAD. It is seen that both RAD and S increase with film
elongation, but the increase of RAD is small beyond λ=3. In
essence, the designed polymer is easy to deform and can be
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Figure 2. (A) Schematic showing how to inscribe an actuation domain in a monodomain-specimen through photocrosslinking and how the polymer actuator can
be processed to exhibit a given shape change either by heating (LC-isotropic) or cooling (isotropic-LC). (B) Photos and corresponding schematics showing
examples of the complex and versatile shape morphing of the polymer actuator through spatially controlled inscription of actuation domains in a flat monodomain-
specimen. Scale bars: 2 mm. (Movies S1-S4)
Examples are given in Fig. 3. The first is the side-exchange-
actuation where the two sides of a specimen exchange their
respective shape upon actuation (Fig. 3A). This can easily be
obtained by programming the two sides to exhibit the same
shape change upon heating and cooling, respectively. In one
example, the rolled side flattens and the flat side rolls (right side
painted black for better viewing); while in the other case, the
helical side unwinds and the flat side coils (Movie S5). Another
unique actuation mode is the reversal-actuation where the
specimen can switch between reversed configurations while
carrying out the same type of shape change (Fig. 3B). In one
example, the downward rolling switches to upward rolling on
heating and vice versa on cooling (film glued to a black tape for
better viewing) (Movie S6). In the other case, a left-handed helix
turns to right-handed helix on heating and the opposite switching
occurs on cooling (Movie S7). The reversed rolling is used to
explain the underlying mechanism. By inscribing the actuation
domain deep on one side of the film (Fig. 2B), it should roll in the
isotropic phase and straighten on cooling to the LC phase.
When the rolling in the isotropic phase reaches about halfway,
the film is intentionally flattened and kept for stress release. On
cooling to the LC phase, the extension of the actuation domain
in the flat film leads to downward rolling, while on subsequent
heating into the isotropic phase, after the contraction in the
actuation domain flattens the film, the halted contraction during
the initial isotropic phase actuation resumes and leads to
upward rolling. Basically, actuators displaying reversed shape-
morphing is obtained by halting the initially programmed shape
change at halfway in the isotropic phase, followed by cooling to
the LC phase. Fig. 3C illustrates the possibility to obtain
sophisticated shapes through the photocrosslinking-enabled
configurability of the actuator. Here, the specimen was subjected
to a two-step biaxial stretching and photocrosslinking. More
specifically, a strip was stretched in the nematic phase to induce
the monodomain and photocrosslinked on one side. After stress
release in the isotropic phase, the strip was stretched a second
Figure 3. Photos showing peculiar actuation modes and sophisticated shapes:
(A) side-exchange-actuation (the two sides exchange their respective shape),
(B) reversal-actuation (the actuator switches between two opposite
configurations), and (C) hyperbolic paraboloid shapes obtained by two-step
biaxial stretching and phosocrosslinking. Scale bars: 3 mm. (Movies S5-S7).
By inscribing the actuation domain on one side of a film whose
orientation of mesogens makes an angle with the long axis of
the strip, actuation results in formation of a helix. While parallel-
patterning of actuation domains on one side of the film produces
wrinkling upon actuation, patterning on both sides in a shifted
fashion for alternating actuation and non-actuation domains
accentuates the periodic deformation and gives rise to “deep
waves”. While the targeted shape change can easily be
programmed, the actuation speed is largely governed by the
heating or cooling rate (Movies S1-S4).
More complex actuation modes or shapes are accessible due
to the combined ease of processing and ability of inducing the
same shape change by either heating or cooling a flat film.
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10.1002/anie.201709528
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time in the nematic phase but in the perpendicular direction, and
under strain it was photocrosslinked again either on the same
side or on the opposite side. On cooling from the isotropic phase
to room temperature, hyperbolic paraboloid plane and tube were
formed. Both geometries are manifestation of two extension
directions, normal to each other, arising from the inscribed
actuation domains with perpendicular LC orientation. While the
extension of the lower side subjected to the first
photocrosslinking bends the strip upward, the extension of the
upper side due to the second photocrosslinking acts to curl the
strip downward. With the second photocrosslinking directly on
the upper side, the perpendicular extension appears more
prominent, rolling the strip into a tube.
capable of complex and controlled shape change, which is a
significant step forward towards applications.
Acknowledgements
Y. Zhao acknowledges financial support from the Natural
Sciences and Engineering Research Council of Canada
(NSERC) and le Fonds de recherche du Québec: Nature et
technologies (FRQNT). R. Yang thanks financial support of
overseas training from Natural Science Foundation of Jiangsu
Province (BK20150257) and Top-notch Academic Programs
Project of Jiangsu Higher Education. D. Fortin is acknowledged
for assisting the XRD measurements. Y. Zhao is a member of
the FRQNT-funded Centre québécois sur les matériaux
fonctionnels (CQMF).
Keywords: liquid crystals • polymers • actuators •
photocrosslinking
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actuation
monodomains
to
generate
differential
contraction/extension force fields required for complex shape
morphing. Making use of the easy polymer processing, a variety
of interesting actuation features and modes, including a given
shape change by either heating or cooling, side-exchange-
actuation, reversal-actuation, hyperbolic paraboloid shapes and,
more importantly, the possibility to program various shape
changes using a single piece of the polymer. Given the ease of
processing and the many light technologies for spatially-resolved
photocrosslinking, the approach of non-uniform inscription of
actuation domains in LCNs simplifies the fabrication of actuators
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This article is protected by copyright. All rights reserved.

10.1002/anie.201709528
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
An easily processable liquid
crystal polymer with only uniaxial
orientation of mesogens can be
“programmed” to display multiple,
complex shape morphing on a
single piece of the material. The
approach consists in using
photocrosslinking to spatially
configure and organize the
actuation domains in
Rong Yang, Yue Zhao*
Page No. – Page No.
Non-uniform Optical Inscription
of Actuation Domains in Liquid
Crystal Polymer of Uniaxial
Orientation: an Approach to
Complex and Programmable
Shape Changes
mechanically stretched specimen.
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