Reversible Micrometer-Scale Spiral Self-Assembly in Liquid Crystalline Block Copolymer Film with Controllable Chiral Response

Article abstract of DOI:10.1002/anie.202101102

The spiral is a fundamental structure in nature and spiral structures with controllable handedness are of increasing interest in the design of new chiroptical materials. In this study, micrometer-scale spiral structures with reversible chirality were fabr

Full text of DOI:10.1002/anie.202101102

Angewandte
Communications
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How to cite:
International Edition: doi.org/10.1002/anie.202101102
German Edition: doi.org/10.1002/ange.202101102
Block Copolymers
Reversible Micrometer-Scale Spiral Self-Assembly in Liquid
Crystalline Block Copolymer Film with Controllable Chiral Response
Jianan Yuan, Xuemin Lu,* Qingxiang Li, Zhiguo Lꢀ, and Qinghua Lu*
Abstract: The spiral is a fundamental structure in nature and
spiral structures with controllable handedness are of increasing
interest in the design of new chiroptical materials. In this study,
micrometer-scale spiral structures with reversible chirality were
fabricated based on the assembly of a liquid crystalline block
copolymer film assisted by enantiopure tartaric acid. Mecha-
nistic insight revealed that the formation of the spiral structures
was closely related to the liquid crystalline properties of the
major phase of block copolymer under the action of chiral
tartaric acid. The chiral spiral structures with controllable
handedness were easily erased under ultraviolet light irradi-
ation and restored via thermal annealing. This facile thermal
treatment method provides guidance for fabrication of chiral
micrometer-scale spiral structures with adjustable chiral prop-
erties.
ments of BCPs with intrinsic or extrinsic chirality enables
formation of different chiral assemblies ranging from nano-
scale to macroscopic. For example, Ho et al. reported the
formation of H* phase by using chiral polystyrene-b-poly(L-
lactide) under thermal annealing.[6] It is also possible to use
achiral block copolymer to construct helical nanostructures
by non-covalent interaction between BCPs and chiral addi-
tives.[7] Although spirals have been obtained by using achiral
block copolymer under the constraint of a nanotemplate, the
obtained spiral films exhibited no chirality owing to the
absence of chiral tectonic units.[8] Recently, Ho et al. reported
the fabrication of a spiral structure based on self-assembly of
chiral block copolymer. They attributed the formation of
spiral structure to chiral deformation of the layered struc-
ture.[9] However, the mechanism for spiral formation is still
a black box. The construction of spiral film with controlled
handedness remains challenging and hampers the in-depth
understanding and functionalization of such a widely occur-
ring natural structure. Furthermore, endowing the spiral
structure with response to external stimuli is of great
importance for the development of novel materials. To the
best of our knowledge, no stimulus-responsive spiral has been
reported until now.
C
hiral assemblies with diverse functions play important
roles in nature and various physiological processes, which
have inspired the scientific community to mimic these
hierarchical structures using synthetic molecules. In-depth
study of these chiral structures provides opportunities for the
design and construction of chiral materials with excellent
properties. Various chiral structures, such as helix,[1] spiral,[2]
gyroid,[3] toroid,[4] and Moebius strips,[5] have been fabricated
based on supramolecular assembly. Among them, the chiral
spirals exhibit great sensitivity to external stimulus, just as
their natural counterparts. However, the evolution of chirality
and morphology from molecules to the micro-scale spiral is
mysterious and it remains a challenge to construct spiral
structures with controlled handedness.
Herein, we report a facile and efficient strategy to
fabricate reversible micrometer-sized three-dimensional
(3D) spirals in liquid crystalline BCP (LC-BCP) film
(Scheme 1). Spirals with controllable handedness were
obtained by co-assembly of LC-BCP and chiral tartaric acid
(TA) molecules under thermal annealing at a temperature
close to the clearing point of the liquid crystal phase. Based on
the liquid crystalline nature of LC-BCP and photosensitivity
of the azobenzene group, the spiral structure can be easily
Block copolymers (BCPs), composed of two or more
blocks linked by chemical bonds, have been intensively
investigated because of their propensity for microphase
separation-induced morphology. Endowing one or two seg-
[*] Dr. J. Yuan, Prof. Q. Lu
School of Chemical Science and Technology, Tongji University
Siping Road No. 1239, Shanghai, 200092 (China)
E-mail: qhlu@sjtu.edu.cn
Prof. X. Lu, Q. Li, Prof. Q. Lu
Shanghai Key Lab of Electrical & Thermal Aging, School of Chemistry
and Chemical Engineering, Shanghai Jiao Tong University
Dongchuan Road No. 800, Shanghai, 200240 (China)
E-mail: xueminlu@sjtu.edu.cn
Prof. Z. Lꢀ
School of Physics and Astronomy, Key Laboratory of Artificial
Structures and Quantum Control, Shanghai Jiao Tong University
Dongchuan Road No. 800, Shanghai, 200240 (China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202101102.
Scheme 1. Diagram showing self-assembly and reversible transforma-
tion of micrometer-scale spiral structures in LC-BCP film doped with
chiral TA. Inset: Bud of Pteridium aquilinum.
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erased by ultraviolet light irradiation and recovered by
thermal annealing. This controllable self-assembly is an
elegant approach for the preparation of reversible spiral
structures, providing a platform to construct new chiroptical
materials.
based on TEM analysis indicated that the spirals were formed
from a bent tube-like structure as shown in the Supporting
Information, Figure S4. The inner and outer diameters of the
tubular structure were ca. 10 and 38 nm, respectively. The wall
of the tube-like structure was 14 nm thick, which is very close
to the diameter of the PEO columnar phase before annealing
(Supporting Information, inset, Figure S4c). Electron diffrac-
tion scanning (EDS) results confirmed that the dark areas in
the Supporting Information, Figure S5 were mainly composed
of PEO segments and the light areas were composed of
PMMA (azo) segments. TEM tomography results of the spiral
structure proved the three-dimensional structure of the spiral
structure (Supporting Information, Figure S6). The three-
dimensional spiral structures showed an average depth of
55 nm as confirmed by AFM height imaging (Supporting
Information, Figure S7).
The molecular order in the spiral structure was inves-
tigated by wide-angle X-ray scattering (Figure S8a). A series
of crystalline peaks were observed in enantiopure TA powder,
while two sharp peaks at 2q = 198 and 23.88, assigned to the
crystal peaks of PEO segments, were visible in neat LC-BCP.
After doping enantiopure TA, all peaks due to the individual
components disappeared, while a new strong peak at 2q = 168
appeared. The absence of TA and PEO peaks indicated that
no individual phase of TA or PEO was present in the LC-
BCP/TA film. The new sharp peak manifests the formation of
an ordered structure of the separated PEO/TA phase in the
liquid phase. Combined with spot EDS results, the molecular
arrangement of block copolymer in the spiral structure can be
depicted as shown in the Supporting Information, Figure S8b.
PEO/TA formed the outer wall and PMMA (azo)/TA formed
the inner wall of the tube structure. Further assembly of these
tube structures in the PMMA (azo) matrix leads to formation
of the spiral structures. Based on a previous report, the TA
molecules not only increase the microphase segregation
tendency of block copolymer, but also provide the driving
force for chiral assembly. The morphology of the block
copolymer depends on the loading level of chiral molecules.[7a]
To gain insight into the formation of the spiral structure, the
effect of the loading level of TA molecules was investigated.
As shown in the Supporting Information, Figure S9, single
circular or bent tube structures were formed when the loading
levels of TA were 13 wt% and 27 wt%. However, the
assembled structures had no obvious handedness, irrespective
of the TA chirality. Considering the difference in morphology
compared with the previously reported helical structure, it is
reasonable to assume that formation of the spiral structure
with controlled handedness is related to the liquid crystalline
properties of the PMMA (azo) segments and the interaction
between TA molecules and LC-BCP.
A liquid crystalline block copolymer, PEO114-PMMA
(azo)46, was synthesized according to the literature.[10] The
detailed procedure is provided in the Supporting Information
(Figure S1). The block copolymer was composed of a poly
(ethylene oxide) (PEO) segment as the minor phase segment
and a liquid crystalline poly (methyl methacrylate) (PMMA)
(azo) segment as the major phase. The PEO segment
dispersed in the PMMA (azo) segment as a cylinder form.
A solution of enantiopure D- or L-TA in tetrahydrofuran was
added to a solution of LC-BCP in the same solvent. The molar
ratio of PEO: TA was fixed at 1.2:1 (the relative weight ratio
of TA to LC-BCP was 54 wt%). An LC-BCP film was
prepared by drop-casting the mixture on a clean quartz glass
slide. The obtained film was then thermally annealed for 2 h
at 1008C, close to the clearing point temperature (988C) of
PMMA (azo)46 (Supporting Information, Figure S2).
The morphology of the LC-BCP and TA hybrid film was
investigated using transmission electron microscopy (TEM)
and atomic force microscopy (AFM). Surprisingly, both TEM
and AFM images showed micrometer-sized tubular multi-
strand spiral structures with clear handedness in the annealed
LC-BCP film (Figures 1a, d and b, e). The spiral structure has
a wide size distribution, the diameter of most spiral structure
reached micrometer level (Supporting Information, Fig-
ure S3). The handedness of the spiral correlated with the
molecular chirality of TA: a clockwise spiral structure was
obtained when D-TA was used (Figure 1c), while anticlock-
wise structures appeared when L-TA was used (Figure 1 f).
Generally, helical nanostructures are obtained by chiral
additive driven self-assembly of block copolymer, which was
ascribed to chiral transfer from the chiral additive to one
segment of the block copolymer.[7,11] Further investigation
To gain insight into the interaction between TA and LC-
BCP, the circular dichroism (CD) spectra of TA-doped LC-
BCP films were recorded. The corresponding UV/Vis spectra
were used to confirm the peak assignments. As shown in
Figure 2a,b, new mirror signals in the range of 275–425 nm
appeared after thermal annealing for 2 h, which corresponded
to the characteristic adsorption of azobenzene groups. This
result indicates that chirality was transferred from the small
TA molecules to liquid crystalline azobenzene-containing
Figure 1. TEM and AFM images of LC-BCP films loaded with 54 wt%
D-TA (a, b) and L-TA (d, e), respectively. Sketch map of spiral
structures with clockwise (c) and anticlockwise (f) configurations. The
slice sample was prepared by microtome with thickness of approx-
imately 100 nm.
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TA hybrid film. For neat LC-BCP, the DSC profile exhibited
a sharp peak at 258C, which was attributed to the crystal-
lization of PEO segments (Figure 2d). This peak degraded
after TA doping, due to the restrictive effect of the TA-PEO
hydrogen bond on the crystallization of PEO chains.[13]
Furthermore, as a consequence of the TA doping, the peak
corresponding to the liquid crystal clearing point of PMMA
(azo) segments became weaker and reduced from 98 to 908C
(Supporting Information, Figure S2). POM observation of
LC-BCP indicated different textures before and after TA
doping (Figure 2e,f). A chiral pattern appeared in the POM
image of LC-BCP/TA hybrid film, which indicated the change
of block copolymer from achiral liquid crystal to chiral liquid
crystal. Considering the effect of TA doping on the properties
of PMMA (azo), it is reasonable to conclude that the chiral
transfer occurred via hydrogen-bonding interaction between
TA molecules and PMMA (azo) segments, leading to the
change from achiral to chiral liquid crystal of the PMMA
(azo) segments. Therefore, for the LC-BCP/TA hybrid film,
hydrogen bonds formed not only between PEO segments and
TA molecules, but also between PMMA (azo) segments and
TA molecules. The hydrogen bond between PMMA (azo)
segments and TA molecules drove the chiral transfer from TA
molecules to the liquid crystalline PMMA (azo) segments,
which caused the formation of chiral liquid crystal phase.
For the chiral BCP self-assembly under annealing, helix is
the most popular structure.[7b,14] More recently, Ho et al.
reported a roll-cake spiral structure in a chiral polystyrene-b-
poly (L-lactide acid) film.[9] They attributed the formation of
spiral structure to the self-assembly of polystyrene layer in the
smectic-liquid crystal like poly (L-lactide acid) phase. The
chiral structures, no matter helix or spiral (or perhaps other
forms), were derived from the deformation of minor phase in
BCP. Akagi et al. reported a thermally invertible chiral spiral
liquid crystal field that could be used in various types of
asymmetric polymerization. The chirality direction of the
chiral spiral liquid crystal field determined the helicity of the
synthetic polymer.[15] In BCP system, Grason et al. used the
orientational self-consistent field theory to simulate the
twisting self-assembly of BCP containing chiral liquid crystal
units.[16] According to the second-order Frank elastic theory,
there are three kinds of deformations including splaying,
twisting and bending in the liquid crystal,[17] of which the
twisting and bending deformations can lead to the formation
of chiral structures.[9,18] Thus, the spiral structures observed
here may derive from the assembly of PEO/TA segments
under the chiral liquid crystal field of PMMA (azo)/TA
matrix. In our case, enantiopure TA interacts with both
segments, endowing the block copolymer with chirality and
leading to the change of liquid crystal PMMA (azo) matrix
from achiral to chiral. Subsequently, the cylinder structures
that are formed by self-assembly before annealing tend to
bend and twist during thermal annealing due to formation of
the chiral liquid crystal field. Finally, the direction of bending
of these spiral structures depends on the chirality of the liquid
crystal fields. The degree of deformation of these spiral
structures is dependent on the force from the chiral liquid
crystal field, which is affected by the loading level of TA,
annealing temperature and time.
Figure 2. CD and UV/Vis spectra of LC-BCP film doped with 54 wt%
D-TA and L-TA before (a) and after (b) thermal annealing treatment.
FTIR of neat LC-BCP, LC-BCP@D-TA and LC-BCP@D-TA after removal
of TA (c). DSC cooling curve of LC-BCP and LC-BCP@D-TA; C:
crystalline range; G: glass transition; LC: liquid crystal phase; Iso:
isotropic phase (d). POM of LC-BCP (e) at 908C and LC-BCP@D-TA
(f) at 808C.
segments. There are two possible mechanisms for chiral
transfer to the azobenzene segments: one is by interaction
between the PEO/TA cylinders and the surrounding PMMA
(azo) phase, and the other is by direct interaction between TA
and the PMMA (azo) segments. To clarify the mechanism,
homopolymer PMMA (azo) (PAZO) was used instead of LC-
BCP to interact with chiral TA molecules in the same
proportion. As shown in the Supporting Information, Fig-
ure S10, after annealing treatment, the CD spectra of the
PAZO/TA hybrid films exhibited the same Cotton effect as
those of LC-BCP/TA films. This result proved that chiral
transfer can be achieved simply by the interaction between
TA and PAZO, which suggests that chiral transfer in the LC-
BCP/TA film might result from interaction between TA and
the PMMA (azo) segments. Furthermore, in the FTIR spectra
=
of LC-BCP film, the C O stretching vibration peak of LC-
BCP at 1721 cmÀ1 was significantly broadened and shifted to
a higher wavenumber (1760 cmÀ1) after TA doping. This
stretching vibration peak returned to its original position after
removing TA by soaking with a methanol solution of lithium
bromide (Figure 2c). Based on these results, the chiral
transfer can be attributed to be the result of hydrogen
bonding between TA and the carbonyl group of PMMA (azo)
segments.[12]
Differential scanning calorimetry (DSC) and polarizing
optical microscopy (POM) were used to further assess the
effect of TA doping on the liquid crystal behavior of LC-BCP/
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To verify the influence of chiral liquid crystal properties of
PMMA (azo)/TA, the evolution of the morphology of LC-
BCP/TA film during annealing at 1008C was tracked by TEM.
As shown in Figure 3a, before annealing, the film contained
irregular linear cylinders composed of PEO/TA dispersed in
the PMMA (azo)/TA matrix. As thermal annealing pro-
ceeded, the cylinder structures became more regular after
15 min (Figure 3b). A twisted tube structure appeared after
thermal annealing for 45 min (Figure 3c). Subsequently, the
twisted tubes deformed and tended to bend in a certain
direction (Figure 3d–f). After thermal annealing for 2 h,
spiral structures appeared (Figure 3g). Further prolongation
of thermal annealing made the spiral structures more regular.
The results clearly indicate that in the early stage of thermal
annealing, because the chiral liquid crystal phase of PMMA
(azo)/TA does not form completely, only cylinder structures
appear due to microphase separation of block copolymers.
Further extending the thermal annealing time allows the
chiral liquid crystal phase of PMMA (azo)/TA to form. In the
chiral liquid crystal matrix, the tube structures tend to be bent
and twisted owing to the twisting force from the chiral liquid
crystal PMMA (azo)/TA matrix, finally leading to the
formation of 3D micrometer-sized spiral structures (Fig-
ure 3i). Because the rotational direction of the resultant chiral
liquid crystal depends on the chirality of the dopant, the
handedness of the 3D spiral structure is regulated by chiral
additives. The stability of the spiral structured was further
investigated by varying the annealing time. As shown in the
Supporting Information, Figure S11a, the spiral morphology
in the hybrid film remained unchanged after thermal anneal-
ing at 1008C for 3 days. We also found that no obvious change
of the spiral structures occurred when the sample was kept at
room temperature for one year (Supporting Information,
Figure S11b). These results proved that the spirals formed in
the hybrid film was thermodynamic stable.
Chirality and liquid crystal properties of LC-BCP/TA play
a key role in the formation of spiral structures, which can be
confirmed by the following facts. When the LC-BCP/TA film
was thermally annealed at 608C, no spiral structure appeared
even when the annealing time was increased to 16 h. This is
because the chiral liquid crystal state cannot be formed at
608C (Supporting Information, Figure S12). On the other
hand, when racemic TA was used as chiral additive instead of
enantiopure TA, as shown in the Supporting Information,
Figure S13, no spiral structure appeared in the LC-BCP/TA
hybrid film. The azobenzene groups in the liquid crystal unit
can also be used to mediate the response of the spiral
structure to external light stimulus. In this work, the LC-BCP
film containing micrometer-scale spiral structures was
exposed to ultraviolet light at 365 nm. We found that both
the CD signals attributed to the azobenzene group and the
spiral structures disappeared after ultraviolet irradiation for
30 s (Figure 4). This is due to isomerization of the azobenzene
group, from trans-state to cis-state, which affects the regular
arrangement of the azobenzene-containing side chains and
disrupts the liquid crystal phase of PMMA (azo) blocks as
shown in Figure 4g. Consequently, the effect of the chiral
liquid crystal field on the PEO/TA cylinders is weakened,
leading to disassembly of the spiral structures. However, when
the cis-azobenzene was annealed and returned to the trans-
azobenzene, the chiral liquid crystal performance of PMMA
Figure 4. a)–c) CD spectra (containing UV/Vis spectra) and d)–f) TEM
images of annealed LC-BCP film doped with 54 wt% L-TA before (a, d)
and after (b, e) UV irradiation (30 s, 365 nm, 150 mWcmÀ2), and the
restoration of the spiral structure after re-annealing for 2 h (c, f);
g) Diagram of the structural change induced by UV irradiation and
thermal recovery.
Figure 3. a)–h) TEM images of the structural evolution of LC-BCP film
doped with 54 wt% L-TA during annealing (1008C) at a) 0 min,
b) 15 min, c) 45 min, d) 60 min, e) 75 min, f) 90 min, g) 2 h and
h) 16 h; i) Diagram of structural evolution.
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(azo)/TA could be recovered (Figure 4c).[19] Furthermore, the
spiral structure of the film can also be reversibly tuned
(Figure 4 f). This discovery will bring new opportunities for
research on chiral spirals and their application.
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In summary, we have demonstrated a facile method for
fabricating micrometer-scale spiral structures with control-
lable handedness using LC-BCP film doped with chiral TA. A
mechanism for formation of the spiral structures is proposed
based on the principle of bending and twisting of the chiral
liquid crystal field towards PEO/TA columns. The chiral
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (grant nos. 21574081, 21975156
and 51733007). X.L. thanks Prof. Ming Li at Shanghai Jiao
Tong University for the discussion on the mechanism of spiral
formation. J.Y. thanks Dr. Q Deng and Prof. Lu Han at Tongji
University for the discussion on 3D tomography of spiral
structure.
Conflict of interest
The authors declare no conflict of interest.
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Keywords: block copolymers · chirality · self-assembly ·
spiral structures
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Manuscript received: January 24, 2021
Revised manuscript received: March 9, 2021
Accepted manuscript online: March 21, 2021
Version of record online: && &&, &&&&
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Communications
Block Copolymers
Micrometer-scale spiral structures with
clear handedness are obtained through
self-assembly of liquid crystalline block
copolymer, poly(ethylene oxide)-b-poly(-
methyl methacrylate) bearing azo-
J. Yuan, X. Lu,* Q. Li, Z. Lꢁ,
Q. Lu*
&&&& — &&&&
Reversible Micrometer-Scale Spiral Self-
Assembly in Liquid Crystalline Block
Copolymer Film with Controllable Chiral
Response
benzene mesogen side chains, with the
aid of enantiopure tartaric acid as chiral
additive. The formation of spiral struc-
tures can be reversibly controlled by
ultraviolet light and heat treatment.
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