# Programmable Liquid Crystal Elastomers Via Magnetic Field Assisted Oligomerization

Rakine Mouhoubi, Vincent Lapinte, Sébastien Blanquer

► To cite this version:

Rakine Mouhoubi, Vincent Lapinte, Sébastien Blanquer. Programmable Liquid Crystal Elastomers Via Magnetic Field Assisted Oligomerization. Advanced Functional Materials, 2025, 10.1002/adfm.202424400. hal-05083410

HAL Id: hal-05083410

https://hal.science/hal-05083410v1

Submitted on 24 May 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Programmable Liquid Crystal Elastomers Via Magnetic Field Assisted Oligomerization

Rakine Mouhoubi, Vincent Lapinte, and Sébastien Blanquer*

Liquid crystal elastomers (LCEs) are highly promising soft actuators due to their ability to undergo significant, rapid, and programmable shape transformations. Achieving this actuation requires precise mesogen alignment before crosslinking. Magnetic field alignment offers an advantage over other techniques as it enables volumetric mesogen alignment. However, magnetic alignment has to be performed in the state where mesogens exhibit anisotropic diamagnetic properties suitable for alignment, typically in the nematic phase. Therefore, it is difficult to obtain a stable nematic liquid crystalline mixture before crosslinking, while preventing mesogen recrystallization. To address this challenge, a method using a magnetic field assisted thio-Michael polyaddition followed by the photo-crosslinking of liquid crystal oligomers (LCOs) is presented. This process is straightforward and allows the synthesis of magnetically aligned high molecular weight LCOs, which maintain a nematic phase before crosslinking. By applying different magnetic field orientations during the polyaddition, three distinct LCE networks are synthesized, each demonstrating unique reversible deformation behaviors, including radial contraction and expansion, thickness contraction, and bending. The potential of this approach is further demonstrated with LCE-based four-leaf clovers exhibiting selective and reversible large-angle bending deformation, despite the high thickness of the material. This simple approach should help researchers in various fields looking for a practical and versatile method for designing robust LCE actuators.

## 1. Introduction

Soft actuators have attracted growing interest for their ability to respond to external stimuli with reversible and controlled actuation, making them ideal for a wide range of applications, including sensors,[1] soft robotics,[2,3] and medical devices.[4] Among

these, liquid crystal elastomers (LCEs) stand out for their significant, rapid, and reversible shape changes, driven by the nematic-isotropic phase transition of mesogens in response to stimuli such as heat or light.[5] Above the nematic-isotropic transition temperature ($T_{N1}$), the initially ordered mesogens lose their alignment and become randomly arranged, resulting in a significant shape change in the direction of mesogen alignment and a moderate expansion in the perpendicular direction.[6]

Controlling mesogen alignment is, therefore, critical to achieving the desired actuation. Several strategies are commonly used to align mesogens, such as mechanical stretching,[7-9] surface-enforced alignment,[10,11] and rheological alignment.[12-16] Once mesogens are aligned, crosslinking is then performed to lock their alignment. However, these methods face limitations when it comes to scaling 3D shapes with precise control of mesogen alignment. For instance, while the two-stage thiol-acrylate Michael addition and photo-polymerization method provides flexibility in mesogen alignment, the mechanical alignment is performed after the LCE sample has been primarily

shaped, restricting its versatility in complex geometries. Similarly, surface-enforced alignment techniques are mostly limited to thin films and struggle with complex 3D shapes due to the constraints imposed by surface anchoring forces.[17] In rheological alignment methods, such as direct ink writing, the alignment direction is restricted by the extrusion pathway.

An alternative method for aligning mesogens is magnetic field alignment. Because of their aromatic structure, most mesogens show positive diamagnetic anisotropy. This means they align their long axis parallel to the applied magnetic field, driven by the minimization of their potential energy.[6] Unlike surface alignment techniques, magnetic field alignment operates volumetrically, orienting mesogens throughout the entire sample thickness, which is particularly advantageous for preparing thick samples. Moreover, by adjusting the orientation of the magnetic field, it is possible to locally control the direction of LC directors without altering the material shape. This approach has already been used to prepare liquid crystal networks using digital light processing, creating sophisticated 3D structures with voxelated molecular patterns.[18] Therefore, magnetic

R. Mouhoubi, V. Lapinte, S. Blanquer
Institut Charles Gerhardt Montpellier (ICGM)
CNRS
Université de Montpellier
ENSCM
Montpellier, France
E-mail: sebastien.blanquer@umontpellier.fr

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202424400

© 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adfm.202424400
alignment has been widely used in the fabrication of LCEs, re-
lying on the alignment of mesogens in the nematic phase be-
fore crosslinking.[19] This technique has proven effective in soft
lithography for creating LCE microactuators.[20] However, its use
for larger-scale LCEs remains limited, despite its potential for
volumetric alignment and compatibility with 3D printing. Re-
cently, honeycomb and iris-shaped LCE actuators have been suc-
cessfully fabricated,[21,22] along with larger LCEs capable of com-
plex and customizable 3D shape changes.[23] Nonetheless, syn-
thesis methods using magnetic field alignment are rarely em-
ployed in standard LCE fabrication, particularly in additive man-
ufacturing, limiting their broader application.[22,23] Indeed, click
reactions, especially chain-extension reactions such as the thio-
Michael addition, have represented a major advancement in LCE
synthesis in recent years.[5] However, their integration with mag-
netic field alignment has been poorly reported.[22,23] This limita-
tion arises from the requirement for mesogens to be aligned and
crosslinked in the nematic state. Specific conditions must there-
fore be set for each system to achieve a stable nematic LC mix-
ture before crosslinking, ensuring no recrystallization of meso-
gens below their melting point.[24] As an alternative, liquid crystal
oligomers (LCOs) can be aligned using a magnetic field. How-
ever, this approach has been rarely used and remains limited to
very low molecular weight oligomers. Aligning high molecular
weight LCOs is particularly challenging, as it requires stronger
magnetic fields and longer relaxation times compared to small
mesogenic units.[20] To date, magnetic field alignment of LCOs
has only been achieved for small mesogenic units, either aligned
in the nematic state or requiring the presence of a liquid crys-
talline solvent.[25,26] As a result, LCEs prepared by this approach
are expected to exhibit mechanical and thermo-mechanical prop-
erties that significantly differ from standard LCEs synthesized us-
ing other alignment techniques.

Herein, we introduce a versatile approach to designing pro-
grammable LCEs via magnetic field alignment. This method
employs a magnetic field-assisted thio-Michael polyaddition fol-
lowed by the photo-crosslinking of liquid crystal oligomers
(LCOs). As illustrated in **Figure 1**, the polyaddition is performed
under two different magnetic field orientations or in the absence
of a magnetic field, resulting in three distinct LCE networks af-
ter photo-crosslinking: monodomain uniaxial planar LCE (m-
LCE), homeotropic LCE (h-LCE) and polydomain LCE (p-LCE).
In m-LCE, mesogens are uniaxially aligned in a planar orienta-
tion; in h-LCE, they are vertically aligned; and in p-LCE, meso-
gens exhibit no global alignment. Each of these networks exhibits
distinctive reversible deformation behaviors resulting from the
specific alignment of mesogens induced by the magnetic field.
This strategy relies on the synthesis of aligned high molecu-
lar weight LCOs that maintain a nematic phase at room tem-
perature, ensuring that the magnetic field-induced alignment of
mesogens is preserved until photo-crosslinking. The process is
straightforward and can be easily tailored for use with commer-
cially available diacrylate mesogens and aliphatic dithiols of dif-
ferent lengths. Furthermore, this method uses the thio-Michael
polyaddition, a well-established chain-extension reaction com-
monly employed in the synthesis of LCEs for its mild condi-
tions, high selectivity, precise control over network architecture,
and ease of scalability.[27–32] To demonstrate the effectiveness of
this approach, we first show how LCOs can be aligned along the

magnetic field through the thio-Michael polyaddition of RM257 and 2,2-(ethylenedioxy)diethanethiol (EDDET). We then investigate how controlling the magnetic field orientation influences the alignment of mesogens and, in turn, impacts the LC order, mechanical properties, and reversible dimensional deformation behaviors of the three LCE networks. Finally, we demonstrate that the monodomain uniaxial planar alignment of mesogens enables reversible bending around an axis perpendicular to their alignment. In particular, the potential of this approach is successfully demonstrated with temperature-responsive LCE-based four-leaf clovers exhibiting selective and reversible bending deformation.

## 2. Results and Discussion

### 2.1. LCOs Prepared Via Thio-Michael Polyaddition

The purpose of this study was to design programmable LCEs via magnetic field alignment. To address this challenge, we developed an approach using a magnetic field-assisted thio-Michael polyaddition followed by the photo-crosslinking of LCOs. As illustrated in Figure 1, an LC mixture composed of a mesogen and a dithiol chain extender is initially made isotropic by heating at T<sub>NI, mesogen</sub>. The polyaddition is then conducted under the influence of a magnetic field. As the temperature is progressively decreased during the polyaddition, the mixture switches from the isotropic to the nematic phase, allowing the short-growing LCOs to align parallel to the magnetic field. Once the polymerization is complete, the high molecular weight LCOs remain in the nematic phase and maintain the magnetic field-induced alignment for subsequent photo-crosslinking. To demonstrate this chemical concept in the most general way, LCOs were synthesized from the mesogenic diacrylate RM257 and the dithiol EDDET via a base-catalyzed thio-Michael polyaddition (Figure 2a). These monomers were selected because they were readily available and widely used in the synthesis of LCEs.<sup>[7,9,28–30,32–41]</sup>

The base-catalyzed thio-Michael polyaddition of RM257 and
EDDET was first investigated to show how LCOs can be aligned
along the magnetic field. An LC mixture of RM257 and EDDET
was initially heated to 128 °C, corresponding to the T<sub>NI, RM257</sub>,
ensuring that the mixture reached the isotropic phase. A mo-
lar excess of RM257 over EDDET at a ratio of 1.15:1 was cho-
sen to obtain acrylate-terminated LCOs. Polyaddition was initi-
ated by the addition of triethylamine (TEA) and conducted in
an oven. The reaction proceeded as the oven temperature (T<sub>oven</sub>)
decreased from 128 to 25 °C over 2 h at a controlled rate of
0.9 °C min<sup>-1</sup>, allowing enough time for mesogen self-assembly
during the isotropic-to-nematic phase transition. The reaction
was then left under a magnetic field at room temperature for
34 h to ensure complete polymerization, resulting in acrylate-
terminated LCOs. The polyaddition was monitored by <sup>1</sup>H NMR
spectroscopy and DSC. <sup>1</sup>H NMR spectra of LCOs at different re-
action times are given in Figure S1 (Supporting Information).
The number-average degree of polymerization (DP<sub>n</sub>) of LCOs at
different reaction times (with one unit represented in Figure 2b)
were determined through end-group detection, as described in
the Experimental Section. These values were subsequently corre-
lated with the T<sub>NI, LCO</sub> measured by DSC. DSC curves of LCOs at
1 min, 60 min, and 36 h of reaction are shown in Figure 2c and
the others are reported in Figure S2a (Supporting Information).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Conceptual scheme describing the different steps involved in obtaining m-LCE, h-LCE, and p-LCE. B refers to the magnetic field vector, whose orientation is represented by grey dotted arrows. The black dotted arrows refer to the direction of LC directors.</figcaption></figure>

As shown in Figure 2c, as the reaction progresses and the DPn of LCOs increases from 1 (1,500 g mol⁻¹) to 12 (10 000 g mol⁻¹), the TNI, LCO decreases from 84 to 55 °C. This is attributed to enhanced chain mobility and increased entropy, which reduce the strength of π-π stacking interactions between mesogenic units. This is confirmed by the reduction in enthalpy change, as reported in Table S1 (Supporting Information).

The values of TNI and Toven were then plotted as a function of reaction time for the first 120 min, as shown in Figure 2d. First, the TNI, LCO drop significantly during the first 10 min of the reaction, with a decrease of 61 °C relative to the TNI, RM257. As expected from a step-growth polymerization, the formation of short oligomers is rapid until the LCOs reach a DPn of 5 (4,500 g mol⁻¹). A TNI, LCO plateau is then reached at ≈67 °C between 30 and 120 min of reaction, with the DPn of LCOs remaining at ≈5. As for the Toven, it reaches this value of 67 °C after 71 min. Therefore, LCOs with a DPn of 5 and a TNI, LCO of 67 °C are the ones that can be aligned along the magnetic field, as they

are those that transition from the isotropic to the nematic phase during oven cooling. To not consider only the peak maximum of TNI, LCO but to capture the full range of this isotropic-to-nematic transition, the onset and the offset of the TNI, LCO peak for LCOs with a DPn of 5 were determined using the DSC curve at 60 min of reaction, as shown in Figure S2b (Supporting Information). Three distinct regions can therefore be identified during the cooling process: 1) as the oven cools from 128 to 82 °C over 54 min, the growing LCOs reach a DPn of 5 while remaining in the isotropic phase, 2) as the oven cools from 82 to 55 °C over 31 min, the LCOs still with a DPn of 5 undergo the isotropic-to-nematic phase transition, during which they can be aligned along the magnetic field, and 3) as the oven cools from 55 to 25 °C over 35 min, the LCOs remain in the nematic phase. After an additional 34 h of reaction at room temperature, corresponding to full conversion, the nematic acrylate-terminated LCOs achieved a DPn of 12, which is close to the theoretical DPn of 14 calculated using Carothers' equation.[42] These LCOs also exhibited a
<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) Reaction scheme illustrating the LCO synthesis via the thio-Michael polyaddition reaction. b) Chemical structure of the resulting acrylate-terminated LCO. c) DSC curves of LCOs at 1 min, 60 min, and 36 h of reaction. d) $T_{NI}$ and $T_{oven}$ plotted as a function of reaction time.</figcaption></figure>

$T_{NI, LCO}$ of 55 °C, ensuring that they remain in the nematic phase. The $T_{NI, LCO}$ are also high enough to allow photo-crosslinking in a UV box, where the temperature can rise to 40 °C due to heat generated by the UV lamps. It is important to note that fully synthesized LCOs with a $DP_n$ of 12 (10 000 g mol⁻¹), which undergo the same isotropic-to-nematic transition under a magnetic field, do not achieve mesogen alignment, and therefore, exhibit no action. This is due to the reduced mobility of the oligomer chains and the increased viscosity of the resin. This highlights the importance of the magnetic field alignment occurring at the initial stages of the thio-Michael polyaddition on short-growing LCOs. It is also important to note that when the magnetic field is removed during the polyaddition, even after aligning the short growing oligomers in the nematic phase, the LCOs lose their alignment. This is due to mesogenic interactions and the dynamic processes of chain growth and relaxation during polymerization, which prevents the preservation of alignment without the continuous presence of the magnetic field. This highlights that the thio-Michael polyaddition is indeed actively assisted by the magnetic field, ensuring that oligomers maintain their alignment as they form. Ultimately, our approach enables high molecular weight LCOs to be aligned using magnetic field alignment, making this method suitable for any $DP_n$ without the risk of mesogen recrystallization.

## 2.2. Influence of Mesogen Alignment on LCE Properties

We then studied the influence of controlling the magnetic field orientation on the alignment of mesogens and its subsequent impact on the LC order, mechanical properties, and reversible deformation behaviors. For this purpose, three LCE networks were synthesized: monodomain uniaxial planar LCE (m-LCE), homeotropic LCE (h-LCE), and polydomain LCE (p-LCE). To prepare m-LCE, the polyaddition was conducted between two permanent NdFeB magnets with a magnetic flux density $B \approx 200$ mT. To prepare h-LCE, the polyaddition was carried out on the surface of a single magnet with a magnetic flux density $B \approx 200$ mT. To prepare p-LCE, the polyaddition was performed without the application of a magnetic field (Figure 1). The resulting acrylate-terminated LCOs were subsequently photo-crosslinked under 365 nm UV light for 40 min, with a duration optimized for sufficient crosslinking. The cured samples were then washed by swelling in toluene and dried in a vacuum oven. Cylindrical samples of m-LCE, h-LCE, and p-LCE with a diameter of $\approx 2.7$ cm and a thickness of $\approx 900$ µm were prepared. All the samples exhibited a gel content of $\approx 70\%$. Indeed, as all LCE samples turn white during photo-crosslinking, the increased light scattering can hinder UV penetration, leading to incomplete crosslinking and lower gel content.
We first examined p-LCE to provide an initial understanding of the thermal and thermo-mechanical properties of the LCE network, as its disordered structure simplifies analysis before investigating the influence of mesogen alignment (Figure S3a, Supporting Information). DSC showed that p-LCE exhibited a $T_{NI, LCE}$ of 75 °C (Figure S3b, Supporting Information). DMA showed that p-LCE had a nematic storage modulus $E'_n$ of 1.6 MPa at 25 °C and a $T_g$ of 7 °C. All these properties are consistent with values reported for other standard LCEs in the literature.[9] However, it is important to note that p-LCE does not show any significant dynamic soft elasticity response found in conventional LCEs.[43] Instead, the storage modulus $E'$ shows a shape more typical to conventional amorphous networks. This behavior is aligned with the properties of polydomain LCEs that are synthesized in the nematic state, as reported in previous studies.[40] Indeed, after 71 min of reaction, when the LCOs transitioned to the nematic phase, the rest of the reaction took place in the nematic state below the $T_{NI, LCO}$, which influenced the final properties of the LCE network.

The influence of the magnetic field on the alignment of the mesogens was then investigated. As illustrated in Figure 1, in m-LCEs, the LC director is parallel to the horizontal plane of the cylinder. This means the mesogenic units are aligned throughout the sample in the same direction parallel to this plane. In contrast, in h-LCEs, the LC director is normal to the horizontal plane, resulting in vertical alignment of the mesogenic units throughout the sample. In p-LCEs, the LC director does not align to any specific direction, leading to mesogenic units that are only locally aligned but not globally aligned throughout the sample. To assess the degree of mesogen alignment induced by the magnetic field, WAXS analyses were performed to estimate the order parameter ($S$) of m-LCE and h-LCE. Both samples exhibited the two characteristic diffuse arcs, indicating mesogen alignment in a preferential direction, as shown in Figure 3a,b. In particular, m-LCE showed more pronounced separated diffuse arcs than h-LCE. This was further confirmed by calculating the order parameters from the WAXS azimuthal intensity distributions (Figure S4, Supporting Information) using Equation (3). The $S$ value for m-LCE is 0.21, while that for h-LCE is 0.10. Therefore, m-LCE exhibits a higher degree of alignment than h-LCE. This difference, despite similar magnetic field densities, could be due to the inhomogeneity of the magnetic field generated by a single magnet, as opposed to the more uniform field generated between two magnets. It should be noted that the setup used was at the laboratory scale and that a more powerful magnetic installation could potentially achieve better alignment, leading to a higher-order parameter.

Uniaxial tensile tests were then conducted to assess the influence of mesogen alignment on the mechanical properties of m-LCE, h-LCE, and p-LCE (Figure 3c). The values of Young modulus, threshold stress (i.e., the stress at which the soft elasticity region begins), and failure strain are given in Table S2 (Supporting Information). First, p-LCE shows a mechanical response typical of polydomain LCEs synthesized in the nematic state,[6] with a low Young modulus, a low threshold stress, and a high failure strain ≈300%. For m-LCE, when stretched parallel to the LC director, the material exhibits a linear elastic response with the highest Young modulus, no soft elasticity region, and a low failure strain (<50%). In contrast, when stretched perpendicu-

lar to the LC director, the material exhibits a non-linear elastic response with a pronounced soft elasticity region between 30% and 90% strain, similar to that reported for main-chain LCEs.[8] It also has a higher threshold stress and Young modulus than p-LCE, but a lower failure strain ≈200%. For h-LCE, similar behavior is observed, where aligned mesogens reorient under applied stress. However, as opposed to previous reports,[5] h-LCE has a higher Young modulus and threshold stress, and a lower failure strain (≈130%) than m-LCE when stretched perpendicular to the LC director. Ultimately, the alignment of mesogens induced by the magnetic field makes the LCEs stiffer and less ductile regardless of the direction of stretching. The reversible dimensional deformation behavior of these LCE networks is also influenced by mesogen alignment, resulting in distinct dimensional changes when reaching the isotropic state. For m-LCE, where the mesogens are uniaxially planar aligned, the main deformation with increasing temperature is a contraction in the horizontal plane (Figure 3d). In particular, the material contracts in length by ≈15% along the LC direction and expands by ≈9% in the perpendicular direction within the same plane (Figure 3f). This gives m-LCE an ellipsoidal shape in the isotropic state. For h-LCE, where the mesogens are vertically aligned, the main deformation with increasing temperature is a contraction normal to the horizontal plane (Figure 3e). In particular, the material contracts in thickness by ≈25% and expands in diameter by ≈10% (Figure 3f,g). In this case, h-LCE keeps its cylindrical shape with a larger diameter and a lower thickness in the isotropic state. For p-LCE, where the mesogens are not globally aligned, no dimensional change is observed when reaching the isotropic state (Figures 3f; Figure S3d, Supporting Information).

## 2.3. Thermal Bending Actuation

In addition to the dimensional changes characteristic of LCEs, we observed that m-LCE generates a selective reversible bending deformation. We attributed this phenomenon to a gradient in mesogen alignment across the thickness of the material, directly resulting from a crosslinking gradient formed during photo-crosslinking. FTIR spectroscopy was used to evaluate the presence of the crosslinking gradient. During photopolymerization, only the top surface of the LCO resin in the silicone mold was exposed to UV light. As a result, FTIR spectra were recorded for the upper side of m-LCE, directly exposed to the UV lamp, and the lower side, which received less UV exposure, before swelling in toluene. The spectra indicate a reduced intensity of the acrylate C=C absorption peak at ≈810 cm⁻¹ on the upper side compared to the lower side (Figures S5a,b, Supporting Information). This indicates a higher level of acrylate conversion and stronger photo-crosslinking in the upper region, confirming the presence of a crosslinking gradient in m-LCE. Such crosslinking gradients have been previously reported as effective strategies to induce controlled bending deformation in LCEs, with the direction of curvature governed by the alignment of mesogens.[44,45] In our approach, the mesogen alignment induced by the magnetic field near the surface becomes more firmly locked during photo-crosslinking compared to the deeper regions, inducing a gradient in mesogen alignment across the thickness of the material.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** a) 2D WAXS image of m-LCE. b) 2D WAXS image of h-LCE. c) Stress-strain curves of m-LCE, h-LCE, and p-LCE. d) Thermo-responsive reversible deformation behavior of m-LCE. e) Thermo-responsive reversible deformation behavior of h-LCE. f) Contraction and expansion in the horizontal plane for m-LCE, h-LCE, and p-LCE as a function of temperature. g) Thickness contraction of h-LCE as a function of temperature. The white scale bars represent 0.5 cm.</figcaption></figure>

Therefore, upon thermal actuation, the upper regions, with better alignment, contract more significantly than the deeper regions, resulting in a bending deformation similar to that observed in bilayer actuators. This bending occurs around the axis perpendicular to the magnetic field-induced mesogen alignment axis, both of which are in the horizontal plane of m-LCE and are directed toward the more crosslinked side (Figure 4a). For h-LCE, no bending deformation was observed as it undergoes radial expansion. For p-LCE, while bending was observed, it occurred around three potential axes, without a clear preference, as the mesogens are disordered and unable to define a consistent axis for deformation (Figure S6b, Supporting Information). It is important to note that the formation of the m-LCE crosslinking gradient is reproducible under these conditions. If crosslinking were nearly complete, resulting in a gel content approaching 100%, the bending deformation would not occur. We took advantage of this selective

bending behavior to develop and fabricate m-LCE-based bending actuators.

The temperature-dependent bending deformation of m-LCE was first examined by determining the bending angle (θ) at different temperatures, as shown in Figure 4b (additional photographs in Figure S6a, Supporting Information). While m-LCE began to exhibit dimensional changes upon heating (Figure 3f), no bending deformation was observed until the temperature of the heating plate reached 90 °C. Beyond this point, the bending angle increased linearly from 18° at 105 °C to 91° at 180 °C, demonstrating that precise control over the bending deformation of m-LCE can be achieved by simply adjusting the temperature. The actuation speed was then assessed at different fixed temperatures by measuring the change in bending angle over time, as shown in Figure 4c. Upon contact with the heating plate at temperatures equal to or higher than 105 °C, m-LCE initially exhibits
<figure><img src="image_6.png" /><figcaption>**Figure 4.** a) Principle of the bending deformation of m-LCE along the mesogen alignment axis. b) Bending angle of m-LCE as a function of temperature. c) Actuation speed of m-LCE at different temperatures. The white scale bars represent 0.5 cm.</figcaption></figure>

rapid linear bending deformation, followed by a slower increase until reaching a plateau, which corresponds to the final bending angle. Actuation speed constants ($k_{a, T}$ (°C)) were calculated by determining the slope of the linear region for each temperature. The speed of bending deformation increased with temperature, from ≈1° min⁻¹ at 105 °C to ≈15° min⁻¹ at 180 °C. Furthermore, m-LCE reached its final bending angle value after more than 40 s at 105 °C, compared to 20 s at 180 °C. Indeed, the actuation speed is dependent on the heat flux through the thickness of m-LCE. As stated by Fourier's law, the heat flux is proportional to the gradient of temperature over the thickness of the material. Therefore, heating m-LCE at a higher temperature leads to a faster thermal bending actuation. To our knowledge, this is the first report of a ≈1 mm thick LCE demonstrating significant and selective large-angle bending deformation within tens of seconds, achieved without relying on mechanical alignment techniques. This performance is enabled by a crosslinking gradient combined with magnetic field-induced mesogen alignment. While the existing literature on LCE actuators shows similar fast bending deformations and high bending angle values, these are typically confined to samples thinner than 100 µm, where low thermal conductivity does not significantly hinder response times and thickness-related constraints on bending are minimal.[30,44] In particular, reported LCE samples prepared via surface or shear alignment with a thickness higher than 300 µm exhibit a bending angle below 45°.[30,46,47] Furthermore, while the achievable thickness of LCEs depends on UV light penetration, this method could be combined with additive

manufacturing techniques, such as stereolithography, to enable photo-crosslinking layer-by-layer. This approach could provide a viable solution for fabricating main-chain LCEs using DLP 3D printing combined with magnetic field alignment, a technique that currently only allows the alignment of small mesogenic units.[26,48]

This reversible bending deformation was then demonstrated by fabricating four-leaf clover-shaped m-LCEs with two distinct mesogen alignments. In the first configuration, the mesogens are aligned parallel to an axis passing through two of the clover's leaves, labeled $\gamma$ and $\beta$. Upon heating, only these two leaves exhibited noticeable bending, while the other leaves, labeled $\alpha$ and $\delta$, remained relatively flat (Figure 5a; Video S1, Supporting Information). This deformation is considered selective, as bending is restricted to only two leaves. In particular, bending occurs around an axis perpendicular to mesogen alignment, passing through the unaffected leaves $\alpha$ and $\delta$. In the second configuration, the mesogens are aligned parallel to an axis passing through the junctions between the left pair ($\alpha$ and $\gamma$) and right pair ($\beta$ and $\delta$) of leaves (Figure 5b). Upon heating, all four leaves were involved in the bending deformation. In this case, bending occurs around an axis passing through the junctions between the top pair ($\alpha$ and $\beta$) and bottom pair ($\gamma$ and $\delta$) of leaves, resulting in a butterfly-shaped geometry. It is important to note that in both configurations, contraction occurs along mesogen alignment. With these specific geometries, the m-LCE-based four-leaf clovers still achieved large bending angles of ≈80° and 75° at 180 °C for each configuration, respectively. The reversibil-
<figure><img src="image_7.png" /><figcaption>**Figure 5.** a) Selective, reversible bending deformation of a m-LCE-based four-leaf clover. Each leaf of the clover is identified by the letters α, β, γ, and δ, respectively. (b) Reversible bending deformation of a m-LCE-based four-leaf clover resulting in a butterfly-shaped geometry. c) Cyclic actuation of the m-LCE-based four-leaf clover exhibiting a selective response. The dotted lines represent the axis around which the bending occurs. The white scale bars represent 0.5 cm.</figcaption></figure>

ity of the four-leaf clover-shaped m-LCE in the first configura-
tion was further investigated. The bending angle was measured
with the same cycle: 1) 40 s at 180 °C and, 2) 40 s at 25 °C.
The measurements were taken during the first five cycles and
again at the 100th cycle. The sample showed highly repeatable
actuation and relaxation over 100 cycles (Figure 5c), maintain-
ing consistent performance and stability over time. This selective
and reversible response highlights the potential of m-LCE for ad-
vanced applications requiring precisely controlled and repeatable
deformation.

**3. Conclusion**

We have introduced a straightforward method for fabricating LCEs using magnetic field alignment. Unlike other approaches that rely on complex synthesis methods, our strategy uses magnetic field-assisted base-catalyzed thio-Michael polyaddition followed by the photo-crosslinking of LCOs. Since this process involves the well-established thio-Michael polyaddition commonly employed in the synthesis of LCEs, it should be accessible to the extended materials science community. The key aspect of our method is the magnetic alignment of short-growing LCOs during the early stages of the polyaddition, as they shift from the isotropic to the nematic phase. When the polymerization is complete, the resulting aligned high molecular weight LCOs

(10 000 g mol⁻¹), with a T<sub>NI, LCO</sub> above room temperature, remain in the nematic phase and maintain the magnetic field-induced alignment for subsequent photo-crosslinking. The versatility of magnetic field alignment is further demonstrated by applying different magnetic field orientations, resulting in LCE networks with distinct reversible deformation behaviors such as radial contraction and expansion, thickness contraction, and bending. The LCEs exhibit mechanical and thermo-mechanical properties comparable to those of standard LCEs. The potential of this strategy is further demonstrated with thick LCE actuators that show large bending deformations, thanks to a crosslinking gradient combined with magnetic field-induced mesogen alignment. In particular, LCE-based four-leaf clovers exhibit selective and reversible bending behaviors, maintaining performance over 100 cycles. While we applied this method to the thio-Michael polyaddition, it can also be extended to other chain extension reactions and adapted to various commercial mesogens and linkers, further highlighting the high versatility and scalability of the approach. This also shows more broadly that chain-extension reactions can be effectively integrated into magnetic field alignment, thereby paving the way for a wide range of magnetically aligned main-chain LCEs with tailored properties. We believe that this facile method opens up new possibilities for using magnetic field alignment to synthesize robust LCEs, offering promising opportunities for the development of soft actuators.
## 4. Experimental Section

**Materials:** 1,4-Bis[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (≥95%, RM257) was purchased from Sm molecule (San Antonio, USA). 2,2-(Ethylenedioxy)diethanethiol (≥95%, EDDET), 2,6-dibutyl-4-methylphenol (≥99%, BHT), Orange G and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (≥97%, PPO) were purchased from Sigma-Aldrich (France). Triethylamine (≥99%, TEA) was purchased from Carlo Erba (France). All chemicals were used as-received without further purification. Permanent NdFeB block magnets (40 × 40 × 20 mm) with a strength of ≈60 kg were purchased from Supermagnete (Germany).

**Synthesis of LCE Samples:** RM257 and EDDET were combined in a molar ratio of 1.15:1. RM257 was added to a 25 mL vial along with BHT (2 wt.%) and PPO (2 wt.%). BHT served as a free-radical inhibitor to prevent thermal crosslinking of RM257 during heating, while PPO acted as the photoinitiator. The vial was then heated at 128 °C and vortex-mixed before adding EDDET. Subsequently, TEA (2 mol%) was added dropwise to initiate the polymerization, and the resulting isotropic mixture was again vortex-mixed.

To prepare monodomain planar LCEs (m-LCEs), the mixture was promptly poured into a cylindrical silicone mold (2.75 cm in diameter) positioned within a setup comprising two permanent NdFeB block magnets, placed 3 cm apart (B≈200 mT), and then transferred to an oven preheated to 128 °C. For homeotropic LCEs (h-LCEs), the mixture was similarly poured into a silicone mold placed on a single permanent Nd-FeB block magnet (B≈200 mT) before being transferred to the oven. For polydomain LCEs (p-LCEs), the mixture was simply poured into a silicone mold and then transferred directly to the oven. In all cases, the reaction proceeded as the temperature of the oven decreased from 128 to 25 °C over a 2 h period at a rate of 0.9 °C min⁻¹, followed by a further 34 h at room temperature to ensure complete polymerization.

Subsequently, the resulting nematic LCO resins were photo-crosslinked under 365 nm UV light (20 mW cm⁻²) for 40 min using a UV Crosslinker Bio-Link chamber (Thermo Fisher, France). The cured samples were then removed from the silicone molds, washed in swelling in toluene for 48 h to remove unreacted polymer residues and dried in a vacuum oven at 90 °C for 24 h.

Measurements of dimensional changes in m-LCE, h-LCE, and p-LCE, as well as bending deformations of m-LCE, were captured at different temperatures and analyzed with ImageJ software. The exception was the thickness measurement of h-LCE at different temperatures, which required a digital thickness gauge from Schmidt (Germany).

**¹H NMR Characterisation:** ¹H NMR spectroscopy was employed to determine the DPn and the number-average molecular weights (Mn) of the LCOs at different reaction times through end-group detection. The ¹H NMR spectra were realized using a 400 MHz Bruker's Avance Spectrometer with CDCl3 as a deuterated solvent. The chemical shifts of protons were relative to CHCl3 residual in CDCl3 at δ = 7.26 ppm. The DPn and Mn values for each LCO were calculated based on the ratio of six protons in the diacrylate end-groups to four aromatic protons in the repetitive mesogen unit using the following equations (1) and (2):

$$DP_n = \frac{\frac{\int_{\delta}^{\delta_8} \frac{1}{l}}{4}}{\frac{\int_{\delta_{5,7}}^{\delta_{8,5}} l + \int_{\delta_{6}}^{\delta_{6,15}} l + \int_{\delta_{6,3}}^{\delta_{6,4}} l}{6}} - 1 \quad (1)$$

$$M_n = DP_n \times M_{\text{repeating unit}} + M_{\text{end-groups}} \quad (2)$$

where Mrepeating unit and Mend-groups are the molar masses of the repeating unit (Mrepeating unit = 772 g mol⁻¹) and the end groups (Mend-groups = 588 g mol⁻¹), respectively.

**Differential Scanning Calorimetry:** Differential scanning calorimetry (DSC) was used to evaluate the thermal properties of the LCOs at different reaction times and p-LCE. DSC analyses were carried out using a Netzsch DSC 3500 Sirius. The ethanol-precipitated and dried LCOs and p-LCE with a mass between 5 and 10 mg were loaded into standard aluminum DSC pans. The samples were equilibrated at -50 °C and heated to 120 °C at a

rate of 10 °C min⁻¹. They were then cooled slowly to -50 °C at a rate of 2 °C min⁻¹ to allow LC self-assembly. They were then heated to 120 °C at a rate of 20 °C min⁻¹. Data were reported from the second heating scans. The glass transition temperature (Tg) was defined at the step change in the slope of the heat flow signal. The nematic-isotropic transition temperature (TNI) was determined at the minimum value of the endothermic peak. The enthalpy change (ΔHNI) was measured by integrating the endothermic energy well of the nematic-isotropic transition.

**Dynamic Mechanical Analysis:** Dynamic mechanical analysis (DMA) was employed to study the thermo-mechanical properties and the phase transitions of p-LCE. DMA test was performed using a Mettler Toledo DMA instrument. A rectangular sample measuring ≈20 × 2 × 0.9 mm³ was tested in tensile mode, with an active length of 10 mm. The sample was equilibrated at -50 °C. It was then subjected to a strain of 0.2% at 1 Hz and heated from -50 to 120 °C at a rate of 2 °C min⁻¹. Tg was defined at the maximum value of the loss tangent (tan δ) curve. TNI was attributed to the lowest value of the storage modulus (E') curve above Tg. The nematic modulus (E'n) was measured using the storage modulus values at 25 °C.

**Uniaxial Tensile Testing:** Uniaxial tensile tests were performed to study the mechanical properties of m-LCE (parallel and perpendicular to the director), h-LCE, and p-LCE. The tensile tests were executed on an Instron 3366L5885 mechanical tester equipped with a 100 N load cell. Rectangular samples measuring ≈20 × 2 × 0.9 mm³ were tested at a displacement rate of 5 mm s⁻¹. The Young modulus was measured by taking the slope in the linear region of the stress-strain curve between 0 and 3% strain. The threshold stress (except for m-LCE parallel to the director) was determined as the stress at the intersection of the tangents of the first linear region and the soft elasticity plateau. The failure strain value was defined as the maximum strain at break.

**Wide Angle X-Ray Scattering:** Wide angle X-ray scattering (WAXS) was employed to measure the orientational order of m-LCE across the horizontal surface and h-LCE through the thickness. The experiments were performed with an in-house setup at Laboratoire Charles Coulomb (L2C), CNRS, Université de Montpellier, France. A high-brightness low power X-ray tube, coupled with an aspheric multilayer optic (GeniX3D from Xenocs), was employed. It delivered an ultralow divergent beam (0.5 mrad, λ = 0.15418 nm). Scatterless slits were used to give a clean 0.6 mm beam diameter with a flux of 35 Mphotons s⁻¹ at the sample. A transmission configuration was used, and the scattered intensity was measured by a 2D "Pilatus" 300 K pixel detector by Dectris (490 × 600 pixels) with a pixel size of 172 × 172 µm² at a distance of 100 mm from the sample for WAXS configuration. The order parameter (S) was calculated using the following equation Equation (3):

$$S = \frac{\int_{0}^{\pi/2} l(\theta) \frac{3\cos^2\theta - 1}{2} \sin\theta d\theta}{-\frac{1}{2} \int_{0}^{\pi/2} l(\theta) \sin\theta d\theta} \quad (3)$$

where θ is the angle between the director and mesogenic units and l(θ) is the azimuthal intensity distribution. The calculation was carried out for θ between 0 and 90°.

**FTIR Spectroscopy:** FTIR spectra were recorded using a ThermoScientific Nicolet i550 spectrometer equipped with a Golden-Specac heating cell and ATR module. Data were collected with a resolution of 4 cm⁻¹ and averaged over 32 scans. The spectra were normalized to the aromatic C=C peak at 1604 cm⁻¹.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

This work received financial support from the State, managed by the National Research Agency (ANR) under the France 2030 program, with
reference ANR-21-RHUS-011 and the "DIADEM Program", grant number ANR-22-PEXD-0016 ("ARTEMIS"). The authors would like to thank Philippe Dieudonné-Georges from Laboratoire Charles Coulomb (L2C), CNRS, Université de Montpellier, France, for performing the WAXS measurements.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

bending deformation, LCE, magnetic field alignment, photo-crosslinking, soft actuators, stimuli-responsive materials, thio-Michael addition

Received: January 16, 2025

Published online:

[1] C. Han, Y. Jeong, J. Ahn, T. Kim, J. Choi, J. Ha, H. Kim, S. H. Hwang, S. Jeon, J. Ahn, J. T. Hong, J. J. Kim, J. Jeong, I. Park, *Adv. Sci.* **2023**, 10, 2302775.

[2] X. Kong, M. Dong, M. Du, J. Qian, J. Yin, Q. Zheng, Z. L. Wu, *Chem Bio Eng.* **2024**, 1, 312.

[3] Y. Chen, H. Zhao, J. Mao, P. Chirarattananon, E. F. Helbling, N. P. Hyun, D. R. Clarke, R. J. Wood, *Nature* **2019**, 575, 324.

[4] M. Li, A. Pal, A. Aghakhani, A. Pena-Francesch, M. Sitti, *Nat. Rev. Mater.* **2021**, 7, 235.

[5] Z.-C. Jiang, Q. Liu, Y.-Y. Xiao, Y. Zhao, *Prog. Polym. Sci.* **2024**, 153, 101829.

[6] K. M. Herbert, H. E. Fowler, J. M. McCracken, K. R. Schlafmann, J. A. Koch, T. J. White, *Nat. Rev. Mater.* **2021**, 7, 23.

[7] C. M. Yakacki, M. Saed, D. P. Nair, T. Gong, S. M. Reed, C. N. Bowman, *RSC Adv.* **2015**, 5, 18997.

[8] D. R. Merkel, N. A. Traugutt, R. Visvanathan, C. M. Yakacki, C. P. Frick, *Soft Matter* **2018**, 14, 6024.

[9] M. O. Saed, A. H. Torbati, C. A. Starr, R. Visvanathan, N. A. Clark, C. M. Yakacki, *J. Polym. Sci. Part B: Polym. Phys.* **2017**, 55, 157.

[10] H.-H. Yoon, D.-Y. Kim, K.-U. Jeong, S. Ahn, *Macromolecules* **2018**, 51, 1141.

[11] T. S. Hebner, B. E. Kirkpatrick, K. S. Anseth, C. N. Bowman, T. J. White, *Adv. Sci.* **2022**, 9, 2204003.

[12] Z. Wang, Z. Wang, Y. Zheng, Q. He, Y. Wang, S. Cai, *Sci. Adv.* **2020**, 6, eabc0034.

[13] X. Song, W. Zhang, H. Liu, L. Zhao, Q. Chen, H. Tian, *Front. Robot. AI* **2022**, 9, 889848.

[14] G. E. Bauman, J. M. McCracken, T. J. White, *Angew Chem Int Ed.* **2022**, 61, 202202577.

[15] Y. Wang, R. Yin, L. Jin, M. Liu, Y. Gao, J. Raney, S. Yang, *Adv Funct Mater.* **2023**, 33, 2210614.

[16] D. J. Roach, X. Kuang, C. Yuan, K. Chen, H. J. Qi, *Smart Mater. Struct.* **2018**, 27, 125011.

[17] J. M. McCracken, J. D. Hoang, J. A. Herman, K. M. Lynch, T. J. White, *Adv Mater. Technol.* **2023**, 8, 2202067.

[18] M. Tabrizi, T. H. Ware, M. R. Shankar, *ACS Appl. Mater. Interfaces* **2019**, 11, 28236.

[19] C. H. Legge, F. J. Davis, G. R. Mitchell, *J. Phys. II France* **1991**, 1, 1253.

[20] S. Li, M. Aizenberg, M. M. Lerch, J. Aizenberg, *Acc. Mater. Res.* **2023**, 4, 1008.

[21] B. Ni, G. Liu, M. Zhang, P. Keller, M. Tatoulian, M.-H. Li, *CCS Chem* **2022**, 4, 847.

[22] S. Schuhladen, F. Preller, R. Rix, S. Petsch, R. Zentel, H. Zappe, *Adv. Mater.* **2014**, 26, 7247.

[23] B. Ni, G. Liu, M. Zhang, M. Tatoulian, P. Keller, M.-H. Li, *ACS Appl. Mater. Interfaces* **2021**, 13, 54439.

[24] B. Ni, M. Zhang, C. Guyon, P. Keller, M. Tatoulian, M. Li, *Macromol. Rapid Commun.* **2020**, 41, 2000385.

[25] M.-H. Li, P. Keller, J. Yang, P.-A. Albouy, *Adv. Mater.* **2004**, 16, 1922.

[26] J. A. Herman, R. Telles, C. C. Cook, S. C. Leguizamon, J. A. Lewis, B. Kaehr, T. J. White, D. J. Roach, *Adv. Mater.* **2024**, 36, 2414209.

[27] Y. Guo, J. Lee, J. Son, S. Ahn, J.-M. Y. Carrillo, B. G. Sumpter, *Macromolecules* **2019**, 52, 6878.

[28] G. Chen, B. Jin, Y. Shi, Q. Zhao, Y. Shen, T. Xie, *Adv. Mater.* **2022**, 34, 2201679.

[29] P. Mainik, L. Hsu, C. W. Zimmer, D. Fauser, H. Steeb, E. Blasco, *Adv Mater. Technol.* **2023**, 8, 2300727.

[30] S. Li, H. Bai, Z. Liu, X. Zhang, C. Huang, L. W. Wiesner, M. Silberstein, R. F. Shepherd, *Sci. Adv.* **2021**, 7, eabg3677.

[31] B. Jin, J. Liu, Y. Shi, G. Chen, Q. Zhao, S. Yang, *Adv. Mater.* **2022**, 34, 2107855.

[32] N. A. Traugutt, D. Mistry, C. Luo, K. Yu, Q. Ge, C. M. Yakacki, *Adv. Mater.* **2020**, 32, 2000797.

[33] Y. Geng, J. P. F. Lagerwall, *Adv. Sci.* **2023**, 10, 2301414.

[34] M. O. Saed, C. P. Ambulo, H. Kim, R. De, V. Raval, K. Searles, D. A. Siddiqui, J. M. O. Cue, M. C. Stefan, M. R. Shankar, T. H. Ware, *Adv. Funct. Mater.* **2019**, 29, 1806412.

[35] X. Peng, S. Wu, X. Sun, L. Yue, S. M. Montgomery, F. Demoly, K. Zhou, R. R. Zhao, H. J. Qi, *Adv. Mater.* **2022**, 34, 2204890.

[36] Y. Zhan, D. J. Broer, J. Li, J. Xue, D. Liu, *M. Horiz.* **2023**, 10, 2649.

[37] J. Lee, J. Bae, J. H. Hwang, M. Choi, Y. S. Kim, S. Park, J. Na, D. Kim, S. Ahn, *Adv Funct Materials* **2022**, 32, 2110360.

[38] Z. Wang, H. Tian, Q. He, S. Cai, *ACS Appl. Mater. Interfaces* **2017**, 9, 33119.

[39] M. Barnes, S. Cetinkaya, A. Ajnsztajn, R. Verduzco, *Soft Matter* **2022**, 18, 5074.

[40] N. A. Traugutt, R. H. Volpe, M. S. Bollinger, M. O. Saed, A. H. Torbati, K. Yu, N. Dadivanyan, C. M. Yakacki, *Soft Matter* **2017**, 13, 7013.

[41] C. Yuan, D. J. Roach, C. K. Dunn, Q. Mu, X. Kuang, C. M. Yakacki, T. J. Wang, K. Yu, H. J. Qi, *Soft Matter* **2017**, 13, 5558.

[42] W. H. CarPeriodicals, *Trans. Faraday Soc.* **1936**, 32, 39.

[43] D. Mistry, N. A. Traugutt, B. Sanborn, R. H. Volpe, L. S. Chatham, R. Zhou, B. Song, K. Yu, K. N. Long, C. M. Yakacki, *Nat. Commun.* **2021**, 12, 6677.

[44] J. Hu, Z.-Y. Kuang, L. Tao, Y.-F. Huang, Q. Wang, H.-L. Xie, J.-R. Yin, E.-Q. Chen, *ACS Appl. Mater. Interfaces* **2019**, 11, 48393.

[45] Y.-Y. Xiao, Z.-C. Jiang, J.-B. Hou, Y. Zhao, *Nat. Commun.* **2021**, 12, 624.

[46] W. C. Han, G. W. Sim, Y. B. Kim, D. S. Kim, *Macromol. Rapid Commun.* **2021**, 42, 2100404.

[47] H. Jiang, C. Chung, M. L. Dunn, K. Yu, *Nat. Commun.* **2024**, 15, 8491.

[48] Y. Wang, J. An, H. Kim, S. Jeong, H. Kim, J. Park, S. Ko, J. Son, H. Lee, *arXiv* **2024**, *arXiv:2401.06590*.