Facile fabrication of patternable and large-area elastic liquid crystal polymer films

ABSTRACT Cholesteric liquid crystal elastomers (CLCEs) can change structural colour in response to mechanical stimuli, which can be applied as sensors and optical devices. Thus, the facile fabrication of large-area CLCE films is critical but challenging. Herein, a simple procedure for the preparation of CLCE films by adding a chain-transfer agent is reported. When the CLCE films are fixed between two thermoplastic polyurethane (TPU) films, the obtained TPU/CLCE/TPU sandwich films exhibit mechanochromic behaviours. Due to the decrease of the helical pitch of CLCE film upon stretching, the selective Bragg reflection band shifts to short wavelength. Based on this structural colour change, a flower pattern can emerge or vanish upon stretching. Moreover, a two-dimensional tetragonal grating with elasticity is prepared. The TPU/CLCE/TPU films can be facilely fabricated over large area and used as car films. GRAPHICAL ABSTRACT


Introduction
Liquid crystal elastomers (LCEs) have received extensive attention as an intelligent material, since they feature both the anisotropy of LCs and the elasticity of polymer networks, and exhibit external responses and optical nonlinearities [1][2][3][4].Unlike regular LCEs, cholesteric LCEs (CLCEs) spontaneously form a supramolecular helix with a helical pitch (P) in the range of hundreds of nanometres [5][6][7].This leads to selective Bragg reflection of circularly polarised light with the same handedness as the CLC helix [8,9].The selective Bragg reflection wavelength (λ) is often defined by 'λ = nP', where n is the average refraction index of CLCE [10][11][12].CLCE can become coloured without adding dyes, as the P value is comparable with the wavelength of visible light [13][14][15].Mechanical deformation can change the reflected colour, which is attributed to the reduction in P driven by the thinning of the elastomer [16][17][18].The mechanochromic behaviour of CLCE materials endows them appealing for a wide range of applications such as sensors [19,20], anti-counterfeiting [21,22], optical devices [23,24], and actuators [25,26].
In addition, mechanochromic CLCE sensors are visible and autonomous, so they do not require electricity and can be designed to operate in a wide temperature range [27][28][29][30].In a recent development, Lagerwall et al CONTACT Hongkun Li hkli@suda.edu.cn;Yonggang Yang ygyang@suda.edu.cnSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2023.2200266.used thiol-acrylate Michael addition to form an oligomer in the first step [31].The oligomer was cast onto a polydimethylsiloxane (PDMS) substrate.Cholesteric ordering occurred in this oligomer upon slow evaporation of the solvent over 24 h.The CLCE was obtained after a second photopolymerisation.Large-area films could be prepared by anisotropic deswelling, though at the cost of increased preparation time and complexity.Bowman synthesised LC diacrylate oligomers by the thiol-Michael addition reaction [32].Next, the chiral monomer and photoinitiators were incorporated at necessary concentrations.Chiral alignment was transferred to the LC oligomers to create a helicoidal superstructure that reflects visible light.An independent CLCE was prepared after secondary photopolymerisation.The method of manual shearing somewhat limited the mechanical properties and the maximum thickness of the film.
In this work, a one-step approach is presented to produce large-area CLCE films with uniform colours, which is reproducible and easily scalable.This approach is based on the property of chain transfer agent which can effectively reduce the molecular weights and crosslinking degrees of polymers.The resultant CLCE films can be applied as car films and in wearable devices.This method shown here will have a significant impact on the commercial application of CLCE films.
Since the haze of CLC film increased with increasing the weight ratios of 2-EHA and MPD, they could not be added too much.Finally, we thought that the mass ratio of MPD and 2-EHA was 8.8/4.4 was more suitable.Herein, a CLC mixture, C6M/RM105/2-EHA/ With decreasing the temperatures, some defects vanished and some merged together to form larger defects [43,44].To obtain high quality CLCE films with low haze, the polymerisation temperature was kept at 25°C.The preparation procedure for CLCE film was briefly illustrated in Figure 2. The CLC mixture was dissolved in a mixture of organic solvents.Then, the solution was coated on the rubbed polyethylene terephthalate (PET) film surface using a Meyer rod.After removing solvents at a high temperature, photopolymerisation was carried out under a highpressure mercury lamp (1000 W) at 25°C for 5 s.A red CLCE film was obtained on the surface of a rubbed PET film.
Since the Diffuse reflectance circular dichroism (DRCD) spectra signal at 645 nm is positive (Figure S3, ESI †), the film should show a right-handed helical supramolecular structure [45][46][47].The cross-sectional Field-emission scanning electron microscopy (FESEM) image indicates a helical pitch of 430 nm (Figure S4, ESI †).It was reported that the average refraction index of CLCE was about 1.50 [48].Therefore, the HTP value of CA-iso should be about 62 μm -1 , which was calculated from the equation of β = (cP) -1 .With increasing the concentration of CA-iso from 3.4 to 5.2 wt%, the colour of CLCE film changed from red to purple in Figure 3(a).The selective Bragg reflection bands shifted from 645 to 410 nm (Figure 3(b)).It was found that the colour of a CLCE film changed from orange to green upon stretching (Video S1, ESI †).However, due to the low elasticity of PET, the colour and shape of the CLCE film could not be restored after releasing (Figure S5, ESI †).
As is known, TPU is a polymer with high transparency, elasticity and self-healing properties [49,50].An orange TPU/CLCE/TPU sandwich film was prepared by transferring the substrate from PET to TPU film according to the procedure shown in Figure 2. The CLCE film was prepared at the CA-iso weight ratio of 3.7 wt%.TPU precursors were covered on the surface of CLCE film and heat-cured to form a TPU film.Here, the precursors used in TPU were 4,4′-diphenylmethane diisocyanate (MDI) and polyhexamethylene carbonate diol mixed in a 10:1 mass ratio.After peeling off the TPU/CLCE film from the PET film, another 100-μm TPU film was prepared on the other side of CLCE film.Then, an elastomeric film with a TPU/CLCE/TPU sandwich structure was obtained.The effect of TPU film thickness on the mechanical discoloration of CLCE was negligible.However, TPU/CLCE films were more susceptible to tensile fracture than TPU/CLCE/TPU films.In this regard, we speculated that it was caused by the fact that the CLCE film loaded on a single layer of TPU film was more prone to peel than the CLCE film loaded between two layers of TPU film.
The colour of the sandwich film changed from orange to green with 32% tensile strain in Figure 4(a).After stress relief, the film immediately returned to its original state, revealing a reversible colour change (Video S2, ESI †).The selective Bragg reflection band shifted from 606 to 566 nm upon stretching (Figure S6, ESI †).The helical pitch was reduced from 400 to 366 nm (Figure 4(b)).Therefore, the average refraction index of the film was about 1.53.It was assumed that the volume remained unchanged and the change in width was negligible during the stretching process [51][52][53][54].When the film was stretched by 32%, the thickness of the film was reduced by 24%.Therefore, the helical pitch should be shortened by 24%.However, in fact, the helical pitch change rate was only ΔP/P = 8.5%.The reasonable justification for this phenomenon was that the helical structure experienced a combination of elongation along with the stretch orientation and shrinkage perpendicular to the stretch direction upon uniaxial stretching [48,55].Therefore, the final change in helical pitch was shortening.
As shown in Figure 5(a), two CLCE films with a flower sign were prepared at the CA-iso weight ratio of 5.4 and 2.8 wt%, respectively.The background was photopolymerised at the isotropic state.Then, patterned films with a TPU/CLCE/TPU sandwich structure were  prepared by fixing the CLCE film between two TPU films (Figure S7, ESI †).For the film with a purple flower pattern, the image disappeared upon 28% stretching.Moreover, the background kept transparent without any colours.The UV-vis spectra indicated that the Bragg reflection band of the pattern shifted from 390 to 356 nm (Figure 6(a)).
When the strain was released, the flower pattern went through a red shift reversibly, during which the pattern became visible again (Video S3, ESI †).For the flower patterned TPU/CLCE/TPU film prepared by adding 2.8 wt% of CA-iso, the pattern was invisible at the initial state, which was visible upon 35% stretching.When the strain was released, the pattern went through a red shift   These films could be applied for the information storage and display in wearable devices.As shown in Figure 5(b), a patterned TPU/CLCE/TPU film with 2.8 wt% of CA-iso was attached to a finger using tape.In the straight state, the Bragg reflection band of the pattern is 731 nm, which made it invisible.Upon bending of the finger, the Bragg reflection band of the pattern blue shifted.The red flower pattern thus gradually appeared (Video S5, ESI †).Therefore, this film can monitor the bending state of the finger.Namely, this flexible and elastic film was expected to be applied as a wearable information sensor.Moreover, these two patterned films could be stuck together by heating to form a multilayered film.Initially, only the purple pattern was visible.With increasing the tensile strain, the purple colour faded out and the red colour appeared gradually.When the strain reached 32%, only the red pattern was identified (Figure 5(c)).After relaxing, the purple pattern was visible again.Namely, a reversible transition from purple to red was achieved (Video S6, ESI †).Moreover, the elastic films could be fabricated in large area.For instance, a car film was prepared based on TPU and CLCE films (Figure 7).The colour can change by changing the view angle (Video S7, ESI †).
An elastic grating was prepared using a mask with a 2D tetragonal structure (Figure S8, ESI †).The POM images indicated that the TPU/CLCE/TPU elastic grating was obviously deformed upon 30% tensile strain (Figure 8(a)).The interference fringes generated by the elastic grating and a mobile phone screen also deform accordingly (Figure 8(b) and Video S8, ESI †).After relaxing, the grating was restored to its original state.Moreover, a high resolution CLCE grating was prepared  using a mask with 2D hexagonal arranged 10-μm pores (Figure S9, ESI †).These elastic gratings were promising for application in mechanically tunable optical devices.

Conclusions
In summary, a simple method to prepare large-area CLCE was presented.For reducing the T g values of polymers, both a chain-transfer agent and an acrylate with a branched alkyl chain were added into the CLC mixtures.Based on the elasticity of TPU, TPU/CLCE/ TPU sandwich films were prepared, which exhibit a reversible mechanochromic property.Large-area films could be used as car films.Films with colourful patterns were prepared using a mask and two-step crosslinking steps at different temperatures.They were expected to be applied as a wearable information sensor.Meanwhile, the elastic gratings with reversible deformation were prepared, which were promising for application in mechanically tunable optical devices.

Materials
C6M and RM105 were given by Wuxi Wanli Adhesive Materials Co., Ltd.(China).2-EHA, MPD, and 907 were purchased from Adamas Chemical Co., Ltd.(Shanghai, China).Ethyl acetate (EA) and cyclohexanone were obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd.(China).TPU precursors and rubbed PET films were given by Wuxi Wanli Adhesive Materials Co., Ltd.The surface of PET film is treated with corona, which changes the surface tension and increases the adhesion performance of PET film.The mask with a flower pattern was prepared by printing using a laser printer on a PET film.The mask with a twodimensional (2D) tetragonal structure and that with a 2D hexagonal structure were bought from Suzhou Research Materials Microtech Co., Ltd.(China).

Characterisation
1 H NMR spectra were recorded on an INOVA-400 spectrometer in CDCl 3 using tetramethylsilane (TMS) as an internal standard.High-resolution mass spectra (HRMS) were measured with an Ultraflextreme MALDI TOF/TOF spectroscope (Bruker, USA).Elemental analysis was measured on a Vario EL III instrument.FT-IR spectra were performed on a VRETEX 70 spectrometer at 4.0 cm −1 resolution by an averaging over 16 scans.Field-emission scanning electron microscopy images were obtained using a Hitachi S-4800 operating (Ibaraki prefecture, Japan) at 10.0 kV.The samples for taking cross-sectional FESEM images were prepared in liquid nitrogen.Diffuse reflectance circular dichroism spectra were measured by using a JASCO 815 spectrometer (Tokyo, Japan).UV-vis reflectance spectra were measured by UV-VIS-NIR spectrophotometer (UV3600).The polarising optical microscopy (POM) images of the target compounds were taken using a Leica Microsystems CMS GmbH fitted with a Linkam LTS420 hot stage.Patterned CLCE films were prepared using UV LED series equipment (UVSF81T, 400 mW) produced by FUTANSI Electronic Technology Co., Ltd.(Shanghai, China).Photopolymerization was carried out under a highpressure Hg lamp (MINHIO 4012-20, 1000 W, input power) produced by MINHIO Intelligent Equipment Co., Ltd.(Shenzhen, China).UV-vis transmission spectra were measured by UV-VIS-NIR spectrophotometer (UV-1900i).

Preparation of the CLCE films
A typical preparation procedure was shown as following.A C6M/RM105/2-EHA/MPD/907/CA-iso mixture was prepared at the weight ratio of 62.2/17.7/8.8/4.4/3.5/3.4, which was dissolved in a cyclohexanone/ethyl acetate (v/v, 4:1) mixture at a solid content of 20 wt%.The solution was coated on the surface of a PET film using a 20-μm Meyer rod.After the solvents were evaporated, a red CLC film was obtained.The polymeric CLC film was prepared by photopolymerisation using a highpressure mercury lamp (1000 W) at 25°C for 5 s.The other films were prepared by gradually increasing the CA-iso weight ratio from 3.4 to 5.2 wt%.

Preparation of the orange TPU/CLCE/TPU films
An orange CLCE film was prepared at 3.7 wt% of CAiso according to the procedure shown above.TPU precursors were covered on the surface of CLCE film and heat-cured to form a 100-μm TPU film.After peeling off the TPU/CLCE film from the PET film, another 100-μm TPU film was prepared on the other side of CLCE film.An elastomeric film with a TPU/CLCE/TPU sandwich structure was obtained.

Preparation of the flower patterned TPU/CLCE/ TPU films
A typical preparation procedure was shown as following.Firstly, a C6M/RM105/2-EHA/MPD/907/CA-iso mixture prepared at the weight ratio of 60.9/17.3/8.6/4.3/3.5/5.4 was coated on the surface of a PET film as above.The CLC coating was locally cured through a mask with a flower image at 25°C for 15 s with a 365-nm UV light intensity of 400 mW cm −2 .After removing the mask, the coating was heated up to 110°C which was higher than the clearing point and cured with a 365-nm UV light with an intensity of 300 mW cm −2 for 60 s.Then, a purple flower patterned CLCE film was obtained.Finally, a purple flower patterned film with a TPU/CLCE/TPU sandwich structure was prepared according to the procedure shown above.Another flower patterned TPU/ CLCE/TPU film was prepared at the C6M/RM105/ 2-EHA/MPD/907/CA-iso weight ratio of 62.2/18.0/9.0/4.5/3.5/2.8.

Preparation of the TPU/CLCE/TPU grating with a two-dimensional (2D) tetragonal structure
A C6M/RM105/2-EHA/MPD/907/CA-iso mixture was prepared at the weight ratio of 62.0/17.7/8.9/4.4/3.5/3.5.The mixture was coated on the surface of a PET film as above.The CLC coating was locally cured through a photomask with a 2D tetragonal structure at 25°C for 5 s with a 365-nm UV light intensity of 400 mW cm −2 After removing the mask, the uncured area was photopolymerised at 110°C using a 365-nm UV light of intensity of 300 mW cm −2 for 60 s.Then, a CLCE elastic grating was prepared.After being fixed between two TPU films, a two-dimensional elastic grating with a TPU/ CLCE/TPU sandwich structure was obtained.

6 Figure 1 .
Figure 1.Molecular structures of the compounds in LC mixtures.

Figure 3 .
Figure 3. (Colour online) (a) Photographs and (b) UV-vis spectra of the CLCE films prepared at different weight ratios of CA-iso.

Figure 5 .
Figure 5. (Colour online) (a) Schematic representation of the preparation of patterned CLCE film, (b) colour change of the flexible sensor caused by a finger bending motion and (c) the photographs of a flower patterned films in the initial and 32% stretched states.

Figure 6 .
Figure 6.(Colour online) Transmittance spectra of (a) the flower patterned film with a purple colour in the initial and 28% stretched states and (b) that with near-infrared reflection in the initial and 35% stretched states.Transmittance spectra were tested at an incidence angle of 0° and under non-polarised light conditions.
and became invisible again (Video S4, ESI †).The Bragg reflection band of the pattern blue shifted from 731 to 663 nm at 35% tensile strain (Figure6(b)).

Figure 7 .
Figure 7. (Colour online) Photograph of a car film prepared based on TPU and CLCE films.

Figure 8 .
Figure 8. (Colour online) (a) POM images of the TPU/CLCE/TPU elastic grating in initial, 30% strain, and relaxation states and (b) the corresponding interference images in front of a mobile phone screen with a white background.