Colourful cholesteric liquid crystal polymer network gratings prepared through nanoimprinting

ABSTRACT Both the colourful cholesteric liquid crystal polymer network (CLCN) patterns and the gratings have attracted much attention for their applications as optical materials. Herein, the photochromic cholesteric liquid crystal (CLC) mixtures were prepared using a photoisomerizable chiral dopant. The structural colour of the CLC mixtures was tunable by changing the chiral dopant concentration and the intensity of the 365-nm irradiation light. The CLCN gratings with a structural colour were prepared using the UV nanoimprinting lithography method. The structural colours of the CLCN gratings originate from both the cholesteric and the grating structures. Moreover, a patterned CLCN grating was prepared using a photochromic CLC mixture, which could be applied for decoration and anti-counterfeiting. GRAPHICAL ABSTRACT


Introduction
A grating is a periodic structure which can change the propagation and splitting spectrum of light [1,2].The ordered structure can be applied in various fields such as data storage, display devices and detector systems [3][4][5].The natural diffraction gratings are present in many insects, marine organisms and plants, which give them brilliant colours and the anti-counterfeiting ability [6][7][8][9][10].Cholesteric liquid crystals (CLCs) can also exhibit the structural colour which originates from the one-dimensional supramolecular helical structure [11,12].For fixing the structural colour, the CLC polymer network (CLCN) films are prepared, which have been applied for decoration and anti-counterfeiting [13,14].Therefore, it is attractive to prepare the CLCN gratings with both the selective Bragg reflection and light diffraction.
The liquid crystal (LC) gratings not only have lower power consumption, longer life and lower cost, but also are optimised in terms of deflection range and programmability.They can be used in manufacture of three-dimensional displays [15], optical communication [16], and beam steering [17,18].The polymer-stabilised LC (PSLC) combines the optical properties of LCs and the stability of polymers [19][20][21][22][23]. Therefore, PSLC gratings have the advantages of mechanical and thermal stabilities.There have been many available fabricating techniques for such LC gratings, including the holographic photolithography, electron beam lithography, solvent assisted microcontact moulding and nanoimprint lithography (NIL) [24][25][26][27][28][29][30][31][32][33].Compared to other methods, the NIL has received interest for low cost and high resolution.And the UV-NIL is an ideal choice with the advantages of large-scale preparation and the intrinsic material characteristic of high thermal endurance [34][35][36][37][38].During the last decades, varieties of CLCN gratings have been prepared through different methods [1].Herein, a patterned CLCN grating was prepared using a photochromic CLC mixture through UV-NIL approach, which was potentially applied for anti-counterfeiting.

Results and discussion
The molecular structures of the compounds are shown in Figure 1.LC242 is an enantiotropic nematic LC with a phase transition sequence of Cr 70°C and 120°C I, which has been used for the preparation of CLCN films [39].C3 is a monotropic nematic LC with a phase transition sequence of Cr 69.9°CI 67.7°CN 46.0°CRecr [33].The lateral methyl substituents effectively decrease the clearing point.CA-1 is a photoisomerizable chiral additive [40,41].The E-Z isomerisation can carried out under the irradiation of a UV light.Then, the helical twisting power (HTP) value decreases with increasing the UV light intensity.Based on this, colourful CLCN patterns were prepared using CA-1.The radical photoinitiator 907 is applied to initiate the photopolymerisation [42].The structurally coloured CLCN films have been prepared using the mixtures of them [33].
The structurally coloured CLCN gratings shown in Figure 2 were prepared through the UV-NIL approach  (Scheme 1).A LC242/C3/CA-1/907 mixture is coated on the surface of a polyethylene terephthalate (PET) film.Then, a polydimethylsiloxane (PDMS) template is placed on the surface of the mixture, which was prepared by transferring the structure of a nickel template.The CLC mixture will fill into the dents of the templates.After photopolymerisation under the parallel 365-nm UV light (400 mW/cm 2 ), the grating structure is fixed.The CLCN grating was obtained by removing the template.For the LC242/C3/CA-1/907 mixtures, the LC242/C3 weight ratios were kept at 1.5/1.0.With increasing the concentration of CA-1 from 4.19 to 6.79 wt%, the colour of the CLCN grating changed from red to purple (Figure 2(a)) and the λ max (the wavelength of the Bragg reflection band at the maximum) value changed from 650 to 420 nm (Figure 2(b)).Moreover, the reflectance of the CLCN gratings increased gradually.The baseline of reflectance was only around 70%, which was caused by the light scattering of the grating structure.To reveal the handedness of the CLCN gratings, the diffuse reflection circular dichroism (DRCD) spectra of the CLCN gratings were taken.For the CLCN grating prepared at the CA-1 concentrations of 4.19 and 6.36 wt%, positive DRCD signals were identified at 624 and 461 nm, respectively (Figure S1, Supporting Information).Therefore, the CLCN gratings should possess a right-handed supramolecular helical structure.
The polarised optical microscopy (POM) image of the PDMS template is shown in Figure 3(a).A twodimensional (2D) tetragonal structure was identified.The POM and field-emission scanning electron microscopy (FESEM) images of the CLCN grating prepared using 6.79 wt% of CA-1 are shown in Figure 3(b-d).The tetragonal structure was successfully transferred to the CLCN grating.The oily streak defects were also identified in the POM image (Figure 3(b)).The square rods with a 2-µm height were identified in the FESEM images (Figure 3(c,d)).A cholesteric structure was clearly identified in the cross-sectional FESEM image.The helical axis was perpendicular to the film surface.The angular-dependence of the Bragg reflection band was studied using the CLCN grating prepared using 5.50 wt% of CA-1 (Figure S2, Supporting Information).With increasing the angle between the incident light and the normal line of the grating from 0° to 45°, the λ max value of the Bragg reflection band underwent blue shift from 500 to 460 nm.Therefore, the colour of the grating originates from the cholesteric structure.
Based on this UV-NIL approach, another CLCN grating with a two-dimensional tetragonal structure was prepared using a CLC mixture with 6.79 wt% of CA-1 and another PDMS template (Figure S3, Supporting Information).The UV-NIL can not only produce the protuberant topography but also the dented one.The oily streak defects were also identified in the POM image (Figure S3(b), Supporting Information).The square dents arranged in a 2D tetragonal structure (Figure S3(c and d), Supporting Information).Therefore, the CLCN films with different grating structures can be prepared on a large-scale by UV-NIL.
Since CA-1 is photoisomerizable, the HTP value decreases with enhancing the irradiation intensity of the 365-nm UV light [40].Colourful CLCN patterns have been prepared using the photochromic CLC mixtures with the addition of CA-1 (Scheme 2).Herein, a photochromic CLC mixture was prepared at the CA-1 concentration of 6.79 wt%.With extending the irradiation time of the 365-nm UV light (80 mW/cm 2 ) from 0 to 15 s, the colour changed from purple to red and the λ max value of the Bragg reflection band shifted from 441 to 695 nm (Figure S4, Supporting Information).Due to polymerisation shrinkage, the Bragg reflection shifted from 441 to 420 nm (Figure 2).For a better understanding the preparation process, FT-IR spectra were taken at different preparation steps (Figure S5, Supporting Information).For the CLC mixture, the absorption bands of the acrylate group were identified at 1410 and 810 cm −1 .After the irradiation of the 365-nm UV light (80 mW/cm 2 ), these two bands did not change, indicating that polymerisation was not carried out.After the irradiation of the 365-nm UV light (400 mW/cm 2 ), these two absorption bands were weakened, indicating the polymerisation of the acrylate groups.
A colourful pattern was prepared using the CLC mixture of above and a photomask through this twostep approach (Scheme 2).Due to oxygen inhibition, only the photoisomerisation of CA-1 was carried out under the irradiation of the parallel 365-nm UV light (80 mW/cm 2 ).The structural colour of the uncovered area changed from purple to green.After removing the photomask, the PDMS template was placed on the surface of the CLC mixture.Without the oxygen inhibition, the structural colours were fixed under the irradiation of the 365-nm UV light (400 mW/cm 2 ).The patterned CLCN grating was obtained by peeling off the PDMS template (Figure 4(c)).The 2-µm-height square rods and the cholesteric structure of the CLCN film were identified in the cross-sectional FESEM images (Figure S6, Supporting Information).Under a point light source, the reflection diffraction pattern showed a tetragonal structure (Figure 4(d)).Therefore, the CLCN patterns with a grating structure are suitably applied for decoration and anti-counterfeiting.

Conclusions
The CLCN films with a 2D tetragonal grating structure were prepared using photochromic CLC mixtures through a UV-NIL approach.Since the selective Bragg reflection band of a CLC mixture was tunable by changing the 365-nm UV light intensity, a CLCN pattern with a 2D tetragonal grating structure was prepared.The structural colours originated from both the cholesteric and the grating structures.Therefore, the CLCN pattern shown here is suitably applied for decoration and anti-counterfeiting.

Materials
LC242 was given by Soochiral Chemical Sci.& Techn.Co., Ltd (Suzhou, China).CA-1 and C3 were synthesised according to the literature [33].The photoinitiator 907 was purchased from Aladdin Chemical Co., Ltd.(Shanghai, China).The rubbing-oriented PET film was given by Wuxi Wanli Adhesive Material Co., Ltd.The nickel templates were given by SVG Optronics Co. Ltd.The Sylgard 184 silicone elastomer base and the Sylgard 184 silicone elastomer curing agent were bought from Dow Chemical (Zhangjiagang) Co. Ltd.

Characterizations
The POM images were taken using a CPV-900C polarisation microscope fitted with a Linkam LTS420 hot stage.The FESEM images were obtained using a Hitachi S-4800 operating (Ibaraki prefecture, Japan) at 5.0 kV.The UV-vis spectra were obtained using a UV-vis spectrophotometer (UV-1900i).The DRCD spectra were measured using the JASCO 815 spectrometer (Tokyo, Japan).The UV LED series equipment (UVSF81T, 400 mW/cm 2 , output power) was produced by FUTANSI Electronic Technology Co., Ltd (Shanghai, China).The UV LED parallel light source is equipped with double aspherical quartz lenses to produce parallel light with a parallel half angle less than 2°.

Preparation of the PDMS templates
A typical preparation procedure was shown as follows.A mixture of the Sylgard 184 silicone elastomer base and the Sylgard 184 silicone elastomer curing agent was prepared at the weight ratio of 10/1.The mixture was coated on the surface of a nickel template at room temperature.Then, the trapped air bubbles were removed under vacuum for 1.0 h.After curing the mixture at 120°C for 4.0 h, the PDMS replica was obtained by being peeled off from the nickel template.

Preparation of the CLCN gratings
A typical preparation procedure was shown as follows.A LC242/C3/CA-1/907 mixture was prepared at the weight ratio of 54.83/36.88/6.79/1.50, which was dissolved in a cyclohexanone/ethyl acetate (v/v, 8/2) mixture to form a solution with a solid content of 20 wt%.Then, the solution was coated on the surface of a rubbing-oriented PET film using a 20-µm Mayer bar.After removing the solvents at 120°C, the PDMS template was placed on the surface of the CLC mixture at 60°C.The CLC mixture was polymerised under the irradiation of the 365-nm LED lamp (400 mW/cm 2 ) for 10 s.The CLCN grating was obtained by peeling off the PDMS template.The other gratings were prepared by keeping the LC242/C3 weight ratios at 1.5/1.0,changing the concentration of CA-1 and the PDMS template.

Preparation of the patterned CLCN grating
A LC242/C3/CA-1/907 (54.83/36.88/6.79/1.50)mixture was coated on the surface of a rubbing-oriented PET film as described above.Then, the photoisomerisation was carried out through a photomask under the irradiation of the 365-nm UV light (80 mW/cm 2 ) for 6.0 s and at 60°C.After removing the photomask, the PDMS template was placed on the surface of the CLC mixture.The CLC mixture was polymerised under the irradiation of the 365-nm LED lamp (400 mW/cm 2 ) for 6.0 s.The patterned CLCN grating was obtained by peeling off the PDMS template.

Figure 1 .
Figure 1.Molecular structures of the compounds in the CLC mixtures.

Figure 2 .
Figure 2. (Colour online) (a) Photographs and (b) UV-vis spectra of the CLCN gratings prepared at different CA-1 concentrations.

Scheme 1 .
Scheme 1. (Colour online) Schematic representation of the preparation of a CLCN grating through the UV-NIL approach.

Figure 3 .
Figure 3. (Colour online) (a) POM image of the PDMS template and (b) POM and (c and d) FESEM images of the CLCN grating.

Figure 4 .
Figure 4. (Colour online) (a) Transmission and (b) reflection diffraction patterns of the CLCN grating prepared using 6.79 wt% of CA-1.(c) Photograph and (d) reflection diffraction pattern of the patterned CLCN grating.