Control the structure of the polyacrylate/epoxy resin film through photopolymerisation

ABSTRACT Double-layered polymer-stabilised cholesteric liquid crystal (PSCLC) films constructed from epoxy resin and polyacrylate were prepared based on photopolymerisation-induced phase separation. Moreover, a PSCLC film with an interpenetrating network was prepared through a one-step photopolymerisation approach. A colourful pattern was prepared based on these two approaches, which was suitably applied for decoration. GRAPHICAL ABSTRACT


Introduction
Structural colours originated hundreds of millions of years ago during the Great Eruption and emerged to meet the optical and visual needs of organisms as the diversity of species progressed.Structural colours are produced by the interaction of light and nanostructures with periodicities.It has been widely observed and studied in animals and plants [1][2][3].For example, the skin of a chameleon consists of two superimposed layers of thick iris cells that can change skin colour by adjusting the average distance between nanocrystals in the skin [4].Some species of squid can also change their reflective colour by adjusting the periodicity of multiple layers of reflectors in special cells to produce camouflage effects [5,6].
During the last decades, much effort has been devoted to prepare the polymer-stabilised cholestericphase liquid crystal (PSCLC) films .D. J. Broer et al. first reported the use of acrylate liquid crystals (LCs) for the preparation of oriented polymer films by in-situ photopolymerisation in 1989 [11].Since then, a large number of PSCLC films have been prepared through this approach [12][13][14][15][16].The PSCLC films with a broad reflection band aroused much attention, which could be used as mirror, circular polariser and for energy-saving [17][18][19].They can be prepared through the light-induced diffusion of molecules [20], by the superimposition of cholesteric liquid crystal films with different reflective bands [21,22] and based on the pretransitional effects [23,24].The diffusion of chiral monomers in the system can also result in a broad reflection band [25].
The PSCLC films with dual-reflectance bands have also attracted much attention.They can be prepared simply by stacking cholesteric LC (CLC) coatings or by using thermochromic and photochromic CLCs [26][27][28].Moreover, the wash and refill methods were also developed [29,30].Polymerisation-induced phase separation (PIPS) is the process of forming phase separation during the polymerisation process [33,34].Polymer-dispersed LC films have been prepared through PIPS [35].Recently, the double-layered and interpenetrating PSCLC films constructed by polyacrylate and oxetane resin were prepared through the PIPS and simultaneous polymerisation approaches, respectively, which can be applied for the preparation of colourful patterns [31].Herein, a two-step photopolymerisation approach for the preparation of the double-layered PSCLC films constructed from polyacrylate and epoxy resin layers was developed through PIPS.The PSCLC films exhibited double reflection bands which originated from different layers.Moreover, the structurally coloured PSCLC films with an interpenetrating network were also prepared through a one-step photopolymerisation approach.The colourful patterns with composite structural colours are suitable for decoration and anti-counterfeiting.

Preparation of double-layered PSCLC films through the two-step polymerisation approach
A typical preparation procedure is shown as following.A mixture of LC242/E11M/CA-Epoxy/CA-Acrylate /1176/ITX/907 was prepared at the weight ratio of 45/ 45/0.9/3.6/2.7/1/1.8,which was dissolved in a cyclopentanone/cyclohexanone (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 30-μm Mayer bar.After removing the solvents, the CLC film was photopolymerised at 105°C under irradiation of the highpressure Hg lamp for 5.0 s.Finally, the double-layered PSCLC film was obtained by photopolymerisation under the irradiation of the 365-nm LED lamp (400 mW/cm 2 ) for 90 s at 110°C.The other double-layered PSCLC films were prepared by changing the weight ratios of CA-Epoxy/CA-Acrylate and LC242/E11M (Table S1).

Preparation of the PSCLC films through the one-step polymerisation approach
A typical preparation procedure is shown as following.The CLC coated film was prepared as above.Then, the PSCLC film was prepared under the irradiation of the 365-nm LED lamp for 90 s at 100°C.

Preparation of the polymer-stabilised cholesteric grating (PSCG)
The CLC coated film was prepared as above.Then, the photopolymerisation was carried out under the irradiation of the 365-nm LED (200 mW/cm 2 ) lamp at 100°C for 5.0 s through the mask with a 1D structure.After the mask was removed, the film was further irradiated under the high-pressure Hg Lamp for 5.0 s at 110°C and the 365-nm LED lamp (400 mW/cm 2 ) for 90 s at 110°C, subsequently.

Preparation of the PSCLC film with a flower pattern
A LC242/E11M/CA-Epoxy/CA-Acrylate/1176/ITX/ 907 (w/w/w/w/w/w/w, 45/45/1.3/3.2/2.7/1/1.8)mixture was coated on the surface of a PET film as above.The photopolymerisation was carried out under the irradiation of the 365-nm LED lamp (400 mW/cm 2 ) for 60 s at 100°C through a photomask.Then, the photomask was removed.Finally, the film was irradiated under the high-pressure Hg lamp for 5.0 s and the 365-nm LED (400 mW/cm 2 ) lamp for 90 s at 110°C, subsequently.

Results and discussion
LC242 shows an enantiotropic nematic phase with the phase transition sequence of Cr 70°C N 120°C I (Figure 1) [36].E11M shows an enantiotropic nematic phase with the phase transition sequence of Cr1 67.5°C Cr2 75.3°-CN 147.9°CI 146.0°CN 49.2°C Recr [32].The chiral dopants, CA-Acrylate and CA-Epoxy, were synthesised according to the literatures [37][38][39].A mixture of the photoinitiators 907, 1176 and ITX was used for photopolymerisation. 1176 can initiate the cationic polymerisation of epoxies, and ITX can accelerate the polymerisation rate of epoxies [40].The structural coloured epoxy resin films have been prepared using a mixture of E11M, 1176, ITX and a chiral dopant [32] 907 can prohibit the oxygen inhibition and initiate the radical polymerisation of acrylates.The structural coloured polyacrylate films have been prepared using LC242, 907 and a chiral dopant [10].
Although 907 and 1176 can initiate the photopolymerisation of acrylate and epoxy monomers, respectively, the photopolymerisation ability of LC242 and CA-Acrylate under a mixture of 1176, ITX and 907 is unclear.Herein, a LC242/CA-Acrylate/1176/ITX/907 mixture was prepared at a weight ratio of 85.5/4.3/5.1/1.7/3.4.It could polymerise under both a high-pressure Hg (1000 W) lamp for 5.0 s and 365-nm LED (400 mW/cm 2 ) lamp for 90 s (Figure S1, Supporting Information).For the polyacrylate film prepared under the high-pressure Hg lamp, the wavelength of the reflection band was identified at 525 nm which was shorter than that of the corresponding CLC mixture.This phenomenon should be driven by the polymerisation shrinkage.For the polyacrylate film prepared under the 365-nm LED lamp, several reflection bands were identified.This wide distribution of helical pitch was proposed to be driven by the oxygen inhibition.To understand the photopolymerisation ability of E11M and CA-Epoxy, an E11M/ CA-Epoxy/1176/ITX/907 mixture was prepared at the weight ratio of 85.5/4.3/5.1/1.7/3.4.E11M could polymerise under the 365-nm LED lamp (Figure S2, Supporting Information).However, it failed to be polymerised under the high-pressure Hg lamp within 5.0 s.This phenomenon was proposed to be driven by the acid-base interaction between 907 and HSbF 6 .The difference between the polymerisation ability of the acrylate and the epoxy monomers under the Hg lamp is proposed to be driven by the fact that the Hg lamp emits both 365-nm and blue light.The blue light can help the polymerisation of the acrylate monomers [31].
For the preparation of a polyacrylate/epoxy resin composite film, a mixture of LC242/E11M/CA-Epoxy/CA-Acrylate/1176/ITX/907 was prepared at the weight ratio of 45/45/0.9/3.6/2.7/1/1.8.An enantiotropic cholesteric phase was identified at 80 � C (Figure S3, Supporting Information).The PSCLC film was first prepared through a two-step approach under the irradiation of the highpressure Hg lamp and the 365-nm LED lamp, subsequently.After being coated on the surface of a rubbingoriented PET film, the CLC mixture exhibited a red structural colour at 100 � C. Single Bragg reflection band was identified at 599 nm (Figure 2(a)).After the CLC mixture was irradiated at 105 � C under the high-pressure Hg lamp for 5.0 s, a purple film with a viscous surface was obtained.Due to the polymerisation shrinkage, the epoxy monomers move to the surface.Two stronger reflection bands were identified at 525 and 839 nm in the UV-vis spectrum.The purple colour was the composite colour of blue and red.Therefore, the PIPS occurred due to the polymerisation of acrylates.After the film was further irradiated at 110° C under the 365-nm LED lamp, the film was solidified, indicating the polymerisation of the epoxy monomers.Two stronger reflection bands were identified at 507 nm and 729 nm (Figure 2(a)).The blue-shifts of the reflection bands should be driven by the polymerisation shrinkage.A double-layered structure was identified in the crosssectional field-emission scanning electron microscopy (FESEM) image (Figure 2(b)).Therefore, the polyacrylate and epoxy resin layers existed at the bottom and top layers, respectively.The helical pitches of the polyacrylate and epoxy resin layers were 310 nm and 503 nm, respectively.
The polymerisations of the monomers were studied by taking FT-IR spectra during the film preparation process (Figure S4, Supporting Information).For the CLC mixture, the absorption bands of the acrylate groups were identified at 1410 and 810 cm −1 , and that of the epoxy groups was identified at 856 cm −1 .After irradiation under the high-pressure Hg lamp, the intensity of the absorption band at 1410 cm −1 decreased sharply, and the absorption band at 810 cm −1 disappeared, indicating the polymerisation of the acrylate monomers.However, the epoxy groups (856 cm −1 ) were kept.After irradiation under the 365-nm LED lamp, the intensity of the absorption band decreased sharply, indicating the polymerisation of the epoxy monomers.Therefore, the acrylate and epoxy monomers can be polymerised step by step.
The PSCLC film can also be prepared through a onestep approach only under the irradiation of the 365-nm LED lamp at 100°C.The PSCLC film prepared using the above CLC mixture exhibited an orange structural colour.Only one Bragg reflection band was identified at 547 nm (Figure 2(a)).The helical pitch was 353 nm (Figure 2(c)).The FT-IR spectrum indicated that the acrylate and epoxy monomers polymerised, simultaneously (Figure S4, Supporting Information).Therefore, an interpenetrating network was formed [36].The results shown here indicate that the structural colour of the PSCLC film was tunable by changing the polymerisation conditions.The formation of the double-layered and interpenetrating PSCLC films is illustrated in Figure 3.
For a better understanding the structures of these two kinds of films, an epoxy resin film was prepared at the E11M/CA-Epoxy/1176/ITX/907 weight ratio of 85.5/ 4.3/5.1/1.7/3.4 under the irradiation of the LED lamp, and a polyacrylate film was prepared at the LC242/CA-Acrylate/1176/ITX/907 weight ratio of 85.5/4.3/5.1/1.7/3.4 under the irradiation of the high-pressure Hg lamp.For the epoxy resin film, the symmetrical and antisymmetrical stretching vibration bands of the alkyl chains were identified at about 2925 and 2853 cm −1 , respectively (Figure S5(a), Supporting Information).The intensities of these two bands were stronger than those of the vibration bands of the polyacrylate, respectively.For the hybrid film with double Bragg reflection bands prepared at the LC242/E11M/CA-Epoxy/CA-Acrylate/1176/ITX/907 weight ratio of 45/45/0.9/3.6/2.7/1/1.8(Figure 2), before taking the reflection FT-IR spectra, it was peeled off from the PET substrate.The reflection FT-IR spectra of the top and bottom layers were similar to those of epoxy resin and polyacrylate films, respectively (Figure S5(b), Supporting Information).Therefore, the PIPS should be carried out during the polymerisation of acrylates.However, the energy-dispersive spectrometer (EDS) image of this hybrid film did not show obvious diffusion of 1176 (Figure S6, Supporting Information).For a hybrid film with a single Bragg reflection band (Figure 2), due to high adhesion force, it failed to be peeled off from the PET substrate.Therefore, only the reflection FT-IR spectrum of the film surface was taken (Figure S5(b), Supporting Information).The intensities of the vibration bands of the alkyl chains were stronger than those of the absorption bands of the polyacrylate, respectively.However, they were weaker than those of the absorption bands of the epoxy resin, respectively.Therefore, the polymerisations of the epoxy and acrylate monomers should be carried out, simultaneously.
Other double-layered films were also prepared by keeping the LC242/E11M weight ratio at 1/1 and changing the CA-Epoxy/CA-Acrylate weight ratio from 0/10 to 10/0 (Figures 4 and S7, Supporting Information).The weight ratios of the reaction mixtures are shown in Table S1 (Supporting Information).When only CA-Acrylate was added into the LC mixture, two selective Bragg reflection bands were identified at 499 and 968 nm, respectively.The helical pitches of the top and bottom layers are 628 and 330 nm, respectively (Figure S8, Supporting Information).Therefore, lots of CA-Acrylate molecules were fixed in the polyacrylate layer and only a little amount of CA-Acrylate molecules were left at the top layer to form a longer helical pitch.The diffuse reflectance circular dichroism (DRCD) spectra of the PSCLC films were shown in Figure S9 With increasing the CA-Epoxy/CA-Acrylate weight ratio, the Bragg reflection band at longer wavelength shifted to short wavelength sharply.However, that at shorter wavelength only shifted slightly to shorter wavelength.For the films prepared at the CA-Acrylate/CA-Epoxy weight ratios of 8/2 and 10/0, only one Bragg reflection band was identified.Therefore, the Bragg reflection band at shorter wavelength was mainly dominated by the total concentration of CA-Acrylate and CA-Epoxy, and that at longer wavelength was mainly dominated by the concentration of CA-Epoxy.
For controlling the structural colour, the doublelayered PSCLC films were also prepared by changing the total concentration of CA-Epoxy/CA-Acrylate (w/w, 4/6).With decreasing the total concentration, the colour of the film changed from purple to red, and then to colourless (Figure 5(a)).Both of the reflection bands shifted to long wavelength (Figure 5(b)).Moreover, the distance between these two reflection bands increased.Therefore, varieties of complex colours can be obtained.Due to the PIPS, the thickness ratio between the polyacrylate and epoxy resin layers was controllable by changing the LC242/E11M weight ratio.The cross-sectional FESEM image of the PSCLC film prepared at the LC242/E11M/CA-Epoxy/CA-Acrylate/1176/ITX/907 weight ratio of 63/27/0.9/3.6/2.7/1/1.8was shown in Figure S10, Supporting Information.The thickness of the top (epoxy resin) layer decreased apparently.For the preparation of the PSCLC films with a broad Bragg reflection band, the weight ratio of the reaction mixture was tuned.The band width of the PSCLC film prepared at the LC242/ E11M/CA-Epoxy/CA-Acrylate/1176/ITX/907 weight ratio of 36.5/54.8/2.3/0.9/2.7/1/1.8reached about 250 nm (Figure S11, Supporting Information).
Structurally coloured PSCLC films were also prepared through the one-step photopolymerisation approach (Figure 6).The films exhibited only one Bragg reflection band.With decreasing the total concentration of CA-Epoxy/CA-Acrylate (w/w, 4/6) from 5.5 to 3.0 wt%, the Bragg reflection band shifted from 548 to 914 nm.Since the structural colour of the PSCLC film can be tuned by changing the photopolymerisation approach, colourful patterns can be prepared.
A PSCLC grating was prepared using a mask with a periodic size of 50 μm (Figure 7(a)).The thinner and wider stripes were prepared through one-step and twostep photopolymerisation approaches, respectively.The  resolution is high enough for the preparation of colourful patterns [41,42].Two reflection bands were identified at 458 nm and 555 nm which should originate from the double-layered structure (Figure S12(a), Supporting Information).A colourful pattern was prepared using a mask (Figure 7(b)) [43,44].The flower and background were prepared through one-and two-step photopolymerisation approaches, respectively.A single Bragg reflection band was identified at 595 nm at the patterned part which originated from an interpenetrating framework (Figure S12(b), Supporting Information).For the background, two Bragg reflection bands were identified at 536 and 710 nm, which originated from a double-layered structure.Based on these, more colourful patterns were able to be prepared.Although the diffusion of the epoxy monomers occurred during the preparation process, the resolution is still very high.This phenomenon might be driven by the high viscosity of the CLC mixture.Moreover, the reflection FT-IR spectra of the flower and background areas were also taken (Figure S13, Supporting Information).Phase separation should exist at the background area, and an interpenetrating network should be formed at the flower area.

Conclusions
In conclusion, the double-layered and interpenetrating PSCLC films were prepared through a two-step and a one-step photopolymerisation approaches, respectively.The double-layered structure was formed by the PIPS.The selectively Bragg reflection bands were tunable by changing the concentrations of the chiral dopants and the weight ratio of the two liquid crystal monomers.Since the structural colour could be tuned by changing the photopolymerisation approach, and the structural colour of the double-layered film is composite, colourful patterns can be prepared, which are suitable for decoration and anti-counterfeiting.Moreover, since the photopolymerisations can be carried out under air, the PSCLC films can be prepared in large area and low price.

Disclosure statement
No potential conflict of interest was reported by the authors.

Figure 1 .
Figure 1.Molecular structures of the LCs, the chiral dopants and the photoinitiators.

Figure 2 .
Figure 2. (Colour online) (a) UV-vis spectra of the CLC mixture and that after polymerisation under different irradiation conditions, and the cross-sectional FESEM images of the PSCLC films prepared through (b) a two-step and (c) a one-step approaches.

Figure 3 .
Figure 3. (Colour online) Schematic representation of the formation of the double-layered PSCLC film through a two-step polymerisation approach and the single-layered PSCLC film with an interpenetrating network through a one-step polymerisation approach.
, Supporting Information.Since all of the DRCD signals of the double-layered films were positive, and the λ max values of the UV-vis spectra and the DRCD signals of the PSCLC films were almost identical, both the top and bottom layers should exhibit right-handed supramolecular helical structures.

Figure 4 .
Figure 4. (Colour online) UV-vis spectra of the double-layered PSCLC films prepared at different CA-Epoxy/CA-Acrylate weight ratios.

Figure 5 .
Figure 5. (Colour online) (a) Photographs and (b) UV-vis spectra of the PSCLC films prepared through the two-step polymerisation approach at different total concentrations of CA-Epoxy and CA-Acrylate (w/w, 4/6).

Figure 6 .
Figure 6.(Colour online) (a) Photographs and (b) UV-vis spectra of the PSCLC films prepared through the one-step polymerisation approach at different total concentrations of CA-Epoxy and CA-Acrylate (w/w, 4/6).