Colorful patterned polyacrylate films prepared using cholesteric liquid-crystalline mixtures with a smectic order

ABSTRACT Polymer-stabilised cholesteric liquid crystal (PSCLC) films with selective circularly polarised light reflection have attracted much attention for their applications as polarisers, energy-saving windows and for displays. Herein, CLC mixtures were prepared using a nematic LC LC242 and a chiral compound S-6 with enantiotropic SmA and SmC* phases, which exhibited a cholesteric phase with a smectic order. After photopolymerisation, the PSCLC films with fingerprint structure at the surfaces and supramolecular helical structure inside were obtained. Due to the existence of short-range smectic order in the cholesteric structure, the Bragg reflection bands were broadened. For a CLC mixture, with increasing temperature, the short-range smectic order was suppressed, and the selective Bragg reflection band shifted to the short wavelength. Based on this thermochromic behaviour, colourful PSCLC patterns and gratings were prepared, which were able to be applied in decoration and anti-counterfeiting. Since the PSCLC films can be obtained within several seconds under air, large-area films can be prepared on coating lines at low-cost. GRAPHICAL ABSTRACT


Introduction
Cholesteric liquid crystals (CLCs) are a special class of photonic materials, which have a submicro-to microscale supramolecular helical structure [1,2].It is because of this special helical structure that CLCs can selectively reflect incident circularly polarised light with the same chirality, while transmitting polarised light with the opposite chirality [3][4][5].The structure of CLCs is often determined by the improved Bragg's law of reflection λ = nPcosθ, where n is the average refractive index, P is the helical pitch length, and θ corresponds to the wavelength reflected by the angle between the incident light propagation direction and the helical axis, and the wavelength of the reflected light (λ) varies as a function of θ and P [6,7].Typically, the bandwidth of a single-pitch CLC material is less than 100 nm and is usually 30-50 nm in the visible region [8][9][10].
During the last decades, much attention has been paid to preparing polymer-stabilised CLC (PSCLC) films which can be used as energy-saving windows, polarisers, sensors and in decoration and displays [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28].For broadening the Bragg reflection band, temperature [15], pressure [16], electric field [17,18], dyes [19], light intensity [20] and molecular structures of the LCs [21] were controlled.For increasing the reflectivity of light, the PSCLC films with both left-and right-handed supramolecular helical structures were prepared [29,30].To enhance the elasticity, LC oligomers were selected as the starting materials [31][32][33].For creating colourful patterns, the E7 mixture was used as an LC ink to locally swell the PSCLC network [34].Up to now, both acrylate-and oxetane-based PSCLC films were prepared through photopolymerisation.Since acrylate LC monomers suffer from oxygen inhibition, the PSCLC films are generally prepared within cells or under N 2 .Recently, it was reported by us that a photoinitiator with a tertiary amino group could suppress oxygen inhibition [35].Based on this and the photochromic property of the CLC mixtures with a photoisomerizable chiral dopant, colourful patterns were prepared, which were suitable for decoration and anti-counterfeiting [36].The colour was controlled by tuning the UV light irradiation time.Moreover, colourful patterns have also been prepared using a cross-linkable thermochromic LC oligomer [37].The colour was controlled by tuning the polymerisation temperature.
Up to now, varieties of CLCs and CLC mixtures with thermochromic property were prepared [38][39][40].The selective Bragg reflection band could blue-and/or redshift with increasing temperature.The thermochromic property is tunable by changing the molecular structures of the LCs and chiral dopants [39].Both the molecular anisometry [38] and solubility [41] of the chiral dopants play important roles in the thermochromic property.During the transition from cholesteric to SmA phase, the helical pitch changes drastically [42].Based on this, the PSCLC films with a broad reflection band were prepared, which can be used as energy-saving windows [43][44][45].Therefore, if the broad Bragg reflection band existents in the visible region, structural coloured PSCLC films are able to be prepared.The broad reflection band can reflect more light which can be observed by naked eyes.Herein, the CLC mixtures with a short-range smectic order were prepared using a nematic LC and a chiral LC with enantiotropic smectic phases.Then, the structural coloured PSCLC films were prepared by controlling polymerisation temperature.Since the oxygen inhibition was suppressed by the addition of the photoinitiator Irgacure 369, the PSCLC films could be obtained within several seconds under air.Therefore, large-area films are able to be prepared on coating lines.Moreover, the colourful patterned PSCLC films were prepared using masks, which showed important applications in decorations and anti-counterfeiting.

Results and discussion
LC242 (Figure 1) is an LC with the phase transition sequence of Cr 70°C N 120°C I, which is usually chosen for the preparation of the PSCLC films [46,47].The synthetic route for S-6 is shown in Scheme S1 (Supporting Information).The phase transition behaviour of S-6 was characterised using polarising optical microscopy (POM), differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) (Figure S1 and Figure S2a and S3a, Supporting Information).Both enantiotropic SmC* and SmA phases were identified.The fan-shaped SmA texture and the broken fanshaped SmC* one with dechiralisation lines were shown in Figure S1a and S1b, respectively.The molecular length of S-6 calculated using Gaussian 09 program [48] is about 3.98 nm (Figure S3b, Supporting Information).For the SAXS pattern taken at 95°C, the sharp peak indicated a SmA structure with a d-spacing of 3.73 nm.The broad peak at ~0.44 nm indicates a fluid smectic phase without in-plane order.When the temperature was decreased to 30°C, the d-spacing decreased to 3.43 nm.The molecules should tilt within the layers.
Before the preparation of PSCLC film, the physical behaviours of the LC242/S-6/369 (w/w/w, 49/45/6) mixture were studied.The phase transition sequence of this mixture is Cr 48.0°C Ch 88.1°CI 86.0°CCh 21.6°C Recr, which was characterised using DSC (Figure S2b, Supporting Information).LC242 was proposed to be partially crystallised out from the CLC mixture at the temperature of 21.6°C (Figure S4, Supporting Information).The POM images taken at temperatures from 30 to 70°C show an oily streak texture, indicating a cholesteric structure (Figure S4, Supporting Information).The SAXS patterns indicated the existence of a smectic order with a d-spacing of 3.79-3.84nm (Figure 2(a)).With increasing temperature, the intensity of the scattering peak decreased, indicating the decrease of the smectic order.And a homogeneous mixture tended to be formed.Due to the disturbance of the smectic order, lots of defects were identified in the POM images (Figure S4, Supporting Information).With increasing the temperature from 30 to 70°C, the Bragg reflection band of this CLC mixture shifted from 633 to 443 nm (Figure 2(b)).Due to the light scattering caused by the oily streak defects, the baselines of these curves were tilted, and the reflection bands were broadened.Namely, the short-range smectic order in the CLCs plays an important role in broadening the Bragg reflection band.
Structural coloured PSCLC films were prepared by the polymerisation of LC242/S-6/369 mixtures under 365-nm UV light and at 70°C (Figure 3(a)).With increasing the concentration of S-6 from 35 to 45 wt%, the reflection band shifted from 627 to 464 nm (Figure 3(b)).Light scattering was also identified.Since the CLC mixtures are thermochromic, the PSCLC films with different and gradient colours were also prepared by controlling the polymerisation temperature (Figure 4).This film can be applied for decoration.With increasing the polymerisation temperature from 30 to 70°C, the colour of PSCLC film changed from red to blue, and the reflection band shifted from 670 to 460 nm (Figure 5(a)).Comparison of the UV-vis spectra of CLC mixtures before and after The increase of the bandwidth was proposed to be driven by the wide distribution of the helical pitches.And the wide distribution of the helical pitch was caused by the existence of the short-range smectic order [42,43].The formation of the PSCLC film with the wide distribution of the helical pitch is shown in Figure 6.Due to the disturbance of a smectic order at    temperatures are shown in Figure S5a (Supporting Information).A fingerprint texture was identified on the surfaces of the CLC films.The helical axes lay in the plane of the surface of CLC film.However, these axes were not orientated in a certain direction.Based on the POM images taken in transmission and reflection mode, the helical axes of the CLC inside are perpendicular to the PET surface, and the helical axes of the CLC at surface are parallel to the PET surface.After photopolymerisation, this structure was fixed, which was identified in the POM and field-emission scanning electron microscopy (FESEM) images (Figure 7, S5b and S6, Supporting Information).The fingerprint and lamellar structures were identified at the surface and inside, respectively (Figure 7).Due to the disturbance of the short-range smectic order, the helical pitches of the PSCLC film inside were not uniform and in 288-442 nm [43].It was also found that the helical pitch inside was shorter than that near the film surface.
Based on the thermochromic property of the CLC mixtures, colourful patterned PSCLC films were prepared by polymerisation at different temperatures and   using photomasks (Figures 8 and 9).For the preparation of an LC coating, a rubbing-oriented PET film was used as the substrate, which can effectively align LC molecules at the interface and reduce defects.For the pattern shown in Figure 9(a), a mask with the words 'Soochow University' was used.After the LC coating was partially photopolymerised using the photomask at 60°C, the background colour was fixed.During the cooling process, the helical pitch of the uncured LC mixture increased.After cooling down to 50°C, the colour of the LC mixture changed to green.After the LC mixture was polymerised at this temperature, a colourful pattern was obtained.The pattern of a tropical fish was prepared according to a similar procedure (Figure 9(b)).For the 1D grating, lots of defects were identified in the POM image taken in reflection mode (Figure 9(c)).The UVvis spectra of the grating were taken by tuning the grating at different angles (Figure S7, Supporting Information).The dispersion of light was clearly identified.Based on the thermochromic behaviour, more colourful patterns and optical materials can be prepared, which are potentially applied in decoration and displays.

Conclusions
In conclusion, CLC mixtures with a smectic short-range ordering structure were prepared using a nematic and a smectic LCs.The CLC mixtures were thermochromic and exhibited a broad reflection band.The PSCLC films with fingerprint structures on the surface and supramolecular helical structure inside were prepared using these CLC mixtures.Due to the disturbance of the short-range smectic order in the cholesteric structure, the helical pitches of the PSCLC films were not uniform.Then, the reflection bands of the PSCLC films were broadened.Since the CLC mixtures can be photopolymerised within several seconds under air, large-area films can be prepared on coating lines at low-cost.Moreover, the colourful PSCLC patterns are able to be prepared based on the thermochromic property, which are potentially applied for decorations and as optical materials.

Characterisation
Optical rotation was measured with RUDOLPH.FT-IR spectra were performed on a Nicolet 6700 spectrometer at 2 cm −1 resolution by averaging over 32 scans. 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 an EA-1106 instrument.The POM images of the target compounds were taken using a CPV-900C polarisation microscope fitted with a Linkam LTS420 hot stage.The DSC measurements were conducted on a TA-Q200 under nitrogen at 1.0°C min −1 .The transition temperatures reported in this paper were mostly the peak values of the transition on DSC traces.The SmC*-SmA transition temperature was the value obtained using POM.UV-Vis reflectance spectra were measured using the UV-VIS-NIR spectrophotometer (UV3600).The DRCD spectra were measured using the JASCO 815 spectrometer (Tokyo, Japan).The FESEM images were obtained using a Hitachi S-4800 operating (Ibaraki prefecture, Japan) at 5.0 kV.The SAXS patterns were taken using an X-ray scattering instrument (SAXSess mc 2 , Anton Paar) equipped with line collimation and a 2200 W sealedtube X-ray generator (Cu-Kα, λ = 0.154 nm).The PSCLC films were prepared using UV LED series equipment (UVSF81T, 400 mW) produced by FUTANSI Electronic Technology Co., Ltd (Shanghai, China).The polymerisation temperature was controlled using the hot plate (YS-200S) maded by Yongxing Co., Ltd (Taiwan, China).

Computation details
All calculations were carried out with Gaussian 09 program [48].Density functional theory (DFT) was carried out at the B3LYP functional level by using 6-311 G(d,p) for all atoms [51][52][53].The geometry was characterised as minima by frequency analysis (N imag = 0).

Preparation of PSCLC films
A typical preparation procedure was shown as following.A mixture of LC242/S-6/369 was prepared at the weight ratio of 49/45/6, which was dissolved in a cyclohexanone/ethyl acetate (v/v, 4/1) mixture.The total concentration of the solutes was about 20 wt%.The solution was coated on a rubbing-oriented PET film using a 20-μm Mayer bar for controlling the thickness.After evaporating the solvents at 100°C for 3.0 min, the temperature was cooled down to 70°C.Then, photopolymerisation was carried out under an LED lamp with 365-nm parallel light (400 mW/cm 2 ) for 5.0 s.The other PSCLC films were prepared by tuning the reaction temperature.The PSCLC film with a gradient of colours was prepared by controlling the film of the CLC mixture at a gradient of temperature.To control the gradient of temperature, one side of the CLC film was put on a hot stage, another side was hung in the air.

Preparation of patterned PSCLC films
The preparation procedure was similar as above.However, a photomask was used during the photopolymerisation step.For the preparation of pattern with the words, 'Soochow University', a photomask with the words was used.The CLC film was partially cured at 60°C under the parallel light.After the photomask was removed and the temperature was cooled down to 50°C, the uncured area was polymerised under the parallel light for 5 s.The pattern with a colourful fish was prepared according to a similar procedure.

Preparation of PSCLC film grating with 1D stripe structure
A photomask with a 1D grating structure was chosen.The LC mixture was partially cured under parallel light for 5 s at 40°C.After the photomask was removed and the temperature was increased to 70°C, the patterned film was irradiated under the parallel light for 5 s.

Figure 1 .
Figure 1.Molecular structures of the LCs.#: the datum was obtained from the POM characterisation.

Figure 6 .
Figure 6.(Colour online) Schematic presentation of the formation of the structure with the wide distribution of the helical pitch.

Figure 7 .
Figure 7. FESEM images of (a) the surface and (b) the crosssection of PSCLC film prepared at 30°C.

Figure 8 .
Figure 8. (Colour online) Schematic representation of the preparation of a colourful pattern.

Figure 9 .
Figure 9. (Colour online) Photographs of the PSCLC films with the patterns of (a) 'Soochow University' and (b) tropical fish, (c) POM image of the 1D grating.