Novel type of combined photopatternable and electro-switchable polymer-stabilized cholesteric materials

Abstract

A novel type of photopatternable polymer network-stabilized cholesteric materials for creation of electrooptical switching was elaborated. For this purpose cholesteric composites based on a commercial low-molar-mass nematic mixture, a chiral dopant, different amount of polymerizable mesogenic diacrylate (1–6 wt%) and a chiral-photochromic substance consisting cinnamoyl and isosorbide fragments (∼4 wt%) were prepared. A small amount of photoinitiator sensitive mostly to light with wavelength of 365 nm was also added. UV irradiation of the complex mixture with this wavelength light results in the polymerization of the diacrylate and formation of the polymer network stabilizing the initial planar texture in the electrooptical cell. Electrical field application induces a transition from the transparent planar cholesteric texture to a scattered focal conic one; at higher voltage the selective light reflection peak disappears. Irradiation of the cell with shorter wavelength light (313 nm) leads predominantly to the E–Z isomerization of the chiral-photochromic dopant followed by irreversible helix untwisting and to the shift of the selective light reflection from blue to green or red spectral regions. In this case, the competing photopolymerization process almost does not take place. Subsequent irradiation of the cell by longer-wavelength UV light (365 nm) allows one to obtain a polymer network and stabilize the new texture. Electrooptical properties of cells with different values of helix pitch were studied in detail. These data demonstrate the new opportunities for creation of photopatternable electrooptically switchable devices, which can be used in photonics and electrooptics.

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Introduction

Dielectric anisotropy inherent to liquid crystals allows one to manipulate their orientation by means of an electric field.¹ In the case of a cholesteric mesophase, at small applied voltages, the planar texture is usually broken into domains where helix axes are orientated in random directions. This focal conic state causes the so-called “blue shift” of selective light reflection and light scattering.¹ Under further increase of the applied voltage, the LC molecules are realigned into a uniform homeotropic transparent texture. The rate of electric-induced reorientation is very high (in the range of milliseconds) when a rather large electric field is applied. After removing electric field the return to the initial planar orientation is inhibited due to the long lifetime of the highly scattering focal conic texture. In order to exclude this undesirable and prolonged lifetime, a small concentration of bifunctional monomer is introduced into the system and this monomer can be polymerized forming a three-dimensional polymer network.^2–5 This network acts as a “memory” due to the helical topology that induces rapid reorientation into the initial planarly oriented aligned cholesteric state. Such so-called polymer-stabilized cholesteric liquid crystals (PSCLC's) are very promising materials for electrooptics and display technology.^2–14

In spite of many publications devoted to PSCLC's only a few papers and one patent describe the possibility to develop photopatternable cholesteric electrooptical cells with different wavelengths of selective light reflection.^10,13–15 For example, in one paper¹⁰ the authors described the possibility of optical recordings in cholesteric mixtures containing photopolymerizable compounds by pattern-wise irradiation of the mixture followed by flood exposure. During the first exposure, polymerization takes place in irradiated areas and diffusion of reactive molecules to these areas occurs owing to formation of a concentration gradient of unreacted species. After flood exposure, the rest of the reactive molecules within the system can be polymerized. As a result, the threshold voltage for switching in various regions of the sample becomes different. Therefore, complicated patterns can be recorded into such a gel and made visible upon application of an electric field.

A patent by Chien et al.¹⁴ deals with pixelated cholesteric materials for display applications. The described cholesteric systems consist of nematic liquid crystals, photopolymerizable bifunctional monomers, chiral dopants and photosensitive chiral dopants capable of photodegradation. The patent includes the description of a large number of mesogenic and chiral compounds, and monomers, as well as some results concerning photochemically induced helical twisting power changing. Unfortunately, any studies of electrooptical properties of such photopatternable devices are not presented in this patent.

Another possibility of tuning selective light reflection wavelength was elaborated by several research groups and was based on the reversible or irreversible E–Z isomerization of chiral-photochromic fragments resulting in changes in their anisometry and helical twisting power.^16–19

Some attempts to combine the phototuning of selective reflection band position with photocrosslinking were undertaken recently.^20,21 In ref. 20 the authors suggested the use of mixtures containing nematogenic diacrylate and chiral-photochromic monoacrylate (based on a menthone derivative) doped with a photoinitiator. Irradiation of the samples with UV light (365 nm) in the presence of air leads only to E–Z isomerization of the chiral-photochromic moiety in respect to C=C double bonds and helix untwisting, whereas subsequent irradiation with light of 405 nm induces photopolymerization and crosslinking. But this method cannot be applied to sandwiched electrooptical cells, because in the latter case contact with oxygen, inhibiting the polymerization, is absent; therefore, both competing processes will occur simultaneously preventing the helix untwisting.

In this paper we prepared and studied photopatternable PSCLC cells applying an approach developed previously in our laboratory²¹ for the creation of cholesteric polymer networks with various positions of the selective light reflection peak.

We have prepared photopolymerizable mixtures containing different concentrations of diacrylateRM257 (1–6%), 25% of right-handed chiral dopant CB15 (Merck), ∼4 wt% of Sorb, ∼1% of Irgacure 651; nematic mixture E48 (Merck) was used as the main component.

The prepared mixtures are quite stable and no phase separation takes place for a long time (several months). The main idea of the approach developed in our work is based on the different positions of the absorbances of the chiral photochromic dopant Sorb and the photoinitiator Irgacure 651. The absorbance maximum of the photoinitiator Irgacure 651 is at 335 nm (Fig. 1). Therefore, the irradiation of the mixture with a mercury lamp line at 365 nm must lead to the photopolymerization of RM257 and formation of a network.The chiral photochromic dopant Sorb absorbs in the short-wavelength region of the spectrum, 315 nm (Fig. 1). Sorb has high twisting ability (i.e. helical twisting power) and can undergo E–Z isomerization leading to decreasing its helical twisting power due to reduction of the anisometry of the molecule.^21,22 The isomerization results in the helix untwisting and the shift of the selective light reflection band to the long-wavelength spectral region. After that the untwisted helical supramolecular structure can be fixed by 365 nm irradiation.

Fig. 1 UV spectra of photoinitiator Irgacure 651 and chiral-photochromic dopant Sorb in dichloroethane solution. In order to demonstrate the real absorbances of the chiral-photochromic dopant and the photoinitiator the concentrations of substances in solution are almost equal to the mixture composition.

Using these features we have suggested the principle of photopatterning and fixation of the cholesteric texture by two steps including (i) irradiation with short-wavelength UV light (313 nm) through the mask and a local untwisting of the helix in the irradiated regions; (ii) irradiation with longer-wavelength UV light (365 nm) inducing the photopolymerization and fixation of photopatterned image. A schematic sketch of this principle is shown in Fig. 2.

Fig. 2 The scheme of the photoinduced processes in films of cholesteric photopolymerizable blends, P - helix pitch, P₂ > P₁; (a) the initial planar cholesteric texture, (b) the helix untwisting in irradiated zones (irradiation wavelength 313 nm), (c) the network formation after irradiation by wavelength 365 nm.

The objectives of the work arose from two main intentions: the elaboration of photopatternable cholesteric mixtures and their usage for creation of electroswitchable LC cells.

The first part of this work deals with a study of the photooptical properties of mixtures under UV irradiation with different wavelengths (313 nm and 365 nm) in order to find optimal conditions for the helix untwisting and shifting of the selective light reflection peak with fixing of the planar texture. For this purpose, the content of diacrylate RM257 was varied in the range 1–6%. The second part of the work is devoted to the detailed study of electrooptical properties, in particular the response of the cholesteric photopatternable cells (having different helix pitches) to an electric field action.

Experimental

Materials, mixture and cells preparation

As nematic liquid crystal host a commercial mixture of cyanobiphenyl derivatives (E48, Merck, Δn = 0.231 at 589 nm) was used. As chiral dopant 4-(2-methylbutyl)-4′-cyanobiphenyl) CB15 (Merck) was selected. Bifunctional monomer RM257 (Merck) containing three aromatic rings and two acrylic bonds was used. Photoinitiator 2,2-dimethoxy-1,2-diphenylethan-1-one, Irgacure 651, was purchased from Chiba company. Chiral-photochromic dopant 2,5-bis(4-methoxycinnamoyl)-1,4;3,6-dianhydro-D-sorbitol, Sorb, was synthesized by us according to ref. 22.

Mixtures were prepared by mixing of weighed components followed by heating to 120 °C (isotropic melt) and stirring. All procedures with prepared mixtures were performed avoiding direct daylight (in order to prevent unnecessary photopolymerization).

For photooptical studies cells were prepared using glass plates for microscopy. Teflon spacers (10 µm) were used for thickness control. Planar orientation of mixtures in cells was realized during flow filling of the mixture inside the cell by capillary forces. Electrooptical studies were performed using two types of cells: custom-made (two ITO glasses with 10 µm spacers between) and commercial cells (5 µm). In latter case planar orientation was provided by a rubbed polyimide coating.

Study of phase behavior

Polarizing microscopy investigations were performed using a LOMO P-112 polarizing microscope; temperature of samples was controlled by a Mettler FP-80 hot stage.

Photooptical investigations

Photochemical investigations were performed using a special optical set-up equipped with a DRSh-250 ultra-high pressure mercury lamp. Using the filter light with wavelengths 365 nm or 313 nm was selected. To prevent the heating of the samples due to the IR irradiation from the lamp, a water filter was used. To obtain a plane-parallel light beam, a quartz lens was applied. During the irradiation, a constant temperature of the test samples was maintained using a Mettler FP-80 heating unit. The intensity of light was measured by LaserMate-Q (Coherent) intensity meter.

The circularly polarized spectra of the film samples were studied with a TIDAS spectrometer (J&M) equipped with a rotating polarizer (Glan-Taylor prism controlled by a computer program) and a broad-band quarter wave plate.

Results and discussion

Photooptical properties of cholesteric mixtures with different concentrations of diacrylate

All prepared mixtures with different concentrations of RM257 form only a cholesteric phase transforming to an isotropic state in the temperature interval about 50–55 °C. No phase separation takes place over a long storage period. Transparent planarly oriented cells are characterized by selective light reflection in the blue region of the spectra as clearly demonstrated by the transmittance spectra in Fig. 3.

Irradiation of mixtures with low content of RM257 (<3%) with long-wavelength UV light (365 nm) induces a noticeable helix untwisting and shift of selective light reflection to longer wavelength (Fig. 3a, 4 and also Fig. S1 in ESI†). This effect is associated with the above-mentioned E–Z isomerization of the chiral-photochromic dopant Sorb accompanied with the decrease in its helical twisting power. Such a shift is irreversible due to the irreversibility of the Sorbisomerization process. Despite the large difference of Sorb absorbance maximum position (315 nm) and wavelength of UV light (365 nm) the dopant molecules still slightly absorb light on the edge of the absorbance band.

Fig. 3 Transmittance logarithm spectra (right-handed circularly polarized light) for mixtures containing 1.0% (a) and 5.9% (b) of RM257 before and during long-wavelength UV irradiation (365 nm, 0.33 mW/cm²). In (a) spectra were recorded each 10 min of irradiation; (b) before (solid line) and after 10 min of UV irradiation (dashed line).

Fig. 4 Kinetics of selective reflection peak wavelength shift during UV irradiation (365 nm, 0.33 mW/cm²) of mixtures containing different concentrations of diacrylate (RM257). Insert shows the dependence of the selective light reflection shift on the concentration of diacrylate.

However, an increase in the concentration of diacrylate RM257 results in the significant suppression of the helix untwisting (Fig. 4). The amplitude of the selective light reflection shift rapidly decreases from ca. 300 nm and becomes almost zero at the concentration of RM257 of about 6 wt% (see insert in Fig. 4). This effect is evidently associated with the photopolymerization process leading to the formation of a three-dimensional cholesteric network. The photopolymerization and network formation processes compete with helix untwisting induced by isomerization of the dopant. At high density of the network the photopolymerization completely suppresses selective light reflection shift. It is noteworthy that the planar texture has no visible strong changes after photopolymerization process (see Fig. S2 in ESI†), probably due to the small size of the polymer network domains.

Completely different photooptical behavior is found under irradiation with shorter wavelength light (313 nm). For all examined mixtures with different concentrations of RM257 (1–6 wt%) helix untwisting takes place (Fig. 5, 6 and also Fig. S3 in ESI†). The amplitude of selective light reflection shift is almost the same, only a slight decrease is found for the mixture with 5.9 wt% of crosslinker. These results show that rates of photopolymerization and network formation are negligible in comparison with the helix untwisting process induced by photoisomerization of the Sorb dopant under the action of short-wavelength UV light (313 nm). The helix untwisting and the selective light reflection shift are irreversible and the shape of the reflection peak remains unchanged during a long period of time.

Fig. 5 Transmittance logarithm spectra (right-handed circularly polarized light) for a mixture containing 5.9% of RM257 during UV irradiation (313 nm, 0.13 mW/cm²). Spectra recorded each 2 min of irradiation.

Fig. 6 Kinetics of the selective reflection peak wavelength shift during UV irradiation (313 nm, 0.13 mW/cm²) for mixtures containing different concentrations of RM257.

For films with a concentration of RM257 higher than 3.0 wt% irradiated by 365 nm light a subsequent action of the short-wavelength light (313 nm) does not lead to any change in the selective light reflection position, i.e. the network density is high enough to prevent further change in the helical structure induced by the isomerization.

Thus, the selection of UV-irradiation wavelength provides an opportunity to realize predominantly one of the photoprocesses—photoisomerization of chiral-photochromic dopant with helix untwisting (313 nm) or photopolymerization of diacrylate with fixation of planar texture (365 nm). The minimum concentration of photopolymerizable diacrylate necessary for complete stabilization of cholesteric structure is ca. 6 wt%.

Electrooptical properties of cholesteric mixtures

Electrooptical studies were performed using two types of electrooptical cells: custom-made cells (two ITO-coated glasses with 10 µm spacers between) and commercial cells (5 µm). In the first case planar orientation was spontaneously achieved during cell filling by capillary forces. In the second case a planar orientation was additionally provided by the rubbed polyimide coating.

First of all let us consider the electrooptical properties of the first type of cells. In all cases before the electrooptical study for stabilization of the planar texture 365 nm light irradiation was performed. Application of the electric field leads to an increase in light scattering from the cell and a small shift of the selective light reflection peak to the shorter-wavelength region (Fig. 7) that is associated with the well-known focal conic deformation of the planar texture.¹ For the mixtures with the concentration of diacrylate below 4 wt% these changes are only partially reversible and after the switching field off, the light scattering still occurs. Good reversibility was found for the mixture with 6 wt% of RM257.

Fig. 7 Spectra of a photocrosslinked film of a mixture containing 4% of RM257 in the OFF state and after application of electric fields of different voltages. Cell thickness is 10 µm.

We also have studied electrooptical response and light scattering growth for the cells irradiated during different periods by short-wavelength UV light (313 nm) followed by photopolymerization of diacrylate by 365 nm light (for 30 min). As was discussed above, the variation of the time of UV irradiation provided a possibility to control the spectral position of the selective light reflection peak. Using this approach three 10 µm cells having different colours of selective light reflection were prepared: nonirradiated (‘Blue’), irradiated by 313 nm during 20 min (‘Green’) and 40 min (‘Red’).

For characterization of electrooptical properties of the cells, namely, the scattered texture formation under voltage application, we have plotted values of transmittance outside the selective light reflection band (800 nm) vs the applied field. As clearly seen in Fig. 8 the threshold voltage of the focal-conic texture appearance strongly depends on the value of helix pitch. For mixtures irradiated before polymerization with 313 nm light films start to scatter light at lower electric field.

Fig. 8 Dependence of transmittance at 800 nm (for right-handed circularly polarized light) on applied voltage (1 kHz) for photocrosslinked film of mixture containing 6% of RM257. Three samples: nonirradiated (‘Blue’), irradiated by 313 nm during 20 min (‘Green’) and 40 min (‘Red’). Cell thickness is 10 µm.

It should be pointed out that switching times of scattering growth and decrease are very short (Fig. 9). For switching “ON” the time of scattering increase is about 5–10 ms, depending on the value of helix pitch. These results are similar to the literature data obtained by different authors^23,24 for nonphotopatternable cholesteric mixtures. Relaxation times are extremely dependent on the helix pitch. These values are very low for samples with selective light reflection in the blue spectral region (ca. 1 ms) and very large for the cell with red selective light reflection (more then 50 ms). The stronger twisted helical structure creates more hindrances to any field-induced textural transformations, whereas the increasing in helix pitch allows one to facilitate the focal-conic texture formation.¹

Fig. 9 Kinetics of electrooptic response (switching on and switching off time) under application of 20 ms voltage pulse (90 V).

Now let us consider the electrooptical behaviour of photocrosslinked films in commercial 5 µm cells. As seen from Fig. 10 the selective light reflection peak has lower bandwidth due to the better planar orientation provided by rubbed polyimide coating (see also Fig. S4 in ESI†). Further evidence of a better and more homogeneous orientation is the appearance of interference fringes in the spectral range outside the selective reflection band. The short wavelength UV-irradiation and the helix untwisting is accompanied by a strong peak widening and decrease in its intensity (Fig. 10). A significant peak widening is a quite unexpected phenomenon in this case because for 10 µm cells this effect does not take place (see Fig. 4 for comparison). It seems strong anchoring and low thickness make helix untwisting difficult and, therefore, this leads to helix deformation and “mosaic” texture formation (with regions having different helix pitch). Complicated peculiarities of helix untwisting under photoinduced shift of selective light reflection and its dependence on cell thickness were studied in detail by Yoshioka et al.²⁵ A stepwise character of the thermally induced shift of the selective light reflection band due to strong anchoring was found by Belyakov et al.^26,27 and Gleeson et al.²⁸ The possible formation of a polymer network even during short-wavelength UV-irradiation (313 nm) also can not be completely excluded. Its presence may results in a gradient of helix pitch in cells.

Fig. 10 Transmittance logarithm spectra for right-handed circularly polarized light of samples irradiated with 313 nm light for different times followed by photopolymerization (365 nm, 30 min).

Application of an electric field to such cells induces the increase in the light scattering in a short-wavelength spectral range (below 450 nm) and to decrease of the selective light reflection intensity (Fig. 11 and S5 in ESI†). This behavior is quite different from the properties of the above-discussed cells (10 µm), for which light scattering was observed in the full visible and near IR spectral ranges.

Fig. 11 Transmittance logarithm spectra for right-handed circularly polarized light under field application (1 kHz). The sample was prepared using a 5 µm electrooptic cell with a polyimide coating.

We have also studied the electrooptical behavior of three different cells: nonirradiated (‘blue’ cell, λ_max∼473 nm), and cells irradiated at 313 nm during 2 min (‘green’ cell, λ_max∼525 nm) and 6 min (‘red’ cell, λ_max∼600 nm). Fig. 12 demonstrates dependences of the transmittance logarithm for right-handed circularly polarized light on the electric field for these three samples. Dependences were plotted for two wavelengths, 400 nm, corresponding to the light scattering contribution, and for the maximum of selective light reflection (which were different for each of the three samples). As seen from Fig. 12a the increasing voltage leads, at first, to an extreme growth of the light scattering. For ‘blue’ and ‘green’ cells the threshold voltage, at which scattering begins to increase, is about 15 V, for the ‘red’ cell this value is lower and equal to ∼10V. Under the further increasing of electric field the scattering approaches a maximum value (at ∼35 V for ‘blue’ and ‘green’ cells and at ∼20V for the ‘red’ one). An additional increase of the voltage results in a significant disappearance of the scattering which is associated with the partial Freedericksz transition to a homeotropic state.

Fig. 12 Changes in transmittance logarithm spectra at 400 nm (a) and at selective reflection maximum (b) for right-handed circularly polarized light under field application (1 kHz). Three samples: nonirradiated (sample ‘Blue’), and the samples irradiated at 313 nm during 2 min (‘Green’) and 6 min (‘Red’). Samples were prepared using a 5 µm electrooptic cell with a polyimide coating.

Polarizing optical photomicrographs (see Fig. S6 in ESI†) demonstrates the transition through these three states under different voltages. It is interesting to note that the picture of planar texture defects remains stable under application of voltage. Evidently, such stability is provided by the polymer network.

More simple behavior was observed for the transmittance at the wavelengths corresponding to the selective light reflection peaks (Fig. 12b). In this case starting at a certain voltage the selective light reflection peak decreases to almost zero. Threshold voltages for ‘blue’, ‘green’ and ‘red’ cells are equal approximately to∼20, ∼20 and ∼10 V, respectively.

Fig. 13 demonstrates a cell photopatterned by UV light irradiation for different periods of time. Just after switching on the electric field a blue square (due to “blue” shift), corresponding to the electrode area, appeared.

Fig. 13 Photopatterned 5 µm cell prepared by UV irradiation (313 nm) for different time periods followed by photopolymerization (365 nm, 30 min) before and after field application.

In conclusion, a novel type of photopatternable polymer network-stabilized cholesteric material for electrooptical switching was elaborated. Obtained data give the opportunity to create photopatternable electrooptically switchable devices for optical processing and storage. Our future efforts will be focused on the creation of cholesteric cells with photopatternable and electro-switchable circular dichroism. For this purpose an absorbing dichroic dye will be added to the cholesteric mixture having a large helical pitch and doped with chiral-photochromic dopant Sorb.

Acknowledgements

This research was supported by the Russian Foundation of Fundamental Research (08-03-00481) and Program COST-D35. Authors thank Dr A.V. Kaznacheev for switching time measurements

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Footnote

† Electronic supplementary information (ESI) available: Transmittance logarithm spectra; texture images; polarised optical micrographs. See DOI: 10.1039/b814455a

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