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Abstract
In this manuscript, photo-sensitive polysulfone thin layers, CMPSF200-HCx thin layers, are utilized to vertically align liquid crystals (LC) accompanied with linearly-polarized UV light (LPUV) exposure. CMPSF200-HCx thin layers are highly transparent and the optical performances of CMPSF200-HCx thin layers are little alternated after LPUV exposing. Vertical alignment of LC between CMPSF200-HC70 thin layers is due to the surface energy alternation generated from [2 + 2] cycloaddition between >C=C< unites, which occurs under LPUV exposing. LC vertically aligned between CMPSF200-HC70 thin layers could be electrically driven to switch at 3.45V, and the total response of LC has been evaluated as 27.941ms. Highly transparent and photosensitive CMPSF200-HC70 thin layers are a competitive candidate in photo-aligning LC.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Liquid crystals (LC) have been successfully utilized in a lot of electro-optic devices, such as LCD [1–3], sensor [4], photonic devices [5,6], lens and optical diffractive optical components [7–10], etc. Alignment of LC exactly determines the optical performances of these devices and nowadays the alignment of LC has been one of the most significant focuses among LC related researches [11–19]. Alignment of LC could be achieved by rubbing, photo-aligning, ion beam spurting and LB thin film methods [20–23], etc, however, photo-aligning is far superior to other methods as it is more convenient and more suitable for domain alignment control. Strategies adopted to photo-align LC could be divided into two categories, one is using photo-sensitive alignment layers [24,25] and the other is doping LC with photo-sensitive materials [26,27]. The above mentioned photo-sensitive materials generally include trans-cis photoisomerization materials, irreversible photo-destruction and photo-cross-linkable materials. Among these photo-sensitive materials, photo-cross-linkable materials are rather good photo-sensitivity and high resistance of LC alignment to heat and light, because of irreversible photochemistry and strongly restricted molecular motions [28–30]. Photo-cross-linkable materials undergoing photo-crosslinking of a cycloaddition stand out with industrial prospect.
In this manuscript, photo-sensitive CMPSF200-HCx thin layers are utilized as alignment layers to align LC accompanied with linearly-polarized UV light (LPUV) exposing. CMPSF200-HCx thin layers are high transparent, and the optical performances of CMPSF200-HCx thin layers are little alternated after LPUV exposing. The [2 + 2] cycloaddition between >C=C< unites at chalcone side chains of CMPSF200-HCx is responsible for LC vertical alignment, and the alignment of LC is found close depends on the cross-link rate of [2 + 2] cycloaddition. LC sandwiched between CMPSF200-HC30 are fractionally aligned even the [2 + 2] cycloaddition between >C=C< unites at chalcone side chains of CMPSF200-HC30 reached the highest cross-link rate, however, by increasing the amount of chalcone side chains, LC sandwiched between CMPSF200-HC70 are completely vertical aligned gradually. LC vertically sandwiched between CMPSF200-HC70 thin layers are electrically switched at 3.45V, and the total response of vertical aligned LC has been determined as 27.941ms.
2. Experimental
Photo-cross-linkable CMPSF200-HCx polymer was synthesized and the corresponding molecule structure is shown in Fig. 1. 200 indicates all the active occupations on CMPSF main chains have been occupied with ClH2C-, and x indicates that among these activated occupations x% has been grafted with chalcone side chains. CMPSF200-HCx solution was prepared by dissolving CMPSF200-HCx in DMF with the concentration of 0.1 mg/ml, and the CMPSF200-HCx thin layers were prepared by generally spin-coating CMPSF200-HCx/DMF solution on glass substrates. The thickness of CMPSF200-HCx thin layer was characterized by a stylus surface profiler (XP-200, Ambios Technology, USA). Photo-cross-linked CMPSF200-HCx thin layers were prepared after LPUV exposing, and the transparency and photo sensitivity of CMPSF200-HCx thin layers was characterized by an ultraviolet-visible transmission spectroscopy (UV-vis, UV-2101, Shimadzu, Japan). The laser across CMPSF200-HCx thin layers was characterized and analyzed by a scanning slit optical beam profilers (638 nm, BP209-IR2, Thorlabs, USA).
Fig. 1. (a) The molecule structure of CMPSF200-HCx and (b) the schematic of cross-link between chalcone side chains.
LC cells with the gaps of 5 µm and 60 µm were assembled from indium tin oxide (ITO) substrates that covered by CMPSF200-HCx thin layers for electro-optic performances and alignment performances evaluation, and nematic LC (MJ98468, Δɛ=8.2, Δn = 0.0987, Tc = 72°C) obtained from Merck Corp. were injected into the cells by siphon. Alignment of LC was realized by exposing the cells with linearly-polarized UV, and the alignment of LC was characterized by polarized optical microscope (POM, Nikon, DXM1200) observation. The pretilt-angle of LC was evaluated by means of a crystal rotation method (TBA 107, Autronic), in which the fluctuating transmittance was recorded while each cell was rotated latitudinally over the range of ± 70°.
The voltage-transmittance (V-T) and response time characteristics of TN cells with crossed polarizers were evaluated using an LCD evaluation system (LCD-700, Otsuka Electronics) with a maximum driving voltage of 5V, a voltage step of 0.2V, a step voltage frequency of 60HZ and an integration time of 200ms. The contact angles of water and diiodomethane droplets on CMPSF200-HCx thin layers were characterized by using the sessile drop technique, and the measurements were conducted by using an angle analyzer (Phoenix 300 Plus, SEO Co., Korea).
3. Results and discussion
Chalcone is sensitive to UV exposure, and by UV exposing, the double bonds of >C=C< units are broken, and the ring structures are generated because of the reconstruction of these >C< chemical radicals [31]. CMPSF200-HCx, containing chalcone components at the side chains are sensitive to UV light and cross-linkable under UV exposing. The UV sensitivity and cross-link of CMPSF200-HCx have been characterized by UV-vis transmission spectroscopy as shown in Fig. 2. The peak located at 340 nm indicates the absorbance of >C=C< units of chalcone at side chains, and the peak decreases after UV exposing.
Fig. 2. UV spectrum of (a) CMPSF200-HC30 and (b) CMPSF200-HC70, and the corresponding decay rate of CMPSF200-HC30 and CMPSF200-HC30, respectively.
As shown in Fig. 2(a) and (b), in the UV spectrum of CMPSF200-HC30 thin layer and CMPSF200-HC70 thin layer, the decrease of the peak located at 340 nm terminated after 596s and 1404s UV exposure, which intimates that the break of double bonds in >C=C< units reaches the maximum rate. The maximum rate of broken double bonds in >C=C< units has been determined as:
A0 and At are ultraviolet absorbance of the peak located at 340 nm that indicates the absorbance of >C=C< unit of chalcone at side chains before and after UV exposing. The maximum rate of broken double bonds in >C=C< units of CMPSF200-HC30 and CMPSF200-HC70 have been determined as 75.7% and 82.3%, respectively.The thickness of CMPSF200-HC30 thin layer and CMPSF200-HC70 thin layer has been characterized as 18.03 nm and 29.76 nm, respectively, and the laser across the CMPSF200-HCx thin layers before and after UV exposing is shown in Fig. 3(a) and (b). Because the laser beam is a Gaussian shape, laser intensity at the center area is relatively strong as shown in Fig. 3(a), and the intensity of laser beam has been evaluated as the numerical intensity in X direction and Y direction as shown in Fig. 3(b). Before UV exposing, the intensity of laser beam across CMPSF200-HCx thin layers are similar in intensity and shape as shown in Fig. 3(a), however, after exposing CMPSF200-HCx thin layers with UV, the intensity of laser beam across CMPSF200-HCx thin layers decreased obviously as shown in Fig. 3(b). The intensity decrease of laser beam finally terminates when the maximum rate of broken double bonds in >C=C< units of CMPSF200-HCx has been reached. Compared with the laser beam across glass substrate, the intensity of laser beam across the glass substrate that coated with CMPSF200-HC30 finally decreased 6.19% at X direction and 5.12% at Y direction, and the intensity of laser beam across the glass substrate coated with CMPSF200-HC70 finally decreased about 16.25% at X direction and 15.89% at Y direction, respectively. Even the thicknesses of CMPSF200-HC30 thin layer and CMPSF200-HC70 thin layer are slightly different, the initial laser across is similar, which indicates the thickness of CMPSF200-HCx thin layers is not a positive cause for laser across decrease. The [2 + 2] cycloaddition of chalcones is generally accompanied with the trans-cis isomerization [32], and the trans-cis isomerization is responsible for the decrease of the laser beam transmission across CMPSF200-HCx thin layers. Contrary to the observed intensity decrease of laser beam after UV exposure, the transmittance increase of CMPSF200-HCx thin layers has been observed, and the transmittance of CMPSF200-HC70 thin layer at 340 nm increased significantly after UV exposing as indicated in Fig. 3(c). The increase of transmittance corresponds to the absorption decease of >C=C< units as shown in Fig. 2(a) and (b).
Fig. 3. (a) The captured laser beam images across CMPSF200-HC30 and CMPSF200-HC70 thin layers, (b) the intensities of laser beam across CMPSF200-HC30 and CMPSF200-HC70 thin layers in X and Y directions, and (c) the transmittance alternation of CMPSF200-HC30 and CMPSF200-HC70 thin layers before and after UV exposing.
Alignment of LC sandwiched between CMPSF200-HCx thin layers has been investigated as shown in Fig. 4. The initial alignments of LC that injected into the cell assembled with CMPSF200-HC30 and CMPSF200-HC70 thin layers are slightly different due to the composition difference of thin layers, and since CMPSF200-HC30 and CMPSF200-HC70 thin layers have been exposed by LPUV, LC start to align. However, after 600s UV exposing, LC cannot be totally aligned even the [2 + 2] cycloaddition in CMPSF200-HC30 thin layer has researched the maximum rate. With the further LPUV exposing on CMPSF200-HC70 thin layers, LC sandwiched between CMPSF200-HC70 thin layers are aligned with seldomly observed light leakages and after 900s, and totally aligned without any observed light leakages after 1200s. The pretilt-angle of LC sandwiched between 1200s UV exposed CMPSF200-HC70 thin layers has been evaluated as 86.2°.
Fig. 4. POM images of LC sandwiched between CMPSF200-HC30 and CMPSF200-HC70 thin layers, respectively.
The surface energies alternation of CMPSF200-HCx thin layer has been evaluated by observing the contact angles of DI-water droplets and diiodomethane droplets. As shown in Fig. 5 and Table 1, after UV exposing a slightly hydrophobic alternation of CMPSF200-HCx thin layers has been confirmed by the increased contact angles of diiodomethane droplets. Generally, the vertical alignment of LC could be obtained when alignment layers are hydrophobic or patterned with columnar structure, ridges and holes. Surface energy of alignment layers is also the key factor relating to LC alignment, and the lower surface energy subserves the vertical alignment of LC. After 600s UV exposing, the surface energy of CMPSF200-HC30 thin layer finally turned to 63.228mJ/m, and after 1200s UV exposing, the surface energy of CMPSF200-HC70 thin layer finally turned to 63.578mJ/m. Compared with the surface energies of CMPSF200-HC30 thin layer and CMPSF200-HC70 thin layer that are free from UV exposing, the surface energy alternation after UV exposing is not obvious.
Fig. 5. (a) The contact angles of DI-water droplets and diiodomethane droplets on CMPSF200-HC30 thin layers and CMPSF200-HC70 thin layers, respectively, and (b) the corresponding calculated surface energy.
Table 1. The contact angles of DI-water droplets and diiodomethane droplets on CMPSF200-HC30 thin layers and CMPSF200-HC70 thin layers, respectively, and the corresponding calculated surface energies, polar energies and dispersive energies.
Table 2. The threshold voltage, rising time and decaying time of cell assembled with rubbed PI alignment layers, LPUV exposed VAPF alignment layers and LPUV exposed CMPSF200-HC70 alignment layers.
The electro-optical performances of LC sandwiched between CMPSF200-HC70 thin layers are presented in Fig. 6. As shown in Fig. 6(a) and Table 2, the threshold voltage of LC sandwiched between CMPSF200-HC70 thin layers is about 3.45 V, which is comparable to LC sandwiched between rubbed homotropic PI thin layers [33]. As shown in Fig. 6(b), LC sandwiched between CMPSF200-HC70 thin layers have the rising time about 19.302 ms, the decaying time about 8.639 ms, and the total response of 27.941 ms. Compared with the LC sandwiched between the thin layers that composed by monomers with vertical-alignment-induced-side chain group and phenanthrene group, LC sandwiched between CMPSF200-HC70 thin layers respond to external electrical field a little slower.
Fig. 6. (a) Voltage-dependent transmittance curves of LC sandwiched between CMPSF200-HC70 thin layers after 1200s LPUV exposing, and the (b) corresponding rising and decaying response time of cell.
4. Conclusion
In conclusion, LC have been vertically aligned between photo-sensitive CMPSF200-HC70 thin layers after LPUV exposing, and the electro-optical performances of LC have been characterized in this manuscript. CMPSF200-HCx thin layers are highly transparent, and after UV exposing, the transparency of CMPSF200-HCx thin layers at the wavelength of 340 cm-1 increases due to the absorbance decrease of >C=C< unites. The [2 + 2] cycloaddition between >C=C< unites at chalcone side chains of CMPSF200-HCx molecules is responsible for LC vertical alignment, and the alignment of LC is found close depends on the cross-link rate of [2 + 2] cycloaddition. Vertically aligned LC between CMPSF200-HC70 thin layers are electrically switched at 3.45V, and have the total response of 27.941ms.
Funding
Shanghai Sailing Program (18YF1400900); National Natural Science Foundation of China (6180030581); Open Project of Jilin Province Key Laboratory of Organic Functional Molecular Design & Synthesis (130028911); Fundamental Research Funds for the Central Universities (2232017D-10, 2232018D3-29, 2232018G-09, CZD19014).
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