Front. Chem. Eng. China 2008, 2(3): 265–268 DOI 10.1007/s11705-008-0054-8
RESEARCH ARTICLE
Effect of polymer structures on electro-optical properties of polymer stabilized liquid crystal films Shoulian WANG, Jie HE, Yu ZENG, Bin YAN, Yinghan WANG (*) College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials and Engineering, Sichuan University, Chengdu 610065, China
E
Higher Education Press and Springer-Verlag 2008
Abstract The polymer stabilized liquid crystal (PSLC) film is a relatively novel electro-optical material, which is generally obtained by dissolving a small amount of a bifunctional photoreactive monomer in a low molecular mass liquid crystal. In this paper, the PSLC films were prepared with photoreactive biphenyl methacrylate monomers by photopolymerization induced phase separation. The effects of liquid crystal concentration, curing time, monomer structures and alignment layer on the electro-optical properties of PSLC films were investigated. The results show that the transmittance in the OFF state (TOFF) increased with the liquid crystal concentration, but the driving voltage decreased. TOFF was also influenced by the curing time. Furthermore, when polyimide was used as alignment layer, the films prepared from the bifunctional monomer shows a higher TOFF, while those from the single functional monomer exhibited a deformed electro-optical curve due to the unsteady polymer networks. Keywords polymer network, liquid crystal, network morphology, electric-optical properties
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Introduction
Polymer stabilized liquid crystal (PSLC) films, in which the liquid crystal represents the continuous matrix with a small amount of a crosslinked polymer dispersed in the anisotropic fluid are novel composite electro-optical materials. Liquid crystal molecules were stabilized by polymer networks via elastic interaction between polymer and liquid crystal. The films have been widely applied in switches, variable optical attenuators, smart windows and displays due to their low threshold voltage, high contrast Translated from Polymer Materials Science and Engineering, 2008, 24(1): 63–66 [译自: 高分子材料科学与工程] E-mail:
[email protected]
ratio and wide viewing angle, etc [1,2]. Unlike normal polymer dispersed liquid crystal (PDLC) films, PSLC appears transparent when no electric field is applied (OFF state) for the homogenous orientation of liquid crystal under the effect of the alignment layer in the polymer network. It turns into an opaque state by AN application of an external electric field (ON state) for parts of reorientation of liquid crystals along the direction of electric field. Because of the birefractive effect of liquid crystal molecules, it will strongly scatter incident light when liquid crystal molecules are randomly distributed in the films. The electro-optical properties of PSLC films depend on the liquid crystal concentration, polymerization condition, interface anchoring strength, etc [3–7]. In recent years, many researchers have focused on the relationships between electro-optical properties and network morphology of PSLC films. Macchione et al. investigated the effect of the alignment layer on the electro-optical properties of PDLC by directly rubbing indium tin oxide (ITO) conductive layer as alignment of liquid crystal [8]. They obtained a reverse mode PDLC and demonstrated that rough surfaces could be used to control the orientation of liquid crystal. Dierking et al. established a quantitative relationship between the polymer network morphology and the electro-optical performance of PSLC by using the fractal dimension as a means to describe the polymer morphology [9]. They have studied in detail some factors (i.e., UV intensity, curing temperature, and time of UV irradiation) influencing polymer network morphology. However, the structures and functional groups of the monomers are closely related to the polymer network morphology. Therefore, further studies on the effect of the structures and functional groups of the monomers on electro-optical properties of PSLC films are still required. PSLC films are generally obtained by dissolving a small amount of a bifunctional photoreactive monomer (5–10 wt-%) together with the photoinitiator in a low molecular
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mass liquid crystal. The monomer is subsequently polymerized by UV irradiation to form a crosslinked polymer networks during polymerization. In this paper, we report the preparation of PSLC films with photoreactive biphenyl methacrylate monomers by photopolymerizationinduced phase separation. The effects of liquid crystal concentration, curing time, monomer structures and alignment layer on the electro-optical properties of PSLC films were also investigated.
2
Experimental
The monomers were synthesized according to the literature [10], which are a bifunctional biphenyl methacrylate (M1) and a monofunctional 4-cyanobiphenyl methacrylate (M2), respectively. Nematic liquid crystal XH (TN1 5 62uC, n0 5 1.527, Dn 5 1.717, De 5 11.4) was purchased from the Yantai XianHua Chem-Tech. Co., Ltd. The monomer, liquid crystal and photoinitiator (1104), which were purchased from the Changzhou LanDing Sci-Tech. Co., Ltd, were mixed in an ultrasonator at a predetermined fraction. A small amount of reactive mixture was filled into test cells consisting of ITO coated glass substrates on the heating stage (60) by capillary action. The cells were assembled at an antiparallel rubbed direction by directly rubbing the ITO layer or spin-coated polyimide to get a homogenous alignment of liquid crystal. The thickness of the cells was set to be about 20 mm by glass spheres. Then, the cells were exposed to a UV light. The electro-optical properties of the samples were measured with the UV 1810 PC (Beijing Purkinje General Instrument Co., Ltd, Beijing, China) spectrometer at 550 nm. The incident He-Ne laser light was polarized in the rubbing direction of the glass substrate. These measurements were corrected by atmosphere directly. The texture of PSLC films was observed on a XP6 (Shanghai Millimeter Precision Instrument Co., Ltd, Shanghai, China) polarized optical microscope (POM) with a camera.
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Results and discussion
We first investigated the effect of the concentration of the monomer M1 on the electro-optical properties of the PSLC films. Samples with different weight ratios of monomer to liquid crystal (5/95, 10/90, 15/85 and 20/80) were prepared by directly rubbing the ITO surface and were cured by UV light for 1 h. The transmittance-voltage (T-V) curves of the samples are shown in Fig. 1. It is apparent that as the concentration of liquid crystal increased, the transmittances of the films in the OFF state (TOFF) also increased, while the driving voltages decreased. This is because the concentration and solubility of the monomer within the liquid crystal
have great influence on the morphology of the PSLC films [11]. The monomer M1 used in our experiment has a good solubility in liquid crystals, and the lower monomer concentration may result in a weaker polymer chain, which would decrease the interaction of the polymer networks and liquid crystals. Thereby, the limitation of the polymer networks to liquid crystal molecules would weaken, so that the liquid crystal molecules will be easy to align preferably by the alignment layer which would lead the liquid crystal molecules in the films to represent the identical refractive index. Therefore, the incident light was less scattered and the higher TOFF would be obtained. When the electrical voltage was applied, the transparencies of the films decreased. The reason is that a part of liquid crystal molecules apart from polymer chain were reoriented by the external electric field. The other part of liquid crystal molecules were still aligned in the rubbing direction. This, leads to scattering and implies that lower monomer concentration can achieve better electro-optical properties.
Fig. 1 Electric-optical curves of PSLC films prepared with different liquid crystal concentration Monomer: M1; ITO was directly rubbed as alignment layer; curing time: 1 h
In order to investigate the effect of curing time on the TOFF of the films, we prepared the PSLC films with the 5/95 monomer (M1)-to-liquid crystal weight ratio at different curing times. As shown in Fig. 2, the TOFF decreased with an increase of the curing time. It can also be seen from Fig. 3, that the liquid crystal was effectively aligned along rubbed direction, because the samples would change from bright to dark alternatively by rotation on POM, where the bright and dark areas are liquid crystal phase and polymer network, respectively. When the curing time is short, the polymer network is sparser. Therefore, the less liquid crystal was limited by network and is aligned easier regularly by alignment layer, the higher TOFF was induced. However, as curing time increased, the molecular weight of the polymer got bigger and the polymer network become denser, and the liquid
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crystals nearby polymer networks would misalign with other parts of liquid crystals in the OFF state, thus leading to scattering.
Fig. 4 Electric-optical curves of PSLC films prepared with different monomer M1 and M2 PI used as alignment layer; XH concentration: 95wt-%; curing time 6 h Fig. 2 Relationships between curing time and TOFF of PSLC films Monomer: M1; XH concentration: 95 wt-%; ITO was directly rubbed as alignment layer
Moreover, the structures of the monomers and alignment layer have a great influence on the electro-optical properties of PSLC films. It is generally considered that the transmission and scattering of incident light are controlled by the alignment of liquid crystal molecules in the polymer networks [12,13]. Figure 4 shows the T-V curves of two kinds of PSLC films formed with the monomer M1 and M2 separately mixed with 95 wt-% liquid crystal curing for 6 h. The sample formed with the monomer M1 has a higher TOFF (78.4%) when polyimide was used as an alignment layer, while those with ITO directly rubbed as the alignment layer shows only 48.3% TOFF. In contrast, when PI was used as the alignment layer, the sample formed with the monomer M2 exhibited a deformed electro-optical curve. It means that, at first, the transmittance of the PSLC films decreased with the applied voltage increasing, and then increased after the applied voltage reached a definite value. Because of these PSLC films
coated polyimide on substrate, it can align more regularly liquid crystal molecules in the OFF state, leading to less scattering. Figure 5 shows the POM photos of the samples formed by the monomers M1 and M2 with polyimide as alignment layer. From Fig. 5(a), we can distinctly observe the polymer networks in the PSLC films. In Fig. 5(b), however, it is apparent that liquid crystal molecules formed a lot of microdomains in the films. This is because the bifunctional photoreactive monomer M1 would form crosslinked polymer networks by polymerization, which can limit the orientation of parts of liquid crystals nearby polymer networks. On the contrary, the monofunctional photoreactive monomer M2 cannot form crosslinked polymer networks. But the pendant groups of the polymer chains would limit the orientation of parts of liquid crystals because the biphenyl groups are similar to the structures of the liquid crystal molecules [14]. Under the applied voltage, the parts of liquid crystal would not align along the direction of the external electric field due to the anchoring force between the pendant groups of the polymer chain and the liquid crystal molecules. At the same time, the disturbed liquid crystal microdomains would
Fig. 3 Polarized optical microscope photos of PSLC films with different curing times Monomer: M1; XH concentration: 95wt%; ITO was directly rubbed as alignment layer
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Fig. 5 Polarized optical microscope photos of PSLC films: (a) M1 (b) M2 PI used as alignment layer; XH concentration: 95wt-%; curing time: 6 h
lead to scattering, and thus decreased the transmittance of the PSLC films. On the other hand, the anchoring force would be broken with the applied voltage increasing. The large number of liquid crystal molecules would reorient along the direction of the external electric field, hence, the transmittance of the PSLC films increases. It implies that the electro-optical properties of the PSLC films prepared with the monofunctional monomer are unstable.
4
Conclusions
In this work, we have successfully prepared the PSLC films with photoreactive biphenyl methacrylate monomers by photopolymerization-induced phase separation. The effects of liquid crystal concentration and curing time on the electro-optical properties of PSLC films were investigated. The results show that the TOFF of the samples increased with the liquid crystal concentration but the driving voltage decreased. As the curing time increased, TOFF decreased. Furthermore, when polyimide was used as alignment layer, the films prepared with the bifunctional monomer did show a higher TOFF, while the films prepared with the monofunctional monomer exhibited a deformed electro-optical curve due to the unsteady polymer networks. Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 50773045) and Talent introduction start-up Found of Sichuan University (No. 0082204127074).
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