反式液晶/聚合物分散薄膜的制备及光电性能研究
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摘要
液晶／聚合物分散(liquid crystal／polymer dispersion，LCPD)光电薄膜是近20年发展起来的新型功能薄膜材料。LCPD光电薄膜可以应用于多种光电器件，如智能变色窗、滤波器、光开关等。并因其经济易制而逐渐表现出较常规扭曲向列型液晶器件更大的发展潜力和优势，因此，引起了液晶界的广泛关注。LCPD光电薄膜根据液晶和聚合物比例不同，可分为聚合物分散液晶(polymer dispersed liquid crystal，PDLC)与聚合物稳定液晶(polymer stabilized liquid crystal，PSLC)两种。
LCPD薄膜光电变色主要依赖于液晶的光、电各向异性。电场下液晶取向从而改变液晶的折射率，造成体系光散射程度的不同达到变色的效果。和通常的LCPD分散薄膜相反，反式LCPD薄膜在电场下为不透明态，在无电场下为透明态。这对大多数时间需要透明态的光电器件来说，更加经济和方便。因此，反式光电薄膜的研究成为当今LCPD薄膜领域的一大热点。但目前反式薄膜都需要特种液晶作为材料，如胆甾液晶、负性液晶、双频液晶等。而目前生产最易，成本最低的却是正介电异向性(Δε＞0)向列液晶。因此本文提出了一种新型反式模式——平行PSLC光电薄膜，利用向列液晶即可制备出成本更加低廉的反式光电薄膜。
本文设想该平行PSLC光电薄膜变色机理为：在无电场时，液晶在取向膜作用下平行基板排列。由于折射率趋近一致使薄膜透光率较高。当在薄膜上施加电场后，部分没有被聚合物网络限制的液晶分子由于介电异向性沿电场排列，另一部分限制在网络中的液晶则依然维持原来的平行基板状态，折射率的差异造成薄膜的透光率下降，实现了光电薄膜的反式效果。
选用5CB做为液晶材料，分别合成了含柔性链的可聚合丙烯酸单体4’-(4-氰基苯基)苯氧基丁基-甲基丙烯酸酯和不含柔性链的甲基丙烯酸-4’-(4-氰基联苯)酯作为薄膜基体材料，制备了平行PSLC薄膜。并通过核磁共振氢谱(~1HNMR)对合成单体进行表征，用紫外—可见分光光度计和偏光显微镜(POM)对薄膜的光电性能进行了测试。本文同时讨论了柔性链、固化时间、温度、配方对光电薄膜性能的影响。
通过对比两种基体对薄膜透光率和光电性质的影响，可以看出使用单体4’-(4-氰基苯基)苯氧基丁基-甲基丙烯酸酯作为基体材料制备的薄膜在断电状态下具有高的透光率，通电状态下有较低的透光率，实现了较好的反式效果。通过偏光POM照片看出，单体4’-(4-氰基苯基)苯氧基丁基-甲基丙烯酸酯中的极性棒状基团可以在固化过程中与液晶共同在取向层作用下取向，而不会破坏液晶分子的排列，使体系具有均一的折射率。因此尽管聚4’-(4-氰基苯基)苯氧基丁基-甲基丙烯酸酯和聚甲基丙烯酸-4’-(4-氰基联苯)酯折射率接近，但使用前者得到的反式薄膜的透光率比后者高得多。薄膜的紫外引发相分散在冰浴中进行，与在常温下进行相分散相比，由于温度较低，使聚合速度减慢，通过8小时以上的固化，可以得到相分散较好的PSLC光电薄膜。随着固化时间增加，聚合物网络变牢固，对液晶锚定能越大，薄膜的阂值电压升高，透光率降低。在液晶含量较高的体系中，则可以通过增加固化时间使薄膜在较高电场下的稳定性提高。在较低的单体含量体系中，聚合物对液晶的影响降低，液晶易于均匀取向使薄膜断电和通电状态下的透光率均增加，阈值电压降低。而对于单体含量较高的体系，在聚合过程中，过多的单体和聚合物使液晶在取向膜作用下的取向程度变差，液晶折射率差异也增加，这就导致了薄膜的透光率较低。
Liquid crystal/polymer dispersion (LCPD) films are of technological importance for electro-optic applications such as large-area flat-panel displays, light shutters and switchable windows. The reasons for continued interest in this area are manifold, encompassing both economic and technological realms. Conventional flat-panel display materials made from twisted and supertwisted nematic liquid crystals (LCs) remain relatively expensive and are moderately difficult to fabricate. LCPD films are divided into two kinds: polymer dispersed liquid crystal (PDLC) film and polymer stabilized liquid crystal (PSLC) film according to LC content.
The operation principle of a switchable LCPD film is the electrically controlled light scattering based on birefringence characteristics and control of refractive index of LC droplets by the use of applied voltage. Reverse mode is the opposite to normal mode LCPD: a clear state is maintained under no applied voltage and an opaque state appeared under applied voltage. It is more economical and convenient for electro-optic devices whose work state is transparent state. Many methods to form reverse mode film have been proposed using a dual frequency addressable LC, cholesteric LC and negative dielectric anisotropy nematic LC. However, few people obtained reverse mode film using normal positive dielectric anisotropy nematic LC which is more economical and available. The purpose of this paper is to propose a new methodology to prepare a new reverse mode system, parallel PSLC, using positive nematic LC.
In this parallel PSLC film, when a positive LC is laminated in a parallel aligned cell at a zero field, the LC molecules are in the planar state. In this state, when the plane of polarized light parallels to the molecule orientation, the refractive index of LC molecules assumed similar which resulted in high transmittance. Upon the application of an electric field, LC molecules out of the polymer network changed their orientation which resulted in the mismatch of LC molecules refractive indexes. In this state, the film appeared opaque.
Two monomers were synthesized and used in this system: 2-methyl-acrylic acid 4-(4'-cyano-biphenyl-4-yloxy)-butyl ester (M1) containing a flexible chain, and 2-methyl-acrylic acid 4'-cyano-biphenyl-4-yl ester (M2) without a flexible chain. The structures of monomers were confirmed by ~1H NMR. The effect of structure, concentration of monomer, curing temperature and the UV-exposure time on the film properties was investigated by polarizing optical microscope (POM) and UV-Visible spectrometer.
Transmittance is a critical parameter of electro-optic film. M1 and M2 were used to prepare PSLC films to investigate the effect of monomer structure on the transmittance of the film without voltage. Although the refractive indexes of polymers were slight different, but transmittance of PSLC film with M1 was much higher than M2 film. Monomer M1 contained a rigid polar group and an alkyl side chain, which was similar with the 5CB. This structure made side chain oriented at the same time with 5CB under the influence of alignment layers in the process of polymerization. For the monomer M2, the scarcity of the flexible chain increased the effect of acrylate main chain on polar group. Random arrangement of the polymer chains made LC oriented irregular, which resulted in low transmittance of the film. To explore the effect of LC content on the electro-optic properties, transmittances dependence on the applied voltage of 17wt.%, 15wt.%, 13wt.% and 11wt.% M1 systems were observed. With the decrease of monomer content, less LC molecules were affected by the polymer network and more LC molecules were easier to orient regularly, which increased the transmittance. Moreover, the lower monomer concentration made the polymer network weaker, which decreased the driving voltage. The extension of the exposure time increased the molecular weight and firmed the polymer network, which increased the driving voltage and decreased the transmittance.
LCPD薄膜光电变色主要依赖于液晶的光、电各向异性。电场下液晶取向从而改变液晶的折射率，造成体系光散射程度的不同达到变色的效果。和通常的LCPD分散薄膜相反，反式LCPD薄膜在电场下为不透明态，在无电场下为透明态。这对大多数时间需要透明态的光电器件来说，更加经济和方便。因此，反式光电薄膜的研究成为当今LCPD薄膜领域的一大热点。但目前反式薄膜都需要特种液晶作为材料，如胆甾液晶、负性液晶、双频液晶等。而目前生产最易，成本最低的却是正介电异向性(Δε＞0)向列液晶。因此本文提出了一种新型反式模式——平行PSLC光电薄膜，利用向列液晶即可制备出成本更加低廉的反式光电薄膜。
本文设想该平行PSLC光电薄膜变色机理为：在无电场时，液晶在取向膜作用下平行基板排列。由于折射率趋近一致使薄膜透光率较高。当在薄膜上施加电场后，部分没有被聚合物网络限制的液晶分子由于介电异向性沿电场排列，另一部分限制在网络中的液晶则依然维持原来的平行基板状态，折射率的差异造成薄膜的透光率下降，实现了光电薄膜的反式效果。
选用5CB做为液晶材料，分别合成了含柔性链的可聚合丙烯酸单体4’-(4-氰基苯基)苯氧基丁基-甲基丙烯酸酯和不含柔性链的甲基丙烯酸-4’-(4-氰基联苯)酯作为薄膜基体材料，制备了平行PSLC薄膜。并通过核磁共振氢谱(~1HNMR)对合成单体进行表征，用紫外—可见分光光度计和偏光显微镜(POM)对薄膜的光电性能进行了测试。本文同时讨论了柔性链、固化时间、温度、配方对光电薄膜性能的影响。
通过对比两种基体对薄膜透光率和光电性质的影响，可以看出使用单体4’-(4-氰基苯基)苯氧基丁基-甲基丙烯酸酯作为基体材料制备的薄膜在断电状态下具有高的透光率，通电状态下有较低的透光率，实现了较好的反式效果。通过偏光POM照片看出，单体4’-(4-氰基苯基)苯氧基丁基-甲基丙烯酸酯中的极性棒状基团可以在固化过程中与液晶共同在取向层作用下取向，而不会破坏液晶分子的排列，使体系具有均一的折射率。因此尽管聚4’-(4-氰基苯基)苯氧基丁基-甲基丙烯酸酯和聚甲基丙烯酸-4’-(4-氰基联苯)酯折射率接近，但使用前者得到的反式薄膜的透光率比后者高得多。薄膜的紫外引发相分散在冰浴中进行，与在常温下进行相分散相比，由于温度较低，使聚合速度减慢，通过8小时以上的固化，可以得到相分散较好的PSLC光电薄膜。随着固化时间增加，聚合物网络变牢固，对液晶锚定能越大，薄膜的阂值电压升高，透光率降低。在液晶含量较高的体系中，则可以通过增加固化时间使薄膜在较高电场下的稳定性提高。在较低的单体含量体系中，聚合物对液晶的影响降低，液晶易于均匀取向使薄膜断电和通电状态下的透光率均增加，阈值电压降低。而对于单体含量较高的体系，在聚合过程中，过多的单体和聚合物使液晶在取向膜作用下的取向程度变差，液晶折射率差异也增加，这就导致了薄膜的透光率较低。
Liquid crystal/polymer dispersion (LCPD) films are of technological importance for electro-optic applications such as large-area flat-panel displays, light shutters and switchable windows. The reasons for continued interest in this area are manifold, encompassing both economic and technological realms. Conventional flat-panel display materials made from twisted and supertwisted nematic liquid crystals (LCs) remain relatively expensive and are moderately difficult to fabricate. LCPD films are divided into two kinds: polymer dispersed liquid crystal (PDLC) film and polymer stabilized liquid crystal (PSLC) film according to LC content.
The operation principle of a switchable LCPD film is the electrically controlled light scattering based on birefringence characteristics and control of refractive index of LC droplets by the use of applied voltage. Reverse mode is the opposite to normal mode LCPD: a clear state is maintained under no applied voltage and an opaque state appeared under applied voltage. It is more economical and convenient for electro-optic devices whose work state is transparent state. Many methods to form reverse mode film have been proposed using a dual frequency addressable LC, cholesteric LC and negative dielectric anisotropy nematic LC. However, few people obtained reverse mode film using normal positive dielectric anisotropy nematic LC which is more economical and available. The purpose of this paper is to propose a new methodology to prepare a new reverse mode system, parallel PSLC, using positive nematic LC.
In this parallel PSLC film, when a positive LC is laminated in a parallel aligned cell at a zero field, the LC molecules are in the planar state. In this state, when the plane of polarized light parallels to the molecule orientation, the refractive index of LC molecules assumed similar which resulted in high transmittance. Upon the application of an electric field, LC molecules out of the polymer network changed their orientation which resulted in the mismatch of LC molecules refractive indexes. In this state, the film appeared opaque.
Two monomers were synthesized and used in this system: 2-methyl-acrylic acid 4-(4'-cyano-biphenyl-4-yloxy)-butyl ester (M1) containing a flexible chain, and 2-methyl-acrylic acid 4'-cyano-biphenyl-4-yl ester (M2) without a flexible chain. The structures of monomers were confirmed by ~1H NMR. The effect of structure, concentration of monomer, curing temperature and the UV-exposure time on the film properties was investigated by polarizing optical microscope (POM) and UV-Visible spectrometer.
Transmittance is a critical parameter of electro-optic film. M1 and M2 were used to prepare PSLC films to investigate the effect of monomer structure on the transmittance of the film without voltage. Although the refractive indexes of polymers were slight different, but transmittance of PSLC film with M1 was much higher than M2 film. Monomer M1 contained a rigid polar group and an alkyl side chain, which was similar with the 5CB. This structure made side chain oriented at the same time with 5CB under the influence of alignment layers in the process of polymerization. For the monomer M2, the scarcity of the flexible chain increased the effect of acrylate main chain on polar group. Random arrangement of the polymer chains made LC oriented irregular, which resulted in low transmittance of the film. To explore the effect of LC content on the electro-optic properties, transmittances dependence on the applied voltage of 17wt.%, 15wt.%, 13wt.% and 11wt.% M1 systems were observed. With the decrease of monomer content, less LC molecules were affected by the polymer network and more LC molecules were easier to orient regularly, which increased the transmittance. Moreover, the lower monomer concentration made the polymer network weaker, which decreased the driving voltage. The extension of the exposure time increased the molecular weight and firmed the polymer network, which increased the driving voltage and decreased the transmittance.
引文
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