丙烯酸酯/液晶体系的光聚合反应动力学对PDLC电光性质的影响

采用光-示差扫描量热法(P-DSC)和光-流变学(P-Rheology)技术,测定了丙烯酸酯/液晶体系的光聚合反应动力学和凝胶化时间.基于自催化模型和凝胶时间-温度关系,计算了体系的光聚合反应速率常数和活化能,探讨了单体结构与组成、反应温度对体系光聚合动力学的影响,并研究了聚合物分散液晶(PDLC)的电光响应行为与相分离结构对光聚合动力学的依赖性.结果表明,升高反应温度、增加体系的单体反应活性和平均官能度,均提高了体系的光聚合速率常数,缩短了光聚合凝胶时间.随着单体反应活性和平均官能度的提高,体系的光聚合反应活化能明显降低,且凝胶化前的光聚合反应活化能低于光聚合全过程的平均反应活化能.当液晶含量为50%时,形成的PDLC呈亚微米尺度的双连续相结构.随着光聚合反应温度的升高,光聚合速率加快,导致凝胶时间缩短、相分离程度降低,使PDLC中液晶相尺寸变小、聚合物网络致密化,PDLC的弛豫时间延长、饱和电压降低,而开启时间和阈值电压变化不大.     

基于偶氮液晶的PDLC膜光调制研究

采用聚苯乙烯和光敏混合向列液晶(5CB＋BMAB), 通过溶剂引发相分离制备了聚合物分散液晶膜(PDLC). 利用偶氮液晶的光致相变, 实现了聚合物分散液晶薄膜的光控开关. 实验结果表明, 在PDLC膜内部液晶微球中, 液晶分子呈双极形分布. 其光控温度区间19～36 ℃之间. 以360 nm附近紫外光照射之后, PDLC膜的最大透光率从6%增加到93%. 在相同的工作距离下, 光调制的时间与PDLC膜内部微孔直径相关. 在相同的液晶浓度下, 直径越小, 调制时间越长.     

全息聚合物分散液晶的结构调控与性能

全息聚合物分散液晶(HPDLCs)是由富聚合物相与富液晶相周期性排列而成的结构有序高分子复合材料.HPDLCs通过单体/液晶复合体系的光聚合诱导相分离而形成,如何调控并定量化描述复合体系的光聚合反应动力学、凝胶化行为和相分离程度,进而获得结构规整、电光性能优异的HPDLCs是关键难题.专论概述了光引发体系、单体结构、纳米无机材料掺杂对HPDLCs结构及性能的影响.光引发阻聚剂通过引发和阻聚的竞争与协同,降低了光聚合反应速率、延迟了凝胶时间,促进形成衍射效率达90%的HPDLCs.超支化单体降低了复合体系黏度和光聚合反应速率,延迟了凝胶时间,促使形成衍射效率达94%、具有一维光子晶体结构的HPDLCs.丙烯酰胺单体优化了相分离结构,将HPDLCs的衍射效率提升至98%.纳米硫化锌掺杂在保持规整结构和高衍射效率的同时,大幅降低了HPDLCs的驱动电压.研究还确定了HPDLCs的相分离程度与凝胶时间的函数关系.构建兼具高衍射效率与低驱动电压的HPDLCs,推进其在彩色3D图像存储等领域的应用仍是重要课题.     

大分子引发剂分子量对接枝聚合物基体的PDLC电光性能的影响

以可逆加成-断裂链转移(RAFT)、引发转移终止(iniferter)活性自由基聚合相结合的方法,用一步法制备了不同分子量的大分子引发剂RAFT-PS-co-PCMSI(MI),并通过紫外光聚合诱导相分离法制备了以接枝聚合物为基体的聚合物分散液晶(PDLC)膜.研究了不同分子量的MI对PDLC的微观形貌,关闭状态透光率,阈值电压,饱和电压以及记忆效应等方面的影响.研究表明,降低PDLC中MI的分子量,会使得液晶微滴粒径增大,阈值电压(Vth)、饱和电压(Vsat)减小,记忆效应、关闭状态透光率升高.     

聚合物分散液晶PMMA-5CB的制备及电光性能

以聚合引发相分离的方法制备了聚甲基丙烯酸甲酯(PMMA)基聚合物分散液晶(PDLC).通过差示量热扫描仪(DSC)、偏光显微镜(POM)对不同液晶5CB(4-氰基-4'-戊基联苯)含量的PDLC热力学行为和液晶分散状态进行了表征.在电压为0～30 V、波长为633 nm处,用紫外可见分光光度计(UV-Vis)对PDLC的电光性能进行了研究.结果表明当w(5CB)达到20%以上时,PDLC发生相分离现象;当w(5CB)=30%时,液晶的分散状态最佳,电光效应最强.     

红外光谱结合线阵列检测技术研究聚合物分散液晶膜分子取向

采用偏振红外光谱和变温红外光谱研究聚合物分散液晶膜中液晶分子取向随外加电场及温度的变化.利用线阵列检测技术表征了聚合物与液晶界面处的成分分布.结果表明,线阵列检测技术能够快速而直观地给出成分分布图,通过该成分分布图可以解释PDLC在温度场作用下分子取向的变化.     

不同醇对聚合物分散液晶光聚合动力学及其电光特性的影响

以丙烯酸-2-乙基己酯(EHA)、二甲基丙烯酸乙二酯(EDMA)/季戊四醇四丙烯酸酯(PETTA)为混合单体、液晶P0616A为液晶相、Irgacure 184为光引发剂,通过UV光引发制备了聚合物分散液晶(PDLCs),研究了不同烷基链长醇,即乙醇(EtOH)、正丁醇(nBA)、正己醇(nHA)、正辛醇(nOA)和正十四醇(nTA)对体系光聚合动力学及其PDLCs液晶相变温度及电光特性的影响.结果表明引入醇分子显著加快了丙烯酸酯/液晶复合体系的光聚合反应速率,提高了单体的最终转化率,其中以正丁醇体系最为明显.随着醇分子烷基链的增长,体系的转化率趋于降低,但依然明显高于不含醇的体系.醇分子的加入降低了PDLCs中液晶相的TNI,且随着醇分子烷基链长的增长,PDLCs液晶相的TNI总体上呈降低的趋势.醇分子的加入增加了PDLCs液晶微区中向列相液晶的含量,而含正丁醇和正十四醇的体系液晶微区中向列相液晶低于其它3个含醇体系.醇分子的加入明显降低了PDLCs的阈值电压和饱和电压以及对比度.结合体系的光聚合速率和单体转化率,正丁醇是改善PDLCs性能的最佳选择.     

木质素接枝偶氮苯液晶聚合物的合成与表征及光响应性能研究

以天然高分子木质素为原料,通过亲核取代反应将木质素改性成为大分子引发剂,引发偶氮苯单体的原子转移自由基聚合(ATRP),得到一系列木质素基光响应聚合物.接枝后的木质素的热稳定性明显改善,且平均接枝率达到72.8%时才能表现出液晶行为.小角X射线散射和偏光显微镜的结果表明所形成的液晶相为近晶C型,层间距为3.21nm.在此基础上,用紫外-可见吸收光谱(UV-Vis)对木质素基液晶聚合物的光响应性进行研究,发现溶液中的光响应性比膜状态的光响应速率快.在紫外光的辐照下,木质素基液晶聚合物表现出快速的光致液晶-各向同性相变行为.     

一种新型的电光显示材料—PDLC

PDLC (Polymer Dispersed Liquid Crystal)是把向列型液晶以微子粒子的方式分散在高分子基质中形成膜,而具有独特性质的一种新型电光显示元件。本文结合我们实验室的最新研究成果,概述了PDLC的制造方法,响应机理,液晶微滴的指向矢构型以及其相转变,简述了PDLC的研究状况,应用范围和发展前景。     

基于星型聚4-甲基-ε-己内酯的光交联网络合成与性能表征

采用季戊四醇(PE)引发4-甲基-ε-己内酯(MeCL)单体开环聚合,得到一系列具有四臂拓扑结构的星型聚4-甲基-ε-己内酯(PMCL),再以丙烯酰氯对PMCL末端羟基进行双键化改性,得到端基双键化星型预聚物,该预聚物与光引发剂混合后可在紫外照射下交联成型。通过1 HNMR、GPC和旋转流变仪表征了PMCL预聚物的微观结构和流变性质。研究表明:成功制备了PMCL预聚物,聚合反应可控,且通过改变臂长可调节预聚物的流变特性。此外,随着预聚物臂长的增加,交联样品的模量下降但断裂伸长率有显著提高。     

Thermotropic and optical behaviour of new PDLC systems based on a polysulfone matrix and a cyanoazomethine liquid crystal

The thermotropic and optical characterisations of a novel polymer dispersed liquid crystal (PDLC) system based on polysulfone UDEL P-1700 as polymer matrix and a low molecular weight liquid crystal compound containing a mesogenic azomethine core and a cyano-substituent were investigated. The PDLC samples were prepared by solvent-induced phase separation (SIPS) and thermally induced phase separation (TIPS) methods using various compositions in the two components. Information on the morphology and phase structure was obtained by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. PDLC systems with well-defined droplets were obtained for the composite with medium content of liquid crystal compound. The optical characterisation of these materials was performed by analysing their UV–visible absorption and photoluminescence emission as a function of the liquid crystal aggregation degree.     

Digital optical memory devices based on polymer-dispersed liquid crystals films: appropriate polymer matrix morphology

ABSTRACT

Permanent memory effect (PME) in polymer dispersed liquid crystal (PDLC) allows a greater applicability range than traditional PDLCs. One of the most interesting application could be the possible storing of optical information, the so-called Digital Optical Memory (DOM) devices. To test this application it would be required a display structure having an array of pixels addresses. Each pixel was filled with PDLC film with PME and electric field can be independently applied to different PDLC elements to define on/off pixel states (transparent or scattering states).PDLC films were obtained from a mixture of E7 nematic liquid crystal and poly(ethylene glycol) dimethacrylate with 875 g mol^-1oligomer as precursor of the polymeric matrix. The effect of the curing temperature and the UV light intensity as well time exposure during the polymerisations on the electro-optical performance of PDLC films were investigated. In this way, a high transparency state (T_OFF=55%) for a long period of time at room temperature even after the applied voltage has been switched off were obtained, started from an opaque state (T₀=0%) and after reaching a transparent state (T_MAX=75%), which causes 73% PME. The application to an 8x8 passive matrix using PDLC with PME is also demonstrated as proof-of-principle.     

Studies on electro‐optical properties of polymer matrix/LC/ITO nanoparticles composites

In this paper, polymer‐dispersed liquid crystal (PDLC) films consisting of liquid crystal (LC)/monomers/indium tin oxide (ITO) nanoparticles with good near‐infrared absorption property had been fabricated, and the influence of the ITO nanoparticles modified with 3‐methacryloxypropyltrimethoxysilane (KH570) on the PDLC films was systematically studied. First, different liquid crystal content was studied to obtain PDLC films with good electro‐optical properties. And then, various weight ratio of ITO nanoparticles was added to samples. While the content of ITO nanoparticles was increased, the saturation voltage increased and the CR decreased. Though the electro‐optical properties of PDLC samples reduced with the addition ITO nanoparticles, the near‐infrared absorption property of films was enhanced.     

Influence of SMA content on the electro-optical properties of polymer-dispersed liquid crystal prepared by monodisperse poly(MMA-co-SMA)/LC microcapsules

Monodisperse poly(methyl methacrylate) (PMMA) particles containing various concentrations of stearyl methacrylate (SMA) were prepared, and a liquid crystal (LC) was swollen into the particles using a solute co-diffusion method (SCM). Phase separation behaviors between the polymer and LC were monitored by utilizing an optical and a polarized microscope (OM/POM). The monodisperse LC microcapsules were then applied to a polymer-dispersed liquid crystal (PDLC), and the electro-optical properties were investigated. As a result, the threshold and driving voltages were improved when the SMA content increased. The long alkyl chains of SMA in the capsules should exist at the interface of the LC and polymer resulting in an enhancement of phase separation between the polymer and LC, which largely influences the electro-optical properties of PDLC.     

Effects of crosslinking agent,cure temperature,and UV flux on the electro‐optical properties of polymer‐dispersed liquid crystal cells

We present results for the effects of a crosslinking agent, cure temperature, and UV flux on the electro‐optical properties of polymer‐dispersed liquid crystal (PDLC) cells. These cells were fabricated using a mixture of a liquid crystal (E8) and an acrylic monomer (CN135). The maximum in the first derivative of the transmission vs. applied, sinusoidal voltage (inflection voltage, V_inf), varies systematically with PDLC formulation and cure‐process conditions. For PDLC cells fabricated with a crosslinking agent (SR295), V_inf increases with increasing the concentration of SR295. However, for cells fabricated without the use of a crosslinking agent, V_inf decreases with increasing the UV flux and decreasing temperature. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 404–410, 2004     

The effect of voltage controlled orientation order of liquid crystals in non-acrylic polymer dispersed liquid crystals films

The nematic liquid crystals (LCs) are randomly dispersed material with random orientation order in polymer dispersed liquid crystal (PDLC) films. The LCs change their orientation from random to vertical as electric field is applied. This transformation of orientation order of nematic liquid crystals in the PDLC films is controlled by many factors operating simultaneously. For instance, some factors like the internal forces of attractions among the neighboring LC molecules, anchoring with polymeric matrix, ITO glass boundaries, and chemical structures of the materials are less studied. The learning of extent of vertical orientation of liquid crystal droplets in an electric field is essential to attain optimum electro optical properties of PDLCs. In this finding, bipolar and radial LCs droplets with random orientation have been observed in non-acrylic polymeric media. It is learned that with small increase of contents of external material, the extent of vertical orientation has been varied intensely. The extent of vertical orientation of LCs molecules increases as the contents of external non-acrylic polymeric material decreased. For this study, the orientations of LCs with respect to material type/contents, external applied force, and restoration of electric filed as hysteresis have been studied in details.     

The effect of mono-sized liquid crystal domains on electro-optical properties in a polymer dispersed liquid crystal prepared by using monodisperse poly(methylmethacrylate)/liquid crystal microcapsules

Highly mono-sized poly(methyl methacrylate) (PMMA)/liquid crystal (LC) microcapsules having a mono-sized single LC domain were prepared by the solute codiffusion method and solvent evaporation. The size of the LC domain in the microcapsules could be controlled by the amount of LC introduced during the swelling stage. The electro-optical properties of the polymer dispersed liquid crystal (PDLC) prepared by using the microcapsules was highly improved. In particular, the threshold voltage was lowered and the switching behaviour with an applied electric field was sharpened drastically compared with PDLC prepared simply by solvent evaporation-induced phase separation.     

The fabrication of polymer dispersed liquid crystal based on SiO2 photonic crystal template

The conductance of polymer matrix is an important factor for the property of the polymer dispersed liquid crystal (PDLC). The nanographites are dispersed into the polymer matrix for optimising the dielectric conductive property. The synthesised nanoparticles SiO₂ was used as photonic crystal (PC) to work as a template for fabricating PDLC films. A mixture of pre-polymer and liquid crystals (LCs) was infiltrated into the void of the PC and polymerised under ultraviolet light. The void of the PC made uniform the dispersion of the liquid crystals in the films. The optical property of the PDLC films was optimised by doped nanographites and negative charge SiO₂ template. The effect of negative charge SiO₂ and nanographites on the threshold voltage and driving voltage was researched. The morphology of the PDLC films was studied by the FTIR image. The dispersed LCs droplets were uniformly affected by the addition of the nanographites. The LCs droplets dispersed in the polymer were located in the void of the SiO₂ photonic crystal.     

Novel techniques of PDLC film preparation furnishing manifold properties in a single device

Three novel techniques of polymer dispersed liquid crystal (PDLC) film preparation have been proposed to obtain/induce systematically varying manifold properties in a single device. These three techniques were used to prepare ‘wedge-shaped’, ‘multi-channelled’ and ‘grating type’ PDLC films. Arrangement and configuration of liquid crystal (LC) microstructures inside these PDLC films, which were conveniently divided into different zones, have been investigated using a polarising optical microscope (POM) and scanning electron microscope (SEM). POM images indicate a predominant bipolar structure in all zones of different types of PDLC films but with varying size and density. Further, the electro-optical (EO) properties of PDLC films for different zones have different morphological characteristics as indicated (observed) in POM and SEM images and were dependent on LC droplet shape, size and distribution. Also different zones show different absorbance/transmittance characteristics in the visible range. Thus, our study proposes a single device with manifold properties. Also, the desired properties can be obtained by selecting the suitable zone from the PDLC composite film.     

<Superscript>2</Superscript>H-NMR investigation after a polymerisation-induced phase separation process*

Polymer-dispersed liquid crystals (PDLC) are composite materials consisting of micron-sized droplets of liquid crystal dispersed in a polymer matrix. The easiest method to obtain a PDLC film is the polymerisation-induced phase separation process (PIPS). The liquid crystal is mixed with a monomer of low molecular weight and polymerisation is induced by heat or UV light. The increasing molecular weight of the polymer causes the phase separation of liquid crystal from the polymer matrix as micron-sized droplets. In this work, we have studied the structural changes induced in the polymer matrix of a PDLC after the PIPS process by deuterium nuclear magnetic resonance. Two different selectively deuterated monomers have been synthesized and investigated: isobutyl methacrylate (IBMA-d₂) and methyl methacrylate (MMA-d₃). The main results were the disappearance of the characteristic two-site hop in poly-IBMA, due to liquid crystal molecules, and the lack of unreacted MMA molecules in the liquid crystal droplets. In this last case, we found that it is possible to confine temporarily the unreacted MMA molecules within liquid crystal droplets.Abbreviations MMA Methyl methacrylate - IBMA Isobutyl methacrylate - PDLC Polymer-dispersed liquid crystal - PIPS Polymerisation-induced phase separation - ²H-NMR Deuterium nuclear magnetic resonance*Dedicated to Professor V. Bertini for his 70^th birthday