Control of the microstructure of polymer network and effects of the microstructures on light scattering properties of UV‐cured polymer‐dispersed liquid crystal films

Polymer‐dispersed liquid crystals (PDLC) films were prepared from ultraviolet (UV) irradiation‐induced polymerization of the photopolymerizable monomers in photopolymerizable monomers/nematic liquid crystal (LC) mixtures. The effects of the composition of the mixtures, the curing temperature, and the UV light intensity on the microstructure of the polymer network in the PDLC films were investigated. Furthermore, the effects of the microstructures on the light scattering properties of the PDLC films in the wavelength region of 300–2500 nm were studied experimentally and theoretically based on the combination of three kinds of classical light scattering theories: the Rayleigh‐Gans (RG) approach, the anomalous diffraction (AD) approach, and the geometrical optics (GO) approach. It was found that the sizes of LC domain in PDLC films increased with the increase of the LC content as well as the decrease of the UV curing intensity, while increased at first and then decreased with the increase of the curing temperature. Moreover, smaller LC domain sizes could exhibit strong scattering properties in a smaller VIS wavelength region and the transmittance in NIR region (especially in the wavelength range of 1300–2500 nm) obviously decreased with the increasing sizes of LC domain. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2090–2099, 2008     

Tunable topographical cellulose matrices for electro-optical liquid crystal cells

In this work we have used acetoxypropylcellulose (APC) to produce free standing solid films (∼60 μm) that were used for assembling electro-optical devices. Thin films were obtained from concentrated lyotropic solutions of cellulose derivatives. Induced by the cast and shearing preparation conditions wrinkles and band textures can be observed in their free-surface plane. In order to eliminate and control these textures we used a process similar to that introducted in literature [1] which consists of storing the films in the same solvent-vapour atmosphere as the solution system. Lyotropic APC liquid crystalline solutions in dymethylacetamide (DMA) with crosslinker were prepared, thin films were obtained by using a shear/casting technique and stored in the solvent-vapour atmosphere until a planar structure was achieved. The dried crosslinked films were analyzed by optical polarised microscopy (POM) and scanning electron microscopy (SEM). The films with different topographies were used to produce optical cells composed by the cellulose derivative film covered on both free surfaces by a layer of the nematic liquid crystal E7 and placed between two transparent conducting substrates. The electro-optical properties of these cells were obtained.     

聚合物分散液晶单体4,4′-二[6-(丙烯酰氧基)己氧基]联苯的合成

利用丙烯酰氯代替丙烯酸钾与4,4′-二(6-溴己氧基)联苯反应改进了制备4,4′-二[6-(丙烯酰氧基)己氧基]联苯(BAB6)的合成路线。改进前的反应温度较高,中间产物溶解度小,产物呈淡黄色,收率仅12.4%;改进后在室温反应,中间产物溶解度较高,产物呈白色,收率达到46.8%。采用1HNMR、13CNMR和元素分析对中间体和BAB6结构进行了表征。由改进后合成的单体,制备了具有快速响应的聚合物分散液晶(PDLC)膜,响应时间约2ms。扫描电子显微镜观察的结果表明,PDLC中形成了聚合物网络结构。     

Effect of external electrical field on phase behavior and morphology development of polymer dispersed liquid crystal

Phase diagrams for mixtures of liquid crystal (LC)/monomer with and without an external electrical field applied have been established using polarized light microscope (PLM).The (isotropic + nematic) coexistent phase region and (isotropic + isotropic) phase boundary of LC/monomer mixtures were observed to shift upward to higher temperatures when the external electrical field exists. It was found that the electrical field applied during the cross-linking polymerization has a significant influence on the phase diagrams for the LC/polymer mixtures by rendering the coexistent phase regions shift upward to higher temperatures. The influence of the external electrical field on the processes of the isotropic-isotropic phase separation and liquid crystal ordering in PDLC formation has also been investigated. The results revealed that both the processes could be highly accelerated by the electrical field.     

紫外光固化PDLC膜性能的温度效应

考察了紫外光固化PDLC膜的电光性能及其温度依赖性,阐述了其作为显示器件的工作稳定性。指出确定PDLC工作温度范围的方法,对PDLC膜中液晶相的N-I转变行为及其与膜的内部结构的关系进行了讨论。     

利用数值拟合研究掺杂纳米Nd2O3聚合物分散液晶(PDLC)的紫外电光特性

对液晶新性能的研究是当前的研究热点。通过非线性数值拟合研究掺杂纳米稀土氧化物Nd2O3聚合物分散液晶(PDLC)在实验给定驱动电压范围20 V~30 V以及在紫外335 nm~375 nm范围内的透射率,给出了符合此实验数据的函数关系,即为一个正弦函数之和的形式。拟合结果中最小相关系数(R)为0.999 8,最大均方误差(RMSE)为0.057。另一方面,通过对吸收带峰值的位置和趋势随着驱动电压的变化进行数学拟合,得到2个数学函数,函数关系为二次多项式的形式。拟合结果的和方差(SSE)分别为1.292e-026和3.944e-030,相关系数均为1。拟合结果表明：数学拟合结果和实验数据接近吻合,能够为进一步研究掺杂纳米Nd2O3聚合物分散液晶(PDLC)在20 V~30 V驱动电压下的紫外电光特性提供一定的理论依据。     

Birefringent hydrogels based on PAAm and lyotropic liquid crystal: Optical, morphological and hydrophilic characterization

In this work, hydrogels of polyacrylamide (or PAAm) with confined lyotropic liquid crystal (potassium laurate-decanol-water, KL-DeOH-H₂O) (or LLC) were synthesized. The hydrogels were characterized by polarized optical microscopy (POM), refractometry, optical transmission, scanning electron microscopy (SEM) and small angle X-ray scattering (SAXS). Besides these techniques, the hydrophilicity of hydrogels was characterized by the degree of swelling. Based on POM, it was observed that the texture of the birefringent hydrogels obtained depends on their cross-linking density, and that it is formed soon after hydrogel synthesis. Refractometry results indicated an behavior antagonist to that obtained for the system constituted by thermotropic liquid crystal inserted into the PAAm lattice in relation to the dependence of Δn on the AAm concentration and the optical transmittance. SEM micrographs show that birefringent hydrogels present rougher surface when compared to the surface of PAAm hydrogels. For the same AAm concentrations, it was observed that the hydrogels with confined LLC present larger swelling values (Q) when compared to those of PAAm hydrogels. The loss of water by birefringent hydrogels is twofold slower than that of PAAm hydrogels. Hydrogels formed by PAAm and lyotropic liquid crystal synthesized in this work can be potentially used in optical devices.     

Effect of electric field on phase separation of polymer dispersed liquid crystal

The kinetics of the polymerization induced phase separation of liquid crystal (LC)/monomer mixture has been investigated by means of depolarized light intensity technique and polarized light microscope (PLM). To examine the effect of the electric field, a DC electric field was applied across the mixtures during the phase separation process. The kinetic study indicates that the phase separation process is accelerated when the electric field is applied. The morphologies of the formed polymer dispersed liquid crystal (PDLC) films were observed by PLM. The electric field applied during the phase separation process yields the PDLC with small LC domains and fine morphologies. The clearing temperature (T_NI) of the formed PDLC films was measured by the PLM and it is found that the T_NI increases with the applied electric field intensity.     

Reduced OFF-axis haze in polymer-dispersed liquid crystals

Polymer-dispersed liquid crystals (PDLCs) are composite materials formed by micron-sized droplets of liquid crystals (LCs) dispersed in a polymer matrix, which can be turned from an opaque state to a transparent one by application of a suitable electric field. PDLCs have been proposed in applications related to the control of light transmittance on large surfaces (light shutters, displays, rear mirrors). Despite several advantages, PDLCs’ main drawback is haze, i.e. the fast decay of transmission at large viewing angles. In this paper, a method for achieving highly transparent PDLC devices over a wide range of viewing angles is proposed. The method is based on the use of PDLCs with tilted elongated LC droplets and driven by opportune electric fields, which are experimentally calculated and able to ensure an almost constant value for OFF-axis transmittance.