气液色谱法研究C8芳烃在N-(对乙氧基苄叉)对氨基苯甲腈液晶中的溶液热力学: 液晶的相变

在不同温度下测量了C_8芳烃在N-(对乙氧基苄叉)对氨基苯甲腈向列相及各向同性相中的活度系数、过量偏摩尔焓及过量偏摩尔熵。引入无因次量σ和τ以处理实验数据,发现所研究的体系,处于相同的τ下,近似具有相同的σ。     

气相色谱法研究某些异构物在对苯二酚芳羧酸酯类液晶中的溶液热力学

在不同温度下测量了二甲苯异构体、二氯苯异构体以及联苯和二苯甲烷在三种对苯二酚芳羧酸双酯类液晶向列相中的活度系数、偏摩尔过最焓和偏摩尔过量熵, 所测结果按溶液统计理论的观点作了定性的解释。     

色谱峰形参数σ、τ与保留值间线性关系的新探讨

本文从质量平衡理论出发考察了色谱参数对柱末端谱带的σ、τ值的影响。结果表明柱末端以长度为量纲的σ、τ值在不同的k′下基本为常数,柱末端的σ值较τ值为大,这也说明在色谱柱内的色谱谱带也并非是完全对称的高斯峰。而柱外因素对σ的影响比对τ的影响要小得多。本文认为导致色谱图上以时间为量纲的σ、τ值与保留值间的线性关系主要由柱末端过程所产生。     

气液色谱法研究醇类在芳烃中的缔合溶液热力学

用气液色谱法测量了不同温度下C~1-C~4醇类的各异构物在二苯甲烷、联苄、二苯乙炔、二苯甲酮及二苯亚砜中的无限稀活度系数γ~i、偏摩尔过量焓H^E~i及偏摩尔过量熵S^E~i。用Kretschmer-Wiebe 模型计算各醇类在二苯甲烷、联苄、二苯乙炔中的自身缔合常数K~A, 得到在同一溶剂中, C~1-C~4醇类, K~A减小次序。用同一模型计算各醇类在二苯甲酮、二苯亚砜中的交叉缔合常数K~A~B, 得到在同一溶剂中, K~A~B减小次序。     

聚氧乙烯与非等摩尔正负离子表面活性剂混合体系的相互作用

通过黏度、动态光散射和透射电镜方法研究了聚氧乙烯(PEO)对正负离子混合表面活性剂癸基磺酸钠(C₁₀SO₃)-癸基三乙基溴化铵(C₁₀NE)聚集体的影响. 实验结果表明, 加入PEO后, 非等摩尔C₁₀SO₃-C₁₀NE混合物的聚集体大小没有明显改变, 只有等摩尔混合物的聚集体发生了明显的长大, 且仍为囊泡, 说明PEO只对囊泡体系有显著的影响, 对胶团体系则没有明显的影响. 在接近等摩尔混合时, 混合体系中C₁₀SO₃与C₁₀NE过量程度相同的情况下, PEO对C₁₀NE过量的体系影响更大. PEO对囊泡体系的影响主要表现为PEO通过对囊泡的“空缺絮凝”作用(Depletion interaction)使囊泡长大, 而对于非等摩尔混合的胶团溶液, 由于胶团之间存在较强的静电排斥作用而无法发生聚集. PEO对C₁₀NE过量的体系影响更大, 主要是由于C10NE过量体系中的聚集体比C10SO3过量体系中的更接近于等摩尔组成.     

气液色谱法测量醇类在惰性溶剂中的无限稀热力学函数

用气液色谱法测量了在不同温度下C_1～C_4醇类的各种异构物在C_(16)～C_(23)正构烷烃、角鲨烷,和角鲨烯中的无限稀活度系数γ_i,偏摩尔过量焓、偏摩尔过量熵。在各种溶剂中γ_i、均大于1,在同一溶剂中γ_i依下列次序减小: 甲醇>乙醇>正丙醇>正丁醇; 正丙醇>异丙醇; 正丙醇>异丁醇>仲丁醇>叔丁醇同一种醇在角鲨烯中的γ_i较在角鲨烷中为低。异构醇类的低于正构醇类。所测的有随的增加而增加的趋势。     

甲壳素类液晶高分子的研究V.取代基个数及长度对羟乙基壳聚糖液晶性的综合影响

采用碱壳聚糖的方法合成了一系列具有不同摩尔醚化度 (DME)的新的液晶性壳聚糖衍生物羟乙基壳聚糖 (HECS) .用酸氧化蚀刻的方法 ,在扫描电子显微镜 (SEM )下证实了其胆甾型的液晶织构 .以甲酸为溶剂 ,研究了摩尔醚化度对羟乙基壳聚糖溶致液晶性的影响 .结果表明 ,在该体系中 ,摩尔醚化度能综合取代基个数及长度两个结构因素的影响 ,当DME较低时 (<～ 2 0 ) ,这种影响关系类似于取代基个数 (即取代度 )的影响 ,当DME较高时 (>～ 2 0 ) ,则类似于取代基长度的影响 .前者对液晶临界浓度的影响很小 ,而后者却有显著影响 .此外还观察到当DME >～ 2 0时出现水溶性 ,HECS的水溶液也呈现典型的胆甾指纹状织构     

功能高分子膜离子选择电极研究 Ⅶ.压片PVC膜氯离子选择电极活化过程动力学机理的探讨

本文研究了以干放后的功能高分子为活性物的压片PVC膜氯离子选择电极与溶液相接触时电阻变化的行为。膜电导随时间变化符合下式σ(t)=σ_1+(σ_0-σ_1)(1-(e)t/τ)其中,σ为膜电导,τ为时间常数,t为电极浸入溶液的时问。当t<τ时,σ(t)=σ＇_1+Kt~(1/t) K为常数由上述公式计算所得值与实验值相吻合。膜电导的上述行为可用离子在膜中的扩散解释,还可用电荷对电极等效线路的充电解释。     

液相色谱动力学过程的研究——色谱参数对σ、τ与保留值间的线性关系的考察

本文在C、Horvath建立的液相色谱柱内质量平衡的基础上考虑到柱外效应的影响,建立起整个色谱系统的质量平衡模型、并求得了色谱系统的一阶距,二、三阶中心距的表达式。同时,通过指数修正的高斯模型中描述色谱峰展宽的参数σ和拖尾因子τ与二、三阶中心距的关系,详细地考察了色谱参数D_m,k_d,d_p和检测池体积对我们以所报导的c、 与保留值之间线性关系的影响。结果表明在一般高效柱液相色谱系统中,σ、τ与保留值间线性关系是成立的,但在K′值小于0.5时与线性关系有一定的偏差;柱外效应对τ的影响比对σ的影响大得多。     

溶剂化的热力学集团展开理论

把二元溶液的过剩内能（excess energy）分成溶剂-溶剂、溶剂-溶质及溶质-溶质相互作用部分。利用集团展开方法给出了二元溶液在正则系综的配分函数的表达式,利用该表达式得到了溶质的偏摩尔内能(partial molar energy)和偏摩尔熵(partial molar entropy)的表达式。在无限稀溶液情形,过剩偏摩尔内能的溶剂-溶剂部分又称重组织内能（reorganization energy）,它反映了溶质存在时对其周围溶剂分子之间的相互作用能的影响。研究表明,在溶质的粒子数密度相对较大时,溶质分子之间的相互作用将影响过剩偏摩尔内能的溶剂-溶剂部分,对于稀溶液,过剩偏摩尔内能的溶剂-溶剂部分与溶质的摩尔分数成线性关系。对低密度二元溶液,溶质的过剩偏摩尔内能和过剩偏摩尔熵也与溶质的摩尔分数成线性关系。     

Sulfur-linked cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of two series of cyanobiphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 4?-[1,ω-alkanediylbis(thio)]bis-[1,1?-biphenyl]-4-carbonitriles (CBSnSCB), and 4?-({ω-[(4?-cyano[1,1?-biphenyl]-4-yl)oxy]alkyl}thio)[1,1?-biphenyl]-4-carbonitriles (CBSnOCB) are described. The odd members of both series show twist-bend nematic and nematic phases, whereas the even members exhibit only the nematic phase. An analogous cyanoterphenyl-based dimer, 3⁴-{6-[(4?-cyano[1,1?-biphenyl]-4-yl)thio]-hexyl}[1¹,2¹:2⁴,3¹-terphenyl]-1⁴-carbonitrile (CT6SCB), is also reported and shows enantiotropic N_TB and N phases. The transitional properties of these dimers are discussed in terms of molecular curvature, flexibility and biaxiality. The same molecular factors also influence the birefringence of nematic phases. Resonant X-ray scattering studies of the twist-bend nematic phase at both the carbon and sulfur absorption edges were performed, which allowed for the determination of critical behaviour of the helical pitch at the transition to the nematic phase, the behaviour was found to be independent of molecular structure. It was also observed that despite the different molecular bending angle and flexibility, in all compounds the helical pitch length far from the N-N_TB transition corresponds to 4 longitudinal molecular distances.     

Schiff碱类向列液晶熔融与清亮过程热力学

用差示扫描量热法及热台显微镜法研究了Schiff碱类向列液晶PEBAB及PBAPB的熔融与清亮过程.给出PEBAB的Tm=378.53K,△Hm=26.48±0.20kJ/mol,△Sm=69.96±0.53J/K.mol,Tc=399.80K,△Hc=929±53J/mol,△Sc=2.32±0.13J/K.mol;对于PBAPB,Tm(II)=352.86K,Tm(I)=354.74K,Tc=384.54K,△Hc=866±25J/mol,△Sc=2.25±0.07J/K.mol.在通过二次加热法消除熔前效应后,得到△Hm(I+II)=12.36±0.17kJ/mol,它不随升温速率改变而变化.本工作测得的清亮焓,在3%的偏差范围内符合平均场理论要求;△H1/△H2=T1/T2;所得清亮熵也与大多数向列型液晶相近.在PBAPB中,在318K左右,还观察到固→固相变等情况,相变焓是13.96±0.35kJ/mol.     

Anchoring of a nematic liquid crystal on a wettability gradient

We have studied the anchoring of the nematic liquid crystal 5CB (4'-n-pentyl-4-cyanobiphenyl) as a function of the surface wettability, thickness of the liquid crystal layer, and temperature by measuring the birefringence of a hybrid aligned nematic cell where the nematic material was confined between octadecyltriethoxysilane-treated glass surfaces, with one surface linearly varying in its hydrophobicity. A homeotropic-to-tilted anchoring transition was observed as a function of the lateral distance along the hydrophobicity gradient, typically in a region corresponding to a water contact angle of approximately 64 degrees. The effect of the nematic layer thickness was measured simultaneously by preparing a wedge cell where the thickness varied along the direction perpendicular to the wettability. The detailed behavior of the onset of birefringence was found to be consistent with a dual-easy-axis model that predicts a discontinuous anchoring transition from homeotropic to planar. The anchoring was independent of temperature, except within 1 degrees C of the nematic-to-isotropic transition temperature (T(NI)). As the temperature approached T(NI), the tendency for planar anchoring gradually increased relative to that for homeotropic anchoring.     

Observation of a chevron hybrid structure in the smectic A phase of a liquid crystal device

Small angle X-ray diffraction experiments show the emergence of a chevron structure on cooling from the nematic phase into the smectic A phase of the commercial mixture S3 (Merck Ltd, UK), in a display device. The chevron angle increases from 0 at the nematicsmectic transition temperature, T NS, to a maximum value of 8.5 , which is reached when the sample is 15 C below T NS. Between 5 C and 15 C below the transition temperature a quasi bookshelf structure emerges, in addition to the prevailing chevron structure; such a structure has not been hitherto reported. There is no further resolvable change in the device structure on cooling lower than T NS -15 C. The chevron structure is due to the combination of layer thinning and fixed surface layers, as confirmed by a comparison of layer spacing calculated from the Bragg angle equation and from the layer thinning equation ( d = d nematic cos delta, where delta is the chevron angle).     

New low viscosity liquid crystal compounds containing the 2,3,4-trifluorophenyl moiety for active matrix displays

The synthetic route and physical properties for new three-ring derivatives, trans -4- n -alkyl-4'-(2,3,4-trifluorophenyl)bicyclohexanes ( n CCHB3F) with alkyl chain length n from 2 to 5 are presented. They exhibit broad range nematic phases exceeding 40 K, and low viscosity with a low activation enthalpy; they also show low dielectric anisotropies and high birefringence. Their dielectric properties are compared with those of their three-ring homologues with a lateral -NCS group. The Maier-Meier theory is used to evaluate the temperature dependence of the value of 'dielectric anisotropy divided by order parameter ( Δ ε/ S )'. The temperature dependence of Δ ε/ S in several phenylbicyclohexane homologues is explained. Furthermore, the viscosity of nematic liquid crystal mixtures containing these materials is significantly reduced. These mixtures are suitable for the improvement of the switching time of liquid crystal displays using TFT Twisted Nematic (TN) modes.     

Influence of the chain length on the nematic-to-isotropic phase transition for the odd members of a highly non-symmetric pyrene-based series of liquid crystal dimers

This paper describes a detailed study of the nematic (N)-isotropic (I) phase transition in the homologous series of liquid crystal dimers, the α-(4-cyanobiphenyl-4?-oxy)-ω-(1-pyreniminebenzylidene-4?-oxy)alkanes (CBOnO.Py) by means of calorimetric and dielectric measurements as a function of temperature. It is concluded that for this transition, the latent heat or the entropy change decreases as the chain length of the odd dimers decreases, and this decrease is consistent with the observed tricritical behaviour.     

Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of several members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkane (CBnCB) and the 1-(4-cyanobiphenyl-4′-yloxy)-ω-(4-cyanobiphenyl-4′-yl) alkane (CBnOCB) homologous series are reported. The new odd members described CB5CB, CB13CB, CB4OCB, CB8OCB and CB10OCB all exhibit twist-bend nematic and nematic phases. The members of these series already reported in literature, CB7CB, CB9CB, CB11CB and CB6OCB, were also prepared in order to allow for a direct comparison of their transitional properties. The properties of these dimers are also compared to those of the corresponding members of the 1,ω-bis(4-cyanobiphenyl-4,-yloxy) alkanes (CBOnOCB). For any given total spacer length, for odd members of these series, the nematic–isotropic transition temperatures and associated entropy changes are greatest for the CBOnOCB dimer and lowest for the CBnCB dimer. These trends are understood in terms of molecular shape. For short spacer lengths, the twist-bend nematic–nematic transition temperature (T_NTBN) is higher for the CBnOCB series than for the CBnCB series but this is reversed as the spacer length increases. Of the CBOnOCB dimers, a virtual value of T_NTBN was estimated for CBO3OCB and T_NTBN was measured for CBO5OCB. These values are considerably lower than those observed for the corresponding members of the CBnCB or CBnOCB series. The dependence of T_NTBN on molecular structure is discussed not only in terms of the molecular curvature but also in the ability of the molecules to pack efficiently. As the temperature range of the preceding nematic phase increases, so the twist-bend nematic–nematic transition entropy change decreases and the transition approaches second order for the longer spacers. For comparative purposes, the transitional behaviour of the even-membered dimers CB6CB, CB5OCB and CBO4OCB is reported and differences accounted for in terms of molecular shape.     

Crystallisation of nematic liquid crystal mixtures

In this paper, we propose a method to accelerate the crystallisation of nematic liquid crystal mixtures based on crystallisation theory. This method is to hold a nematic liquid crystal sample at a temperature suitable for crystal growth after aging it at a temperature suitable for nucleation. After we specified these temperatures of a nematic liquid crystal mixture using differential scanning calorimetry, we demonstrate that the two-temperature aging method is effective for the crystallisation of other nematic liquid crystal mixtures in which the crystal-liquid crystal transition temperature has so far been undetectable.     

Theory of nematic–smectic phase separation in thin twisted liquid crystal cells

Recently it has been shown experimentally by the authors that a highly twisted thin nematic cell at low temperatures can separate into a smectic A region in the middle of the cell surrounded by twisted nematic layers at the boundaries. In this case the twist is expelled into the nematic layers and the nematic–smectic A transition temperature is strongly depressed. We present a thermodynamic theory of such a phase transition in a twisted nematic cell, taking into account that the smectic A slab inside the nematic cell can be stable only if the decrease of free energy in the smectic region overcomes the increase in distortion energy of the twist deformation in the nematic layers plus the energy of the nematic–smectic A interface. In such a system the equilibrium thickness of the smectic A slab corresponds to the minimum of the total free energy of the whole cell, which includes all the bulk and surface contributions. Existing experimental data are at least qualitatively explained by the results of the present theory. This opens a unique possibility to study the properties of the nematic–smectic interface which is perpendicular to the smectic layers.     

Nematic-isotropic interfacial tension anisotropy of a calamitic lyotropic liquid crystal

In a planar oriented sample of a calamitic nematic lyotropic system (mixture of sodium lauryl sulphate/water/decanol), isotropic pretransitional domains appear at the nematic to isotropic transition. The domains are oblong in shape with the long axis along the orientational direction. We show experimental evidence that this oblong shape is determined by the nematic-isotropic interfacial tension anisotropy. Two uniparametric models of simple angular dependences for the interfacial tension are tested. Using the differential system obtained from the Young-Laplace condition at the nematic-isotropic interface, the domain shape can be numerically calculated for each value of the interfacial tension anisotropy. By processing the values of the transmitted light through both an isolated isotropic domain and its surrounding nematic zone, we obtain the anisotropy of the interfacial tension as the main fitting parameter. An estimation of the ratio of the extreme values for the interfacial tension is given.