乙基醋酸纤维素溶致性液晶的研究

乙基醋酸纤维素溶于三氟乙酸、二氯乙酸、醋酸中.在浓度大于临界浓度时,形成胆甾型液晶态.溶液随浓度的增加,从各向同性态经两相共存转变成液晶态.在加热液晶溶液时,溶液转变成各向同性态.降低温度,液晶相再生成.在形成液晶过程中,存在过冷现象.在一定的条件下,液晶相以球状的形式存在.溶液的双折射△n在两相共存与完全液晶态间相互转变时会发生较大变化.乙基醋酸纤维素大分子链的刚性随有机酸溶剂体系的酸强度的增大而增大,使临界浓度C_1~*随溶剂酸强度的增大而减小.     

Mesomorphic solutions of cellulose triacetate in halogenated organic acids and mixtures of trifluoroacetic acid and dichloromethane

Cellulose triacetate (CTA) forms liquid-crystalline solutions in trifluoroacetic acid, dichloroacetic acid, and mixtures of trifluoroacetic acid and dichloromethane. Brilliant iridescent coloration, high optical rotations, and birefringent regions with swirl-like fingerprint patterns suggested the formation of cholesteric liquid crystals. Trifluoroacetic acid–dichloromethane is a particularly excellent solvent mixture to form liquid-crystal solutions of cellulose acetate. Crude bulk viscosity measurements confirm the formation of an anisotropic phase, and the temperature dependence of the critical concentration for formation of the anisotropic phase, obtained by bulk viscosity measurements at various temperatures, provides confirming evidence. Viscosity decreases with aging, suggesting that CTA is slowly degraded in the solvents studied. This is also confirmed by optical rotatory measurements and by polarizing microscope observations. The miscibility gap is observed as a function of molecular weight, and the differences between the experimental data and Flory's prediction are discussed.     

Morphological characteristics of the lyotropic and gel phases in the cellulose/NH3/NH4SCN system

Solutions of cellulose in the ammonia/ammonium thiocyanate (24.5/75.5 w/w) solvent form several stable phases. Of particular interest in this work are the temperature-dependent liquid crystalline and gel phases which are stable at cellulose concentrations above 6% w/v. While the temperature-composition conditions yielding these phases are reasonably well established, very little is currently known about the morphological characteristics of lyotropic and gelled cellulose. Polarized light microscopy is employed here to demonstrate that solutions at temperatures above the gel melting point are birefringent, composed of liquid crystals. Field-emission scanning electron microscopy has been used to (i) examine the three-dimensional network in cellulose gels, and (ii) correlate network morphology with cellulose molecular weight and solution concentration. Results obtained from two complementary sample preparation techniques (i.e., critical point drying and freeze drying) are compared to identify and minimize artifacts, and reveal that gel formation occurs as the solutions phase-separate into polymer-rich anisotropic and solvent-rich isotropic phases. The polymer-rich phase is highly interconnected and forms a fibrillar network, with fibrils measuring 20–70 nm in diameter. © 1996 John Wiley & Sons, Inc.     

Solution properties of cellulose triacetate. II. Solubility and viscosity studies

The solubility of cellulose triacetate in a range of solvents was measured, and the results for tetrachloroethane, chloroform, and acetic acid were compared with those from initial phase separation in solvent–nonsolvent mixtures and viscosity–concentration studies. The correlation found between solubilities, precipitation values, and values of the Huggins viscosity constant is discussed with reference to the type of polymer–solvent interaction proposed previously to explain fractionation behavior. A qualitative comparison of solubility–swelling behaviour was also made for a very low molecular weight cellulose triacetate sample in a wide range of solvents. Results are compared with those for higher molecular weight samples and discussed with regard to the cohesiveenergy densities of solvent and polymer. Some attempt has been made to predict suitable solvents for cellulose triacetate, based on consideration of their molecular structures.     

Thermoreversible gelation of cellulose acetate solutions studied by differential scanning calorimetry

Thermoreversible gels of cellulose acetate can be obtained by cooling concentrated cellulose acetate solutions in solvent-nonsolvent mixtures of dioxane and water. Upon heating the gels, endothermic effects were observed with differential scanning calorimetry. The heat effects are ascribed to the melting of a crystalline phase consisting of cellulose triacetate units. The endothermic peaks appear only after long aging periods of up to several days. Melting points generally decrease and heats of melting increase with increasing polymer concentration and with increasing nonsolvent content. The maximum degree of crystallinity is estimated as 8%. The kinetic effects of varying the water content in the solvent mixture are discussed.     

Solution properties of cellulose triacetate. I. Fractional precipitation

Using petroleum ether as a precipitant and chloroform–acetone mixture as solvent, six fractions of cellulose triacetate were obtained by fractional precipitation. The molecular weight of each fraction was obtained from osmotic pressure measurements carried out on chloroform solutions. It was shown that virtual nonfractionability of cellulose triacetate with regard to molecular weight occurred in this system. This behavior was also observed in attempts to fractionate it from tetrachloroethane or acetic acid solutions. These results are explained by the hypothesis that specific polymer–solvent interaction takes place due to hydrogen bonding.     

Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation

Suspensions of rod-like cellulose crystallites of axial ratio ≈ 20-40, prepared by acid hydrolysis of natural cellulose fibres with sulphuric acid, give stable ordered fluids that display well-formed textures and disclinations characteristic of chiral nematic liquid crystalline phases. The critical volume fraction for phase separation of salt-free suspensions is typically 0.03, with a relatively narrow biphasic region. Because of the negative diamagnetic susceptibility of cellulose, the ordered phase becomes oriented in a magnetic field with its chiral nematic axis parallel to the applied field.     

Structural investigations of polymer liquid-crystalline solutions: Aromatic polyamides,hydroxy propyl cellulose,and poly(γ-benzyl-L-glutamate)

A comparative structural investigation of the characteristics of polymer liquid-crystalline solutions including Kevlar® (PPD-T)/sulfuric acid, poly(Cl-p-phenylene terephthalamide) (Cl-PPD-T)/sulfuric acid, poly(γ-benzyl-L-glutamate) (PγBLG)/dioxane, and hydroxypropyl cellulose (HPC)/water was undertaken. Experimental procedures included polarized light microscopy, light scattering, absorption spectra, and x-ray diffraction on solutions at various concentrations and temperatures. Both the two-phase region at the onset of liquid-crystal formation and the wholly anisotropic phase were investigated. Each solution exhibited distinctive characteristics. The PPD-T and Cl-PPD-T solutions were nematic, and the PγBLG and HPC solutions were cholesteric. In the two-phase region the PPD-T, Cl-PPD-T, and PγBLG (but apparently not the HPC) exhibited negatively birefringent spherulites and aggregates of spherulites. The HPC solutions only exhibited spherulitic structures in the single-phase anisotropic system. The structures and orientations in the anisotropic phase for the various polymer solutions is considered. The helicoidal structural characteristics of the PγBLG and HPC solutions are contrasted.     

Lyotropic mesophase of cellulose in ammonia/ammonium thiocyanate solution

Solutions of cellulose in a mixture of 27:73 (w/w) of liquid ammonia and ammonium thiocyanate become liquid crystalline at room temperature above a certain critical concentration which depends on the degree of polymerization of the dissolved cellulose. The high optical rotations of the solution suggest that the cellulose mesophase is cholesteric in nature. In the two-phase region, the cellulose solutions exhibit negatively birefringent spherulites that possess both ringed and nonringed internal structures. The anisotropic solutions can be oriented by shear, indicating high potential for spinning them into useful fibers.     

乙基氰乙基纤维素溶致性液晶的研究

纤维素和纤维素衍生物在适当的溶剂中可以形成溶致性液晶, 乙基氰乙基纤维素是纤维素的一种混合醚类衍生物, 本文研究了其溶致性液晶的形成, 结构以及不同溶剂对形成液晶所产生的影响.     

Multivariable structural characterization of semicrystalline polymer blends by small‐angle light scattering

A new multi‐variable‐measurement approach for characterizing and correlating the nanoscale and microscale morphology of crystal‐amorphous polymer blends with melt‐phase behavior is described. A vertical small‐angle light scattering (SALS) instrument optimized for examining the scattering and light transmitted from structures ranging from 0.5 to 50 μm, thereby spanning the size range characteristic of the initial‐to‐late stages of thermal‐phase transitions (e.g., melt‐phase separation and crystallization) in crystal‐amorphous polymer blends, was constructed. The SALS instrument was interfaced with differential scanning calorimetry (DSC), and simultaneous SALS/DSC/transmission measurements were performed. We show that the measurement of transmitted light and SALS under H_V (cross‐polarized) optical alignments during melting can be used to reliably measure the thermodynamic (e.g., crystal melting and melt‐phase separation temperatures) and structural variables (e.g., crystalline fraction within the superstructures and volume fraction of superstructures) necessary for describing the multiphase behavior of crystal‐amorphous blends in one combined measurement. We also evaluate the orientation correlations of crystalline volume elements within the superstructures. Our results indicate that simultaneous measurement of transmitted light can provide a reliable estimate of the total scattering from density and orientation fluctuations and the melt‐phase separation temperature of polymer blends. For solution‐cast poly(?‐caprolactone)/poly(D,L‐lactic acid) blends, our multivariable measurements during melting provide the parameters necessary to generate a crystal–liquid and liquid–liquid phase diagram and characterize the solid‐state morphology. This opens up the challenge to explore use of our vertical SALS instrument as a rapid and convenient method for developing structure–property relationships for crystal‐amorphous polymer blends. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2714–2727, 2002     

氰乙基纤维素溶致性液晶的研究

氰乙基纤维素在二甲基甲酰胺、二甲基乙酰胺、二甲基亚砜、乙腈以及丙酮等溶剂中可以形成溶致性液晶。随浓度增加,溶液从各向同性状态经两相共存态转变成为完全的液晶态。升高温度到T_c,液晶相消失;降低温度到T′_c,液晶相再生成。T_c总大于T′_c。而且,浓度越高,过冷温度△T=T_c—T′_c越小。在各向同性,两相共存或完全的液晶状态,溶液平均折射率和消光度均与浓度呈线性关系。但在两相间相互转变时,即在C_1~*和C_2~*处,n-C和 A-C 曲线上出现转折点。高聚物与溶剂的相互作用参数X_(12)愈小,临界浓度C_1~*愈小。把描述大分子链柔顺性的参数f与X_(12)联系起来,可用 1956年 Flory的理论定性地解释溶剂对高聚物溶致性液晶形成的影响。     

Phase behavior of lyotropic rigid-chain polymer liquid crystal studied by dissipative particle dynamics

The phase behavior of lyotropic rigid-chain liquid crystal polymer was studied by dissipative particle dynamics (DPD) with variations of the solution concentration and temperature. A chain of fused DPD particles was used to represent each mesogenic polymer backbone surrounded with the strongly interacted solvent molecules. The free solvent molecules were modeled as independent DPD particles, where each particle includes a lump of solvent molecules with the volume roughly equal to the solvated polymer segment. The simulation shows that smectic-B (S(B)), smectic-A (S(A)), nematic (N), and isotropic (I) phases exist within certain regions in the temperature and concentration parameter space. The temperature-dependent S(B)∕S(A), S(A)∕N, and N∕I phase transitions occur in the high concentration range. In the intermediate concentration range, the simulation shows coexistence of the anisotropic phases and isotropic phase, where the anisotropic phases can be the S(B), S(A), or N phases. Mole fraction and compositions of the coexisted phases are determined from the simulation, which indicates that concentration of rigid rods in isotropic phase increases as the temperature increases. By fitting the orientational distribution function of the systems, the biphasic coexistence is further confirmed. From the parameter α obtained for the simulation, the distribution of the rigid rods in the two coexistence phases is quantitatively evaluated. By using model and simulation methods developed in this work, the phase diagrams of the lyotropic rigid-chain polymer liquid crystal are obtained. Incorporating the solvent particles in the DPD simulation is critical to predict the phase coexistence and obtain the phase diagrams.     

4. Characteristics of cellulose acetates 4.1 Characterization and physical properties of cellulose acetates

An overview is provided of the basic features of cellulose acetate of various degree of substitution in the solid and liquid crystalline state as well as in solution. These features represent a necessity for an understanding of the properties of these cellulose derivatives and further for mixed esters, which are not presented in this paper. Specifically, the crystal structure of cellulose triacetate will be addressed as well as structures in dilute and semi-dilute solutions. Thermal, viscoelastic and further properties in the solid state are discussed as well as flow behavior of solutions and their application in molecular weight determination, including false viscosity of commercial 2.5 cellulose acetates.     

Light-scattering studies on solutions of cellulose in the ammonia / ammonium thiocyanate solvent system. II. Quasielastic light-scattering and liquid crystal study

This is the second part of a two–part study of the NH₃NH₄SCN cellulose solvent system. Quasielastic light scattering was used to determine the diffusion coefficients of cellulose in solution and the effective hydrodynamic radius of the dissolved molecules. Additionally, the system was studied using light microscopy to determine the minimum critical volume fraction or liquid crystal formation. Very little change was found in the diffusion coefficients with change in cellulose concentration indicating little interaction between the chains in solution. Values of 7.69 and 2.66 × 10⁸ cm²/s were measured for samples having a degree of polymerization of 153 and 969. The value of the coefficient relating the hydrodynamic volume to the radius of gyration was found to be in the range of 0.33 to 0.53, indicating an extended coil conformation according to the Kirkwood-Riseman theory. The minimum critical volume fractions necessary for liquid crystal formation, υ₂′ were 0.039, 0.038, and 0.048 for the three solvent compositions studied. The values calculated for υ₂′ based on the measured persistence lengths were much larger than the predicted values, indicating strong deviation from theory or possible aggregation in the system.     

LYOTROPIC LIQUID CRYSTALLINE BEHAVIOR OF FIVE CHITOSAN DERIVATIVES*

Five chitosan derivatives, i.e. O-butyryl chitosan, O-benzoyl chitosan, N-phthaloyl chitosan, N-maleoyl chitosan and O-cyanoethyl chitosan, were prepared from chitosan. All of them had better solubilitythan chitosan, and demonstrated lyotropic liquid crystalline behavior in various solvents. The critical liquidcrystalline behavior of three O-substituted chitosan derivatives was evidently different from two N-substituted analogues. Typical fingerprint textures of cholesteric phase were only observed in three O-substituted derivatives. The critical concentration (v/v%) of three O-substituted derivatives does not dependon the acidity of acidic solvents.     

Scattering from polymer films and its application to optical properties of blends and display devices

The application of the technique of observing the light scattering from solid polymers for the study of amorphous phase separation, spherulitic crystallization, phase separation in crystalline polymers, and coarsening of liquid crystal polymers is reviewed. The effect of shear and electrical and magnetic fields on liquid crystal orientation is discussed and its application for constructing light control and display devices is considered.     

Morphology of polymer networks polymerized in highly ordered liquid crystalline phases

The morphology and optical properties of polymer stabilized liquid crystals formed in a more highly ordered low molecular weight liquid crystal solvent were studied. Tetrafunctional, mesogenic monomers (with and without flexible spacers) were polymerized in isotropic, nematic and smectic phases of the LC solvent (4′-octyl-4-cyanobiphenyl) and studied with scanning electron microscopy and cross-polarized light microscopy. The network morphology of the nematic and isotropic phase polymerizations showed strong similarities with the corresponding polymerizations in other solvents. Polymerization in the smectic phase, however, resulted in marked increases in network order and directionality. Most dramatically, even the polymer without flexible spacer formed a fibrous network of rodlike units, in contrast to the random, beaded texture formed by the same polymer in nematic or isotropic conditions. Correspondingly, a large increase in birefringence demonstrated significant polymer orientation and more effective orientational interaction with the liquid crystalline solvent.     

Hydrogels from phospholipid vesicles

It is shown that phospholipid dispersions with a few percent of diacylphosphocholine PC in water can be swollen to single-phase lyotropic liquid crystalline L_α-phases by the addition of co-solvents like glycerol, 1,3-butyleneglycol BG or 1,2-propyleneglycol PG. The birefringent L_α-phases contain small unilamellar and multilamellar vesicles if the temperature of the samples is above the Krafft-Temperature T_m of the phospholipid. When such transparent birefringent viscous samples are cooled down below T_m the samples are transformed into birefringent gels. Cryo-TEM and FF-TEM measurements show that the bilayers of the vesicles are transformed from the liquid to the crystalline state during the transformation while the vesicle structure remains. The bilayers of the crystalline vesicles form adhesive contacts in the gel. Pulsed-field gradient NMR measurements show that two different kinds of water or co-solvent can be distinguished in the gels. One type of solvent molecules can diffuse like normal solvent in a continuous bulk phase. A second type of water diffuses much more slowly. This type of solvent is obviously trapped in the vesicles. The permeability of the crystalline vesicles for water and solvent molecules is much lower in the crystalline state than in the fluid state.     

Resolution of enantiomers by HPLC on cellulose derivatives

Summary Various polysaccharide derivatives, particularly cellulose derivatives, were synthesized and used as chiral stationary phases for optical resolution by HPLC after being adsorbed on macroporous silica gel. Cellulose triacetate (CTA-II), which was synthesized under homogeneous conditions, showed a chiral recognition ability for many racemates. Other cellulose derivatives such as cellulose tribenzoate (OB), cellulose-trisphenylcarbamate (OC), cellulose tribenzyl ether (OE), and cellulose tricinnamate (OK) also showed unique chiral recognition. Among other polysaccharide derivatives, curdlan triacetate was also exhibited an effective chiral recognition. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984