Phase transitions of hydroxypropylcellulose liquid-crystalline solutions in magnetic field

The phase transitions and the phase state of hydroxypropylcellulose-DMAc and hydroxypropylcellulose-ethanol solutions both under an applied magnetic field and in its absence have been studied via the cloud-point method, polarization microscopy, and polarization-photoelectric measurements. The magnetic field changes the structure of solutions and increases the phase transition temperature. The higher the field strength, the more pronounced this effect. As the molecular mass of the polymer grows, the ability of its macromolecules to orient in the magnetic field tends to increase. Hydroxypropylcellulose solutions fall into the family of memory systems. When the magnetic field is switched off, the orientation of macromolecules and the increased phase transition temperature are preserved for many hours.     

Phase transitions in solutions of hydroxypropoyl cellulose and cyanoethyl cellulose

LC phase transitions in mixtures of cyanoethyl cellulose and hydroxypropyl cellulose with different solvents have been studied by the cloud point and polarization microscopy methods and polarization photoelectric measurements. It has been found that, as the solvent polarity is decreased and the molecular mass of a polymer is increased, boundary curves delimiting the regions of isotropic and anisotropic solutions shift to the region of lower concentrations.     

Phase transitions of liquid crystalline cyanoethyl cellulose solutions under static conditions and in shear field

Phase transitions in the systems cyanoethyl cellulose-DMF, cyanoethyl cellulose-DMAA, and cyanoethyl cellulose-(trifluoroacetic acid + methylene chloride) were studied by means of the cloud-point and polarization microscopy techniques, as well as with a photoelectric polarization unit and a modified plasticorder. It was shown that the LC phase appears at higher concentrations and lower temperatures as the polarity of solvent molecules increases. The shear deformation of cyanoethyl cellulose solutions in DMF and DMAA results in the expansion of the temperature-concentration region of existence of the LC phase. The effect of shear field on phase transitions in cyanoethyl cellulose solutions is nonmonotonic in character.     

Effect of magnetic field on phase transitions in solutions of cellulose derivatives

LC transitions occurring in mixtures of cyanoethyl cellulose with DMAA or DMF and hydroxypropyl cellulose with ethanol, DMAA, or water in the presence and absence of magnetic field have been studied. With an increase in the polarity of solvent molecules and a decrease in the molecular mass of the polymer, the LC phase develops at higher concentrations and lower temperatures. Under application of magnetic field, the domain structure is formed in solutions and the temperature-concentration region of the LC phase widens. Cyanoethyl cellulose and hydroxypropyl cellulose solutions are found to possess memory: after the magnetic field is switched off, the orientation of macromolecules and the increased temperature of phase transitions are preserved for many hours. As the molecular mass of the polymer is increased, the ability of macromolecules to orient themselves in the magnetic field declines. The threshold mechanism governing the effect of magnetic field on LC transitions in polymer solutions has been discovered. The critical value of magnetic intensity that brings about a shift in boundary curves is consistent with the value of H _cr necessary for the cholesteric liquid crystal-nematic liquid crystal phase transition.     

Self-organization of macromolecules and liquid-crystalline phase transitions in solutions of cellulose esters

For hydroxypropyl cellulose-ethanol, hydroxyethyl cellulose-water, and cyanoethyl cellulose-DMAA systems, phase diagrams are constructed and the regions of existence of isotropic and anisotropic phases and the dimensions of macromolecules and supramacromolecular particles in a wide composition range are determined through the use of the cloud-point method, polarization microscopy, the turbidityspectrum method, dynamic light-scattering measurements, and a polarization optical apparatus. It is shown that the formation of LC phases with an increase in the polymer concentration is associated with a significant enlargement of supramolecular particles.     

Phase transitions in cellulose solutions. The kinetics of precipitation of cellulose from diluted cadoxen solutions

Solutions of cellulose (degree of polymerization: 296 ± 16) in tris(ethylenediamine)cadmium(II)hydroxide-ethylenediamine-water (cadoxen), 1.015% (weight per volume) were diluted 1:1.5 with water. The ensuing isothermal precipitations were followed dilatometrically at temperatures ranging from 27.50 to 48.45°C. The precipitation data were characterized by a positive temperature coefficient. They generally conformed to a one-dimensional nucleation rate law; the Avrami exponents were close to unity. Alternatively, they fit a first-order reaction rate expression with respect to cellulose concentration. The low crystallinity of the precipitated cellulose leads to the interpretation of the kinetics in terms of the dissociation of a cellulose-cadoxen complex, rather than phase separation, as the rate-determining step. The nucleation rate law suggests a one-dimensional nonrandom cooperative chemical process, while the first-order rate law suggests a random process.     

Phase transitions in liquid crystalline solutions of hydroxypropyl cellulose under deformation

Phase transitions and the physical state of the hydroxypropyl cellulose-dimethylacetamide system under static conditions and in a shear field were studied by the cloud-point and polarized optical microscopy techniques with a polarization-photoelectric setup and a modified plasticorder. The deformation of solutions leads to a change in their structure and elevation of liquid-crystalline phase formation temperatures, a result that is due to the additional orientation of macromolecules in the flow direction. The ability of macromolecules to be oriented in a shear field decreases with an increase in the molecular mass of the polymer. The influence of deformation on phase transitions in hydroxypropyl cellulose solutions is nonmonotonic in character.     

Morphological studies of liquid-crystalline cellulose derivatives. II. Hydroxypropyl cellulose films prepared from liquid-crystalline aqueous solutions

A morphological study of hydroxypropyl cellulose (HPC) was performed on solid films prepared by casting from a liquid-crystalline aqueous solution at rest or under shear. Electron microscopic observations reveal that many round particles composed of stacked disks are densely packed in the interior of a quiescently cast HPC film, while on the film surface formation of fibrous textures is also noted. Shear-deformed HPC films exhibit some interesting morphological features according to the shearing conditions. It is found by electron microscopy that the originally round particles become more and more elongated as shear stress increases. The resulting rodlike fibrillar entities are considerably aligned in the shear direction (SD), but form a banded structure with periodic discontinuities of molecular orientation distribution along the SD. A new mechanism of structural transformation is proposed in order to interpret these results.     

Phase transitions in polystyrene solutions induced by a longitudinal hydrodynamic field

The flow of solutions of polystyrene with various molecular weights in a longitudinal hydrodynamic field was studied. The stepwise variation of such state parameters as volume and entropy of flexible chain macromolecules was observed on the skein-unrolled chain transition. To describe this transition, a modified Van der Waals’ equation taking into consideration the deviation of the system from ideality was proposed.     

Phase liquid-crystalline transitions in hydroxypropylcellulose-ethanol and hydroxypropylcellulose-acetic acid systems under deformation

The phase transitions and phase state of hydroxypropylcellulose mixtures with ethanol and acetic acid under static conditions and in the shear field have been studied by the turbidity-point method and polarization microscopy with the use of a polarization-photoelectric setup and a modified plastoviscometer. The deformation of solutions leads to a decrease in the temperature of mesomorphic phase formation and to a change in the type of liquid crystals from cholesteric to nematic.     

Thermodynamics of liquid-crystalline solutions of hydroxypropyl cellulose in water and ethanol

Phase diagrams were constructed and comprehensive thermodynamic analysis was performed for hydroxypropyl cellulose-water and hydroxypropyl cellulose-ethanol systems with the use of the static sorption, calorimetry, cloud-point, polarization microscopy, and X-ray diffraction analysis techniques and the measurement of transmitted polarized light intensity. The concentration dependences of the enthalpy, entropy, and Gibbs energy of the formation of liquid-crystalline phases in the systems were determined. It was found that the formation of liquid-crystalline solutions of hydroxypropyl cellulose in water is associated with the energy term of interaction between the components and that in ethanol solutions is due to changes in combinatorial entropy.     

Effect of magnetic field on phase transitions in solutions and melts of flexible polymers

The effect of the magnetic field on the phase diagrams of flexible-chain polymer–solvent systems is observed for the first time. Phase transitions in systems with the crystalline-phase separation (PE–o-xylene, PE–n-hexane, PE–chloroform, PE–o-dichlorobenzene, PEG–1,4-dioxane, PEG–toluene) and the amorphous demixing (PS–methyl acetate, PVA–ethanol, PDMS–butanone) are studied. The magnetic field increases the temperatures of crystallization of PE and PEG from solutions and melts but has no effect on phase transitions in PS, PVA, and PDMS solutions. The structures of polymer entities isolated from solutions and melts are studied. Under application of the magnetic field to PEG solutions, spherulites of substantially smaller sizes than those formed outside the field appear. The magnetic field increases the degree of crystallinity of PEG, but the degree of crystallinity and size of PE spherulites remain unchanged.     

Investigation of molecular order in liquid-crystalline solutions of cellulose triacetate using PMR spectroscopy

Liquid-crystalline solutions of cellulose triacetate (CTA) in trifluoroacetic acid (TFA)–CH₂Cl₂, TFA–1.2-dichloroethane (1,2-DCE) solvent mixtures were examined by means of PMR spectroscopy. CTA forms both cholesteric and nematic phases in these solvents depending on the CTA concentration. In cholesteric solutions the CH₂Cl₂ signal is initially a singlet and then splits into a doublet. The time dependence of the splitting and the effect of CTA concentration are reported. The results suggest that the cholesteric phase slowly changes into a nematic phase in the magnetic field. The splitting of the CH₂Cl₂ proton signal into a doublet and the 1,2-DCE signal into a quartet are due to direct magnetic dipole-dipole interactions. Rotation of the sample in the magnetic field results in the disappearance of the doublet or quartet and suggests that the solvent molecules are originally oriented in the direction of the magnetic field. In the biphasic region, immediate splitting of the CH₂Cl₂ proton signal suggests that the anisotropic phase is nematic.     

Effect of a magnetic field on the rheological properties of cellulose ether solutions

Application of a magnetic field is shown to be accompanied by an increase in the viscosity of hydroxyethyl cellulose and ethyl cellulose solutions and an additional assembly of macromolecules, as is evident from a gain in the radii of light-scattering particles. The concentration dependences of supramolecular-particle radius and solution viscosity in the presence of a magnetic field are described by curves with maxima.     

Rheology of liquid-crystalline solutions of hydroxylpropyl cellulose filled with layered silicate particles

The rheological properties of nanocomposites with an anisotropic matrix are systematically studied. As a matrix, a 60% solution of hydroxypropyl cellulose in PEG is used. In accordance with the phase diagram, it demonstrates the LC-isotropic-state transition along the temperature scale. The solution is filled with Na-montmorillonite particles (1.0–7.5%). The rheological characteristics of solutions under steady-state shear flow, periodic (harmonic) oscillations with different amplitudes, and uniaxial extension at the constant stretching rate are investigated. Experiments are performed at various temperatures, and the properties of the system are compared with the phase state of the matrix. The viscoelastic properties of the material are described by a model with a single relaxation time. The abnormal behavior of the storage modulus in the low-frequency range is observed. When the matrix is transformed into the LC state, the yield point becomes well-defined and the shear viscosity abruptly increases with an increase in the content of the LC phase. The above-described effects are discussed in terms of the ideas that two competing structures exist in the system, one of which is formed by the LC domains of solution, while the other structure is based on the nanofiller capable of forming ordered structures. Deformation, especially longitudinal flow, facilitates self-organization of the system.     

Superfluorescent transitions in an external magnetic field

Polarisation properties of the superfluorescence in the near-infrared regime have been investigated between high-lying levels of Sr and Ba under the influence of a static homogeneous external magnetic field. In some transitions the time-resolved measurements show a change of the polarisation of the superfluorescence depending on the magnetic field strength. In suitable experimental conditions intensity modulations were observed. These were assigned as Zeeman quantum beats or indirectly observed Zeeman superfluorescent beats. The experimental findings of superfluorescence in two-level, three-level, or multi-level configurations in dependence on the magnetic field strength can be explained well in a semiclassical model of multi-level superfluorescence.     

Phase transitions and rheology of aramid solutions

Clearing temperatures of solutions of poly(4-4′-benzanilidylene-terephthalamide) in concentrated sulphuric acid are presented as a function of polymer concentration and average molecular weight. The orientational order parameter <P ₂> is obtained from birefringence measurements. The experimental results are explained by a mean-field type theory similar to the Maier-Saupe model for thermotropic liquid crystals. Molecular flexibility, concentration and molecular weight are taken into account by using simple scaling factors. The birefringence induced by shear flow in an isotropic solution of poly(para-phenylene-terephthalamide) shows a strong pretransitional behaviour. This indicates the occurrence of a flow-induced phase transition.     

Phase and structural transformations of liquid-crystalline polymer systems in the mechanical field

Phase LC transitions and the structure of blends based on cyanoethyl cellulose with DMA and DMF, poly-(γ-benzyl-L-glutamate) with DMF, and hydroxypropyl cellulose with ethanol, acetic acid, DMA, DMF, and water have been studied under static conditions and in a mechanical field. With decreasing molecular mass of the polymer, the boundary curves separating isotropic from anisotropic solutions are shifted to higher concentrations and lower temperatures, in agreement with the Flory theory. When solutions are deformed, the cholesteric type of liquid crystals is transformed into the nematic type; this process is accompanied by the formation of domains in solutions, and the corresponding temperature-concentration boundaries of the LC phases are changed. As the molecular mass of the polymer increases, the ability of macromolecules to orient under the shear field decreases.     

Structural evolution of liquid-crystalline solutions of hydroxypropyl cellulose and hydroxypropyl cellulose-based nanocomposites during flow

The phase state and orientation and dissipative characteristics of biphasic LC HPC-water solutions and filled systems formed on their basis during shear flow are studied by various methods. The concentration of solutions is selected on the basis of the corrected phase diagram constructed with the use of optical interferometry. Flow curves and concentration dependences of viscosity provide additional information about the phase state and structure of the samples and the role of fillers in the rheological properties of solutions. X-ray diffraction data are obtained with the use of a Couette cell consisting of two coaxial capillaries. In the case of a clay suspension in water, practically no orientation is attained. However, in the isotropic 30% solution, clay particles easily orient, a result that indicates an important role of the viscoelasticity of a medium in the orientation process. The development of orientation of HPC macromolecules and clay particles in relation to the shear rate is analyzed separately for systems with the biphasic matrix (LC + isotropic phase). In addition, the time decay of the orientation parameter during relaxation is investigated. It is shown that higher shear rates cause a more rapid relaxation of orientation, for which recovery of the cholesteric helix typical for LC solutions of cellulose derivatives in the equilibrium state plays an important role. Order parameters (separately for the two components) are calculated, and their evolution with the shear rate and total deformation is investigated for systems containing clay nanoparticles (also the structure-active component) in LC solutions. On the basis of these data, it is hypothesized that clay particles form the columnar mesophase, which, under certain conditions, may transform into the discotic mesophase. This transition is responsible for a certain decrease in the order parameter of HPC apparently due to the instability effect of the director. It is found that shearing substantially affects the structure of the system composed of two mesophase species; specifically, it either facilitates the reinforcement of one of them or provokes structural transitions.