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 liquid-crystalline cyanoethyl cellulose solutions in magnetic field

The cloud-point method and polarization microscopy have been used to investigate the phase transitions and the phase state of cyanoethyl cellulose-dimethylacetamide and cyanoethyl cellulose-dimethylformamide systems in the presence and absence of magnetic field with the help of polarization photoelectric and magnetic setups. The temperature-concentration region of the existence of the liquid-crystalline phase in solutions widens in the magnetic field; the higher the field strength and polymer concentration, the more pronounced this widening. Cyanoethyl cellulose solutions are found to possess “memory”: after the magnetic field is switched-off, the orientation of macromolecules and the increased phase transition temperature are preserved for many hours.     

Phase equilibria in polymer–solvent systems: Progress in the field of fibers

The progress in phase equilibria of polymer-solvent systems is briefly reviewed. The features of phase diagrams intended for production of fibers with high strength, deformation, and thermal properties are considered. Some attention is given to the fundamental role of S.P. Papkov in the creation and development of scientific concepts on phase equilibria in fiber-forming polymers involving solvents.     

Phase equilibrium in polymer systems

The topological analysis of phase equilibria in polymer systems, which was developed by S.P. Papkov; the classification of the types of phase equilibrium; the principle of mutual independence of the two types of equilibrium; the concept of the generality of the phase equilibrium in polymer-solvent, polymer-plasticizer, polymer-oligomer, and polymer-polymer systems; and the processes of water sorption by polymers are considered. It is shown that the problems that Papkov dealt with remain topical and the concepts that he elaborated can underlie new studies in the field of phase equilibria in polymer systems.     

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.     

Adhesion of liquid-crystalline polymer systems to substrates of varied roughness

Evolution of the adhesion behavior of a liquid crystalline system under various conditions of the adhesion joint formation was studied. A blend of hydroxypropyl cellulose with propylene glycol was selected as a model liquid crystalline system. Influence of the substrate roughness on the adhesion of the liquid crystalline systems was studied for the first time. It was found out that the roughness influence on the adhesion characteristics of the systems studied was much more pronounced than in case of the common pressure-sensitive adhesives. High values of the debonding energies for the systems studied (up to 280–330 J/m²) were obtained. These values are similar or even exceed the values obtained for the common pressure-sensitive adhesives.     

Bistable dynamic coordination polymer showing reversible structural and functional transformations

A bistable dynamic coordination polymer [Ni(pca)(bdc)(0.5)(H(2)O)(2)] having a two-dimensional (2D) zigzag sheet structure is synthesized solvothermally. Topological analysis revealed that the frameworks have an hcb type of uninodal net. The compound exhibits guest specific reversible structural transformations accompanying reversible changes in physical properties driven by inherent flexibility and transformability.     

Phase equilibria of polymer dispersed liquid crystal systems in the presence of an external electrical field

The effect of an external electrical field on phase behaviors is reported for polymer dispersed liquid crystal films of 4′‐pentyl‐4‐biphenylcarbonitrile/poly(methyl methacrylate) binary mixtures with various polymer molecular weights. The experimental results show that increasing the molecular weight of the polymer or the electrical field intensity can give rise to an increase in the phase‐transition temperature and a widening of the binary phase region. The lattice theory, regarding a binary system consisting of a rigid nematic liquid crystal and a random polymer, has been extended to the case in which an external electrical field is present. A comparison of the theoretical predictions with the experimental results has been carried out, and satisfactory agreement has been found. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1898–1906, 2007     

Effect of spatial irregularities on the temperature and field dependence of the mobility in liquid-crystalline conjugated polymer films

We have performed simulations of time-of-flight measurements via the Monte Carlo approach for films made of conjugated polymers in the liquid-crystalline phase. In spatially regular films with a distribution of on-site energies the mobility is affected by the interplay between electrostatic and thermal energy. When the width of the on-site energy distribution is comparable to the thermal energy, the mobility increases with temperature at low fields, but shows the opposite behaviour at larger fields. However, when spatial irregularities in the arrangement of the film are introduced, the mobility is enhanced at all temperatures, as the electrostatic energy plays less of a role in charge transport than thermal activation over energy barriers.     

Homeotropic and planar structures in liquid-crystalline polymer brushes

The ordering in polymer brushes formed by macromolecules with mesogenic groups in the main chain is investigated. The numerical method of self-consistent field approximation was used. The existence of two different liquid crystalline nematic states is shown: homeotropic (HLC) and planar (PLC) states. The free energy of the HLC state is always less than that of the PLC state. However, with the increase of energy of anisotropic interactions, (with decrease in temperature) our numerical procedure leads us to either one or another state depending on the grafting density. The results obtained show that both brush surfaces, play an essential role in establishing the concrete LC state structure. The grafting surface and the external surface force the planar order.     

Deformation-induced structural transformations in Si and the initial stage of mechanical alloying of Si and Fe

The methods of X-ray diffraction, Mössbauer spectroscopy, IR spectroscopy, and laser diffractometry are employed to study the changes in the structure and phase transformations that accompany mechanical alloying of Si and ⁵⁷Fe used in an atomic ratio of 99: 1. It is established that the process comprises the development of a nanocrystalline state of Si with crystallite sizes smaller than 10 nm; the formation of an amorphous phase of Si at particle surfaces and in near-boundary distorted zones of interfaces in Si nanostructure; the penetration of ⁵⁷Fe atoms along grain boundaries; and the formation of Si-Fe clusters, the local environment of Fe atoms in which is typical of a deformed α-FeSi₂ phase, with these clusters being located in the interfaces.     

Elastic deformations in a magnetic field of a liquid-crystalline polymer with mesogenic groups in the main and side chains

Orientational elastic deformations in a magnetic field and phase transition temperatures of a thermotropically mesogenic “combined” aromatic polyester with mesogenic groups, both in the main and in the side chains, have been investigated in the range of 900 to 10,200 mol. wt. It was shown that in this molecular weight range the birefringence of a completely oriented nematic and correspondingly, the degree of its orientational order S are independent on molecular weight. When the relative temperature ΔT changes from −2 to −30°C the orientational order parameter increases from 0.35 to 0.55. The bend elasticity constants K₃ coincide in the order of magnitude with those for high and low molecular weight nematics investigated previously and their dependence on molecular weight has not been detected.     

Applications of self-consistent field theory in polymer systems

1Introduction Owing to the specificity of the long chain,polymers present complexity and versatility.These molecules in the system can be various in their topological struc-tures,such as linear,star,comb or circle structures;meanwhile they can be polymeri…     

Phase separation of polymer blends under shear field

The effect of shear flow on phase separation in critical polymer blends has been studied. For a low-molecular-weight blend, the response is in good agreement with the theoretical predictions of Onuki and Kawasaki. The breakup of large-scale critical fluctuations by the flow leads to a drop in the temperature T_c at which phase separation begins. For a high-molecular-weight blend, the data suggest that the mode-coupling contribution to the decay rate of composition fluctuations is significant in both the miscible and immiscible phases. However, the lack of change of structure factor in the vorticity direction implies that there is no apparent shear induced shift in T_c.     

Size-dependent structural transformations of hematite nanoparticles. 1. Phase transition

Using Fourier Transform InfraRed (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM), we characterize the structure and/or morphology of hematite (alpha-Fe(2)O(3)) particles with sizes of 7, 18, 39 and 120 nm. It is found that these nanoparticles possess maghemite (gamma-Fe(2)O(3))-like defects in the near surface regions, to which a vibrational mode at 690 cm(-1), active both in FTIR and Raman spectra, is assigned. The fraction of the maghemite-like defects and the net lattice disorder are inversely related to the particle size. However, the effect is opposite for nanoparticles grown by sintering of smaller hematite precursors under conditions when the formation of a uniform hematite-like structure throughout the aggregate is restricted by kinetic issues. This means that not only particle size but also the growth kinetics determines the structure of the nanoparticles. The observed structural changes are interpreted as size-induced alpha-Fe(2)O(3)<-->gamma-Fe(2)O(3) phase transitions. We develop a general model that considers spinel defects and absorbed/adsorbed species (in our case, hydroxyls) as dominant controls on structural changes with particle size in hematite nanoparticles, including solid-state phase transitions. These changes are represented by trajectories in a phase diagram built in three phase coordinates-concentrations of spinel defects, absorbed impurities, and adsorbed species. The critical size for the onset of the alpha-->gamma phase transition depends on the particle environment, and for the dry particles used in this study is about 40 nm. The model supports the existence of intermediate phases (protohematite and hydrohematite) during dehydration of goethite. We also demonstrate that the hematite structure is significantly less defective when the nanoparticles are immersed in water or KBr matrix, which is explained by the effects of the electrochemical double layer and increased rigidity of the particle environment. Finally, we revise the problem of applicability of IR spectroscopy to the lattice vibrations of hematite nanoparticles, demonstrating that structural comparison of different samples is much more reliable if it is based on the E(u) band at about 460 cm(-1) and the spinel band at 690 cm(-1), instead of the A(2u)/E(u) band at about 550 cm(-1) used in previous work. The new methodology is applied to analysis of the reported IR spectra of Martian hematite.     

Self-consistent field theory and its applications in polymer systems

This review article addresses the widely used self-consistent field theory (SCFT) in interacting polymer systems. The theoretical framework and numerical method of solving the self-consistent equations are presented. In this paper, different structures of polymer can be considered, such as homopolymer, block copolymer, polydisperse polymer and charged polymer. Several systems, micro/macro phase separation, interface, self-assembly, are presented as examples to demonstrate its applications in details. Besides, the fluctuation effects are considered. The first order is Gaussian fluctuation theory, which can be used to determine the stability of the mean-field solution and predict the kinetics of unstable structure. The derivation and applications of Gaussian fluctuation theory are presented as well.     

Phase transformations in hydrogen sorption-desorption by hydrides of intermetallic compounds of the CrB structural type

State Institute of the Nitrogen Industry. M. V. Lomonosov State University, Moscow. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 5, pp. 74–81, September–October, 1991.     

Reorientation of a liquid-crystalline side chain polymer

The reorientation times of a side chain nematic polymer have been measured. The polymer samples were oriented in magnetic fields of 2·1 T and the reorientation experiments were carried out using a NMR spectrometer operating at 1·5 T. Temperature and twist angle dependencies of the proton NMR spectra were studied in detail and discussed with regard to the alignment of the mesogenic molecule groups in the polymer medium. An additional result concerns the homogeneous or inhomogeneous reorientation.     

Fast switching ferroelectric side-chain liquid-crystalline polymer and copolymer

New ferroelectric side-chain liquid-crystalline polymers, a copolymer and a homopolymer, with siloxane backbone and a triaromatic mesogen as the side group have been synthesized. The materials exhibit a chiral smectic C phase over a large temperature range extending to room temperature. They possess high values of spontaneous polarization: 105 nC cm^-2 for the homopolymer and 180 nC cm^-2 for the copolymer. The electro-optic switching time in the chiral smectic C phase is extremely fast (150 μs). In the smectic A phase, an electroclinic effect with switching times less than 100 μs and with field induced tilt angles of 18° is observed.