V. V. Makarova

Phase state and rheology of polyisobutylene mixtures with decyl surface modified silica nanoparticles

The miscibility of linear polyisobutylene and silica nanoparticles with surfaces modified by decyl groups is studied. The phase state of these systems corresponds to the amorphous equilibrium and may be described by a binodal with the UCST. As the radius of the inorganic core of nanoparticles and the molecular mass of polyisobutylene increase, the insolubility region on the phase diagram becomes wider. The addition of nanoparticles to the polymer leads to decreases in the viscoelastic characteristics of homogeneous media and provides the non-Newtonian behavior of the composition in the two-phase region. Shear deformation causes shifts of the phase equilibrium lines in the direction depended on the sizes of the nanoparticles.

Mechanical and thermal properties of polymer micro- and nanocomposites

Abstract We have analyzed the thermal conductivity and the tensile modulus of composite materials within the framework of the Halpin-Tsai and Lewis-Nielsen models. The parameter linking thermal conductivity and tensile modulus together is the shape factor of the (nano)filler. Model analysis based on experimental data shows that particle aggregation into a weak mechanical network may be required to achieve good correlation between thermal conductivity and the Young’s modulus when analyzing the data within the framework of a single model and requiring the same value of the shape factor. We believe this approach will make quantitative analysis of nanocomposite thermal properties possible.

Interchain exchange and interdiffusion in blends of poly(ethylene terephthalate) and poly(ethylene naphthalate)

The interchain exchange and interdiffusion in blends of poly(ethylene terephthalate) and poly(ethylene naphthalene-2,6-dicarboxylate) are investigated with reprecipitated commercial samples (M η ∼ 104) and samples containing no polycondensation catalyst (M η ∼ 103) synthesized in the course of this study. The kinetics of multiblock copolymer formation and gradual reduction of the mean block length in quasi-homogeneous blends were shown to fit a simple theoretical model of a second-order reaction. The increase of the reaction-rate constants on the transition from commercial samples to synthesized ones revealed a significant role of chain ends in interchain exchange. The detected activation energy of the interchange in the absence of catalysts (97 kJ/mol) was noticeably less than that previously reported for the polymer pair under study (120–170 kJ/mol). The obtained data were applied for analysing the interdiffusion between melts of the same polymers accompanied by the interchain exchange. By means of the microinterference method, the interdiffusion in the synthesized samples was shown to be much faster than that in the reprecipitated commercial samples, a result that may be due to the better compatibility of the initial polyesters as their molecular mass decreased. In later stages of the process in both systems, the interpenetration of components was slower than that predicted by Fick’s law, owing to formation of copolymer species that diminished the thermodynamical factor of mixing.

Rheological properties and phase behavior of a hydroxypropyl cellulose-poly(ethylene glycol) system

The relaxation and phase behavior of solutions of hydroxypropyl cellulose in poly(ethylene glycols) of various molecular masses has been studied by dynamic mechanical analysis. The dynamic mechanical data are compared with the results of microinterferometry and polarization-microscopy measurements. The combination of optical and mechanical characteristics makes it possible to construct generalized phase- relaxation diagrams for the binary systems under study. Solutions based on lowmolecular-mass poly(ethylene glycol) are characterized by LC equilibrium. With an increase in the molecular mass of poly(ethylene glycol) in a certain temperature-concentration region, amorphous phase separation takes place and the phase diagram is the superposition of LC and amorphous equilibria. The relaxation properties of the systems are sensitive to the phase state and its transformation.

Phase equilibria in solutions of cellulose derivatives and the rheological properties of solutions in various phase states

Phase equilibria in solutions of hydroxypropyl cellulose in such solvents as water, propylene glycol, poly(ethylene glycols) of various molecular masses, triethyl citrate, and DMSO are analyzed. The phasetransition lines are constructed with the use of microinterference and polarization microscopy methods. In a certain temperature-concentration range, all solutions undergo a sequence of transitions typical for solutions of stiff-chain crystallizable polymers: isotropic and LC states separated by the two-phase corridor and, in some cases, the crystalline state. Of special interest are systems in which the superposition of amorphous (with the LCST or UCST), LC, and crystalline equilibria is realized. The features of the phase states of solutions are compared with the physicochemical characteristics of solvents and the intensity of their interaction with hydroxypropyl cellulose macromolecules. Solutions occurring in various phase states are tested by rheological methods. These solutions demonstrate the typical rheological behavior of anisotropic solutions: the presence of yield stress and the extremum concentration dependence of viscosity with a maximum, when the LC phase appears, and a minimum, when it transforms into the 100% phase. In the case of single-phase solutions, viscosity increases with concentration, while for two-phase solutions, viscosity decreases with an increase in the fraction of the LC phase. The rheological data are found to be sensitive to phase transitions observed in solutions.

Miscibility and rheological properties of epoxy resin blends with aromatic polyethers

The miscibility of the bisphenol A epoxy oligomer with a number of aromatic polyethers is studied via optical interferometry. It is shown that polysulfone and polycarbonate are unlimitedly soluble in the oligomer. The dissolution of polycarbonate is accompanied by a chemical interaction and the formation of a new phase. For poly(ether sulfone) and poly(ether imide), amorphous phase separations with the LCST and the UCST, respectively, are observed. The polymer solutions are Newtonian liquids, and the systems in which phase separation occur feature the yield stress. A shift of the phase-equilibrium lines during the action of shear is revealed.

Diffusion and phase behavior of a hydroxypropylcellulose-poly(ethylene glycol) system

The solubility and interdiffusion between hydroxypropylcellulose samples of various molecular masses (M w = 8 × 104, 14 × 104, 37 × 104, 85 × 104, and 115 × 104) and poly(ethylene glycol) (M w = 400 and 1500) in the range 18–210°C have been studied by optical interferometry and polarization microscopy methods. Oligomeric poly(ethylene glycols) have been considered as solvents for hydroxypropylcellulose. Phase diagrams have been constructed, and Flory-Huggins thermodynamic interaction parameters have been calculated. For the hydroxypropylcellulose-poly(ethylene glycol) 400 system, an LC and crystalline equilibria have been realized. An increase in the M w of hydroxypropylcellulose to 1500 leads to the appearance of a wide region of amorphous phase segregation with a UCST, whereas the liquidus line is conserved at high concentrations of hydroxypropylcellulose. Such a superposition of two kinds of phase equilibrium that is achieved only with a change in M w of the oligomeric solvent has been observed for the first time. For all the systems under examination, the kinetics of diffusion mixing has been estimated and the activation energies of the process have been calculated. The concentration dependences of diffusion coefficients demonstrate jumps in the mesomorphic-transition region.

Phase-equilibrium and cellulose-coagulation kinetics for cellulose solutions in N-methylmorpholine-N-oxide

The dissolution of cellulose in N-methylmorpholine-N-oxide monohydrate and the dissolution of N-methylmorpholine-N-oxide monohydrate in water have been studied via optical interferometry. A part of the phase diagram for the cellulose/N-methylmorpholine-N-oxide system has been constructed. The phase diagram is characterized by crystalline equilibrium, hysteresis of the melting temperatures of the solvents, and a region of anisotropy. Optical interferometry has been used for the first time to study the kinetics of cellulose coagulation during the interaction of cellulose solutions in N-methylmorpholine-N-oxide with water and water solutions of N-methylmorpholine-N-oxide. Information on the values of interdiffusion coefficients and the morphologies of the resulting cellulose films has been obtained. The possibility to use optical interferometry to analyze the interaction of a solution with the coagulating agent in the case of cellulose fiber and film formation has been demonstrated. The influences of temperature, the nature of the coagulating agent, and the cellulose content on the kinetics of the process and morphologies of the formed films have been shown. The use of N-methylmorpholine-N-oxide as a part of the coagulation system decreases the rate of interdiffusion of solutions, thereby resulting in a more uniform and dense morphology of cellulose films. Increased temperature causes diffusion acceleration, thereby leading to the formation of an anisotropic morphology of the cellulose films.

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.

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.