Adsorption-active Medium Research Articles

Kinetics of Low Molecular Substances Sorption by the Polymer Mixtures

The sorption capacity of polymer blends was analyzed on the basis of absorption kinetics. The study was carried out in an adsorption-active medium with different degrees of deformation. Heptane and butanol were used as adsorption-active media. The influence of the chemical nature of the adsorbate on the sorption process was studied. The free volume fractions for each sample of the composition were calculated. The ability of materials to absorb different amounts of substances allows us to identify the behavior peculiarities of the interfacial regions of different chemical compositions.

Stabilization of the Mesoporous Structure of Poly(ethylene terephthalate) Deformed by the Crazing Mechanism

Deformation of amorphous glassy poly(ethylene terephthalate) (PET) films in liquid adsorption-active media (AAMs) by the crazing mechanism has been studied. The crazing has been shown to be an efficient method for the formation of a finely dispersed mesoporous structure in the polymer. A technique has been developed for stabilizing the structure of crazed PET by its annealing in the presence of an AAM under conditions that prevent the polymer from shrinkage. The porosity and vapor permeability of the obtained PET-based materials have been studied as depending on the deformation and annealing conditions. It has been revealed that the stability of the mesoporous structure formed in PET films after their annealing in the AAM results from polymer crystallization. The obtained structure is characterized by anisotropic relief and wettability of the surface.

Low-Temperature Features of the Micromechanical Properties of Polystyrene

The microindentation of thin polystyrene plates, which is an amorphous linear thermoplastic polymer with a glass transition temperature Tg ≈ 373 K, was carried out in the 77–295 K temperature range. A decrease in the temperature from 295 K to 140 K generated a two-fold linear increase in microhardness, which is in good agreement with the temperature dependence of the forced elasticity limit of polystyrene measured by other authors during uniaxial compression deformation. At temperatures of T = 77–140 K the local deformation of polystyrene was reversible: during heating to room temperature the impressions on the sample surface disappeared completely. Partial deformation reversibility was observed upon indentation in the temperature range of 140–190 K, but at indentation temperatures T > 190 K the impressions remained. The microstrain reversibility of polystyrene in nitrogen at, and close to, 77 K is due to the effect of the adsorption-active medium on the formation of crazes (fibrillar-porous nanostructures), the glass transition temperature of which is tens of degrees below that of bulk polymer.

CRACKING OF MONOLITHIC POLYCARBONATE SHEETS IN A STRESSED STATE

The accelerated stress cracking of monolithic polycarbonate sheets was investigated. Cracking of polycarbonate organic glasses during operation leading to a drop in impact strength and transparency is their main disadvantage. The main role in the acceleration of the process of cracking is played by the stresses that arise when they are formed and mounted. The aim of this work was to investigate the dependence of the cracking start time on the stress applied to the sheet monolithic polycarbonate. This makes it possible to predict the lifetime of polycarbonate products. The experiments were carried out in air and in contact with an adsorption-active medium (a mixture of toluene and n-propanol). The stresses applied to the samples varied in the range from 25 to 55 MPa. It is shown that the dependence of the cracking start time of a polycarbonate on the applied stress can be described by Zhurkov exponential equation. The coefficients of this equation for the processes of cracking in air and in contact with the adsorption-active liquid are calculated. It is established that the activation energy of the polycarbonate cracking process (129.5 kJ/mol) is close in magnitude to the activation energy of the thermal-oxidative destruction (about 145-155 kJ/mol). In contact with the adsorption-active mixture of toluene (25% mass.) and n-propanol, the activation energy of the cracking is reduced to 98.5 kJ/mol. At the same time, the structural coefficient in the Zhurkov equation ("activation volume") increases from 1.45 to 2.45 nm3. The analysis of the obtained results made it possible to predict the lifetime of monolithic polycarbonate sheets and products made of them at various operating stresses

Evolution of the Nanoporous Structure of High-Density Polyethylene during Drawing in Supercritical Carbon Dioxide

The process of open-porous structure development in high-density polyethylene (HDPE) films during uniaxial deformation in supercritical carbon dioxide (SC-CO2) fluid at 35 °C and 10 MPa has been studied and visualized by means of atomic force microscopy. We suggest that the supercritical fluid act as adsorption-active medium, and the porous structure is developed via the crazing mechanism due to the increasing the distance between of lamellae and the formation of oriented separate fibrils in the intercrystallite space. Effective bulk porosity of the films has been up to 40%. Small-angle X-ray scattering studies and ethanol permeability measurements have revealed that the pores and fibrils are about 10 nm in diameter. The prepared nanoporous materials exhibit good vapor permeability. Structural and mechanical behavior of the prepared porous films has been investigated. Large reversible deformation (up to 80%) of HDPE in the SC-CO2 has been observed. Repeated drawing of the shrunk films in air under ambient...

Effect of orientation on the mechanism of plastic yielding of poly(ethylene terephthalate) in adsorption-active media

The effect of the preliminary orientation on the formation of crazes in poly(ethylene terephthalate) during straining in adsorption-active liquids is studied. Poly(ethylene terephthalate) is oriented by drawing at a temperature of 80°C, which is somewhat higher than its glass-transition temperature (∼75°C). After orientation, samples are tested in tension in organic liquids at room temperature. At low degrees of preliminary drawing, the shear yield stress during straining in air does not increase significantly. However, the stress of craze widening rises in proportion to the degree of preliminary drawing. Thus, the orientation suppresses crazing and leads to the transition to shear flow. A model is proposed to explain the effect of orientation on crazing. According to this model, craze widening and pulling of a nonoriented polymer into the craze volume result from the formation of pores in the bases of fibrils. The formation of fibrils is caused by straining of the polymer between pores.

Regularities of the Rehbinder effect in polymers. Review

Data are analyzed that are related to particular details of manifestations of the Rehbinder effect in the case of polymer deformation in adsorption-active media (AAMs). It is shown that, as a result of such a deformation, a vitreous polymer is dispersed, forming a system of fibrillar aggregates of oriented macromolecules with a diameter of 1–20 nm separated in space by microcavities of similar size (the crazing phenomenon). It is shown that polymer crazing in AAMs is a complex multistage process. Its first stage is initiation of crazes related to the microscopic surface defect structure of the actual polymer and is controlled by the Griffith criterion. Craze growth and broadening are typical thermally activated processes of plastic deformation of the polymer. The main areas for practical application of the Rehbinder effect in polymers are pointed out.

Dyeing polyester fibers using the crazing method

As we know, finished polyester fibers are very difficult to dye, especially at low temperature (including room temperature). An exception is processes for dying undrawn fibers. We have studied use of the crazing method to dye polyester fibers at low temperatures with water-soluble dyes not having an affinity for polyethylene terephthalate. We have studied the effect on sorption of cationic dyes from the composition, concentration, and temperature of the adsorption-active medium, and the dye concentration in the dye bath. The results of the study are the basis for developing a production process for obtaining colored special-purpose polyester fibers.

Polypropylene/Silicate Composites on the Basis of Crazed Polymer and Hyperbranched Polyethoxysiloxane

Hyperbranched polyethoxysiloxane (HPEOS) is suggested as an adsorption-active medium in the crazing of commercial polypropylene film and as a precursor of silica. As a result of delocalized crazing of polypropylene, the nanoporous materials characterized by the effective volume porosity to 60 vol % are received. The reaction of HPEOS hydrolysis into pores of a polymer at the presence of acidic and basic catalysts is studied. Obtained silica phase is shown to stabilize the nanoporous structure of crazed polypropylene. The morphology of polymer/silicate composite is fully determined by the structure of crazed polymer matrices, and it is a system of organic/inorganic interpenetrative nets. Because of this structure, the tensile modulus of the composite material exceeds the modulus of the initial polymer by 50-100%.

Incorporation of an abnormally high amount of poly(ethylene glycol) into poly(ethylene terephthalate) during tensile drawing in liquid media

Tensile drawing of PET via the mechanism of solvent crazing in adsorption-active media containing poly(ethylene glycol) with M < 1 × 106 is accompanied by their penetration into the porous structure of the matrix polymer. In this case, the amount of PEG in PET exceeds its concentration calculated on the assumption that the porous structure is filled with the polymer solution. This excess is evidently due to the adsorption of PEG on the highly developed surface of crazes.

Effect of an adsorption-active medium on quasimicroplastic deformation in solids

An adsorption-active medium (distilled water, which is chemically neutral under normal conditions) is capable of influencing the quasimicroplastic (QMP) deformation in solids—a fundamental effect of irreversible deformation in the range of low strains (e≈10−6). The results of experiments with heulandite (a zeolite mineral) showed that the adsorption-active medium reduces the energy barrier for the QMP deformation and increases the rate of this process. The mechanism of the QMP deformation in the presence of the adsorption-active medium remains unchanged, being mediated by single structural relaxation events in the vicinity of point defects.

Reversible and irreversible evincing of Rehbinder effect in polymer systems

Changing polyethyleneterephthalate film properties by means of their treatment with solvents comprising crystallizing and non-crystallizing components has been studied. It has been found that under the influence of the solvent a microheterogeneous defect structure (and in a narrow range of modifying solvent concentrations, microporous structure) appears. Studying the structure is of a special interest because it opens up an opportunity to intensify transmembrane fluxes. The cause of micropore appearance is developing of internal stretching tensions locally exceeding material’s strength lowered by the adsorption active medium. Permeability and selectivity autooscillations have been detected in the process of evaporation of binary mixtures of low molecular weight solvents through an asymmetric polyvinyltrimethylsilane membrane. The autooscillations were caused by a reversible evincing of Rehbinder effect: periodic emergence and collapse of micropores in the membrane’s active layer. Reversible structural transitions play the role of a feed-back relay controlling switching the fluxes in the pores on and off. Conductometric measurements of swelling of chitosan in water vapour have been carried out. Gradual evolution of kinetic instabilities and macrofluctuations (stochastic autooscillations) of local (belonging to infinite clusters) sorbate concentrations were observed as variations of electrical current. The phenomenon is connected with a course of relaxation processes resulting in induced porosity genesis. There were no macrofluctuations if the polymer had a relaxed structure. Wavelet analysis was applied for characterization of the swelling dynamics.

Electron-microscopic study of structurization of low-molecular inorganic compounds introduced by counter-diffusion into the porous structure of a polymer strained in adsorption-active medium

Samples were prepared of polyethyleneterephthalate (PETP) films stretched in adsorption-active medium, with their porous structure filled with silver chloride by the method of counter-diffusion from solutions of silver nitrate and sodium chloride in a dialysis cell. By subsequent treatment of this film with a photodeveloper, highly disperse blends of PETP with metallic silver were obtained. The applied counter-diffusion method is more effective than procedures previously used for filling the porous craze structure in the polymer.

Aspects of the formation of the porous structure of polymer films based on the two-component system PVC plus rubber modifier on their strain in a liquid adsorption-active medium

Aspects of the formation of a porous structure on strain of films of PVC and mixtures of it with the graft copolymer methylmethacrylate-butadiene-styrene in a medium of ethyl alcohol have been studied. The investigations were made with stretching at constant speed and on exposure to a constant load. The parameters of the porous structure depend on the level of the applied load indicating the applicability of the theory of the unstable meniscus for describing the systems studied. Widening of the crazes occurs by the mechanism of surface stretching. The porous structure appearing in the course of stretching is prone to typical colloidal processes such as coagulation of the fibrillized material. The role of the second component of the two phase system during crazing is discussed.

Effect of a plasticizer on the structure of PA-6 deformed in an adsorption-active medium

The permeation of a liquid under the action of a pressure gradient has been used to investigate how the evolution of the parameters of the porpous structure of PA-6 with various moisture-contents depends on the degree of extension in an adsorption-active medium. As the plasticizer concentration in the polymer is increased, a transition is shown to occur from classical localized crazing to delocalized crazing.

On the nature of spontaneous elongation during the irradiation of polymers with prior stretching in adsorption-active media

A study has been made of the nature of the spontaneous extension effect occurring during fast electron irradiation of polymers subjected to prior stretching in adsorption-active media. This effect is associated with radiation heating of microporous polymer samples. Manifestation of the effect in amorphous PETP is necessarily related to the presence of crazes with a well developed microfibrillar structure. It was found that the spontaneous extension effect is due to crystallization of partially oriented material in transitional regions relating the oriented material of microfibrils inside crazes to unstrained polymer in interspaces between crazes.

Phase transitions of p-butoxybenzylideneaminobenzonitrile in oriented polymer matrices

Polymeric composite systems containing the liquid crystalline (LC) compound p-butoxybenzylideneaminobenzonitrile have been prepared by way of polymer drawing in an adsorption-active medium containing the desired additive. Orientation and phase transitions of the low-molecular compound in oriented matrices of polytetrafluoroethylene (PTFE), high density polyethylene (HDPE) and poly(ethylene terephthalate) (PETP) have been studied by IR polarization spectroscopy and DSC. The LC compound contained in the porous structure of the polymer can form highly ordered microdomains in which orientation is sufficiently sharply and reversibly changed during phase transitions. The phase transitions of the LC compound in polymer micropores are spread out and occur at lower temperatures than in the free state.

Influence of stress and the nature of the adsorption-active medium on the amount of crazes appearing in polyethylene terephthalate during uniaxial stretching of the polymer under creep conditions

The influence of stress and of the nature of the adsorption-active medium on the amount of crazes formed in PETP during uniaxial stretching has been investigated under the action of a constant load in liquid media. Critical conditions for the development of crazes were determined in relation to stress and the degree of reduction of surface energy at the polymer-liquid interface. Superposition of the influence of stress and of surface energy at the polymer-liquid interface was found to affect both the total number of crazes and the critical conditions of their formation.

Effect of filler type on the mechanical properties of latex films of natural rubber strained in liquid media

The structural mechanical properties of natural rubber with various types of fillers and different degrees of dispersion (chalk, TiO 2, polystyrene) subjected to strain in air and in the presence of an adsorption-active medium (alcohol) are studied. It is shown that in alcohol the strain mechanism depends on the nature of the filler. The structure of crazes formed in natural rubber filled with polystyrene subjected to strain in alcohol is studied. Preliminary cyclic loading can make the action of the adsorption-active medium on filled rubber less efficient.

On the structure of the surface layer in crazes of glassy polyethyleneterephthalate, deformed in adsorption-active medium

A study is presented of the structure and permeability of glassy PETP films deformed in adsorption-active media. The permeability of such films depends in an extremal manner on the degree of polymer extension. It is assumed that the permeability growth is connected with an increase in the porosity of the polymer; films dried under isometric conditions contain enclosed micropores, isolated from the surrounding medium by a dense surface layers consisting of fibrillar macromolecular aggregates.