The membranes (Nafion and LDPE) in binary liquid mixtures benzene + methanol - sorption and swelling

Sorption studies provide valuable information about the interactions of the components of the liquid mixture with the polymer. In the present paper, the behaviour of Nafion and low-density polyethylene membranes in binary mixtures benzene + methanol was examined with respect to their application in separation processes. The individual sorption isotherms, the separation factors, and the composition of the swollen membranes were derived from the experimental data. The results confirm that Nafion as a polar material sorbs the more polar component of the mixture (methanol) preferentially to the less polar component (benzene) whereas non-polar polyethylene prefers non-polar benzene in the whole concentration range. Volume measurements of the swollen membranes indicate that the ideal sorption behaviour cannot be considered for the selected systems.     

Flow microcalorimetry and competitive liquid sorption II. Liquid sorption and wetting on macroreticular hydrophilic/hydrophobic networks

Three different types of polymer networks (polymer resins) were investigated by flow microcalorimetry and selective liquid sorption from 1-propanol(1)-water(2) mixtures. Type 1 network structure is formed by non-polar, non-swelling, macroporous resins (Amberlite XAD-2, Amberlite XAD-4). The composite isotherms for alcohol sorption were S-shaped. 1-Propanol is preferentially sorbed up to x₁≈0.5. Macroreticular non-ionic resins, Amberlite XAD-7 and XAD-8, represent the second type of polymer structure. These particles have a considerable extent of swelling in the binary liquid mixture and 1-propanol was preferentially sorbed by the polymer. The composite isotherms were U-shaped and exhibit maxima and minima. The third type of network structure is attached to macroreticular polar, hydrophilic ion-exchange resins (Chelite-S, Amberlyste A-21). Depending on the composition range of the binary liquid mixture, the resins may swell to a different extent and water is preferentially embeded in the porous polymer network. The swelling of the polymer networks was monitored by the gravimetric technique in separate experiments. The sorption capacity determined from the liquid uptake of the resins was related to the sorption capacity derived from the reduced surface excess. The enthalpy effect accompanying the sorption process was determined by flow and immersion microcalorimetry. The enthalpy of displacement isotherms reveals differences in polarity and swelling ability of the polymer network fairly well. Structural changes in the adsorption layers and formation of alcohol-water clusters on the surfaces play an important role.     

Mass and momentum transfer in polymers. III. Effects of liquid penetrants on tensile stress relaxation of amorphous polymers

Sorption, diffusion, swelling, and tensile stress relaxation measurements were made at room temperature (23°C) for the systems poly(n-butyl methacrylate) (PBMA) with liquid methanol and ethanol, and poly(methyl acrylate) (PMA) with liquid water. Stress relaxation curves for the fully swollen polymers could be superimposed approximately with those for the dry polymers by appropriate shifting along the long axes. For PMA–water the measured curve for stress relaxation with concurrent sorption could be predicted accurately by using a moving boundary theory with data measurements of stress relaxation of the unswollen and swollen polymer combined with sorption data. The modified moving boundary theory is generalized to include the effects of dimension changes through swelling and the larger effects of plasticization associated with sorption of liquids. This improved theory accurately predicts measured curves of stress relaxation with concurrent sorption for the PBMA–alcohol systems from individual stress relaxation, sorption, diffusion and swelling data. The general approach should be applicable to other amorphous polymer–liquid swelling agent systems. The anisotropic nature of swelling of polymer films and its effect on calculated diffusion coefficients are discussed briefly.     

Sorption and swelling in ultra-thin polymer films

Different polymers were investigated with respect to the sorption of solvents and gases. Depending on the chemical nature of the polymers this sorption leads to polymer swelling. The degree of swelling D/D₀ was measured utilizing Small angle X-ray scattering (SAXS) as well as Surface plasmon resonance (SPR). From the change in film thickness after swelling in different solvents Hildebrand parameters of the polymers were determined. By crosslinking of the polymer films the degree of swelling can be controlled. In the case of ultra-thin polyimide films a higher degree of crosslinking led to a decreased selectivity of the transport of gases through the membrane. Reptation of macromolecules was also investigated and the influence of polymer swelling in different solvents has a great influence on the selectivity and diffusion coefficient.     

Effect of swelling on radiation-induced grafting of styrene to polyethylene

The radiation-induced grafting of low-density polyethylene in contact with styrene solution was studied. The effect of the degree of swelling of the polymer on the rate of grafting was investigated by diluting the styrene with methanol and with n-octane. For styrene-methanol solution, the rate of grafting was found to increase with degree of swelling, passing through a maximum when the sorbed solvent reaches 6.2 wt-% (70 vol-% methanol in the outside solution) and decreasing therafter. The methanol fraction of the sorbed liquid is far too small to cause precipitation of the grafted chains and inhibition of their termination rate. The dilution of styrene by octane has no effect on the swelling of polyethylene, but it decreases the grafting rate over the entire concentration range. The results are explained in terms of the concentration of sorbed monomer and the viscosity of the amorphous region of the polyethylene swollen by nonpolar liquids. Supporting evidence for the mechanism is presented in the form of grafting kinetic data as a function of dose rate (2.8 × 10²?9.5 × 10⁴ rad/hr), and post-irradiation grafting measurements for polyethylene in methanol-styrene (70/30, v/v). The data indicate that at the maximum grafting rate an optimum is achieved between a high concentration of sorbed monomer and a low viscosity for the poorly swelled polymer matrix.     

Comparison of permeability coefficients of organic vapors through non-porous polymer membranes by two different experimental techniques

The permeability coefficients of saturated and non-saturated vapors of benzene, hexane and cyclohexane through flat polymer membranes (low density polyethylene BRALEN FB2-30 and polyether-block-amide PEBA 4033-PE) by two different experimental techniques at 298.15 K are reported. The permeation data have been obtained using the differential flow permeameter and sorption ones by glass sorption apparatus with McBain’s spiral balance. The so-called stationary (steady) diffusion theory has been applied for evaluating the permeability coefficients from sorption (equilibrium) data and obtained values have been compared with the permeability coefficients from permeation (steady-state) measurements. In the case of relative lower vapors sorption in polymers (hexane and cyclohexane) good agreement between permeability coefficients from sorption and permeation is obtained. Hence, this paper proves the possibility to estimate the permeability coefficients of organic vapors from sorption data without need of performing the permeation experiments.     

Molecular Filtration of Hydrocarbon Isomers Through Polymer/[Liquid Crystal] Composite Membranes

Ultrathin membranes of a polymer/(liquid crystal) mixture were prepared by spreading a single drop of a casting solution on the water surface. The thickness and the aggregation state of the water-cast membrane can be controlled by the kind of solvent and the concentration of the solution. In the case of a liquid crystalline state above the crystal-nematic phase transition temperature, T _KN, the polymer (liquid crystal) composite membrane follows Henry's law for the sorption isotherm of hydrocarbon gases and, also, Fickian sorption for the sorption-desorption kinetics. These results indicate that hydrocarbon gases permeate through a homogeneous medium composed of liquid crystalline molecules. Therefore, the permeability coefficients of hydrocarbon gases can be controlled by the dimensions of the channels through which the gas molecules diffuse. The channel for diffusion is generated by thermal or fluctuating molecular motion which opens up the intermolecular distance between liquid crystalline molecules. In the case of a self-supported liquid crystalline membrane, the channel dimension can be controlled in the range of several Å by both the intermolecular distance and the degree of thermal molecular motion of the liquid crystalline molecules. Separation of hydrocarbon isomers was investigated by use of composite membranes composed of a polymer matrix and self-supported liquid crystalline molecules.     

Correlation between environmental stress cracking and liquid sorption for low-swelling liquids

The environmental stress cracking (ESC) of polyethylene has been studied under conditions of dynamic equilibrium with the liquid for low-swelling liquids. Even among active ESC agents, there is a clear relative order of efficiency. It has been shown that the liquid becomes less efficient with increasing equilibrium swelling. This fact has been attributed to local plasticization of the crack front leading, in turn, to a reduction in the high-stress concentrations associated with a wedge-shaped crack. Some semiquantitative ideas are proposed in an attempt to explain the relation between ESC efficiency and volume/volume sorption.     

Effect of Chain Length of Polyethylene Glycol and Crosslink Density (NCO/OH) on Properties of Castor Oil Based Polyurethane Elastomers

The equilibrium swelling study of polyurethanes (PU) was carried out in various solvents in order to calculate their solubility parameter. The kinetics of swelling and sorption have also been studied in 1,4‐dioxane at 30°C. The PU was synthesized by reacting a novel polyol (castor oil derivative and epoxy based resin, EpxR) and one of the polyethylene glycols (PEG 200, PEG 400, PEG 600) with different weight compositions, with a toluene diisocyanate (TDI) adduct (derived from toluene diisocyanate and R60 polyol). Different NCO/OH ratio viz. 1, 1.3 and 1.7 were employed in the study. The results were found to vary with the weight composition of polyol components, as well as the crosslink density of the samples. The sorption behavior is also found to vary with the molecular weight of polyethylene glycol employed in the preparations of the polyurethanes. Kinetic studies of swelling revealed that the sorption is anomalous in nature. The diffusion coefficient (D) increased with an increase in the NCO/OH ratio and decreased with an increase in chain length of polyethylene glycol. The sorption coefficient (S) decreased with an increase in crosslink density (NCO/OH) and increased with increasing polyethylene glycol (i.e., PEG 200, PEG 400, and PEG 600) moieties in the polyurethanes. The molecular weight between two crosslink points was calculated using the Flory Rehner equation (24), and hence, the number of chains per unit volume (N) and degree of crosslinking (ν) in all the samples were determined.     

Preparation and characterizations of asymmetric sulfonated polysulfone membranes by wet phase inversion method

This paper presents an original approach to prepare the asymmetric sulfonated polysulfone membranes by using wet phase inversion method and their applications for dehydrating a water/ethanol mixture by pervaporation. The separation performances of sulfonated membranes were strongly affected by the degree of sulfonation and the degree of swelling of membranes. The substitution degree of sulfonic group enhanced the permselectivity of sulfonated polysulfone membranes by increasing the hydrophilicity of polymer backbone. Based on the observations of membrane morphology and light transmittance measurements, the degree of sulfonation of polysulfone presented less influence on the membrane formation pathway and the final structure of membrane in wet phase inversion process. It was also found that the sulfonated membranes showed well hydrophilic properties and facilitated water adsorption in the membranes. The sorption and permeation properties also showed that the permselectivity of asymmetric membrane was dominated by the permeate diffusion rather than the permeate sorption in the skin layer. The high separation performance of pervaporation membrane can be achieved by phase inverse method with sulfonated polysulfone.     

Swelling behavior of the filling-type membrane

A model that can express the swelling of the filling-type membrane was developed by modifying a model that was developed for a crystalline polymer. The filling-type membrane is composed of two different polymers. One is porous substrate and another is a polymer that filled pores of the substrate. The filling technique can effectively suppress polymer swelling due to the substrate matrix. The model needs two parameters: one is a unit ratio of tie segments in the substrate to the filling polymer, f, which can express a mechanical strength of the substrate, and another is the Flory interaction parameter, χ, between the filling polymer and a solvent that expresses a mixing energy. A porous high-density polyethylene film was used as a porous substrate, and plasma-graft filling polymerization technique could make the filling-type membrane. Methylacrylate was used as a grafting monomer that filled the pores of substrate. A swelling behavior of the filling-type membrane and pure poly(methylacrylate) were measured by the vapor sorption method at different solvent activities. The model was in good agreement with experimental results for the filling-type membrane. Using the model, swelling of the filling-type polymer was compared with a crosslinked polymer, which can be expressed by Flory and Rehner model. The comparison showed that the filling technique is a good way to suppress polymer swelling, and a high crosslinking density is needed to obtain the same level of swelling suppression effect the filling type membrane showed. © 1997 John Wiley & Sons, Inc.     

The influence of cellulose complexite swelling on its protolytic properties in aqueous-organic mixtures

The swellability of cellulose complexite containing hydroxamic acid and amidoxime groups and its matrix (grafted copolymer of cellulose and polyacrylonitrle) in variable-composition water-methanol and water-dimethylsulfoxide mixtures was studied by pH metry, refractometry, and atomic absorption spectroscopy over the pH range 5.30–12.87 corresponding to the dissociation of the functional groups of fibers. The solvation characteristics of fibers, including specific volume, degree of sorption, and interphase distribution coefficients of mixture components, were determined experimentally. The Gibbs energies of sample swelling and the dissociation constants of complexite groups were calculated taking into account the contribution of the solvation parameter. The suggestion was made that the acid properties of the polymer were determined by the predominant influence of the specific solvation of its matrix caused by the formation of H-bonds in complexite-solvent systems.     

Paradoxes of thermodynamics of aqua-vapor-polymer interface equilibrium

Schroeder’s paradox is the different equilibrium degree of swelling for a hydrophilic polymer in saturated aqueous vapor and in liquid water; it is experimentally verified by the registration of increasing volume of spherical polymer samples upon the processes of vapor and liquid water sorption. Specimens of three main classes of hydrophilic cross-linked polymers were tested. These had gel, hypercrosslinked, and macroporous structures that differed by the scale of the Schroeder effect. A similar effect is typical also for hydrophobic polymers upon swelling in liquid organic solvents and their saturated vapor. The paradox is explained by the lower activity of a sorbate in a saturated vapor compared to its activity in a liquid phase. The ability of many samples of cross-linked polymers with different degrees of saturation with a sorbate to be at equilibrium with the saturated vapor of a sorbate is explained by the differences in the inner structure of these samples, i.e., by differing in the swelling combinations and the intensity of the interchain interactions in a polymer network.     

The influence of transverse differential swelling stresses on the kinetics of sorption of penetrants by polymer membranes

The influence of transverse differential swelling stresses on the kinetics of sorption of a penetrant in a polymer membrane exhibiting linear viscoelasticity is described by a model developed from the much simpler one of Crank. Sorption and transverse swelling kinetic curves are computed numerically. The character of absorption and desorption curves is examined systematically mainly as a function of (i) the magnitude of the stresses set up and of the stress-dependence of the diffusion coefficient, (ii) the relative rates of stress relaxation and of diffusion, and (iii) the degree of plasticization or “softening” of the polymer by the penetrant. It is shown that important general features of experimental sorption kinetic curves can be reproduced satisfactorily under well defined conditions. Attention is also given to transverse swelling kinetic curves. Their correlation with the corresponding sorption curves is examined briefly but systematically and discussed with reference to experimental data.     

Strength-strain,barrier, thermal,and fire-resistance properties of nanocomposites based on linear polyethylene with montmorillonite

Strength-strain, sorption, thermal, and fire-resistance properties of nanocomposites based on linear low-density polyethylene with montmorillonite modified with a quaternary phosphonium salt were studied. It is shown that the diffusion, sorption, and permeability of the nanocomposites depends on the type of a montmorillonite and its concentration in the polymer, and also on the nature of a penetrant and degree of its thermodynamic affinity for the polymer. Introduction of up to 3 wt % organomontmorillonite improves the thermal stability by more than 90°, raises the oxygen index to 20 vol %, and makes the elastic modulus 47% larger relative to the base polymer due to the nanostructural morphology formed in the course of mixing.     

非水体系中磺酸铜型树脂对有机碱的配位吸附

研究了磺酸铜型树脂在乙醇、乙酸乙酯和正己烷中对苯胺、N 甲基苯胺和N ,N 二甲基苯胺的吸附规律 ,并与水中的结果相对照 .树脂在正己烷中对苯胺等的吸附量最大 ,在乙醇、乙酸乙脂和水中依次减小 .在4种不同的介质中 ,树脂对苯胺的吸附均表现出相同的吸附选择性 ,即对苯胺的吸附亲合性最大 ,N ,N 二甲基苯胺最小 .在不同的介质中 ,树脂对苯胺的吸附基本符合Freundlich吸附方程 ,并计算了在不同介质中对苯胺吸附的吸附焓 .在水中 ,吸附速率最高 ,在乙醇、乙酸乙酯和正己烷中 ,吸附速率依次降低     

Vapour permeation and sorption in fluoropolymer gel membrane based on ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide

The emissions of hydrocarbons from fossil fuels into atmosphere entail both an economic loss and an environmental pollution. Membrane separations can be used for vapour recovery and/or vapour removal from the permanent gas stream, given that the appropriate membrane is identified. A neat poly(vinylidene fluoride-co-hexafluoropropylene) membrane is impermeable to both the representatives of aliphatic hydrocarbons and branched hydrocarbons, namely hexane and isooctane, whereas the permeation flux is enhanced by the presence of 80 mass % of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide in the membrane, as detailed in this work. The permeabilities of hydrocarbon vapours were determined from the binary mixture containing hydrocarbon and nitrogen to simulate the real input of an air stream containing a condensable hydrocarbon. The diffusion coefficient determined from sorption measurements was higher for hexane, as would be expected for a smaller molecule, whereas both the sorption isotherms and permeabilities of the hydrocarbons studied were found to be almost identical. It is possible that the sorption effect predominates in the transport mechanism for VOCs/N₂ separations.     

Swelling and sorption in polymer–CO2 mixtures at elevated pressures

Experimental data on gas sorption and polymer swelling in glassy polymer—gas systems at elevated pressures are presented for CO₂ with polycarbonate, poly(methyl methacrylate), and polystyrene over a range of temperatures from 33 to 65°C and pressures up to 100 atm. The swelling and sorption behavior were found to depend on the occurrence of a glass transition for the polymer induced by the sorption of CO₂. Two distinct types of swelling and sorption isotherms were measured. One isotherm is characterized by swelling and sorption that reach limiting values at elevated pressures. The other isotherm is characterized by swelling and sorption that continue to increase with pressure and a pressure effect on swelling that is somewhat greater than the effect of pressure on sorption. Glass transition pressures estimated from the experimental results for polystyrene with CO₂ are used to obtain the relationship between CO₂ solubility and the glass transition temperature for the polymer. This relationship is in very good agreement with a theoretical corresponding-states correlation for glass transition temperatures of polystyrene-liquid diluent mixtures.     

An analysis of the swelling equilibrium of an ionic polymer network in a binary liquid mixture

An equation which represents the swelling equilibrium of an ionic polymer network in a binary liquid mixture is introduced and evaluated numerically. Discontinuous volume changes are obtained with pertinent values of the parameters. From two types of dependence of the degree of ionic dissociation on the composition of a liquid mixture, two types of volume transitions of an ionic gel are illustrated. One is the transition typically seen in acrylamide gels, and the other is a re-entrant transition typical of isopropylacrylamide gels. The selective dissolution factor of two liquids into a swollen polymer network also becomes discontinuous in accordance with the discontinuous volume change. Transition points and the spinodal line are calculated from a generalized form of the free energy change of the swollen gel system. © 1994 John Wiley & Sons, Inc.