Equilibrium Water Sorption Research Articles

Water sorption and diffusion studies in an epoxy resin system

The kinetics of water absorption in epoxide materials was studied by the aid of a diglycidyl ether of bisphenol-A–triethylenetetramine (DGEBA–TETA) epoxy–resin system containing various amounts of the plasticizer THIOCOL (0–40 phr). The presence of plasticizer permits the formation of products with different crosslinking densities and hydrophilic characters. Dynamic water absorption experiments were carried out at 15, 40, and 70°C. For the fitting of the experimental results, a new model was used, based on a model proposed earlier by Jacobs and Jones. This model considers epoxide product as a two-phase system consisting of a master phase (where the major part of the water is absorbed), which is homogeneous and nonpolar (phase 1), and of a second phase with different density and/or hydrophilic character (phase 2). By making the assumption that water diffusion can take place independently in the different phases of the material in accordance with Fick's second law, we can calculate the diffusion coefficient D and the water content at saturation M∞ for each phase separately. Equilibrium water sorption measurements were performed at 40°C, and the data were analyzed and discussed based on the Guggenheim–Anderson–de Boer (GAB) equation, the results being in support of the two-phase model used in the analysis of absorption kinetics. The linear expansion coefficient and the glass transition temperature of the materials, employed in the discussion of the results, were measured by thermomechanical analysis. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1165–1182, 1999

Evaluation of Calorimetric and Gravimetric Methods to Quantify the Amorphous Content of Desferal

The amorphous content of different Desferal samples was quantified by recording its recrystallization using isothermal microcalorimetry in a static as well as in a flowing humid atmosphere. Furthermore water vapor sorption gravimetry was performed for the same purpose. These analytical methods result in a quantitative signal directly dependent on the content of the amorphous phase (recrystallization, water sorption equilibrium). Their sensitivity allows the detection of amorphous content below 1%. Methods are compared and advantages and disadvantages are discussed.

THERMOPROCESSIBLE HYDROGELS. I. SYNTHESIS AND PROPERTIES OF POLYACRYLAMIDES WITH PERFLUOROALKYL SIDE CHAINS

ABSTRACT Copolymers composed of acrylamide (AM), N,N-dimethylacrylamide (DMAM), N-isopropylacrylamide (NIPAM) and 2-(N-ethyl-perfluorooctanesulfonamido) acrylamide (FOSA) were synthesized by free radical polymerization. The chemical structure of the resulting polymers was characterized with NMR spectroscopy and thermal properties were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). 1H-NMR spectra of the copolymers of NIPAM with FOSA showed that FOSA was incorporated quantitatively. The glass transition temperature (Tg) of the copolymers and the terpolymers decreased with increasing FOSA content. The Tgs, however, were higher than predicted for a random copolymer by the Fox equation, which was attributed to microphase separation of the hydrophobic, fluorinated species. Copolymers of AM and FOSA became discolored above 180°C due to formation of cyclic imide and nitrile moieties through cyclization or dehydration of amide groups. The equilibrium water sorption of the copolymers decreased with increasing FOSA content, but increasing FOSA suppressed the water desorption kinetics. Water sorption and thermal stability were improved by terpolymerization of AM, NIPAM, DMAM and FOSA.

Evaluation of thin defect-free epoxy coatings using electrochemical impedance spectroscopy

The water sorption of thin defect free epoxy films and coatings on aluminium electrodes was studied using the gravimetric method and electrochemical impedance spectroscopy. The results show that the double layer capacitance of the wetted surface of the aluminium electrode under the epoxy film is considerably smaller than that of a bare aluminium electrode, except in the early period of immersion. The resistivity of the solution absorbed in the epoxy coating is much higher than that of the bulk solution. A method for approximately determining the equilibrium water sorption of the epoxy coatings on the metal surface from the double layer capacitance is reported.

Polymer-water interactions in poly(hydroxyethyl acrylate) hydrogels studied by dielectric, calorimetric and sorption isotherm measurements

Abstract Poly(hydroxyethyl acrylate) hydrogels with water contents up to 0.96 (g water/g dry material) are studied with equilibrium and dynamic water sorption isotherm measurements, differential scanning calorimetry, thermally stimulated depolarization current measurements and broadband a.c. dielectric relaxation spectroscopy. Our main interest is focused on comparing results on the state of water in the hydrogel obtained with the different techniques on the same sample and on discussing in a quantitative way the relations among the different descriptions imposed by the different techniques. The results show that for water contents lower than about 0.20-0.30 the hydrogel is a homogeneous system in the temperature range down to −100 °C. At higher water contents phase separation occurs. Several critical water contents have been obtained by means of the different techniques used.

In Vitro Characterization of Sodium Glycocholate Binding to Cholestyramine Resin

Cholestyramine resin in a bile acid sequestrant which binds with bile salts in the intestinal lumen to increase the fecal excretion of bile salts and, thus, lower blood serum cholesterol. In order to gain a better understanding of the lowin vivopotency of cholestyramine,in vitroequilibrium binding studies, water sorption studies, and resin capacity measurements were performed using cholestyramine and the bile salt sodium glycocholate. Equilibrium binding and water sorption studies entailed equilibrating cholestyramine (1.0–20 mg/mL) with solutions which varied in glycocholate anion concentration (0.20–16.5 mM) and chloride anion concentration (15–150 mM). The resin's practical specific capacity for glycocholate was lower than the practical specific capacity for chloride. This difference suggests that the rigid, bulky bile salt was pore excluded from 10% of the resin's ionogentic sites. A fundamental parameter called the capacity‐corrected molar selectivity coefficient,KCl‐GC−, was postulated to describe the underlying binding phenomena and was determined by measuring the free glycocholate and chloride anion concentrations;KCl‐GC−ranged from 9.8 (±0.7) to 18.6 (±0.2) and depended on the square of the free chloride concentration. The capacity‐corrected molar selectivity coefficient was larger than the molar selectivity coefficient due to pore exclusion of glycocholate. A more simple method to calculate the capacity‐corrected molar selectivity coefficient which required less data gave similar values to the more rigorous method (r2= 0.955).

Poly(Urethane)-Crosslinked Poly(HEMA) Hydrogels

Abstract Hydrogels have been prepared from 2-hydroxyethyl methacrylate polymerized in the presence of isocyanate-terminated poly(ethylene glycol) (PEG) crosslinking agents. PEGS of molecular weights 200, 400, and 1000 were investigated. The crosslinked nature of the hydrogels was demonstrated by their insolubility in solvents which normally dissolve poly(HEMA). Hexamethylene diisocyanate (HDI) was mainly used as the isocyanate. The molecular weight of the PEG and the crosslinker content significantly influenced the equilibrium water sorption and mechanical properties of the saturated networks. It was observed that as the molecular weight of the PEG increased, the water sorption increased and the nominal modulus decreased. However, for higher levels of cross-linker, water sorption decreased and modulus increased at low molecular weight PEG; for PEG 1000, water absorption increased as crosslinker content increased. These results are explained by the competing effects of flexibility, crosslink density, and h...

The influence of relative humidity on polyarylate hydrolysis

AbstractThe influence of relative humidity (RH), between 55 and 100%, on the rate of hydrolytic degradation of polyarylate was investigated at 100°C. The equilibrium water sorption in weight percent, C∞, was determined to be C∞ = 0.0043 X (RH)1.2. For specimens 0.32 cm thick, it takes about 24 h to reach 0.9C∞ at 100°C. The decrease in molecular weight (M̄w) due to the hydrolytic attack of water was linear with time, suggesting a zero‐order process with respect to M̄w. The rate of decrease was found to be proportional to C∞. A 30% decrease in M̄w was observed after 21 days at 55% RH and 100°C and, at that point, the material has become brittle.

Water sorption and hydrolytic stability of polycarbonates

AbstractThe hydrolytic stability of a new commercial polycarbonate (Calibre 300, Dow Chemical USA) was investigated and compared with that of other commercial polycarbonates. The tests were conducted between 56% and 95% relative humidity (R. H.) at 100°C. Also performed were water immersion tests at 80 and 100°C. The water diffusivity was found to be 8.7 × 10−7 cm2/s at 100°C with an activation energy of 7.9 kcal/mole. These values are similar to other glassy polymers. The equilibrium water sorption, C∞, was found to increase with temperature and R.H. The isotherm at 100°C was determined to be: C∞ = 0.005945 [R.H.]. When samples immersed in a water bath at 100°C were transferred into room‐temperature water, visible aqueous microcavities were formed due to the condition of super‐saturation, and under stress may become crack initiation sites. For the polycarbonate investigated here, it was found that the decrease in weight‐average molecular weight (M̄)w was a first‐order process under a constant R.H. and temperature, and that hydrolytic embrittlement, i. e., (M̄)w <34,000, was reached after ca. 188, 143, 99, and 66 days under 56%, 73%, 87%, and 95% R.H., respectively, at 100°C. A comparison with reported hydrolytic stability data for other polycarbonates showed large differences in their stability which are believed to be due to the extent of end‐group capping (over 95% in Calibre 300) and resin purity: both phenolic end‐groups and some additives (i.e., fire retardants, thermal stabilizers) are known to accelerate hydrolytic degradation.

Hydrolysis of 1,4‐cyclohexanedimethanol‐based copolyester

AbstractThe solid state hydrolysis of a copolyester based on a mixture of 1,4‐cyclohexanedimethanol and ethylene glycol condensed with terephthalic acid was studied at 100°C and 57 to 96 kPa water vapor partial pressure (55% to 95% relative humidity). The equilibrium water sorption in weight percent (C∞) was found to be where P is the water vapor partial pressure in kPa. For specimens 0.32‐cm thick, it took about 24 h to reach 0.9C∞. The intrinsic viscosity (IV) was measured and used to calculate the relative change in molecular weight (M̄w) from the relationship IV ∝︁ (M̄w)0.7. The decrease in molecular weight was linear with time, and the rate of decrease was found to be proportional to C∞; the empirical correlation is where the rate constant, k, is in day−1. A decrease of 50% in M̄w was observed after 22 days at 95% relative humidity.

A New Water Sorption Equilibrium Expression for Solid Foods based on Thermodynamic Considerations

ABSTRACTSolution thermodynamics was used to derive a new semitheoretical expression to represent sorptional equilibrium in food products having both temperature and water content as independent variables. A number of correlations have been studied and compared with the proposed equation using different requirements, necessary for their application in mathematical modeling and engineering calculations. These criteria were their capabilities to fit the experimental data, to evaluate the derivatives and the heat of sorption, and to predict the behavior at high moistures. On this basis, it was concluded that the present expression provided an accurate description of the sorptional behavior for many food systems.

Synthesis and characterization of poly(urethane- g-acrylic acid)s

The free-radical copolymerization of a completely hard-block unsaturated polyurethane, based on methylene bis(4-phenylisocyanate), erythrol and butane-1,4-diol, with acrylic acid in the presence of 2,2′-azobisisobutyronitrile at 60°C under various reaction conditions is described. Grafting of poly(acrylic acid) onto the unsaturated polyurethane with pendant alkene groups was shown to be successful. Some of the characteristics of the ungrafted and grafted polyurethane copolymers were studied using microanalysis, gel permeation chromatography (g.p.c.), nuclear magnetic resonance, infra-red spectroscopy and differential scanning calorimetry. The resultant poly(urethane- g-acrylic acid)s, poly(U- g-AA)s, and the original unsaturated polyurethane were also studied viscometrically in a mixture of tetrahydrofuran and dimethylformamide (DMF) over the temperature range 26–56°C. Different-sized particles, formed in a solution of graft copolymer in water or DMF, were confirmed by photocorrelation scattering. These measurements indicated that predominantly two types of particle were present in both solutions, and values of the hydrodynamic radii and diffusion coefficients were obtained. The number-average molecular weights M ̄ n and weight-average molecular weights M ̄ W of the polyurethanes and poly(U- g-AA)s were determined by g.p.c. relative to polystyrene as a standard. Equilibrium water sorption measurements of the ungrafted polyurethanes were also carried out at room temperature.

Water sorption isotherms on sucrose and starch by modified inverse frontal gas chromatography

A modification of the inverse frontal gas chromatography method was made to allow for obtaining equilibrium water sorption isotherms at around room temperature. Modification consists of allowing sufficient wet-gas contact time for samples to reach equilibrium then measuring the water uptake by increasing the column temperature to 70°C at the later phase of water desorption. Good agreement was observed between the water sorption isotherms of starch and crystaline sucrose obtained with the modified inverse gas chromatography method and the static method.

Free‐radical synthesis of poly(urethane‐g‐N‐vinyl pyrrolidone)

AbstractRelatively high‐molecular‐weight segmented polyurethanes based on methylene bis(4‐phenyl‐iso‐cyanate), poly(propylene glycol), butane‐1,4 diol, and cis‐2‐butene‐1,4 diol have been synthesized and characterized. These unsaturated polyurethanes were successfully grafted using N‐vinyl pyrrolidone as monomer and 2,2′‐azobisisobutyronitrile as free‐radical initiator. However, grafting experiments involving benzoyl peroxide as initiator were unsuccessful. The graft copolymers were isolated from the ungrafted polyurethane and poly(N‐vinyl pyrrolidone) by selective solvent extraction. Elemental microanalysis, IR, NMR, thermogravimetric analysis, and equilibrium water sorption measurements were used to characterize the graft copolymers.

SORPTION OF WATER VAPOR BY ETHYLENE/VINYL ALCOHOL COPOLYMERS

Equilibrium water sorption isotherms were obtained for ethylene-vinyl alcohol copolymers (EVA) with a wide variety of hydroxyl contents to examine the sorptive capacity of EVA polymers as a function of hydroxyl content. The state of water molecules sorbed by EVA was investigated using the clustering function.The larger was the hydroxyl content of EVA, the higher was the sorptive affinity toward water. The molar ratio of water sorbed to hydroxyl content varied linearly with hydroxyl content over the whole range of relative humidity studied. The BET sorption mechanism was obeyed by this EVA-water system in the relative humidities ranging from about 20 to 44%.At low humidities, the clustering function was larger than -1 for each EVA. This indicates that there is a tendency for water molecules to cluster in these polymers. The clustering function showed a minimum at about 30% relative humidity where the amount of water sorbed by EVA is equal to that required for monolayer to be formed.

WATER SORPTION FOR ACRYLONITRILE/ACRYLAMIDE COPOLYMERS AND POLYACRYLONITRILE/POLYACRYLAMIDE BLENDS

Equilibrium water sorption isotherms for acrylonitrile (AN)/acrylamide (AAm) random copolymers and polyacrylonitrile (PAN)/polyacrylamide (PAAm) blend polymers were obtained and the sorptive capacity of these polymers was discussed.At low humidities, the sorptive capacity of random copolymers was higher than that of blend polymers. With increasing relative humidities the sorptive capacity of blend polymers approached that of random copolymers. BET sorption mechanism could be applied to these cases in the relative humidities 10% to 45%. Nm, water content per a CONH2 group, was constant at every composition of polymer. Moreover, Nm of blend polymers was not different from that of random copolymers.Clustering function showed that polyacrylonitrile had no clustering water over the whole range of humidity, while on polyacrylamide water molecules began to cluster at about 40% relative humidity. On random copolymers there was no clustering at low humidities, and the more acrylamide groups presented in the polymer matrix, the higher was its clustering tendency towards water. Whereas blend polymers had much clustering water even at low humidities for their strong hydrogen bonds between CONH2 groups, its clustering function showed a minimum at about 30% relative humidity where the polymer sorbed the water required to form monolayer.

Equilibrium Water Sorption and Volumetric Behavior of Ion-Exchange Resin Spheres

Equilibrium Water Sorption and Volumetric Behavior of Ion-Exchange Resin Spheres

WATER SORPTION AND WATER VAPOR PERMEABILITY OF NYLON 4

Equilibrium water sorption on Nylon 4 and diffusion process of water in the polymer were studied. Water sorptive ability of Nylon 4 is lower than that of regenerated cellulose, higher than that of cotton and three times of that of Nylon 6. The temperature dependence of the sorption isotherm is not significant.Diffusion coefficient obtained from the permeation experiment depends remarkably on the water content, which is common for the lyophilic polymers. In the case of water content 0.19g/g, Arrhenius_??_ plot of diffusion coefficients has an inflection at about 26°C, which is lower than the glass transition temperature. Sorption rate curve is not of Fickian type even at temperatures higher than the glass transition determined by dilatometry.

The effect of vinyl monomer grafting on the water sorption kinetics and equilibria in cellulose

AbstractWood pulp cellulose was grafted with styrene and a number of other vinyl monomers using the mutual radiation technique. The effects of the level of grafting, type of monomer, and the grafting conditions on the water sorption equilibria and kinetics were determined. Styrene was studied in considerable detail. Styrene grafting reduced the equilibrium water content but water diffusivities increased as a consequence of styrene grafting. Conversely, the grafting of polar monomers such as acrylonitrile, ethyl acrylate, and acrylic acid led to somewhat higher water regains but to decreased diffusion constants.The swelling conditions used during the mutual grafting process with styrene significantly affected the grafting rates and the properties of the resulting grafts. For example, the equilibrium water contents at the same level of grafting, was a function not only of percent graft but also of the specific swelling conditions. These moisture regains were inversely related to the density of the grafted fibers. These results are discussed in terms of the conformation of the grafted side chains which is determined by the various grafting conditions.

Membrane-water structural interactions in reverse osmosis transport

Abstract Reverse-osmotic water permeabilities, equilibrium water sorption levels, and rates of approach to sorption equilibrium were measured for a series of polymers, including hydroxyethyl methacrylate (HEMA), copolymers of HEMA and ethyl methacrylate (EMA), cellulose acetate, cellulose nitrate, and poly(urethans). Pronounced equilibrium solvent clustering behavior was observed for these systems as vapor saturation was approached in sorption experiments. However, clustering tendency was not found to be a function of total membrane water content at saturation but rather appears to be a function of the chemical nature of the polymer in question. Moreover, clustering of water molecules in (relatively) hydrophobic membranes resulted in low effective diffusivities (reverse osmotic permeability divided by equilibrium water content) whereas clustering in hydrophilic membranes led to higher effective water diffusivities. Clustering tendency was not as strong in the case of the weakly interacting membranes (i.e., the cellulose acetates). These conclusions were supported by theoretical diffusivity calculations. Predictions were based on analyses of transient sorption data, employing a dual-mode sorption model, and considering ordinary Fickian diffusion with simultaneous first-order reversible penetrant localization at water-binding sites in the polymer matrices. Means were found for correcting these diffusivity predictions to those values obtained experimentally under reverse osmosis conditions by accounting for the nonideality of the water flux under the latter conditions.