Sequential vapor sorption of additional penetrants in preswollen polymers

Uptake and removal of a second penetrant in polymer samples preexposed to another vapor dictate many industrially important processes. Both the kinetics and equilibrium sorption can be strongly affected by the presence of the first permeant in the polymeric matrices. A novel experimental setup was constructed to study penetrant uptake in sequence. Sorption of the second vapor took place while the partial vapor pressure of the first vapor was maintained, so that diffusion into the preswollen polymer approximated transport in a pseudobinary system. Polybutylene was chosen in this work to illustrate the capability of this versatile experimental system. Both the rate of diffusion and equilibrium sorption of the second vapor were found to depend on the prevailing composition of the preswollen polymer for all penetrant pairs studied.     

Gas and vapor permeability of perfluoroalkylated polymers

Summary Three polymers bearing benzene rings: polystyrene, poly(2,6-dimethyl-p-phenylene oxide) and poly(diphenylacetylene), were chemically modified with bis(heptafluorobutyryl) peroxide. The heptafluoropropylated products have higher gas permeabilities than the parent polymers. In the pervaporation of aqueous ethanol, modified poly(diphenylacetylene) showed ethanol permselectivity.     

Modeling of vapor sorption in glassy polymers using a new dual mode sorption model based on multilayer sorption theory

Conventional dual mode sorption (CDMS) model is one of the most effective models in describing vapor sorption isotherms with a concave towards the activity axis in glassy polymers, while engaged species induced clustering (ENSIC) model has been approved to be highly successful in modeling vapor sorption isotherms in polymers with a convex to the activity axis (BET type III) over a wide range. However, neither of them is effective to describe other types of vapor sorption isotherms, especially sigmoidal isotherms. The Guggenheim–Anderson–de Boer (GAB) model fits extremely well with sigmoidal isotherms such as some vapor especially water vapor sorption data in food and related natural materials. However, one assumption of the GAB model for vapor sorption in glassy polymers is inconsistent with the fact that there are two species of sorption sites as the CDMS model assumes. Based on multilayer sorption theory on which the Guggenheim–Anderson–de Boer (GAB) model is based, a new dual mode sorption (DMS) model for vapor sorption in the glassy polymers is deduced. The mathematical meanings and the physicochemical significances of the parameters in the new model are analyzed. The new model has been verified experimentally by some special cases. Comparisons of the new DMS model with the CDMS and the ENSIC models prove that only the new model fits extremely well with all types of vapor sorption isotherms in the glassy polymers.     

Gas sorption in polymers,molecular sieves,and mixed matrix membranes

Gas sorption has been an underutilized technique for characterizing organic–inorganic hybrid (mixed matrix) membranes. Sorption in these membranes, which are composed of rigid inorganic domains, such as zeolites, dispersed in a polymer matrix, should be approximately additive. Sorption in the neat polymers and zeolites were first measured to demonstrate that sorption in mixed matrix membranes is approximately additive in the absence of other effects. Sorption in mixed matrix membranes was demonstrated to be additive. This extends to cases where sorption in one or both phases of the mixed matrix membrane is affected by an outside contaminant. For example, zeolite 4A is extremely hydrophilic and easily affected by contaminants from processing or from the test gases. Zeolite 4A encapsulated within a polymer matrix can still be affected by these same components, and this causes sorption lower than predicted based on that in unaffected polymers and sieves. This sorption analysis has proven to be very important in understanding the permeabilities and selectivities of mixed matrix membranes. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 4053–4059, 2007     

Tg/Tm for unsubstituted polymers

Polymers under discussion can be represented by [CH₂)_nR]_p where R is CH₂, CF₂, O, S or CH = CH; and where n can vary from 0 to ∞. The copolymer, P(ethylene-alt-TFE) is also included. These polymers tend to be highly crystalline with resultant confusion about their Tg and hence their Tg/Tm values and a clarification scheme is proposed in which it is considered that each such polymer has a double glass transition, Tg(L)/f(χ_c), Tg(U) = F(χc); and a sub-glass T<Tg(L) at 0.75 Tg(L), where F(χc) signifies a function of fractional crystallinity, χ_c. T<Tg(L) and Tg(L) increase linearly with Tm. The ratio, Tg(L)/Tm, is therefore, not a constant but is close to 0.5. T<Tg(L)/Tg(L) is also not a constant but is close to 0.75. Tg(U) lies within a wedge whose top and bottom sides increase with Tm. The premelting, intracrystalline transition temperature, Tα_c, is also a linear function of Tm. It is suggested that the scheme outlined above should help to resolve uncertainties in assigning transition temperatures for unsubstituted polymers. The low Tg/Tm values for these polymers is discussed in terms of their small da/dc ratios and small crosssectional areas per chain.     

Gel–water relationships in hydrophilic polymers: Thermodynamics of sorption of water vapor

A thermodynamic study was conducted of water vapor adsorption on four hydrophilic polymers (agar, carboxymethyl cellulose, gelatin, and maize starch) at 12 and 25°C. Monolayer coverage amounted, after correction for crystallinity, respectively, to 0.93, 1.46, 0.51, and 0.77 mol water/mol monomer. Evidence is adduced from the Bradley equation and thermodynamic data to indicate that at least during coverage with the second layer of water, the energy of adsorption is greater than that due to condensation alone. Differences in the amount of sorption and in the trend of values of ΔS?° and ΔH?° with the amount of sorbed water are related with differences in the strength of intermolecular association as affected by steric hindrances.     

Water vapor sorption and diffusivity in bark

Physical properties of importance in drying processes have been compiled for bark, including spruce, pine, and birch in Sweden. Water vapor sorption isotherms were determined for the bark of these trees. At 95% relative humidity and 25°C, outer birch bark reached a moisture ratio (kg water/kg dry mass) of 5%, whereas inner birch bark, spruce bark, and pine bark reached moisture ratios of 21, 28, and 25%, respectively. The transverse water vapor diffusivity in samples of spruce bark, pine bark, inner birch bark, and outer birch bark were determined to be 1.1–1.7 · 10^?6, 2.2–9.2 · 10^?7, 1.5–2.6 · 10^?6, and 4.3–13 · 10^?8 m²/s, respectively. The in-plane diffusivity was of the same magnitude as the transverse diffusivity in inner birch bark, whereas in outer birch bark the in-plane diffusivity was several times higher than the transverse diffusivity. These data can be used to model the drying behavior and can thus aid in the design of efficient bark drying processes.     

Differential sorption in glassy polymers

Anomalous sorption curves have often been observed for differential sorption experiments in glassy polymers. A model is proposed to describe this non‐Fickian behavior. This model is based on the presence of interfacial resistance caused by slow rate processes at the phase boundary. Predictions of the model are compared with general experimental observations. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1431–1440, 1999     

Gas permeation related to the moisture sorption in films of glassy hydrophilic polymers

The purpose of this article is to elucidate the effect of integral sorption of moisture on gas permeation in glassy hydrophilic polymers. The oxygen and the simultaneous moisture sorption into various hydroxypropyl methylcellulose (HPMC) films were measured under a wide range of relative humidities using sorption analyzer equipment. Correspondingly, the oxygen permeability at different ambient conditions was measured using an oxygen detector. The solubility of oxygen in the HPMC film was found to be affected by the amount of water and therefore by the water state. At low moisture content, the water molecules are present as bound water, which promotes the sorption of oxygen in the HPMC films. At moisture content higher than 5%, water clusters are rapidly formed, which increase the affinity of HPMC polymer towards water rather than towards oxygen molecules, resulting in a decrease of oxygen solubility in the polymer. This was found to be the governing factor for the reduction in the oxygen permeability in glassy HPMC films at high water activity. This proposes a specific interaction between moisture sorption and oxygen transport in coating films like HPMC, which is of important aspect in the coating design and formulation. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Free radical formation in oriented polybutadiene during mechanical deformation below Tg

The tensile behavior of oriented polybutadiene at 83°K is systematically studied as a function of strain rate and pre-test orientation. Electron spin resonance studies of radical formation are made in conjunction with the mechanical tests. Three different modes of mechanical behavior are observed (brittle, crazing and a second ductile behavior without crazing), depending on test conditions. Radical formation is observed in association with the two ductile modes of behavior. The ESR spectra obtained are attributed to a combination of allyl radicals formed by chain scission between the α-methylene groups and peroxy radicals. The relative quantity of the two radical species present is thought to be related to the ratio of cis/trans-isomerism. Stability of the radicals observed with time and with an increase in temperature is studied. Further studies are made of the quantity of environmental test gases absorbed during crazing.     

Modified BET models for modeling water vapor sorption in hydrophilic glassy polymers and systems deviating strongly from ideality

The use of two modified BET models for modeling water vapor sorption in various nonideal systems involving strong interactions (textiles, polyamides, polyimide, vinyl polymers, proteins, and related compounds) has been investigated. Contrary to the original BET model derived for multimolecular adsorption and restricted to low water activities only, the three-parameter BET model, which considers sorption on a limited number of sorption layers, allowed the modeling for activities up to 0.9. This model has been shown to be an important extension of the original model but remains limited, due to the divergence of the model for a → 1. Releasing an other important assumption of the BET model, the GAB model was then investigated and its striking efficiency, in terms of high correlation coefficients and low residual sums of squares, was demonstrated for activities up to 0.95, thus covering all the usual experimental range. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1415–1430, 1998     

Gas sorption and transport in poly(phenylene oxide) and comparisons with other glassy polymers

The sorption and transport of several gases in poly(phenylene oxide) were measured at 35°C, and the results have been analyzed in terms of the dual sorption/mobility models which have been successfully employed for this purpose for other glassy polymers. Both the extent of sorption and rate of permeation of gases are quite large for poly(phenylene oxide) compared to other glassy polymers with rigid chain backbones. It is shown that the high extent of sorption is owing to the high glass transition temperature of this polymer, but this is not a significant factor in its high permeability to gases. The latter stems from large diffusion coefficients. It is shown that the capacity of the Langmuir mode of sorption inherent to glassy polymers is related to the value of the glass transition temperature in a general way for a wide variety of polymers. Observations about the diffusion coefficients for numerous gas–polymer pairs are discussed.     

Slow relaxations in semicrystalline poly(butylene succinate) below and above Tg

The slow molecular mobility in the amorphous part of the semi‐crystalline polymer poly(butylene succinate) (PBS) has been studied by the thermally stimulated depolarization current (TSDC) technique. Experiments were carried out in the temperature range, which includes the glassy state, the glass transformation region, and the rubber state. A broad and low intensity secondary relaxation was observed in the temperature region from ?140°C up to the glass transition region; the activation energy of the motional modes of this secondary relaxation was in the range between 35 and 55 kJ mol^?1. The glass transition temperature of PBS, provided by the TSDC technique, was T_g = ?40 °C, and the fragility index was found to be m = 43. The aging behavior of the main and of the secondary relaxations was analyzed. A strong relaxation above T_g was observed, whose molecular origin was discussed. The thermal behavior of the PBS was also characterized by differential scanning calorimetry. POLYM. ENG. SCI., 55:1873–1880, 2015. © 2014 Society of Plastics Engineers     

Mechanism of iodine vapor sorption in polyacetylene

Simultaneous measurements of weight uptake and conductivity variation during iodine vapor sorption of polyacetylenes (PA) at 20–25°C show that the sorption is a two-stages process. For the first stage, iodine diffusion to the fibrilar surface of the cis-rich and trans-rich PA is of Knudsen type pore diffusion as supported by the sorption measurements of hexane for the cis-rich PA, and has a diffusion coefficient in the order of 10⁷ cm²/sec. Conductivity of the PA rises rapidly and reaches a maximum at end of the stage. For the second stage, the diffusion is more restrictive and has a diffusion coefficient lower than the first stage by a factor of about 10 due to multilayer sorption of iodine, which leads to a decrease in the pore diameter and therefor the diffusion rate. In addition to the restricted diffusion in the second stage, diffusion of the iodine molecules adsorped on the fibrilar surface into the interior of the fibrils is appreciable for cis-rich PA (leading to a conductivity drop) and is negliglible for tran-rich PA (leading to insignificant variation in conductivity).     

Water vapor sorption and transport in dense polyimide membranes

The sorption and transport of water vapor in five dense polyimide membranes were studied by thermogravimetry. The sorption isotherms of water vapor in the polyimides could be successfully interpreted by both the dual‐mode sorption model and the Guggenheim–Anderson–de Boer equation. The water vapor diffusion behavior was found to be nearly Fickian at higher water vapor activities, whereas non‐Fickian diffusion was observed at lower water activities. The phenomena could be well described by the mechanism of combined Fickian and time‐dependent diffusion. The diffusion coefficient and water vapor uptake in the polyimides were strongly dependent on the polymer molecular structure. Except for the polyimide prepared from 3,3′,4,4′‐diphenylsulfone tetracarboxylic dianhydride and 1,3‐bis(4‐aminophenoxy) benzene, the permeability of water vapor in the dense polyimide membranes predicted from the sorption measurement at 30°C corresponded well with the water vapor permeability measured at 85°C. Among the polyimides studied, pyromellitic dianhydride–4,4′‐diaminophenylsulfone (50 mol%)/4,4′‐oxydianiline (50 mol%) showed both high water sorption and diffusion and, therefore, high water vapor permeability, which for vapor permeation membranes is necessary for the separation of water vapor from gas streams. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2306–2317, 2003     

Water vapor sorption by amylose and cellulose acetates

Experimental sorption isotherms for amylose acetate (ADA) and cellulose acetate (CDA) are discussed in terms of the B.E.T. theory and the Hailwood–Horrobin theory. The B.E.T. theory predicted a good fit to most of the experimental isotherms up to 0.8 relative vapor pressure. A somewhat better fit was found with the solution theory of Hailwood–Horrobin over the entire range of relative vapor pressures. Annealing had little effect on the sorption properties of CDA, but significantly reduced sorption in ADA. This was attributed to a higher degree of ordering in the amylose polymer.     

Methanol vapor sorption in plasma polymerized thin films

Methanol vapor sorption isotherms for methyl acrylate (MA) and styrene plasma polymer thin films are measured and compared with those for conventionally polymerized analogs. Isotherms are collected for the series of thin films between 30 and 75 °C using a quartz crystal microbalance. Rubbery poly(methyl acrylate) (PMA) and plasticized poly(styrene) (PS) conscribe to the classical Flory-Huggins-type sorption mechanism, whereas the sorption isotherms of the plasma polymer analogs have a distinct Langmuir-type sorption component. Plasma polymers are analyzed using the dual mode sorption model commonly applied to conventional glassy polymers. The heat of sorption of the ‘glassy’ ppMA films (−56 kJ/mol) is far above that measured for PMA (−20 kJ/mol) and much closer to that measured for glassy poly(methyl methacrylate) (PMMA) (−48 kJ/mol). The work shows that both the plasticization effect of sorbed penetrants and relaxation phenomena associated with slow cooling are suppressed in the plasma polymer materials when compared to the conventional polymer analogs.     

Energetics of gas sorption in glassy polymers

The molar enthalpy for gas sorption in glassy polymers at a fixed concentration, often called the isosteric enthalpy of sorption, exhibits a clearly discernable minimum when plotted as a function of penetrant concentration. This unexpected behaviour has been observed in several glassy polymer systems including poly(ethylene terephthalate), polyacrylonitrile and polycarbonate. The behaviour can be modelled by analysing the temperature dependence of the various equilibrium parameters comprising the so-called dual mode sorption model for gas sorption in glassy polymers. The fundamental significance of the various enthalpies describing the temperature dependence of the Henry's law solubility constant, the Langmuir affinity constant and the Langmuir capacity constant are included in the discussion. Provision is made for non-ideal vapours and gases by introduction of the compressibility factor in the expression for the isosteric enthalpy. Application of relationships for calculating both the isosteric and the isothermal enthalpies of sorption is made to the case of CO₂ in poly(ethylene terephthalate) in the temperature range 35° to 115°C. These results and analyses complement the wealth of equilibrium and transport data which are consistent with the dual mode sorption model for penetrant sorption in glassy polymers.