Polymer-gas Systems Research Articles

Polymer materials for solving actual problems of membrane gas

Membrane gas separation using polymeric membranes is one of rapidly developing energy-conserving technologies. This review describes the scope of membrane gas separation problems and demonstrates the relevance of membrane methods for their solution. The basic concepts and regularities of diffusion gas transfer and principles and characteristics of membrane gas separation are considered. The main physicochemical approaches to the selection of membrane materials considering both the properties of the gas–polymer system and the structure and physical properties of polymers are discussed. The considered issues are combined within a common approach to substantiate the choice of both existing commercial polymers and advanced polymeric materials to address important gas separation problems such as separation of air components; carbon dioxide, hydrogen and helium recovery from natural and industrial gas mixtures; and separation of a nitrogen and methane mixture. The avenues of development of the membrane materials science to solve each of the mentioned problems of membrane gas separation are demonstrated. The review is intended for specialists in the synthesis and physics of polymers for planning the research and assessing the applicability of polymeric materials for various practical gas separation problems and for specialists in the membrane materials science and membrane technology. <br> The bibliography includes 165 references.

A modified non-equilibrium lattice fluid model based on corrected fractional free volume of polymers for gas solubility prediction

We propose a model based on non-equilibrium lattice fluid (NELF) theory and corrected fractional free volume of polymers to effectively and accurately predict the solubility of gases in different polymers. The method to achieve this purpose is based on the utilization of NELF model infinite dilution solubility coefficient (S0) as the base of predictive calculations. To account for the isolated pore in the polymer matrix in density estimation, a fractional free volume correction factor (β) was introduced in NELF model. The modified NELF model was successfully applied for prediction of solubility of C3H8 and CO2 in polyethylene oxide (PEO) and CO2 in polyethylene terephthalate (PET), isotactic polypropylene (i-PP), polyetherimide (PEI), polymethyl methacrylate (PMMA) and polyethyl methacrylate (PEMA) with adjustments in β value and depth of diffusion of gases in polymer matrix (ζ) at different pressures and temperatures. This work involves multi-objective optimization using genetic algorithm of MATLAB toolbox with adjusted settings. It applies to find the optimum temperature at which the minimum standard deviation of β for different gas-polymer systems is obtained. β showed the same trend of change with temperature as the constrained pressure imposed on the amorphous phase in semi-crystalline polymers. A cubic correlation for standard deviation for β versus temperature was obtained which was able to anticipate the changing trend of β at different temperatures. The chi-square test results verified that compared with original NELF model, a more accurate model for prediction of gas solubilities in polymers has been proposed.

Improved prediction of solubility of gases in polymers using an innovative non-equilibrium lattice fluid/Flory-Huggins model

A combination of Flory-Huggins and non-equilibrium lattice fluid models are utilized for predicting the solubility coefficients of CO2, CH4, N2, n-C4H10 and i-C4H10 in low and high-density polyethylene, polysulfone and polycarbonate. The genetic programming has been used to acquire the appropriate function for this model. The solubility coefficients at infinite dilution are calculated based on non-equilibrium lattice fluid theory and the gas-polymer interaction is expressed by the Flory-Huggins interaction parameter. The solubility coefficients at infinite dilution, Flory-Huggins interaction parameter and pressure were selected as terminal sets and some simple mathematical functions and operators such as multiplication, division, summation, power and absolute value were regarded as mathematical function sets. The adjustable parameters of the proposed function were determined for each gas-polymer system based on nonlinear data fitting. The first adjustable model parameter was in the form of constant power and the second was obtained as a variable coefficient in the form of quadratics function of temperature. The results of the presented model demonstrate improved ability to predict the solubility of investigated gases in the considered polymers at high pressures in comparison to non-equilibrium lattice fluid and Sanchez-Lacombe equation of state models. In some cases, the absolute errors between experimental and predicted values of solubility coefficients were below 1% at the considered conditions.

CO2/CH4 Sorption Behavior of Glassy Polymeric Membranes Based on Dual Mode Sorption Model

Among the various transport models for gas separation via membrane, the best description is done by solution-diffusion model. The main parameters of this model are the sorption and diffusion of the penetrant through the membrane. Studies conducted by various researchers in the field of gas separation indicate that the thermodynamic interaction (sorption effects) in glassy polymers has the major role in the diffusivity, permeability, and selectivity of the membrane, especially in the multi-component gas mixture. In glassy polymers, dual-mode sorption model is frequently used to describe the equilibrium sorption behavior of a polymer-gas system and, based on this sorption model; the permeation behavior is described by the partial immobilization model. In this study, the difference between the sorption mechanism of CO2 and CH4 in glassy polymeric membranes was analyzed by separating the sorption mode and introducing P50/50 parameter (the pressure at which the contribution of both Henry and Langmuir sorption is equal), which was done using the available experimental data in the scientific references for the sorption values of CO2 and CH4. The contribution of the sorbed molecules in Henry and Langmuir sites was investigated and its changes with pressure were evaluated for CO2 and CH4 permeation. This study was also attempted to provide a correct definition of F factor, used in references, in the form of the multiplication of mobile concentration ratios (CHmobile/CH) by the diffusivity coefficients in two modes (DH/DD).

Determination of solubility coefficients and sorption isotherms of gases in polymers by means of isothermal desorption with a chromatographic detection

A new method is developed for determining the solubility coefficients of gases in polymers that combines the advantages of the static and dynamic approaches to sorption estimation and allows us to determine the equilibrium characteristics of sorption for small quantities of samples (0.1–0.2 g) and low (<0.5 atm) partial pressures of the investigated gas. Sorption isotherms and solubility coefficients of nitrogen, oxygen, carbon dioxide, methane, ethane, and propane in polyvinyltrimethylsilane are obtained, and in poly[3,4-bis(trimethylsilyl)-tricyclononene-7], polyhexafluoropropylene, and OH-containing polyimide for the first time ever. It is shown that the sorption isotherms of gases for all of the gas-polymer systems in the investigated range of pressures are linear. The obtained solubility coefficients are compared to data for other polymers studied earlier.

Biopolymer foaming with supercritical CO2—Thermodynamics, foaming behaviour and mechanical characteristics

Polymer foams, especially those based on biodegradable polymers, are in high demand for energy saving applications, for example as thermal insulations or packaging materials. To understand and predict the quality and material properties of polymer foams, concise knowledge of the factors influencing the foaming behaviour, especially pressure and temperature, is required. Therefore, three biodegradable polyesters, namely poly (lactide) (PLA), poly (butylene succinate) (PBS) and a blend of poly (lactide) and poly (hydroxy butyrate) (PLA-PHB), have been subjected to a direct foaming procedure using compressed carbon dioxide as blowing agent, studying the influence of saturation temperature (ranging from 95°C to 175°C) and applied pressure (ranging from atmospheric pressure to 30MPa) on the resulting foam material. As these results are strongly depending on the melting behaviour of the polymers, all three polymers were subjected to calorimetric analysis in a scanning transitiometer that allows for applying pressure levels of up to 45MPa. The created porous materials were characterized by determining their density, porosity and morphology, using SEM analysis. Their mechanical behaviour was investigated by using compressive strength tests. It is shown that the quality of the produced foam structures and its properties is strongly depending on the foaming conditions. In order to obtain foams with a high quality, the saturation temperature and pressure have to be adapted to the phase transition liquid–solid of the polymer–gas system. The results obtained via scanning transitiometer represent the SLG-line of the binary systems polymer and CO2, and their influence on the foaming behaviour enabled the identification of ideal foaming conditions for the three polymers regarded in this study.

Quantitative study of diffusion jumps in atomistic simulations of model gas–polymer systems

Microscopic mechanisms underlying the diffusion of particles in polymeric and other systems include ‘jumps’ that are said to provide for a substantial contribution to the overall particle displacement. Such jumps have been observed in molecular simulations and experimentally, leading to important qualitative conclusions. An efficient method has been proposed for the identification and quantitative processing of jumps, and successfully employed in simulations of gas–liquid alkane systems. In the present work, the same method is applied in equilibrium Molecular Dynamics simulations of methane-like molecules dispersed in polymer-like alkanes, at atmospheric pressure and temperature well above the polymer melting point. The systems studied were prepared and equilibrated and a linear diffusion regime was confirmed by means of various criteria. The occurrence of distinct jump events was clearly revealed and their average length and frequency were calculated. In this way, a random-walk-type diffusion coefficient, D s, jumps, of the penetrants, was obtained and found to be lower than the overall diffusion coefficient D s, MSD calculated by the mean square displacement method. This is a strong indication that the overall diffusion is a combination of longer jumps with other microscopic mechanisms such as smoother and more gradual displacements effected upon the diffusing particle by its surroundings.

The sorption behaviors in PLLA-CO2 system and its effect on foam morphology

The sorption behavior, physical properties, and foam morphologies in poly(L-lactic acid) (PLLA)-CO2 system were studied in this paper. The solubility and diffusion coefficient of CO2 in PLLA in the range of 0 °C to 45 °C and pressure up to 5.5 MPa were investigated. The diffusion coefficients were analyzed to determine the plasticization glass transition temperature (Tg) of the PLLA-CO2 systems. The data of Tgs of PLLA at various CO2 pressures demonstrated that PLLA-CO2 system exhibited a retrograde vitrification behavior, which has never been reported on semi-crystalline polymer-gas system by systematic measurement of solubility data. The sorption curves of PLLA at certain temperatures and pressures exhibited a characteristic keen which indicated the rejection of CO2 from the polymer matrix due to CO2-induced crystallization. The fundamental understanding of PLLA-CO2 interactions was utilized to control the CO2 solubility and crystallinity in PLLA thus the physical properties, in order to develop various unique foam structures.

Effect of High Pressure CO2on the Structure of PMMA: A FT-IR Study

Conformational changes in polymer films exposed to high-pressure CO(2) have been investigated with Fourier transform infrared (FT-IR) spectroscopy. The experimental setup, based on a custom-made stainless steel optical cell with CaF(2) windows, allows measurements in a CO(2) environment for pressures up to 6 MPa, in a temperature range from 293 to 353 K and in the mid-infrared (1000-4000 cm(-1)). Poly(methyl methacrylate) (PMMA), a polymer with a side group (C-type), was studied to monitor the spectral changes as a function of CO(2) pressure and was compared to poly(D,L-lactic-co-glycolic acid) (PLGA), a polymer without a side group (B-type). By monitoring the characteristic carbonyl bands, conformational changes that occur due to molecular interactions between the high-pressure CO(2) and the polymers were explored at a constant pressurization rate (0.02 MPa/min) and temperature. Spectral changes are observed only for PMMA, where the vibrational band at 1680 cm(-1) disappears with increasing pressure. The spectra of PLGA do not show any significant change in the presence of high pressure CO(2) in the investigated range. The behavior of the absorbance peak as a function of pressure and temperature highlights the presence of dynamic cross-links (DCs) between the side groups of PMMA films obtained by solvent casting below the glass transition temperature of the polymer. The spectral features are correlated using a model that accounts for CO(2) diffusion and the relaxation kinetics of the polymer chains in the thin film. The disappearance of the vibrational band attributed to the DCs for PMMA is related to the glass transition temperature, and a retrograde vitrification phenomenon is observed. This approach can be considered a useful alternative to magnetic suspended balance for the study of polymer-gas systems.

Foaming of amorphous polymers and blends in supercritical CO2: Solubility versus block copolymers addition

Supercritical CO2 (scCO2) is used as a medium for foaming amorphous polymers. Astudy of the solubility of supercritical CO2 in several amorphous polymers (PS andPMMA) and blends is performed, followed by an investigation of the foaming behavior of the polymer–gas systems. Nano-structuring triblock copolymers (styrene-co- butadiene-co-methylmethacrylate SBM and methylmethacrylate-co-butylacrylate-co- methylmethacrylate MAM) were blended as additives to PS or PMMA by extrusion. The addition of these triblock copolymers results in an important weight gain ratio of gas in a wide range of temperatures (from 25 to 80°C), relating this weight gain ratio to the foaming behavior of the blends (CO2 is preferentially located in the micro or nano-domains issued from the structuration of the block copolymer). Foaming is carried out in a batch one-step scCO2 process, keeping constant the saturation pressure and depressurization rate (300 bar and 60 bar/min, respectively). Influence of saturation temperature (25–85 °C) on the final porous structure is shown. In spite of the influence of the terpolymer on the weight gain ratio, the structuration is believed to provide a good control of microcellular foams in PS and PMMA.

Predictive Model for the Solubility of Fluid Mixtures in Glassy Polymers

The estimation of the solubility of fluid mixtures in polymeric phases is an essential prerequisite for membrane separations and in the design of several devices, such as chemical sensors: the presence of swelling and strongly interacting components in the fluid mixture can often cause rather large deviations from the pure component sorption behavior. For rubbery polymers, the usual equilibrium tools such as the multicomponent versions of the equation of state (EoS) models can be adopted, while for the case of glassy polymers, which are not at equilibrium, such an approach cannot be followed. The general model called nonequilibrium thermodynamics of glassy polymers (NET-GP) has proved to be a successful tool for predicting the solubility of pure gases, vapors and liquids in glassy polymers, and it is here applied to the prediction of multicomponent fluid solubility. In particular, three gas–gas–polymer systems, which show significant deviations from the ideal behavior, are examined at room temperature and...

Microcellular and nanocellular solid-state polyetherimide (PEI) foams using sub-critical carbon dioxide II. Tensile and impact properties

Microcellular foams, closed-cell polymer foams with cells of order 10 μm, have been studied for over two decades. These foams have shown significant improvements in mechanical properties, such as strength-to-weight ratio, over conventional foams with cells on the order of 1 mm. Will an additional 100-fold reduction in cell size yield further improvement in properties? Here we answer this question in the solid-state PEI–CO 2 system, a unique gas–polymer system in which cellular structures can be created throughout the polymer volume at either micro or nano scales. The tensile and impact behaviors of microcellular (cells in 2–5 μm range) and nanocellular (cells in the 50–100 nm range) structures are experimentally compared for foams with relative densities, to that of the starting solid, in the range of 0.75–0.90. We found that nanofoams show a significantly higher strain to failure, resulting in an improvement in the modulus of toughness by up to 350% compared to microcellular foams. Falling weight impact tests show evidence of a brittle-to-ductile transition in nanofoams resulting in impact energies that are up to 600% higher compared to microcellular foams. These results point to the promise of nanofoams as an important new class of materials.

Solid‐state microcellular polycarbonate foams. I. The steady‐state process space using subcritical carbon dioxide

AbstractThe process parameters for production of solid‐state microcellular polycarbonate using subcritical CO2 were explored. Sufficiently long foaming times were used to produce foams, where cell growth had completed, resulting in steady‐state structures. A wide range of foaming temperatures and saturation pressures below the critical pressure of CO2 were investigated, establishing the steady state process space for this polymer–gas system. Processing conditions are presented that produce polycarbonate foams where both the foam density and the average cell size can be controlled. The process space showed that we could produce foams at a constant density, while varying the cell size by and order of magnitude. At a relative density of 0.5, the average cell size could be varied from 4 to 40 μm. The ability to produce such a family of foams opens the possibility to explore the effect of microstructure, like cell size on the properties of cellular materials. It was found that the minimum foaming temperature for a given concentration of CO2, determined from the process space, agrees well with the predicted glass transition temperature of the gas–polymer solution. A characterization of the average cell size, cell size distribution, and cell nucleation density for this system is also reported. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers

Fabrication of Microcellular HDPE Foams in a Sub-Critical CO2 Process

In this paper processing conditions are presented to fabricate HDPE foams with a microcellular structure. The conventional microcellular foams are normally produced by heating a gas-saturated thermoplastic to a foaming temperature near the glass transition of the gas-polymer system. Efforts to foam HDPE in the conventional process have largely been unsuccessful, due to the low glass transition temperatures and low gas solubility in these polymers. We have found that HDPE can be successfully foamed with a microcellular structure at foaming temperatures above the melting point. Density reductions of up to 65% were obtained using sub-critical carbon dioxide as blowing agent. A unique bimodal structure was observed, consisting of closed microcells with an interconnected nanoporous interior.

Polymer–CO2 systems exhibiting retrograde behavior and formation of nanofoams

AbstractThe sorption of compressed gases in polymers causing a reduction in the glass transition temperature (Tg) is well established. There is, however, limited information on polymer–gas systems with favorable interactions, producing a unique retrograde behavior. This paper reports on using a combination of established techniques of in situ gravimetric and stepwise heat capacity (Cp) measurements using high‐pressure differential scanning calorimetry (DSC) to demonstrate the occurrence of this behavior in acrylonitrile–butadiene–styrene copolymer (ABS)–CO2 and syndiotactic poly(methyl methacrylate) (sPMMA)–CO2 systems. The solubility and diffusion coefficient of CO2 in the range 0 to 65 °C and pressures up to 5.5 MPa were determined, which resulted in a heat of sorption of − 15.5 and − 15 kJ mol−1, and an activation energy for diffusion of 28.3 and 32.1 kJ mol−1 in the two systems, respectively. The fundamental kinetic data and the changes in Cp of the polymer–gas systems were used to determine the plasticization glass transition temperature profile, its relationship to the amount of gas dissolved in the polymer, and hence the formation of nano‐morphologies. Copyright © 2006 Society of Chemical Industry

Experimental Study on Filling Stage of Microcellular Injection Molding Process

An experimental research work was carried out to produce microcellular foam in an injection molding process. Microcellular foams are identified by a porous structure containing cells on the order of 10 μm in size. In this work, a conventional molding system was modified and a gas injection module was devised and integrated to the system. The experiments were carried out to attain the suitable processing windows with main emphasis on the cell growth stage (or foam expansion) via variation of shot size. While obtaining a sound microstructure, the results indicated that, in microcellular injection molding process, cell formation and growth depend strongly on the shot size. Further, it was found that a single phase solution of gas-polymer system is resulted with regulating a high shot size. Finally a foaming characteristics diagram is introduced for (microcellular) foaming process.

Evaluation of HFC-245fa as an Alternative Blowing Agent for Extruded Low-Density Polyethylene

HFC-245fa was investigated as a blowing agent in low-density polyethylene (LDPE). Ultrasonic monitoring showed that this blowing agent has degassing pressures systematically lower in LDPE than those reported in polystyrene, in agreement with solubility measurements performed on other polymer-gas systems. Based on these results, HFC-245fa would have solubility properties similar to HCFC-22 both in polyolefins and in polystyrene. Foams were produced in extrusion using gas concentrations ranging from 3 to 7.7 wt.%. Extrusion profiles show that foam growth in LDPE is very slow and that adding a nucleating agent is required to trigger the expansion process. Using an optimal concentration of gas and talc, it was possible to produce foam samples below 100 kg/m3.

Methods for the calculation of occupied volumes in glassy polymers: The lattice integration and the monte carlo methods

AbstractA new model for characterizing the free volume of a glassy polymer—gas systems is proposed. An improved method for the calculation of occupied volume per monomer unit was developed within the limits of this model. The model assumptions, error estimates and algorithm efficiencies are described. Using the example of polyvinyltrimethylsilane, it is shown that linear dependences of logarithms of the diffusion and the permeability coefficients on specific accessible volume for inert gases exist.

Occupied and Accessible Volumes in Glassy Polymers and Their Relationship with Gas Permeation Parameters

AbstractIn order to relate the microstructure of polymer chains to the gas permeation properties, a novel method is proposed for the calculation of the van der Waals (Vw), occupied (Vocc), and accessible (Vacc) volumes of polymers, using polyimides as examples. In this approach,VoccandVaccvalues are obtained as characteristics of polymer–gas systems. Correlations betweenVoccand the gas permeability (P) and diffusion (D) coefficients are demonstrated. Accessible volume,Vacc, determined experimental densities, andVocccalculated using a Monte Carlo technique, as well as the fractional accessible volume (FAV) (Vacc/Vsp), whereVspis the specific volume of polymers, also correlate well with thePandDvalues. The slopes of these correlations increase with increasing molecular size of the diffusing molecules. In addition, the parameters of the equations forD(FAV) andP(FAV) strongly depend on conformation rigidity (Kuhn segment) of the polyimides. An interpretation of the results obtained is presented.Definition of occupied volume.imageDefinition of occupied volume.

Penetrant-induced plasticization phenomenon in glassy polymers for gas separation membrane

Penetrant-induced plasticization of gas separation membranes has been the subject of a number of recent investigations. The plasticization of the polymer matrix by penetrant gases can be attributed to the swelling stresses on the polymer network. It is well known that sorption of carbon dioxide in glassy polymers can facilitate the local segmental organization with a reduction in the permselectivity and substantially affect membrane morphological performance. Hence, plasticization is a phenomenon that most frequently encountered in polymer–gas systems for commercial CO2/CH4 separation applications where the membranes are exposed to high CO2 concentration in the feed stream. This paper attempts to present an overview of the plasticization phenomenon and the alteration of physical properties resulting from the polymer plasticization by the sorbed penetrant molecules. The plasticization suppression methods in polymeric membrane for gas separation and the future direction of research in this area are also given.