Effect of structure on the temperature dependence of gas transport and sorption in a series of polycarbonates

The permeabilities and solubilities of five gases are reported for bisphenol-A polycarbonate (PC), tetramethyl polycarbonate (TMPC), and tetramethyl hexafluoro polycarbonate (TMHFPC) at temperatures up to 200°C. The temperature dependence of permselectivity is discussed in terms of solubility and diffusivity selectivity changes with temperature for CO₂/CH₄ and He/N₂ gas separations. The activation energies for permeation and diffusion and the heats of sorption are also reported for each gas in the three polycarbonates. Analysis of these values provides a better fundamental understanding of the effect of polymer-penetrant interactions and polymer backbone structure on the temperature dependence of the transport and sorption properties of gases in membrane separation processes. Important factors affecting the solubility and diffusivity selectivity losses or gains with increased temperature are also identified through correlation of these data with physical properties of the gases and polymers. These conclusions provide a framework for choosing the most promising membrane materials for particular gas separations at elevated temperatures. © 1994 John Wiley & Sons, Inc.     

Effect of isopropylidene replacement on gas transport properties of polycarbonates

The gas sorption and transport properties of a series of polycarbonates in which the isopropylidene unit of bisphenol A polycarbonate has been replaced with another molecular group are presented. Two new materials, bisphenol of norbornane polycarbonate (NBPC) and bisphenol Z polycarbonate (PCZ), are compared with several polymers which have been studied previously in this laboratory, including bisphenol A polycarbonate (PC), hexafluorobisphenol A polycarbonate (HFPC), and bisphenol of chloral polycarbonate (BCPC). The effect of molecular structure on chain mobility and chain packing is related to the gas transport properties. Dynamic mechanical thermal analysis and differential scanning calorimetry are used to judge chain mobility, while x-ray diffraction and free volume calculations give information about chain packing. Permeability measurements were made for He, H₂, O₂, N₂, CH₄, and CO₂ at 35°C over a range of pressures up to 20 atm. Sorption experiments were also done for N₂, CH₄, and CO₂ under the same conditions. The permeability coefficients of these polymers rank in the order HFPC ? NBPC>PC>BCPC ? PCZ for all of the gases. With the exception of BCPC, this order correlates well with fractional free volume. The low gas permeability of BCPC is attributed to a polarity effect. In general, bulky and relatively immobile substituents, as in HFPC and NBPC, can yield improved separation characteristics. The polar group of BCPC and the flexible cyclohexyl substituent of PCZ result in relatively low gas permeability.     

Gas transport properties of poly(2,2,4,4-tetramethyl cyclobutane carbonate)

Gas transport properties of semicrystalline films of poly(2,2,4,4-tetramethyl cyclobutane carbonate) (TMCBPC) were studied. Permeability coefficients for He, O₂, N₂, CH₄, and CO₂ at 35°C for pressures between 1 and 20 atm are reported as well as sorption isotherms for N₂, CH₄, and CO₂ at the same conditions. The permeability coefficients for TMCBPC are larger than corresponding values for the aromatic bisphenol-A polycarbonate (PC) and tetramethyl bisphenol-A polycarbonate (TMPC), even though the TMCBPC films are semicrystalline. These results are explained on the basis of the larger free volume available for permeation in this polymer. Significant TMCBPC plasticization by CO₂ was also observed and this causes typical time-dependent behavior. The plasticization process starts at very low pressures compared with the behavior of aromatic polycarbonates PC and TMPC. This early onset of plasticization seems to be related also to the larger free volume in the amorphous phase of TMCBPC which favors high gas sorption. The diffusion coefficients for TMCBPC are also larger than those reported for the aromatic polycarbonates PC and TMPC. Ideal gas separation factors were found to follow the usual trend; that is, as permeability increases, the ideal separation factor decreases. © 1993 John Wiley & Sons, Inc.     

Evaluation of substituted polycarbonates and a blend with polystyrene as gas separation membranes

The balance between the rate and the selectivity of transport of various gas pairs in a series of polycarbonates has been examined. Replacement of the four available hydrogens on the aromatic rings of the bisphenol-A unit with CH₃, Cl, or Br groups gives materials with a better balance of these two characteristics than the unsubstituted polycarbonate (PC). For example, using CH₃ substitution increases the permeability to O₂ by nearly a factor of four with no loss in O₂/N₂ selectivity compared with PC, while using Br substitution increases O₂/N₂ selectivity by 50% without any loss in O₂ permeability compared with PC. While these substitutions affect the permeability through both its mobility and solubility components, the remarkable selectivity effects are caused primarily by changes in relative mobility since the changes in solubility characteristics are nearly the same for all gases. These substitutions alter chain motions, cohesion, and packing as discussed. The tetramethylbisphenol-A polycarbonate forms miscible blends with polystyrene. These blends show absolute permeability coefficients which are lower than additivity while the selectivity of transport is greater. These effects are a result of the interactions between the two polymers.     

Dynamic mechanical and gas transport properties of blends and random copolymers of bisphenol-A polycarbonate and tetramethyl bisphenol-A polycarbonate

Dynamic mechanical and gas transport properties for homogeneous homopolymer blends and random copolymers of bisphenol-A and tetramethyl bisphenol-A polycarbonates (PC-TMPC) were determined. The gas transport measurements were performed at 35°C for the gases He, H₂, O₂, Ar, N₂, CH₄, and CO₂. The results show that the copolymers have lower permeability, apparent diffusion, and solubility coefficients than the blends. Permeability coefficients for blends follow a semilogarithmic ideal mixing rule while copolymers exhibit negative deviations from this. Specific volume measurements show that the free volume available for gas transport is slightly larger in copolymers than in blends of the same composition. These apparently contradictory results may relate to the differences in local mode chain motions observed for the copolymer and blend series. The γ relaxation processes in PC and TMPC seem to operate independently in the blends (no intermolecular coupling) while there is clear evidence for intramolecular coupling in the copolymers. © 1992 John Wiley & Sons, Inc.     

Gas sorption and transport properties of bisphenol-I-polycarbonate

Gas sorption and transport characterization of a new polymer in the polycarbonate family, based on the bisphenol of 3,3,5-trimethylcyclohexane-1-one (BPI) is reported at 35°C. By comparison with properties of other known polycarbonates, the effects of inhibition of both packing and segmental motion due to the introduction of the bulky substituent in the backbone are elucidated. The T_g of the material was measured with differential scanning calorimeter (DSC) and was found to be unusually high for a polycarbonate (233°C). This indicates a successful inhibition of the large-scale segmental mobility of the polymer. Variable ¹³C NMR analysis indicated that rotation of one phenylene ring has an unusually high (ca. 10 kcal) energy barrier, whereas the other phenylene ring has a more typical rotation profile (barrier < 3 kcal). The density was measured and found to be low (1.107 g/cm³), indicating a high fractional free volume (FFV) for the polymer. Consistent with expectations, the introduction of the bulky-substituted cyclohexane group gave high permeabilities for the various gases tested (N₂, O₂, He, CH₄, CO₂) compared to most standard polycarbonates. On the other hand, the permselectivities were typical for standard polycarbonates. The solubility coefficients of all gases were rather high, as expected for a polymer with such an open structure. © 1994 John Wiley & Sons, Inc.     

Gas permeability of cardo-poly(ether-ether-ketone) and poly(phenylene sulfide)

The gas permeability and sorption of CO₂ and N₂O was measured on cardo-poly(ether-ether-ketone) (C-PEEK) and poly(phenylene sulfide) (PPS) at 298 K. The results are discussed on the basis of the dual-mode model. Results obtained from measurements at 308 K are compared with literature data of poly(phenylene oxide) (PPO), poly(sulfone) (PSU) and poly(carbonate) (PC). While C-PEEK shows similar transport properties as PC and PSU, the values of PPS are distinctly lower. The solubility of CO₂ in the various polymers as well as the correlation of the permeability coefficients of the same polymers for He, Ar, CO₂, N₂, and CH₄ with the kinetic molecular diameter of the gases indicate a simple relation of the transport properties with the polymer density.     

Gas transport in a thermotropic liquid-crystalline polyester

A preliminary report is given on the gas transport characteristics of a thermotropic liquid-crys talline copolyester that is fully aromatic. At 35°C the permeability coefficients for He, H₂, O₂, Ar, N₂, and CO₂ in this polymer are comparable to or smaller than those for polyacrylonitrile, which is one of the least permeable polymers known. This low permeability seems to stem from low solubility of these gases in the liquid-crystalline polymer rather than low transport mobility. Activation energies for transport are somewhat larger than those found for conventional amorphous polymers. Determination of the detailed mechanism of transport in polymers with liquid-crystalline order requires further study.     

Permeation and sorption in polynorbornenes with organosilicon substituents

Gas sorption properties, permeability coefficients, and diffusion coefficients of a series of norbornene polymers are presented. Introduction of the Si(CH₃)₃ group into the polynorbornene (PNB) backbone chain results in significant increases in glass transition temperature, permeability, and diffusion coefficient for a number of gases (H₂, O₂, N₂, CO₂, CH₄, C₂H₆). The transport properties and sorption isotherms for poly(5-trimethylsilyl norbornene) (PTMSNB) are very similar to those for poly(vinyltrimethyl silane) (PVTMS), which contains the same side-chain group but differs from PTMSNB by the structure of its main chain. For another silicon-containing polymer poly[5-(1,1,3,3-tetramethyl-1,3-disilabutyl) norbornene] (PDSNB) having a bulkier side-chain group, the glass-transition temperature is decreased in comparison with that of PNB, presumably owing to self-plasticization. Both silicon-containing norbornene polymers (PTMSNB and PDSNB) have permeability coefficients for “rapid” gases like H₂ or CO₂ of about 10² Barrer. The high values of the Langmuir sorption capacity C′_H for PTMSNB and PVTMS, as well as the high diffusivity and mobility of spin probes in these polymers, were attributed to a large free volume related to the bulky Si(CH₃)₃ groups attached directly to the main chain. © 1993 John Wiley & Sons, Inc.     

Gravimetric study of high-pressure sorption of gases in polymers

A gravimetric method for determining precisely the solubility of gases in polymers at high pressure is described. The solubilities of N₂ and CO₂ in low-density polyethylene (LDPE); CO₂ in polycarbonate (PC); and N₂, CH₄, C₂H₆, and CO₂ in polysulfone (PSUL) have been measured as a function of pressure up to 50 atm. Most of the measured sorption isotherms agreed closely with published data, but reproducible and time-dependent hysteresis in the sorption of CO₂, C₂H₆, and CH₄ in glassy polymers, PC, and PSUL, was observed in this study for the first time. Like the well known conditioning effect of high-pressure CO₂ on the sorption capacity of glassy polymers, these hysteresis phenomena are believed to be due to the plasticizing effect of sorbed gases. On the basis of the current data, the dual-mode sorption model including the plasticization by sorbed gas is discussed and a primitive equation for the concentration of sorbed gases in a quasiequilibrium state of sorption or desorption is proposed.     

The effect of plasticization on the transport of gases in and through glassy polymers

The effects of plasticization on the transport of gases and vapors in and through glassy polymers are examined from the viewpoint of the “dual-mode” sorption model with partial immobilization. The analysis assumes the existence of two penetrant populations with different mobilities in the Henry's law and Langmuir domains of the glassy polymers. These mobilities are characterized by their mutual diffusion coefficients D_D and D_H. The plasticization of the polymer by penetrant gases is reflected in the concentration dependence of D_D and D_H. Expressions for the effective (apparent) diffusion and permeability coefficients are derived assuming that D_D and D_H are exponential functions of the penetrant concentration in the polymers. The results of this study are compared with a similar analysis which assumed the existence of a single mobile penetrant population. The present analysis provides information on the effects of plasticization on the penetrant transport in the Henry's law and Langmuir domains separately. The effects of antiplasticization or clustering of penetrant molecules on the effective diffusion and permeability coefficients are also examined.     

Gas sorption and transport in miscible blends of tetramethyl bisphenol-A polycarbonate and polystyrene

The permeability coefficients for He, O₂, N₂, CH₄, and CO₂ in miscible blends of polystyrene (PS) and tetramethyl bisphenol-A polycarbonate (TMPC) at 35°C and 1 atm driving pressure are reported. Sorption isotherms for CO₂ and CH₄ are also presented. The isotherms were fitted to the dual sorption model. The Langmuir capacity factor was found to follow an earlier correlation based on unrelaxed volume. For each gas, the permeability was found to go through a minimum when plotted against blend composition. This behavior is primarily the result of the volume change on mixing observed for this system. The attractive interaction between TMPC and PS is relatively strong as indicated by density and solubility data. The value of the binary interaction parameter was found to be of the same magnitude as that for poly(phenylene oxide) (PPO)-polystyrene (PS) blends. Considering the similarity of structure between PPO and TMPC, it is concluded that similar phenyl-phenyl interactions and conformational changes on blending may prevail in TMPC/PS blends.     

Gas permeation properties of phenylene oxide polymers

Gas permeability (P_i) and diffusion (D_i) coefficients in respect to several gases (H₂, O₂, N₂, CO, CO₂, CH₄) have been measured for poly(2,6-dimethylphenylene oxide) (PMPO), poly(2,6-diphenylphenylene oxide) (PPPO), and phenylene oxide copolymers containing methyl, phenyl, and allyl radicals as side groups. X-ray diffraction study shows that both homopolymers are semicrystalline materials, whereas all the copolymers are completely amorphous. The results show that a replacement of methyl by phenyl groups in PMPO/PPPO pair is accompanied by decrease in the P values. A transition from semicrystalline PMPO to amorphous copolymers results in a decrease in permeability and solubility coefficients and not in a growth of these parameters as can be expected on the basis of the behavior of other semicrystalline polymers (e.g. polyolefins). It is supposed that the crystallites of PMPO, and possibly of PPPO are packed loosely and, hence, take part in sorption and gas transport. This assumption is in agreement with numerous X-ray data as well as the results of positron annihilation study of these polymers.     

Transport properties of polyimides containing phenylquinoxaline moieties

The gas permeabilities of a number of new structurally related polyimides containing phenylquinoxaline moieties were studied for the first time. The test polymers had different dianhydride units, whereas their diamine components differed in the presence of flexible ether bonds-O-in the main chain, a structure that is reflected in the effective packing of chains and, as a result, in transport parameters. The permeability, diffusion, and solubility coefficients for the gases H₂, He, O₂, N₂, CO, CO₂, and CH₄, as well as the ideal separation factors for gas pairs, were determined. The transport characteristics of polymers were compared within the given polymer series and with published data for other polymer series.     

Gas transport in polymers based on bisphenol-A

Permeation measurements for CO₂, CH₄, O₂, N₂, and He were made with three polymers based on bisphenol-A, namely a polyhydroxyether, a polyetherimide, and a polyarylate. Measurements were also made for CO₂ and CH₄ in polysulfone. The data for CO₂, CH₄, and N₂ plus previous data for these gases in polycarbonate and polysulfone were combined with equilibrium gas sorption data and analyzed with the dual mode/partial immobilization model and the more recent gas-polymer-matrix model. A comparison of the two models was done on the basis of physical interpretations of the resulting parameters. The diffusion coefficient for the Henry's law population was related to the kinetic diameter of the gas. The infinite dilution, Henry's law, and Langmuir diffusion coefficients were related to the free volume of the polymer. The work suggests a means for order-of-magnitude estimation of diffusion coefficients from polymer density and molecular structure.     

Reversible isopentane-induced depression of carbon dioxide permeation through polycarbonate

Permeability data are reported for carbon dioxide in Lexan polycarbonate at 35°C. Measurements were made for both pure carbon dioxide and for a mixed feed consisting of carbon dioxide with a 117.8-torr (0.155-atm) Partial pressure of isopentane. The effects of varying upstream CO₂ driving pressure from 1 up to 20 atm were studied. The permeability to CO₂ is reduced significantly in the presence of isopentane; however, the fractional depression of the CO₂ permeability due to the isopentane at low driving pressures is much more significant than at high CO₂ driving pressures. The well-known pressure dependence of carbon dioxide permeabilities in glassy polymers, therefore, is largely diminished by introducing isopentane to the pure carbon dioxide feed. These observations are consistent with a model for transport in glassy polymers which explains the observed trends in terms of competition between the two penetrants for microvoid sorption sites existing in the non-equilibrium glassy polymer. Exclusion of carbon dioxide from microvoid sorption sites by the more condensable isopentane preempts transport through the microvoid regions, resulting in the observed depression of the CO₂ permeability.     

High-precision gravimetric technique for determining the solubility and diffusivity of gases in polymers

An in situ gravimetric technique, employing an electrobalance, is described for determining the solubility and diffusivity of gases in polymers over extended ranges of temperature and pressure. Solubilities of CO₂ in polystyrene at 35°C were measured as a test case; the results are in excellent agreement with the literature values determined by the pressure decay method. Solubility and diffusivity results are also reported for PVC-CO₂ at 35°C and for PS-1,1,1,2-tetrafluoroethane at 30, 90, and 120°C. A comparison with other studies shows the in situ method to be more efficient and precise than the ones based on weighing the gas-saturated polymer under ambient conditions. The kinetics of gas sorption were analyzed in terms of two data reduction techniques to derive diffusion coefficients. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2025–2032, 1998     

Transport properties of poly(phenylquinoxalines) containing heterocyclic moieties

The transport behavior of a new class of membrane materials—a series of poly(phenylquinoxalines) containing heterocyclic fragments in the backbone—has been studied. These polymers contain moieties of a common chemical structure. Therefore, it is possible to follow how the transport parameters change upon introduction of various moieties into the backbone chain. The coefficients of permeability, diffusion, and solubility for H₂, He, O₂, N₂, CO, CO₂, and CH₄ along with the separation factors for the corresponding pairs of gases have been determined. The results are compared with the data for previously studied polymers of the poly(phenylquinoxaline) series.     

Gas transport in poly [bis(trifluoroethoxy) phosphazene] above the T(1) transition

Gas permeation measurements were performed by the time-lag method for 13 gases in poly [bis(trifluoroethoxy) phosphazene] above the mesophase transition, T (1) transition, of the crystals. Hysteresis was found in the temperature dependence of the permeability coefficients for CO₂, O₂, and N₂ in the transition region as observed by dilatometry and differential scanning calorimetry. Independently the solubility of CO₂ was determined gravimetrically above and below T(1). Noticeable change and hysteresis were also observed in the temperature dependence of the solubility coefficients. Diffusion of CO₂ in the measophase is suggested from the permeation and sorption measurements. Gas transport parameters of CO₂ in the mesophase are estimated, and the contributions of the mesophase and the crystalline phase to gas transport are considered.