B. Kruczek

Development and characterization of homogeneous membranes de from high molecular weight sulfonated polyphenylene oxide

Abstract The high molecular weight polyphenylene oxide (PPO) was sulfonated to different ion exchange capacity (IEC) values using chlorosulfonic acid. The physico-chemical properties along with the gas transport properties of the membranes prepared from sulfonated PPO (SPPO) were evaluated. Sulfonation of PPO results in a linear increase of density with the IEC value while the average d-spacing in polymer remains constant. Sulfonic groups attached to the aromatic ring in the PPO backbone are not thermally stable. On the other hand, when tested with CO2 at room temperature, the SPPO membranes maintained a constant permeability over the period of 60 days. An increase in IEC value of SPPO results in an increase in O2/N2 and CO2/CH4 ideal selectivities and a decrease in O2 and CO2 permeabilities. The combination of permeability and ideal selectivity for both gas pairs places the SPPO membranes below the respective upper-bound lines for polymeric membranes. However, an increase in the IEC value brings the permeability versus ideal selectivity relationship closer to the upper-bound line, especially for the O2/N2 gas pair.

Characterization of membranes prepared from PPO by Raman scattering and atomic force microscopy

Abstract The surface structures of dense (homogeneous) and asymmetric (integrally skinned) membranes made from poly(2,6-di-methyl-1,4-phenylene oxide) (PPO) in chloroform and in 1,1,2-trichloroethylene were investigated by Raman spectroscopy and by tapping mode atomic force microscopy (TM AFM). Results revealed by Raman spectroscopy indicated that the “state of the polymer” in the polymer powder and in the membranes prepared from PPO by using different solvents was not identical. A significant perturbation in the Raman scattering was observed in the asymmetric membrane prepared from PPO by using CHCl3 (PPO-CHCl3). A difference in the morphology of the surfaces (top and bottom) was also observed by TM AFM. It is believed that faster evaporation rates resulting from the use of a more volatile solvent, results in preserving more of the polymer structure present in the solution. Therefore, nodules produced from polymer dissolved in more volatile solvents might contain more free volume entrapped inside nodules, which results in larger dimensions of nodules compared to those produced from polymer dissolved in a less volatile solvent. This was confirmed by the larger dimensions of nodules and the higher permeation rate of CO2 in the membranes prepared from PPO-CHCl3 solution compared to those prepared from PPO-TCE solution.

Surface morphology of homogeneous and asymmetric membranes made from poly(phenylene oxide) by tapping mode atomic force microscope

Surface morphology of asymmetric and homogeneous membranes prepared from poly(phenylene oxide) (PPO) was studied by tapping mode atomic force microscopy (TM AFM). As expected, a significant difference in the morphology between the top and the bottom surfaces of the asymmetric membrane was observed. The images of the top surface revealed a small variation in the vertical direction (6.7 nm), compared to the mean diameter of nodules (62 nm), while the images of the bottom surface were very porous (microfiltration structure). On the other hand, the observed difference in morphology between the top and the bottom surfaces of the membrane prepared by the complete evaporation of the solvent (homogeneous membrane) was rather unexpected. The nodules on the bottom surface were twice as large as those on the top surface. These studies also revealed some differences in the morphology of the top surface of asymmetric and homogeneous membranes. Both surfaces were made up of nodules having a similar size (62-64 nm) ; however, roughness parameters calculated for the top surface of the asymmetric membrane were approximately two times greater than those for the top surface of the homogeneous membrane.

Effect of metal substitution of high molecular weight sulfonated polyphenylene oxide membranes on their gas separation performance

Abstract High molecular weight sulfonated polyphenylene oxide in protonated form (HSPPO) was modified by substitution of the proton in sulfonic groups by various metal cations, including Na+, Mg2+ and Al3+. Physico-chemical properties along with gas transport properties of the modified polymer — sulfonated polyphenylene oxide in metal form (MeSPPO), were determined and compared with the properties of the unmodified polymer. For a given degree of sulfonation (DS), MeSPPO is more hydrophilic than HSPPO. After substitution of the proton by a metal cation sulfonic groups become more thermally stable. Despite a greater density, MeSPPO is more permeable to gases than HSPPO. Permeability ratios for MeSPPO of higher DS are the same or greater than for the corresponding HSPPO. Consequently, substitution of the proton in sulfonic groups by metal cations improves gas transport properties of MeSPPO.

Characterization of the PPO dense membrane prepared at different temperatures by ESR, atomic force microscope and gas permeation

Abstract Dense (homogeneous) membranes were prepared from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) by using 1,1,2-trichloroethylene as a solvent at different solvent evaporation temperatures (22,4 and −10°C). The effect of temperature used during evaporation of solvent on the characteristics of the membrane was studied by using electron spin resonance, atomic force microscopy and gas permeation rate. The morphology of the surfaces of the membrane, the shape of spin probe in the membrane, and the selectivity of gases depend on the temperature of evaporation of solvent. The permeation rate of CO2 increased with the decrease in the temperature used for the preparation of the membrane. However methane permeation rate increased in the membrane prepared at −10°C. It is suggested that Langmuir sites could be favorable for the CH4 permeation.

Limitations of a constant pressure-type testing system in determination of gas transport properties of hydrophilic films

Abstract Permeabilities of dense films made from high molecular weight sulfonated polyphenylene oxide in protonated (HSPPO) and sodium (NaSPPO) forms were determined by constant pressure (CP) and constant volume (CV) techniques in appropriate testing systems. The comparison of permeability data of similar films determined by the two testing systems reveals considerable differences. In general, for any film the CO 2 /CH 4 permeability ratio obtained by the CP system is larger than that obtained by the CV system. In case of the O 2 /N 2 permeability ratio the situation is opposite. The observed differences are attributed to the fact that the water vapor pressure in the laboratory might have influenced the tests performed in the CP system. In addition, the order and duration of gas permeation tests, in particular the duration of the test with CO 2 , also influences permeabilities determined by the CP system. The effects of the water vapor pressure and film hydrophilicity on the apparent gas permeabilities are discussed.

Separation of SF6 from Binary Mixtures with N2 Using Commercial Poly(4-Methyl-1-Pentene) Films

Systematic studies on gas permeation of pure SF6 and N2 as well as their mixture in poly(4-methyl-1-pentene) (PMP) at different temperatures and pressures, using commercially available thin PMP films, are reported in this article. The effective separation of SF6 from binary mixtures with N2 is critical for the proposed replacement of pure SF6, used as an insulating gas in high power industry, by the mixtures of these two gases. This replacement is driven by the fact that SF6 is the most potent greenhouse gas, with a global warming potential of 22,200 times that of CO2. The experiments with a 1:1 mixture of N2 and SF6 revealed the permselectivity of PMP as high as 476 with the corresponding N2 permeability coefficient of 7.6 Barrer. These properties, which are much better than those of other glassy polymers considered for this separation, were not affected by a long-term exposure to SF6, which indicates the excellent resistance of PMP to plasticization by this gas. Using a single stage membrane system utilizing the PMP membrane would allow separating the above gas mixture into a 99% pure SF6 product with the corresponding recovery rate of SF6 greater than 99%.

Effect of Pressure and Membrane Thickness on the Permeability of Gases in Dense Polyphenylene Oxide (PPO) Membranes: Thermodynamic Interpretation

Abstract The permeability data of oxygen and nitrogen were obtained from both air and individual gas permeation experiments under different pressures using dense polyphenylene oxide (PPO) membranes of different thicknesses, after the membranes were fully stabilized. Opposite trends were observed with respect to oxygen and nitrogen as their permeabilities changed with changes in feed pressure and membrane thickness. Moreover, the pressure effect on oxygen permeability was reversed when the membrane was flipped upside down. Attempts were made to interpret the above experimental observations within a framework of entropy and enthalpy effect on the energy barrier for gas permeation and the change in the void space in the cross‐sectional direction of a dense PPO membrane.

Thermal properties of silica/poly(2,6-dimethyl-1,4-phenylene oxide) films prepared by emulsion polymerization

Thermal properties of the silica/poly(2,6-dimethyl-1,4-phenylene oxide) films prepared via emulsion polymerized mixed matrix (EPMM) method are investigated, and the impact of the synthesis protocol on the silica content, compatibility between the organic and inorganic phases, and the thermal stability of these nanocomposites is studied. Two series of films, namely EPMM-1S and EPMM-2S, synthesized in one- and two-step process, respectively, with different combinations of surfactant and compatibilizer were prepared. The polymerization of the silica precursor in the films was confirmed by 29Si nuclear magnetic resonance, and its content was investigated by inductively coupled plasma mass spectroscopy analysis. Thermal properties of the EPMM films were investigated by differential scanning calorimetry and thermogravimetric analysis. The glass transition temperature (Tg) of EPMM films was greater compared to the neat PPO film. However, an increase in Tg was not related to the concentration of silica in the film, but rather to the quality of dispersion of synthesized nanoparticles. Despite a lower inorganic loading, EPMM-1S films had a greater Tg than EPMM-2S films. On the other hand, both the decomposition temperature and the activation energy for the decomposition were directly related to the silica content in the EPMM films. In general, regardless of the synthesis protocol, the presence of compatibilizer (ethanol) leads to greater inorganic content and improved thermal properties of the EPMM films.

Separation of CO2 and N2 on Zeolite Silicalate-1 Membrane Synthesized on Novel Support

This paper reports on the properties of an MFI-type zeolite (silicalite-1) membrane synthesized on a novel tubular support with a 0.45 µm-pore size active layer consisting of zirconium and titanium oxides. Even though the membrane was synthesized by a pore plugging method, apart from penetrating into the support, the silicalite-1 crystals formed a 1.5 µm layer on top of the support. After the zeolite synthesis, the Si constituted more than 35% of the active layer of the support, which implies small size and close packing of the silicalite-1 crystals in the pores of the active layer. Single gas permeation tests with N2 and CO2 revealed comparable N2 and CO2 permeances. On the other hand, CO2/N2 gas separation tests performed at different total feed pressures and feed compositions lead to CO2/N2 permselectivities as high as 26.0, with the corresponding CO2 permeance of 6 × 10−8 mol/m2 Pa s. The effects of changing the partial pressure gradient of CO2 across the membrane by means of varying the total feed pressure and the feed composition on the CO2 permeance and CO2/N2 permselectivity are discussed.