Michael Langsam

High performance polymers for membrane separation

Abstract Polymers have been recognized to exhibit selective permeation rates to gases for more than a century. The commercial reality of this characteristic, however, occurred in the late 1970s. There has been significant commercial activity in this area which has also brought about a rapid increase in the study of polymeric structural variations to optimize the combination of gas separation and permeability (permselectivity). It has been recently noted that upper bound limits exist for the separation of common gas pairs. These limitations will be discussed along with the structural features necessary to achieve the best combination of high permeability combined with high selectivity. One of the speciality polymers receiving significant attention in the past decade is poly(trimethylsilylpropyne) (PTMSP) primarily due to a permeability to common gases an order of magnitude higher than silicone rubber. Structural variations, solvent variations, non-solvent swelling effects and flux decline of PTMSP are discussed. Flux decline, which has been reported in detail in the literature, is believed to be due to two factors. Contamination can significantly decrease permeability which comprises the reason behind many literature citations. Another factor involves a slow collapse of the structure (as cast) which can depend on the casting solvent. Non-solvent swelling promotes an instantaneous increase in permeability which slowly decays back to original values. High glass transition temperature engineering polymers (e.g. polyimides, polyarylates, polycarbonates) yield permselectivity characteristics of interest for gas separation. Structural features and variations of these polymers to achieve high permeability combined with selectivity (e.g. upper bound properties) will be discussed. Surface modification techniques comprise another route to achieve high selectivity for permeable polymers. These methods (e.g. fluorination, plasma modification, u.v. exposure) offer an additional route towards meeting the requirements for separation of gases with polymeric membranes.

A group contribution approach to predict permeability and permselectivity of aromatic polymers

Membrane separation of gases has evolved into an important separation technology for various gas mixtures (specifically O2N2). Aromatic engineering polymers such as polysulfones, polycarbonates, and polyimides comprise commercially utilized membranes for these applications. The ability to predict permeability and permselectivity from polymeric structural units is highly desired in order to streamline synthetic approaches to optimum membrane candidates. A group contribution methodology is outlined in this paper which demonstrates excellent predictability of permeability (for O2, N2 and He) and good prediction of permselectivity for the O2N2 and HeN2 gas pairs. This procedure utilizes the basic equation: ln P = Σi=1n φi ln Pi where φi=volume fraction of a structural unit i and Pi=the permeability contribution of the structural unit. Experimental permeability data are employed to set up an array of equations (of the above equation) solved by least squares fit. The values of φi are calculated using computer software programs to predict molar volume contributions. The structural units are chosen around the chemical bond. This procedure shows promising results when applied to aromatic polymers chosen from the classes of polysulfones, polycarbonates, polyarylates, poly(aryl ketones) and poly(aryl ethers). This procedure has been utilized to determine the contributions of 24 structural units employing 65 polymers which comprise the database. Excellent agreement within the database is observed and good agreement outside the database is also demonstrated. This procedure allows for a quantitative assessment of the structure/permeability (permselectivity) relationships for polymers of interest for membrane separation, and thus demonstrates group contribution methodology can be applied to both polymer permeability and permselectivity. Further refinements by addition of other polymeric classes (e.g. polyimides and polyamides) as well as additional expansion of the database should prove to be a valuable technique to predict the membrane separation pottential of a wide variety of polymeric materials.

Highly soluble polyimides from sterically hindered diamines

Polyimides with enhanced solubility have been synthesized from various aromatic tetracarboxylic dianhydrides and sterically hindered diamines. Intrinsic viscosities in 1-methyl-2-pyrrolidinone (NMP) ranged from 0.28 to 1.05 dL/g. Most of the polyimides were soluble in common solvents such as N,N-dimethylacetamide, NMP, chloroform and tetrahydrofuran. Polyimides derived from thianthrene-2,3,7,8-tetracarboxylic dianhydride (TDAN) and diamino mesitylene (DAM) or diethyltoluene diamine (DETDA) were insoluble in all solvents indicating that polyimide solubility decreased as anhydride rigidity increased. Glass transition temperatures ranged from 252 to 398°C and above with the polymers showing little or no weight loss by TGA up to 400°C in both air and nitrogen. The glass transition temperatures of the polyimides increased 15 to 98°C (compared to unhindered polyimide analogs) when one or more methyl group was placed ortho to the imide nitrogen, hindering backbone rotation, chain packing and flexibility. Tough, transparent films of the soluble polyimides were cast from solution.

Multicomponent gas separation by selective surface flow (SSF) and poly-trimethylsilylpropyne (PTMSP) membranes

Abstract A selective surface flow (SSF) membrane consisting of a thin layer of a nanoporous carbon was produced in a tubular form using a macroporous alumina support. The membrane was tested for hydrogen enrichment applications. Simulated waste gases from a petrochemical refinery and a hydrogen pressure swing adsorption unit were used as the feed gas to the membrane. Very high rejections of C 1 C 3 hydrocarbons (saturated and unsaturated) and carbon dioxide over hydrogen were exhibited by the membrane at low feed gas pressures. The hydrogen enriched stream was produced at the feed gas pressure. The separation characteristics of a polymeric poly-trimethylsilylpropyne (PTMSP) membrane in a tubular form was also tested for the same applications using identical conditions of operation. This membrane also selectively rejected heavier components of the feed gas mixture over hydrogen and produced the hydrogen enriched stream at the feed gas pressure. The SSF membrane exhibited much higher hydrogen recovery and hydrocarbon rejections than the PTMSP membrane for these applications under identical conditions of operations using identical support materials.

SELECTIVITY ENHANCEMENT VIA PHOTOOXIDATIVE SURFACE MODIFICATION OF POLYIMIDE AIR SEPARATION MEMBRANES

Abstract The scope and limitations of photooxidative surface modification of polyimide films were investigated as means of improving the selectives of air separation membranes without drastically reducing their permeabilities. Membranes of polyimides that contained the phthalimide chromophore and abstractable hydrogens showed significantly higher oxygen/nitrogen selectivities after being irradiated with ≈ 200–300 nm light in the presence of oxygen for 0.5·-30 min. Not only were benzylic hydrogens found to be abstractable, but also certain t -butyl hydrogens could be abstractable uder these conditions. A mechanism for the phtochemical reaction that is based on the photochemistry of structurally similar monomeric phthalimides is proposed to explain the observed selectivity enhancement.

Effect of Cellulose Suspension Agent Structure on the Particle Morphology of PVC. Part II. Interfacial Properties

Abstract The particle structure of polyvinyl chloride is controlled by the shear field imposed on the monomer droplet and the interfacial behavior of the vinyl chloride/water phases during polymerization. The inter facial tension in the presence of hydroxypropyl methylcellulose (HPMC) was measured as a function of concentration and temperature. The molecular weight distribution of HPMC was determined by coupled GPC-LALLS (low angel laser light scattering) technique. By monitoring the concentration of HPMC in the aqueous phase during polymerization, the coverage powers of HPMC were calculated and compared with the theoretical value based on Langmuir layer consideration. The effects of agitation on resin porosity were also examined. These results are discussed with respect to the particle structure.

Synthesis and gas transport properties of random amide imide copolymers

Fully imidized random amide imide copolymers (rPAI) can be prepared in an aprotic solvent from trimellitic anhydride chloride (TMAc) and mixtures of various aromatic diamines via condensation polymerization. The polymers are soluble in a number of aprotic organic solvents including 1-methyl-2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and dimethylsulfoxide (DMSO). The gas transport properties of the rPAI materials are governed by the local structure around both the amide and imide linkages and can be tuned by the choice and ratio of diamines used. Significant improvement in selectivity relative to polyimides can be achieved. When inorganic carbonate salts are used to scavenge byproduct hydrogen chloride, the amount of residual salt in the dense films has a substantial effect on their gas transport properties. A fugitive salt process was identified, which eliminated this problem of residual inorganic salts. The activation energy for O2, N2, He, CO2, and CH4 permeability was determined for one of the copolymers.

Poly(Trimethylsilylpropyne) Utility as a Polymeric Absorbent for Removal of Trace Organics from Air and Water Sources

Poly(trimethylsilylpropyne), PTMSP, is well known to exhibit the highest permeability for gas and vapors of all dense polymeric systems. The high free volume observed yields extremely high diffusion coefficients for penetrating species. These properties have yielded interest for various gas and pervaporation membrane separation processes. It has been found that PTMSP also exhibits unique characteristics as a polymeric absorbent for removal of trace organics from air and water sources. The distribution coefficient for organics between the PTMSP phase and the water phase is extremely high for aliphatic, aromatic, and chlorinated hydrocarbons. In fact, PTMSP approaches activated carbon adsorbents in efficiency (much closer than other polymeric species). The removal of organics from PTMSP proceeds easier than activated carbon, and applications involving simple regeneration of a fixed bed may indeed be possible.

Soluble poly(amide-imide)s prepared by one-pot solution condensation

A new one-pot procedure for imide-acid monomer synthesis and polymerization is reported for four new poly(amide-imide)s. Bisphenol A dianhydride (BPADA) was reacted with twice the molar amount of 3-aminobenzoic acid (3ABA) or 3-amino-4-methylbenzoic acid (3A4MBA) in 1-methyl-2-pyrrolidinone (NMP) and toluene mixture, and the amic acid intermediates cyclized in solution to give two diimide-containing dicarboxylic acid monomers. Without isolation, the diacid monomers were then polymerized with either 1,3-diaminomesitylene (DAM) or 1,5-diaminonaphthalene (1,5NAPda) using triphenyl phosphite-activation to give a series of four soluble poly-(amide-imide)s, PAI. Isolation and purification of the dicarboxylic acid monomers was not necessary for formation of high molecular weight polymers as indicated by intrinsic viscosities of 0.64-1.04 dL/g determined in N,N-dimethylacetamide (DMAc). All of the PAI were soluble in polar aprotic solvents such as NMP, DMAc, and dimethyl sulfoxide (DMSO). Glass transition temperatures ranged from 243 to 279°C by DSC, and 5% weight loss temperatures were above 400°C in both air and nitrogen. Flexible films cast from DMAc were light yellow, transparent, and tough.