Lloyd Mahlon Robeson

Correlation of separation factor versus permeability for polymeric membranes

The separation of gases utilizing polymeric membranes has emerged into a commercially utilized unit operation. It has been recognized in the past decade that the separation factor for gas pairs varies inversely with the permeability of the more permeable gas of the specific pair. An analysis of the literature data for binary gas mixtures from the list of He, H2, O2, N2, CH4, and CO2 reveals an upper bound relationship for these mixtures. The upper bound can be represented by a log-log plot of αij (separation factor = Pi/Pj) versus Pi (where Pi = permeability of the more permeable gas). Above the linear upper bound on the log-log plot, virtually no values exist. The slope of this line (n) from the relationship Pi=kαnij can be related to the difference between the gas molecular diameters Δdji (djdi) where the gas molecular diameter chosen is the Lennard-Jones kinetic diameter. This relationship yields linearity for a plot of −1/n versus Δdji, and the line passes through (0,0) for the x–y plot thus providing further verification of this analysis. These results indicate that the diffusion coefficient governs the separating capabilities of polymers for these gas pairs. As the polymer molecular spacing becomes tighter the permeability decreases due to decreasing diffusion coefficients, but the separation characteristics are enhanced.

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.

Mechanical characterization of vinyl acetate based emulsion polymer blends

Emulsion blends comprise an important commercial area of polymer blend utility. Surprisingly, the fundamental study of emulsion blends is rarely noted in the literature. This study investigates emulsion blends of poly(vinyl acetate) (PVAc) and vinyl acetate-ethylene copolymers (VAE), where both components employ poly(vinyl alcohol) (PVOH) as the protective colloid. PVOH comprises the continuous phase in the emulsion cast films for both the individual components and the blends. This provides an example whereby excellent adhesion can be expected between the particles comprising the blend. The combination of low Tg/high Tg emulsion blends has been noted to be of interest, and the PVAc/VAE emulsion blends noted here offer an excellent model to study. The PVAc/VAE blends protected with PVOH exhibit poor mechanical compatibility even though there is good adhesion. Conventional theory based on polymer/filler combinations predicts a rapid loss in elongation as filler content increases if excellent adhesion is observed. The PVAc/VAE blends (where PVAc is the filler) also exhibit similar behavior. This result implies excellent adhesion may not be desired where a compliance mismatch occurs for emulsion blends. The polymer/filler theories do not properly predict PVAc/VAE blend tensile strength results. A newer approach termed the equivalent box model (EBM) employing percolation theory agrees well with experimental results. Melt mixing of the low/high compliance PVAc/VAE emulsion blends yields a significant improvement in mechanical compatibility. This indicates that a heterogeneous mixture of the same components yields better mechanical results than an array of particles with excellent adhesion between the particles.

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.

Novel ionomers based on blends of ethylene-acrylic acid copolymers with poly(vinyl amine)

The polymerization of N-vinyl formamide followed by hydrolysis yields a linear, water-soluble poly(vinyl amine). The high concentration of pendant primary amine groups leads to a polymer with an interesting set of properties. Complexation with water-soluble anionic polyelectrolytes in water solutions leads to a highly water-insoluble material. The study described herein investigated the phase behavior/properties of melt blends of poly(vinyl amine) with ethylene-acrylic acid (EAA) copolymers of less than 10 wt % acrylic acid. The calorimetric and dynamic mechanical analyses of the resultant blends show that the vinyl amine groups are accessible to the acrylic acid groups of the copolymers and the major property changes occur up to the stoichiometric addition of vinyl amine/acrylic acid. At higher levels of vinyl amine (vinyl amine/acrylic acid mol ratio > 4), additional poly(vinyl amine) forms a separate phase. The mechanical, dynamic mechanical, and calorimetric properties of these blends below the stoichiometric ratio show analogous trends as with typical alkali/alkaline metal neutralization. These characteristics relative to the base EAA include improved transparency, lower melting and crystallization temperature, lower level of crystallinity, and increased modulus and strength. The emergence of the β transition in dynamic mechanical testing is pronounced with these blends (as with alkali/alkaline metal neutralization), indicative of microphase separation of the amorphous phase into ionic-rich and ionic-depleted regions. A rubbery modulus plateau for the blends exists above the polyethylene melting point, demonstrating ionic crosslinking. Above 150°C exposure, further modulus increases occur presumably due to amide formation. This study demonstrates that the highly polar poly(vinyl amine) can interact with acrylic acid units in an EAA copolymer comprised predominately of polyethylene (>90 wt %). The thermodynamic driving force favoring ionic association overrides the highly unfavorable difference in composition.

Reactive Extrusion Compatibilization of Poly(Vinyl Alcohol) — Polyolefin Blends

Reactive extrusion compatibilization of highly incompatible blend combinations has been an active area of research for almost two decades. Polyamide-polyolefin reactive extruded blends involving peroxide grafting of anhydride or acid groups onto polyolefin followed by reaction with terminal polyamide amine groups has been well demonstrated as a method for achieving mechanical compatibilization. Poly(vinyl alcohol)-polyolefin blends comprise a combination of polar, hydrophilic polymers with non-polar hydrophobic polymers and expectedly poor mechanical compatibility. A similar procedure involving reactive extrusion grafting of maleic anhydride onto the polyolefin followed by thermoplastic poly(vinyl alcohol) addition allows for anhydride-alcohol reactions and thus compatibilization.

An equation of state for polymer/polymer systems

Abstract Polymer blends offer an extremely promising method to produce polymer materials with commercially desirable properties. In this work, we assess the capability of equations of state to describe and predict the properties of polymer mixtures, particularly those that have specific interactions. A promising approach is a lattice model that uses a site-site quasichemical theory to describe nonrandomness.