Glassy poly membranes

Poly(4-methyl-2-pentyne) [PMP] is a glassy, disubstituted, purely hydrocarbon-based polyacetylene. PMP has a density of only 0.78 g/cm and a high fractional free volume of 0.28. The polymer has very high hydrocarbon permeabilities for example, the /i-butane permeability of PMP in a mixture of 2 mol% n-butane in methane is 7,500 X lO l cm3(STP) cm/cm2 s cmHg at 25 C. In contrast to conventional, low-free-volume glassy polymer membranes, PMP is significantly more permeable to n-butane than to methane in gas mixtures. In this paper, we present the gas permeation properties of PMP in mixtures of -butane with methane. The mixed-gas permeation and physical aging properties of PMP are compared to those of poly(l-trimethylsilyl-l-propyne), the most permeable polymer known. 

VOP Vopieka, 0., DeAngelis, M.G., and Sarti, G.C., Mixed gas sorption in glassy polymeric membranes I. CO2/CH4 and n-C4/CH4 mixtures sorption in poly(l-trimethylsilyl-l-propyne) (PTMSP), J. Membrane Sci., 449, 97, 2014. 

The consensus form the membrane literature is that thin glassy polymeric membranes exhibit an increase in the rate of physical aging with decreasing film thickness. The range of polymeric membranes that have been shown to exhibit this anomalous aging response include polysulfone, [42, 43, 125, 126] polyimides, [42, 43, 50, 125, 126] polynorborene [127] and poly(phenylene oxide)s [43, 125, 126]. Figure 3.8 shows the impact of film thickness on the physical aging of polysulfone... 

Poly(substituted acetylene)s such as PTMSP and PMP, amorphous fluoro-polymers like Teflon AF and Hyflon AD, polymers with intrinsic microporosity, and thermally rearranged (TR) polymers are the candidate polymers for highly permeable glassy polymer membranes. The high free volume in glassy polymers contributes to enhanced diffusion and permeation of small gas molecules. The gas permeation performances of these highly permeable polymers even surpass upper bounds for CO2/N2, CO2/CH4 and H2/CO2 separations. 

The selectivity of pervaporation membranes varies considerably and has a critical effect on the overall separation obtained. The range of results that can be obtained for the same solutions and different membranes is illustrated in Figure 41 for the separation of acetone from water using two types of membrane (89). The figure shows the concentration of acetone in the permeate as a function of the concentration in the feed. The two membranes shown have dramatically different properties. The siUcone mbber membrane removes acetone selectively, whereas the cross-linked poly(vinyl alcohol) (PVA) membrane removes water selectively. This difference occurs because siUcone mbber is hydrophobic and mbbery, thus permeates the acetone preferentially. PVA, on the other hand, is hydrophilic and glassy, thus permeates the small hydrophilic water molecules preferentially. 

Glassy polymers with much higher glass transition temperatures and more rigid polymer chains than rubbery polymers have been extensively used as the continuous polymer matrices in the zeolite/polymer mixed-matrix membranes. Typical glassy polymers in the mixed-matrix membranes include cellulose acetate, polysul-fone, polyethersulfone, polyimides, polyetherimides, polyvinyl alcohol, Nafion , poly(4-methyl-2-pentyne), etc. 

A tetraruthenated porphyrin was electropolymerised onto glassy carbon and used to catalyse the oxidation of nitrite to nitrate, with the resultant current giving a selective measure of the concentration of nitrite ion [81]. As an alternative method, soluble poly(3-octyl thiophene) [82] was cast along with tridodecylmethylammonium chloride onto glassy carbon, to give electrodes with superior selectivity over PVC-based membranes to lipophilic ions such as bromide or nitrate. 

The preceding structural characteristics dictate the state of polymer (rubbery vs. glassy vs. semicrystalline) which will strongly affect mechanical strength, thermal stability, chemical resistance and transport properties [6]. In most polymeric membranes, the polymer is in an amorphous state. However, some polymers, especially those with flexible chains of regular chemical structure (e.g., polyethylene/PE/, polypropylene/PP/or poly(vinylidene fluoride)/PVDF/), tend to form crystalline... 

In contrast, organophilic PV membranes are used for removal of (volatile) organic compounds from aqueous solutions. They are typically made of rubbery polymers (elastomers). Cross-linked silicone rubber (PDMS) is the state-of-the-art for the selective barrier [1, 43, 44]. Nevertheless, glassy polymers (e.g., substituted polyacetylene or poly(l-(trimethylsilyl)-l-propyne, PTMSP) were also observed to be preferentially permeable for organics from water. Polyether-polyamide block-copolymers, combining permeable hydrophilic and stabilizing hydrophobic domains within one material, are also successfully used as a selective barrier. 

Binyamin, Chen and Heller reported that wired enzyme electrodes constituted of glassy carbon electrodes coated with poly(4-vinylpyridine) complexed with [Os(bpy)2Cl] and quarternized with 2-bromoethylamine or poly[(iV-vinylimidazole) complexed with [Os(4,4 -dimethyl-2,2 -bypyridine)2Cl] or poly(vinylpyridine) complexed with [Os(4,4 -dimethoxy-2,2 -bypyridine)2Cl] quaternized with methyl groups lost their electrocatalytic activity more rapidly in serum or saline phosphate buffer (pH 7.2) in the presence of urate and transitional metal ions such as Zn and Fe " " than in plain saline phosphate buffer (pH 7.2). It was reported that as much as two-thirds of the current is lost in 2 h in some anodes. However, when a composite membrane of cellulose acetate, Nafion, and the polyaziridine-cross-linked co-polymer of poly(4-vinyl pyridine) quaternized with bromoacetic acid was applied, the glucose sensor stability in serum was improved and maintained for at least 3 days [27,50]. 

Figure 5. Hydrogen peroxide calibration curve for three different electrode systems MP and ferrocene-immobilized polyion complex membrane (a), ferrocene-immobilized poly-ion complex membrane (b). and bare glassy electrodes (c).

In contrast to the LCP results just presented, in glassy polymers used as gas separation membranes, free volume influences diffusion coefficients much more than solubility coefficients. Figure 6 provides an example of this effect. In this figure, the solubility, diffusivity, and permeability of methane in a series of glassy, aromatic, amorphous poly(isophthalamides) [PIPAs] are presented as a function of the fractional free volume in the polymer matrix. (More complete descriptions of the transport properties of this family of materials are available elsewhere (59, 40)). The fractional free volume is manipulated systematically in this family of glassy polymers by synthesizing polymers with different substituent and backbone elements as shown in... 

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