Crosslinking and stabilization of high fractional free volume polymers for the separation of organic vapors from permanent gases
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The removal of higher hydrocarbons from natural gas streams is an important separation that has been identified as a growth area for polymer membranes. An ideal membrane material for this separation would be more permeable to higher hydrocarbons (i.e., C3+ compounds) than to CH₄. This allows the CH₄ rich permeate to be retained at or near feed pressure, thus minimizing the requirement for repressurization followingmembrane separation. A polymer which demonstrates the ability to separate vapor from gases with high efficiency is poly [1-(trimethylsilyl)-1-propyne] (PTMSP). PTMSP is a stiff chain, high free volume glassy polymer well known for its very high gas permeability and outstanding vapor/gas selectivity. However, PTMSP is soluble in many organic compounds, leading to potential dissolution of the membrane in process streams where its separation properties are of greatest interest. PTMSP also undergoes significant physical aging, which is the gradual relaxation of non-equilibrium excess free volume in glassy polymers. Crosslinking PTMSP with bis(azide)s was undertaken in an attempt to increase the solvent resistance and physical stability of the polymer. A fundamental investigation into crosslinking PTMSP with a bis(azide) crosslinker was the focus of this thesis. Pure gas transport measurements were conducted with N₂, O₂, CH₄, C₂H6, C₃H₈, and n-C₄H₁₀ over temperatures raging from -20°C to 35°C and pressures ranging from 0 to 20 atm. Mixed gas permeation experiments were conducted using a 98 mol % CH₄, and 2 mol % n-C₄H₁₀ mixture. The mixed gas permeation experiments were conducted at temperatures ranging from -20°C to 35°C, and pressures ranging from 4 to 18 atm. Inorganic nanoparticles such as fumed silica (FS) were added to uncrosslinked and crosslinked PTMSP, and the effects of their addition on the transport properties were investigated. Crosslinking PTMSP with bis(azide)s increases its solvent resistance, and crosslinked films are insoluble in common PTMSP solvents such as toluene. At all temperatures, the initial pure and mixed gas permeabilities of crosslinked PTMSP films are less than those of uncrosslinked PTMSP. This decrease in permeability is consistent with the fractional free volume (FFV) decrease that accompanies crosslinking. Pure gas solubility coefficients are relatively unaffected by the crosslinking process, so the decrease in permeability is caused by decreases in diffusivity. The addition of FS nanoparticles increases the initial pure and mixed gas permeabilities of uncrosslinked and crosslinked PTMSP. The pure gas permeabilities and solubilities of all PTMSP films increase when the temperature decreases, while the diffusivities decrease. The rates of change in pure gas transport properties with temperature is similar for all films, so the temperature dependence of pure gas transport properties of PTMSP is unaffected by the addition of crosslinks or FS. The aging of uncrosslinked and crosslinked PTMSP films was investigated by monitoring N₂, O₂ and CH₄ permeabilities and FFV over time. The FFV and permeabilities of crosslinked films decreased over time, so crosslinking did not arrest the physical aging of PTMSP, as has been previously reported, and these differences in aging observations are likely to be a consequence of differences in post film casting thermaltreatments. The addition of 10 wt % polysiloxysilsesquioxanes (POSS) nanoparticles decreases the permeabilities of uncrosslinked and crosslinked PTMSP by approximately 70 %, and the permeability and FFV values of the resulting nanocomposite films were stable over the course of 200 days. In all PTMSP films, the mixed gas permeabilities of n-C₄H₁₀ increase with decreasing temperature, while the mixed gas CH₄ permeabilities decrease with decreasing temperature. As a result, the mixed gas n-C₄H₁₀/CH₄ permeability selectivities increase with decreasing temperatures. The addition of crosslinks and FS nanoparticles to PTMSP decreases the mixed gas n-C₄H₁₀/CH₄ permeability selectivities, and changes in the free volume characteristics of PTMSP caused by crosslinking and FS nanoparticles are thought to reduce the blocking of CH₄ permeation by n-C₄H₁₀.