Glassy polymers PTMSP

The authors of this paper have proposed the application of novel high performance membranes with the dense thin top-layer made of the glassy polymer PTMSP with the highest gas permeance among known polymers. It was already shown that PTMSP as a membrane material possesses long-term chemical and mechanical stability at typical MGD conditions - amine-based solvent, trans-membrane pressure up to 40 bar and temperature 100 °C [7]. Furthermore, PTMSP is a barrier material towards chemical solvents such as aqueous solutions of alkanolamines [7] and some physical solvents like water [8] and ionic liquid [9]. Details about the development of these membranes are described elsewhere [10], In this paper, the focus is on experimental work on using these membranes in contactors and the implications for application in natural gas processing. 

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. 

Two of these are under extensive investigation and are currently being studied for gas separation on a pilot scale. These are DuPonfs 2,2-bistrifluoromethyl-4,5-difluoro-l,3-dioxole/tetrafluorethylene copolymer (Teflon AF 2400 ) and poly(4-methyl-2-pentyne) (PMP). All three polymers, PTMSP, PMP and Teflon AF2400, are glassy with glass transition above 230°C and have a very high fractional free volume (FFV). Figure 7.4 shows the chemical structure and fractional free volume of these three polymers. 

It has been found, for a wide range of solntes in glassy and rubbery polymers, that S varies with T, where Ti is the critical tanperature of the solute [31]. This correlation was found to apply for PlM-1, as can be seen in Figure 2.6, which includes S values from IGC and S values derived from gas permeation experiments. Figure 2.6 also includes data for the previous champion amongst monbiane polymers, PTMSP [32], and the results show that PlM-1 exhibits the largest solubility coefficients of all polymers studied in this way. This confirms the exceptional affinity that PlM-1 has for small molecules. 

Figure 7.17 Influence of porous aromatic frameworks (PAFs) on the CO2 permeability coefficient of different glassy polymers, polyftrimethylsilyl propyne) (PTMSP), PMP, and polymers of intrinsic microporosity (PIM)-l, with respect to the aging phenomenon.

Figure 9. Pure gas propane permeability and propane/methane selectivity for a series of selected organic liquids (O), rubbery siloxane-based polymers ( ), and glassy polymers ( ). The glassy polymers include PI, a polyimide (79), PC, polycarbonate (80), PS, polystyrene (81), and PTMSP (82), Data for the siloxane-based rubber polymers are from Stem et al (83), The solubility of propane and methane in selected organic liquids (hexane, heptane, octane, acetone, benzene, methanol, and ethanol) is from the compilation by Fogg and Gerrard (72). Diffusion coefficients of propane and methane in these liquids were estimated using the Tyn and Calus correlation (46 48),...

This work offers a contribution to the understanding of some fundamental aspects of sorption and diffusion in glassy polymers. The research focuses on an extensive experimental study of sorption and mass transport in a specific polymeric matrix. A high free volume polymer, (poly l-trimethylsilyl-l-propyne) [PTMSP], has been used here in order to emphasise aspects of sorption and transport which are peculiar to polymer/penetrant mixtures below the glass transition temperature. The discussion of the experimental data presented in this work permits a clarification of concepts which are of general validity for the interpretation of thermodynamic and mass transport properties in glassy systems. 

As also observed by Morisato et al. (10) and by Doghieri and Sarti (75), the solubility isotherms of n-pentane in PTMSP have the usual downward curvature, which is typical of the sorption of most gases and vapours in glassy polymers. 

From the data in Figure 1, a significant temperature effect on the solubility of n-pentane in PTMSP is also evident at all penetrant activities. As is typical for the sorption in glassy polymers, the solubility of the penetrant increases as the temperature decreases at each activity value. This effect is obviously associated with negative values of the mixing enthalpy, and it appears to be significantly pronounced in the case of n-pentane. 

In Figure 7, diffusivity isotherms at 300 K are presented for n-pentane, n-hexane, ethanol and methanol. The measured values of D are orders of magnitude higher than the typical values of diffusion coeffidents in glassy polymers. The diffusivity of the two alkanes in PTMSP show similar trends as a function of the penetrant content. For both n-pentane and n-hexane, the diffusion coefficient exhibits a maximum variation of about 40% and a rather flat maximum value at intermediate concentrations. 

It is worthwhile to notice that the value of the activation energy measured for the penetrants in PTMSP is significantly lower than that in the other glassy polymers used as gas separation membranes. Indeed, this result is consistent with previous results for the activation energy of diffiirion coefficients of gases in PTMSP (l8). The previously reported activation energies of diffusion were evauated based on gas permeability and sorption data. 

PTMSP has a glass transition temperature of more than 250 C (2). In aU glassy polymers, small-scale polymer segmental motions lead to relaxation of non-equhibrium excess free volume and, as a result, the physical properties of glassy polymers drift over time. Because PTMSP has more free volume than other glassy polymers, a dramatic decline in gas permeability occurs when the non-equilibrium excess volume in PTMSP relaxes (3,4). Membrane contamination via absorption of organics (such as pump oil vapor) also decreases gas permeability of PTMSP membranes (4). In the absence of such contaminants, the decrease in gas permeability... 

The PAL spectrum of polystyrene, which is considered as a conventional glassy polymer, is also presented in Table I for comparison. It is seen that much longer lifetimes (5-7 ns for X4) are characteristic of high permeability materials. Interestingly, such long lifetimes have been observed as well in silicagels and zeolites (76) or even in porous UF membranes as is shown below. All of this information is in line with the assunq)tion that high free volume polymers like PTMSP are akin to porous sorbents (probably with open or closed porosity). 

---