Methanol PTMSP

Figure 5.10 Double logarithmic plots of selectivity versus permeability for (top) OJ N2 and (bottom) CO2/CH4, showing (solid line) Robeson s 1991 upper bound and (dashed line) the 2008 upper bound, as well as data for (A) pol (trimethylsilyl prop me) (PTMSP), (A) indan-based polyacetylene 2e, (x) Teflon AF2400, (+) addition-type poly(trimethylsilyl norbornene), ( ) PIM-1, ( ) PIM-1 after methanol treatment, (O) 6FDA-DMN pol dmide, (O) PIM-PI-8 and ( ) PIM-PI-8 after methanol treatment. Reproduced from Ref. 15 with permission from The Royal Society of Chemistry.

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),...

Five different penetrants were considered n-pentane (n-Cs), n-hexane (n-Ce), n-heptane (n-C ), ethanol (EtOH) and methanol (MeOH). For the cases of n-pentane, ethanol and methanol, solubility and difiiisivity isotherms at three different temperatures 285, 300 and 330 K were examined. Sorption experiments for n-hexane in PTMSP were performed both at 300 and 330 K, while only the highest temperature was used for n-heptane. 

The peculiar features shown by the sorption isotherms of ethanol in PTMSP are followed also by the solubility curves for methanol in PTMSP, which are presented in Figure 5. As in the case of ethanol, the methanol solubility coefBcient in the low pressure limit. So, is orders of magnitude lower than for alkanes, and the sorption isotherms are characterised by the previously mentioned S shape. 

Diffusivity and Mobflity Isotherms. The temperature and penetrant concentration dependence of the diffusion coefficient, D, has been measured for binary mixtures of PTMSP with n-pentane, n-hexane, ethanol and methanol Indeed, for each sorption step in the sequences of isothermal sorption experiments, the kinetics of mass uptake have been analysed and estimates of the corresponding diffusivity have been obtained from both small and long time sorption data, as indicated in a previous study (6). The relative difference in the values of the diffusion coefficients obtained through the use of long time and short time sorption data is less than 5% in the great majority of cases and is less than 10% in the worst cases. Indeed, when one compares the diffusion coeffident results obtained from two different experiments performed under the same conditions, the diffusivity values are only determined to within approximately 10%. 

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. 

The diffusivity isotherms obtained for ethanol and methanol are similar to each other and quite different from those of the alkanes. Very large diffusion coefficients have been measured in the low penetrant concentration range. Then, the diffusion coefficients decrease to a minimum value at approximately 7% penetrant weight fraction. Afterwards, the diffusivity increases again and shows a local maximum value at o ll%. The latter trend is also observed for diffusivity coefficients measured at temperatures of 285 and 330 K. However, for the sake of conciseness, these data are not presented explicitly. The present data also confirm the diffusivity behaviour of ethanol in PTMSP reported previously (6). 

We observe in this case that, contrary to what could be expected based simply on molecular weight considerations, the values of methanol diffusivity in PTMSP are the lowest among all of the penetrants considered. 

In Figure 8, the mobility isotherms at 300 K are reported for binary mixtures of PTMSP with n-pentane, n-hexane, ethanol and methanol. Despite some scatter in the data due to the uncertainty in the values of the diffusion coefficient and to some error propagation due to the use of Eq.(7), it is quite evident that all the isotherms show the same shape. In all cases the mobility coefficient decreases exponentially with increasing penetrant weight fraction. These results confirm the data obtained for... 

The solubility of several penetrants in PTMSP indicate relevant differences between polar and non-polar components. In particular, for the alcohols, unusual S shaped solubility isotherms are observed which cannot be explained by the dual mode model. Variations of solubility with temperature were also considered in this work, and the mixing enthalpy for n-pentane, n-hexane, ethanol and methanol in PTMSP was calculated. Significant negative mixing enthalpies were measured in all cases, and absolute values of the excess enthalpy of the sorption is much higher for the alkanes than for the alcohols. 

Membrane Preparation. All PTMSP, PTBA and PTMSP/PTBA blend polymer membranes were cast on a horizontal glass plate from polymer solutions in toluene. They were immersed in methanol until just before beginning experimental measurements to prevent hysteresis of the membranes. 

Membranes. PTMSP membranes with uniform thicknesses of 60 to 200 pm were prepared by casting on a horizontal glass plate from a solution of the polymer dissolved in toluene. To prevent physical aging and contamination, all membranes were immersed in methanol until just before beginning gas permeation, gas sorption, and NMR experiments. The methanol treatment was performed at least 48 hours at room temperature in order to attain the equilibrium sorption of methanol in the PTMSP membranes (Le, to prepare samples with the same initial condition). PTMSP membranes swell when immersed in methanol. The experiments were started after die macroscopic contraction of the swollen membrane was finished. The detailed... 

Gas Permeability. Figure 1 presents Nj permeability coefficients for as-cast and aged PTMSP membranes synthesized using various catalysts. The initial value of an as-cast membrane was measured between 24 and 48 hours after the membrane was removed from methanol. 

---