PTMSP permeability coefficients

Properties for several TFE/PDD copolymers and PTMSP are compared in Table IV. Density and glass transition temperatures for the TFE/PDD copolymers were obtained from Buck and Resnick (44), and the density and glass transition temperature for PTMSP are from the study of Nakagawa et al.(l). Among the fluoropolymers in this table, PTFE homopolymer exhibits 5ie lowest glass transition temperature, the lowest oxygen permeability coefficient, and the lowest fractional free volume. In the polymers in Table IV, the PALS results suggests a bimodal distribution of free... 

These composite results suggest that the distribution and availability of free volume in PTMSP and the TFE DD copolymers are very different. Both PTMSP and the TFE/PDD copolymers are high Tg, stiff chain materials, so it is unlikely that the vast differences in accessible free volume and permeability coefficients is solely related to great differences in segmental dynamics between these materials which would render the free volume in PTMSP much more accessible on the time scales appropriate for PALS and permeation. Rather, it seems more likely that free volume elements in PTMSP are interconnected and span the sample, providing extremely efficient pathways for penetrant diffusion. In fret, the notion of interconnected free volume elements in ITMSP has been invoked to explain the unusual transport... 

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.

Nagase et al. have studied the grafting of polyacetylene and its derivative onto poly(dimethylsiloxane) (PDMS) for membrane applications [118-121]. Poly(l-trimethyl-silyl-l-propyne) (PTMSP) is a polymer known to have excellent gas permeability but suffers from relatively low selectivity and a decrease in gas permeability over time. Graft copolymers of poly(l-phenyl-1-propyne) (PPP) onto PDMS were found to have improved gas permeability and selectivity, and the performance was related non-linearly to the PDMS content. A minimum oxygen permeability coefficient was found for a copolymer with 55 mol% PDMS [118]. The same membrane series was also foimd to be permselective for a range or organic liquids, including an ethanol/water mixture, and was used in pervaporation applications... 

Si(CH3)3 substituent can be as large as two orders. The copolymer of SiMe3NB and HexNB characterized by lower gas permeability coefficients than homopolymer of SiMesNB, but higher, than homopolymer of HexNB. Permselectivity or separation factors a(Mi/M2) = P(Mi)/P(M2) are compared in Table 7 for addition poly(5-trimethylsilylnorbomene) and some other polymers. It is seen that in the most eases addition poly(5-trimethylsilyl-2-norhomene) is more permseleetive than high permeability polymer polytrimethylsilylpropyne (PTMSP). 

An example of the extraordinary separation properties of PTMSP for hydrocarbon/hydrogen mixtures is given in Figures 8a and 8b, which present propane and hydrogen permeability coefficients and mixture separation factors as a fonction of propane relative pressure in the feed gas. As the propane relative pressure increases, propane permeability increases by less than a factor of two, from... 

Figure 8. Hydrocarbon/light gas separation properties of PTMSP. (a) Effect of propane relative pressure on propane and hydrogen permeability coefficients in PTMSP at 25°C. (b) Effect of propane relative pressure on C3H8/H2 mixture selectivity. The feed pressure of the propane/hydrogen binary mixture was 200 psig and the permeate pressure was atmospheric (0 psig) (9). Propane relative pressure is p/psa, where p is the partial pressure of propane in the feed and p, is the saturation vapor pressure of propane.

Effect of Physical Aging on Gas Permeability. Literature values of the oxygen permeability coefficient of PTMSP are strikingly variable. At room temperature, the minimum and maximum values are 3,000 and 12,000 Barrers (11J2). The unusually... 

Figure 7 shows the effect of PTBA content on Ch (obtained from CO2 sorption) and oxygen permeability coefficient at 30 C. As expected, both and Oj I rmeability decrease monotonically upon increasing Ae concentration of PTBA since PTBA has a lower fractional free volume than PTMSP. 

Figures 14 and 15 show the relationship between PTB A content and carbon spin-lattice relaxation time (Ti) of the PTMSP side-chain and backbone carbon atoms, respectively. The effect of PTBA content on Ch is also shown in these figures. Ti of the methyl group carbon atoms (a) in the side-chain of PTMSP became longer with increases in PTBA content. This result indicates that the molecular motion of the trimethylsilyl groups became slower. On the other hand, Ti values for the carbon atoms of the other methyl group (b) and the backbone chain show a maximum at 20 vol % PTBA, which suggests perfect mixing of PTMSP and PTBA. These results coincide with the maximum stability of the gas permeability coefficient (c/. Figure 10).

PTMSP exhibits significant decreases in gas permeability with age. During aging, the nonequilibrium excess volume of glassy PTMSP relaxes. The reasons for the decrease in gas permeability were not only relaxation of the nonequilibrium excess volume in the Langmuir mode, but also reduction of the permeability coefficient in the Henry s law mode. This latter effect appears to be correlated with relaxation of the nonequilibrium excess volume. 

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. 

Permeability coefficients of nitrogen in ascast and aged PTMSP membranes at 30 C. 

The diffusion of gases through a polymer matrix is determined by the mobility of gas molecules through the matrix. The diffusion coefficient is therefore, at least partially determined by the free volume size of the polymer. It has been shown, for example, that there is a correlation between the free volume measured by PAL and the diffusivity of carbon dioxide in a seriers of polycarbonates [58], In a study of poly (trimethylsilyl propyne) (PTMSP), which has an extremely high gas permeability and diffusion coefficients, it was found that the lifetime data could be resolved into four components [59]. The longest lifetime component (T4) had a lifetime of... 

---