PTMSP properties

A. Morisato, B.D. Freeman, I. Pinnau and C.G. Casillas, Pure Hydrocarbon Sorption Properties of Poly(l-trimethylsilyl-l-propyne) [PTMSP] and Poly(l-phenyl-l-propyne) [PPP] and PTMSP/PPP Blends, J. Polym. Sci., Polym. Phys. Ed. 34, 1925 (1996). 

Table 1.1 contains typical solubility prediction data for an ultrahigh free-volume polymer (PTMSP) and a polymer with more conventional transport properties (PTMSS). 

Unfortunately there is an aging problem for PTMSP, resulting in deterioration of its transport properties with time. As shown in Figure 9.16, permeability decreased by one order of magnimde during the first 3 months, whereupon permeability stabilized. Selectivity remained stable during this period. But even the aged PTMSP is still a viable alternative to mbbery polymers for heavy hydrocarbon removal. 

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... 

Table IV. Physical properties, permeability and PALS properties for PTFE, TFE/PDD copolymers, and PTMSP at 25 C...

Because the so-called ultrahigh free volume polymers aroused much interest during the last 10 years, they will be briefly described in this introductory chapter. The publication of the physical properties of poly(l-trimethylsilyl-l-propyne) (PTMSP) in 1983 [281] aroused much interest in the field of membrane research. Up to this time it had been believed that the rubbery poly(dimethyl si-loxane) has by far the highest gas permeability of aU known polymers. Very surprisingly, the glassy PTMSP showed gas permeabilities more than 10 times higher than PDMS. This could be attributed to its very high excess-free volume and the interconnectivity of the free volume elements. Since then a number of... 

Table 7.5 Mixed gas-permeation properties of PTMSP and PMP. Feed 2% butane in methane, feed pressure 10 bar, permeate pressure atmospheric, temperature 25 °C. From I. Pinnau et al. In Polymer Membranes for Gas and Vapor Separation, ACS Symposium Series 733 (1999), 56-67.

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.

The sorption of several penetrants in PTMSP has been studied as a function of temperature and pressure. For both solubility and diffiisivity isotherms, the experimental results show significant differences between n-alkanes and alcohols. A discussion of the experimental data is presented, considering the glassy matrix as a homogenous phase, and using thermodynamic arguments commonly applied to standard mixtures. It is thus possible to offer a unique description of the thermodynamic properties of the various mixtures examined, in spite of their rather different behaviour. Simple isotherms for the mobility coefficient are also calculated for all penetrants as a function of composition. Remarkably, they show very similar trends for both n-alkanes and alcohols. 

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. 

A clear interpretation of the concentration and temperature dependence of the mass transport properties of both alkanes and alcohols in PTMSP can be obtained when it is recognised that the diffusion coefficient, A results from the product of the mobility, L, and a pure thermodynamic factor, a, defined as... 

A similar highly permeable, disubstituted polyacetylene, poly(4-methyl-2-pentyne) [PMP], was synthesized more than a decade ago by Masuda et al (6). The chemical structure of PMP is shown in Figure 1. The pure-gas permeation properties of poly(4-methyl-2-pentyne) are similar to those of PTMSP die gas permeabilities increase as the size, or condensability, of the penetrants increase (7). The physical properties, nitrogen permeabilities, and oxygen/nitrogen selectivities of PMP and PTMSP are shown in Table I. 

Table II. Mixed-gas permeation properties of high-free-volume, glassy PTMSP and PMP. Feed 2 mol% n-butane in methane feed pressure 150 psig permeate pressure atmospheric (0 psig) temperature 25 C.

In this paper, physical aging of gas permeability properties of PTMSP is described using the dud mode sorption and transport model, lliis model is also used to rationalize the increase in stability of gas permeation properties as a result of blending PTMSP with poly(tert-butyl acetylene) (PTBA). 

Similar behavior has been described in the literature for other polyacelylenes. Indeed, different solubility properties, originating from differences in the polymer microstructure, have been reported for PTMSP (5,6,20,21) and poly(trimethylsilylacetylene) (5). 

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 catalyst used for polymerization influences several properties such as the solubility of PTMSP in various solvents (7). Even though the cis-trans content of PTMSP has not been quantitatively determined, Costa et al (5) and Izumikawa et al. (6) reported that the polymerization catalyst NbClj produced a more regular chain confi ation than TaClj. In addition, PTMSP synthesized using NbClj had a cis-rich structure relative to that prepared with TaClj. In this study, Sie effects of physical aging on gas permeability and molecular motion of PTMSP membranes synthesized using various catalysts are reported. 

Commercial forms of the amorphous glassy perfluorinated polymers AF2400 and AF1600 purchased from DuPont Co in powder form were used as received. Some of the properties of these materials are described in (13), The films were cast from 2 wt.% solutions in perfluorotoluene and dried in the same way as the PTMSP films. 

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