Free PTMSP

Figure 2.23 Structure of two high-free-volume substituted polyacetylenes, PTMSP and PMP. The carbon-carbon double bond is completely rigid, and depending on the size of the substituents, rotation around the carbon-carbon single bond can be very restricted also. The result is very stiff-backboned, rigid polymer chains which pack very poorly, leading to unusually high fractional free volumes...

These high-free-volume polymers also have unusual permeability characteristics with mixtures of condensable and noncondensable gases. For example, in the presence of as little as 1200 ppm of a condensable vapor such as the per-fluorocarbon FC-77 (a perfluoro octane-perfluoro decane mixture), the nitrogen permeability of PTMSP is 20 times lower than the pure nitrogen permeability [71], as shown in Figure 2.41. When the condensable vapor is removed from the feed gas the nitrogen permeability rapidly returns to its original value. The best... 

Figure 2.41 The change in nitrogen flux through a PTMSP membrane caused by the presence of a condensable vapor in the feed gas [71]. This behavior is characteristic of extremely finely porous microporous ceramic or ultrahigh-free-volume polymeric membranes such as PTMSP. The condensable vapor adsorbs in the 5- to 15-A-diameter pores of the membrane, blocking the flow of the noncondensable nitrogen gas...

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

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... 

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... 

X3 l3+X4 l4), in PTFE, only 0.4% in TFE/PDD 65 and TFE/PDD87 and only 0.2% in PTMSP. Thus, the vast majority of the free volume accessible to oPs is in the larger free volume elements. 

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... 

However, the fast physical aging limits the practical application of PTMSP membranes. One solution is the cross-linking of PTMSP, which stabilizes the large excess free volume elements and hence improves physical stability [86-88]. Cross-linking generally reduces gas permeability due to free volume reduction, while the polymer network becomes more size selective and gas selectivity increases. 

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... 

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. 

Fig. 7.4 Chemical structure and fractional free volume of PTMSP, PMP and Teflon AF2400.

It has been reported recently that flux and even selectivity of PMP and PTMSP can be enhanced by the addition of nanoparticles (285, 286]. Merkel et al. [285] added fumed sihca to PMP and observed a simultaneous increase of butane flux and butane/methane selectivity. This unusual behavior was explained by fumed-silica-induced disruption of polymer chain packing and an accompanying increase in the size of free volume elements through which molecular transport occurs. Gomes et al. [286] incorporated nanosized sihca particles by a sol-gel technique into PTMSP and found also for this polymer a simultaneous increase in flux and selectivity. It has to be studied, if physical aging of the polyacetylenes is reduced by the addition of nanoparticles. 

In a previous work [4] we already proposed a method to predict the enhancement in diffusivity due to the addition of fumed silica particles to high free volume matrices such as PTMSP and Teflon AF 2400. The model was tested only on the bases of available literature data while in this work we performed a detailed characterization of Teflon AF 2400 mixed matrices to further inspect and document the validity of the approach proposed. 

Another possible approach to indirectly characterize the membrane morphology is based on the investigation of the free volume within the matrix. Density measurements [119,120] and positron annihilation lifetime spectroscopy evaluation [47] are common methods. Typically, the comparison between the theoretical density or free volume (calculated by simple additivity rules) and the experimental one can reveal the presence of a good interfacial morphology or the presence of interface voids or clustering formation. Fig. 7.13 shows the influence of filler content on the morphology of poly(trimethylsilyl propyne) (PTMSP)/Ti02 NCMs in terms of the volumetric fraction of interface voids as calculated from a comparison of the expected and measured membrane density [119],... 

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