PTMSP free volume elements

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... 

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. 

Figure 2. Free-volume hole radius distribution R pdf(R) (relative units) of PTMSP (pdf(R) is probability density function) obtained from the two right-hand peaks in Fig. 1. The dashed line shows the calculated dependence (3) of annihilation rate, (in ns ) versus radius of FV elements used in confuting the size distribution.

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