Diffusion PTMSP

Table 1.3 Results of application of the Cusev-Suter method for the diffusion constants of different penetrants in PTMSP.

The results given in Table 5-1 show that the agreement between the diffusion coefficients predicted from MD simulations and experimental ones ranges from reasonable to excellent. At temperatures around 300 K this is found both for polymers which are above their glass transition temperature, Tg, (PDMS, PIB, PE and aPP) and for polymers which are below Tg (PET, PS, PTMSP, PI and PAI). As a trend one can notice, and this not only from Tab. 5-1 but also from other works published in the last six or seven years, that the agreement between MD simulations of diffusion and solvation of small penetrants in polymers and experiment steadily improved. These are encouraging developments, showing that modern softwares (some of them available for... 

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... 

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... 

J. H. Zhou, R. X. Zhu, J. M. Zhou, M. B. Chen, Molecular dynamics simulation of diffusion of gases in pure and silica-filled poly(l-trimethylsilyl-l-propyne) [PTMSP], Polymer, 47, 5206-5212 (2006). 

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. 

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),...

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. 

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. 

Remarkably, the temperature dependence of the alcohol diffusivity in PTMSP does not show a unique tr d in the entire concentration range an increase in temperature results in an increase or a decrease in the diffusivity according to the value of the composition of the polymer/penetrant mixture. 

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... 

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... 

It is worthwhile to notice that the value of the activation energy measured for the penetrants in PTMSP is significantly lower than that in the other glassy polymers used as gas separation membranes. Indeed, this result is consistent with previous results for the activation energy of diffiirion coefficients of gases in PTMSP (l8). The previously reported activation energies of diffusion were evauated based on gas permeability and sorption data. 

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