Mobility PTMSP

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... 

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. 

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... 

As the thermodynamic factor a can be determined from the solubility isotherms described in the previous section, we are able to estimate the mobility for all of the penetrant-PTMSP mixtures considered in this study. The thermodynamic factor a at the mean value of the concentration for a given sorption step can be estimated from the following relation... 

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... 

The measured concentration dependence of the mobility coefficient for PTMSP is opposite to that commonly observed for rubbery polymers. The reason for this difference is due to the decrease in the specific free volume of PTMSP mixtures as the penetrant concentration increases, at least as long as the swdling of the polymer matrix is negligible (6). 

Similar isotherms for the mobility coefficient are observed at aU t peratures and, moreover, the ratio between the mobility coefficients at different temperatures for the same component are essentially independent of concentration. This observation permits the evaluation of a unique activation energy, E., for the mobility coefficient of a given penetrant in PTMSP. The values of the activation energy of the mobility coefficient in PTMSP for n-alkanes and alcohols are reported in Table I. 

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