Pentane PTMSP

Five different penetrants were considered n-pentane (n-Cs), n-hexane (n-Ce), n-heptane (n-C ), ethanol (EtOH) and methanol (MeOH). For the cases of n-pentane, ethanol and methanol, solubility and difiiisivity isotherms at three different temperatures 285, 300 and 330 K were examined. Sorption experiments for n-hexane in PTMSP were performed both at 300 and 330 K, while only the highest temperature was used for n-heptane. 

Solubility Isotherms. In Figure 1, the solubility of n-pentane in PTMSP is reported for three different temperatures. The zero pressure solubility coefficient So, defined as the limit of the ratio between the penetrant weight fraction a)i and the corresponding... 

As also observed by Morisato et al. (10) and by Doghieri and Sarti (75), the solubility isotherms of n-pentane in PTMSP have the usual downward curvature, which is typical of the sorption of most gases and vapours in glassy polymers. 

From the data in Figure 1, a significant temperature effect on the solubility of n-pentane in PTMSP is also evident at all penetrant activities. As is typical for the sorption in glassy polymers, the solubility of the penetrant increases as the temperature decreases at each activity value. This effect is obviously associated with negative values of the mixing enthalpy, and it appears to be significantly pronounced in the case of n-pentane. 

In Figure 2 the solubility isotherms of n-hexane in PTMSP are reported at temperatures of 300 K and 330 K. The behaviour is qualitatively similar to that observed for n-pentane sorption. Both isotherms exhibit a downward curvature and the solubility coefficient in the low pressure limit. So, is high. For n-hexane. So 25 and 16 at 300 and 330 K, respectively. Also for n-hexane sorption, the effect of temperature on solubility indicates a large negative enthalpy of mixing since the solubility decreases substantially with increasing temperature. 

Remarkably, upon comparing the solubility of n-pentane and n-hexane in PTMSP, it is observed that, at the same temperature and penetrant activity, the solute mass ratio dissolved in PTMSP is higher for the higher molecular weight component. The same trend is confirmed when the analysis is extended to the case of n-heptane, and it is clearly apparent from Figure 3, where the solubility isotherms for the three alkanes at 330 K are simultaneously reported. From the data shown in Figure 3, the solubility coefficient in the low pressure limit for n-heptane in PTMSP may be evaluated as So 27. 

The quantitative differences observed between the two families of penetrants are due both to enthalpy and to entropy contributions. Based on a qualitative consideration of the interaction energies between the polymer matrix and the aliphatic or the alcoholic groups of the different penetrants, the enthalpic term in Equation 1 is mostly responsible for the quantitative differences observed in the isotherms. This was also the conclusion drawn in a previous study (75) where the solubility of ethanol and n-pentane in PTMSP were qualitatively compared with each other. Based on the solubility isotherms at different temperatures for n-alkanes and alcohols, we can now directly evaluate the excess enthalpy of mbdng in Eq.(l). In a previous discusdon of the comparison of solubility data for ethanol and n-pentane in PTMSP (6), it was assumed that the difference in chemical potential for the two penetrants was essentially due to the enthalpic term. TMs hypothesis is reasonable since the interaction energy of penetrant molecules with PTMSP segments should be significantly different, relative to interaction energies of pure penetrant molecules, between polar and non polar penetrants. 

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. 

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 solubility of several penetrants in PTMSP indicate relevant differences between polar and non-polar components. In particular, for the alcohols, unusual S shaped solubility isotherms are observed which cannot be explained by the dual mode model. Variations of solubility with temperature were also considered in this work, and the mixing enthalpy for n-pentane, n-hexane, ethanol and methanol in PTMSP was calculated. Significant negative mixing enthalpies were measured in all cases, and absolute values of the excess enthalpy of the sorption is much higher for the alkanes than for the alcohols. 

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