Alcohols PTMSP

AA acrylic acid LDPE low density polyethylene NBR poly (butadiene-acrylonitrile) PA polyamide PAA poly(acrylic acid) PAN polyacrylonitrile PB polybutadiene PC polycarbonate PDMS polydimetylsiloxane PE polyester PEBA polyetheramide-block-polymer PI polyimide PMA poly(methyl acrylate) POUA poly(oxyethylene urethane acrylate) PP polypropylene PPO poly(phenylene oxide) PTMSP poly(trimethylsilylpropyne) PUR polyurethane PVA poly(vinyl alcohol) PVC poly(vinyl chloride). 

Their applications and improved long-term stability for gas separation and pervaporation were further investigated. A maximum in the separation factor and rate was obtained for the PTMSP copolymer with 12 mol% PDMS. The membrane was able to convert a 7 wt% ethanol mixture to over 70 wt% in ethanol [120]. Good oxygen permeability and stability of over a month was also achieved for the PTMSP copolymers with over 60% PDMS [121]. In a separate study, PPP was foimd to have good permselectivity to water while PTMSP is alcohol permselective. These copolymers as well as a series of poly(phenyl acetylene) graft PTMSP copolymers were further studied for pervaporation applications [122]. 

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. 

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. 

From the experimental data presented herein, we are now able to estimate the mixing enthalpy for n-alkanes and alcohols in PTMSP and, thus, to evaluate the... 

In summary, the enthalpy of mixing is always negative and thus represents an important contributions in favour of the solubility of penetrants in PTMSP. For alcohols in PTMSP, the excess enthalpy is significantly smaller than for the n-alkanes and even shows negligible values in the low concentration range where the solubility... 

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... 

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. 

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. 

By considering the solid glassy mixture as a homogeneous phase, the apparent qualitative differences between the solubility of alkanes and alcohols in PTMSP may be easily interpreted as simple quantitative differences in energetic and entropic contributions to the free energy of mixing. Indeed, the chemical potentials of the penetrant species in the mixture show similar isotherms for every case considered. The difference in the value of the chemical potential for alkanes and alcohols in PTMSP is mainly due to differences in energetics of the sorption process, and not to entropic effects. 

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