Free Volume and Transport Properties in Highly Selective Polymer Membranes

Abstract

Varying systematically the structure of glassy poly(amide imide), poly(ester imide), and polyimide, we have studied the correlation between free volume and transport properties of highly selective polymer membranes. Free volume data were determined by means of positron annihilation lifetime spectroscopy (PALS) while transport properties originate from time-lag measurements of permanent gases. We find a good correlation between PALS average hole size and transport coefficients. The correlation is much better than with free volume data from group contribution methods. It is shown that the permeation properties are controlled not only by free volume fluctuations but also by energy barriers. A modified transport model taking into account the effect of the cohesive energy density on the energy barriers further improves the correlation significantly.

Methods for the calculation of occupied volumes in glassy polymers: The lattice integration and the monte carlo methods

“…Volume corrections are calculated by formula (14), but now m is the total of the points, satisfying all conditions (16)(17)(18)(19).…”

“…In order to understand this, it is necessary to bear in mind that di®usion coe±cient does not depend only on free volume. A more rational picture is given by models using both methods, namely the activated di®usion and the free volume theories [18,19,20,21]. The generalized formula, following this approach is given by,…”

Free Volume of PVA/SSA Proton Exchange Membrane Studied by Positron Annihilation Lifetime Spectroscopy

“…In general, the fuel permeability through PEM is believed to be correlated to the structure of the membrane and the open free volume in the PEM material [5][6][7], hence, the structure with less free volume is desirable for PEM. In order to develop advanced PEM materials with low fuel permeability, it is important to gain a deeper * corresponding author; e-mail: mahmoud_gomaa19@mu.edu.eg insight into the nanostructure and, in particular, into the free volume hole size distribution.…”

Free-Volume Studies in Polycaprolactone/Poly(propylene oxide) Urethane/Urea Membranes by Positron Lifetime Spectroscopy

“…This is not fully surprising, as, e.g., the fractional free volume seen by o-Ps must exhibit a saturation limit when full trapping is reached, which is probably the case here. Studies on gas permeation [19] clearly indicate the influence of many factors, e. g. chain flexibility, chemistry at the free-volume walls, diffusion barriers, etc. The interconnectivity must play a crucial role in the gas permeation and definitely needs further systematic studies, by PALS and other techniques.…”