Positron annihilation lifetime spectroscopy high free volume polymers

Peng, Z. L., Olson, B. G., Srithawatpong, R., McGervey, J. D., Jamieson, A. M., Ishida, H., Maier, T. M., and Halasa, A. R, Study of free volume in high vinyl-polybutadiene/cis-polyisoprene blends by positron annihilation lifetime spectroscopy, J. Polym. Sci. Polym. Phys., 36, 861-871 (1998). 

Positron annihilation lifetime spectroscopy (PALS) is an efficient tool for measuring free volume and sizes of free volume elements in polymeric materials. This is particularly inq)ortant for studies of membrane materials, since free volume determines the permeation rate of small molecules. Free volume was studied by means of PALS in polymers characterized by extremely high permeability poly(l-trimethylsilyl-l-propyne) and copolymers of 2,2-bistrifluoromethyl-4,5-difluoro-l,3-dioxole and tetrafluoroethylene. The results obtained were compared with those observed for conventional glassy polymers. For the first time, the size distribution of free volume has been determined for these membrane materials. 

A variety of methods, both direct and indirect, have been developed for the measurement of free volume. Most simply the change in free volume can be measured from the macroscopic change in volume of a polymer on a change in temperature. Small angle X-ray and neutron diffraction are methods used to directly measure free volume by the determination of static density fluctuations in the materials (7). The rates of photo-isomerization of photochromic dyes imbedded within a polymeric material has been shown to be highly dependent upon the available free volume (3, 8). The techniques used in this study, Xe NMR and positron annihilation lifetime spectroscopy (PALS) have recently been successfully applied to the measurement of pore sizes in nanoporous solids and more recently in polymers. 

Free volume present in nanocomposite systems plays a major role in determining the overall performance of the membranes. Positron annihilation lifetime spectroscopy (PALS) is an efficient technique used for the analysis of free volume. The diffusion of permeant through polymeric membranes can be described by two theories, namely, molecular and free-volume theories. According to the free-volume theory, the diffusion is not a thermally activated process as in the molecular model, but it is assumed to be the result of random redistributions of free-volume voids within a polymer matrix. Cohen and Turnbull developed the free-volume models that describe the diffusion process when a molecule moves into a void larger than a critical size, Vc- Voids are formed during the statistical redistribution of free volume within the polymer. It is found that the relative fractional free volume of unfilled polymer decreases on the addition of layered silicates. The decrease is attributed to the interaction between layered silicate and polymer because of the platelet structure and high aspect ratio of layered silicates. The decrease is explained to the restricted mobility of the chain segments in the presence of layered silicates. This results in reduced free-volume concentration or relative fractional free volume [49]. 

The results summarized in Table 9 show that intrinsic porosity of the polymer, i.e. the fraction of its total volume accessible to N2 molecules at 77 K, can exceed 20% for the sample conditioned at high propylene pressure and room temperature, decreasing down to ca. 6% upon annealing in vacuum at 373 K. These results allow assuming that the variations in propylene permeability through a membrane described above apparently stem from the changes in the free volume structure, i.e. accessibility of the intrinsic micropores for the gas molecules. Similar conclusion on the expansion of the free volume hole size and the increase in the number of holes upon sorption of CO2 in polycarbonate has been made in [47] on the basis of positron annihilation lifetime spectroscopy investigations. 

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