Comparative Study of Different Probing Techniques for the Analysis of the Free Volume Distribution in Amorphous Glassy Perfluoropolymers

A comparative study of five different experimental and computational methods is presented for the characterization of the overall free volume (FV) and the free volume element (FVE) size and shape distribution in amorphous glassy perfluoropolymers (PFPs). Experimental results from the photochromic probe (PCP) method, positron annihilation lifetime spectroscopy (PALS), and inverse gas chromatography (IGC) were confronted with literature data from Xe-129 NMR spectroscopy, and the experimental data were further compared with molecular dynamics (MD) simulations and a combination of MD studies and the well-known Bondi method as well as a modified Bondi method. An evaluation of the advantages and the limits of each method is presented. This is the first reported study on a so vast number of complementary techniques applied on a single glassy polymer, in this case Hyflon AD perfluoropolymer, and is also the first successful application of the photochromic probe technique in such materials. In two different grades of Hyflon AD, the polymer with the highest content of the stiff cyclic comonomer was found to have a slightly larger average FVE size but a lower void concentration, explaining the nearly identical density and fractional free volume (FFV) of the two samples. PALS furthermore demonstrated a similar trend for solution-cast samples in comparison with melt-pressed samples, the latter having FVEs with a smaller size but a higher concentration. The data from IGC seem to correspond most closely to those of the PCP method. Differences between the results from the individual techniques derive mainly from the fundamentally diverse nature of the various probing methods but also from the different capacity to take into account the FVE shape. Only MD Simulation studies, using detailed atomistic packing models, can give such deep insight into the spatial arrangement of the FVEs directly. Besides giving the highest level of detail, MD simulations can thus help to understand the possible limits of the experimental methods. Knowledge of their free volume distribution is of fundamental importance to gain more insight into the mass transport phenomena in these materials, relevant for their successful application in the emerging Field of synthetic membranes for gas and vapor separations.