ANALYSIS OF THE MECHANICAL PROPERTIES OF POLYMERIC MEMBRANES BY AFM AND TENSILE TESTS

In the last years, the development of novel materials with superior gas transport properties enhances the exploitation of polymer membrane gas separation technology in new markets or in replacement of traditional technology1. The transition from lab-scale membrane to real industrial use requires a fully physical-chemical characterization of the materials. A promising material should couple the superior gas transport properties with a mechanical stability. The mechanical properties are commonly investigated by tensile test or rheological characterization. However, these techniques present two main drawbacks: they are destructive and require an amount of sample, which is not always available. The Atomic Force Microscopy (AFM) worked in Force Spectroscopy mode is a technique to investigate the mechanical and viscoelastic properties of materials 2. This technique is non-destructive, requires a small amount of material, and it makes possible the investigation of the mechanical properties at nano and micro scale. The aim of this work is to assess the comparability of the mechanical properties analysed through these two different techniques. The mechanical properties of a rubbery polymer, Pebax®1657, added with different amount of the ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], and the mechanical properties of glassy polymers of intrinsic microporosity (PIMs), are investigated. The tensile test data, obtained for Pebax samples, are in agreement with the ones obtained with AFM on micro scale. On nano scale, the probe has dimensions comparable with polymer domains and it can distinguish their different rigidity. The force spectroscopy analysis on PIMs showed how the thermo-mechanical history influences the mechanical properties, and the correlation with the diffusion and permeability coefficients.