Positron annihilation spectroscopy for the free volume depth profile analysis of multilayer and 2D materials composite membranes: A review

Previous techniques probed only static defects in polymeric membranes. However, spaces within polymer chains must be dynamic, which allows them to move freely. The advent of positron annihilation spectroscopy (PAS) has dispelled the lack of a suitable probe for free volume, which has both a subnanometer dimension (Å–nm) and a very short timescale (10^–6–10^–13). Since then, PAS has introduced an influx of experimental data on free volume, voids, depth profile, and elemental analysis in 2D and polymeric membranes. But PAS is still in the developing stage. The following requisites propel the development of PAS: (1) more accurate measurement of free volume and voids; (2) characterization of depth profile in composite membranes; (3) analysis of elemental and chemical compositions of membranes; (4) advancement of PAS technology. PAS results are correlated with pervaporation, nanofiltration, microfiltration, ultrafiltration, and gas separation in membrane application. This review article discusses PAS techniques, data analysis, and instrumentation in detail in each requirement. We identify PAS limitations and challenges and comment on past studies that suggested and applied solutions. Furthermore, we introduce the characteristics of PALS technology in polymer composite membranes and describe the application potential of advanced two-dimensional materials.