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Description
The ePDF method allows the measurement of the average local structure of an amorphous or semicrystalline material$.^1$ Applied to the class of micro- and nanoplastic particles, which are considered a possible threat to the environment, it could serve as a means for the distinction of polymer types in mixtures of (sub-)micron particles. In particular for nanoplastics of sizes below 100 nm, contemporary methods such as Raman microscopy are limited by spatial resolution.
Polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyamide (PA) microparticles, as well as polystyrene (PS) nanoparticles were examined by transmission electron microscopy and electron diffraction (ED) under low dose, non-cryo conditions. Reduced pair density functions (RDF) were calculated from the processed and background subtracted ED data and compared to simulated RDF. Beam induced effects were determined by serial ED at different beam dose rates and analysis of the time dependent ED.
High quality serial ED suitable for ePDF analysis was measured on polymer microparticles. The ePDF method yields a qualitative agreement of experimentally determined and simulated RDFs for PE, PP, PET and PA. The RDFs of the examined polymers allow a distinction of the materials, granted that the differences between PE and PP are small. In the short distance region (1-5 Å) all experimental peaks can be assigned to C-H, C-C or C-O bond lengths or remote distances by comparison to the model RDFs. The beam induced effects on the structures are visible in the temporal evolution of the RDFs. The change in structure depends on the polymer type and the total dose/dose rate. The observed changes in the RDFs can be rationalized by considering the chemical structures of the materials. In the cases of PE and PP, first C-C-bond scission dominates, leading to a loss of medium range (5-10 Å) order, then C-H bond scission becomes more prevalent and graphitization occurs. In contrast, PET shows little change in the RDF in the lower dose rate experiment and only a slight shift in peak positions to longer distances in the higher dose experiment. This can be rationalized by a lack of initial medium range order in the glassy material, and by the presence of the extended π-electron system which provides more resilience to the beam effects compared to the single bonded PE, PP and PA. Nanobeam diffraction was performed on 100 nm PS and silica nanospheres. RDFs gained from this technique confirm the applicability of ePDF to nanosized materials.
We show that detailed structural information can be gained on highly beam sensitive polymer materials by following a low dose, ePDF approach without cryo or staining techniques. The approach should be extendable to 4D STEM methods and thus allow for automated handling of environmental samples if combined with computational techniques. This methodology may also deliver new insights into transient, stationary as well as amorphous phases that form during catalysis.
(1) T.E. Gorelik, et al. Acta Cryst. 2019;75:532–49.
