Electron ptychographic imaging of polyvinyl alcohol ordering Botao Hao, Zhiyuan Ding, Xudong Tao, Peter D. Nellist, Hazel E. Assender University of Oxford, UK Polyvinyl alcohol (PVA) is considered to have great potential in medical, pharmaceutical, and packaging applications due to its outstanding biocompatibility, water solubility, low density and relatively low cost. PVA crystallinity, central to the material properties, has been studied by X-ray diffraction, but two possible crystal structures are mooted [1][2] . This work uses an atomic-resolution imaging technique to ascertain the local crystal structure of PVA. Electron microscopic techniques can potentially image PVA at high resolution. Still, it is challenging for conventional electron microscopies because of the severe beam sensitiveness of PVA and the poor contrast of light elements [3] . Electron ptychography makes use of a 4D STEM dataset comprising the intensity in the STEM detector plane and has lower sample damage and better phase contrast compared to traditional techniques, which is helpful to overcome the existing challenges for the structural study of polymers [4] . In this work, PVA was first annealed to achieve a higher crystallinity for electron ptychographic study. XRD results reveal the overall crystallinity of PVA. Furthermore, some characteristic spacings related to the [010] and [-110] directions were found in HRTEM images. Electron ptychographic data with the simultaneous HAADF-STEM results on the above directions were then acquired. Compared to HRTEM and HAADF-STEM images, electron ptychography brings more information on PVA, and the image shows how the light atoms order in the material. However, the previous models derived from X-ray diffraction cannot fully explain the observations. To interpret the data, without changing unit cell dimensions, we did a series of modifications based on the previous models and the electron ptychographic images of PVA. The multislice simulation was finally used to confirm the modifications of the PVA model. The simulated results are essentially in accord with the experimental data, which supports our hypothesis to modify the structure. Overall, our results show that the local microstructure of PVA is much more disordered than previous models suggest, and this can be attributed to the flexibility of polymer backbones and the randomness of hydrogen bonding directions.
In summary, by using electron ptychography, we managed to directly image the atomic-scale structure of PVA, which has not previously been achieved by any conventional methods. And the results bring us a new and deeper understanding of PVA crystal structure. In the future, the methodology can be expanded to other polymeric systems for better microstructural insights.
Figure 1. Scheme of the key technique and results of this work References 1. Bunn, C. W. (1948). Nature ,161(4102), 929-930. 2. Sakurada, I., et al. (1950). Bull. Inst. Chem. Res., Kyoto Univ ., 23, 78-79. 3. Sawyer, L., et al. (2008). Polymer microscopy . 4. Yang, H., et al. (2016). Nature Communications ,7, 12532.
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