NSRRC Activity Report 2022

Soft Matter 037 Fig. 2 : (a) SAXS spectra of Ni-bpyPTD, prepared with different cosolvent ratio of DMF/THF. (b) Normalized XANES spectra for Ni-bpyPTD, prepared with THF and DMF/THF. (c) Phase-corrected Ni K-edge EXAFS pattern, fitted using the first coordination shell of 4N and 2O atoms. (d,e) Schematic illustrations of the nanostructure of the Ni-bpyPTD films fabricated by THF system and the cosolvent system. [Reproduced from Ref. 1] Ho-Hsiu Chou (National Tsing Hua University) unraveled the mystery of ML interactions within the polymer matrix. The team fabricated a series of polyurethane with poly(propylene glycol) as the soft segments with the tolylene 2,4-diisocyanate (TDI) and the 4,4’-Bis(hydroxymethyl)-2,2’-bipyridine (bpy) moiety as the hard segments and coordination sites. Figure 1 shows the illustration of the design concept and chemical structures of Ni-bpyPTD, a high-stretching and self-healing polymer. Furthermore, they introduce Ni 2+ (from metal salt, Ni(OTf) 2 ) as the center metal ions. Small-angle X-ray scattering (SAXS) was collected at TLS 23A1 . In Fig. 2(a) , SAXS was used to elucidate the nanostructure of Ni-bpyPTD in different solvent systems. It was found that the SAXS peaks of Ni-bpyPTD in the co-solvent of mixed dimethylformamide (DMF) / tetrahydrofuran (THF) (solvent ratio of 3:1) shifted to higher q in comparison with Ni-bpyPTD in THF. It might indicate denser crosslinking sites are formed in the co-solvent system and thus lead to better nano-structural polymer networks with enhanced mechanical and self-healing properties ( Figs. 2(d) and 2(e) ). X-ray absorption near edge fine structure (XANES) spectra was measured in a transmission mode at TPS 44A . These results suggest the oxidation state of Ni ions in Ni-bpyPTD systems is closer to +2.0, compared with the standards NiO ( Fig. 2(b) ). Correspondingly, as shown in Fig. 2(c) , the extended X-ray absorption fine structure (EXAFS) pattern of Ni-bpyPTD indicate the first-shell metal coordination environments, including four nitrogen (from two bpy) and two oxygen atoms (from the triflate anion). This phenomenon is consistent with the +2.0 valence state observed from XANES. The bond length for Ni-N and Ni-O in Ni-bpyPTD from the EXAFS analysis is 2.06 Å. In summary, ML interactions and the co-solvent effect influence the mechanical and self-healing properties of stretchable and self-healable polymers. This work demonstrated that the nanoscale phase-separated and ordered networks play a crucial role in the field of dynamic ML interaction polymers. The ability to probe nanoscale structures of E-skin materials makes synchrotron-based SAXS and XANES/EXAFS important techniques that provide significant insight into mechanisms related to mechanical and electrical responses. (Reported by Yi-An Chen, National Tsing Hua University)