writer：Yongsheng Luo, Chu Wang, Ai-Ping Pang, Xiang Zhang, Dayang Wang, and Xiaolin Lu*
keywords：Polymer brushes, SFG, Salt ions, Low concentration
source：期刊
specific source：Macromolecules 2021, 54, 13, 6006–6013
Issue time：2021年

The interfacial molecular structures of polyelectrolyte brushes grafted on solid substrates are generally influenced by the electrostatic interaction and osmotic pressure. Although the hydrated structures of the strong polyelectrolyte brushes are considered less sensitive to salts present in the surrounding environment at low concentrations, the salts may still influence the distribution of polyelectrolyte counterions and interfacial network structures of water molecules nearby, thereby being correlated to the dissociation of the polyelectrolyte chain segments bound on solid substrates and the entire chain conformation. In this study, sum-frequency generation (SFG) vibrational spectroscopy and coarse-grained molecular dynamics (MD) simulation are combined to investigate the interfacial molecular structures of a strong polyelectrolyte, polystyrene sulfonate (PSS), grafted on silica substrates. The dynamic responses of the OH stretching signals of the interfacial water for the PSS brushes in low-concentration salt solutions bearing monovalent (Na+ and K+), divalent (Ca2+), and trivalent (Y3+) cations are monitored via a connected SFG/microfluidic chip system. First, the salts enhanced the ordering of interfacial water structures irrespective of the nature of cations, reflecting the enlarged electric potential owing to the diffusion of ions and the dissociation of the PSS segments near the solid substrate. Second, the dynamics to reach the final interfacial structural equilibrium appears to be dependent on the type of cations, showing a two-step process and thus presenting evidence for the initial ion diffusion and subsequent dissociation of the chain segments near the substrate wall. Meanwhile, it was found that divalent and trivalent cations caused electrostatic bridging among the polyelectrolyte brush segments, leading to a more disordered chain conformation. We therefore believe that this report provides a fundamental understanding of the interfacial structural and structural transition features with respect to the polyelectrolyte brushes at the molecular level.

https://pubs.acs.org/doi/10.1021/acs.macromol.1c00119