Regulating polystyrene glass transition temperature by varying the hydration levels of aromatic ring/Li⁺ interaction

Polymer properties can be altered via lithium ion doping, whereby adsorbed Li⁺ binds with H₂O within the polymer chain. However, direct spectroscopic evidence of the tightness of Li⁺/H₂O binding in the solid state is limited, and the impact of Li⁺ on polymer sidechain packing is rarely reported. Here, we investigate a polystyrene/H₂O/LiCl system using solid-state NMR, from which we determined a dipolar coupling of 11.4 kHz between adsorbed Li⁺ and H₂O protons. This coupling corroborates a model whereby Li⁺ interacts with the oxygen atom in H₂O via charge affinity, which we believe is the main driving force of Li⁺ binding. We demonstrated the impact of hydrated Li⁺ on sidechain packing and dynamics in polystyrene using proton-detected solid-state NMR. Experimental data and density functional theory (DFT) simulations revealed that the addition of Li⁺ and the increase in the hydration levels of Li⁺, coupled with aromatic ring binding, change the energy barrier of sidechain packing and dynamics and, consequently, changes the glass transition temperature of polystyrene.