Tuning Zwitterionic Polymer Interactions with Water and Ice through Side-Chain Engineering

Abstract

Ice formation on solid surfaces presents major engineering challenges. Although zwitterionic polymer coatings show promising anti-icing performance, the fundamental role of the side-chain carbon spacer in governing polymer–water and polymer–ice interactions remains insufficiently understood. Here, we used density functional theory (DFT) calculations to investigate how spacer length between zwitterionic groups affects these interactions, focusing on poly(2-methacryloyloxyethyl phosphorylcholine) (pMPC), a known anti-icing and anti-fouling polymer model system. To guide molecular design, the -CH2- spacer length was systematically varied to generate five representative polymers: pMPC-1, pMPC, pMPC+1, pMPC+2, and pMPC+3, with one, two, three, four, and five -CH2- units between zwitterionic moieties, respectively. A comprehensive evaluation of polymer–water interactions, spanning electronic structure, side-chain conformations governing water accessibility, the number and strength of bound water molecules, adsorption energetics, and charge transfer, was performed to elucidate how spacer-length–dependent molecular features control hydration. These analyses show that pMPC+1 and pMPC+2, containing three- and four-carbon spacers, exhibit the most favorable hydration behavior by achieving an optimal balance between side-chain flexibility and crumpling, which enables both strong and sufficiently abundant water binding. Interaction analysis with ice clusters further indicates that pMPC+1 provides the best anti-icing performance, displaying the greatest resistance to ice formation and thereby promoting the retention of water in the liquid state. Overall, these molecular-level insights underscore the critical role of carbon spacer architecture in dictating polymer–water/ice interactions, offer valuable guidance for designing zwitterionic polymeric materials via side-chain engineering.