Poly(ethylene glycol) Becomes a Supra-Polyelectrolyte by Capturing Hydronium Ions in Water

Abstract

Poly(ethylene glycol) (PEG) has been widely used for decades as a "gold standard" in bioconjugation, nanomedicine, and antifouling. Although being extensively studied since 1859, PEG remains mysterious, as can be exemplified by the facts that PEG is the only polyether showing excellent water solubility, and the molecular structure of PEG is surprisingly simple if the fantastic properties are considered. Since PEG is usually used in an aqueous medium, the interactions between PEG and water should be the key to understanding the mechanism. Here, we find that by capturing hydronium ions (H 3 O + ) in water, PEG changes from a neutral polymer to a supra-polyelectrolyte, which is a new category of polymer that becomes a polyelectrolyte when an external ion is dynamically bonded to the polymer via intermolecular interactions. This conclusion is supported by multiple experimental methods from the ensemble to single-molecule level. This finding casts new light on the relationship between the simple structure and fantastic functions of PEG. With known species of polymers and ions, numerous novel supra-polyelectrolytes can be prepared, which may present exciting properties in water.

Structure of Self-Initiated Photopolymerized Films: A Comparison of Models

“…A detailed discussion and calculation may be found in Zhulina et al In the present case the monomers do not contain charged groups, and infrared spectroscopy on identically prepared samples show no indication of ionizable residues in the films . However, recent work suggests that hydronium ions bound to PEG chains can form a so-called supra-polyelectrolyte, where PEG behaves like a pH-dependent polyelectrolyte rather than a conventional neutral polymer in an aqueous solution . This could potentially explain a polyelectrolyte-like behavior, but further data are needed to support such a hypothesis; leaving thisat this stage somewhat speculativehypothesis aside, the behavior could be attributed to the entropic contribution of water molecules participating in hydrogen bonding with the side chains.…”

“… 29 However, recent work suggests that hydronium ions bound to PEG chains can form a so-called supra-polyelectrolyte, where PEG behaves like a pH-dependent polyelectrolyte rather than a conventional neutral polymer in an aqueous solution. 62 This could potentially explain a polyelectrolyte-like behavior, but further data are needed to support such a hypothesis; leaving this—at this stage somewhat speculative—hypothesis aside, the behavior could be attributed to the entropic contribution of water molecules participating in hydrogen bonding with the side chains. Poly(ethylene glycol) is known for interacting strongly with water, 63 , 64 resulting in strong hydration, which supports this hypothesis, but neutron reflectometry data from PEG brushes are usually modeled with parabolic profiles rather than Gaussian profiles.…”

Single-Chain Inherent Elasticity of Macromolecules: From Concept to Applications

“…Poly­(ethylene glycol) (PEG), a linear polymer having a C–C–O skeleton, is a “superstar” in the biological related fields. It is reported that the elastic behavior of PEG depends largely on the local environment. − To quantify the impact of solvent on the single-chain mechanics of PEG, its inherent elasticity has been investigated by single-molecule AFM . It is found that the inherent elasticity of PEG can be achieved in nonpolar small-sized solvents (e.g., nonane).…”

Neutron Reflectometry Studies of the Hydrated Structure of Polymer Thin Films

“…Here it is attributed to the interaction between the PEG10MA sidechains in a bottlebrush-like manner 149 . However, recent results indicate that PEG may bind with hydronium ions, forming a so-called supra-polyelectrolyte 150 . If these results are confirmed, that may explain the observed stretched profiles, due to electrostatic repulsion of the PEG-containing sidechains.…”