Unraveling the Molecular Magic: AI Insights on the Formation of Extraordinarily Stretchable Hydrogels

TL;DR

This work introduces span networks as a design paradigm for ultra-stretchable hydrogels, where multiple N,N-dimethylacrylamide networks are interconnected by crosslinkers and bridged by a single linear polyethylene oxide chain. An AI-driven radical reaction predictor maps plausible APS- and radical-initiated pathways, revealing that either sulfate radical anion SO4·- or hydroxyl radical OH· can initiate monomer/crosslinker activation, with PEO integration occurring deeper in the reaction tree to connect networks and enable unprecedented stretchability, exemplified by ~ $260\times$ elongation. FTIR supports ester and carboxyl formation and other functional-group changes predicted by the mechanism, while tensile tests quantify the extraordinary extensibility and modest strength (≈ $9\ \mathrm{kPa}$), underscoring a novel structure-property relationship in span-network hydrogels. The study demonstrates how AI-driven mechanistic insights can guide the rational design of highly stretchable soft materials, with implications for soft robotics, tissue engineering, and responsive actuators.

Abstract

The deliberate manipulation of ammonium persulfate, methylenebisacrylamide, dimethyleacrylamide, and polyethylene oxide concentrations resulted in the development of a hydrogel with an exceptional stretchability, capable of extending up to 260 times its original length. This study aims to elucidate the molecular architecture underlying this unique phenomenon by exploring potential reaction mechanisms, facilitated by an artificial intelligence prediction system. Artificial intelligence predictor introduces a novel approach to interlinking two polymers, involving the formation of networks interconnected with linear chains following random chain scission. This novel configuration leads to the emergence of a distinct type of hydrogel, herein referred to as a "Span Network." Additionally, Fourier-transform infrared spectroscopy (FTIR) is used to investigate functional groups that may be implicated in the proposed mechanism, with ester formation confirmed among numerous hydroxyl end groups obtained from chain scission of PEO and carboxyl groups formed on hydrogel networks.

Figures (79)

• Figure 1: Structural Representation of Span Network Hydrogels: The hydrogel structure illustrates the interconnection of Dimethylacrylamide chains through methylene bisacrylamide crosslinkers, forming networks. These networks are further linked by polyethylene oxide (PEO). In the pre-stretch state, the PEO exhibits a coiled conformation, constituting a stable structure. Under tension, the networks align parallel to each other. This redistribution of stress ensures that pressure is uniformly distributed throughout the hydrogel, allowing for the unrestricted movement of PEO.
• Figure 2: The Initial reactants: N, N-Dimethylacrylamide as the monomer, N, N’-Methylenebisacrylamide as a crosslinker, Ammonium Persulfate as initiator, and Polyethylene Oxide as polymer.
• Figure 3: Products Attainable from Ammonium Persulfate (APS) Decomposition: a) Sulfate Radical Anion (SO4·-); b) Sulfur Trioxide; c) Hydroxyl Radical; d) Hydroxyl Ion; e) Oxygen Radical Anion; f) Superoxide Anion; g) Peroxymonosulfate; h) Sulfite Radical Anion; i) Sulfuric Acid; j) Bisulfate; k) Dianion Bisulfate; l) SO5 Anion Diradical; m) SO5 Dianion.
• Figure 4: Reaction Mechanisms and Proposed PEO Involvement: A) Gelation process initiated by sulfate radical anion. B) Gelation process initiated by hydroxyl radical (OH·). C) Two proposed mechanisms for participating in polyethylene oxide (PEO): 1. Acquisition of hydrogen from PEO end groups. 2. Chain scission involving PEO. D) First Mechanism: Radical PEO attaches to the hydrogel network. E) Interaction between water and superoxide ion. F) Second Mechanism: Radical carbonyl group and hydroxyl group lead to the emergence of a carboxyl and ester.
• Figure 5: FTIR spectrum of drawn hydrogel left for several months to be air-dried.