Water accelerated self-healing of hydrophobic copolymers

Placing monomer units in an orderly fashion into a macromolecule may facilitate self-healing because upon mechanical damage, neighboring polymer chains return to their original conformations due to enhanced van der Waals interactions. This approach is advantageous because it eliminates chemical and physical alterations and enables multiple recovery of thermoplastic polymers upon mechanical damage, thus expanding their functionality and sustainability. Obtaining materials with a longer life span also requires consideration of external environments to which polymers are exposed, for example, water. Hydrophobic nature of the majority of polymers though suggests that the presence of hydrophilic water should not impact self-healing properties. For that reason, to achieve water-induced self-healing, multilayered polyelectrolytes and redox-switchable supramolecular were proposed or sugar moieties incorporated into polymer networks. Considering that the hydrophobic effect is critical in many diverse phenomena, from the cleaning of laundry to emulsion synthesis or the assembly of proteins into functional complexes, theoretical studies have taught us that this typically multifaced effect depends on whether hydrophobic molecules are individually isolated or reassembled into larger hydrophobic structures. For example, water molecules can readily participate in four H bonds with a single methane molecule, but in larger hydrophobic aggregates, such as polymers, hydration of water is significantly diminished. Here, we show that if a polymer is physically damaged resulting in a chain separation, water molecules may disrupt vdW interactions and participate in self-H-bonding, thus affecting self-healing. When mechanical load is removed, unfavorable polymer-water interactions within hydrophobic domains will lead to the expulsion of water from the system and rapid regeneration of polymer-polymer interactions due to enhanced interchain cohesive energies, thus leading to potentially faster self-repair.

The study situates itself within prior work indicating that copolymer composition can tune self-healing via enhanced vdW interactions and that water-induced self-healing in hydrophobic systems typically requires special chemistries. Previous approaches to enable self-healing in aqueous environments involved multilayered polyelectrolytes, redox-switchable supramolecular systems, or incorporation of sugar moieties into polymer networks. Foundational theories of hydrophobic effects suggest behavior depends on isolated versus aggregated hydrophobes; in large hydrophobic assemblies like polymers, water hydration is diminished, influencing how water might mediate chain interactions post-damage. The current work explores how confined water can modulate inter- and intrachain interactions in hydrophobic acrylic copolymers without introducing new chemical functionalities.

Materials: MMA, n-butyl acrylate (nBA), chloroform, methyl ethyl ketone (MEK), and AIBN (Sigma-Aldrich), hexane and THF (Thermo Fisher), and chloroform-d (ACROS).

Copolymerization: Poly(methyl methacrylate/n-butyl acrylate) [p(MMA/nBA)] with 40/60, 50/50, and 60/40 MMA/nBA molar ratios synthesized by solution polymerization in THF. Typical synthesis: total 0.042 mol monomer in 5 mL THF, 0.5% w/w AIBN, N₂ purge 30 min, reacted at 75 °C for 8 h. Product precipitated in hexane and redissolved in THF (300 mg/mL) before film formation. The same method used for poly(methyl methacrylate/n-pentyl acrylate) [p(MMA/nPA)].

Characterization:

• GPC: Waters GPC with RI detector; calibrated with polystyrene standards; samples in HPLC-grade chloroform, 0.2-µm filtered.
• DSC: TA Q100; 20 °C/min from −100 to 100 °C; Tg: 50/50 p(MMA/nBA) = 7 °C; 40/60 = −5 °C; 60/40 = 28 °C.
• Tensile analysis: Instron 5500 R 1125; gauge length 1.0 cm; strain rate 40 mm/min; film thickness ~0.5 mm. Self-repair protocol: 3 × 1 × 0.025 cm films damaged in air with stainless-steel razorblade (cuts ~20 µm width, ~50 µm depth), healed under ambient air or aqueous conditions at 25 °C up to 150 min; stress–strain recorded every 30 min for undamaged, air-cut/air-healed (3 h), and air-cut/water-healed (3 h) films.
• TGA: TA Hi-Res TGA 2950; films 3 × 3 mm, 200–300 µm thick; immersed in DI water for 0, 30, 60, 90, 120, 150, 1440 min; heated 25–150 °C at 20 °C/min.
• Solution ¹H NMR: 500-MHz Bruker Avance; 5.0 mg/mL in CDCl₃; 64 scans; processed with MestReNova.
• 2D NOESY NMR: Bruker NEO Avance 500 MHz; specimens prepared by cutting 0.5 × 0.5 × 0.025 cm films into 441 pieces, dissolved in CDCl₃ at 15 mg/mL without agitation for 10 min; relaxation time 1.5 s; mixing time 0.6 s; 8 scans co-added; multiple damage-repair cycle experiments for reproducibility; assignments validated against literature.
• ATR-FTIR: Agilent Cary 680 µATR-FTIR; 4 cm⁻¹ resolution; 64 scans/spectrum; damaged 50/50 p(MMA/nBA) films (0.5 × 0.5 × 0.03 cm; 20 vertical and horizontal cuts, 30–50 µm depth) exposed to water for varying times (0–150 min); monitored O–H stretching region and carbonyl region.
• DLS: Malvern Zetasizer Nano-SZ Zen3600.
• MD simulations: Materials Studio v5.5 (BIOVIA); seven identical chains per unit cell, each with degree of polymerization Xn = 60; initial density 1.125 g/cm³; Dreiding force field with TIP3P water model. Cells filled with discrete numbers of water molecules to achieve H₂O:MMA/nBA repeat unit ratios Rw of 1:4, 1:2, 3:4, 1:1, 3:2 (4:3 in figures), 7:4, and 2:1 (corresponding to 105, 210, 315, 420, 630, 735, and 840 water molecules per 420 repeat units). NVT at 298 K for 100 ps, time step 0.33 fs, Berendsen thermostat; followed by NVE equilibration as stated. Cohesive energies (total, vdW, H-bond) and cohesive energy densities, radius of gyration (Rg), and end-to-end distances (req) computed with Forcite. Compositions simulated: 50/50, 60/40, 40/60 (step of 2 units due to chain length constraints).

• Water accelerates autonomous self-healing of hydrophobic 50/50 p(MMA/nBA) by approximately threefold relative to ambient air conditions.
• Mechanical performance recovery: After 30 min, air-cut/water-healed specimens recovered ~80% of stress at break (σ_break) and maximum elongation (ε_max); air-healed recovered ~70% (σ_break) and 46% (ε_max). After 150 min, water-healed recovered ~100% of both σ_break and ε_max, while air-healed recovered 87% and 83%, respectively.
• 2D ¹H NOESY/COSY NMR: Through-space CH₃δ–CH₂β and CH₃η–CHγ cross-peak intensities initially increased post-damage (shorter through-space distances due to chain compression), then decreased as healing progressed. In water-healed specimens, intensities indicated chains were driven into closer proximity, reducing repair time. Longer water exposure diminished interchain side-group interactions, likely due to H-bonding near hydrophilic ester groups.
• ATR-FTIR: Upon exposure to water during healing, free water bands (3619 cm⁻¹) decreased with time; dimerized water (~3546 cm⁻¹) remained relatively constant; small (≈3342 cm⁻¹) and large (>4) clusters (≈3282 cm⁻¹) increased. Carbonyl C=O at ~1728 cm⁻¹ remained essentially unchanged, indicating minimal C=O…H₂O H-bonding; slight broadening near ~1710 cm⁻¹ and increased OH bending at ~1643 cm⁻¹ suggested limited interaction.
• MD simulations (50/50 p(MMA/nBA)): Total cohesive energy and H-bonding cohesive energy increased with added water; vdW cohesive energy (CE_vdw) exhibited a maximum at Rw = 1:1 (one H₂O per repeat unit) with CE_vdw ≈ 7.70 × 10³ kJ. Without water, CE_vdw ≈ 8.32 × 10³ kJ; at low water (1:2), CE_vdw decreased to ~7.41 × 10³ kJ with chains adopting more globular conformations (req ≤ 32 Å from 34 Å). At Rw = 1:1, chains returned to extended-helical conformations (req ≈ 41.9 Å; Rg ≈ 15.8 Å). At higher water contents (e.g., 2:1), interchain vdW forces weakened (CE_vdw ≈ 4.19 × 10³ kJ) and chains became more globular.
• Optimal water content: One H₂O per MMA/nBA repeat unit favored faster self-healing, correlating with denser packing similar to dry state and enhanced interchain vdW interactions.
• Side-group conformation: At Rw = 1:1, a larger fraction (~42%) of nBA side groups adopted L-shaped conformations, strengthening dipole–dipole interactions and reducing interchain distances; this coincided with the CE_vdw maximum.
• Composition dependence: The accelerated self-healing and vdW enhancement were prominent for self-healable 50/50 p(MMA/nBA); non-self-healable 60/40 and 40/60 compositions did not exhibit the same behavior (weaker NOESY cross-peaks; different CE_vdw trends). Increasing alternating dyads to ~37% in 50/50 compositions increased CE_vdw (~8.45 × 10³ kJ) compared to off-stoichiometric ratios.
• Generality: Similar water-accelerated healing observed in more hydrophobic 50/50 p(MMA/nPA).

The findings demonstrate that confined water can accelerate self-healing in hydrophobic acrylic copolymers by modulating noncovalent interactions during the damage-repair cycle. Mechanical damage perturbs interchain vdW contacts; the presence of limited water content (optimal near one H₂O per repeat unit) leads to self-association of water near ester functionalities without significant carbonyl H-bonding, and induces nBA side groups to adopt L-shaped conformations. This increases dipole–dipole interactions and enhances interchain vdW cohesive energies while decreasing interchain distances. When load is removed, hydrophobic expulsion of water and the strengthened interchain interactions favor rapid restoration of the original conformations, thereby accelerating self-repair. Spectroscopic evidence (NOESY, ATR-FTIR) and MD simulations converge on an optimal hydration level that recovers extended-helical packing and maximizes vdW cohesion; excess water disrupts these contacts and reduces vdW cohesion, while too little water yields globular conformations and lower cohesion. The effect is strongly composition-dependent, aligning with previously identified self-healable 50/50 p(MMA/nBA) that forms interdigitated key-and-lock associations; off-stoichiometric copolymers do not show the same enhancement. The mechanism suggests a route to leveraging confined water as a reversible modulator of chain conformation and packing to speed autonomous healing in hydrophobic systems.

This work reveals that introducing confined water into hydrophobic acrylic copolymers dramatically accelerates autonomous self-healing—by about threefold—without chemical modification. In 50/50 p(MMA/nBA), an optimal hydration level of approximately one water molecule per repeat unit maximizes interchain vdW cohesion, promotes L-shaped nBA side-group conformations, decreases interchain distances, and restores extended-helical packing, enabling near-complete recovery of mechanical properties within 150 minutes. The conclusions are supported by tensile recovery data, 2D NOESY NMR, ATR-FTIR analyses of water speciation, and MD simulations correlating cohesive energies and chain conformations. The concept appears generalizable to more hydrophobic analogs (p(MMA/nPA)) and may extend to other self-healing mechanisms (reversible covalent bonds, supramolecular assemblies, phase-separated systems) where controlled water confinement could modulate intermolecular forces. Future work could explore broader copolymer chemistries and architectures, control of water uptake and confinement, long-term cycling stability under varying humidity/temperature, and coupling with other dynamic bonding motifs to synergistically enhance healing speed and efficiency.

• The acceleration effect is composition-dependent, being pronounced for 50/50 p(MMA/nBA) and not observed for 60/40 or 40/60 compositions, limiting generalizability across copolymer ratios.
• Extended exposure to water diminishes interchain side-group interactions (via increased H-bonding near ester groups), indicating a narrow optimal hydration window; excessive water reduces vdW cohesion and slows healing.
• Fully alternating topologies are desirable but are constrained by MMA/nBA reactivity ratios, limiting control over sequence distribution.
• Spectroscopic and MD insights rely on model conditions and specific force fields (Dreiding, TIP3P) and simulation scales; real-world environments may introduce additional complexities (e.g., impurities, variable humidity profiles).
• Experiments were conducted at ambient temperature (25 °C) and on films of specific dimensions; different geometries, thicknesses, or temperatures may alter water transport and healing kinetics.