Enhanced toughness in highly entangled hydrogels via non-covalent molecular hooks

Abstract

Hydrogels with enhanced toughness have received increasing attention due to their potential for load-bearing applications. Recent findings have demonstrated that highly entangled networks are capable of forming such materials, yet their fabrication generally necessitates a covalent cross-linker, limiting their processability and applicability. In this work, we found that poly(ethylene glycol) methyl ether acrylate (PEGmeAc) can serve as a non-covalent cross-linker for highly entangled poly(acrylamide) networks in dilute state, imparting extreme stretchability (6600%) and toughness (26 MJ m^-3) to the hydrogels that otherwise flow within minutes. Oscillatory and rotational rheology demonstrates that the incorporation of 1 mol% of PEGmeAc is sufficient to provide structural stability to the hydrogel, increase relaxation time and improve elastic response, and severely inhibits disentanglement under shear. We hypothesized that this effect was caused by a combination of hydrogen bonding and topological entanglements, making PEGmeAc act as "hooks" within the network. The mechanism was confirmed through a combination of dissolution assays and nuclear Overhauser enhancement spectroscopy (NOESY), which showed that hydrogen bonding and topological entanglements are at play. Consequently, the incorporation of small quantities of such monomers into hydrogels may open new pathways to enhance the mechanical properties of soft materials.

Fig. 1. (a) Synthesis of highly entangled AAm-based hydrogels showing the proposed mechanism and the role of PEGmeAc through hydrogen bonding and topological entanglements. (b) Different UHMW pAAm hydrogel samples obtained through thermo-initiated FRP: pAAm, the poly(AAm-stat-PEGmeAc) copolymer gel (1 mol%), and a blend of pAAm and 1 mol% PEG dimethyl ether (PEGdme).

Fig. 2. True stress–strain curves measured through uniaxial tensile tests for pAAm hydrogel samples with 1 mol% PEGmeAc (red), 5 mol% PEGmeAc (green), 1 mol% PEGDA (black) and 0.05 mol% PEGDA (blue). The inset graph presents an enlarged window of the main graph (small box between 0 and 500% strain).

Fig. 3. Rheological comparison between pAAm and copolymer samples with 1 mol% PEGmeAc: (a) frequency sweep (ω = 100–0.1 rad s⁻¹, γ = 0.5%, n = 3), (b) shear strain sweep (γ = 0.1–100%, ω = 10 rad s⁻¹, n = 3), (c) estimated relaxation times for hydrogels with different concentrations of PEGmeAc, (d) stress relaxation modulus after applying 10% shear strain, (e) shear rate sweep ( = 0.1–100 s⁻¹) (the inset shows the image of the extruded sample), and (f) rotational shear experiments with increasing shear strain at a constant shear rate of 0.001 s⁻¹.

Fig. 4. Dissolution assays (n = 3) of highly entangled hydrogels: (a) poly(AAm-stat-PEGmeAc) and (b) pAAm, in water with pH = 1, 7, and 14 as well as NaCl_(aq) (5 M) and urea (5 M).

Fig. 5. ¹H NMR spectra of (a) poly(AAm-stat-PEGmeAc) and (c) poly(AAm) blended with 1 mol% PEGdme, recorded in a 95 : 5 mixture of 50 mM acetate buffer and D₂O. Excitation sculpting was used to suppress residual solvent signals. Gradient selected ¹H NOE experiments (mixing time d₈ = 0.1 s) for (b) poly(AAm-stat-PEGmeAc) and (d) poly(AAm) blended with 1 mol% PEGdme when selectively inverted at δ = 7.65, 6.91, and 3.61 ppm, respectively. No ¹H NOE effects were observed upon selective inversion of the blend.