Strong and Tough Polyion-Complex Hydrogels from Oppositely Charged Polyelectrolytes: A Comparative Study with Polyampholyte Hydrogels

Abstract

Oppositely charged homopolyelectrolytes were found to form strong, tough, and self-healing polyion-complex (PIC) hydrogels, similar to polyampholytes (PA) which have opposite charges randomly distributed on the same polymer chains. The excellent mechanical performances of these two novel hydrogels are the results of dynamic ionic bonds formation between entangled polymer chains. For the PIC system, only interchain bonding occurs, while for the PA system both inter- and intrachain bonding exist. In addition, the ion pairs are expected to form stronger bonding in the PIC system than those in the PA system. In this work, we performed a comparative study of PIC hydrogels with the PA hydrogels. The PIC hydrogels are synthesized by sequential homopolymerization of cationic and anionic monomers at varied formulation, and their swelling and mechanical properties are systematically studied in comparison to the PA hydrogels that were synthesized from random copolymerization of anionic monomers and cationic monomers of the similar formulation. Different from the PA system which only forms tough hydrogels around zero net charge composition without chemical cross-linking, the PIC system forms tough physical hydrogels even at weakly offbalanced charge composition. At the charge-balanced composition, the low entanglement concentration of homocharged polyelectrolyte chains leads to tough PIC hydrogels formation at much lower concentrations than that of PA hydrogels. As a result, the PIC hydrogels are much tougher than the PA hydrogels prepared at the same monomer composition. In similar to PA hydrogels, the PIC hydrogels also exhibit broad dynamic mechanical spectra, indicating the formation of ion complexes with widely ranged bond strength. The PIC hydrogels have strong viscoelasticity in comparison with PA hydrogels. However, the two systems show the similar activation energies of the dynamic mechanical spectra. The SEM microstructural observation shows that the PIC hydrogels have segregated structure while PA hydrogels are more homogeneous

Biomimetic Supramolecular Polymer Networks Exhibiting both Toughness and Self‐Recovery

“…Notably, the Young's modulus increases by 17× from 4.6 kPa (10 mm min −1 ) to 77.5 kPa (1000 mm min −1 ). These viscoelastic characters of the CB [8] dual networks are similar to polyampholyte hydrogels reported by Gong and co-workers, [30,[32][33][34] alginate hydrogels by Suo and co-workers, [29] and Fe 3 + -acrylic acid hydrogels by Zhou and co-workers, [35] to cite only a few. This can be explained by the dynamic dissociation and reassociation of the CB [8] host-guest complexes during the low-rate stretching, which result in the relaxation of overstretched chains under the stress, further maintaining the fracture stress and strain, thus a high strain-at-break detected.…”

“…Both fracture stress and strain change slightly at low deformation rates (10-50 mm min −1 ), however, decrease at higher rates afterward. Even though the modulus is not directly comparable to those previously reported supramolecular hydrogel systems, [29,30,[32][33][34][35][36] it is surprising that a small amount of supramolecular cross-linking (2.5 mol%) and trace amount of covalent crosslinking (0.05 mol%), along with an overall mass fraction of only 10 wt%, could effectively impart the dual network with remarkable stretchability and strength. [28][29][30][31] At higher stretching rates (>100 mm min −1 ), however, the dual network looses its toughness presumably since the host-guest complexes do not have sufficient time to reform after dissociation at timescale of the mechanical deformation, leading to stress propagating at the crack tip.…”

“…Stretching-retraction cycles at different strains are shown in Figure 3a, demonstrating significant quasiplastic deformation and pronounced hysteresis, for which the supramolecular interactions play a critical role. [29][30][31][32][33][34][35] Through a quantitative measurement, a toughness of 750 ± 40 J m −2 was estimated from a Rivlin-Thomas pure shear test (see Figure S11b, Supporting Information), comparable to the toughness of cartilage (1000 J m −2 ), [29] promising potential use as structural biomaterials for regenerative tissue engineering. [29][30][31][32][33][34][35] Through a quantitative measurement, a toughness of 750 ± 40 J m −2 was estimated from a Rivlin-Thomas pure shear test (see Figure S11b, Supporting Information), comparable to the toughness of cartilage (1000 J m −2 ), [29] promising potential use as structural biomaterials for regenerative tissue engineering.…”

One‐Step Preparation of Tough and Self‐Healing Polyion Complex Hydrogels with Tunable Swelling Behaviors

“…using these ionic bond interactions, and their distinction lies in the arrangements of the opposite charges, where hydrogels with opposite charges in the same polymer chains are known as polyampholyte (PA) hydrogels and those with opposite charges in separated polymer chains are known as PIC hydrogel. Here, we focused on the relatively stable PIC gels . Benefitting from the charge interaction features, the PIC hydrogels show an impressive performance in their dynamic properties, including self‐healing, reprocessability, and saline responsiveness, and they also share the typical mechanical properties of other physically cross‐linked tough hydrogels, such as enhanced toughness, fatigue resistance, and deformation‐recovery abilities .…”

“…These hydrogels can also be prepared in high concentration (50 wt%) without a change in volume during the preparation. In comparison, the conventional method prefers preparations of relatively low concentrations, and high concentrations result in a decrease in mechanical properties . Notably, as a limitation of the PSSA preparation method, this PSSA/PMPTB hydrogel has a protonation ratio of 0.9, which means that 10% of the PSSA repeating units have sodium ions as the counter ions and that the PIC hydrogel has a 0.1 equivalent of residual NaHCO 3 salt inside the hydrogel.…”