Dually cross-linked single network poly(acrylic acid) hydrogels with superior mechanical properties and water absorbency

Abstract

Poly(acrylic acid) (PAA) hydrogels with superior mechanical properties, based on a single network structure with dual cross-linking, are prepared by one-pot free radical polymerization. The network structure of the PAA hydrogels is composed of dual cross-linking: a dynamic and reversible ionic cross-linking among the PAA chains enabled by Fe(3+) ions, and a sparse covalent cross-linking enabled by a covalent cross-linker (Bis). Under deformation, the covalently cross-linked PAA chains remain intact to maintain their original configuration, while the Fe(3+)-enabled ionic cross-linking among the PAA chains is broken to dissipate energy and then recombined. It is found that the mechanical properties of the PAA hydrogels are significantly influenced by the contents of covalent cross-linkers, Fe(3+) ions and water, which can be adjusted within a substantial range and thus broaden the applications of the hydrogels. Meanwhile, the PAA hydrogels have excellent recoverability based on the dynamic and reversible ionic cross-linking enabled by Fe(3+) ions. Moreover, the swelling capacity of the PAA hydrogels is as high as 1800 times in deionized water due to the synergistic effects of ionic and covalent cross-linkings. The combination of balanced mechanical properties, efficient recoverability, high swelling capacity and facile preparation provides a new method to obtain high-performance hydrogels.

Recent Advances in Metal‐Containing Polymer Hydrogels

“…An example of a metal-containing hydrogel formed by ionic and covalent interactions was given by Xie and co-workers. [28] An iron-containing hydrogel was prepared by one-pot free radical polymerization of acrylic acid (AA) with N, N′-methylenebisacrylamide as the covalent crosslinker and Fe(NO 3 ) 3 as the ionic crosslinker. Many reversible crosslinks are formed as a result of coordination interactions between Fe 3+ ions and carboxyl groups of PAA within the sparse, covalently crosslinked network (Figure 9a).…”

Mechanical and Rheological Behavior of Hybrid Cross‐Linked Polyacrylamide/Cationic Micelle Hydrogels

“…The fabricated hydrogel, even in the swelling equilibrium state, keeps extraordinary mechanical and recoverable properties . Xie and co‐workers synthesized poly(acrylic acid) hybrid hydrogels by introducing ionic cross‐linking interaction between Fe(III) and carboxyl groups into the chemical cross‐linking network, and the resulting hydrogels achieve tensile stress of ≈1.07 MPa and toughness of ≈11.7 MJ m −3 . In these network structures, the covalent cross‐linking preserved the initial state of the network and the recoverable physical cross‐linking could well disperse the stress concentration during deformation process, resulting in excellent mechanical property of these hydrogels.…”

In situ polymerized hyperbranched polymer reinforced poly(acrylic acid) hydrogels

“…[7][8][9] Among various types of hydrogels, poly acrylate polymers such as poly acrylic acid (PAA), with distinctive absorptivity properties, have the most versatile structure to enhance their practical utility in everyday life applications such as filtration, water remediation, diapers and hygiene products, cosmetics, wound dressings, medical waste solidification and metal ion removal. [10][11][12][13][14] From extensive studies evaluating different properties of PAA hydrogels, such as swelling, 15 adhesion, 16 diffusion, 17 physico-chemical 18 and mechanical 19 properties, it has been revealed that the greatest concern about these types of hydrogels is their poor mechanical properties. 11,[20][21][22] The poor mechanical properties of PAA hydrogels, which relate closely to their large volume change subsequent to swelling, count as a marked weakness in consideration for adoption for high-tech applications, and have limited their specific applications.…”

“…[10][11][12][13][14] From extensive studies evaluating different properties of PAA hydrogels, such as swelling, 15 adhesion, 16 diffusion, 17 physico-chemical 18 and mechanical 19 properties, it has been revealed that the greatest concern about these types of hydrogels is their poor mechanical properties. 11,[20][21][22] The poor mechanical properties of PAA hydrogels, which relate closely to their large volume change subsequent to swelling, count as a marked weakness in consideration for adoption for high-tech applications, and have limited their specific applications. 23,24 Typically, PAA hydrogels exhibit a low Young's modulus (o200 kPa) and low fracture energy (o20 J m…”