AMPS-based H-bonded superabsorbent hydrogels

Abstract

Water is the most abundant molecule in living tissues, and organisms. The presence of soluble ions and water provides regulation, protection, and conductivity to the nature of the systems. Different bodily parts store varying amounts of water according to their place and duty. The bones and teeth contain the lowest percentage of water in the body, whereas the brain and kidneys have the highest percentage. Materials that resemble the mechanical properties and behavior of biological tissues are now possible thanks to new polymeric networks. The distinctive qualities of hydrogels, like their high water content, softness, and flexibility that can mimic those of human tissues and organs, as well as biocompatibility have made them particularly desirable. These properties can be tailored according to the purposed area however poor mechanical properties restrict the efficient application of the hydrogels. Many hydrogel network structures, such as double network hydrogels, macromolecular microsphere composite hydrogels, physically cross-linked networks, and nanocomposite hydrogels, have been produced to address this issue. The objective of this study is to prepare hydrogels that could absorb a high amount of water without compromising their structural integrity and exhibit desirable mechanical characteristics (such as high stiffness, and toughness). Particularly strong, dynamic hydrogels can be obtained by using physical crosslinks in the hydrogel network structure which give an energy dissipation mechanism to avoid fracture and gain reversibility under stress. Today many studies are inspired by the mechanical features and high water content of biological tissues for the preparation of hydrogels. Because macromolecules naturally interact with one another through noncovalent interactions, supramolecular assemblies are frequently found in biological systems. The hydrogels based on 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) have enormous and pH-independent water absorption capacity and electro-sensitive characteristics when swollen in water. Because they are frequently produced using chemical cross-linkers, their network structures cannot effectively dissipate energy and poly(AMPS) (PAMPS) hydrogels with poor mechanical properties are obtained. A physically crosslinked dynamic PAMPS network dissipating energy under stress can be made by replacing chemical crosslinks with cooperative H-bonds to improve mechanical properties. Multiple H-bonding interactions are necessary for the creation of strong physical hydrogels that are stable in water. Therefore, careful consideration must be given while choosing monomers that will form cooperative H-bonds. Preliminary experiments were conducted with N,N-dimethylacrylamide (DMAA), methacrylic acid (MAAc), and acrylic acid (AAc) to investigate comonomer effect. Because AMPS/DMAA and AMPS/MAAc copolymer hydrogels have good mechanical properties and the carbonyl of DMAA and the carboxyl group of MAAc are known to produce multiple H-bonds connecting primary polymer chains, AMPS/MAAc/DMAA terpolymer hydrogels were prepared without using any chemical cross-linker. Effect of terpolymer composition was investigated by preparing hydrogels at various MAAc/DMAA molar ratio at a constant water content (w) of 25 wt%. The terpolymer hydrogels exhibited the best mechanical properties at a MAAc/DMAA molar ratio of 4:1 (xMAAc = 0.8). Then, by fixing the MAAc/DMAA molar ratio at 4:1, we increased the water content (w) from 25 to 95 wt% to observe the effect of water amount at gelation on the hydrogel properties. Visual inspection showed that the transparent terpolymer hydrogels formed at a water content lower than 40 wt% become translucent at 50 wt%, and finally opaque at 75 wt%. A phase-separated system formed of a viscous solution and an opaque gel was obtained by further increasing the water content w to 93 wt%. Both fracture stress f and the modulus E three orders of magnitude decrease (from 26±2 MPa to 7±1 kPa, and 10.4±0.7 MPa to 15±1 kPa, respectively) with increasing w from 25 to 75 wt%. Detailed studies revealed the formation of cooperative H-bonds that link MAAc and DMAA segments leading to a phase separation of the highly H-bonded regions. Their network structure is characterized by rheological tests, FTIR, and elemental analyzes. Furthermore, swelling tests also showed that the superabsorbent behavior of AMPS-based terpolymer hydrogels was improved by the addition of comonomers to the structure. In short, better mechanical strength and high water absorption were obtained by the addition of comonomers MAAc and DMAA to the AMPS hydrogel network structure.