Shape Stable Poly(vinyl alcohol) and Alginate Cross-Linked Hydrogel under Drying-Rewetting Cycles: Boron Substitution

Abstract

Hydrogels of poly­(vinyl alcohol) (PVA) and alginate that are cross-linked with boric acid (PVA-B) hydrogels can be stable in water when they are further cross-linked with a nucleophile complex that replaces the boron complex. The B­(OH)3 dimer cores, rather than the BO4 – cores, constitute the skeleton of the PVA-B matrix under neutral or acidic conditions. The nucleophiles SO4 2–, NO3 – and H2PO4 –, can substitute up to 52, 44, and 59% of the boron cores in the PVA-B matrix to form shape-stable PVA hydrogels with isotactic segments at d:B ratios of 3.0:1, 2.6:1, and 3.5:1, respectively. The boron substitution is proposed to be achieved by allowing nucleophiles to attack carbon atoms that are linked to the B3 complex and replace one B­(OH)3 from the dimer core via Walden inversion mechanisms at 1:1 stoichiometry. Shape-stable PVA-S immobilized copper hexacyanoferrate (CuHCF) hydrogels were synthesized to effectively adsorb cesium (Cs) ions from a water.

Investigation of Hydronium Diffusion in Poly(vinyl alcohol) Hydrogels: A Critical First Step to Describe Acid Transport for Encapsulated Bioremediation

“…PVA crosslinked in boric acid has been examined in recent years by Itou et al , Tsai et al , and Wang et al. to determine the possible PVA–borate complexes formed under acidic and basic conditions. ,, The synthesized PVA–borate complex is pH-dependent with the formation of a 3-coordinate complex under acidic conditions and a 4-coordinate complex under basic conditions. , The crosslinking structures shown in Figure are the most probable complexes formed when PVA is crosslinked with boric acid under acidic conditions. ,, Itou et al found that Figure b,c were the most probable complexes based on Raman analysis of 2,4-pentanediol reacted with boric acid.…”

“…PVA crosslinked in boric acid has been examined in recent years by Itou et al , Tsai et al , and Wang et al. to determine the possible PVA–borate complexes formed under acidic and basic conditions. ,, The synthesized PVA–borate complex is pH-dependent with the formation of a 3-coordinate complex under acidic conditions and a 4-coordinate complex under basic conditions. , The crosslinking structures shown in Figure are the most probable complexes formed when PVA is crosslinked with boric acid under acidic conditions. ,, Itou et al found that Figure b,c were the most probable complexes based on Raman analysis of 2,4-pentanediol reacted with boric acid. Tsai et al found that Figure a,c were the most probable complexes formed based on Raman and NMR analysis.…”

“…Therefore, longer crosslinking times lead to additional PVA−borate complex formation, which in turn causes additional expansion of the water domains within the hydrogel and improves the hydronium transport efficiency. PVA crosslinked in boric acid has been examined in recent years by Itou et al, 25 Tsai et al, 26 and Wang et al 31 to determine the possible PVA−borate complexes formed under acidic and basic conditions. 25,26,31 The synthesized PVA− borate complex is pH-dependent with the formation of a 3coordinate complex under acidic conditions and a 4-coordinate complex under basic conditions.…”

“…Crystalline regions are impermeable to solute diffusion and have been shown to hinder solute diffusion, while amorphous regions are more flexible but act as roadblocks to solute diffusion. Poly­(vinyl alcohol) (PVA) hydrogels are commonly used in cryogel and chemically crosslinked hydrogel studies. − Previous work by Schuszter et al investigated hydronium diffusion in non-PVA hydrogels, and Choudhury et al , Ajith et al , and Wang et al measured hydronium conductivity in PVA hydrogels but did not quantify hydronium diffusivity, a critical knowledge gap in these materials. NMR analysis of hydronium diffusion in aqueous systems is common, but this technique is infeasible in hydrogel systems because the high water content in the hydrogel would mask the hydronium signal …”

Tough and Conductive Dual Physically Cross-Linked Hydrogels for Wearable Sensors

“…Hydrogels, containing abundant water inside three dimensional cross-linked networks, have gained much attention in different areas such as actuators & sensors, − tissue engineering, − drug delivery, and water disposal. − Nevertheless, traditional hydrogels are usually cross-linked by permanent covalent bonds and display weak mechanical performances (e.g., toughness, strength, modulus, and recoverability) because of the inhomogeneity of chemically cross-linked structures and ineffective dissipation mechanisms that are subject to deformation, which largely limits their potential applications. − Recently, enormous efforts have been made for designing different hydrogels with favorable mechanical performances, such as HA gels, − nanocomposite gels, − double network gels, − macromolecular microsphere gels, , ionically cross-linked gels, − and hydrogen bond- and dipole–dipole-enhanced gels. − …”