Hydrogen Bonding Enables Polymer Hydrogels with pH‐Induced Reversible Dynamic Responsive Behaviors

Driven by pH-induced alteration of hydrogen bonds, when placed in an acid solution, the transparent PAN hydrogel quickly becomes opaque and then gradually regains its transparency; while after being switched into DI water, the same dynamic response of fast transparency drop followed by spontaneous slow recovery is recreated.

Abstract

Poly(acrylic acid-co-N-vinylcaprolactam) (PAN) hydrogels containing multiple hydrogen bonds can exhibit pH-induced reversible dynamic responsive behaviors. When placing a transparent hydrogel in an acid bath, as hydrogen bonds between comonomer units involving protonated COOH groups are formed faster than water diffusion, a nonequilibrium light-scattering state is formed to turn the hydrogel opaque, while as the swelling equilibrium is reached over time, the hydrogel regains its transparency. Likewise, when the transparent, hydrogen-bonded hydrogel is subsequently immersed in DI water, faster water absorption occurs in where more COOH groups are deprotonated, which also generates a light-scattering state leading to opacity, while the transparency is slowly recovered after equilibrium. Using such two-way dynamic transparency evolution, a PAN-based hydrogel material is prepared to demonstrate a dynamic memory system for information memorizing-forgetting and recalling-forgetting.

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