Multi-Stimuli Responsive Viologen-Imprinted Polyvinyl Alcohol and Tricarboxy Cellulose Nanocomposite Hydrogels

Abstract

Photochromic inks have shown disadvantages, such as poor durability and high cost. Self-healable hydrogels have shown photostability and durability. Herein, a viologen-based covalent polymer was printed onto a paper surface toward the development of a multi-stimuli responsive chromogenic sheet with thermochromic, photochromic, and vapochromic properties. Viologen polymer was created by polymerizing a dialdehyde-based viologen with a hydroxyl-bearing dihydrazide in an acidic aqueous medium. The viologen polymer was well immobilized as a colorimetric agent into a polyvinyl alcohol (PVA)/tricarboxy cellulose (TCC)-based self-healable hydrogel. The viologen/hydrogel nanocomposite films were applied onto a paper surface. The coloration measurements showed that when exposed to ultraviolet light, the orange layer printed on the paper surface switched to green. The photochromic film was used to develop anti-counterfeiting prints using the organic hydrogel composed of a PVA/TCC composite and a viologen polymer. Reversible photochromism with strong photostability was observed when the printed papers were exposed to UV irradiation. A detection limit was monitored in the range of 0.5-300 ppm for NH_3(aq). The exposure to heat (70 °C) was found to reversibly initiate a colorimetric change.

Keywords: cellulose; hydrogels; polyvinyl alcohol; trichromism; viologen polymer.

Figure 1

Preparation of tricarboxy cellulose.

Scheme 1

Synthesis process of viologen polymer.

Figure 2

TEM analysis of viologen polymer particles at different magnifications and different positions in the tested sample (a–f).

Figure 3

XRD spectrum of polymer nanoparticles.

Figure 4

SEM analysis of cast hydrogel film; VP₆ (a–c).

Figure 5

SEM images of printed paper (VP₆) at different positions on the sample surface (a–c).

Figure 6

Absorption spectra of VP₆ below visible (Vis) and ultraviolet (UV) irradiation.

Figure 7

Reversibility of absorbance of VP₆ over numerous cycles of ultraviolet (598 nm) and visible (430 nm) illumination.

Figure 8

Effect of viologen content on mechanical performance of stamped samples, including tensile strength, strain, and Young’s modulus.

Figure 9

Effect of shearing rate on the viscosity of VP₆.

Figure 10

Absorption spectra of VP₆ at different temperatures.

Figure 11

Thermochromism of VP₆ showing a change in color from orange (a) to greenish (b) with heating from 25 °C to 70 °C, respectively.

Figure 12

Absorbance spectra of paper (VP₆) under air and different concentrations of NH_3(g).

Figure 13

Absorption intensity of VP₆ versus various concentrations of NH_3(aq).