Adjusting Some Properties of Poly(methacrylic acid) (Nano)Composite Hydrogels by Means of Silicon-Containing Inorganic Fillers

Abstract

The present work aims to show how the main properties of poly(methacrylic acid) (PMAA) hydrogels can be engineered by means of several silicon-based fillers (Laponite XLS/XLG, montmorillonite (Mt), pyrogenic silica (PS)) employed at 10 wt% concentration based on MAA. Various techniques (FT-IR, XRD, TGA, SEM, TEM, DLS, rheological measurements, UV-VIS) were used to comparatively study the effect of these fillers, in correlation with their characteristics, upon the structure and swelling, viscoelastic, and water decontamination properties of (nano)composite hydrogels. The experiments demonstrated that the nanocomposite hydrogel morphology was dictated by the way the filler particles dispersed in water. The equilibrium swelling degree (SDe) depended on both the pH of the environment and the filler nature. At pH 1.2, a slight crosslinking effect of the fillers was evidenced, increasing in the order Mt < Laponite < PS. At pH > pKaMAA (pH 5.4; 7.4; 9.5), the Laponite/Mt-containing hydrogels displayed a higher SDe as compared to the neat one, while at pH 7.4/9.5 the PS-filled hydrogels surprisingly displayed the highest SDe. Rheological measurements on as-prepared hydrogels showed that the filler addition improved the mechanical properties. After equilibrium swelling at pH 5.4, G’ and G” depended on the filler, the Laponite-reinforced hydrogels proving to be the strongest. The (nano)composite hydrogels synthesized displayed filler-dependent absorption properties of two cationic dyes used as model water pollutants, Laponite XLS-reinforced hydrogel demonstrating both the highest absorption rate and absorption capacity. Besides wastewater purification, the (nano)composite hydrogels described here may also find applications in the pharmaceutical field as devices for the controlled release of drugs.

Keywords: Laponite; hydrogel; montmorillonite; nanocomposites; poly(methacrylic acid); pyrogenic silica; water decontamination.

Figure 1

FT-IR spectra of (nano)composite hydrogels swelled at different pH values. (a) H; (b) HXLG; (c) HXLS; (d) HMt; (e) HHDK; (f) HFS.

Figure 2

XRD spectra of both hydrogels investigated and reinforcing agents used. (a) HXLG; (b) HXLS; (c) HMt; (d) HFS; (e) HHDK.

Figure 3

TEM images for hydrogels reinforced with various reinforcing agents.

Figure 4

TGA curves for the composite xerogels obtained.

Figure 5

Equilibrium swelling degree in four different pH media.

Figure 6

Frequency sweep rheological measurements on the (nano)composite hydrogels (a) in the as-prepared state and (b) after swelling at equilibrium.

Figure 7

Frequency sweep rheological measurements on both as-prepared and equilibrium-swelled (pH = 5.4) hydrogels: (a) storage modulus; (b) loss modulus; (c) loss factor. Values obtained at 1 Hz.

Figure 8

Time dependence of the percentage of dye absorbed by the (nano)composite hydrogels: (a) methylene blue; (b) crystal violet. Initial dye concentration = 20 mg/L; xerogel amount = 0.003 g; dye solution volume = 30 mL; temperature = 25 ± 0.5 °C.

Figure 9

Dependence of the percentage of dye absorbed on reinforcing agent. Initial dye concentration = 20 mg/L; xerogel amount = 0.003 g; dye solution volume = 30 mL; temperature = 25 ± 0.5 °C; time = 30 h.

Figure 10

Dependence of the absorption capacity on the filler of the (nano)composite hydrogels. Initial dye concentration = 20 mg/L; xerogel amount = 0.003 g; dye solution volume = 30 mL; temperature = 25 ± 0.5 °C; time = 30 h.