Cross-Linked Hydrogels Based on PolyNIPAAm and Acid-Activated Laponite RD: Swelling and Tunable Thermosensitivity

The effects of acid activation of Laponite RD (Lap) on the structure and properties of activated Lap nanoparticles (aLap) and the properties of polyNIPAA_m hydrogels physically cross-linked by aLap have been studied. The acid activation of Lap by the sulfuric acid was done using the concentration of sulfuric acid within the interval C_a = 0.525-14.58% for 10 h. For slightly activated samples (C_a ≤ 1.25 wt %), the significant increase of the specific surface area (by ≈1.56 times) was accompanied with a significant decrease in both the values of the specific heat of immersion in water and n-decane. However, the hydrophilic properties of all samples S₀-S₅ were still observed. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) data, Fourier transform infrared (FTIR) spectra, and X-ray diffraction (XRD) patterns demonstrated that the acid activation resulted in the destruction of the crystal lattice of Lap, leaching of magnesium and lithium, and formation of the amorphous phases. Moreover, the acid activation significantly affected aggregation and negative surface charges of the aLap faces in aqueous suspension. The effects of aLap on the swelling properties and cooperativity in the phase transitions of polyNIPAA_m hydrogels cross-linked by aLap are also discussed. It was demonstrated that an increase in C_a resulted in a significant increase in the equilibrium degree of swelling of the hydrogels and a decrease in the hydrogel phase-transition temperature from the swollen phase to the shrunken phase.