Comparison of Structural, Water-Retaining and Sorption Properties of Acrylamide-Based Hydrogels Cross-Linked by Physical and Chemical Methods.

Two series of hydrogels based on acrylamide and its copolymers with acrylonitrile and acrylic acid were synthesized by two cross-linking methods - chemical (using N,N'-methylene bis-acrylamide) and physical (using montmorillonite (MMT)) ones. The structure of the gels was characterized by Fourier Transform Infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The swelling and sorption properties were analyzed as a function of both the monomer composition and the cross-linking method. The shift of the band corresponding to Si-O (995-1030 cm-1 ) confirmed the formation of intercalation structures for MMT-cross-linked gels. Moreover, physically cross-linked gels demonstrated a non-monotonic dependence of the swelling degree on the MMT concentration, and acrylamide-acrylic acid copolymer MMT-cross-linked gels showed pH sensitivity and the highest swelling degree of 150 g/g. The highest sorption capacity towards cadmium(II) ions was demonstrated by acrylamide-acrylic acid copolymer gels, both covalently cross-linked (30 mg/g) and MMT-cross-linked (8.9 mg/g).

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 polyNIPAAm 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 Ca = 0.525-14.58% for 10 h. For slightly activated samples (Ca ≤ 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 polyNIPAAm hydrogels cross-linked by aLap are also discussed. It was demonstrated that an increase in Ca 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.

Thermoresponsive hydrogels physically crosslinked with magnetically modified LAPONITE® nanoparticles.

Recently, considering the potential applications of hydrogel nanocomposites in biomedical engineering, there has been a growing interest in the synthesis of hydrogels with improved mechanical properties. Among magnetic materials, iron oxides are of particular interest due to their magnetic properties and biocompatibility. At the same time, LAPONITE®, a synthetic clay, can be used to improve the mechanical properties of polymer-based nanocomposites. In this study we report the effects of hydrogel composition and structure on its thermoresponsive properties and hydrogel sorption and release of a model anticancer drug - 5-fluorouracil. Using one-step coprecipitation method we synthesized magnetic LAPONITE® (LAM) nanoparticles with magnetite-to-LAPONITE® weight ratios from 2 : 1 to 1 : 8. With increase in magnetite concentration the ferrofluidic properties of LAM nanoparticles are getting improved, while fluorouracil absorptivity - decreases. Exfoliation of the clay is observed when the magnetite content exceeds the LAPONITE® content. Physical crosslinking of poly(N-isopropylacrylamide) with LAM nanoparticles yields magnetic thermosensitive hydrogel nanocomposites with controllable temperature-induced drug release. All hydrogel nanocomposites have a distinct volume phase transition from a swollen state to a collapsed state upon heating within the physiologically acceptable temperature range of 33-36 °C.