Swelling properties of copolymer hydrogels in the presence of montmorillonite and alkylammonium montmorillonite

Abstract

Copolymers of different compositions were produced by polymerization of various acrylic compounds [ N-isopropyl-acrylamide (NIPAAm), acrylamide (AAm) and acrylic acid (AAc)]. The swelling stability of the gels was enhanced by the addition of fillers [Na-montmorillonite (Na-m.) and montmorillonites hydrophobized with alkylammonium ions (C n-m.), n = 4, 12, 18]. Verified by X-ray diffraction (XRD) measurements that exfoliated and/or intercalated nanocomposites were formed. In the course of the experiments the monomer composition (0/100–100/0 mol% of the initial monomers) of the gels and, in the case of composites, the quality (Na-montmorillonite and hydrophobized montmorillonite) and quantity (1–25 wt.%) of fillers were varied. The extent of swelling was manipulated through the ratios of these parameters. In the course of swelling measurements the relationship between the composition of the gels and their swelling characteristics was established. In view of the intended use of the samples (skin extender), the difference between swelling measured in distilled water and under physiological conditions was also studied and the swelling kinetics of the gels was also analyzed. In the case of polymer and composite gels, values of desorption enthalpy (Δ H m) corresponding to the actual water contents were also determined by thermoanalytical measurements (DSC). Swelling values determined by gravimetry and enthalpies calculated from DSC measurements were found to be in good correlation. Even in the case of the relatively hydrophobic poly(NIPAAm)-based gels an enthalpy value of 98.41 kJ/mol was obtained, which is twice the value measured in pure water (41.74 kJ/mol). The extent of swelling was found to be controllable through the hydrophilicity of the initial monomers: in the case of NIPAAm-AAm and NIPAAm-AAc gels, the samples containing hydrophilic monomers swelled more extensively, whereas in the case of AAm-AAc gels the copolymer exhibited more pronounced swelling than did the gels synthesized from pure polymers. The swelling of gels was enhanced by the addition of fillers, but only at relatively low filler concentrations (1–5 wt.%). Hydrophilic Na and C 4-m. fillers increased the swelling of hydrophilic polymers and compolymers, whereas the addition of the hydrophobic C 12- and C 18-m. improved swelling of hydrophobic polymers. The effect of changes in pH was most pronounced in the case of poly(AAc) gels, which exhibited maximal swelling at pH = 9, swelling at this pH exceeds 250 g/g. The samples most sensitive to electrolyte content (physiological saline) were poly(AAm-co-AAc) gels, whereas the lowest reaction to electrolyte content was observed in the case of AAm-based gels.