Investigation of hydrophobically modified polyacrylamide gels prepared in micellar solutions of a cationic surfactant

Abstract

The present study has been focused preparation of hydrophobically modified polyacrylamide gels in micellar solution of the cationic surfactant (CTAB) and their characterization. The investigation is considered as two stages. The first stage includes preparation of solutions and determination of optimum amount of each compound through which micellar solutions of water / CTAB / NaNO3 were prepared. Viscosity of solutions containing different amounts of salt (NaNO3) was measured and the results showed growth of the micelles versus salt concentration (Csalt) up to 0.5 M. In addition, the effect of different amounts of stearylmethacrylate (SM), as hydrophobic monomer, on the behavior of solutions was investigated via viscosity measurement and it was found that formation of hydrophobic blocks occurs as a consequence of solubility of SM within micelles. This phenomenon is an evidence of hydrophobic interactions that is believed to act as cross-links that form the network of physical hydrogels. Dynamic Light Scattering (DLS) technique was also employed in investigation of solutions that confirmed the enhancement of aggregation and formation of hydrophobic associations in the presence of salt and SM. The second stage of this study included formation of gels and evaluation of their mechanical and structural properties. In this stage also the effect of different compounds and temperature on gelation process and properties of gels were investigated. The effect of salt on solubilization of SM within the micelles and formation of physical cross-links was observed again. Increasing the initial monomer concentration (C0) positively affected the gelation so that the elastic modulus exhibited a strong increase with raising the concentration, while its frequency dependence was decreasing. Presence of reversible cross-links resulting from the fact that hydrophobic associations can be locally solubilized also was evidenced from relaxation modulus, G (t), of the gels as a function of time. The normalized intermediate scattering function, , as well as the distribution of relaxation rates, G (Γ), of the gels showed that the monomer concentration has only little effect on the fast relaxation modes. DLS of gels showed two characteristic correlation times corresponding to two diffusive modes with coefficients the Dfast and Dslow. Plotted Dfast and Dslow versus initial monomer concentration also indicated the smaller mesh size of the network and longer life time of the physical cross-links. All in all, gels with a high degree of toughness and self-healing property have been obtained through this investigation.