No abstract

Polyacrylamide hydrogels formed via hydrophobic interactions between stearyl groups in aqueous micellar solution of sodium dodecyl sulfate (SDS) present two faces depending on which state they are. The gels containing SDS micelles exhibit, in addition to the fast mode, a slow relaxation mode in dynamic light scattering (DLS) and timedependent elastic moduli, indicating the temporary nature of the hydrophobic associations having lifetimes of the order of seconds to milliseconds. The gels where SDS had been removed after their preparation behave similar to chemically cross-linked ones with time-independent elastic moduli, a high degree of spatial inhomogeneity, and a single relaxation mode in DLS. Because of this drastic structural change, the physical gels are insoluble in water with a gel fraction close to unity. In surfactant containing gels, a large proportion of physical cross-links dissociate under force, but they do so reversibly, if the force is removed they reform again. The reversible disengagements of the hydrophobic units building the physical cross-links leads to a self-healing efficiency of nearly 100%, while no such healing behavior was observed after extraction of SDS due to the loss of the reversible nature of the cross-linkages.

The free-radical polymerization of acrylamide in aqueous clay dispersions and the structure of the resulting polymer-clay nanocomposite hydrogels have been investigated by rheometry using oscillatory deformation tests. Laponite was used as clay particles in the hydrogel preparation. The reactions were carried out with and without the presence of the chemical cross-linker N,N′-methylenebis(acrylamide) (BAAm). In the absence of BAAm, increasing clay concentration from 0.2 to 7% results in 3 orders of magnitude increase of the elastic modulus G′ of the hydrogels. At a clay concentration of 5% or above, all the reaction systems, with or without BAAm, exhibit similar elastic moduli, indicating that clay mainly determines the rubber elasticity of the hydrogels. The loss factor tan δ was found to be around 0.1, indicating that the nanocomposite hydrogels are much more viscous than the conventional hydrogels. Increasing the amount of clay also increases the viscous, energy dissipating properties of the nanocomposite hydrogels, which are responsible for their improved mechanical properties. Dynamics of the nanocomposite hydrogels was also investigated by dynamic light scattering. The ensembleaveraged scattered intensity of the hydrogels varies nonmonotonically with the clay concentration due to the action of clay both as a cross-linker and as an ionic component during the formation of the nanocomposite hydrogels.

Static light scattering measurements performed on poly(N-isopropylacrylamide) hydrogels having a polymer volume fraction around 0.05−0.09 reveal that spatial inhomogeneities grow stronger with rising preparation temperature of the gels, increasing cross-linker concentration, and decreasing monomer concentration. Quantitative analysis of the angular dependence of the excess scattering intensity via the Debye−Bueche method results in correlation lengths of several 10 nm. Relative mean-square concentration fluctuations on this length scale are around 15−35% depending on preparation conditions. The cross-linking efficiency as determined by shear modulus measurements declines when the degree of inhomogeneity is enhanced. Comparison of the experimental results with Panyukov−Rabin (PR) theory indicates that the general trends are well predicted and that thermal concentration fluctuations can be calculated almost quantitatively. However, the theory significantly underestimates the static spatial concentration fluctuations. This discrepancy may be a consequence of preparing the gels by cross-linking polymerization, which leads to much stronger heterogeneity than presumed in PR theory.