Effects of Temperature and Shear History on the Network Properties of Amphiphilic Hydrogels

Abstract

We report on thermally irreversible hydrogels made from un-cross-linked hydrophobically modified (hydroxyethyl)cellulose in solution with sodium dodecyl sulfate (SDS). The effects of temperature and shear history on bulk and microstructural properties of these gels were investigated over a temperature range 25-70°C. The bulk properties investigated were composition, gel volume, and dynamic storage and loss moduli. Microstructural properties of the networks were investigated using two spectroscopic probes, pyrene and ET(30), which gave complementary information about the packing density and surface hydrophobicity, respectively, of the hydrophobic microdomains in the gels. The hydrogels exhibit an irreversible transition in both their bulk properties and the structure of the microdomains at a temperature in the range 35-50°C. Below the transition, thermal gelation is observed as the polymer chains expand with temperature and SDS is driven to associate with the side chains. Above the transition temperature the gels collapse, and the dynamic storage modulus of the gels goes down. At the same time the hydrophobic microdomains become increasingly porous. A mechanism for these changes is proposed, and the properties of these materials are contrasted with those of other temperature-sensitive hydrogels.

Physical Gelation of Hydrophobically Modified Polyelectrolytes. 1. Homogeneous Gelation of Alkylated Poly[acrylamide-co-sodium acrylate]

“…16 Water-insoluble HMHECs undergo exclusively heterogeneous gelation in anionic surfactant solutions below the cmc [20][21][22] and in ethanol/water mixtures 23,24 and exhibit a peak in dynamic storage modulus (G′) with temperature occurring at ∼50 °C, coinciding with the collapse of the already precipitated gel phase. 25 Hydrophobically modified (R) polyacrylamide (AM), or RAM, forms homogeneous non-Newtonian solutions or gels containing hydrophobic aggregates in both water 26,27 and sodium dodecyl sulfate (SDS) solutions below the cmc. 28 When the polymer is partially hydrolyzed (HRAM) to introduce charge onto the backbone, the bulk rheological properties depend on both the blockiness of the distribution of hydrophobes and the hydrolysis (i.e., charge) level, and are strongly influenced by the presence and concentration of added NaCl and SDS below the cmc.…”

“…The disparate phase behavior in the presence of surfactants (see ref vs 17 and 18) has been attributed to different degrees of water solubility as measured by different hydroxyethyl molar substitution levels on the polymer backbones . Water-insoluble HMHECs undergo exclusively heterogeneous gelation in anionic surfactant solutions below the cmc − and in ethanol/water mixtures , and exhibit a peak in dynamic storage modulus ( G ‘) with temperature occurring at ∼50 °C, coinciding with the collapse of the already precipitated gel phase …”