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

Abstract

Stiff, viscoelastic hydrogels have been fabricated from hydrophobically modified poly(acrylamide-cosodium acrylate) (HRAM). These materials are macroscopically homogeneous but microscopically heterogeneous, containing hydrophobic aggregates that bridge polymer chains in a three-dimensional network. The charge level at which the dynamic storage modulus is maximized for a given polymer concentration corresponds closely with the charge level giving a maximum in hydrophobicity of the aggregates, suggesting a link between the mechanical stability of these aggregates and their ability to exclude water effectively. The gel point, where the system first exhibits viscoelastic behavior, may also be identified from discontinuities in the hydrophobicity (as measured by the fluorescence spectra of solubilized pyrene) and equivalent conductivity with polymer concentration. The effects of various aspects of polymer architecture on hydrogel properties have been investigated. The dynamic storage modulus of the network exhibits a maximum with backbone charge at constant polymer concentration, with a charge level in the range 3.58-4.81 mol % Na acrylate producing the stiffest gels in C8-substituted HRAMs. An increase of two methylene units in the length of the side chains results in up to 1 order of magnitude increase in the dynamic storage modulus of the gels at constant polymer molecular weight. The anionic surfactant sodium dodecyl sulfate (SDS) exhibits a synergistic effect on gel formation by promoting the formation of intermolecular hydrophobic aggregates despite the presence of negatively charged groups on the backbone. In contrast, the effect of sodium chloride on gel properties is not as pronounced. A literature review on the effects of polymer architecture on the macroscopic phase behavior of hydrogel-forming hydrophobically modified water-soluble polymers is presented.

Facile fabrication method of hydrophobic-associating cross-linking hydrogel with outstanding mechanical performance and self-healing property in the absence of surfactants

“…As the driving force of the selfassembly of the amphiphilic multiblock copolymers, the hydrophobic-associating interaction of the hydrophobic microdomains can significantly improve the homogeneity of the hydrogel network and provide dynamic and mobile properties to the hydrogels [29]. When the hydrogels are subjected to external force, the hydrophobic microdomains can deform at a large scale or even dissociate into many small-scale microdomains to dissipate energy [30].…”

Controlled drug release from gels using surfactant aggregates: I. Effect of lipophilic interactions for a series of uncharged substances

“…These kinds of aggregates have been previously discussed for lipophilically modi®ed polyelectrolytes. 27,28 Stoichiometric calculations are, however, hard to perform because the required Carbopol physicochemical data could not be obtained from the manufacturer.…”

Network structure and mechanical properties of hydrophobic association hydrogels: Surfactant effect I

“…As a matter of fact, physical gels could be formed at concentrated regime, in which the mixed micelles of hydrophobic groups and surfactants would act as crosslinking points. 21,22 On the basis of this point of view, novel hydrophobic association hydrogels (HA-gels) were synthesized via a free-radical micellar copolymerization technique in our laboratory. [23][24][25][26][27] In the process, the octylphenol polyoxyethylene acrylates (OP$AC) as hydrophobic monomers were used to copolymerize with acrylamides (AM) in the presence of anionic surfactant sodium dodecyl sulfate (SDS).…”