ABSTRACT
Nonionic surfactant-activated microgels (SAMs), composed of hydrophobic alkyl acrylates and hydrophilic hydroxyalkyl esters that utilize the effects of surfactant mediated swelling and interaction to provide pH-independent rheological properties, were previously reported as a new pathway to the rheology modification of surfactant solutions. Crosslinking was shown to play an important role in the properties of these soft microgel systems. To understand the impact of crosslinking chemistry on SAM polymers, we have compared two types of SAM polymers: a conventionally crosslinked SAM polymer via allyl pentaerythritol and a novel SAM polymer, where the surface is self-crosslinked via a reactive surfactant. We have systematically characterized the polymer's swelling, rheology and microstructure in a model system containing the polymer, sodium dodecyl sulfate (SDS) and water. Surface self-crosslinking is demonstrated to be a more effective crosslinking approach to create surfactant-mediated interactions between the microgel particles, resulting in more effective rheology modification. Internal crosslinking hinders both the full swelling of the SAM polymer as well as inter-particle bridging interactions, and is therefore less effective. To our best knowledge, this is the first report on creating a novel surface self-crosslinked microgel via a dual-functional reactive surfactant that interacts with a non-reactive surfactant to create a yield stress fluid.
ABSTRACT
Alkali swellable microgels are widely used to control rheology of formulated products containing surfactants. However, formulations based on these pH-responsive polymers show undesirably large changes in yield stress in a range of pH close to the pKa of the acid group. Analysis of the behavior of a cross-linked copolymer of ethyl acrylate and methacrylic acid in the nonionized form (at pH below the pKa of methacrylic acid) in the presence of sodium dodecyl sulfate shows surfactant-mediated swelling (an increase in particle diameter by over 2.5×) and a peak in zero-shear viscosity versus surfactant concentration indicating surfactant-mediated interaction of the swollen microgels. On the basis of these results, we demonstrate a new class of nonionic microgels composed of hydrophobic alkyl acrylates and hydrophilic hydroxyalkyl esters that utilize the effects of surfactant-mediated swelling and interaction to provide pH-independent rheological properties.
