A “Limited Aggregation Model” to Predict the Size of Acrylamide‐Based Microgels Synthesized with Ionic Surfactants

Abstract

The size of acrylamide‐based microgels can be decreased by addition of ionic surfactants during the classical dispersion polymerization. Nevertheless, the mechanism of such syntheses is not well understood yet. Here, a “Limited Aggregation Model” is proposed by analogy with the limited coalescence mechanism occurring for Pickering emulsion stabilization. In such a model, nuclei aggregate until a constant, high enough surfactant surface coverage is reached, which ensures colloidal stability. Consequently, the total surface of the growing particles, linked to the inverse of their size, is linearly dependent on the surfactant concentration. This law is verified if the surfactant/polymer particle interaction is high enough to guarantee a “total adsorption” of the surfactants onto the particles. This simple model fits very well with all the data extracted from the literature, including very different synthesis conditions. Finally, it not only permits to predict the microgels size, but it is also an interesting tool to investigate the role of each synthesis parameter like initiator, solvent or polymer. For instance, it shows that the surfactant role is not linked to its charge, proving that a phenomenon complementary to the electrostatic repulsion, related to the surfactant tail, ensures the colloidal stability of the growing collapsed microgels.

Multiresponsive Supramolecular Poly(N-isopropylacrylamide) Microgels

“…In most studies, N , N ′-methylenebis­(acrylamide) (BIS) is used as a chemical cross-linker . Besides, surfactants can also be added to the reaction mixture to tailor the size of the microgels. − Apart from the temperature responsiveness obtained by the synthesis of NIPAM/BIS microgels, other properties can be obtained by the copolymerization of NIPAM with other comonomers. For instance, ionic comonomers like acrylic acid or N -(3-aminopropyl) methacrylamide hydrochloride have been used in order to endow ionic strength and pH sensitivity. − Another way to introduce interesting properties is to replace the conventional BIS cross-linkers with cleavable ones to synthesize redox-triggered degradable microgels. − Recently, a new type of cross-links based on supramolecular chemistry has emerged using host–guest interactions or hydrogen bonds for instance. ,− Such new supramolecular microgels are extremely interesting since they can be cleaved on demand.…”