A fluorescence probe study on effects of surfactants on cloud points in aqueous poly(N-isopropylacrylamide) solutions.

Fluorescence probe methods were applied to investigate micelle formation of poly(N-isopropylacrylamide) (PNIPA) with two kinds of surfactants, anionic sodium n-dodecyl sulfate (SDS) or cationic n-dodecyltrimethylammonium chloride (DTAC), in aqueous solutions using pyrene or 1-pyrenecarboxaldehyde as fluorescence probes. Two PNIPA samples, one having a hydrophobic chain-end group and the other having a negatively-charged hydrophilic chain-end group, were used to investigate effects of the chain-end group on formation of the micelles. It was found that the critical aggregation concentration, at which the surfactant molecules start to bind to the PNIPA chains to form the micelles, is much lower for the PNIPA solutions containing SDS than for the solutions containing DTAC. This is consistent with the previous result that the cloud point in the PNIPA solutions containing SDS starts to increase from its value in the surfactant-free solutions at much lower concentration of added surfactant than that in the solutions containing DTAC. It was also found that there is a discrepancy in emission spectra for the solutions containing SDS between the two PNIPA samples but not for the solutions containing DTAC, indicating that the chain-end group of PNIPA may affect the microenvironmental polarity in the micelles composed of PNIPA and the surfactants

A Versatile Synthesis Platform To Prepare Uniform, Highly Functional Microgels via Click-Type Functionalization of Latex Particles

Advances in the understanding of microgel properties and exploitation of their full potential for applications require control of the extent and type of functionalization, while at the same time providing control of particle shape, that is, size and uniformity, and intrinsic particle properties such as hardness and degree of crosslinking. However, versatile and simple synthetic approaches to prepare highly uniform and densely functionalized microgels based on functional groups unsuitable for aqueous precipitation polymerization are still scarce. As an alternative platform approach, herein we report on the synthesis of uniform particles based on classical batch emulsion polymerization, which are later postfunctionalized via Cu-mediated Huisgen-type alkyne/azide click chemistry. We use propargyl acrylate (PGA) as a monomer and ethylene glycol dimethacrylate (EGDMA) as a crosslinker for the synthesis of narrowly dispersed latex particles in the size regime of 50–200 nm in radius. We demonstrate how particle hardness and swelling can be tuned as a function of the used ratio of monomer/crosslinker. Postmodifications in the interior of the particles are conducted in the swollen state in DMSO, and we add pH-responsive cationic moieties as a first attractive model functionality. Combined Raman spectroscopy and elemental analysis reveal the kinetics and degree of modification. Both depend on the degree of crosslinking, and we find densely functionalized particles exhibiting a conversion of the alkyne functionalities of up to 90%. After modification, the resulting microgels display a pH-dependent ionization and swelling behavior in water. The suggested route opens up new and versatile ways to prepare narrowly dispersed water-dispersible microgels with tailored hardness and high density of functional groups, based on readily available building blocks