Interparticle cycloaddition reactions for morphology transition of coumarin-functionalized stimuli-responsive polymer nanoparticles prepared by surfactant-free dispersion polymerization

Stimuli-responsive polymer nanoparticles were synthesized by surfactant-free dispersion polymerization of styrene and 2-(dimethylamino)ethyl methacrylate (DMAEMA) in three different contents of divinylbenzene intraparticle crosslinking agent. Incorporation of the hydrophilic DMAEMA comonomer during the polymerization yielded pH-responsive nanogels with a higher size than the polystyrene nanoparticles. Surface of the nanoparticles was decorated with 7-hydroxyl-4-methylcoumarin (HMC) via an esterification reaction to yield stimuliresponsive fluorescent nanogels. The monodispersed polymer nanoparticles with spherical morphology were turned into chain-like coupled structures because of interparticle cycloaddition reactions between the surface HMC moieties upon UV irradiation, as shown by scanning and transmission electron microscopies. Dimerization of the HMC-functionalized nanoparticles was investigated by ultraviolet-visible spectrophotometer and fluorescence spectroscopy. Size and its distribution for the nanoparticles before and after light irradiation were determined using dynamic light scattering. From a different view, decreasing the intraparticle crosslinking density and also purging the colloidal dispersions with CO2 were resulted in a higher mean particle size. Variation of particle size by purging CO2 and consequently at different pH values was resulted in different fluorescence characteristics. Therefore, these stimuli-responsive fluorescent nanoparticles were finally used as a pH indicator in the presence of CO2 gas or even at different pH values using fluorescence spectroscopy.