Response of micelles formed by smart terpolymers to stimuli studied by dynamic light scattering

A series of novel terpolymers of sodium 2-acrylamido-2-methyl-1-propanesulfonate (AMPS), N-isopropylacrylamide (NIPAM) and cinnamoyloxyethylmethacrylate (CEMA) have been synthesized in a radical polymerization process using AIBN as an initiator in DMF at 60 °C. Five terpolymers were obtained by copolymerization of the monomer mixtures containing a fixed amount of 10 mol% of AMPS while the content of CEMA ranged from 5 to 25 mol% and was changed in 5 mol% increments. The terpolymers obtained are soluble in water. Due to their amphiphilic nature the terpolymers undergo self-organization in the aqueous solution with the formation of micelles. The terpolymers are sensitive to three stimuli, i.e. temperature, UV light and ionic strength. Terpolymers with small and medium content of CEMA formed both small (Rh=8–11 nm) and large (Rh=22–51 nm) micelles, while the terpolymer with the highest content of CEMA formed only large and relatively monodisperse micelles (Rh=28 nm), as found in the DLS measurements. The size of the micelles increases in response to both temperature and irradiation with UV light at λ=280 nm. The sensitivity to temperature is due to the presence of NIPAM which imposes the occurrence of lower critical solution temperature (LCST) while the sensitivity to UV light is due to the content of cinnamoyl chromophores which undergo photodimerization resulting in the photocrosslinking of the micelles. Irradiation of the micelles in aqueous solution of relatively high ionic strength results in the loss of their temperature-sensitivity.