Preparation, characterization and mechanism study of small size core-shell polymer nanoparticles dissociated from poly(N-isopropylacrylamide) ionic microgels

Abstract

Preparation of small size core-shell polymer nanoparticles has significant applications in photonic crystal materials and drug delivery systems. Although core-shell nanoparticles could be easily prepared by seeded emulsion polymerization, their size was generally larger than that of seeds, which limited their further applications in some areas. In this work, a new method was utilized to prepare small size core-shell polymer nanoparticles. In detail, poly(N-isopropylacrylamide) ionic microgels (encoded as PNI microgels) with controllable diameter were firstly prepared and then used as seeds in seeded emulsion polymerization of styrene. After polymerization, core-shell nanoparticles which were smaller in size than that of seeds were obtained. In order to reveal this abnormal phenomenon, the inner structure, components, element distribution, thermo-sensitivity and deformability of the obtained nanoparticles were characterized by TEM, FTIR, DSC, EDS, DLS and AFM. It was found that the core composed of polystyrene and the shell composed of PNI microgels in the core-shell nanoparticles. The detailed formation process was then studied systematically. It was found that polystyrene nanoparticles formed within PNI microgels and peeled off from the seeds when they were large enough. Considering the weak ionic interaction of PNI microgels, mechanism for this phenomenon was proposed. Factors, such as initiator type, microgel type, cross-linking degree and stirring speed, also had significant effects on the formation of core-shell nanoparticles.