Reaction Mechanisms of N-isopropylacrylamide Soap-Free Emulsion Polymerization Based on Two Different Initiators

Abstract

Poly(N-isopropylacrylamide) (PNIPAM) microgels were prepared through soap-free emulsion polymerization using 2,2′-azobisobutyronitrile (AIBN) or potassium persulfate (KPS) as initiator. UV-Vis spectrophometer was used to monitor the polymerization kinetics at very low monomer contents (≤0.2 wt% based on water). The absorbance at 220 nm was investigated, being proportional to the NIPAM concentration, but not PNIPAM, and thus was chosen to characterize the residual monomer concentration. The results showed the effect of the initiator was particularly noticeable in the initial polymerization period. The KPS-initiated polymerization started with a shorter induction time, while the AIBN initiated one started more slowly but had a higher rate in the later stage of polymerization. The change of particle mean Dh (hydrodynamic diameter) and Dh distribution and molecular weight during the polymerization processes were examined by dynamic light scattering (DLS) and gel permeation chromatography (GPC), respectively, to investigate the reaction mechanisms of the NIPAM soap-free emulsion polymerization based on the two different initiators. The reaction mechanisms based on AIBN and KPS are discussed in detail in the article. The results indicated that the mechanisms of forming PNIPAM microgels based on AIBN and KPS are different. When AIBN was used as initiator, the reaction mainly took place in the monomer droplets which were stabilized by the formation of PNIPAM, and with high reaction rate. But when KPS was used as initiator, the nucleation and the PNIPAM microgels forming field both were in the aqueous phase.