Preparation of poly(N-isopropylacrylamide) microgels using different initiators under various pH values

Ammonium persulfate (APS), 2,2'-azobis( amidinopropane) dihydrochloride (V50) and 4,4'-azobis(4-cyanovaleric acid) (ACVA) were utilized to prepare temperature-sensitive poly(N-isopropylacrylamide) ( PNIPAM) microgels by precipitation polymerization under various reaction pH conditions. Their particle sizes and swelling ratios depended on the reaction pH due to the pH dependence on the ionization degree of the decomposed fragments originating from the initiators and their hydrophilicity-hydrophobicity. The more hydrophobic initiator, under the reaction pH conditions used, could be partitioned to a greater extent into the microgel particles due to the hydrophobicity of PNIPAM chains at the reaction temperature, which led to a more cross-linked structure inside the microgels resulting in their smaller swelling ratio. pH dependence of surface charge density of the microgels with amidino groups or carboxylic acid groups on their surfaces was evidenced by the variation of their zeta potentials as a function of pH.