PH- and thermo-responsive behavior of PNIPAM star containing terminal carboxy groups in aqueous solutions

Abstract

In the present work, a core-crosslinked poly(N-isopropylacrylamide) (PNIPAM) star polymer with 17 polymer arms was prepared via reversible addition–fragmentation chain transfer (RAFT) radical polymerization. Here, the outer chain end of the PNIPAM arm contained a carboxy group. The pH- and thermo-responsive aggregation behavior of this polymer was studied in aqueous solutions using UV–Vis spectrophotometry, dynamic light scattering, static light scattering (SLS), and transmission electron microscopy (TEM) techniques. By comparing the light scattering intensities of the aggregate and unimer states, the aggregation number was estimated at various pH values below and above the phase transition temperature (Tp); these findings were supported by SLS and TEM measurements. Below the Tp, two star polymer molecules aggregated due to the presence of hydrophobic interactions at pH 4. The star polymer molecule existed as a unimer at pH 5. The arm chains in the unimer state star polymer expanded at pH 10. Above the Tp, the PNIPAM stars were densely aggregated with about 57 polymers at pH 4, whereas six polymers were loosely aggregated at pH 5, and the unimer state polymer shrank at pH 10. This study is useful for understanding the aggregation behavior of stimuli-responsive star polymers.