Surface Activity and Structure of Temperature-Responsive Polymer Surfactants Based on PNIPAm at the Air/Solution...

Thermally sensitive polymers have attracted tremendous interest in the design of stimulus-responsive surfactants. In this article, poly(propylene oxide)-b-poly(N-isopropylacrylamide) (PPO-b-PNIPAm) with different block lengths of PNIPAm was synthesized through atom transfer radical polymerization (ATRP). Different from commercial Pluronic surfactants, four distinct sections appeared in the decrease of surface tension with concentration. First, with increasing concentration, the amount of adsorbed polymers increased and the surface tension decreased sharply until a plateau was reached, which was caused by the rearrangement of methyl groups. The increasing adsorbed amount of PPO-b-PNIPAm resulted in the rearrangement of isopropyl groups, which changed from a lying down or horizontal conformation to a standing up or vertical conformation. This behavior led to the decrease in surface tension in part III until the critical micelle concentration (CMC) was reached. The surface tension of PPO-b-PNIPAm was thermally responsive. Except for the hysteresis observed in the first cycle, the surface tension was reversible during the heating-and-cooling cycles. At low concentrations, the low surface tension at higher temperatures was mainly caused by the increasing adsorption amount and ordered arrangement of methyl groups, while the standing up conformation of isopropyl groups at higher concentrations resulted in the low surface tension observed at high temperatures.