Controlled Thermoresponsive Aggregation of Core-Cross-Linked Multiarm Star Poly( <i>N</i> -isopropylacrylamide) for a Highly Dispersed Hydrophobic Nanospace in Water

Core-cross-linked multiarm star polymers, with their densely packed arm chains and an internal microgel core, provide platforms for the construction of dispersed functional nanospaces in solutions, which contribute to a wide range of applications. In this study, we systematically investigated the thermoresponsive solubility and aggregation behavior of multiarm star poly(N-isopropylacrylamide) (PNIPAAm) in water with varying numbers of arm chains (Narms), arm molecular weights, and terminal structures. Turbidity measurements and particle size analyses showed that the solubility of star PNIPAAms was predominantly governed by Narms, and that appropriately large Narms led to high dispersibility without noticeable aggregation. Such unique behavior is most likely to result from a densely packed structure and a high degree of hydration, as demonstrated by terahertz time-domain spectroscopy. Moreover, star PNIPAAms modulated their internal polarity through temperature changes, forming a designed hydrophobic nanospace in water at elevated temperatures.