Controlled Thermoresponsive Aggregation of Core-Cross-Linked Multiarm Star Poly( N -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 ( N arms ), arm molecular weights, and terminal structures. Turbidity measurements and particle size analyses showed that the solubility of star PNIPAAms was predominantly governed by N arms, and that appropriately large N arms 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.