Dynamic Behavior of Heavy Water Inside and Outside of Poly (N-Isopropylacrylamide) (PNIPAM) Microgel Using Broadband Dielectric Spectroscopy

Abstract

One-pot free radical precipitation polymerization was used to synthesize an aqueous suspension of nonionic poly (N -isopropylacrylamide) (PNIPAM) microgel particles. For dielectric measurements in the frequency range of 100 MHz-50 GHz and the temperature range of 288-323 K, 10 wt% of freeze-dried PNIPAM microgels dispersed in heavy water were used. The relaxation process was seen at around 15 GHz, which corresponds to the entire rotational motion of the heavy water, both within and outside the microgel. Furthermore, in the high-frequency relaxation spectrum, heavy water outside the microgel (h1 -process with a relaxation time of h1, τ h1, fixed as that of pure heavy water) and confined heavy water within the microgel (h2-process with a relaxation time of h2, τ h2) were evaluated in view of the two-water model's assumption and contribution. Under volume phase transition temperature (VPTT), τ h2 is about 4 to 5× larger than τ h1, and it does not change much even though the temperature is raised. However, τ h2 rapidly increases above VPTT from 7 to 14, which reveals that the dynamics of heavy water are severely constrained inside the shrunken state of the PNIPAM microgel. The obtained results are compared to the dynamics of pure water in PNIPAM microgels aqueous suspension to better understand the effect of deuterium substitution for hydrogen on the dynamics of heavy water inside and outside of PNIPAM microgels.