Dynamics of the Poly(N-Isopropylacrylamide) Microgel Aqueous Suspension Investigated by Dielectric Relaxation Spectroscopy

Free-radical precipitation polymerization was used to make non-ionic poly(N-isopropylacrylamide) (PNIPAM) microgel particles. On the synthesized PNIPAM microgel particles, a dynamic light scattering experiment was performed, and hydrodynamic radii were determined to be roughly 240 and 125 nm for temperatures of 298 and 313 K, respectively. Dielectric experiments were carried out on a 10 wt % PNIPAM microgel aqueous suspension at temperatures extending from 288 to 323 K, including volume phase transition temperature (VPTT) at 305 K in the frequency range of 40 Hz to 50 GHz. At frequencies of about 3-5 MHz and 16-18 GHz, two distinct relaxation processes were detected, in addition to electrode polarization and the contribution of dc conductivity. The local chain motion of PNIPAM (p-process) and the average relaxation mode of water located at the bulk solution and also within the microgel (w-process) are assumed to be the origins of the two relaxation processes. Furthermore, based on the idea of two kinds of water models, contributions of each of the two kinds of water, both free water outside the microgel (w1, with its relaxation time of tau(w1)) and confined water within the microgel (w2, with its relaxation time of tau(w2)), to the high-frequency relaxation spectrum were evaluated. The tau(w2) is only 2-2.7 times larger than tau(w1) above VPTT. This means that rotational motion of water molecules is not significantly constrained inside the microgel particle even above VPTT. The NMR rotational correlation time tau(c), which is comparable to the dielectric relaxation time, was estimated using Bloembergen-Purcell-Pound (BPP) theory. The 3 tau(c) value for the microgel suspension obeys BPP theory only up to VPTT; above that, due to anisotropy and/or loss of translational mobility of water induced by microgel shrinkage, precondition of BPP theory is broken. Furthermore, we obtained the concentration of PNIPAM in microgel particles using both the relaxation times and relaxation strengths of w1 and w2 above and below VPTT. Below VPTT, the p-process locates at the MHz region, and it shifts toward the lower-frequency side above VPTT due to the hindrance by microgel structural changes. The dynamics of the polymer and water inside and outside microgel particles in the solution bulk are observed simultaneously by the same physical quantities through the volume phase transition.