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

Abstract

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 τ w1 ) and confined water within the microgel (w2, with its relaxation time of τ w2 ), to the high-frequency relaxation spectrum were evaluated. The τ w2 is only 2−2.7 times larger than τ 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 τ c , which is comparable to the dielectric relaxation time, was estimated using Bloembergen−Purcell−Pound (BPP) theory. The 3τ 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.

Water Dynamics in Aqueous Poly-N-Isopropylacrylamide Below and Through the Lower Critical Solution Temperature

“…In addition, experiments and MD simulations indicate that there is also dehydration of the hydrophobic moieties, which leads to a favorable entropy gain. ,,, However, it is clear that water is critical to the underlying transition mechanism and plays a crucial role in thermo-responsive polymers and, more generally, hydrogel materials. The limited studies on the water structure and dynamics using techniques such as dielectric relaxation, , NMR, − and quasi-elastic neutron scattering , show a reduction in bound or hydrating water near the polymer through the LCST, which exhibits much slower dynamics than bulk water.…”

Separation of Micro-Brownian Motion and Side-Group Rotational Motion for Poly(N-isopropylacrylamide) in 1,4-Dioxane Studied by Dielectric Relaxation Spectroscopy

“…In addition, the previously observed asymmetric relaxation processes in the MHz region of PNiPAM in aqueous systems comprise two individual processes, although the side-group rotational motion cannot be discerned when water is the solvent. Several studies used dielectric relaxation spectroscopy to study aqueous PNiPAM solutions and PNiPAM microgel aqueous suspensions in the vicinity of LCST. − Water processes near the GHz region were analyzed and used to interpret the PNiPAM–water interactions as well as molecular structures of the system. Using the low-polarity solvent 1,4-dioxane in this study, we revealed that the PNiPAM has two individual relaxation modes in the MHz region.…”

“…There are several reports on the dielectric relaxation spectroscopy analysis of PNiPAM. In most cases, water has been considered the solvent, and the concentration or temperature dependency has been measured precisely. − These reports have examined influences of the cross-linking density of PNiPAM microgel, charge effects on the chain, Maxwell–Wagner relaxation, and DC conductivity. Although dielectric spectra reflect the relaxation of both water and PNiPAM, the weaker relaxation signal from PNiPAM is hidden by the much stronger ones from water, DC conductivity, and electrode polarization, making it difficult to further analyze the PNiPAM chain dynamics.…”

Dielectric relaxation of ice in a partially crystallized poly(N-isopropylacrylamide)microgel suspension compared to other partially crystalized polymer–water mixtures

“…Furthermore, based on the idea of two kinds of water models, the contributions of each of the two kinds of water, both free water outside the microgel and confined water within the microgel, to the high-frequency relaxation spectrum were evaluated. Despite the extensive research on PNIPAM, [48][49][50][51][52][53][54][55] low temperature investigations of PNIPAM dispersions remain mostly unexplored. In this study, we used BDS to investigate the dielectric relaxation of ice of a frozen 10 wt% PNIPAM microgel aqueous suspension and compared it to other frozen polymer-water mixtures such as BSA, gelatin, and PVP of the same concentration in order to understand the mechanism of the temperature dependent dielectric relaxation behavior of ice in such partially crystallized polymer-water mixtures.…”