Investigation on Temperature Dependent Dynamic behaviour of Poly N Isopropylacrylamide Pnipam Microgel Aqueous suspension using Broadband Dielectric Spectroscopy

Abstract

Dielectric experiments were performed on a synthesized 10 wt. newline% PNIPAM microgel aqueous suspension, covering temperatures from newline288 K to 323 K and a frequency range of 40 Hz to 50 GHz. Analysis of newlinethe data revealed two relaxation processes at frequencies around 3-5 newlineMHz and 16-18 GHz, attributed to the local chain motion of PNIPAM newline(p-process) and the average relaxation mode of water (w-process). newlineTwo kinds of water model were considered to evaluate the newlinecontribution of each type of water to the high frequency relaxation newlinespectrum: free water outside the microgel (w1) and confined water newlinewithin the microgel (w2). The relaxation time of water confined within newlinethe microgel (w2) was found to be only 2-2.7 times larger than that of newlinefree water outside the microgel (w1) above the volume phase transition newlinetemperature (VPTT). Furthermore, by utilizing the relaxation times and newlinestrengths of w1 and w2, the concentration of PNIPAM in microgel newlineparticles was determined. The p-process shifted towards lower newlinefrequencies above VPTT due to microgel structural changes. These newlinefindings allowed for the simultaneous observation of the dynamics of newlinethe polymer and water inside and outside the microgel particles during newlinethe volume phase transition. newlineAdditionally, dielectric measurements were performed on a newlinefreeze-dried PNIPAM microgel dispersion in heavy water, covering a newlinefrequency range of 100 MHz to 50 GHz and a temperature range of 288 newlineK to 323 K. The relaxation process detected at approximately 15 GHz newlinevii newlinecorresponded to the average rotational motion of heavy water, both newlinewithin and outside the microgel. The relaxation time of heavy water newlineconfined within the microgel (h2) was found to be approximately 4 to newline5 times larger than that of heavy water outside the microgel (h1) under newlineVPTT, but increased rapidly from 7 to 14 above VPTT, indicating newlinesignificant constraint on heavy water dynamics within the shrunken newlinestate of the PNIPAM microgel.