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.

No abstract

No abstract

Temperature-dependent relaxation time and dielectric strength of the ice process in partially crystalized 10 wt% PNIPAM (green), PVP (blue), BSA (red) and gelatin (orange) water mixtures.

In this study, ZnO–Fe2O3 nanocomposites were prepared by high-energy ball milling technique and characterized through X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), UV–visible spectroscopy and dielectric spectroscopy. The amount of Fe2O3 in the ZnO–Fe2O3 nanocomposites was varied at the rates of 1[Formula: see text]wt.%, 3[Formula: see text]wt.% and 5[Formula: see text]wt.% in order to investigate its influence on the structural, optical and dielectric properties of the nanocomposites. XRD patterns of nanocomposites revealed no shift in peak positions and hence confirmed the formation of composites after ball milling. Further, it was observed from FESEM analysis that Fe2O3 particles were distributed randomly on the ZnO matrix of the nanocomposites. ZnO–Fe2O3 nanocomposites reveal extended optical absorption in the range of 400–600[Formula: see text]nm from UV studies. The dielectric constant and loss of the nanocomposites decrease exponentially with increase in frequency. The composition and frequency dependences of the dielectric constant, dielectric loss and AC conductivity are explained based on the Maxwell–Wagner effect and Koop’s theory.

Composites of polypropylene with different weight percentages of ZnO-TiO2 core-shell nanoparticles were prepared by the combination of solution and mixture melting methods. Dielectric properties of polypropylene composite films were studied at frequencies ranging from 50 Hz to 5 MHz at four different temperatures (313, 333, 353, and 373 K). It is observed that the dielectric constant reduces quickly in the low-frequency range followed by a near frequency independent behavior above 1 KHz. The dielectric properties of composites at low frequency can be explained by interfacial polarization or Maxwell-Wagner-Sillars effect. It is also observed that the dielectric constant reaches the maximum value at 3 wt% of ZnO-TiO2, which is the percolation threshold of nanocomposite. As the weight percentage of ZnO-TiO2 increases beyond the percolation threshold up to 7%, the dielectric constant of the nanocomposites decreases. The dielectric loss of the composites follows the similar trend with frequency as the dielectric constant. A sharp increase in the dielectric loss of the nanocomposite observed near the percolation threshold is due to leakage current produced by the formation of conductive network by ZnO-TiO2 core-shell nanoparticles. Further, peaks in the loss tangent observed for the nanocomposite systems indicating the appearance of a relaxation process. These relaxations peaks were shifted to higher frequencies as the particle content increased, since relaxation processes were influenced by the interfacial polarization effect which generated electric charge accumulation around the ZnO-TiO2 core-shell nanoparticles.

No abstract

Dielectric relaxation studies of acetate buffer solutions of Sodium Dodecyl Sulphate (SDS- anionic), Cetyl Trimethyl Ammonium Bromide (CTAB- cationic), Tween 80 (TW-80-non-ionic), Betaine Anhydrous (BA- zwitterionic) surfactants have been examined in the frequency region between 1GHz and 25GHz for various concentrations of surfactants at the temperatures of 283, 288, 293 and 298K using time domain dielectric spectroscopy. The obtained corrected loss spectra of all the amphiphiles except betaine anhydrous in acetate buffer solution depicted peaks near 1-2GHz and 15GHz, respectively. For betaine anhydrous, expected peak was not observed in the 1-2GHz frequency region. The peak ascertained near 15GHz, and another peak about 1-2GHz was accorded to free water relaxation and bound water reorientation of the surfactant micelles, and has acquired the reliance of temperature with concentration in detail. Single Debye and Cole-Cole function was employed to compute the relaxation times of free water and bound water, respectively. The Arrhenius plot was used to calculate the enthalpy and entropy for the micelle forming surfactants.