The dielectric signature of poly(N-isopropylacrylamide) microgels at the volume phase transition: dependence on the crosslinking density

Abstract

Temperature sensitive poly(N-isopropylacrylamide) (pNIPAM) microgels are prepared and investigated using dielectric spectroscopy in a frequency range from 10 À1 Hz to 10 6 Hz at temperatures from 15 C to 50 C. The microgels were synthesized with different crosslinker molar ratios resulting in microgels with structural differences. From the dielectric response of the pNIPAM microgels the swelling/ deswelling behaviour is monitored by both the temperature (T) and the frequency ( f ) dependence of the conductivity spectra s*(f, T). The volume phase transition (VPT) at the lower critical solution temperature (LCST) is deduced by a change in the T-dependence of the DC conductivity s 0 DC . It can be explained by a decrease in the effective charge mobility and a reduction in the effective charge number contributing to s 0 DC at T > LCST. Addressing the f-dependence of the real part of the conductivity s 0 , a pronounced frequency dependence at temperatures above the LCST can be observed whereas at temperatures below the LCST the conductivity spectra resemble that of the pure solvent (water) which is frequency independent. The f-dependence of s 0 at T > LCST is assigned to the collapse of the microgel particles. At the interfaces of the collapsed particles charge carriers are blocked and/or entrapped giving rise to Maxwell-Wagner-Sillars (MWS) polarization effects. The dependence of the MWS effect on the crosslinker amount is studied in detail and conclusions concerning the internal structure of the microgels with respect to their crosslinking density are drawn. Moreover the dielectric data are related to dynamic light scattering data. A correlation between the MWS polarization effect and the swelling/ deswelling ratio expressed by the hydrodynamic radius R h at different temperatures is established for the first time.

Maxwell-Wagner-Sillars Dynamics and Enhanced Radio-Frequency Elastomechanical Susceptibility in PNIPAm Hydrogel-KF-doped Barium Titanate Nanoparticle Composites

“…As the onset of EP is indicated by an ϵ ′ ( f ) saturation plateau, it is not a significant contributor to the dielectric behavior and MWS is the dominant contributor to observed polarization effects in this work. Figure 1 a and c show a strong decrease in conductivity at lower frequencies due to MWS as observed in other works [27]. PNIPAm is a heterogeneous system due to free and bound water molecules.…”

“…Mesoscopic properties of ionic substances can be effectively probed using dielectric spectroscopy. Examination of the complex permittivity ϵ ∗ = ϵ ′ − iϵ ", complex conductivity σ ∗ = σ ′ + iσ ", and other derived factors can reveal mechanisms related to charge transport [26] and molecular structure [27] among a host of other properties [28–31]. The modification of the synthesis process for the composites in this work was aimed at maintaining the volumetric phase change in PNIPAm-based hydrogels while enhancing dielectric responsiveness for actuating a volumetric phase change using RF.…”

A comparison of the network structure and inner dynamics of homogeneously and heterogeneously crosslinked PNIPAM microgels with high crosslinker content

“…19,20 Although supporting the conclusion of a homogeneous inner structure, these methods provide no direct access to the inner structural properties. Theoretical considerations and experiments using dielectric spectroscopy 21,22 predict an influence of the microstructure of the microgels on their physico-chemical properties, e.g., swelling kinetics 23 Besides this, the interaction potential between them, their ability to interpenetrate and their behavior at very dense packing conditions are influenced, e.g., by the microgel softness and the internal crosslinker density distribution. 25,26 Hence, a thorough and direct comparison of the internal structure and dynamics of both types of microgels in the often investigated sub micrometer size range is of importance but still missing.…”

Inner structure and dynamics of microgels with low and medium crosslinker content prepared via surfactant-free precipitation polymerization and continuous monomer feeding approach

“…38 Moreover, a decrease of the collective diffusion coefficient of the microgels with the crosslinker concentration increase was obtained. 10 Despite the number of works dedicated to the structural investigation of PNIPAM/BIS microgels and to the prediction of the influence of the microgel structure on their physico-chemical properties, [39][40][41] the direct comparison of microgels with different internal crosslinker distributions is very limited. 21 Recent investigations of the inner dynamics and network structure of PNIPAM microgels with different internal inhomogeneities with high cross-linker content (10 mol% BIS) were reported.…”