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 10⁻¹ Hz to 10⁶ 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 σ*(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 σ^′_DC. It can be explained by a decrease in the effective charge mobility and a reduction in the effective charge number contributing to σ^′_DC at T > LCST. Addressing the f-dependence of the real part of the conductivity σ′, 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 σ′ 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.