Influence of the structure on the collapse of poly(N-isopropylacrylamide)-based microgels: an insight by quantitative dielectric analysis

Abstract

The collapse of poly(N-isopropylacrylamide)/poly(acrylic acid) semi-interpenetrating polymer network (PNIPAM/PAA SIPN) and poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAM-co-AA)) microgel suspensions is studied by dielectric spectroscopy in a frequency range from 40 Hz to 110 MHz as a function of temperature. Dielectric measurements show that the structure affects the relaxation behavior of microgels: two relaxations (micro-Brownian motion and interfacial polarization at low frequency and counterion polarization at high frequency) are observed in the SIPN microgel whose charges mainly exist in domains and one relaxation (interfacial polarization) is observed in the copolymer microgel whose charges distribute in the whole network. A dielectric model is proposed to describe the collapsed microgel suspensions, from which some parameters, such as the volume fraction and the permittivity of microgels, were calculated using Hanai's equation. The temperature dependencies of these parameters show that the SIPN microgel has better low-temperature swelling properties and thermal responsiveness. This is caused by different polymer-solvent and electrostatic repulsion interactions in different microgels. Compared with pure PNIPAM, the relationship of volume phase transition temperature (VPTT) is VPTT_{P(NIPAM-co-AA)} > VPTT_{PNIPAM/PAA SIPN} > VPTT_PNIPAM, and it is explained from the viewpoint of interaction. Besides, the activation energy data prove that the structure influences the electrical properties of microgels, which is consistent with the results obtained from quantitative dielectric analysis.