Anomalous Volume Phase Transition Temperature of Thermosensitive Semi-Interpenetrating Polymer Network Microgel Suspension by Dielectric Spectroscopy

A new experimental result from dielectric spectroscopy of poly(N-isopropylacrylamide)/poly(acrylic acid) semi-interpenetrating polymer network (PNIPAM/PAA SIPN) microgel, which undergoes significant volume phase transition, is reported. Two significant dielectric relaxations were observed around 0.1-0.5 MHz and 1-5 MHz, respectively. The high-frequency relaxation is attributed to the migration of counterions tangentially and radially along the domain formed by linear PAA chains (counterion polarization). The temperature dependence of the domain size obtained from this relaxation shows that the SIPN microgel with higher content of PAA has better thermal response and swelling property. The low-frequency relaxation shows two separate mechanisms below and above the volume phase transition temperature (VPTT), which are dominated by different relaxation processes, respectively: micro-Brownian movement of solvated side groups of PNIPAM dominates when T < VPTT, while the interfacial polarization does when T > VPTT. A dielectric model was proposed to describe the collapsed microspheres suspension, from which the electrical parameters of microgel were calculated. The permittivity of microgel shows that a special ordered arrangement of water molecules is formed in microgel with less PAA. Thermodynamic parameters obtained from Eyring equation reveal that the difference in PAA content has a great influence on the thermodynamics of the phase transition process. Besides, it was found that the VPTT of the SIPN microgel was significantly increased compared with pure PNIPAM hydrogel microspheres. The essence of anomalous VPTT revealed by relaxation mechanism is the difference in composition content leading to different hydrophilic/hydrophobic and electrostatic interaction. Determining the reason for anomalous VPTT is of instructive significance to understand the volume phase transition of complex polymer materials.