Influence of Charged Groups on the Structure of Microgel and Volume Phase Transition by Dielectric Analysis

The thermally sensitive charged poly­(N-isopropylacrylamide-co-methacrylic acid) (P­(NIPAM-co-MAA)) spherical microgel was prepared and their temperature-dependent volume phase transition behavior was systematically studied by analyzing the dielectric spectroscopy theoretically over a frequency range from 40 Hz to 110 MHz. It was found that the dielectric relaxation of the charged P­(NIPAM-co-MAA) microgel drastically changed between 30 and 35 °C, and was significantly different from that of neutral PNIPAM microgels recently published in Soft Matter. The relaxation mechanism was speculated and the relaxation parameters were fitted after successfully eliminating the electrode polarization at low-frequency. On the basis of this, the differences in the structure, swelling ability and dehydration dynamics between charged P­(NIPAM-co-MAA) and neutral PNIPAM microgels were compared. Although both of the two microgels showed the same changing trend around the volume phase transition temperature (VPTT), VPTT of charged P­(NIPAM-co-MAA) microgels is less markedly than that of neutral PNIPAM. This suggests that the interaction between the PNIPAM chains and the solvent was changed significantly because of the introduction of charged groups. The volume phase transition behavior of P­(NIPAM-co-MAA) microgels controlled by a delicate balance between the hydrophobic attraction of NIPAM and the electrostatic repulsion of the carboxylate group of methacrylic acid (MAA). These interactions are the essential reasons of the changes of the charged microgels in structure, swelling ability, and dehydration dynamics.