Electrophoretic behavior of microgel-immobilized polyions

Electrophoretic behavior was studied for N-isopropylacrylamide (NIPA) microgels, into which different amounts of poly(acrylic acid) (PAAc) were physically entrapped. Copolymer microgels of NIPA with acrylic acid (AAc) were also studied as a control. Electrophoretic mobility was measured in 0.1 M NaCl solution at 25 °C as a function of pH, using an electrophoretic light scattering technique. The mobility of the copolymer microgel whose COOH groups are fully ionized agreed with that of PAAc when its ionization degree (α(n)) is close to the mole fraction (f(AAc)) of the AAc unit in the copolymer gel. There was good agreement between the mobility values of the copolymer microgel and the linear NIPA/AAc copolymer when their AAc contents are very close to each other. However, the mobility of the microgel with immobilized PAAc was higher than that of the copolymer microgel, even when there was no difference in the AAc content for both microgels. Moreover, the immobilized PAAc showed a higher mobility than the free PAAc when its α(n) is equal to f(AAc) in the immobilized system. No correlation was observed between the mobility and the hydrodynamic radius. These results were discussed in terms of the free draining model (FDM) for the electrophoresis of polyelectrolytes. It became apparent that the mobility difference depending upon whether (i) the PAAc ions are in the cage of the NIPA network or (ii) the AAc units are copolymerized with the network chain is due to the structural difference of the segments considered in the FDM.