Erosion behavior of ionically crosslinked polyampholyte gels under DC electric fields

An ionic complex of anionic and cationic monomers was obtained when (N,N-diethylamino)ethylmethacrylate (DEAEM) was protonated with acrylic acid (AAc). Through the free radical copolymerization of the ionic complex and acrylamide (AAm) in aqueous solution, a novel ionically crosslinked polyampholyte gel (poly(AAc-DEAEM- AAm), designated PADA) was prepared without chemical crosslinker. An investigation of swelling behavior indicated that PADA gels exhibited peculiar erosion phenomena under contact and non-contact DC electric fields. Under contact DC electric fields, PADA gels of swollen equilibrium experienced a dual response to DC fields, i.e., swelling first and then erosion. This phenomenon has not been described elsewhere. In contrast to with the contact DC electric fields, PADA gels did not undergo a swelling process, but eroded completely within 20 min under non-contact DC fields. The NaCl concentration in electrolyte solutions greatly influenced the swelling/eroding behavior of PADA gels because the ionic crosslinkages of PADA gels could be weakened and screened by salt ions. The combined effects of higher NaCl concentration and local pH change near the electrode under DC electric fields resulted in crosslinkages quickly dissociating into cationic and anionic groups. In this case, PADA gels did not experience the swelling process, but directly eroded. Moreover, the erosion rate of PADA gels was related to the voltage of the electric field. The swelling/eroding rate for the gels in contact with the cathode was faster than that for the gels in contact with the anode. This difference can be attributed to varying local pH near different electrodes.