Micelle formation and inversion kinetics of a schizophrenic diblock copolymer

It is known that the zwitterionic diblock copolymer, poly(4-vinylbenzoic acid)-b-poly(N(morpholino) ethyl methacrylate) (VBA-b-MEMA), exhibits interesting schizophrenic micellization behavior (see: Liu, S.; Armes, S. P. Langmuir 2003, 19, 4432- 4438). The kinetics of the pH-induced formation and dissociation of VBA-core micelles, the salt-induced formation and dilution-induced dissociation of MEMA-core micelles at pH 10, and the pH-induced micellar inversion between VBA- and MEMA-core micelles in the presence of 0.8 M Na2SO4 were studied in detail using stopped-flow apparatus equipped with a light scattering detector. A pH jump from 12 to 2 in the absence of salt leads to the formation of VBA- core micelles; upon a pH jump from 2 to 12, the breakup of VBA- core micelles into unimers occurs within the dead time of the stopped-flow apparatus (similar to 2-3 ms). At pH 10, addition of Na2SO4 (> 0.6 M) induces the formation of MEMA-core micelles. Compared to the pH-induced formation and dissociation of VBA- core micelles, the salt-induced formation of MEMA-core micelles is faster, while the dilution-induced dissociation of MEMA-core micelles into unimers is considerably slower. This partially reflects the block length asymmetry of this VBA-b-MEMA copolymer and also the fact that the MEMA-core micelles are denser and larger than the VBA-core micelles. The structural inversion from VBA-core micelles to MEMA-core micelles upon a pH jump from 2 to 12 in the presence of 0.8 M Na2SO4 proceeds first with the fusion of VBA-core micelles into lose aggregates due to the insolubility of MEMA shell immediately after pH jump, then the dissociation of VBA-core micelles into unimers and partial disintegration of initially formed loose aggregates, which is followed and/or accompanied by the reaggregation of unimer chains into MEMA core-micelles. The structural inversion from MEMA-core micelles to VBA-core micelles on jumping from pH 12 to 2 in the presence of 0.8 M Na2SO4 exhibits different kinetics. The scattering intensities decrease monotonically with time and then stabilize out. All the relaxation curves at different copolymer concentrations can be well-fitted using a single-exponential function and the characteristic relaxation time for the structural inversion of the micelles (Yi) is similar to 0.3 s, which slightly decreases with increasing copolymer concentrations. We tentatively propose that the structural inversion from MEMA-core to VBA-core micelles proceeds first with the splitting of large MEMA-core micelles into small VBA-core micelles, followed and/or accompanied by the redistribution of unimer chains between appearing small VBA-core micelles.