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 Na 2 SO 4 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 (∼ 2−3 ms). At pH 10, addition of Na 2 SO 4 (> 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 Na 2 SO 4 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 Na 2 SO 4 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 (τ i ) is ∼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.