Micelle Formation and Inversion Kinetics of a Schizophrenic Diblock Copolymer

Abstract

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 MEMAcore 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 MEMAcore 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.

Cononsolvency-induced micellization kinetics of pyrene end-labeled diblock copolymer of N-isopropylacrylamide and oligo(ethylene glycol) methyl ether methacrylate studied by stopped-flow light-scattering and fluorescence

“…The micellization kinetics was investigated by stopped-flow light scattering and analyzed in the context of Dormidontova theory. We also studied the micellar formation and inversion kinetics of a schizophrenic diblock copolymer of poly(4-vinylbenzoic acid)-b-poly(N-(morpholino)ethyl methacrylate) (PVBA-b-PMEMA), induced by a combination of pH and ionic strengths [36]. However, though several kinetic studies have been conducted, more systems with different characteristics should be needed to reach a general consensus concerning the micellization kinetics.…”

Production of Biopolymer Composites by Particle Bonding

“…2 readily yields inversion of micelle at around 90% aqueous ethanol. This behavior resembles pH-induced inversion of micelle that was formed by diblock copolymer unimers (Wang et al, 2006). It is expected that zein molecules would surround hydrophilic or hydrophobic particles depending on the percentage of ethanol in the solution.…”

Micellization of a Siloxane‐Based Segmented Copolymer in Organic Solvents and its Use as a Tool for Metal Complex Nanoparticles

“…Surface tension measurements in organic solvents are not currently used to determine the CMC, although this method is very sensitive at low concentrations. This is because of the dynamics of the micelle formation [2,12] and also the fact that many organic solvents have low surface tensions themselves. It is known that siloxane-containing amphiphilic compounds substan- ), because of the very low cohesive energy of PDMS.…”