Purely salt-responsive micelle formation and inversion based on a novel schizophrenic sulfobetaine block copolymer: structure and kinetics of micellization.
A novel sulfobetaine block copolymer, poly(N-(morpholino)ethyl methacrylate)-b-poly(4-(2-sulfoethyl)-1-(4-vinylbenzyl)pyridinium betaine) (PMEMA-b-PSVBP), was synthesized via reversible addition-fragmentation chain transfer polymerization. In aqueous solution, PMEMA homopolymer becomes insoluble in the presence of Na2SO4 (>0.6 M), whereas PSVBP homopolymer molecularly dissolves in the presence of NaBr (>0.2 M). Thus, PMEMA-b-PSVBP diblock copolymer exhibits purely salt-responsive "schizophrenic" micellization behavior in aqueous solution, forming two types of micelles with invertible structures, that is, PMEMA-core and PSVBP-core micelles, depending on the concentrations and types of added salts (Scheme 1). The equilibrium structures of these two types of micelles were characterized via a combination of 1H NMR and laser light scattering (LLS). We further investigated the kinetics of salt-induced formation/dissociation of PMEMA-core and PSVBP-core micelles and the structural inversion between them employing the stopped-flow light scattering technique. In the presence of 0.5 M NaBr, the addition of Na2SO4 (>0.6 M) induces the formation of PMEMA-core micelles stabilized with well-solvated PSVBP coronas. Dilution-induced dissociation of PMEMA-core micelles into unimers occurs within the dead time of the stopped-flow apparatus (approximately 2-3 ms) when the final Na2SO4 concentration drops below 0.3 M, while salt-induced breakup of PSVBP-core micelles is considerably slower. The structural inversion from PMEMA-core to PSVBP-core micelles proceeds first with the dissociation of PMEMA-core micelles into unimers, followed by the formation of PSVBP-core micelles. On the other hand, structural inversion from PSVBP-core to PMEMA-core micelles exhibits different kinetic sequences. Immediately after the salt jump, PMEMA corona chains are rendered insoluble, and unstable PSVBP-core micelles undergo intermicellar fusion; this is accompanied and/or followed by the solvation of PSVBP cores and structural inversion into colloidally stable PMEMA-core micelles.
Wang
Di
D
Department of Polymer Science and Engineering, Joint Laboratory of Polymer Thin Films and Solution, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China.
Wu
Tao
T
Wan
Xuejuan
X
Wang
Xiaofeng
X
Liu
Shiyong
S