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Probing the micellization kinetics of pyrene end-labeled diblock copolymer via a combination of stopped-flow
Jingyan Zhang1, Yuting Li, Steven P Armes
1Department 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.
Abstract:
A pyrene end-labeled double hydrophilic diblock copolymer, poly(2-(diethylamino)ethyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate) (Py-PDEA-b-PDMA), was synthesized by sequential monomer addition via oxyanionic polymerization using a 1-pyrenemethanol-based initiator. This diblock copolymer exhibits reversible pH-responsive micellization behavior in aqueous solution, forming PDEA-core micelles stabilized by the soluble PDMA block at neutral or alkaline pH. Taking advantage of the pyrene probe covalently attached to the end of the PDEA block, the pH-induced micellization kinetics of Py-PDEA-b-PDMA was monitored by stopped-flow light scattering using a fluorescence detector. Upon a pH jump from 4.0 to 9.0, both the scattered light intensity and excimer/monomer fluorescence intensity ratios (IE/IM) increase abruptly initially, followed by a more gradual increase to reach plateau values. Interestingly, the IE/IM ratio increases abruptly within the first 10 ms: a triple exponential function is needed to fit the corresponding dynamic trace, leading to three characteristic relaxation time constants (tau(1,fluo) < tau(2,fluo) < tau(3,fluo)). On the other hand, dynamic traces for the scattered light intensity can be well-fitted by double exponential functions: the resulting time constants tau(1,scat) and tau(2,scat) can be ascribed to formation of the quasi-equilibrium micelles and relaxation into their final equilibrium state, respectively. Most importantly, tau(1,scat) obtained from stopped-flow light scattering is in general agreement with tau(2,fluo) obtained from stopped-flow fluorescence. The fastest process (tau(1,fluo) approximately 4 ms) detected by stopped-flow fluorescence is ascribed to the burst formation of small transient micelles comprising only a few chains, which are too small to be detected by conventional light scattering. These nascent micelles undergo rapid fusion and grow into quasi-equilibrium micelles and then slowly approach their final equilibrium state. The latter two processes can be detected by both techniques.
