Probing the Micellization Kinetics of Pyrene End-Labeled Diblock Copolymer via a Combination of Stopped-Flow Light-Scattering and Fluorescence Techniques

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.