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

Cononsolvency-induced micellization kinetics of a pyrene end-labeled diblock copolymer of N-isopropylacrylamide and oligo(ethylene glycol) methyl ether methacrylate, Py-PNIPAM-b-POEGMA, was investigated in detail via a combination of stopped-flow light-scattering and fluorescence techniques. Upon a stopped-flow jump from pure methanol to proper methanol/water mixtures, scattered light intensity exhibited an initial increase and then stabilized out; whereas the time-dependence of monomer to excimer fluorescence intensity ratios (I_E / I_M) revealed an abrupt increase followed by a gradual decrease to plateau values. The dynamic traces of scattered intensity can be well fitted by double exponential functions, the obtained τ_{1, scat} and τ_{2, scat} can be ascribed to processes of forming quasi-equilibrium micelles and their relaxation into final equilibrium states, respectively. On the other hand, a triple exponential function was needed to fit the dynamic traces of I_E / I_M, leading to three characteristic relaxation times (τ_{1, fluo}, τ_{2, fluo}, and τ_{3, fluo}). It was found that the time scales of τ_{1, scat} and τ_{2, scat} obtained from stopped-flow light scattering were in general agreement with τ_{2, fluo} and τ_{3, fluo} obtained from stopped-flow fluorescence. Considering that excimer fluorescence is extremely sensitive to small aggregates, the newly detected fast process (τ_{1, fluo} ∼ 10 ms) by stopped-flow fluorescence should be ascribed to the early stage of micellization, i.e., the burst formation of small transient micelles, in which light scattering detection was still not sensitive enough. These small transient micelles fused and grew into quasi-equilibrium micelles, which then slowly relaxed into the final equilibrium state.