Chain-length dependence of diblock copolymer micellization kinetics studied by stopped-flow pH-jump

A series of well-defined poly(ethylene oxide)-b-poly(2-(diethylamino)ethyl methacrylate) (PEO-b-PDEA) diblock copolymers containing PEO block of identical chain length and PDEA block with varying degrees of polymerization (DP, in the range of 32-154) were prepared via atom transfer radical polymerization (ATRP) employing a PEO-based macroinitiator (DP = 113). Upon a pH-jump from 3 to 12 under highly efficient stopped-flow mixing conditions, PEO-b-PDEA copolymers spontaneously form spherical micelles of increasing sizes and aggregation numbers (N-agg) with increasing PDEA chain lengths. Stopped-flow light scattering technique was used to probe the pH-induced micellization kinetics of PEO-b-PDEA copolymers, aiming to elucidate the PDEA chain-length effects on the unimer-to-micelle transition process. Upon a stopped-flow pH-jump from 3 to 12, the obtained dynamic traces can be well-fitted with double exponential functions. The calculated fast and slow characteristic relaxation times (tau(1) and tau(2)) can be ascribed to the formation of quasi-equilibrium micelles (fast process) and subsequent relaxation into final equilibrium micelles (slow process), respectively. For PEO113-b-PDEA(32) and PEO113-b-PDEA(61), tau(2) is almost independent of polymer concentrations, suggesting that the relaxation from quasi-equilibrium micelles into final equilibrium micelles mainly proceeds vi a insertion/expulsion of unimer chains. Upon increasing the DP of pH-responsive PDEA block to 89, 117, and 154, the obtained slow relaxation time, tau(2), tends to decrease with increasing polymer concentrations, suggesting that the slow process is dominated by the micelle fusion/fission mechanism. The apparent activation energy (E-a) associated with tau(2) has also been determined from ternperature-dependent micellization kinetics for five PEO-b-PDEA copolymers. It was found that during micellization, copolymers with longer PDEA blocks exhibit much lower E-a compared to those with shorter blocks. Thus, we observed experimentally for the first time that increasing the hydrophobic block length in double hydrophilic block copolymers (DHBCs) can transform the mechanism of the slow process from unimer insertion/expulsion to micelle fusion/fission.