The effect of micellization-induced deprotonation on the associative behavior of a carboxyl modified Pluronic P85

Abstract

Thermoresponsive polymeric surfactant CAE85 is a telechelic carboxyl group derivative of Pluronic P85 and its carboxyl end-groups undergo deprotonation into carboxylate groups upon micellization. Micelle formation and disintegration were studied here by means of small angle X-ray scattering, FTIR and Raman spectroscopy, quantum mechanical calculations and dynamical mechanical analysis. The deprotonation was observed in aqueous solutions of CAE85 for concentrations from 5 wt% to 30 wt% at temperatures above the corresponding critical micellization temperature (CMT). The most likely cause is a difference between the proton dissociation constant of the micelle (pK_m) and the proton dissociation constant of the unimers in solution (pK_a); our observations indicate that pK_m < pK_a. For concentrations up to 15 wt%, the presence of carboxylate groups in CAE85 lowered the CMT in comparison with P85. In addition, the behavior of CAE85 was generally not thermo-reversible and reproducible upon cooling. Quantum chemical calculations showed that, in the dense micelle corona, the deprotonated states were more stable than hydrogen-bonded states of neutral molecules, which is likely to affect the equilibrium processes in the micelle. In contrast to the unmodified P85, no gelation was observed in the case of CAE85. By studying the processes at all the levels of organization from nanoscale charge formation through micellization to the macroscale process of gelation, our understanding of polymeric micelle formation may be advanced.