An isotopic effect in self-assembly of amphiphilic block copolymers: the role of hydrogen bonds

Abstract

An isotopic effect was observed in solutions of self-assembling (SA) amphiphilic block copolymers: it was found that the micellization enthalpy, temperature and the size of the formed micelles are affected by replacing H₂O by D₂O. The SA of solvated block-copolymers (poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide)) in H₂O, D₂O and their mixtures was investigated as a function of temperature. High sensitivity differential scanning calorimetry revealed that the micellization temperature is reduced, and the enthalpic penalty of the transition and size of the formed micelles increase when H₂O is exchanged by D₂O. Molecular theory calculations suggest that the difference in the hydrogen bond strength of the solvent, H₂O or D₂O, is the origin of the different structural and conformational properties of the solvated block copolymers. The differences in the solvent properties were predicted to modify the solubility and consequently the SA of the polymers in the two solvents, as experimentally measured. The study provides an insight into the role of hydrogen bonding in systems of amphiphilic block copolymers, and suggests that in SA polymers small differences in hydrogen-bonding strength of the solvent may result in observable macroscopic effects.