Role of Solvent Water in the Temperature-Induced Self-Assembly of a Triblock Copolymer

Water-soluble triblock copolymers have received much attention in industrial applications and scientific fields. We here show that femtosecond mid-IR pump-probe spectroscopy is useful to study the role of water in the temperature-induced self-assembly of triblock copolymers. Our experimental results suggest two distinct subpopulations of water molecules: those that interact with other water molecules and those involved in the hydration of a triblock copolymer surface. We find that the vibrational dynamics of bulk-like water is not affected by either micellation or gelation of triblock copolymers. The increased population of water interacting with ether oxygen atoms of the copolymer during the unimer to micelle phase transition is important evidence for the entropic role of water in temperature-induced micelle formation at a low copolymer concentration. In contrast, at the critical gelation temperature and beyond, the population of surface-associated water molecules interacting with ether oxygen atoms decreases, which indicates important enthalpic control by water. The present study on the roles of water in the two different phase transitions of triblock copolymers sheds new light on the underlying mechanisms of temperature-induced self-aggregation behaviors of amphiphiles that are ubiquitous in nature.