Hydration State on Poly(ethylene glycol)-Bearing Homopolymers and Random Copolymer Micelles: In Relation to the Thermoresponsive Property and Micellar Structure

Abstract

Poly(ethylene glycol) (PEG)-bearing (co)polymers often show lower critical solution temperature-type solubility in water; the aqueous solution becomes clouded and/or phase-separated upon heating to a certain temperature (cloud-point temperature). In general, it has been interpreted that the thermoresponsive properties of (co)polymers in water originate from the dehydration of the polymer chains, whereas the hydration state of these polymers has not yet been evaluated. Herein, we report the amount of hydration water on PEG-bearing homopolymers and random copolymer micelles in water, analyzed by terahertz time-domain spectroscopy, and micellar structures (molecular packing in the hydrophilic layer) determined by small-angle X-ray scattering. Systematic investigation by changing the ratio of PEG side chains and alkyl side chains revealed a clear correlation between the thermoresponsive properties, molecular and micellar structures, and the amount of hydration water. The random copolymers induce self-folding or intermolecular self-assembly to form unimer or multichain micelles in water; the size of the micelles increases with an increasing number of hydrophobic monomer units. As the micelle size increases, the PEG chains tend to be densely packed in the hydrophilic shell layers, leading to a reduction in the amount of hydration water per ethylene glycol unit of PEG. Thus, the dehydration of the PEG chain likely becomes easier, and the cloud-point temperatures decrease with increasing hydrophobic monomer content.