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.

Controlled Amphiphilicity and Thermo-Responsiveness of Functional Copolymers Based on Oligo(Ethylene Glycol) Methyl Ether Methacrylates

“…On the other hand, the 50% moles GMA content in P(PEGMA 500 -co-GMA50) suffices to observe a lower cloud point (Tcp = 62 °C), in agreement with other findings in the literature. For example, for copolymers of PEGMA 500 with dodecyl methacrylate or butyl methacrylate with 50% moles content in these hydrophobic units, cloud points at 61–62 °C have been found [ 47 ].…”

Efficient design and synthesis of an amorphous conjugated polymer network for a metal-free electrocatalyst of hydrogen evolution reaction

“…THz-TDS is used to analyze the hydration state, including water weakly affected by solutes, and the method is applied to study the hydration behavior of macromolecules and small molecules. [73][74][75][76][77][78][79][80][81][82][83][84] In the THz frequency band, the collective rotational relaxation dynamics of water (slow relaxation), dynamics of water isolated from hydrogen bonds (fast relaxation), and stretching vibrations between water molecules exist. [85][86][87] Hydration amounts were obtained with the same method in the literature.…”

“…[85][86][87] Hydration amounts were obtained with the same method in the literature. 75,77,81,82 When water is bound to a solute, the slow relaxation becomes slower than the bulk water, and the intensity of the slow relaxation of the bulk water is reduced by the decrease in the amount of bulk water. Primarily, the amount of hydration water can be quantied from this decrease in bulk water.…”

Response strategies and biological applications of organic fluorescent thermometry: cell- and mitochondrion-level detection

“…Temperature changes induce alterations in the spatial conformation of such polymers, coupled with hydration and dehydration. 49 According to existing reports of intracellular fluorescent temperature probes, N -isopropylacrylamide (NIPAM) is the most commonly used such thermoresponsive polymer. With a low critical solution temperature (LCST) of 32 °C, 50 NIPAM undergoes a phase transition within the physiological temperature range.…”