Thermoresponsive Hydrogel of Diblock Methylcellulose: Formation of Ribbonlike Supramolecular Nanostructures by Self-Assembly

Abstract

This article provides detailed insight into the thermoresponsive gelation mechanism of industrially produced methylcellulose (MC), highlighting the importance of diblock structure with a hydrophobic sequence of 2,3,6-tri-<i>O</i>-methyl-glucopyranosyl units for this physicochemical property. We show herein, for the first time, that well-defined diblock MC self-assembles thermoresponsively into ribbonlike nanostructures in water. A cryogenic transmission electron microscopy (cryo-TEM) technique was used to detect the ribbonlike nanostructures formed by the diblock copolymers consisting of hydrophilic glucosyl or cellobiosyl and hydrophobic 2,3,6-tri-<i>O</i>-methyl-cellulosyl blocks, methyl β-d-glucopyranosyl-(1→4)-2,3,6-tri-<i>O</i>-methyl-celluloside <b>1</b> (G-236MC, DP_<i>n</i> = 10.7, DS = 2.65), and methyl β-d-glucopyranosyl-(1→4)-β-d-glucopyranosyl-(1→4)-2,3,6-tri-<i>O</i>-methyl-celluloside <b>2</b> (GG-236MC, DP_<i>n</i> = 28.2, DS = 2.75). Rheological measurements revealed that the gel strength of a dispersion of GG-236MC (<b>2</b>, 2.0 wt %) in water at 70 °C was 3.0 times stronger than that of commercial MC SM-8000, although the molecular weight of GG-236MC (<b>2</b>) having <i>M</i>_w = 8 × 10³ g/mol was 50 times smaller than that of SM-8000 having <i>M</i>_w = 4 × 10⁵ g/mol. Cryo-TEM observation suggested that the hydrogel formation of the diblock copolymers could be attributed to the entanglement of ribbonlike nanostructures self-assembled by the diblock copolymers in water. The cryo-TEM micrograph of GG-236MC (<b>2</b>) at 5 °C showed rectangularly shaped nanostructures having a thickness from 11 to 24 nm, although G-236MC (<b>1</b>) at 20 °C showed no distinct self-assembled nanostructures. The ribbonlike nanostructures of GG-236MC (<b>2</b>) having a length ranging from 91 to 864 nm and a thickness from 8.5 to 27.1 nm were detected above 20 °C. Small-angle X-ray scattering measurements suggested that the ribbonlike nanostructures of GG-236MC (<b>2</b>) consisted of a bilayer structure with a width of ca. 40 nm. It was likely that GG-236MC (<b>2</b>) molecules were oriented perpendicularly to the long axis of the ribbonlike nanostructure. In addition, wide-angle X-ray scattering measurements revealed that GG-236MC (<b>2</b>) in its hydrogel formed the same crystalline regions as 2,3,6-tri-<i>O</i>-methylcellulose. The influence of the DP of diblock MC with a DS of around 2.7 on the gelation behavior will be discussed