How does Poly(ethylene glycol) with varied chain length affect the thermo-responsive behavior of methyl cellulose in aqueous solutions?

In this study, by using a set of narrowly distributed polyethylene glycol (PEG) samples with varied average molar mass (PEGn, 9 < n < 454), we have systematically studied how PEG molar mass and solution concentration influence the thermal response behavior of methyl cellulose (MC) aqueous solutions (Mw = 4.3 × 105 g/mol, = 71 nm) by a combination of isothermal titration calorimetry (ITC), rheometer, UV–vis spectroscopy, laser light scattering (LLS) and atomic force microscopy (AFM). At room temperature, ITC result clarifies the existence of extremely weak hydrophobic interactions between MC and PEG chains. In addition, for MC/PEG mixed solution with shorter PEGs (PEG22 and PEG90), experimentally measured zero-shear viscosity (ηexp) is confirmed to be 5–10% higher than the theoretical values (ηcalc), further supporting the interchain association. Whereas for MC/PEG454, insoluble aggregates are formed, which results in ηexp < ηcalc. Further, dynamic laser scattering (DLS) results further reveal how the percentage of peak area of aggregates depends on the chain length of PEG. During the heating process from T = 20 to 70 °C, a combination of UV, LLS and AFM results clearly illustrates that the shorter PEG chains can effectively stabilize the formed MC fibers and prevent the fiber extension; whereas for longer PEG chains, they can not only promote the nucleation process, but also destabilize the formed fibers and promote the fiber extension. Our result unambiguously reveals the macromolecular effect of PEGs on the thermal response behavior of MC in solutions.