Effect of poly(<i>N</i>-isopropylacrylamide) grafting on the temperature dependent properties of methylcellulose solutions

As a water-soluble cellulose ether, methylcellulose (MC) is widely used as a binder and viscosity modifier in a variety of applications from pharmaceuticals to construction materials. Many applications exploit the ability of MC to gel thermoreversibly above its lower critical solution temperature (LCST) (~50 °C), correlated to the formation of nanofibrils of uniform diameter. Our recent work has shown that the fibril structure can be altered by grafting poly(ethylene glycol) (PEG) chains onto MC, and that fibrils are suppressed at high grafting densities. To expand our understanding of fibril formation, we have chemically modified MC with poly(N-isopropylacrylamide) (PNIPAm) at various grafting densities. PNIPAm also displays an LCST in water, but at a lower temperature (~32 °C). In this study, we present the effect of PNIPAm on the chain conformation and fiber formation of MC. Using dynamic and static light scattering, the temperature dependent changes in polymer size and solvent quality are investigated. Cryogenic electron microscopy and small-angle X-ray scattering are used to observe and quantify changes in fibril structure. Similar to PEG-grafted MC, at 80 °C the fibril length decreases with increasing PNIPAm grafting density, until fibril formation is entirely suppressed.