Gelation and fibril formation of poly(<i>N</i>-isopropylacrylamide)-grafted methylcellulose

Methylcellulose (MC) is a commercially relevant cellulose ether. As a water-soluble polymer, MC is used in a variety of applications from food to construction materials. Many applications exploit the thermoreversible gelation of MC at ca. 60 °C, which has been correlated with the formation of nanofibrils upon heating. Previously, we have shown that grafting poly(ethylene glycol) (PEG) chains onto MC modifies the fibril structure, including suppression of fibrils at high enough grafting densities. To expand our understanding of fibril formation, we have grafted poly(N-isopropylacrylamide) (PNIPAm) onto MC at various grafting densities. PNIPAm is water-soluble at room temperature, however, it displays a lower critical solution temperature at ca. 32 °C and phase separates upon heating. The chain conformation of PNIPAm-grafted MC was studied using dynamic and static light scattering as a function of temperature and grafting density. Cryogenic electron microscopy and small-angle X-ray scattering revealed changes in fibril structure and formation. Utilizing small-amplitude oscillatory shear, we characterized the change in modulus and gelation behavior. The effects of PNIPAm-grafting on MC gelation and fibril formation will be compared to the results obtained with PEG-grafted MC.