Temperature Dependence of Chain Conformations and Fibril Formation in Solutions of Poly(N-isopropylacrylamide)-Grafted Methylcellulose

As a water-soluble cellulose ether, methylcellulose (MC) is used in a variety of applications that take advantage of its thermoreversible gelation. Recent work has shown that MC gelation is due to the formation of nanofibrils with a relatively uniform diameter (ca. 15 nm) and that gelation and fibril formation can be suppressed through the addition of low-molecular-weight poly(ethylene glycol) as grafts along the backbone. In this work, we modify MC similarly with thiol-terminated poly(N-isopropylacrylamide) (PNIPAm, Mw ≈ 3 kg/mol) using thiol–ene click chemistry and investigate the resulting influence on aqueous MC solution properties. From static and dynamic light scattering, it is apparent that the coil dimensions increase with grafting density (up to 0.11 grafts/anhydroglucose repeat unit), which leads to an increase in the persistence length inferred from the Kratky–Porod wormlike chain model. The data are consistent with a model based on the incorporation of graft–graft and graft–backbone excluded volume interactions. Interestingly, grafting PNIPAm leads to an increase in the theta temperature, even though PNIPAm typically has a lower critical solution temperature (LCST) that is lower than bare MC. Small-angle X-ray scattering and cryogenic transmission electron microscopy reveal that fibril formation still occurs at high temperature for the grafted chains.