Recent experimental studies demonstrated that the aqueous methylcellulose (MC) polymer chains in water can form nanoscale fibrils (diameter similar to 14 nm, persistence length similar to 60 nm), and those fibrils can organize into networks at higher temperatures and/or concentrations, forming the commonly observed gel. Here we propose that the fibrils are one-dimensional self-assemblies of stacked, fused polymer rings that are formed at elevated temperatures due to the changing nature of the MC-water hydrogen bonding. This mechanism is analogous to the coil-helix transition in polypeptides, although it is not clear whether the MC fibrils possess chirality. We perform coarse-grained molecular simulations of MC chain structure at temperatures both above and below the hypothesized coil-to-ring transition, with CG forcefield tuned by atomistic molecular dynamics simulations, and observe the expected conformational change. We then develop a statistical mechanical theory to predict the fibril self-assembly, gelation and rheology as function of temperature and concentration. The findings are in reasonable agreement with experimental data and could be generalized to other carbohydrate polymers. (C) 2016 Wiley Periodicals, Inc.