Anisotropic self‐assembly and gelation in aqueous methylcellulose—theory and modeling

Abstract

Recent experimental studies demonstrated that the aqueous methylcellulose (MC) polymer chains in water can form nanoscale fibrils (diameter $14 nm, persistence length $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 onedimensional 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 struc-ture 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.

Inverse Thermoreversible Mechanical Stiffening and Birefringence in a Methylcellulose/Cellulose Nanocrystal Hydrogel

“…Interestingly, CD signal was also observed in pure 1.0 wt % MC at 60 °C ( Figure S14 ), possibly related to the existence of ring-like fibrillar structures. 47 − 49 , 51 , 52 Remarkably, CD signal was detected from MC/CNC gels at as low as 0.5 wt % of CNC in 1.0% MC. Further, the signals of MC/CNC were amplified by increasing CNC weight fractions as well as upon increasing the temperature.…”

A Simple Analytical Model for Predicting the Collapsed State of Self-Attractive Semiflexible Polymers

“…For example, it was recently shown that methylcellulose gel contains fibrillar structures with a uniform diameter [ 13 ]. Such structures are likely to be initiated by the collapse of individual methylcellulose chains when the solution temperature increases [ 14 ]. This collapsed state can be qualitatively predicted based on the structural properties of methylcellulose, as explained in a recent simulation study of methylcellulose by Huang et al [ 11 ].…”

1,4-D-Glucan block copolymers: synthesis and comprehensive structural characterization

“…Studying the self-assembly of model diblock copolymers having unsubstituted glucose/cellobiose as hydrophilic heads and regioselectively methylated or permethylated glucose units as hydrophobic tails with a total DP of ≤ 28, Nakagawa et al [5] showed the necessity of a sequence of at least 10 permethylated glucosyl units for gelation of methylcellulose to happen. Fibril formation prior to gelation was also observed and further studied using a systematic coarsegrained model [6][7][8][9][10][11][12][13][14]. Taking into account the role of highly methylated glucose units in thermogelation of methylcellulose, Adden et al [15] suggested the cationic ring-opening polymerization of differently substituted cyclodextrins as a potential method for synthesis of glucan ether block copolymers having blocks of permethylated glucose units.…”