Thermodynamics of Aqueous Methylcellulose Solutions

Abstract

The weight-average molecular weight M w , z-average radius of gyration R g , and second virial coefficient A 2 have been determined between 15 and 52 °C for dilute aqueous solutions of methylcellulose (MC) with three different molecular weights and constant degree of substitution (DS) of 1.8 using static light scattering. These measurements, conducted within 1 h of heating the homogeneous solutions from 5 °C, reveal that the theta temperature for MC in water is T θ = 48 ± 2 °C, with A 2 < 0 for T > T θ , indicative of lower critical solution temperature (LCST) behavior. However, after annealing a solution for 2 days at 40 °C evidence of high molecular weight aggregates appears through massive increases in the apparent M w and R g , a process that continues to evolve for at least 12 days. Cryogenic transmission electron microscopy images obtained from a solution aged for 3 weeks at 40 °C reveal the presence of micron size fibrils with a diameter of 16 ± 4 nm, structurally analogous to the fibrils that form upon gelation of aqueous MC solutions at higher concentrations and elevated temperatures. Growth of fibrils from a solution characterized by a positive A 2 indicates that semiflexible MC dissolved in water is metastable at T < T θ , even though the solvent quality is apparently good. The minimum temperature required for MC solutions to aggregate is estimated to be 30 °C, based on the rate-independent gel-to-solution transition determined by small-amplitude oscillatory shear measurements conducted while cooling 0.5 and 5.0 wt % solutions. These results cannot be explained based solely on separation into two isotropic phases upon heating using classical Flory−Huggins solution theory. We speculate that the underlying equilibrium phase behavior of aqueous MC solutions involves a nematic order parameter.

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

“…The last step in the modeling is describing the rheology. As already discussed by Arvidson et al, the elasticity of the MC fibrillar network at high temperature should be similar to that of the actin network or any other network of semi‐flexible fibers. The elasticity of semi‐flexible fibrillar network is often described by the model of MacKintosh et al, resulting in a strong dependence of the shear storage modulus, G’ , on the polymer concentration, c , as G’ ∼ c 2.5 This result is indeed consistent with experiments for the case of gelatin networks, as demonstrated by Joly‐Duhamel et al We use the same modeling approach to compute the dependence of G’ ( T ) for various concentrations of aqueous MC, while estimating the crosslink density as function of temperature from thermodynamic theory.…”

“…Here, ϕ is the methylcellulose volume fraction in water, and α ≈ 0.4 is the volume fraction of polymer in the tube (we assume that it is the same as in experiments of Arvidson et al). The free energy density of the ensemble of rings/tubes in water can be written as follows, …”

“…Over the years, a number of studies have been devoted to the phase behavior of aqueous MC . Very recently, studies by Bodvik et al, and independently by Kobayashi et al provided unexpected evidence of anisotropic polymer self‐assembly into high‐aspect‐ratio fibrils of MC. In particular, Bates et al used a combination of experimental techniques (SANS, cryo‐TEM, optical, and rheology) to find the following features for a set of water‐soluble MC materials (polymer concentration in water ranged from 0.014 to 12 wt %) prepared by the same commercial heterogeneous process with similar degrees of methylether substitution (1.8 mol MeO/mol anhydro glucose unit (AGU)).…”

Self-Organization of Spindle-Like Microtubule Structures

“…This may be due to the crowding agent, methylcellulose, we employed. Methylcellulose is a large, viscous polymer that can form bundles on its own at high concentrations 45 . Here, we use it at 0.3% in the chamber, which should be in the isotropic network phase 45 .…”

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.…”