Thermoreversible Gelation of Aqueous Methylcellulose Solutions

The thermoreversible gelation of methylcellulose in aqueous solution has been studied by static and dynamic light scattering (DLS), small-angle neutron scattering (SANS), and rheology. At 20 °C, dilute solution light scattering establishes the molecular weight, second virial coefficient, radius of gyration, and hydrodynamic radius of the polymer. Semidilute solutions exhibit two relaxation modes in DLS, one reflecting cooperative diffusion and the other attributable to pregel clusters. Rheological measurements in this regime also suggest a weak supermolecular association. The gelation of semidilute solutions proceeds in two stages with increasing temperature above 20 °C, consistent with previous reports. The first stage is attributable to clustering of chains, driven by hydrophobic association, and extends up to approximately 50 °C. This process is accompanied by an increase in the low-frequency dynamic elastic modulus, G‘, and an increase in both light and neutron scattered intensity. The DLS properties of these solutions, and the angular dependence of the scattered intensity, is not greatly affected by this association. The second stage of gelation occurs rather abruptly above ca. 50 °C and is attributed to phase separation accompanied by gelation. The elastic modulus increases rapidly with temperature, the samples become visibly turbid, and the scattered intensity increases markedly over a wide range of scattering wavevector, q. In contrast, dilute solutions do not gel but give clear evidence of aggregation in the high-temperature regime, consistent with crossing a phase boundary. The SANS structure factor S(q) in the gel state is well described by a sum of two terms, corresponding to two power-law regimes. The lower q regime follows S(q) ∼ q-1.8 in both the pregel and gel states, consistent with chains intermediate between good and ϑ solvent conditions. At higher q the exponent evolves from ca. −2.5 to −4 upon gelation. The latter exponent indicates a sharp boundary between the gel structure and the intervening fluid, consistent with liquid−liquid phase separation that is arrested by gelation.