Properties of Chemically Cross-Linked Methylcellulose Gels

Methylcellulose (MC) is widely used as a rheology modifier because, upon heating in aqueous solutions, MC reversibly self-assembles into ∼7-10 nm radius fibrils that percolate into a network, resulting in physical gelation. Here, we have chemically cross-linked both MC solutions at room temperature and MC physical fibril gels at 80 °C and compared the swelling and shear modulus properties of both materials. To achieve this, hydroxyl moieties on MC (M _w ≈ 150 kDa) were substituted with allyl groups, with a degree of substitution of about one pendant carbon-carbon double bond per nine anhydroglucose repeat units. The allyl groups undergo cross-linking in the presence of a photoinitiator and UV light. Chemically cross-linking MC fibril gels ("xfib-MC") at 80 °C results in opaque solid materials and locks in the fibril structure, which persists even on cooling back to room temperature. From small-angle X-ray scattering analysis, the fibril radius is larger at room temperature ∼20 nm and decreases to ∼10 nm at 80 °C. While the fibrils themselves shrink upon heating, the total volume change of xfib-MC gels is minimal. The dynamic shear modulus G′ increases modestly with increasing temperature despite the lack of volume change, and the volume fraction scaling of the modulus is consistent with previous results for fibril gels. On the other hand, chemically cross-linking MC solutions ("xsol-MC") at room temperature leads to clear, solid hydrogels, which no longer form fibrils upon heating. Instead, swelling measurements show that the xsol-MC gels shrink by an order of magnitude in volume when the temperature is increased from 25 to 80 °C. The equilibrium polymer volume fraction, φ _e, and G′ are consistent with established theories for cross-linked polymer chains. We conclude that the origin of elasticity at 80 °C for the two solid materials is totally different and highly tunable. For xsol-MC gels, the modulus arises from conformational entropy of the chains, and for xfib-MC gels, the modulus is attributed to the bending modulus of the individual fibrils.