Internal Structure of Methylcellulose Fibrils

Methylcellulose (MC) is a water-soluble cellulose derivative with a wide range of commercial applications. Upon heating, MC solutions reversibly form ∼15–20 nm diameter fibrils, which percolate into a fibrillar network, resulting in macroscopic gelation. Using mid-angle X-ray scattering (MAXS) and wide-angle X-ray scattering (WAXS), we have analyzed MC chain organization within fibrils aligned in dried films that have been stretched by over 300%. MAXS and WAXS show distinct anisotropic scattering features, which we interpret as reflecting crystalline domains within the fibrils. The scattering peaks are consistent with a body-centered monoclinic unit cell, with similar dimensions as other cellulosic crystals, a = 11.4 Å, b = 8.9 Å, and c = 10.2 Å, and γ in the range of 90–100°, with MC chains oriented along the long axis of the fibril. Phase-plate cryogenic transmission electron microscopy images of MC fibrils contribute to a more comprehensive picture. Along the long axis of MC fibrils, there is evidence of dense twisted domains, which are interpreted as regions containing semicrystalline MC, interspersed with looser, less organized amorphous domains. Together, these two techniques provide the most complete interpretation of MC subfibril structure currently available.