Fibrillar structure in aqueous methylcellulose solutions and gels

The fibrillar structure of aqueous methylcellulose (MC) gels was probed using a combination of small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), and cryogenic transmission electron microscopy (cryo-TEM). The effect of molecular weight (M_w) and concentration on the gel structure was explored. The fibrillar morphology was consistently observed at elevated temperatures (≥70 C), independent of concentration and M_w. Moreover, the fibril dimensions extracted from SANS by fitting to a scattering function for semiflexible cylinders with disperse radii revealed that the fibril diameter of ca. 14 ± 1 nm is constant for a mass fraction range of 0.01%-3.79% and for all M_w investigated (49-530 kg/mol). Comparison of the measured SANS curves with predicted scattering traces revealed that at 70 C the fibrils contain an average volume fraction of 40% polymer. Taking linear combinations of low temperature (solution state) and high temperature (gel state) SANS traces, the progression of fibril growth with temperature for aqueous MC materials was determined. At low temperatures (≤30 C) no fibrils are present, whereas in the vicinity of 40-50 C a small fraction begins to form. For temperatures ≥70 C, virtually all of the chains are incorporated into the fibrillar structure. The persistence of the fibril structure during cooling was probed by SANS and cryo-TEM. The well-established rheological hysteresis upon cooling is directly correlated to the persistence of the fibril structures. Furthermore, cryo-TEM images taken upon heating to 50 C showed no fibrils, whereas images for samples that were first heated to 70 C and then cooled to 50 C clearly display the fibrillar structure. USANS measurements revealed that heterogeneities in the gels persist beyond the largest length scale accessed in scattering experiments (∼20 μm), consistent with the observed optical turbidity.