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.

Aqueous dispersions of a poly(ethylene-alt-propylene)-bpoly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PON) triblock terpolymer with block molecular weights of 3 000−25 000−10 000 and polymer concentrations ranging from 1 to 5 wt % were investigated at several temperatures from 25 to 55°C using cryogenic scanning electron microscopy (cryo-SEM), cryogenic transmission electron microscopy (cryo-TEM), and small-angle neutron scattering (SANS). The cryo-SEM and cryo-TEM micrographs revealed that PON triblock terpolymer selfassembled into spherical micelles with PEP cores and PEO−PNIPAm coronae at room temperature and subsequently formed a twocompartment micellar network consisting of distinct spherical PEP and PNIPAm cores upon heating above the critical gelation temperature (42°C). The formation of two discrete spherical PEP and PNIPAm hydrophobic domains was supported by detailed SANS analysis of the PON triblock samples in D 2 O, as the resulting intensity profiles can be successfully fitted using a scattering equation based on the two-compartment network structure. The two-compartment structure was further confirmed using contrast-matching SANS experiments on a PON d7 triblock sample with similar block molecular weights and a partially deuterated PNIPAm block. An important result of the SANS profiles in the gel state was the emergence of two distinct scattering peaks, which could be accounted for by considering spatial correlations between PEP and PNIPAm micellar cores. This study confirms the hypothesis that the formation of two-compartment networks in ABC terpolymer hydrogels results in better gelation properties, in comparison to other physically associated hydrogels, and can further guide the design and development of advanced hydrogel systems with enhanced performance.

We measure the spatially resolved microstructure and concentration in the plane of flow for a viscoelastic solution of polymer-like micelles comprised of mass fraction 6.0% (volume fraction 6.6%) solution of 2:1 molar ratio cetylpyridinium chloride/sodium salicylate in 0.5 mol/L NaCl/D2O through the shear banding transition. Spatially resolved flow small-angle neutron scattering measurements in the velocity–velocity gradient (1–2) plane of flow establish the local microstructure, and scanning narrow-aperture flow ultrasmall-angle neutron scattering (SNAFUSANS) measurements indicate no flow-induced concentration gradients within measurement accuracy. These results show shear banding in this solution is not associated with an isotropic–nematic transition and are fundamentally important for validating models of shear-banding complex fluids. Improvements in the SNAFUSANS method are also documented.