High-strength bacterial cellulose-polyacrylamide hydrogels: Mesostructure anisotropy as studied by spin-echo small-angle neutron scattering and cryo-SEM

Submicron- and micron-scale structures of composite hydrogels based on bacterial cellulose (BC) and polyacrylamide (PAAm) were studied by spin-echo small-angle neutron scattering (SESANS) and cryo-scanning electron microscopy (cryo-SEM). These hydrogels possessing the structure of interpenetrating polymer network were synthesized via free-radical polymerization of acrylamide carried out in the pellicle of BC swollen in the reaction solution. No neutron scattering was observed for the samples swollen in heavy water to the equilibrium state, but the SESANS signal appeared when TbCl3 salt was added to the solvent. It is the unusual effect, which may be very helpful for SESANS studying of other hydrogel systems. The SESANS dependences obtained for these samples revealed the anisotropy of mesostructure for the hydrogels under investigation. Density inhomogeneities on the characteristic scale of 11.5 ± 0.5 μm were detected in one fixed orientation of the sample, i.e., with the growth plane of BC parallel to the plane formed by the neutron beam and the spin-echo length. The uniaxial anisotropy revealed agrees with a recently proposed model, which attributes this behavior to the existence of tunnel-like oriented structures inside BC. The evidence of such type of mesostructure anisotropy of BC and BC-PAAm hydrogels was obtained by using the cryo-SEM method.