High-strength cellulose-polyacrylamide hydrogels: Mechanical behavior and structure depending on the type of cellulose

Two types of stiff and high-strength composite hydrogels possessing the structure of interpenetrating polymer networks were synthesized via free-radical polymerization of acrylamide carried out straight within the previously formed physical network of regenerated plant cellulose or bacterial cellulose (PC and BC respectively) that was swollen in the reactive solution. The mechanical behavior of synthesized hydrogels subjected to the action of compressive deformations with different amplitude values was studied. The analysis of the stress-strain curves of compression tests of the hydrogels of both types obtained in different test conditions demonstrates the substantial difference in their mechanical behavior. Both the PC- and BC-based hydrogels withstand successfully the one-shot compression with the amplitude up to 80%, but in the conditions of the multiple compression tests (cyclic compressions) during the subsequent compression acts the dramatic increase in the stiffness of the BC-based hydrogels was observed at the deformation region beyond 60%. This effect can be explained by the deep reorganization of the intermolecular structure of the material with the stress-induced reorientation of BC micro-fibrils. Submicron- and micron-scale specific features of structures of composite hydrogels of both types were studied by cryo-scanning electron microscopy to explain the peculiarities of the mechanical effects observed.