Neutron scattering studies on molecular origin of tough double network hydrogels

Double-network hydrogels (DN-gels) are a new class of cross-linked polymer networks having high mechanical properties while containing ∼90 wt.% water. To investigate the origin of the toughness, small-angle neutron scattering (SANS) and neutron spin echo (NSE) were applied. The SANS results in both deformed and undeformed conditions for a tough DN-gel made of poly- (2-acrylamido-2-methyl-1-propane sulfonic acid) (PAMPS) and polyacrylamide (PAAm), are presented. The SANS measurements indicate that the deformation of DN-gels results in periodic and mesoscale (∼1.5 μm) compositional fluctuations in both PAMPS and PAAm. In addition, SANS measurements also indicate that the DN-gel constituents favorably associate with each other in water. This favorable association between PAMPS and PAAm is consistent with the rheological and NSE results on solution blends of these two polymers. The implications of the above observations to toughening mechanisms are discussed. (author)