Design of ultra-stretchable physical hydrogels cross-linked by cubosomes: structural changes revealed by SANS during <i>in situ</i> polymerisation and mechanical deformation.

Abstract

We report a novel class of nanocomposite physical hydrogels based on polyacrylamide networks, where the crosslinking is achieved through complex nanostructures. For the first time, the cross-linkers consist of lyotropic liquid-crystalline cubic phases (cubosomes) stabilized by LAPONITE® clay nanoplatelets. The hydrogels are synthesized through a three-step <i>in situ</i> polymerisation process. This starts by formulating the dispersion of LAPONITE®-stabilised cubosomes (step 1), followed by the addition of acrylamide monomers (step 2), and finally the polymerisation is carried out by adding the catalyst and the initiator in step 3. Small-angle neutron scattering (SANS) reveals significant structural evolution throughout the polymerization process. Initially exhibiting a <i>Pn</i>3<i>m</i> phase (double diamond cubic phase), the cubosomes undergo swelling upon insertion of acrylamide monomers into the linker inner structure. The subsequent polymerization triggers a remarkable morphological transition from a <i>Pn</i>3<i>m</i> to an <i>Ia</i>3<i>d</i> cubic phase (gyroid cubic phase), accompanied by a clear contraction of the cubosomes below their original size. The resulting hydrogels, containing a high loading of <i>Ia</i>3<i>d</i> cubosomes, demonstrate exceptional mechanical properties with stretchability up to 500%. SANS analysis during deformation reveals that the cubosomes maintain their structural integrity while reorienting along the stretching direction. Simultaneously, free LAPONITE® particles align perpendicular to the applied strain. The deformation mechanism primarily involves the stretching of polymer chains adsorbed onto LAPONITE® particles, whether they are cubosome-bound or free in solution. This is consistent with previous observations in polyacrylamide-based hydrogels cross-linked by only LAPONITE®.