Effect of a Hydrophobic Branching Agent in Physically Cross-Linked Hydrogels with Shape-Memory and Self-healing Abilities

Physically crosslinked hydrogels consisting of polyacrylamide (Aam) and polyoctadecylacrylate (C18A) blocks, leading to hydrophilic and hydrophobic domains respectively, are prepared by micellar polymerization. A multifunctional monomer (divinylbenzene, DVB), is also employed to create branched chains. The effect of the polymer composition and the amount of the branching agent on the material characteristics are evaluated. The melting enthalpies of the hydrophobic domains increase as the n-octadecyl acrylate unit amount increases and, in a parallel fashion, an increase in the mechanical modulus G' of the hydrogel is observed. Furthermore, G′ increases as the DVB concentration increases. Self-healing behavior is also demonstrated both visually and by rheological measurements. Finally, once heated above the melting temperature (T_m), these hydrogels can be deformed into different shapes that can be fixed on cooling. Then, heating again above T_m, the original shape is recovered thus indicating shape memory characteristics.