Crack Blunting and Advancing Behaviors of Tough andSelf-healing Polyampholyte Hydrogel.

Recently, we have reported that polyampholytes,synthesized fromfree radical copolymerization of anionic monomer and cationic monomer,form physical hydrogels of high toughness and self-healing. The randomdistribution of the opposite charges forms ionic bonds of a wide distributionof strength. The strong bonds serve as permanent cross-links, impartingelasticity, whereas the weak bonds serves as reversible sacrificialbonds by breaking and reforming to dissipate energy. In this work,we focus on the rupture behaviors of the polyampholyte physical hydrogel,P(NaSS-co-MPTC), copolymerized from sodium p-styrenesulfonate (NaSS) and 3-(methacryloylamino)propyltrimethylammoniumchloride (MPTC). Tensile test and pure shear test were performed atvarious stretch rates in the viscoelastic responses region of thematerial. Tensile test showed yielding, strain softening, and strainhardening, revealing the dually cross-linked feature of the gel. Pureshear test showed crack blunting at the notched tip and a large yieldingzone with butterfly shaped birefringence pattern ahead of the cracktip. After blunting, crack advanced at steady-state velocity witha constant angle. The conditions for the occurrence of crack bluntingand variables governing the crack advancing angle are discussed. Wefound that even for these highly stretchable samples, significantblunting only occurs when the tensile fracture stress σfis larger than modulus Eby a factor of about 2, in consistent with Hui’s theoreticalprediction for elastic materials. The crack advancing angle θwas found to be proportional to σy/Eover a wide stretch rate range, where σyis the yielding stress. In addition, thefracture energy was correlated to small strain modulus by a powerlaw in the viscoelastic response region. This systematic study willmerit revealing the fracture mechanism of tough viscoelastic materialsincluding biological tissues and recently developed tough and highlystretchable hydrogels. [ABSTRACT FROM AUTHOR]