2, University of Hokkaido, Sapporo, Hokkaido, Japan
3, Cornell University, Ithaca, New York, United States
We have investigated the mechanical and fracture properties of dual crosslink hydrogels, i.e. gels containing two types of crosslinks: permanent covalent crosslinks and dynamic crosslinks. Although there are clear similarities in linear rheology with a well-defined single relaxation time, the non-linear behavior and fracture properties of the two different gel systems display significant differences when tested at different applied strain rates. The first system has been previously reported1 and is composed of polyvinyl alcohol chains chemically crosslinked with glutaraldehyde and physically crosslinked with borate ions while the second system is a random copolymer of polyacrylamide and vinyl-imidazole chemically crosslinked with methyl bisacrylamide and physically crosslinked with metal ions. Both systems show a strong dependence of the fracture properties on the applied strain rate. However for the PVOH/Borax system the material becomes increasingly brittle with increasing strain rate while for the imidazole system the material becomes tougher with increasing strain rate. More interestingly we show that even if the fracture energy is reported as a function of a reduced strain rate (product of strain rate and rheological relaxation time) their behavior remains markedly different at high strain rates and we will discuss these differences in terms of a recently proposed model2. We also obtain the very interesting result that, at low strain rate, the dual crosslink gels have a much larger extensibility than the simply chemically crosslinked gels even in a strain rate regime where the reduced strain rate << 1 and the physical crosslinks have very little influence on the stress-strain curve.
1. Mayumi, K.; Guo, J.; Narita, T.; Hui, C. Y.; Creton, C., Fracture of dual crosslink gels with permanent and transient crosslinks. Extreme Mechanics Letters 2016, 6, 52-59.
2. Long, R.; Mayumi, K.; Creton, C.; Narita, T.; Hui, C.-Y., Time Dependent Behavior of a Dual Cross-Link Self-Healing Gel: Theory and Experiments. Macromolecules 2014, 47 (20), 7243-7250.