Importance of entanglement between first and second components in high-strength double network gels

The double network (DN) gels, composed of a minor component, chemically cross-linked polyelectrolyte (first network), and the major one, linear neutral polymer (second network), exhibit anomalously high mechanical strengths. In this study, adopting the concentration of the second polymer as the experimental parameter, we investigate the relation between the mechanical strength and the dynamics of polymer concentration fluctuation. The former is measured by compression and tearing tests; the latter is measured by a special dynamic light scattering technique (called SMILS) suitable for polymer gels. The mechanical strength is enhanced when the concentration is so high that the second polymer chains strongly entangle each other. The main finding of SMILS is that the diffusion constant of the concentration fluctuation of the second polymer is increased by the existence of the first network despite the minority of the first component (<(1)/(10) in monomer ratio to the second component). This indicates that there is entanglement also between the first and the second polymers. The inter-species entanglement and inhomogeneities of the first network suggested in our previous study [Macromolecules 2004, 37, 5370] give support to the crack model concerned with yielding around the crack tip, which explains the high mechanical strength.