Introduction of anisotropy in double network elastomers by orientation and two-step curing

Overview & Synopsis

Double network elastomers are composed of two interpenetrated polymeric networks. A highly-crosslinked and isotropically prestretched filler network is embedded into a soft matrix network. The properties of such a molecular composite primarily depend on the state of stretch of the filler network chains, which dictates their maximum extensibility. Here, we introduce some anisotropy into the double network architecture by using uniaxially pre-oriented filler networks. This prestretching is obtained by crosslinking the filler network in two steps, the second one being performed in the stretched state. Two dual-curing systems are designed, activated either by heating or by UV. Prestretched filler networks are then prepared and do not show any anisotropy in modulus nor swelling ratio. However, double networks prepared from prestretched filler networks show a pronounced anisotropic behavior in large strain. In particular, the transverse direction is not predicted by mean-field models. We propose a new model based on the non-additivity of the finite extensibility for both populations of crosslinks to predict semi-quantitatively the large strain behavior of these anisotropic tough materials. Interestingly we find also that the damage in small and large strain are decoupled, as opposed to conventional filled rubbers where a single damage parameter is sufficient for both regimes. All in all, these new double network elastomers based on more complex network topologies represent a new step toward practical applications, materials and situations that are closer to real life situations.