Mechanical behaviors and the equivalent network model of self-similar multinetwork elastomers

Multinetwork (MN) is a common topological structure in polymerization related manufacturing and artificial high-performance polymers. Currently the experimental and theoretical researches of MNs are generally focused on the deformation and failure mechanism of self-dissimilar tough hydrogels or tough elastomers, in which the topological structures of each sub network are significantly different. In naturally formed MNs the networks are highly entangled and the interactions between the self-similar sub networks dominate the overall mechanical behaviors. Examples of such self-similar MNs include the case when monomers diffuse into the polymer network during nonuniform polymerization and forms additional interpenetrating chains. In this paper we present the experiment results for a series of self-similar MNs. The networks were created by sequentially immersing the MNs in the same monomers, during which the base network gradually expanded until saturation. Different from tough hydrogels or tough elastomers, here all of the compositional networks deformed simultaneously without obvious damage of any individual network before ultimate breaking. With the increase of interpenetration cycle, the MN was gradually stiffened until the introduction of interpenetrating chains exerted no effect on the mechanical behaviors. With the gradual hydrostatic stretching of each sub network, the viscoelasticity of the MN decreased in the sequential interpenetration process. By introducing the idea of equivalent network, we developed a theoretical model incorporating both the phenomenological treatment and the micromechanical description of MNs, with special attention payed to the interactions of different sub networks in the self-similar MN, the extensibility of chain segments during interpenetration, as well as the alteration of network viscoelasticity. The sequential stiffening phenomenon, the saturation of network elasticity and the disappearance of viscoelasticity could be well captured by the theoretical model.