  Here, the general design and properties of new multiple network elastomers
with an exceptional combination of stiffness, toughness, and elasticity are
reported. In this paper, it is reported in more detail how the increase in
strain at break resulting from the toughening can be used to provide great
insight in the large strain properties of otherwise brittle acrylic well
crosslinked networks. The networks have been prepared by sequences of
polymerization and swelling with monomers. The parameters that have been varied
are the nature of the base monomers and the degree of crosslinking of the first
network. Here, the small strain properties, equilibrium swelling, and large
strain properties in uniaxial tension are characterized. It is shown here that
the large strain properties of the multiple networks are quantitatively
controlled by the large strain properties of the stretched first network which
acts as a percolating filler, while the small and intermediate properties are
controlled by the entanglement density which can be largely superior to that of
homogeneous networks. Different brittle and prestretched elastomer networks are
embedded at a low volume fraction in a soft extensible matrix. The increase in
toughness of the final material is directly controlled by the nonlinear elastic
properties of the prestretched network and its volume fraction, providing a
general design rule for tough soft materials
