Toughness and fracture energy of PDMS bimodal and trimodal networks with widely separated precursor molar masses

End-linked PDMS bimodal and trimodal networks display enhanced mechanical properties in uniaxial extension over those of unimodal networks with similar modulus when the molar masses of their precursor chains are widely separated. These multimodal networks have optimal mechanical properties when the short chains are near their overlap concentration and sustain most of the load, but the volume of the system is still dominated by the ductile long chain component. Such elastomers can be stretched to large elongations before fracture while displaying an upturn in stress at high strain. Improvement in fracture energy of pre-cut bimodal and trimodal networks over that of unimodal networks is much less pronounced and appears to be dictated by the average molar mass of the effective elastic strands in each network.