Toughening Limit of Hybrid Nanocomposites

The intimate mixing of the polymer and inorganic phases can lead to spatial confinement of the polymer phase resulting in an exceptional mechanical properties of the nanocomposites. In this paper the authors have probed the mechanical and fracture properties of polymers in the extreme limits of molecular confinement, where a stiff inorganic phase confines the polymer chains to dimensions far smaller than their bulk radius of gyration. To realize the conditions of molecularly confined polymers experimentally, the authors fabricated hybrid nanocomposites composed of a polymeric phase, in wide range of molecular masses, confined within a nanoporous organosilicate matrix with a mean pore diameter of 7 nm. The results show that the polymers confined at molecular length scales dissipate energy through a confinement-induced molecular bridging mechanism that is different from existing entanglement-based theories of polymer deformation and fracture. The toughening is controlled by the molecular size and the degree of confinement, but is ultimately limited by the strength of individual molecules.