From Melts of Graft Polymers to Supersoft and Hyperelastic Materials

We use scaling analysis and molecular dynamics simulations to study relationship between mechanical properties of graft polymer networks and their molecular architecture. The elastic response of such networks can be described by replacing the graft polymer strands with worm-like strands characterized by the effective Kuhn length b_k. Simulations of graft polymer melts show that b_k of graft polymers is controlled by the degree of polymerization of side chains n_sc and their grafting density 1/n_g to the backbone. These results are used to obtain the structural shear modulus G and strands extension ratio β in terms of the network architectural triplet [n_sc, n_g, n_x] (n_x is the degree of polymerization of the backbone between crosslinks). Simulations show that G of graft polymer networks could decrease with decreasing β. This behavior for linear chain networks known as a “golden rule” reflects that softer materials are more deformable, G∝β. However, graft polymer networks could break this rule showing an increase of G with decreasing β such as G∝β^-2. This can be achieved by changing n_g and keeping n_x and n_sc constant. This peculiar mechanical response of graft polymer networks agrees with experimental studies of PDMS graft polymer elastomers.