Reversible modulus reinforcement of end-linked polydimethylsiloxane ionomer networks

We report the mechanical behavior and swelling properties of covalently end-linked networks of polydimethylsiloxane with tailored number of monomers between side carboxyl groups and number of carboxyl groups along the chain. Imperfect carboxyl networks are reinforced by neutralization of the carboxyl groups with gallium ions via conversion of pendent chains into elastically active strands due to formation of inter-molecular ionic cross-links. These networks swell in non-polar solvents to a similar degree as unmodified end-linked PDMS networks of comparable moduli. For unannealed samples, the modulus reinforcement is reversed by a polar THF:water solution that breaks the ionic cross-links. Neutralization with transition metal cations such as cobalt causes no initial reinforcement due to weak intra-molecular interactions. Reinforcement in annealed gallium, cobalt or carboxyl networks is not readily reversible.