Polymer films prepared using ionically crosslinked soft core-shell nanoparticles: a new class of nanostructured ionomers

Although conventional ionomers have been studied for more than 40 years, reports of ionomers prepared using aqueous core-shell nanoparticle dispersions are lacking. In this study we examine the structure and properties of new soft core-shell elastomeric nanoparticles and ionomer films. The nanoparticles consist of a soft poly(Bd) (Bd is 1,3-butadiene) core and a poly(Bd-co-MAA) shell (MAA is methacrylic acid). The nanoparticles were prepared by sequential emulsion polymerisation and were characterised using photon correlation spectroscopy, TEM, PSDA (particle size distribution analysis) and potentiometric titration. Robust ionomer films were formed by casting mixed core-shell nanoparticle/ZnO dispersions at room temperature. The ZnO particles provided aqueous Zn2+ which neutralised the carboxylate groups and imparted ionic crosslinking. The mechanical properties of the films were investigated using DMTA (dynamic mechanical thermal analysis) and stress-strain measurements. Neutralisation substantially increased the film modulus values. This was ascribed to the formation of a poly(Bd-co-MAA)/Zn2+-rich layer that moved inwards from the periphery of the nanoparticles as the degree of neutralisation increased. In contrast to conventional ionomers, the mechanical properties of the present films depend on ionic crosslinking within a honeycomb-type continuous phase that percolates the film. The storage and tensile moduli for the films increased linearly with neutralisation. The films studied here are a new class of nanostructured ionomers and the results obtained should be generally applicable to other films comprising soft-core-shell nanoparticles containing surface carboxylate groups.