Tuning the mechanical properties of nanostructured ionomer films by controlling the extents of covalent crosslinking in core-shell nanoparticles

Abstract

Ionomers are polymers containing a low mole fraction of ionic groups bound to the polymer backbone. These ionic groups produce major changes in their structure and mechanical properties. Recently, we introduced a new family of crosslinked poly(Bd)/poly(Bd-co-MAA) core shell nanoparticles (1,3butadiene and methacrylic acid) that could be ionically crosslinked and cast as nanostructured ionomer films from aqueous dispersions [Pinprayoon et al., Soft Matter, 2011, 7, 247]. The MAA units in the core-shell particles were neutralised by Zn 2+ . Here, we explore the structure-property relationships for these new architecturally controlled nanocomposites by investigating 6 new poly(Bd)/poly (Bd-co-MAA) dispersions and films. In this study we varied the extent of covalent crosslinking in the core and the shell at constant ionic crosslinking for the first time. We used dynamic mechanical thermal analysis to establish a general phase map for the new nanostructured ionomers. Stress-strain data show that our nanostructured films have well controlled, and adjustable, modulus and strain at break values. The data show that the core-shell nanoparticle geometry allows the often observed trade-off between elasticity and ductility to be tuned in a manner that is not possible for conventional ionomers. We show that the chain transfer agent (CTA) concentrations used during the preparation of the nanoparticle cores and shells can be used to independently tune the mechanical properties of the films. This is due to variation of the extents of covalent crosslinking. The results of this study should apply to other covalently crosslinked core-shell nanoparticles containing RCOOH groups in the particle shells.

Tuning the modulus of nanostructured ionomer films of core–shell nanoparticles based on poly(n-butyl acrylate)

“…7c -e Table S1) reveals that an increase of the φ s value by a factor of ~ 2 increased the E values by a factor of ~ 2 with only a small decrease of the strain-at-break (ε b ), and hence ductility. This result is consistent with a property tuning advantage reported earlier for lower modulus nanostructured ionomer films constructed from Bd-based core-shell nanoparticles where stiffness and ductility could be independently tuned 24 .…”

“…This trend has also been observed for Bd-based nanostructured ionomer films 24 and has its origin in the inverse relationship between the number-density of elastically (1)) had the highest E s values. Consequently, film ductility increased when the stiffness decreased for these systems and was controlled by the ionic crosslinks formed in the shells.…”

“…The method for pH adjustment, ZnO neutralisation and film preparation was similar to that described earlier 34 . The nominal extent of neutralisation is the mol.…”

“…AN is known to increase the surface hardness 25 , modulus 26 and tensile strength 27 and thermal stability 28,29 of copolymers due to the strong polarity of nitrile groups 28 and strong molecular interactions 30 . However, all of our previous nanostructured ionomer films used Bd as the main structural monomer 9,23,24 . For the coreshell systems, Bd compromised more than ~ 70 wt.% of the nanoparticles and the Young;s modulus values were less than 12 MPa 8, 9, 24 .…”

“…Previously, we explored the structure-property relationships for low modulus Bd-based coreshell nanostructured ionomer films and probed methods for controlling their mechanical properties 9,23,24 . For those soft films it was shown that inclusion of AN was an effective method for tuning the modulus of the films 8 .…”