Interlayer structure and self-healing in suspensions of brush-stabilized nanoplatelets with smectic order

Abstract

We have investigated the rheology of an uncured epoxy fluid containing high aspect ratio (length/thickness ≈ 160) α-zirconium phosphate (ZrP) nanoplatelets with smectic order. The nanoplatelets were exfoliated into monocrystalline sheets with uniform thickness using a monoamine-terminated oligomer. The oligomers were densely grafted to the plate surfaces and behave as a molecular brush. Suspensions containing ∼ 2 vol.% ZrP and above show liquid crystalline order with scattering peaks characteristic of a smectic (layered) mesophase. At much higher loading, ∼ 4 vol.% ZrP, there is a sharp transition in visual appearance, steady shear rheology, and linear and non-linear viscoelasticity that is attributed to the reversible interdigitation of oligomer chains between closely spaced layers. The oligomers are proposed to serve as inter-lamellar bridges that store elastic stresses for intermediate rates of deformation, but are able to relax on longer time scales. Under steady shearing conditions, the smectic suspensions with "overlapped" microstructure show a discontinuous flow curve characteristic of shear banding that is attributed to the dynamic pull-out of oligomer chains from the overlap region. At high shear rates, the limiting viscosity of the concentrated suspensions is on the same order of magnitude as the unfilled suspending fluid. When the rate of deformation is reduced below a critical time scale, the original network strength, and corresponding microstructure, is recovered through a passive self-healing process. The unique combination of concentration-dependent yield stress, low post-yield viscosity, and self-healing is potentially useful for various applications in the liquid state, and desirable for scalable processing of nanocomposite materials for structural applications.

Hierarchically Ordered α-Zirconium Phosphate Platelets in Aqueous Phase with Empty Liquid

“…Accordingly, the stabilization of nano-platelets with polymer brushes and their exfoliation into solvent-borne, lyotropic smectic phases 15–17 has been used for the formulation of organic coatings via the addition of molecularly dissolved monomers and polymers 17–21 . Such coatings have been proposed for anti-corrosive 6,22 and anti-scratch 23 applications.…”

Imidazoles-Intercalated α-Zirconium Phosphate as Latent Thermal Initiators in the Reaction of Glycidyl Phenyl Ether (GPE) and Hexahydro-4-Methylphthalic Anhydride (MHHPA)

“…These residues might be attributed to the deterioration of the resin. Amine-intercalated α-ZrP can act as latent thermal initiators [17,18] and the blocking residues of α-ZrP can act as additives for reinforcement of resins [19][20][21].…”

Epoxy nanocomposites with reduced coefficient of thermal expansion

“…Polyetheramine and TBA are surfactants well‐known as good exfoliating agents for ZrP . We investigated the use of mixtures of low‐molecular‐weight surfactants to achieve our goal of significant reductions of CTE with minimal drop of T g .…”

“…An epoxy monomer plus hardener are then added to this dispersion, and the solvent is removed. We have shown that a long‐range order smectic structure is produced, which is maintained after cure . Although the polyetheramine works well as an exfoliating agent, it causes a substantial T g drop in comparison with the neat resin.…”