Kevin L. White

Large-scale self-assembled zirconium phosphate smectic layers via a simple spray-coating process

The large-scale assembly of asymmetric colloidal particles is used in creating high-performance fibres. A similar concept is extended to the manufacturing of thin films of self-assembled two-dimensional crystal-type materials with enhanced and tunable properties. Here we present a spray-coating method to manufacture thin, flexible and transparent epoxy films containing zirconium phosphate nanoplatelets self-assembled into a lamellar arrangement aligned parallel to the substrate. The self-assembled mesophase of zirconium phosphate nanoplatelets is stabilized by epoxy pre-polymer and exhibits rheology favourable towards large-scale manufacturing. The thermally cured film forms a mechanically robust coating and shows excellent gas barrier properties at both low- and high humidity levels as a result of the highly aligned and overlapping arrangement of nanoplatelets. This work shows that the large-scale ordering of high aspect ratio nanoplatelets is easier to achieve than previously thought and may have implications in the technological applications for similar materials.

Rheology of Disentangled Multiwalled Carbon Nanotubes Dispersed in Uncured Epoxy Fluid

The rheological behaviors of clustered and disentangled multiwalled carbon nanotubes (MWCNTs) dispersed in a stable, non-reactive Newtonian epoxy fluid have been investigated. Suspensions of untreated and chemically oxidized MWCNTs in epoxy show strong rate-dependent behavior and long-time elastic response that is characteristic of a flocculated microstructure. Suspensions of disentangled MWCNTs in epoxy were prepared by a functionalization reaction with nitrobenzene, and show distinct rheological behavior that is attributed to the motion and rotation of the individual nanoparticles. Characteristic features of the disentangled MWCNTs in epoxy are a lack of low-frequency plateau in storage modulus and shear thickening behavior at high shear rate. The rheological behavior is discussed on the basis of continuum-level predictions for the motion of individual, semiflexible fibers under shearing flows. Implications of MWCNT disentanglement on fundamental study and applications of MWCNT-filled systems are discussed.

Mechanical reinforcement of epoxy with self-assembled synthetic clay in smectic order.

Epoxy films containing self-assembled 2D colloidal α-zirconium phosphate nanoplatelets (ZrP) in smectic order were prepared using a simple, energy-efficient fabrication process suitable to industrial processing. The ZrP nanoplatelets form a chiral smectic mesophase with simultaneous lamellar order and helical arrangements in epoxy. The epoxy nanocomposite films are transparent and flexible and exhibit exceptionally high tensile modulus and strength. The findings have broad implications for development of multifunctional materials for engineering applications.

Highly effective anti-corrosion epoxy spray coatings containing self-assembled clay in smectic order

Epoxy nanocomposite coatings containing self-assembled α-zirconium phosphate nanoplatelets (ZrP) in smectic order were successfully prepared by spray-coating on an aluminum substrate using a simple, energy-efficient fabrication process that is suitable for industrial practices. Aluminium substrates with smectic epoxy/ZrP coatings showed a substantial improvement in corrosion resistance compared to the metal surface coating with unfilled epoxy alone. Lyotropic liquid crystal behaviour of ZrP nanoplatelets was observed in a number of different solvents, which suggests the phenomena are largely entropically driven and would be suitable for various coating systems. The results show that the epoxy coatings containing highly aligned ZrP nanoplatelets in smectic order may be used to effectively protect metal surfaces from corrosion using a scalable and simple approach.

Effect of surface modifier on flow properties of epoxy suspensions containing model plate-like nanoparticles

The influence of inter-particle interaction on the rheology of an uncured epoxy containing model α-zirconium phosphate (ZrP) nanoplatelets with aspect ratio of 160 is reported. Epoxy suspensions containing nanoplatelets exfoliated with tetra-n-butyl ammonium hydroxide (TBA), a low molecular weight quaternary ammonium cation, show short-range repulsive potential with weak elastic response at low concentration. At semi-dilute concentrations, the suspensions are solid-like at intermediate frequency and transition to viscous flow for time scales longer than the rotary diffusion process. The weak elasticity at intermediate frequency is attributed to the effect of Brownian motion on the rotational motion of the isolated plates. Suspensions containing nanoplatelets exfoliated with hydrophilic polyetheramine oligomers show similar behaviour to the ZrP-TBA system, but shifted to lower concentration. The rheological behaviour is attributed to steric stabilization of the nanoplatelets by extended oligomer brushes with short-range repulsive interactions. For suspensions containing nanoplatelets exfoliated with hydrophobic polyetheramines with shorter length, there is evidence for elastic response on local length scales and the flow behaviour shows strong history and temperature dependence. Rheological signatures associated with equilibrium nanoplatelet dispersions with repulsive interactions are discussed.

Spray-coated epoxy barrier films containing high aspect ratio functionalized graphene nanosheets

Epoxy nanocomposite spray-coatings containing large aspect ratio modified graphene (MG) were successfully prepared in a facile manner. The individually exfoliated MG nanosheets form liquid crystalline mesophases in the epoxy precursor at low loadings. The epoxy/MG nanocomposite films exhibit significantly improved gas barrier properties even in humid conditions. The spray-coating method allows for the preparation of high MG content nanocomposites in large scale without compromising processability. Implication of the present finding for fabrication of high-performance graphene-based packaging films with multi-functionality is discussed.

Effects of aspect ratio and concentration on rheology of epoxy suspensions containing model plate-like nanoparticles

Hexagonal 2-dimensional α-zirconium phosphate crystals were prepared with lateral diameters ranging from 110 nm to 1.5 μm to investigate the effect of particle size on suspension rheology. The nanoplatelets were exfoliated to individual sheets with monodisperse thickness and dispersed in a Newtonian epoxy fluid. The steady shear response of dilute and semi-dilute suspensions was measured and compared to expressions obtained from theory for infinitely dilute suspensions. For suspensions containing the smaller nanoplatelets, aspect ratio ∼160, the low shear rate viscosity and transition to shear thinning behavior were well described by theory for loadings up to 0.5 vol. %. The agreement was improved by assuming a moderate polydispersity in lateral diameter, ∼30%–50%, which is consistent with experimental observation. For the higher aspect ratio nanoplatelets, good agreement between theory and experiment was observed only at high shear rates. At lower shear rate, theory consistently over-predicted viscosity, which was attributed to a progressive shift to non-isotropic initial conditions with increasing particle size. The results suggest that at a fixed Peclet number, there is an increasing tendency for the nanoplatelets to form transient, local stacks as particle size increases. The largest particles, aspect ratio ∼2200, showed unusual shear thinning and thickening behaviors that were attributed to particle flexibility. The findings demonstrate the surprising utility of theory for infinitely dilute suspensions to interpret, and in some cases quantitatively describe, the non-Newtonian viscosity of real suspensions containing high aspect ratio plate-like particles. A simple framework is proposed to interpret deviations from ideal behavior based on the local and collective behavior of the suspended nanoplatelets.

Preparation of High-Density Polymer Brushes with a Multihelical Structure

It is well-known that a mixture of isotactic and syndiotactic polymethyl methacrylate (PMMA) forms a stereocomplex consisting of a multihelical structure in which an isotactic chain is surrounded by a syndiotactic chain. Here, we report the basic structure of the stereocomplex formed when the syndiotactic PMMA chains are tethered to a silicon substrate and form a high-density polymer brush. The influence of geometric confinement was investigated by preparing the high-density polymer brushes on a flat and spherical substrate. In both cases, mixing the untethered isotactic PMMA with the grafted syndiotactic PMMA led to the formation of a stereocomplex with a multihelical structure. Static contact angle measurements showed a hindered surface mobility at the outermost surface of the polymer brush, indicating that the stereocomplex forms a crystalline structure. A syndiotactic polymer brush with substituted fluoroalkyl groups was prepared to increase the contrast for grazing incidence wide-angle X-ray diffraction (GIWAXD) measurements. The GIWAXD results verified that the stereocomplex forms a crystalline structure oriented perpendicular to the substrate with a relatively low degree of orientation.