Hierarchically Ordered \u03b1-Zirconium Phosphate Platelets in Aqueous Phase with Empty Liquid

Xi Lin,Sadaf Imanian,Dirk Schmelter,Horst Hintze-Bruening

doi:10.1038/s41598-019-51934-y

Abstract

Platelets of α-zirconium phosphate (α-ZrP) obtained from the reflux method in H3PO4 are successfully exfoliated into water via the intercalation of alkanol amines. With volume fractions greater than 0.02 they are stacked into tactoids of few layers with a repeat distance in the order of 10 nm. The tactoids align into nematic liquid crystalline phases with irregularly wide interstices of empty liquid. Colloidal processing involves the freeze-drying of such anisotropic fluids and the dispersion of the restacked tacoids into aqueous dispersions of colloidal polymer particles of largely varying size which occupy the otherwise empty liquid between the α-ZrP tactoids and induce piling of the tactoids into columns. Real-time SAXS on drying films and TEM of the obtained coatings demonstrate that the stacked α-ZrP platelets and the polymer particles comprising liquid dry separately without polymer intercalation, while the morphology of the obtained composites can be tuned primarily by the size of the polymer colloids. Concomitant α-ZrP hydrolysis in the exfoliation step is scrutinized as a function of amine basicity and temperature. The role of zirconium based hydrolysis products in the hierarchical α-ZrP assembly is indirectly though consistently confirmed by opposing impacts of ultra-filtration and added oxoanions on the platelets’ spacing, smoothness and aggregation. HAADF-TEM imaging of scattered, singular platelets and XRD peak analysis of the pristine solid shed light on the α-ZrP synthesis. Coexisting flakes and lacunae, both similar in size to the intra-layer crystal domains, suggest the stitching of proto-α-ZrP flakes into extended layers in accordance with our observations on the aging behaviour of α-ZrP dispersions as well as with literature data on related systems.

Highlights

Over the last decade the concept of empty liquids has been discussed in the context of the directional assembly of patchy colloids[1,2,3]
We show that the colloidal state of the exfoliated platelets is rooted in the hydrolysis of the α-zirconium phosphate (α-ZrP) which is known to accompany its intercalation by amines[36,37,38]
From numerous lyotropic liquid crystal phases that are based on charge stabilized 2D nanoparticles in water and reported in the literature, α-ZrP exfoliated by TBA33,34 and phosphatoantimonic acid (PA)[32] can be regarded as reference points for the results presented here

Summary

Introduction

Over the last decade the concept of empty liquids has been discussed in the context of the directional assembly of patchy colloids[1,2,3]. In volume fractions sufficient for the percolation of such clusters, these are commonly used to render aqueous preparations like cosmetics or paints shear-thinning, often associated with distinct yield points Contrary to these tiny, randomly oriented hectorite platelets, homogeneous alignment of 2D particles with large aspect ratios has been shown to provide a barrier function[5,6,7] and mechanical reinforcement[8] to polymer films as well as to enhance effect pigment orientation in organic coatings[9]. The stabilization of nano-platelets with polymer brushes and their exfoliation into solvent-borne, lyotropic smectic phases[15,16,17] has been used for the formulation of organic coatings via the addition of molecularly dissolved monomers and polymers[17,18,19,20,21]. Our results add new facets to the pioneering work of Kaschak et al who studied the hydrolysis of α-ZrP in the course of TBA intercalation[39]

Methods

Results

Conclusion