Alignment of layered double hydroxide platelets

A new generation of ordered, mineral-reinforced polymer nanocomposites is emerging for which assembly of platelets into planar-random array provides the reinforcement structure. Motivated by this goal, Co–Al layered double hydroxide (LDH) tactoids (diameter ∼3 μm, thickness ∼30 nm) were synthesized and their colloidal stability in water as a function of pH was characterized by sedimentation and zeta-potential measurements. The tactoids in both stable and unstable suspensions assembled into sheet materials with a layered planar random arrangement during filtration presenting a putative reinforcement structure. Exfoliated nanosheets, characterized by the disappearance of diffraction peaks in XRD patterns and by AFM, were also assembled and produced a region of 10 μm next to the filtration membrane which was highly ordered but in which the order reduced at further distances because of higher entropic undulation of very thin platelets at much reduced filtration rates. Reassembly of these exfoliated platelets confirmed that more ordered parallel structures are obtained with tactoids. Such structures resemble that of nacre and could serve as reinforcements for subsequent infiltration by matrix resins in the same way that carbon fibre woven mat is prepared for fibre reinforced polymers.