Maurice C. D. Mourad

Sol-gel transitions and liquid crystal phase transitions in concentrated aqueous suspensions of colloidal gibbsite platelets

In this paper, we present a comprehensive study of the sol-gel transitions and liquid crystal phase transitions in aqueous suspensions of positively charged colloidal gibbsite platelets at pH 4-5 over a wide range of particle concentrations (50-600 g/L) and salt concentrations (10(-4)-10(-1) M NaCl). A detailed sol-gel diagram was established by oscillatory rheological experiments. These demonstrate the presence of kinetically arrested states both at high and at low salt concentrations, enclosing a sol region. Birefringence and iridescence show that in the sol state nematic and hexagonal columnar liquid crystal phases are formed. The gel and liquid crystal structures are studied in further detail using small-angle X-ray scattering (SAXS) and cryo-focused ion beam/scanning electron microscopy (cryo-FIB-SEM). The gel formed at high salt concentration shows signatures of a sponge-like structure and does not display birefringence. In the sol region, by lowering the salt concentration and/or increasing the gibbsite concentration, the nematic phase gradually transforms from the discotic nematic (ND) into the columnar nematic (NC) with much stronger side-to-side interparticle correlations. Subsequently, this NC structure can be either transformed into the hexagonal columnar phase or arrested into a birefringent repulsive gel state with NC structure.

Formation of Nematic Liquid Crystals of Sterically Stabilized Layered Double Hydroxide Platelets

Colloidal platelets of hydrotalcite, a layered double hydroxide, have been prepared by coprecipitation at pH 11-12 of magnesium nitrate and aluminum nitrate at two different magnesium to aluminum ratios. Changing the temperature and ionic strength during hydrothermal treatment, the platelets were tailored to different sizes and aspect ratios. Amino-modified polyisobutylene molecules were grafted onto the platelets following a convenient new route involving freeze-drying. Organic dispersions in toluene were prepared of the particles with the largest size and highest aspect ratio. The colloidal dispersions prepared in this way showed isotropic-nematic phase transitions above a limiting concentration in a matter of days. The number density at the transition and the width of the biphasic region were determined and compared to theory. The orientation of the platelets in nematic droplets (tactoids) and at the isotropic-nematic interface were analyzed by polarization microscopy. It was observed that sedimentation induces a nematic layer in samples that are below the limiting concentration for isotropic-nematic phase separation. No nematic phase was observed in the initial aqueous suspensions of the ungrafted particles.

Antireflective Coatings for Glass and Transparent Polymers

Antireflective coatings (ARCs) are applied to reduce surface reflections. We review coatings that reduce the reflection of the surface of the transparent substrates float glass, polyethylene terephthalate, poly(methyl methacrylate), and polycarbonate. Three main coating concepts exist to lower the reflection at the interface of a transparent substrate and air: multilayer interference coatings, graded index coatings, and quarter-wave coatings. We introduce and discuss these three concepts, and zoom in on porous quarter-wave coatings comprising colloidal particles. We extensively discuss the four routes for introducing porosity in quarter-wave coatings through the use of colloidal particles, which have the highest potential for application: (1) packing of dense nanospheres, (2) integration of voids through hollow nanospheres, (3) integration of voids through sacrificial particle templates, and (4) packing of nonspherical nanoparticles. Finally, we address the remaining challenges in the field of ARCs, and elaborate on potential strategies for future research in this area.

Gelation versus liquid crystal phase transitions in suspensions of plate-like particles.

Gelation is a common effect in aqueous suspensions of charged colloidal clay platelets at concentrations as low as 1 wt%. However, in systems of charged gibbsite [Al(OH)3] platelets, gelation can be delayed to concentrations as high as 50 wt% depending on the ionic strength. We investigated the phase behaviour of this system approaching the state of gelation in the delicate region between attractive and repulsive states that originate from competition between Coulomb repulsion and van der Waals attraction. As a function of the ionic strength, isotropic–nematic, nematic–columnar and isotropic–columnar phase separations were observed. Moreover, compression by gravitational forces allowed us to observe phase separation that is arrested by gelation in the homogeneous suspensions.

Lyotropic Hexagonal Columnar Liquid Crystals of Large Colloidal Gibbsite Platelets

We report the formation of hexagonal columnar liquid crystal phases in suspensions of large (570 nm diameter), sterically stabilized, colloidal gibbsite platelets in organic solvent. In thin cells these systems display strong iridescence originating from hexagonally arranged columns that are predominantly aligned perpendicularly to the cell walls. Small angle X-ray scattering and polarization microscopy indicate the presence of orientational fluctuations in the hexagonal columnar liquid crystal phase. The presence of decoupling of the average platelet orientation and the column axis as well as column undulations leading to a decrease of the effective column diameter are discussed. The fact that these phenomena are particularly pronounced in the vertical direction and are enhanced toward the bottom part of the system points to the role of gravitational compaction on the structure.

Structure of the repulsive gel/glass in suspensions of charged colloidal platelets

Rheological, optical and structural properties of colloidal suspensions of charge-stabilized gibbsite platelets across the sol–gel transition region are investigated. In this work we focus on samples with a low salt content (10−4 M). While at a gibbsite concentration of 300 g l−1, a nematic–columnar phase separation is observed, an arrested state with a nematic signature and highly elastic response has been observed for a concentration of 400 g l−1. A temporal evolution of the structure of the arrested state, which leads to stronger interparticle correlations, has been observed on a timescale of 20 months. The results suggest that the arrested state develops into a glass with a columnar nematic structure.

Experimental observation of fractionated crystallization in polydisperse platelike colloids.

We have discovered that the long-term aging of the hexagonal columnar liquid-crystal phase of polydisperse gibbsite platelets leads to fractionated crystallization, that is, to the formation of coexisting columnar crystals with different periods. This process was revealed by microradian X-ray diffraction demonstrating the splitting of the Bragg intercolumnar reflections into sequences of sharper reflections. The fractionated crystallization was observed in a number of samples of sterically stabilized as well as charge-stabilized polydisperse gibbsite platelets.

Columnar liquid crystals of gibbsite platelets as templates for the generation of ordered silica structures

In this paper we describe the use of columnar liquid crystals of silica coated colloidal gibbsite platelets as templates for the generation of ordered silica structures. The colloidal liquid crystal phases can be formed in a few hours by modest centrifugation forces (100–200g). Base catalysed hydrolysis of tetraethoxysilane (TEOS) is used to deposit silica in the space between the silica coated gibbsite particles. The size of the gibbsite platelets is varied between 200 and 500 nm and the thickness of the silica coating between 5 and 15 nm making the system a very versatile templating material. Acid leaching is used to selectively remove the gibbsite material.