Liquid Crystals Structure of liquid crystalline dendrimersLiquid crystals (or mesophases) generally consist of elongated or flat molecules that pack into an ordered fluid phase. The order is less complete than in a crystal but more than in a simple liquid. For rod shaped molecules (called mesogenic units) the tendency for the molecules to lie parallel, to produce orientational order, is responsible for the formation of the nematic phase. Other intermolecular interactions lead to the layers that are characteristic of a smectic phase. Many crystals melt to smectic and nematic mesophases before the normal liquid is reached.
Figure 1. Schematic showing orientational order of mesophases during a melt. |
The nematic phase is widely used in displays in computers, televisions etc. However there is considerable interest in combining the orientational order of the nematic with other types of molecular architecture with a view to developing new materials for display and other applications. We have been studying the structure and properties of materials formed by joining mesogenic units to the outer groups of a dendrimer to for a giant molecule. A dendrimer is a hyperbranched polymer and typically has 2N+2 terminal chains where N is known as a the generation number. For a 3rd generation liquid crystal dendrimer (shown in figure 2) there are 32 mesogenic units.
Figure 2. A 3rd generation liquid crystal dendrimer. |
We have been studying the structure of liquid crystal dendrimers using X-ray and neutron scattering to identify the phases formed. The parallel tendency of the mesogenic units is in completion with the radial tendency induced by the dendritic core. Increasing the generation number changes the molecular packing, driving the phase structure from smectic to columnar. A neutron diffraction pattern from a magnetically aligned columnar phase of a 5th generation dendrimer and the corresponding structure are shown below (a ~ 5 nm, c ~ 10 nm).
Figure 3. Neutron diffraction pattern from a magnetically aligned columnar phase of a 5th generation dendrimer. |
Figure 4. The structure corresponding to the diffraction pattern in figure 3. |
Particle suspensions in nematic hosts|
Nematic liquid crystals find widespread application because it is possible to change the
direction of their optic axis (director) by the application of a modest electric field.
We are investigating the stability and physical properties of nanoparticles suspended in
liquid crystal hosts. These offer the possibility of new responsive materials with
properties such as dichroism that can be reoriented by the application of a field.
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Figure 1. Suspension of platelike particles of clay in nematic 5CB with magnetic alignment. |
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In this case, the clay is treated with a surfactant so that the director is anchored
perpendicular to the clay and so the orientation of the director by the applied magnetic
field forces the clay to lie perpendicular to the field. Figure 2 illustrates this for a
field that is not sufficient to force the platelet completely perpendicular. (Detailed
theory in Connolly et al., J Phys-Cond. Mat. 19 156103 (2007)).
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Figure 2. Forcing clay to lie perpendicular to the applied magnetic field. |
Figure 3. Trends in the interplate distance for surfactants with different hydrocarbon chain lengths. |
Liquid crystal interfacesThe interface between a liquid crystal and a solid substrate is central to the operation of many liquid crystal devices. An alignment agent is often used to define the orientation of the director with respect to the surface. Typically the device is switched "on" by application of an electric field but, when the field is removed, it switches "off" because the director remains anchored to the surface. Thus the surface structure of nematic liquid crystal is important technologically as well as scientifically. We have been measuring the strength and amplitude of smectic layers that are induced at the surface of a nematic using neutron reflection. Many solid materials such as silicon or quartz are transparent to neutrons so the technique is ideal for probing the structure of buried interfaces. The experiments have been done using the D17 reflectometer at ILL, Grenoble and the CRISP reflectometer at ISIS, Oxon. The data from the neutron reflection experiment is complex, containing several components as shown below (figure 1). Sophisticated data analysis is required to separate the components. The structure that is defined by the data is also shown (figure 2).
Figure 1. Neutron reflection data from a liquid crystal - solid interface. |
Figure 2. Schematic of the structure defined by the data in figure 1. |
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It is found that the smectic layers are more strongly developed at certain surfaces (eg CTAB
coated silicon) and only weakly present for others (eg at the air interface). This is
quantified by the surface smectic order parameter shown below (figure 3) which shows a high and
temperature independent value for CTAB. (Details in: Lau et al., J Chem. Phys. 124, 234910
and Lau et al., Liquid Crystals 34, 333)
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Figure 3. Variation of surface smetic order parameter with temperature. |