Phases of Liquid Crystals
Just as there are many varieties of solids and liquids, there is also a variety of liquid crystal substances. Depending on the temperature and particular nature of a substance, liquid crystals can be in one of several distinct phases. Most liquid crystal compounds exhibit polymorphism. The term mesophase is used to describe the “Subphases” of liquid crystal materials. Mesophases are formed by changing the amount of order in the sample, either by imposing order in only one or two dimensions, or by allowing the molecules to have a degree of translational motion. The liquid crystals have many phases such as nematic,smectic,cholesteric and columnar phases.
The mechanism by which liquid crystalline order is favored can be illustrated through an analogy between molecules and grains of rice. Collisions of molecule’s require energy, so the greater the energy, the greater the tolerance for collisions. If rice grains are poured into a pan, they fall at random positions and orientations and tend to jam up against their neighbours. This is similar to the liquid state illustrated in Fig.7.2D. After the pan is shaken to allow the rice grains to readjust their positions, the neighbouring grains tend to line up. The alignment is not perfect across the sample owing to defects, which also can occur in nematic liquid crystals. When all grains align, they have greater freedom to move before hitting a neighbour than they have when they are disordered. This produces the nematic phase, illustrated in Figure.7.2C. The freedom to move is primarily in the direction of molecular alignment, as sideways motion quickly results in collision with a neighbour. Layering the grains, as illustrated in Figure.7.28, enhances sideways motion. This produces the smectic phase. In the smectic phase some molecules have ample free volume to move in, while others are tightly packed the lowest-energy arrangement shares the free volume equitably among molecules. Each molecular environment matches all others, and the structure is a crystal like that illustrated in Figure.7.2A.
Fig.7.2. Arrangement of molecules in different phases.
Optical polarizing microscopy is a standard tool in the identification of liquid crystal phases and phase transitions but requires considerable experience. particularly in the study of new and less familiar materials. X-Rays provide a much more definitive means for the identification of mesophase. Differential Scanning Calorimetry (DSC) is a useful tool which complements optical methods in the study of liquid crystal phase transitions. It is utilized in determining the heat supplied or extracted during a process such as a phase transition.
A phase diagram bar illustrating the heating process is placed just above the temperature axis (at one atmosphere pressure). This includes a crystal to liquid crystal (smectie A) transition at 55°C followed by a barely detectable smectic to nematic transition at 67°C and finally the nematic to isotropic (NI) transition near 80° C. The upper cooling, curve shows a slight displacement of the NI transition, partially due to super cooling and partially instrumental hysteresis attributable to the temperature scan rate. The smectic A to crystal transition is depressed strongly due to super cooling of the smectic A phase. Thus, the phase diagram for the cooling process would not be identical to that for heating.
Fig.7.3. DSC plot with phase diagram
The nematic liquid crystal phase is characterized by molecules that have no positional order but tend to point in the same direction (along the director). In the following diagram, notice that the molecules point vertically but are arranged with no particular order.
Ncmatic liquid crystals are subdivided into the ordinary nematic and the cholesteric-nematic. The molecules in the ordinary nematic structure maintain a parallel or nearly parallel arrangement to each other along the long molecular axes. They are mobile in three directions and can rotate about one axis. This structure is one-dimensional. When the nematic structure is heated, it is generally transformed into the isotropic liquid. The nematie structure is the highest-temperature mesophase in the thermotropic liquid crystals. The energy required to deform a ncmatic liquid crystal is so small that even the slightest perturbation caused by a dust particle can distort the structure considerably.
In the cholcsteric-nematic structure, the direction of the long axis of the molecule in a given layer is slightly displaced from the direction of the molecular axes of the molecules in an adjacent layer.
Smectic the most common arrangement, creates layers of molecules. The word “smectic” is derived from the Greek word for soap. This seemingly ambiguous origin is ‘explained by the fact that the thick, slippery substance often found at the bottom of a soap dish is actually a type of smectic liquid crystal.
The smectic state is another distinct mesophase of liquid crystal substances. Molecules in this phase show a degree of translational order not present in the nematics. In the smectie state, the molecules maintain the general orientational order of nematics, but also tend to align themselves in layers or planes. Motion is restricted to within these planes, and separate planes are observed to flow past each other. The increased order means that the smectic state is more “solid-like” than the’nematic. The term smevic covers all thermotropic liquid crystals that are not nematics. Many compounds are observed to form more than one type of smectic phase. As many as 12 of these variations have been identified. The alphabetic subscripts only indicate the order in which the smectic structures were first recognized and identified. The molecules (except in smectic D) are arranged in layers with their long axes parallel to each other. They can move in two directions in the plane and can rotate about one axis. Those within layers can be in neat rows or randomly distributed.
Smcctic liquid crystals may have structured or unstructured strata. Structured smectic liquid crystals have long-range order in the arrangement of molecules in layers to form a regular two-dimensional lattice. The most common of the structured liquid crystals is smectie B. Molecular layers are in well-defined order, and the arrangement of the molecules within the strata is also well-ordered. The long axes of the molecules lie perpendicular to the plane of the layers. In the smectic A structure, molecules are also packed in strata, but the molecules in a stratum are randomly arranged. The long axes of the molecules in the smectic A structure lie perpendicular to the plane of the layers. Molecular packing in the smectic C is the same as that in smectic A, except the molecules in the stratum are tilted at an angle to the plane of the stratum.
There is also a unique kind of liquid crystal known as the smectic D which is isotropic, but nevertheless shows three-dimensiontional order in the molecular packing of the structure. In the smectic-A mesophase, the director is perpendicular to the smectic plane, and there is no particular positional order in the layer. Similarly, the smectic-B mesophase orients with the director perpendicular to the smectic plane, but the molecules are arranged into a network of hexagons within the layer. In the smectic-C mesophase, molecules are arranged as in the smcctic-A mcsophase, but the director is at a constant tilt angle measured normally to the smectic plane.
As in the nematic, the smectic-C mesophasc has a chiral state designated C*. Consistent with the smectic C. the director makes a tilt angle with respect to the smeetic layer. The difference is that this angle rotates from layer to layer forming a helix. In other words, the director of the smectic C* mesophase is not parallel or perpendicular to the layers, and it rotates from one layer to the next
Cholesteric liquid crystals are also known as chiral nematic. In this phase, the molecules twist slightly from one layer to the next, resulting in a spiral formation. These liquid crystals consist of series of stacked layers within each of which the molecules are packed with their long axes in the plane of the layer similar to nematic liquid crystals. The molecular axes in adjacent, layers are no parallel but they point in slight different directions above and below. These liquic crystals received their names from the fact that many derivatives of cholesterol form this structure. The slight twist in the planes of these structures tends to make these crystals coloured. Cholesteric liquid crystals show variation in colour due to changes in the amount of twisting. These changes depend upon temperature, pressure. magnetic and electric fields and trace additives.
The cholesteric (chiral nematie) liquid crystal phase is typically composed of nematie mesogenic molecules containing a chiral center which produces intermolecular forces that favour alignment between molecules at a slight angle to one another. This leads to the formation of a structure which can be visualized as a stack of very thin 2-D nematic-like layers with the director in each layer twisted with respect to those, above and below. In this structure, the directors actually form in a continuous helical pattern about the layer normal.
Chiral compounds show the cholesteric-nematic structure (twisted nematic). for example the cholestric esters. If a twist is applied to a molecular packing. a helical structure is formed. The helix has a pitch which is temperature-sensitive. The helical structure serves as a diffraction grating for visible light.