Determination of Liquid Crystal Phase Transitions

Visual characterization of phase transitions
The liquid crystal state is a distinct phase observed between the crystalline (solid) and isotropic (liquid) states. Several different transition phases occur with liquid crystals which include the Nematic, Smectic and Cholesteric phases.

Differential scanning calorimetry (DSC) is an excellent tool for investigating the phase transitions of liquid crystals. It can quickly and easily measure the temperatures at which melting and phase transitions occur.

Hot-stage polarization microscopy is an advanced technique that is widely used for the visual characterization of phase transitions. This technique allows the user to directly observe morphological changes in a sample as it is heated or cooled. Changes in the shape, structure and color of crystals can be observed along with their size and number. This is very valuable information within research and development and quality control.

Achieving excellent results with the right combination
The following example shows how cholesteryl myristate can be easily investigated and characterized using the METTLER TOLEDO DSC 1 combined with the FP82 hot-stage microscopy system.

The DSC curve in Figure 1 illustrates the phase behavior of cholesteryl myristate. Three liquid-liquid transitions can be observed upon heating. If the sample is directly observed under polarized light then the individual transitions can be identified.

When the FP82 hot-stage microscopy system is used in combination with the DSC 1 it is possible to directly observe and identify the individual phase transitions which correspond to each DSC peak. Figure 2 shows that the sample has changed from the solvent-crystallized form to the smectic phase at 75.5 °C. Figure 3 shows that at 79.1 °C the crystals have changed to the cholesteric phase and present a structureless gray image.

The DSC 1 combined with the FP82 hotstage microscopy system is a powerful tool for identifying and characterizing liquid crystal phase transitions. The FP82 hotstage microscopy system yields a wealth of information that is complementary to the data obtained from the DSC 1. Furthermore, polarization microscopy and DSC measurements provide a complete thermal picture of each sample.

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