POLARIZING MICROSCOPE

Polarizing microscope Jenapol of Carl Zeiss Jena Company is used to measure the transition temperatures by texture observation as well as to identify the liquid crystalline phases. In addition to standard microscope optics, there is a polarizer in the condenser (P) and another mounted in a slider, in the tube above the objective, called analyzer (A) (see Figures below). The polarizer and analyzer are crossed, i.e. their polarization plains are perpendicular.

Scheme of transmission of light by crossed polarizes without any birefringent material

Scheme of transmission of light by liquid crystal (LC) under polarizing microscope: polarizer (P) and analyzer (A)

If there is not any birefringent material between crossed polarizers, the light will not go through the system. If there is a birefringent substance, e.g. liquid crystal (LC) or crystal, between polarizer (P) and analyser (A), the transmission of the light will be govern by equation (1) and one can observe a characteristic texture.

I=I₀sin²(2φ)sin²(δ/2)           (1)

where φ is the angle between the optic axis of liquid crystal and a polarization plane of the polarizer and δ is phase shift given by equation (2):

δ=(2πdΔn)/λ         (2)

where Δn= n_e-n_o is the birefringence, d is the thickness of the sample, and λ is the wavelength of incoming light.

Each liquid crystalline phase gives its characteristic interference pattern known as texture, which is seen under the polarizing microscope as specially modulated and colourful picture for non-aligned liquid crystal. Texture observations are done versus temperature using mettler-toledo F82HT hot stage. Because properties of substances studied vary during phase transition therefore it causes changing of the textures observed and based on this one can identify liquid crystalline phases. During the first order transition (e.g. melting, clearing point) texture changes are very well visible contrary to the second order transition (e.g. SmC - SmA or SmC* - SmA*) where texture changes may be very subtle.