Plastic Crystals Versus Liquid Crystals
Like liquid crystals, plastic crystals can be considered a transitional stage between real solids and real liquids and can be considered soft matter. Another common denominator is the simultaneous presence of order and disorder. Both types of phases are usually observed between the true solid and liquid phases on the temperature scale:
- true crystal -> plastic crystal -> true liquid
- true crystal -> liquid crystal -> true liquid
The difference between liquid and plastic crystals is easily observed in X-ray diffraction. Plastic crystals possess strong long range order and therefore show sharp Bragg reflections. Liquid crystals show no or very broad Bragg peaks because the order is not long range. The molecules that give rise to liquid crystalline behavior often have a strongly elongated or disc like shape. Plastic crystals consist usually of almost spherical objects. In this respect one could see them as opposites.
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