Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages Liquid Crystals Order and disorder – molecular position
Liquid Crystals AimsBefore you startIntroductionOrder and disorder – molecular orientationOrder and disorder – molecular positionDefectsOptical properties – birefringence in nematicsOptical properties – birefringence in chiral nematicsOptical properties – observing defectsObserving phase transitionsCommercial usesSummaryQuestionsGoing further
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Order and disorder – molecular position

In the introduction we stated that whilst liquid crystals have high orientational order, their positional order is very low. However certain positional arrangements are possible. In general, calamitic liquid crystals can be divided into three different mesophases:

Nematic:
Nematic liquid crystals have no positional order – they only have orientational order.

 Diagram of nematic crystals

Smectic:

Smectic liquid crystals consist of molecules arranged into separate layers. However, there is no further positional order within the layers themselves.		 Diagram of smectic crystals

Chiral Nematic:
In chiral nematic liquid crystals we see a helical structure, where the director vector is rotated slightly in each subsequent layer of molecules – the distance along the axis between two molecules with parallel director vectors is called the pitch of the liquid crystal.

Their name derives from the fact that they are easily made by mixing a nematic with a chiral substance (which does not have to be a liquid crystal itself). Historically, they were also known as cholesteric liquid crystals as the first molecules found to display these properties were those related to cholesterol. 		Diagram of choleristic crystals

As we will later see, the different degrees of positional ordering lead to very different optical properties.

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