Liquid crystals have orientational order and properties that are directionally dependent, existing in states between solid and liquid. They are classified into lyotropic, polymeric, and thermotropic types. Thermotropic liquid crystals, which are most widely used, can exist in nematic, cholesteric, or smectic phases depending on temperature. The properties of liquid crystals are determined by their molecular structure, particularly the arrangement of aromatic rings, linking groups, terminal groups, and any attached side chains. Mixtures and composites are important for applications as they allow for tailoring of properties within broader temperature ranges than single component liquids.

1. Introduction to Liquid Crystals
States of Matter
What is Liquid Crystal?
Classification

2. Isotropic
vs.
Anisotropic
Liquids and gases
(uniform properties in
all directions).
Liquid Crystals
have orientational order

3. Classifications
• Lyotropic, Polymeric, and Thermotropic Liquid
Crystals
• Nematic, cholesteric, smectic, and ferroelectric.
• Small molecular, polymer

4. 2. Molecular structure and Chemical
Composition
•A side chain R, two or more aromatic rings A and
A’, connected by linkage groups X and Y, and at
the other end connected to a terminal group R’.

5. Examples
N-(4- Methoxybenzylidene)-4-butylaniline
(MBBA) molecule

6. Consequence
• All the physical and optical properties of liquid
crystals are governed by the properties of these
constituent groups and how they are chemically
synthesized together: Dielectric constants, elastic
constants, viscosities, absorption spectra, transition
temperatures, existence of mesophases, anisotropies,
and optical nonlinearities
• Molecules are quite large and anisotropic, practically
impossible to treat all the possible variations in the
molecular architecture and the resulting changes in
the physical properties.
• Some generally applicable observations on the
dependence of the physical properties on the
molecular constituents.

7. Lyotropic, Polymeric, and
Thermotropic Liquid Crystals
• Lyotropic Liquid Crystals are obtained when
an appropriate concentration of a material is
dissolved in some solvent; Ex: Soap; Are of
interest in biological studies.
• Polymeric Liquid Crystals are basically the
polymer versions of the monomers

8. Thermotropic Liquid Crystals
• Most widely used and extensively studied
Liquid Crystals
• There are 3 basic phases : Nematics,
Cholesterics, and Smectics

9. Liquid Crystal Phases
Nematic, Smectic & Cholesteric
Anisotrpic substances may go through one or
several Liquid Crystal Phases

10. Nematic and Cholesteric

11. Smectic-A and Smectic-C

12. 4. Mixtures and Composites
• temperature ranges for pure liquid crystals are
quite limited.
• Industrial applications employ mostly mixtures,
composites, or doped liquid crystals with tailor-
made physical and optical properties.
• The optical properties, dielectric anisotropies,
and viscosities are very different from those of
the individual mixture constituents.
• Creating mixtures is an art, guided of course by
some scientific principles.