Polymeric Liquid Crystals- macromesogens. CHM3T1 Lecture- 4. M. Manickam School of Chemistry The University of Birmingham M.Manickam@bham.ac.uk. Out line of This Lecture. Introduction Structure-Property Relations Synthesis of PLCs Strategies and Methods Application PLCs Final comments .
School of Chemistry
The University of Birmingham
After completing this lecture you should have an understanding of and be able to demonstrate, the following terms, ideas and methods.
Polymers are substances containing a large number of structural units joined by the same type of linkage.
These substances often form into a chain-like structure.
Polymers in the natural world have been around since the beginning of time.
Starch, cellulose, and rubber all possess polymeric properties.
Man-made polymers have been studied since 1832. Today, the polymer industry has grown to be larger than the aluminium, copper and steel industries combined
Polymers already have a range of applications that far exceeds that of any other class of materials available to man.
Current application extend from adhesives, coatings, foams, and packaging materials to textile and industrial fibers, composites, electronic devices, biomedical devices, optical devices, and precursors for many newly developed high-tech ceramics.
One of the most common types of polymer reactions is chain-reaction
This type of polymerization is a three step process involving two chemical entities.
The first, known simply as a monomer, can be regarded as one link in a polymer chain.
It initially exists as simple units. In nearly all cases, the monomers have
at least one carbon-carbon double bond.
Ethylene is one example of a monomer used to make a common polymer.
The other chemical reactant is a catalyst.
In chain-reaction polymerization, the catalyst can be a free-radical peroxide added in relatively low concentrations.
A free-radical is a chemical component that contains a free electron that forms a covalent bond with an electron on another molecule.
The formation of a free radical form an organic peroxide is shown below:
With (.) representing the free electron
In this chemical reaction, two free radicals have been formed from the
one molecule of R2O2.
Now that all the chemical components have been identified, we can begin to look at the polymerization process.
The first step in the chain-reaction polymerization process, initiation, occurs when the free-radical catalyst reacts with a double bonded carbon monomer, beginning the polymer chain.
The double bond breaks apart, the monomer bonds to the free radical, and the free electron is transferred to the outside carbon atom in this reaction.
The next step in the process, propagation, is a repetitive operation in which the physical chain of the polymer is formed.
The double bond of successive monomers is opened up when the monomer is reacted to the reactive polymer chain.
The free electron is successively passed down the line of the chain to the outside carbon atom.
This reaction is able to occur continuously because the energy in the chemical system is lowered as the chain grows. Thermodynamically speaking, the sum of the energies of the polymer is less than the sum of the energies of the individual monomers.
Simply put, the single bonds in the polymeric chain are more stable than the double bonds of the monomer.
New polymer chain
Termination occurs when another free radical (R-O. ), left over from the original splitting of the organic peroxide, meets the end of the growing chain.
This free-radical terminates the chain by linking with the last CH2. component of the polymer chain.
This reaction produces a complete polymer chain.
Termination can also occur when two unfinished chains bond together.
Both termination types are below. Other types of termination are also possible.
Completed polymer chain
Completed polymer chain
Step-reaction (condensation) polymerization is another common type of polymerization.
This polymerization method typically produces polymers of lower molecular weight than chain reaction and requires higher temperatures to occur.
Unlike addition polymerization, step-wise reactions involve two different types of difunctional monomers or end group that react with one another, forming a chain.
Condensation polymerization also produces a small molecular by-product (water, HCl etc.).
Below is an example of the formation of Nylon 66, a common polymeric clothing material, involving one each of two monomers, hexamethylene diamine and adipic acid, reacting to form a dimer of Nylon 66.
loss of water
This polymer is known as nylon 66 because of the six carbon atoms
in both the hexamethylene diamine and the adipic acid.
Loss of water
Dacron or Terylene
The polymerization process rarely creates polymer molecules all of which have the same number of monomers.
Therefore, any sample of the polymer materials contains polymer molecules made from different numbers of monomers.
To describe a polymer sample, we must state the average number of monomers in a polymer molecule (called the degree of polymerization) and state by how much the majority of the polymer molecules differ from this average number.
Polymers can also be made from a chemical reaction in a mixture of two types of monomers.
The result of this process is called a copolymer.
If the two types of monomers (M and m) combine at random to form the polymer, a random copolymer result ( MmMMmMmmmMmMM).
If the two monomers form short sequences of one type first( MMMM or mmmmm), which then combine to form the final polymer (MMMMmmmmMMMMMmmmm), a block copolymer result.
Finally, if short sequence of one monomer (mmmmm) are attached as side chains to a very long sequence of the other monomer (MMMMMMMMM), a graft copolymer is formed.
A general template for main chain liquid crystal polymers
g 65 N 135 I
MCLCPs have repeating mesogenic units
Flexible alternating hydrocarbon spacers
Discotic cores of polymer are
separated by long flexible chains
which again give the polymer a
sufficiently low melting point for
mesogenic behaviour. In this case,
as is common in discotic systems,
a hexagonal columnar mesophase
is exhibited (confirmed by X-ray)
The M.Wt of polymer 24,000.
C 98 Dh 118 I
units, with ester
or ether (for attachment)
A general template for side chain liquid crystal polymers
Third class of liquid crystal polymers is called combined liquid crystal polymers
These polymers, combine the features of MCLCPs and SCLCPs.
Figure - B
A general template for combined liquid crystal polymers
Side chain mesogenic units can be attached, via a spacer unit, to a mesogenic
main chain either at the linking unit Figure - A or at the mesogenic unit Figure- B
A range of different types of SCLCPs
A template structure for possible mesogenic side chain units
Typical template for some
possible mesogenic units
commonlyemployed in SCLCPs
(m and n areusually one or two)
Effect of spacer length on mesomorphic behaviour
The influence of the flexible spacer that is normally essential for the generation of
mesophases in SCLCP is of great interest.
In general, the increased ordering generated on polymerisation means that smectic phases predominate and the nematic phase is only exhibited by polymers with a short spacer and a short terminal chain.
Where the polymers without
spacer units exhibit liquid crystalline
phases, they are of the smectic type
however, a short spacer usually
generates a nematic phase (b)
Which gives way to the smectic phases as the spacer length increases
(c and d )
R = terminal chains
n = spacer
Cyanobiphenyl units have commonly
been incorporated into SCLCP polymers
in order to generate polymers with a +ve
Polymers 1-3 differ only in the unit which links the
spacer to the mesogenic unit.
Polymer 1 has a particularly high clearing point
because of the enhanced polarisability, whereas
Removal of the ether oxygen in polymer 2 has
reduced the clearing point.
The clearing point recovers by the use of an ester
linkage 3 but not to the level of polymer 1 because
of the kink in the structure.
Glass transition temperature (Tg) relates to the
polarity of the connecting unit, highest for the polar
ester unit 3 and lowest for the hydrocarbon unit 2
g 40 SA 121 I
g 30 SA 81 I
g 45 SA 93 I
The increased polarisability and increased molecular length in going from
two to four phenyl rings considerably enhances the clearing points of
these nematic polymers.
The nematic phase is probably exhibited in preference to the smectic phase
because the spacer and terminal chain lengths are short.
Polymer become more crystalline as the mesogen length increases;
again this is expected.
Common, non- mesogenic polymers
Natural rubber: cis-2-
Super glue: methyl
Methyl group and X could be the point of mesogenic unit attachment
Unusual polymer backbones that been used in SCLCPs
Composed of hexamethenediamine and adipic acid
Natural rubber: cis-2-
Super glue: methyl α- cyano
Common, non- mesogenic polymers
The backbone flexibility dominates for three polymers (1-3) with identical
mesogenic side chains but with methacrylate, acrylate and siloxane backbones,
Here Tg and TN-I values fall with increasing backbone flexibility.
The nature of liquid crystals polymers means that there are two
aspects to the synthesis
liquid crystals polymers
Kevlar exhibits a namatic phase when dissolved in sulfuric acid, and extrusion
in the nematic phase provides the great strength. It is well-known polymer material that is extremely strong and is used in bullet-proof vests in construction.
Poly (ethylene terephthalate)
4-hydroxybenzoic acid units randomly within the new polymer chain to generate a MCLCPS
This polymer prepared by transesterification
LCPs have been the subjects of much research since their realisation nearly twenty years ago.
However, no commercial application has yet been found for the more commonly encountered side chain liquid crystal polymers.
However, the combination of polymeric and liquid crystal properties is very special and further research is required to exploit fully LCPs in commercially viable new technologies.
MCLCPs have found application in high strength plastics for use in construction.
Plastics owe their strength to the orientation of the polymer chains during the extrusion process.
Polymers in a LC phase have inherently ordered chains. Accordingly, when extruded in the LC phase, polymers with extremely high strength are generated.
For example, Kevlar is produced from a lyotropic liquid crystal polymer that is extremely strong and is used in many items, such as bullet-proof vests, mooring cables and car body panels.
Further research into MCLCPs will provide designer polymers for a wide range of applications.