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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 .

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polymeric liquid crystals macromesogens

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
Out line of This Lecture
  • Introduction
  • Structure-Property Relations
  • Synthesis of PLCs Strategies and Methods
  • Application PLCs
  • Final comments
learning objectives
Learning Objectives

After completing this lecture you should have an understanding of and be able to demonstrate, the following terms, ideas and methods.

  • What are polymers?
  • The different types of polymerization reactions.
  • The different types of liquid crystal polymers.
  • The importance of structure-property relationship in polymers.
  • Synthesis of liquid crystal polymers.
  • Application of liquid crystal polymers.
what are polymers
What are Polymers?

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

application of polymers
Application of Polymers

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.

  • Agriculture and Agribusiness
  • Medicine and Consumer Science
  • Industry and Sports
polymerization reactions
Polymerization Reactions
  • The chemical reaction in which high molecular mass molecules
  • are formed from monomers is known as polymerization.
  • There are two basic types of polymerization,
  • Chain-reaction (or addition) and
  • step-reaction (or condensation) polymerization.
polymerization reactions7
Polymerization Reactions

Chain-Reaction Polymerization

One of the most common types of polymer reactions is chain-reaction

(addition) polymerization.

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.

Ethylene

chain reaction polymerization
Chain-Reaction Polymerization

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.

step 1 initiation
Step 1: Initiation

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.

Polymer chain

step 2 propagation
Step 2: Propagation

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.

Propagating

Polymer chain

monomer

New polymer chain

step 3 termination
Step 3: Termination

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.

Leftover

free radical

Propagating

Completed polymer chain

polymer chains

Propagating

Completed polymer chain

examples polymerisation
Examples: Polymerisation

Addition Polymers

polymerisation

Ethane

gas

Poly(ethylene), Solid

polymerisation

Methylacrylate ester

Poly(methy methacrylate)

second type step reaction polymerization
Second Type: Step-Reaction Polymerization

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.

step reaction polymerisation example nylon
Step-Reaction Polymerisation: Example: Nylon

Condensation Reaction

Hexamethylene diamine

Adipic acid

loss of water

polymerisation

Nylon 66

This polymer is known as nylon 66 because of the six carbon atoms

in both the hexamethylene diamine and the adipic acid.

example dacron or terylene
Example: Dacron or Terylene

Condensation Reaction

Polymerisation

Loss of water

Dacron or Terylene

degree of polymerization
Degree of Polymerization

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.

copolymer
Copolymer

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.

main chain liquid crystal polymers mclcps
Main Chain Liquid Crystal Polymers (MCLCPs)
  • Basically, there are two types of liquid crystal polymers;
  • Main chain liquid crystal polymers (MCLCPs)
  • 2. Side chain liquid crystal polymers (SCLCPs)
  • MCLCPs consist of repeating mesogenic (liquid crystal like) monomer
  • units (see below).
  • The monomer unit must be aniostropic and bifunctional (one function at each end)
  • to enable polymeristaion and the generation of mesophases.
  • For example, one end of a long, lath-like mesogenic unit might be a carboxylic acid
  • and other end might be an amine; condensation would sequentially link the
  • mesogenic unit together to give a liquid crystalline poly(amide)

Mesogenic unit

Linking unit

A general template for main chain liquid crystal polymers

examples of main chain polymers
Examples of Main Chain Polymers

g 65 N 135 I

MCLCPs have repeating mesogenic units

Flexible alternating hydrocarbon spacers

Racemic form

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

side chain liquid crystal polymers sclcps
Side Chain Liquid Crystal Polymers (SCLCPs)

Terminally Attached

Laterally Attached

Polymer

backbone

Spacer unit

Several methylene

units, with ester

or ether (for attachment)

Discotic

mesogenic

unit

Calamitic

mesogenic unit

A general template for side chain liquid crystal polymers

third class combined liquid crystal polymers
Third Class: Combined Liquid Crystal Polymers

Third class of liquid crystal polymers is called combined liquid crystal polymers

These polymers, combine the features of MCLCPs and SCLCPs.

Linking unit

Main chain

mesogenic

uints

spacer

Side chain

mesogenic

uints

Figure - B

Figure- A

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

types of side chain liquid crystals polymers
Types of Side Chain Liquid Crystals Polymers

Linking units

backbone

Homopolymers

Side chain

copolymers

Spacer unit

Mesogenic unit

BB (backbone)

e.g., siloxanes,

Acrylates

Methylacrylates

Ethylenes

Epoxides

BackBone copolymers

SC/BB copolymers

A range of different types of SCLCPs

mesogenic unit on mesomorphic behaviour
Mesogenic Unit on Mesomorphic Behaviour

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)

flexible spacers used in sclcps
Flexible Spacers used in SCLCPs

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.

influence of spacer length on mesomorphic properties
Influence of Spacer Length on Mesomorphic Properties

Methacrylate polymers

Where the polymers without

spacer units exhibit liquid crystalline

phases, they are of the smectic type

(a);

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 )

influence of terminal chain on mesomorphic properties
Influence of Terminal Chain on Mesomorphic Properties

R = terminal chains

n = spacer

Acrylate polymers

mesogenic side chain units
Mesogenic Side Chain Units

Cyanobiphenyl units have commonly

been incorporated into SCLCP polymers

in order to generate polymers with a +ve

dielectric anisotropy.

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

1

g 40 SA 121 I

2

g 30 SA 81 I

3

g 45 SA 93 I

length of mesogenic unit on mesomorphic properties
Length of Mesogenic Unit on Mesomorphic Properties

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.

polymer backbone on mesomorphic behaviour
Polymer Backbone on Mesomorphic Behaviour

Common, non- mesogenic polymers

Natural rubber: cis-2-

Methylbuta-1,3-diene

Super glue: methyl

α- cyanoacrylate

alkenes

Methyl group and X could be the point of mesogenic unit attachment

Poly(nitriles)

Poly(phosphazenes)

Unusual polymer backbones that been used in SCLCPs

polymer backbone on mesomorphic behaviour30
Polymer Backbone on Mesomorphic Behaviour

Nylon 6,6:

Composed of hexamethenediamine and adipic acid

Natural rubber: cis-2-

Methylbuta-1,3-diene

alkenes

Super glue: methyl α- cyano

acrylate

Common, non- mesogenic polymers

backbone flexibility on mesomorphic properties
Backbone Flexibility on Mesomorphic Properties

1

2

3

The backbone flexibility dominates for three polymers (1-3) with identical

mesogenic side chains but with methacrylate, acrylate and siloxane backbones,

repectively.

Here Tg and TN-I values fall with increasing backbone flexibility.

synthetic routes to polymeric mesogens
Synthetic Routes to Polymeric Mesogens

The nature of liquid crystals polymers means that there are two

aspects to the synthesis

  • Firstly, conventional synthesis to provide the monomer units.
  • Secondly, the polymerisation reaction that yield the desired

liquid crystals polymers

kevlar nematic phase
Kevlar: Nematic phase

diamine

Dicarboxylic acid

heat

Kevlar

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.

main chain liquid crystals polymer
Main Chain Liquid Crystals Polymer

New ester

Dimethyl terephthalate

transesterification

2000C

2800C

Ethylene glycol

Poly (ethylene terephthalate)

transesterification

heat

4-hydroxybenzoic acid

4-hydroxybenzoic acid units randomly within the new polymer chain to generate a MCLCPS

This polymer prepared by transesterification

siloxane backbone based lcp
Siloxane Backbone Based LCP

Siloxane

backbone

Alkenic moiety

polysiloxanes

final comments
Final Comments

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.