Polymeric Liquid Crystals-macromesogens

M. ManickamSchool of Chemistry

The University of BirminghamM.Manickam@bham.ac.uk

CHM3T1

Lecture- 4

Out line of This Lecture

Introduction

Structure-Property Relations

Synthesis of PLCs Strategies and Methods

Application PLCs

Final comments

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?

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

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

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 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.

C C

H

H

H

H

Ethylene

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:

R O O R R O. O. R+

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

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.

R O.

+ C C

H

H

H

HR O C C

H

H

H

H

.

Polymer chain

Step 2: PropagationThe 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.

R O CH2 CH2.

CH2 CH2+ R O CH2 CH2 CH2 CH2.

PropagatingPolymer chain

monomer New polymer chain

Step 3: TerminationTermination 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.

R O CH2 CH2 CH2 CH2.

R O+ . R O CH2 CH2 CH2 CH2 O R

R O CH2 CH2 R O CH2 CH2.. + R O CH2 CH2 CH2 CH2 O R

Propagating Leftover free radical

Completed polymer chain

Propagating polymer chains Completed polymer chain

Examples: Polymerisation

C CH

H

H

H

H H

H

H

H

H

HH

H

H

H

H

C CCH3H

H O

CH3O

CH2C

CH3

H3CO

O

n

polymerisation

polymerisation

Poly(ethylene), Solid

Poly(methy methacrylate)

Addition Polymers

Ethanegas

Methylacrylate ester

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

HO H2NOH

O

O

NH2+

HN

HN

HN

HN

O O6

4

6O

4

On

polymerisation

Nylon 66

Hexamethylene diamineAdipic acid

loss of water

This polymer is known as nylon 66 because of the six carbon atomsin both the hexamethylene diamine and the adipic acid.

Condensation Reaction

Example: Dacron or Terylene

OHHO

OO

HOOH

OO

OO

O

OO

O

n

+

PolymerisationLoss of water

Dacron or Terylene

Condensation Reaction

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

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)

Mesogenic unit Linking unit

A general template for main chain liquid crystal polymers

Basically, there are two types of liquid crystal polymers;

1. 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)

Examples of Main Chain Polymers

OC6H13C6H13O

O

OC6H13

O

C6H13OOC6H13

O

OC6H13C6H13O

O

C6H13O

(CH2)11(CH2)11

n

OO (CH2)6

n

g 65 N 135 I

C 98 Dh 118 I

MCLCPs have repeating mesogenic units Flexible alternating hydrocarbon spacers

Racemic form

Discotic cores of polymer areseparated by long flexible chainswhich again give the polymer asufficiently low melting point for mesogenic behaviour. In this case,as is common in discotic systems,a hexagonal columnar mesophaseis exhibited (confirmed by X-ray)The M.Wt of polymer 24,000.

Side Chain Liquid Crystal Polymers (SCLCPs)

A general template for side chain liquid crystal polymers

Calamitic mesogenic unit

Spacer unit

Polymerbackbone

Discotic mesogenic unit

Terminally Attached Laterally Attached

Several methyleneunits, with ester or ether (for attachment)

Third Class: Combined Liquid Crystal Polymers

A general template for 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.

Side chain mesogenic units can be attached, via a spacer unit, to a mesogenicmain chain either at the linking unit Figure - A or at the mesogenic unit Figure- B

Figure- A Figure - B

Side chain mesogenic uints

Main chain mesogenic uints

spacer

Linking unit

Types of Side Chain Liquid Crystals Polymers

(X)m

(X) (X)m n

(X) (Y)m n

(X) (Y)m n

A range of different types of SCLCPs

Homopolymers Side chain copolymers

BackBone copolymers SC/BB copolymers

BB (backbone)e.g., siloxanes,AcrylatesMethylacrylatesEthylenesEpoxides

Mesogenic unit

Spacer unit

Linking unitsbackbone

Mesogenic Unit on Mesomorphic Behaviour

AO

OO

O NN

CN

H

B-CN -OR -R -NO2 -F -Cl

X -CN -CH3 -R -NO2 -F -Cl

A B

X X

m n

spacer

A template structure for possible mesogenic side chain units

Typical template for somepossible mesogenic units commonlyemployed in SCLCPs (m and n areusually one or two)

Flexible Spacers used in SCLCPs

O

O

O O

O O O O

O

O O

OEffect 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

O(H2C)O nC

OCH2

H3COCH3

Transition Temperatures (0C)Spacer Length (n)

(a) 0

(b) 2

(c) 6

(d) 11

S 255 I

g 120 N 152 I

C 119 S 136 I

g 54 SC 87 SA 142 I

Where the polymers without spacer units exhibit liquid crystallinephases, they are of the smectic type(a);

however, a short spacer usuallygenerates a nematic phase (b)

Which gives way to the smectic phases as the spacer length increases (c and d )

Methacrylate polymers

Influence of Terminal Chain on Mesomorphic Properties

O

O

RO(H2C)O n

C

OCH2

H

R n Transition Temperatures (0C)

CH3O

C4H9O

CH3O

CH3O

C4H9O

CH3O

CH3O

C4H9O

C6H13O

0

0

2

2

2

6

6

6

6

g 110 C 180 S 296 I

g 120 C 180 S 321 I

g 25 C 55 S 116 I

g 62 N 116 I

g 30 C 64 S 119 S 154 I

g 5 C 20 S 86 S 104-118 I

g 35 S 97 N 123 I

g 5 C 30 S 103 N 114 I

g 28 S 130 I

Acrylate polymers

R = terminal chainsn = spacer

Mesogenic Side Chain Units

O(H2C)O11

C

OH2C

H3CCN

(H2C)O11

C

OH2C

H3CCN

O CNO

10C

OH2C

H3C(CH2)

O

g 40 SA 121 I

g 30 SA 81 I

g 45 SA 93 I

Cyanobiphenyl units have commonlybeen incorporated into SCLCP polymersin order to generate polymers with a +vedielectric anisotropy.

Polymers 1-3 differ only in the unit which links thespacer 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 esterlinkage 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 polarester unit 3 and lowest for the hydrocarbon unit 2

1

2

3

Length of Mesogenic Unit on Mesomorphic Properties

O

O

OCH3

OSiO

(CH2)3

n

n Transition Temperatures (0C)

1 g 15 N 61 I

2 C 139 N 319 I

3 C 200 N 360 I

The increased polarisability and increased molecular length in going fromtwo to four phenyl rings considerably enhances the clearing points of these nematic polymers.

The nematic phase is probably exhibited in preference to the smectic phasebecause 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

H2CCN

O

CH3

OH2CX

H

Common, non- mesogenic polymers

Natural rubber: cis-2-Methylbuta-1,3-diene

Super glue: methyl α- cyanoacrylatealkenes

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

N C

R

N P

R

R'

Unusual polymer backbones that been used in SCLCPs

Poly(phosphazenes) Poly(nitriles)

Polymer Backbone on Mesomorphic Behaviour

HN N

H

O

O

H2CCN

O

CH3

O H2CX

H

Common, non- mesogenic polymers

X Name

H

CH3

Cl

CN

Ph

polyethylene

polyproplene

PVC

Orlon, Acrilan

polystyrene

Nylon 6,6: Composed of hexamethenediamine and adipic acid

Natural rubber: cis-2-Methylbuta-1,3-diene

Super glue: methyl α- cyanoacrylate

alkenes

Backbone Flexibility on Mesomorphic Properties

CH3C

CH2

O R

O

CH

CH2

O R

O SiH3C

O

CH2R

g 96 N 121 I g 47 N 77 I g 15 N 61 I

O(H2C)OCH3O

OR =

1 23

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

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

NH2H2NOHHO

OO

NHOO

OHN

NH

HN O

+

heat

Kevlar

Kevlar exhibits a namatic phase when dissolved in sulfuric acid, and extrusionin 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.

Dicarboxylic aciddiamine

Main Chain Liquid Crystals Polymer

OMeMeO

O

O

OHHO

OOO

OOH

HO

OO

OO

HO OH

O

OOO

O

OO

+ 2000C2800C

heat

Poly (ethylene terephthalate)

Dimethyl terephthalate

Ethylene glycol

New ester

4-hydroxybenzoic acid

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

This polymer prepared by transesterification

transesterification

transesterification

Siloxane Backbone Based LCP

CNOO

O(H2C)

H

HH

4Si

O

HMe

H2PtCl6

+

CNOO

O

SiMe

O

6

polysiloxanes

Alkenic moiety

Siloxane backbone

Final CommentsLCPs 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.