Transcript Polymeric Liquid Crystals

Polymeric Liquid Crystalsmacromesogens
CHM3T1
Lecture- 4
M. Manickam
School of Chemistry
The University of Birmingham
[email protected]
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.
H
H
C
C
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 +
H
C
H
H
C
H
R
O
H
C
H
H
.
C
H
Polymer chain
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.
R O CH2
Propagating
Polymer chain
.
CH2 + CH2
CH2
monomer
R O CH2
.
CH2 CH2 CH2
New polymer chain
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.
R O CH2
Propagating
R O CH2
Propagating
.
.
CH2 CH2 CH2 + R O
Leftover
free radical
.
CH2 + R O CH2
.
CH2
polymer chains
CH2 CH2 CH2 O
R O CH2
R
Completed polymer chain
R O CH2
CH2 CH2 CH2 O
Completed polymer chain
R
Examples: Polymerisation
Addition Polymers
H
H
H
C
H
H
Ethane
gas
H
C
C
H
HH
polymerisation
C
H
HH
H H
H
H H
Poly(ethylene), Solid
CH3
CH3
polymerisation
H2C
H
C
O
O
O
CH3
Methylacrylate ester
H3CO
n
Poly(methy methacrylate)
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 byproduct (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
Condensation Reaction
O
NH2
OH
HO
+
O
Adipic acid
Hexamethylene diamine
loss of water
polymerisation
H
N
H2N
4
H
N
6
H
N
H
N
4
6
O
O
O
O
n
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
Condensation Reaction
O
O
OH
+
HO
HO
OH
Polymerisation
Loss of water
O
O
O
O
O
O
O
O
n
Dacron or Terylene
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)
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)
Mesogenic unit
Linking unit
A general template for main chain liquid crystal polymers
Examples of Main Chain Polymers
g 65 N 135 I
MCLCPs have repeating mesogenic units
Racemic form
Flexible alternating hydrocarbon spacers
O
O
(CH2)6
n
C6H13O
O
C6H13O
OC6H13
C6H13O
O (CH2)11 O
OC6H13
C6H13O
C 98 Dh 118 I
OC6H13
O (CH2)11
OC6H13
n
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.
Side Chain Liquid Crystal Polymers (SCLCPs)
Terminally Attached
Polymer
backbone
Laterally Attached
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 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
Figure- A
Side chain
mesogenic
uints
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
Types of Side Chain Liquid Crystals Polymers
Linking units
backbone
Homopolymers
(X)m
Side chain
copolymers
(X)m
(X)n
(X)m
(Y)n
Spacer unit
Mesogenic unit
(X)
m
(Y)
n
BackBone copolymers
BB (backbone)
e.g., siloxanes,
Acrylates
Methylacrylates
Ethylenes
Epoxides
A range of different types of SCLCPs
SC/BB copolymers
Mesogenic Unit on Mesomorphic Behaviour
A template structure for possible mesogenic side chain units
X
spacer
X
A
Typical template for some
possible mesogenic units
commonlyemployed in SCLCPs
(m and n areusually one or two)
B
m
n
O
O
A
B
X
O
-CN
-CN
-OR
-CH3
H
N
O
-R
-R
-NO2
-NO2
N
-F
-F
C
-Cl
-Cl
N
Flexible Spacers used in SCLCPs
O
O
O
O
O
O
O
O
O
O
O
O
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
Methacrylate polymers
H3C
C
O
O
CH2
Spacer Length (n)
(H2C) O
n
Transition
Temperatures (0C)
(a) 0
S 255 I
(b) 2
g 120 N 152 I
(c) 6
C 119 S 136 I
(d) 11
g 54 SC 87 SA 142 I
OCH3
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
H C
O
CH2
O
O
(H2C) O
n
O
R
R = terminal chains
n = spacer
Acrylate polymers
R
n
Transition Temperatures (0C)
CH3O
0
g 110 C 180 S 296 I
C4H9O
0
CH3O
CH3O
2
g 120 C 180 S 321 I
g 25 C 55 S 116 I
2
g 62 N 116 I
C4H9O
2
g 30 C 64 S 119 S 154 I
CH3O
6
g 5 C 20 S 86 S 104-118 I
CH3O
6
C4H9O
6
g 5 C 30 S 103 N 114 I
C6H13O
6
g 28 S 130 I
g 35 S 97 N 123 I
Mesogenic Side Chain Units
H3 C
C
O
(H2C)
11
O
O
H2C
H3C
CN
1
C
O
g 40 SA 121 I
(H2C)
O
Polymers 1-3 differ only in the unit which links the
spacer to the mesogenic unit.
CN
11
H2C
2
C
g 30 SA 81 I
O
O
(CH2)
O
CN
10
H2C
3 g 45 SA 93 I
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.
O
H3 C
Cyanobiphenyl units have commonly
been incorporated into SCLCP polymers
in order to generate polymers with a +ve
dielectric anisotropy.
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
Length of Mesogenic Unit on Mesomorphic
Properties
Transition
Temperatures (0C)
n
O
Si
O
(CH2)3 O
1
O
g 15 N 61 I
OCH3
2
C 139 N 319 I
n
3
C 200 N 360 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.
Polymer Backbone on Mesomorphic
Behaviour
Common, non- mesogenic polymers
CN
H
H2 C
H2C
O
X
Natural rubber: cis-2Methylbuta-1,3-diene
O
CH3
Super glue: methyl
α- cyanoacrylate
alkenes
Methyl group and X could be the point of mesogenic unit attachment
R
R
N P
N C
R'
Poly(phosphazenes)
Poly(nitriles)
Unusual polymer backbones that been used in SCLCPs
Polymer Backbone on Mesomorphic Behaviour
O
H
N
O
N
H
Nylon 6,6:
Composed of hexamethenediamine and adipic acid
CN
H2C
O
Natural rubber: cis-2Methylbuta-1,3-diene
X
H
O
H2C
X
CH3
Super glue: methyl α- cyano
acrylate
alkenes
H
CH3
Cl
Name
polyethylene
polyproplene
PVC
CN
Orlon, Acrilan
Ph
polystyrene
Common, non- mesogenic polymers
Backbone Flexibility on Mesomorphic
Properties
O
R=
(H2C) O
1
O
OCH3
2
3
O R
H3C
C
CH2
g 96 N 121 I
O
O
O R
H
C
CH2
g 47 N 77 I
O
H3C
Si CH2R
g 15 N 61 I
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
HO
O
OH
H2N
+
O
NH2
diamine
Dicarboxylic acid
heat
O
O
H
N
HN
NH
NH
O
O
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
O
MeO
OMe
O
HO
Dimethyl terephthalate
OH New ester
O
O
transesterification
2000C
+
HO
O
O
2800C
OH
Ethylene glycol
O
O
O
O
Poly (ethylene terephthalate)
HO
OH
O
transesterification
heat
4-hydroxybenzoic acid
O
O
O
O
O
O
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
H
H
O
Me
Si
H
+
H
Alkenic moiety
O
4 (H2C) O
CN
O
H2PtCl6
O
Me
Si
6
O
polysiloxanes
O
O
CN
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.