Transcript Slide 1

DEFINITION:
The word polymer is derived from the two greek words
poly
e.g.
many
and
mers
parts or units
mer
H H H H H H
C C C C C C
H H H H H H
mer
H H H H H H
C C C C C C
H Cl H Cl H Cl
Polyethylene (PE)
Polyvinyl chloride (PVC)
mer
H H H H H H
C C C C C C
H CH3 H CH3 H CH 3
Polypropylene (PP)
Polymers are macro molecules formed by linking smaller
molecules repeatedly, called monomers.
Examples:
Polyethylene is formed by linking a large number of
ethylene molecules
H H
H
H
Polymerisation
C C
n
C C
H
H
n
H H
Ethylene
polyethylene
polystyrene is formed by linking styrene molecules
H
n C
C
H
H
styrene
Polymerisation
H
H
C
C
H
n
polystyrene
Classification of Polymers
Polymers can be classified in several ways, based on
origin
structure
methods of formation
response to heat
properties (or applications)
The number of repeating units (n) in the chain is
known as the degree of polymerization.
e.g.,
D.P.
Polymers with high degree of polymerization are called
high polymers and these have very high molecular weights
(104 to 106).
Polymers with low degree of polymerization are called
oligomers.
Based on the origin
polymers can be classified as
Natural polymers
synthetic polymers
Natural polymers are those which are obtained naturally
e.g., Cellulose, Silk, Starch, RNA, DNA, Proteins etc.,
Synthetic polymers are those which are made by man
e.g., polyethylene, polystyrene, PVC, polyester, etc.,
semi-synthetic polymers which are chemically modified
natural polymers
e.g., cellulose acetate, cellulose nitrate, halogenated
rubbers etc.,
Based on the molecular structure
polymers can be classified as
Linear
Branched
Cross-linked
In linear polymers,
the monomeric units combine linearly with each other
secondary bonding
Branch polymers
Cross linked polymers
Based on the method of formation
Addition polymers
Condensation polymers
Addition polymers are formed by self-addition of monomers
The molecular mass of a polymer is an integral multiple of
the molecular mass of a monomer
Condensation polymers are formed by condensation reaction
i.e., reaction between two or more monomer molecules
with the elimination of simple molecules like water,
ammonia, HCl etc.,
Based on the response to heat
Thermo softening
Thermosetting
thermosoftening or thermoplastics
soften on heating and can be converted into any shape
and can retain its shape on cooling
thermosetting polymers
under go chemical change on heating and convert
themselves into an infusible mass
Based on the properties or applications
Plastics
Elastomers
Fibers
Resins
Plastics
The polymers, which are soft enough at some temperature
to be moulded into a desired shape and hardened on cooling
so that they can retain that shape.
e.g., polystyrene, polyvinylchloride, polymethylmethacrylate etc.,
Elastomers
The polymers in which the structural units are either zig zag or
in helical chains.
They undergo elastic changes when subjected to an external force
but readily regain their original shape when the force is withdrawn
e.g., natural rubber, synthetic rubbers, silicone rubbers etc.,
Fibers
In these polymers, the molecular chains are arranged parallel to
each other in a spiral or helical pattern and
the molecular length is at least 100 times its diameter
e.g., nylons, terylene, etc.,
Resins
These polymers have a glossy appearance
These constitutes the major essential part of the plastics
These suffers the polymerization reactions and impart
different properties to plastics
e.g., polysulphide sealants, epoxy adhesives, etc.,
Functionality
the number of reactive sites or bonding sites
Some mono functional hydrocarbons
Alcohols
Methyl alcohol
Ethers
Dimethyl Ether
Acids
Acetic acid
Aldehydes
Formaldehyde
Aromatic
hydrocarbons
Phenol
Some bi functional hydrocarbons
adipic acid (hexanedioic acid)
Terephthalic acid
ethylene glycol
1,6-hexanediamine
Stereo regular polymers (or) Tacticity of Polymers
Isotactic
On one side
Syndiotactic
Alternating sides
Atactic
Randomly placed
- Conversion from one stereoisomerism to another is not possible by simple
rotation about single chain bond; bonds must be severed first, then reformed!
Types of
Polymerisation
Polymerisation occurs basically in two different modes.
• addition (chain growth) polymerization
• condensation (step growth) polymerization
• Addition
– monomers react through stages of initiation,
propagation, and termination
– initiators such as free radicals, cations, anions opens
the double bond of the monomer
– monomer becomes active and bonds with other such
monomers
– rapid chain reaction propagates
– reaction is terminated by another free radical or
another polymer
• condensation
- two monomers react to establish a covalent bond
- a small molecule, such as water, HCl, methanol or CO2 is
released.
- the reaction continues until one type of reactant is used
up
DISTINGUISHING FEATURES OF
ADDITION AND CONDENSATION POLYMERISATION
ADDITION
Monomers undergo self addition to
each other without loss of by products
It follows chain mechanism
CONDENSATION
Monomers undergo intermolecular
condensation with continuous elimination
of by products such as H2O, NH3, HCl, etc.,
It follows step mechanism
Unsaturated vinyl compounds undergo Monomers containing the functional
addition polymeristion
groups (-OH, -COOH, -NH2, ….) undergo
this polymerization
Monomers are linked together
Covalent linkages are through
through C – C covalent linkages
their functional groups
High polymers are formed fast
Linear polymers are produced
with or without branching
e.g., polystryrene, plexiglass,
PVC, etc.,
The reaction is slow and the polymer
molecular weight increases steadily
throughout the reaction
Linear or cross linked polymers
are produced
e.g., nylons, terylene, PF resins, etc.,
CoPolymers
• Random, Alternating, Blocked, and Grafted
• Synthetic rubbers are often copolymers.
e.g., automobile tires (SBR)
Styrene-Butadiene Rubber random polymer
Addition polymerization can be explained on the basis of
free radical mechanism
It involves three stages
viz.,
(i) Initiation
(ii) Propagation
(iii) termination
Initiation
I
(Initiator)
D or
u.v.light
R*
(Free radical)
R*
+
(Free radical)
H
H
C
C
X
H
Vinyl monomer
R
H
H
C
C*
H
X
(new free radical)
Propagation
The new free radicals attack monomer molecules further in quick
succession leading to chain propagation
H H
R
C
C*
H X
(Free radical)
H H H H
H H
+
C C
X H
Vinyl monomer
R
C C C C*
H X H X
(new free radical)
H
H H H H
R
H H H H
H
C C C C* + C C
H X H X
(new free radical)
R
X H
Vinyl monomer
H H
C C C C C
C*
H X H X
X
H
(another new free radical)
at m th stage,
R
H H
H
H
H H
H
C C
C
C
C
C* +
C C
H X
H
X
m-2 H
X
X H
R
H
H H
H
H
H H
C C
C
C
C
H X
H
X
m-1 H
C*
X
At some stage this chain propagation is terminated when the
free radicals combine either by coupling (combining) of the two radicals
or by disproportionation
coupling
R
R
H
H
H H
C
C
C
H
X
m-1 H
H
H
H
H
C* +
C*
C
C
C
X
X
H
X
H
H
H
H H
H
H
H
H
C
C
C
C
C
C
C
C
H
X
m-1 H
X
X
H
X
H
saturated highpolymer (dead polymer)
R
m-1
R
m-1
disproportionation
H
H
H
C* +
C*
C
C
C
X
X
H
X
H
H
H H
C
C
C
H
X
m-1 H
R
R
H
H
H
H
H
C
C
C
C
H
X
m-1 H
X
unsaturated oligomer
(dead polymer)
+
H
R
m-1
H
H
H
H
C
C
C
C
X
H
X
H
R
m-1
saturated oligomer
(dead polymer)
TECHNIQUES OF POLYMERISATION
Addition polymerization is brought about using four different
techniques
• Bulk or Mass polymerization
• Solution polymerization
• Suspension polymerization
• Emulsion polymerization
Bulk or Mass Polymerization
only the monomer and the initiator are involved
monomer is taken in the liquid state
the initiator should dissolve in the monomer
Initiation can be done either by heating or by exposing
to radiation
the reaction is exothermic
As the reaction proceeds, the reaction mixture becomes
viscous
the polymer molecules with wide range of molecular
masses will be obtained
Advantages:
The method is simple
It needs simple equipments
The percentage of conversion is high
Product obtained is pure with high optical clarity
Disadvantages:
as the reaction proceeds stirring become difficult as the
product becomes more and more viscous
Uncontrolled temperature rise may lead to
discoloration
thermal degradation
branching
cross linking
and some times explosion also
Solution Polymerization
the medium chosen is an inert solvent
the monomer, the initiator and a chain transfer agent
should dissolve in an inert solvent
The solution is heated with constant agitation
After the reaction is over,
the polymer formed may dissolve in the solvent
along with the monomer or may be precipitated
Advantages:
Solvent will reduce the viscosity of the reactant mixture
heat transfer will be better
Disadvantages:
the polymer will not be pure and has to be isolated
by chemical techniques
high molecular mass polymers will not be obtained
Suspension Polymerization
Water is used as a solvent
the monomer is suspended in water as droplets of
colloidal size
Initiators used are soluble in monomer droplets
protective colloids are added to suppress the coagulation
of the monomer molecules
The reaction mixture is heated or exposed to radiation
with constant stirring.
Polymerisation takes place inside the droplet
the polymer formed being insoluble in water, produce
spherical pearls or beads
Advantages:
Process is cheap since it uses water as a solvent
instead of costly solvents
Viscosity increase is negligible
Agitation and thermal control are easy
Product isolation is easy since the product is
insoluble in water
Product formed is pure
Disadvantages:
the method can be used only for water insoluble monomers
it is difficult to control polymer size
Emulsion Polymerisation
This method is used for water insoluble monomers
Emulsion of water and the monomer is allowed to form
Emulsion is the colloidal dispersion of a liquid in
another immiscible liquid
To maintain the system stable, a small amount of an
emulsifier will be added
Soaps and detergents are examples for emulsifiers
Emulsifier contains
a hydrophilic (water loving) polar end group (head) and
a hydrophobic (water hating) non polar end group (tail)
At very low concentration, the soap or detergent (emulsifier)
dissolves completely in water
at slightly higher concentrations, the emulsifier molecules
form aggregates, called miscelles
The monomer molecules dissolve in the hydrocarbon centre
of the miscelles
water soluble initiator will be added and the system is kept
agitated at the required temperature.
The initiator molecules diffuse into the centre of miscelles
through its polar head
Reaction takes place at the centre of the miscelles
The polymer is formed and the miscelles begins to swell
The monomer consumed inside the miscelles is replenished
by diffusion from aqueous phase
This continues till the size of the polymer is big enough
to come out of the miscelles
Advantages:
Rate of polymerization is high
polymers with higher molar masses are formed
thermal control is easy
control over the polymer molar mass is possible
no viscosity build up and hence agitation is easy
Disadvantages:
the polymer formed may contain impurities such as
the emulsifiers and coagulants
It needs further purification by other chemical techniques
GLASS TRANSITION TEMPERATURE (Tg):
Amorphous polymers do not have sharp melting points
They possess softening point
At low temperature, polymers exist as glassy substances
Since the molecular chains can not move at all easily in
this state, the solid tends to shatter, if it is hit
If the solid polymer is heated, eventually it softens and
becomes flexible
This softness and flexibility is obtained at the
glass transition temperature
So the glass transition temperature can be defined as
the temperature below which an amorphous polymer is
brittle, hard and glassy and above the temperature
it becomes flexible, soft and rubbery
Glassy state
(Hard brittle plastic)
rubber state
(soft flexible)
In the glassy state of the polymer, there is
neither molecular motion nor segmental motion
When all chain motions are not possible, the rigid solid results
On heating beyond Tg segmental motion becomes possible
but molecular mobility is disallowed. Hence flexible
The glassy state and the glass transition
• In general for ordinary compounds of low molar
mass:
• crystalline solid
melting
• liquid
A
V
B
C
• increase in volume at Tm;
F
Tm T
• slopes of FC and BA: expansion coefficients of
crystalline phase and liquid, respectively.
Non-crystallisable materials
Some materials CANNOT
crystallize, e.g. ordinary glass
Why?
Molecular structure is too
irregular
liquid
rubber
amorphous or
glassy phase
A
V
D
E
B
C
F
Tg
Tm T
•Cooling of liquid via AB continues until D
•The area BD has elastomeric properties and is the rubbery state
•D is called the glass-rubber transition, Tg = glass transition
temperature
•DE has the same slope as CF
Crystalline vs. Amorphous
Phase transitions for long-chain polymers.
Factors influencing the glass transition temperature
Glass transition temperature of a polymer depends on
parameters such as
• chain geometry
• chain flexibility
• molecular aggregates
• hydrogen bond between polymer chains
• presence of plasticizers and
• presence of substrates in the polymer chains
A polymer having regular chain geometry show high
glass transition temperature
crystalline polymers have higher Tg s than amorphous polymers
HIGH-DENSITY POLYMERS
Linear polymers with chains that can pack closely
together. These polymers are often quite rigid.
LOW-DENSITY POLYMERS
Branched-chain polymers that cannot pack together as
closely. There is often a degree of cross-linking.
These polymers are often more flexible than highdensity polymers.
the bulky groups on chain, increases the Tg of the polymer
Polyethylene
Tg = -110 0C
Polypropylene
Tg =
R
Polystyrene
Tg = 100 0C
The presence of H-bonds between the polymer molecules
increases the Tg
• e.g., the Tg of nylon 6,6 (Tg = 50 0C) is higher than PE (Tg = -110 0C)
nylon

O H 
O
H 
||  | 
||
6,6  | 
 
 N  C   N C  C   N C 
 
 
| |
|
|
|
H  H
H H 6 H
4
+
+
+
+
Hydrogen
bonds
+
+
H H
C C
H

polyethylene
H
+
+
+ bonds
+ Waals
Van der
+
+
H H
C C
O H 
O
H 
 
||  | 
||
|
 
 N  C   N C  C   N C 
 
 
| |
|
|
|
H  H
H H 6 H
4
H
H


With H-bonds vs vdW bonds, nylon is expected to have (and does) higher Tg.
The presence of a plasticizer reduces the Tg of a polymer
The plasticizers are usually dialkyl phthalate esters,
such as dibutyl phthalate, a high boiling liquid.
O
C
C
O
O CH2CH2CH2CH3
O CH2CH2CH2CH3
dibutyl phthalate
The plasticizer separates the individual polymer chains
from one another. It acts as a lubricant which reduces
the attractions between the polymer chains.
The Tg of a polymer is influenced by its molecular weight
With increase in molecular mass, the Tg increases
However, it is not significantly affected if molecular weight is
around 20000
e.g.,
PE (low Mw)
-110 0C
PE (high Mw)
- 90 0C
The glass transition temperature helps in choosing the right
processing temperature
It also gives the idea of
thermal expansion
heat capacity
electrical and mechanical properties
T
mobile
liquid
viscous
liquid
crystalline
solid
Callister,
rubber
Fig. 16.9
tough
plastic
Tm
Tg
partially
crystalline
solid
Molecular weight
Tm: melting over wide range of T depends upon history of sample,
a consequence of lamellar structure
thicker the lamellae, higher the Tm.
Tg: from rubbery to rigid as T lowers
STRUCTURE – PROPERTY RELATIONSHIP OF POLYMERS
Macromolecules show a wide range of properties which are
quite different from those of respective monomers
They may be
elastic or rigid
hard or soft
transparent or opaque
have strength of steel but can have very light weight
soften on heating or
can set to a hard mass on cooling the melt
These properties may vary from one type of polymer to
another and even among the same type
The fundamental parameters which influence the
structure-property relationship are
molecular mass
polarity
crystallinity
molecular cohesion
the nature of polymeric chains and
stereochemistry of the molecules
The properties like tensile strength, crystallinity,
elasticity, resistance to chemicals, wear and tear
depend mostly on the polymer structure
Tensile Strength
This can be discussed based on
the forces of attraction and
slipping power
Based on forces of attraction:
Strength of the polymer is mainly determined by
the magnitude and distribution of attraction forces
between the polymer chains
These attractive forces are of two different types
primary or covalent bond
secondary or intermolecular forces
In straight chain and branched chain polymers,
the individual chains are held together by
weak intermolecular force of attraction
strength increases with increase in chain length
(in turn increase in molecular weight)
as the longer chains are entangled (anchored) better
In cross-linked polymers, monomeric units are held together
only by means of covalent forces
Increase in Strength
Examples:
Linear Polymers: Polyethylene, polyvinyl chloride (PVC),
polystyrene, polymethyl methacrylate (plexiglass), nylon,
fluorocarbons (teflon)
Branched Polymers: Many elastomers or polymeric rubbers
Cross-linked Polymers: Many elastomers or polymeric
rubbers are cross-linked (vulcanization process); most
thermosetting polymers
Network Polymers: Epoxies, phenol-formaldehydes.
Based on slipping power:
It is defined as the movement of molecules one over the other
It depends on the shape of the molecule
E.g., polyethylene molecule is simple and uniform
the movement of molecules one over other is easy
i.e., slipping power is high
Hence it has less strength.
in case of poly vinyl chloride (PVC), the bulky chlorine atoms are
present along the chain length hence movement is restricted
i.e., slipping power is less
Hence PVC has higher strength than PE
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Plastic deformation
When a polymer is subjected to some stress in the form
of heat or pressure or both, permanent deformation in
shape takes place, which is known as plastic deformation
Slippage is more in case of linear molecules than branched
and cross-linked, because of the presence of only the weak
intermolecular forces
at high pressure and temperature, the weak
Vander waal’s forces between molecules become
more and more weak
Hence linear molecules show greatest degree of
plastic deformation, under pressure
Such type of materials are called thermoplastic materials
No slippage occurs in case of cross-linked molecules,
because of only primary covalent bonds are present
throughout the entire structure
i.e., plasticity does not increase with rise in temperature
or pressure or both
Such type of polymers are known as thermosetting polymers
However, when considerable external force
or temperature exceeding the stability of material is applied,
it will result in total destruction
Crystallinity
Polymers are part crystalline and part amorphous
An amorphous state is characterized by complete
random arrangement of molecules
crystalline form by regular arrangement of molecules
crystalline
region
amorphous
region
• A linear polymer will have a high degree of
crystallinity, and be stronger, denser and more rigid.
• The more “lumpy” and branched the polymer, the less dense
and less crystalline.
• The more crosslinking the stiffer the polymer.
And, networked polymers are like heavily crosslinked ones.
• Polymers with a long repeating unit or
with low degree of symmetry do not crystallize easily
• Isotactic and syndiotactic polymers are stronger and
stiffer due to their regular packing arrangement.
• Optical properties: crystalline -> scatter light (Bragg)
amorphous -> transparent.
Most covalent molecules absorb light outside visible spectrum,
e.g. PMMA (lucite) is a high clarity tranparent materials.
Which polymer more likely to crystallize? Can it be decided?
Networked
Phenol-Formaldehyde
(Bakelite)
Linear and highly crosslink
cis-isoprene
+
H
+ H20
• Networked and highly crosslinked structures are near impossible
to reorient to favorable alignment.
• Not possible to decide which might crystallize. Both not likely to do so.
Polystyrene (PS) possess greater strength when compared to
PE and PVC because of the presence of bulky phenyl group.
R
Which polymer more likely to crystallize? Can it be decided?
alternating
Poly(styrene-Ethylene)
Copolymer
random
poly(styrene - ethylene) copolymer
• Alternating co-polymer more likely to crystallize than random ones, as they
are always more easily crystallized as the chains can align more easily.
Which polymer more likely to crystallize? Can it be decided?
Linear syndiotactic polyvinyl chloride
Linear isotactic polystyrene
• For linear polymers, crystallization is more easily accomplished as
chain alignment is not prevented.
• Crystallization is not favored for polymers that are composed of
chemically complex mer structures, e.g. polyisoprene.
• Linear and syndiotactic polyvinyl chloride is more likely to crystallize.
• The phenyl side-group for PS is bulkier than the Cl side-group for PVC.
• Generally, syndiotactic and isotactic isomers are equally likely to crystallize.
Chemical Resistance
Chemical resistance of the polymer depends upon the
•
•
chemical nature of monomers and
their molecular arrangement
As a general rule of dissolution,
‘like materials attract’
‘unlike materials repel’
and
Thus a polymer is more soluble in structurally similar solvent
e.g.,
polymers containing polar groups like – OH, - COOH etc.,
usually dissolve in polar solvents like water, ketone, alcohol etc.,
but these are chemically resistant to non-polar solvents
Similarly non-polar groups such as methyl, phenol dissolve
only in non-polar solvents like benzene, toluene, etc.,
polymers of more aliphatic character are more soluble in
aliphatic solvents, hence chemical resistance is less in
aliphatic solvents and more in aromatic solvents
polymers with more aromatic groups dissolve more in
aromatic solvents, hence chemical resistance is less in
aromatic solvents and more in aliphatic solvents
Polymers containing ester groups (e.g., polyesters) undergo
Hydrolysis with strong alkalis at high temperature
Implies less chemical resistance in alkalies
Polyamides like nylon containing –NHCO– group can undergo
easily the hydrolysis by strong acid or alkali
Polymers containing residual unsaturation e.g., rubbers
(natural and some synthetic) easily undergo degradative
oxidation in air in presence of light or ozone
Because of the dissolution of polymers in suitable solvents,
there occurs softening, swelling and loss of strength of
polymer material
The tendency of swelling and solubility of polymers
in a particular solvent decreases with increase in
molecular weight
Linear polymers have lower resistivity than branched chain
and cross linked polymers
Permeability of the solvents in the polymers also depends
on crystallinity
crystalline polymers exhibits higher chemical resistance than
less crystalline polymers because of denser packing
Elasticity
Elasticity of the polymer is mainly because of the uncoiling and
recoiling of the molecular chains on the application of force
a polymer to show elasticity the individual chains should not
break on prolonged stretching
Breaking takes place when the chains slip over the other
and get separated
So the factors which allows the slippage of the molecules
should be avoided to exhibit an elasticity
The slippage can be avoided by
• introducing cross-linking at suitable molecular positions
• introducing bulky side groups such as aromatic and
cyclic groups on repeating units
• introducing non-polar groups on the chains
a polymer to show elasticity, the structure should be
amorphous
By introducing a plasticizer the elasticity of polymer
can enhance
to get an elastic property, any factor that introduces
crystallinity should be avoided
Molecular Weight of Polymers
A simple compound has a fixed molecular weight
e.g., acetone has mol. Wt. of 58 (regardless of how it is made)
in any given sample of acetone, each molecule
has the same molecular weight
This is true for all low molecular weight compounds
In contrast, a polymer comprises molecules of different
molecular weights
hence, its molecular weight is expressed in terms of an
‘average’ value
e.g., ethylene gas, which is a low mol. wt. compound
each of its molecules have the same chemical structure and
hence, a fixed molecular weight of 28
upon polymerization, it forms polyethylene and we encounter
an indefinite chemical structure of --(-CH2 – CH2 -)n—
where ‘n’ can change its value from one polyethylene
molecule to another present in the same polymer sample
When ethylene is polymerized to form polyethylene,
a number of polymer chains start growing at any instant,
but all of them do not get terminated after growing to the
same size
The chain termination is a random process
hence, each polymer molecule formed can have a
different number of monomer units and thus
different molecular weights
So, a sample polymer can be thought of as a mixture of
molecules of the same chemical type, but of different
molecular weights
In this situation, the molecular weight of the polymer can
only be viewed statistically and expressed as some average
of the Mol. Wt.s contributed by the individual molecules that
make the sample
the molecular weight of a polymer can be expressed by
two most and experimentally verifiable methods of averaging
(i) Number – average
(ii) weight – average
Number average molecular mass of a polymer can be defined
as the total mass of all the molecules in a polymer sample
divided by the total number of molecules present
The molecular mass of a polymer can use either
number fractions or the weight fractions of the
molecules present in the polymer
In computing the number average molecular mass of a
polymer, we consider the number fractions
Assume that there are n number of molecules in a
polymer sample
n1 of them have M1 molecular weight (each)
n2 of them have M2 molecular weight
ni of them have Mi molecular weight
Total no. of molecules (n) is given by
n = n1 + n2 + n3 + n4 + n5 + n6 + …………+ ni
No. of molecules in fraction 1 = n1
Number fraction of fraction 1  n1
 ni
Molecularweight contribution by fraction1 
n1M1
 ni
Similarly,
Molecular weight contribution by other fractions are
n1M1 n2M2 n3M3
;
;
;
 ni  ni  ni
n iM i
 ni
Number average molecular mass of the whole polymer
is given by
Mn 
Mn
n1M1 n2M2 n3M3 n4M4
niMi



 ............................
 ni  ni  ni  ni
 ni
n M


n
i
i
i
In computing the weight average molecular mass
of a polymer, we consider the weight fractions
Total weight of the polymer (W) is given by
W = S ni Mi
Weight of fraction 1 = W1= n1M1
weight fractionof fraction1 
n1M1
n1M1

W
 niMi
 n1M1 
M1
Molecularweight contribution by fraction1  
  niMi 


n1M12

 niMi
Molecular weight contribution by other fractions are
n1M12 n2M2 2 n3M3 2
;
;
;
 niMi  niMi  niMi
niMi 2
 niMi
Weight average molecular mass of the whole polymer
is given by
n1M12
n2M2 2
n3M32
n4M4 2
niMi 2
Mw 



 .................
 niMi  niMi  niMi  niMi
 niMi
Mw 
2
n
i
M
i

nM
i
i
Polymers: Molecular Weight
Ni: no. of molecules with degree of polymerization of i
Mi: molecular weight of i
• number average, Mn
• weight average, Mw
• Ratio of Mw to Mn is known as the polydispersity index
(PI)
– a measure of the breadth of the molecular weight
– PI = 1 indicates Mw = Mn, i.e. all molecules have equal
length (monodisperse)
– PI = 1 is possible for natural proteins whereas
synthetic polymers have 1.5 < PI < 5
– At best PI = 1.1 can be attained with special
techniques
The number-average molecular mass (Mn)is determined
by the measurement of colligative properties such as
lowering of vapour pressure
osmotic pressure
depression in freezing point
elevation in boiling point
The weight-average molecular mass (Mw) is determined by
light scattering
and
ultra-centrifugal techniques
i
Ni
Mi
NiMi
NiMi2
1
50
500
25000
12500000
2
100
1000
100000
1E+08
3
300
1500
450000
6.75E+08
4
400
2000
800000
1.6E+09
5
600
4000
2400000
9.6E+09
6
400
5000
2000000
1E+10
7
300
10000
3000000
3E+10
8
100
15000
1500000
2.25E+10
9
50
30000
1500000
4.5E+10
SUM
2300
69000
11775000
1.19E+11
M n=
5119.565
M w=
10147.56
PDI=
1.982113
Polymers: Molecular Weight
• Biomedical applications:
25,000<Mn<100,000 and
50,000<Mw<300,000
• Increasing molecular weight increases
physical properties; however, decreases
processibility
TEFLON or FLUON or Polytetrafluoroethylene (PTFE):
Preparation
F
F
C
n
F
C
F
Water emulsion
polymerization
F
F
C
C
F
F
peroxide
n
Properties
• a highly regular and linear polymer without branching
• a highly crystalline polymer with a melting point of
around 330 oC
• Its mechanical strength remains unchanged over a wide
temperature range from -100 oC to 350 oC
• It does not dissolve in any of the strong acids including
hot fuming nitric acid
• It is resistant to corrosive alkalies and known organic solvents
• It reacts with only molten alkali metals (to any significant
extent) probably, this is because fluorine atoms from the
polymer chain get removed by the alkali metals
• It has very low dielectric constant
• The conventional techniques used for the processing of
other polymers can not be applied to Teflon because
of its low melt flow rates
• The strong attractive forces between the polymer chains
gives the extreme toughness, high softening point,
exceptionally high chemical resistance
• It has high density 2.1 to 2.3 gm/cm3
• It has low coefficient of friction (low interfacial forces
between its surface and another material)
• It has very low surface energy
Uses
• It is used in making articles such as pump valves and
pipes where chemical resistance is required
• It is used in non-lubricated bearings
• It is used in non-sticking stop-cocks like burettes etc.,
• It is used for coating and impregnating, glass fibers,
asbestos fibers (to form belts), filter cloth etc.,
• It is used for products where resistance to acid and
alkalies are needed
• It is used as catheters, artificial vascular grafts etc.,
NYLON 6, 6
The aliphatic polyamides are generally known as nylons
The nylons are usually indicated by a numbering system
The nylons obtained from dibasic acids and diamines
are usually represented by two numbers
the first one indicating the number of ‘C’ atoms in the
diamine and the second that in the dicarboxylic acid
Nylons made by the self condensation of an amino acid
or by the ring opening polymerization of lactams are
represented only by a single number as in the case of
nylon 6
Polyamides are prepared by the melt poly condensation
Preparation
n
+n
Heat
- 2n H2O
Properties
• It has a good tensile strength, abrasion resistance and
toughness upto 150 oC
• It offers resistance to many solvents. However, it
dissolves in formic acid, cresols and phenols
• They are translucent, wheatish, horny, high melting
polymers (160 – 264 oC)
• They possess high thermal stability
• Self lubricating properties
• They possess high degree of crystallinity
• The interchain hydrogen bonds provide superior
mechanical strength
(Kevlar fibers stronger than metals)
• Its Hardness is similar to tin
Uses
• It is used as a plastic as well as fiber
• This is used to produce tyre cord
• It is used to make mono filaments and roaps
• Nylon 6,6 is used to manufacture articles like brushes
and bristles
• Nylon 6,6 used as sutures
P – F Resins
These are formed by condensation polymerization and
are thermosetting polymers
The phenol ring has three potential reactive sites
while the formaldehyde has two reactive sites
The polycondensation reaction between these two
are catalyzed by either acids or alkalies
The nature of the product formed depends largely
on the molar ratio of phenol to formaldehyde and also
on the nature of the catalyst
There are two important commercial PF resins
• Novolacs
• Resoles
Both novolacs and resoles are linear, low molecular
weight, soluble and fusible prepolymers
During moulding operations, these two undergo
extensive branching leading to the formation of highly
cross linked, insoluble, hard, rigid and infusible products
Novolacs
When P/F molar ratio is > 1 and the catalyst used is an acid,
low mol. wt. polymers formed are called Novolacs
The first step in the reaction is the addition of
formaldehyde to phenol to form ortho or para methylol
phenols
OH
H
+
C=O
H
Phenol (excess)
H+
formaldehyde
OH
OH
CH2OH
and
o-methylol phenol
CH2OH
p-methylol phenol
These methylol phenols condense rapidly to form Novolacs
OH
OH
CH2OH
or
o-methylol phenol
OH H2
C
HO
OH
H2
C
OH H2
C
CH2OH
p-methylol phenol
H2
C
OH
OH
Novolacs
These novolacs are linear and low mol. wt. polymers
About 5 – 6 phenol rings per molecule are linked through
methylene bridges
They are soluble and fusible
Since they contain no active methylol groups, they
themselves do not undergo cross linking
However, when heated with formaldehyde or hexamine, they
undergo extensive cross linking, resulting in the formation
of infusible, insoluble, hard and rigid thermosetting product
OH H2
C
H2
C
HO
Novolacs (prepolymer)
OH H2
C
H2
C
OH
OH
Curing with
Formaldehyde or
hexamine
Resoles
When the molar ratio of P/F is < 1 and the catalyst used is a
base, the polymer formed are called Resoles
The first step in the reaction is the formation of mono,
di and trimethylol phenols.
They undergo condensation to form resoles
OH
H
+
C=O
H
Phenol
OH--
Formaldehyde
(excess)
OH
CH2OH
OH
OH
OH
+
+
+
CH2OH
o-methylol phenol
CH2OH
CH2OH HOH2C
p-methylol
phenol
CH2OH
CH2OH
di methylol
phenol
tri methylol phenol
Curing
The resoles in which phenols are linked through
methylene bridges are soluble and fusible
Since they contain alcoholic groups, further reaction
during curing leads to cross linking, resulting in a
network, infusible and insoluble product
Properties
• These are (bakelite) set to rigid and hard
• They are scratch-resistant
• They are infusible
• They are water-resistant
• They are insoluble solids
• They are resistant to non-oxidizing acids, salts and
many organic solvents
• but are attacked by alkalis, because of the presence
of free hydroxyl group in their structures
• They possess excellent electrical insulating character
• Their Hardness is similar to copper
• These are usable up to 400 °F (204°C)
• These tends to be brittle
• The properties can be modified by fillers
& reinforcements
• These have the highest compressive strength
• These are machinable
• Phenolics are the resin in plywood
Uses
• For making electric insulator parts like switches, plugs,
switch-boards, heater-handles etc.,
• For making moulded articles like telephone parts,
cabinets for radio and television
• For impregnating fabrics, wood and paper
• As adhesives (e.g., binder) for grinding wheels
• In paints and varnishes
• As hydrogen-exchanger resins in water softening
• For making bearings, used in propeller shafts for
paper industry and rolling mills
Epoxy resins
Preparation
CH3
n Cl
OH
C
CH2 + HO
CH2 CH
O
CH3
epichlorhydrin
bis phenol
Alkaline catalyst
60 OC
-n HCl
CH3
O
C
CH3
O
CH2
CH
OH
CH2
n
In epoxy resins, n ranges from 0 to 20
The molecular weight of the epoxy resin depends upon
the relative proportions of the reactants
The epichlorhydrin acting as a chain stopper
Molecular weight ranges from 350 to 8000
It is a mobile and easy flowing liquid at a mol. Wt. of 350
It is a solid at higher mol. wt. with a melting range
of 145 oC - 155 oC
Linear epoxy resins are converted into 3D polymers
by curing with some chemicals like diethylene triamine,
triethylene tetramine and meta-phenylene diamine
Properties
• Epoxy resins have ability of getting cured, without
application of heat
• They have good resistance to chemicals
• They have less shrinkage during curing process
• They may be used in solid or liquid form
• They possess excellent electrical resistance
• Epoxy resins stick well to a number of substances
including metal and glass
• Their properties can be modified by adding compounds
like unsaturated fatty acids or amines and
some of the solvents
• No size-change upon cross-linking/hardening
This means they make ideal adhesives
Shrinkage causes adhesive failures
Adhesives require no dimensional change
• Resins can be changed to modify epoxy properties
Uses
• epoxy resins are mainly used as adhesives
• They are used for surface coatings
• Moulds are made with epoxy resins, which are used for
the production of metallic components of aircrafts
and automobiles
• They are used as laminating and casting materials
• Epoxy resins are used as potting compounds for
electrical equipment
• They are used as stabilizers for PVC resins
• Epoxy resins are used for skit-resistant surfaces, for
highways rendering a number of advantages
• Delayed wearing of road surfaces in hot and cold climates
• Excellent resistance to freezing conditions,
de-icing salts, solvents and water
• Non-porosity which protects the original pavements
from scaling and spalling
• Permanent high traction even under wet or oily conditions
• Fast curing, causing minimum interruption to the flow of
traffic
• Light weight, especially useful for surfacing bridges
• Epoxy resins are applied over cotton, rayon and
bleached fabrics to impart crease resistance and
shrinkage control
ELASTOMERS
Elastomer is defined as a long chain polymer which
under stress undergoes elongation by several times and
regains its original shape when the stress is fully released
Stretched
Returned to
randomization
Elastomers are high polymers, which have elastic
properties in excess of 300 %
The elastic deformation in an elastomer arises due to
the fact that the molecule is not a straight chained
in the unstressed condition and is in the form of a coil
Hence, it can be stretched like a spring
So, the unstretched rubber is in an amorphous state
As stretching is done, the macromolecules get partially
aligned with respect to another, thereby causing
crystallization
Consequently, stiffening of material (due to increased
attractive forces between these molecules) taking place
On releasing the deforming stress, the chains get reverted
back to their original coiled state and the material again
becomes amorphous
Natural rubber is an addition polymer formed from the
monomer called isoprene i.e., 2-methyl-1,3-butadiene
The average D.P. (n) of rubber is around 5000
Addition between molecules of isoprene takes place by
1,4 addition and one double bond shifts between 2nd and
3rd positions
As each isoprene unit contains C = C bond,
polyisoprene exists in two isomeric forms
viz., cis and trans
Cis-polyisoprene
trans-polyisoprene
where R= CH3
Natural rubber contains the cis isomer while the
gutta percha contains the trans isomer
Natural rubber consists of basic material latex, which is
a dispersion of isoprene
During the treatment, these isoprene molecules polymerize
to form long-coiled chains of cis-polyisoprene
The mol. wt. of raw rubber is about 100,000 – 150,000
Natural rubber is made from the saps of a wide range of
plants like havea brasillians and guayule, found in
tropical countries (such as Indonesia, Malaysia, Thailand,
Ceylon, India, South America, etc.,)
The rubber latex (or milky liquid rubber ) is obtained by
making incisions in the bark of the rubber trees and
allowing the saps to flow out into small vessels
Tapping is, usually done at intervals of about six months
The latex is emptied into buckets and transferred to a
factory for treatment
Gutta Percha is trans-polyisoprene and is obtained from
the mature leaves of dichopsis gutta and palagum gutta
trees (belonging to sapetaceae family)
These trees are grown mostly in Broneo, Malaya and Sumatra
Gutta percha may be recovered by solvent extraction
Alternatively, the mature leaves are ground carefully;
treat with water at about 70 oC for half an hour and
poured into cold water, then the gutta percha floats on
water surface and can be easily removed
Deficiencies of natural rubber
Natural rubber is addition product of isoprene units
and still contains a large number of double bonded
carbon atoms
Hence it exhibits a large number of deficiencies
• At low temp. it is hard and brittle but as the temp.
rises it becomes soft and sticky
• It gets oxidized easily in air and produces bad smell even
if kept as such for a few days
• It is soluble in many organic solvents
• It absorbs large quantities of water
• Its chemical resistivity is low and is attacked by acids,
alkalies, oxidizing and reducing agents
• Its tensile strength, abrasion resistance wear and tear
resistances are low
• It possesses marked tackiness
i.e., when two fresh raw rubber surfaces are pressed
together, they coalesce to form a single piece
• It has little durability
• When stretched to a great extent, it suffers permanent
deformation, because of the sliding or slippage of
some molecular chains over each other
Synthetic rubbers have slightly modified structures from
natural rubber they exhibit properties that are more
conducive for their technical uses
A comparative account of the properties of
natural and synthetic rubbers
Property
Tensile
strength
Natural rubber
Low (only 200 kg/cm2)
Synthetic rubber
High
Chemical
resistivity
Low – gets oxidized
even in air
High – not oxidized in
air
Action of
heat
Cold condition it is hard
and brittle, at higher
temp.s soft and
sticky
Withstand effect of
heat over a range of
temperature.
With organic
solvents
Swells and dissolves
Do not swell and
dissolve
Ageing
Undergoes quickly
Resists ageing
Elasticity
On increased stress
undergoes permanent
deformation.
Has high elasticity.
Vulcanization of rubber
This process was discovered accidentally by Goodyear
when he dropped rubber and sulfur on a hot stove
To improve the properties of rubber, it is compounded
with some chemicals like sulphur, hydrogen sulphide,
benzoyl chloride etc., It is known as vulcanisation of rubber
The process consists of heating the raw rubber with
sulphur at 100 – 140 oC
The added sulphur combines chemically at the double
bonds of different rubber springs
Thus this process serves to stiffen the material by a sort of
anchoring and consequently, preventing the intermolecular
movement of rubber springs
The extent of stiffness of vulcanized rubber depends on
the amount of sulphur added
e.g., a tyre rubber may contain 3 to 5% sulphur,
but a battery case rubber may contain as much as 30% sulphur
H
H
H
H
C
C
C
H
H
H
C
H
H
C
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
C
C
C
H
C
H
H
+S+
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
C
C
H
H
H
C
H
H
H
C
C
H
H
H
C
H
H
H
C
H
H
H
H
C
C
C
C
C
H
H
H
H
H
C
C
H
HH
H
H
HH
H
H
S
C
H
H
C
C
HH
H
C
C
H
H
H
C
C
H
C
S
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
C
H
HH
H
C
C
H
H
C
H
HH
H
C
C
C
HH
H
H
H
H
Advantages of vulcanization
Vulcanized rubber
• has good tensile strength and extensibility, when a
tensile force is applied, can bear a load of 2000 kg/cm2
before it breaks
• has excellent resilience
i.e., article made from it returns to the original shape,
when the deforming load is removed
• possesses low water-absorption tendency
• has higher resistance to oxidation and to abrasion
• has much higher resistance to wear and tear as
compared to raw rubber
• is better a electrical insulator, although it tends to
absorb small amounts of water
• is resistant to organic solvents (such as petrol,
benzene, and carbon tetrachloride), fats and
oils. However, it swells in these liquids
• is very easy to manipulate the vulcanized rubber to
produce the desired shape articles
• has useful temperature range of - 40 to 100 oC
• has only slight tackiness
• has low elasticity and is depending on the extent of
vulcanization
e.g., vulcanite (32% Sulphur) has practically no elasticity
Compounding of rubber
Compounding is mixing of the raw rubber (synthetic or
natural) with other substances so as to impart the
specific properties to the product, which are suitable
for a particular job
Besides rubber, the following materials may be incorporated
• Softners and plasticizers
These are added to give the rubber greater tenacity
and adhesion. Important materials are vegetable
oils, waxes, stearic acid, rosin, etc.
•Vulcanizing agents
The main substance added is sulphur
Depending on the nature of the product required, the
% of sulphur added varies between 0.15 and 32.0%
Many other vulcanizing agents are now-a-days added to
rubber, among them are sulphur monochloride,
hydrogen sulphide, benzoyl chloride, trinitrobenzene and
alkylphenol sulphides
• Accelerators
These materials drastically shorten the time required
for vulcanization
The most used accelerators are 2-mercaptol,
benzothiozole and zinc alkyl zanthate
•Antioxidants
Natural rubber has a tendency to perish, due to oxidation
For this reason, anti oxidation materials, such as complex
amines like phenyl naphthylamine and phosphates are
added
•Reinforcing fillers
These are added to give strength and rigidity to the
rubber products
Common reinforcing fillers are carbon black, zinc oxide,
calcium carbonate and magnesium carbonate
•Colouring matter
These are added to give the desired colour to the
rubber product
for white colour
titanium dioxide
Green
chromium oxide
red
ferric oxide
Crimson
antimony sulphide
yellow
lead chromate
---- pigments are added
Styrene rubber (GR-S or Buna-S or SBR)
Preparation
This is produced by copolymerization of butadiene
(about 75% by wt.) and styrene (about 25% by wt.)
H2C
n x H2C
H2C
CH
CH
CH
CH
CH
CH2 + n
H2C
CH2
CH
x
n
Styrene-butadiene copolymer
Styrene domains act as
anchors or junctions
Butadienes provide
flexible linkages
The desire to maximize the ways you can arrange the flexible
links is what causes rubbers to return to given shapes
Properties
 It possess high abrasion-resistance
 It possess high load-bearing capacity and resilience
 It gets readily oxidized, especially in presence of
traces of ozone present in the atmosphere
 It swells in oils and solvents
 It can be vulcanized in the same way as natural
rubber either by sulphur or sulphur monochloride
However, it requires less sulphur, but more
accelerators for vulcanization
 Styrene rubber resembles natural rubber in
processing characteristics as well as the quality
of the finished products
Uses
It is used for the manufacture of
• motor tyres
• floor tiles
• shoe soles
• gaskets
• wire and cable insulations
• carpet backing
• adhesives
• tank-lining
etc.,
Silicone rubber
Silicone resins contain alternate silicone – oxygen
structure, which has organic radicals attached to
silicone atoms
H
H C
O
H
H
Si
H C
O
H
Si
H C H
H C H
H
H
O
Dimethyl silicone dichloride is bifunctional and
can yield very long chain polymer
CH3
n Cl
Si
CH3
Cl
Hydrolysis
- HCl
n HO
Si
OH
CH3
CH3
unstable
CH3
CH3
Si
CH3
H 2O
polymerization
O
n
( Si
O)
CH3
unstable
Vulcanized silicone rubbers are obtained by mixing
high molecular weight linear dimethyl silicone polymers
with filler
The fillers are either a finely divided silicon dioxide
or a peroxide
It may also contain the curing agents
Peroxide causes the formation of dimethyl bridge
(cross link) between methyl groups of adjacent chains
CH3
CH3
Si
H
CH2
O
+
H
CH2
Si
CH3
O
O
Si
CH3
O
Si
CH3
O
Si
CH3
O
Si
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Si
CH3
O
Si
O
CH3
Si
CH3
H 2O
O
Si
CH3
O
O
CH3
CH3
Si
O
Si
CH3
O
Si
CH3
O
Si
CH3
O
Si
CH2
CH3
CH3
CH3
CH3
CH2
CH3
CH3
CH3
CH3
Si
CH3
O
Si
CH3
O
Si
CH3
O
Si
CH3
O
Si
CH3
O
O
Properties
They possess exceptional resistance to
• prolonged exposure to sun light
• weathering
• most of the common oils
• boiling water
• dilute acids and alkalies
They remain flexible in the temp. range of 90 – 250 OC
hence, find use in making tyres of fighter aircrafts,
since they prevent damage on landing. Ordinary rubber
tyre becomes brittle and hence disintegrates
silicone rubber at very high temp. s (as in case of fibers)
decomposes; leaving behind the non-conducting silica
(SiO2), instead of carbon tar
Uses
• as a sealing material in search-lights and ain aircraft engines
• for manufacture of tyres for fighter aircrafts
• for insulating the electrical wiring in ships
• In making lubricants, paints and protective coatings for
fabric finishing and water proofing
• as adhesive in electronics industry
• For making insulation for washing machines and electric
blankets for iron board covers
• For making artificial heart valves, transfusion tubing and
padding for plastic surgery
• For making boots for use at very low temp., since they are
less affected by temperature variation
e.g., Neil Armstrong used silicone rubber boots when he
walked on the moon
Reclaimed rubber
Reclaimed rubber is rubber obtained from waste rubber
articles
like worn out tyres, tubes, gaskets, hoses, foot-wears etc.,
The waste is cut to small pieces and powdered by using a
cracker, which exerts powerful grinding and tearing action
The ferrous impurities, if any, are removed by the
electro-magnetic separator
The purified waste powdered rubber is then digested with
caustic soda solution at about 200 oC under pressure for
8 – 15 hours in steam-jacketed autoclave
By this process, the fibers are hydrolyzed
After the removal of fibers, reclaiming agents like petroleum
and coal-tar based oils and softeners are added
Sulphur gets removed as sodium sulphide and rubber
becomes devulcanized
The rubber is then thoroughly washed with water sprays
and dried in hot air driers
Finally, the reclaimed rubber is mixed with small
proportion of reinforcing agents like clay, carbon black etc.,
Properties
The reclaimed rubber has
• less tensile strength
• has lower elasticity
• possesses lesser wear-resistance than
natural rubber
• it is much cheaper, uniform in composition
and has better ageing properties
• it is quite easy for fabrication
Uses
 for manufacturing tyres
 tubes
 automobile floor mats
 belts
 hoses
 battery containers
 mountings
 shoes, heals etc.,