DEFINITION: The word polymer is derived from the two greek words polyandmers Polymers are macro...

DEFINITION:

The word polymer is derived from the two greek words

poly and mers

Polymers are macro molecules formed by linking smallermolecules repeatedly, called monomers.

parts or unitsmany

C C C C C CHHHHHH

HHHHHH

Polyethylene (PE)

mer

ClCl Cl

C C C C C CHHH

HHHHHH

Polyvinyl chloride (PVC)

mer

Polypropylene (PP)

CH3

C C C C C CHHH

HHHHHH

CH3 CH3

mer

e.g.

HIGH POLYMERS

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1

Examples:

Polyethylene is formed by linking a large number ofethylene molecules

nC C

H

H H

H

C CH

H H

Hn Polymerisation

Ethylene polyethylene

polystyrene is formed by linking styrene molecules

H

styrene polystyrene

C CH

Hn Polymerisation

nC C

H

H

H


2

Classification of Polymers

Polymers can be classified in several ways, based on

origin

structure

methods of formation

response to heat

properties (or applications)


3

The number of repeating units (n) in the chain is known as the degree of polymerization.

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.

e.g.,

D.P.


4

Based on the origin

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 modifiednatural polymers

polymers can be classified as

e.g., cellulose acetate, cellulose nitrate, halogenated rubbers etc.,


5

Based on the molecular structure

polymers can be classified as

Linear Branched

Cross-linked

the monomeric units combine linearly with each other In linear polymers,

secondary bonding


6

Branch polymers

Cross linked polymers


7

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.,


8

Based on the response to heat

Thermo softening

Thermosetting

soften on heating and can be converted into any shapeand can retain its shape on cooling

thermosoftening or thermoplastics

under go chemical change on heating and convert themselves into an infusible mass

thermosetting polymers


9

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 coolingso that they can retain that shape.

e.g., polystyrene, polyvinylchloride, polymethylmethacrylate etc.,


10

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 forcebut 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 andthe molecular length is at least 100 times its diameter

e.g., nylons, terylene, etc.,


11

Resins

These polymers have a glossy appearance

These constitutes the major essential part of the plastics

These suffers the polymerization reactions and impartdifferent properties to plastics

e.g., polysulphide sealants, epoxy adhesives, etc.,


12

Functionality

the number of reactive sites or bonding sites

Alcohols Methyl alcohol

Ethers Dimethyl Ether

Acids Acetic acid

Some mono functional hydrocarbons


13

Aldehydes Formaldehyde

Aromatic hydrocarbons Phenol

Some bi functional hydrocarbons

adipic acid (hexanedioic acid)


14

1,6-hexanediamine

Terephthalic acid

ethylene glycol


15

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!


16


17


18


19

Types of Polymerisation

Polymerisation occurs basically in two different modes.• addition (chain growth) polymerization• condensation (step growth) polymerization

• Additiono monomers react through stages of initiation,

propagation, and terminationo initiators such as free radicals, cations, anions

opens the double bond of the monomer o monomer becomes active and bonds with other

such monomerso rapid chain reaction propagateso reaction is terminated by another free radical or

another polymerwww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

20

- 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

• condensation


21

DISTINGUISHING FEATURES OF ADDITION AND CONDENSATION POLYMERISATION

ADDITION CONDENSATION

Monomers undergo self addition to each other without loss of by products

Monomers undergo intermolecular condensation with continuous eliminationof by products such as H2O, NH3, HCl, etc.,

It follows chain mechanism It follows step mechanism

Unsaturated vinyl compounds undergoaddition polymeristion

Monomers containing the functional groups (-OH, -COOH, -NH2, ….) undergothis polymerization

Monomers are linked together through C – C covalent linkages

Covalent linkages are throughtheir functional groups

High polymers are formed fast The reaction is slow and the polymermolecular weight increases steadily throughout the reaction

Linear polymers are produced with or without branching

Linear or cross linked polymers are produced

e.g., polystryrene, plexiglass, PVC, etc.,

e.g., nylons, terylene, PF resins, etc., www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

22

• Random, Alternating, Blocked, and Grafted

CoPolymers

• Synthetic rubbers are often copolymers.

e.g., automobile tires (SBR)

Styrene-Butadiene Rubber random polymer


23

Addition polymerization can be explained on the basis of free radical mechanism

It involves three stages

viz., (i) Initiation

(ii) Propagation

(iii) termination

oru.v.light

I(Initiator)

R*(Free radical)

Initiation


24

C C

H

X H

H

+R*

(Free radical)

Vinyl monomer

C C *

H

H X

H

R

(new free radical)

The new free radicals attack monomer molecules further in quick succession leading to chain propagation

Vinyl monomer

C C

H

X H

H

C* +C

H

H X

H

R

(Free radical)

C C

H

H X

H

R C C*

H

H X

H

(new free radical)

Propagation


25

Vinyl monomer

C C

H

X H

H

+

(new free radical)

C C

H

H X

H

R C C*

H

H X

H

(another new free radical)

C*C

H

H X

H

C C

H

H X

H

R C C

H

H X

H

at m th stage,

C C

H

X H

H

+C

H

H

R C

X

H

C

H

H

C

X

H

m-2

C*C

H

H X

H

C

H

H

R C

X

H

C

H

H

C

X

H

m-1

C*C

H

H X

H


26

At some stage this chain propagation is terminated when the free radicals combine either by coupling (combining) of the two radicals or by disproportionation

R C

H

H

C

X

H

m-1

C*C

H

H X

H

RC

H

H

C

X

H

m-1

C*

X

H

C

H

H

+

R C

H

H

C

X

H

m-1

CC

H

H X

H

RC

H

H

C

X

H

m-1

C

H

H

C

X

H

saturated highpolymer (dead polymer)

coupling


27

R C

H

H

C

X

H

m-1

C*C

H

H X

H

+ RC

H

H

C

X

H

m-1

C*

X

H

C

H

H

+R C

H

H

C

X

H

m-1

CC

H X

H

H RC

H

H

C

X

H

m-1

C

H

H

C

X

H

saturated oligomerunsaturated oligomer(dead polymer) (dead polymer)

disproportionation


28

TECHNIQUES OF POLYMERISATION

Addition polymerization is brought about using four different techniques

• Bulk or Mass polymerization

• Solution polymerization

• Suspension polymerization

• Emulsion polymerization


29

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


30

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

Advantages:

The method is simple

It needs simple equipments

The percentage of conversion is high

Product obtained is pure with high optical clarity


31

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 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

32

Suspension Polymerization

the monomer is suspended in water as droplets of colloidal size

Water is used as a solvent

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


33

Advantages:

Disadvantages:

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

the method can be used only for water insoluble monomers

it is difficult to control polymer size


34

Emulsion Polymerisation

This method is used for water insoluble monomers

Emulsion is the colloidal dispersion of a liquid in another immiscible liquid

Emulsion of water and the monomer is allowed to form

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


35

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 www.bookspar.com | Website

for Students | VTU NOTES | QUESTION PAPERS

36

Advantages:

Disadvantages:

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

the polymer formed may contain impurities such as the emulsifiers and coagulants

It needs further purification by other chemical techniques


37

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


38

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 rubber state(Hard brittle plastic) (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


39

The glassy state and the glass The glassy state and the glass

transitiontransition

• In general for ordinary compounds of low molar mass:

• crystalline solid

• liquid

• increase in volume at Tm;

• slopes of FC and BA: expansion coefficients of crystalline phase and liquid, respectively.

V

TTm

A

B

C

F

melting


40

Non-crystallisable materials Non-crystallisable materials

V

TTg Tm

A

B

C

F

Some materials CANNOTcrystallize, e.g. ordinary glass

Why?Molecular structure is too irregular

liquid amorphous or glassy phase

rubber

•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

D

E


41


42

Crystalline vs. Crystalline vs. AmorphousAmorphous

Phase transitions for long-chain polymers.


43

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


44

HIGH-DENSITY POLYMERSHIGH-DENSITY POLYMERS

LOW-DENSITY POLYMERSLOW-DENSITY POLYMERS

Linear polymers with chains that can pack closelytogether. These polymers are often quite rigid.

Branched-chain polymers that cannot pack together as closely. There is often a degree of cross-linking.These polymers are often more flexible than high-density polymers.

crystalline polymers have higher Tg s than amorphous polymers


45

PolypropyleneTg =

R

PolystyreneTg = 100 0C

the bulky groups on chain, increases the Tg of the polymer

PolyethyleneTg = -110 0C


46

• e.g., the Tg of nylon 6,6 (Tg = 50 0C) is higher than PE (Tg = -110 0C)

N|

H

H

|

C

|

H

6

N|

H

O

||

C

H

|

C

|

H

4

N|

H

O

||

C

N|

H

H

|

C

|

H

6

N|

H

O

||

C

H

|

C

|

H

4

N|

H

O

||

C

+++

+++

H

C

H

H

C

H

H

C

H

H

C

H

+++

+++

nylon 6,6 polyethylene

Hydrogen bondsVan der Waals bonds

With H-bonds vs vdW bonds, nylon is expected to have (and does) higher Tg.

The presence of H-bonds between the polymer molecules increases the Tg


47

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.

C

C

O

O

O

O

CH2CH2CH2CH3

CH2CH2CH2CH3

dibutyl phthalate

The plasticizer separates the individual polymer chainsfrom one another. It acts as a lubricant which reducesthe attractions between the polymer chains.


48

The Tg of a polymer is influenced by its molecular weight

However, it is not significantly affected if molecular weight is around 20000

With increase in molecular mass, the Tg increases

PE (low Mw)

PE (high Mw)

-110 0C

- 90 0C

e.g.,


49

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


50

Callister, Fig. 16.9

T

Molecular weight

Tg

Tmmobile liquid

viscous liquid

rubber

tough plastic

partially crystalline solid

crystalline solid

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 lowerswww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

51

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


52

The fundamental parameters which influence the structure-property relationship are

molecular mass

polarity

crystallinity

molecular cohesion

stereochemistry of the molecules

the nature of polymeric chains and

The properties like tensile strength, crystallinity, elasticity, resistance to chemicals, wear and tear depend mostly on the polymer structure


53

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 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

54

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 Strengthwww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

55

Linear Polymers: Polyethylene, polyvinyl chloride (PVC), polystyrene, polymethyl methacrylate (plexiglass), nylon, fluorocarbons (teflon)

Branched Polymers: Many elastomers or polymeric rubbersCross-linked Polymers: Many elastomers or polymeric rubbers are cross-linked (vulcanization process); most thermosetting polymers

Network Polymers: Epoxies, phenol-formaldehydes.

Examples:


56

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 easyi.e., slipping power is high

Hence it has less strength.


57

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


58

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

Hence linear molecules show greatest degree of plastic deformation, under pressure

at high pressure and temperature, the weak Vander waal’s forces between molecules becomemore and more weak

Such type of materials are called thermoplastic materials www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

59

No slippage occurs in case of cross-linked molecules,because of only primary covalent bonds are presentthroughout 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 forceor temperature exceeding the stability of material is applied,it will result in total destruction


60

Crystallinity

Polymers are part crystalline and part amorphous

An amorphous state is characterized by completerandom arrangement of molecules

crystalline form by regular arrangement of molecules

crystalline region

amorphous region


61

• 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.

• Isotactic and syndiotactic polymers are stronger andstiffer due to their regular packing arrangement.

• Polymers with a long repeating unit or with low degree of symmetry do not crystallize easily

• 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.


62

Which polymer more likely to crystallize? Can it be decided?

Linear and highly crosslink cis-isoprene

• Not possible to decide which might crystallize. Both not likely to do so.

• Networked and highly crosslinked structures are near impossible to reorient to favorable alignment.

H+

+ H20

Networked Phenol-Formaldehyde

(Bakelite)


63

Polystyrene (PS) possess greater strength when compared toPE and PVC because of the presence of bulky phenyl group.

R


64


alternating 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.

random poly(styrene - ethylene) copolymer


65


Linear syndiotactic polyvinyl chloride Linear isotactic polystyrene

• 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.

• 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.


66

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’ and‘unlike materials repel’

Thus a polymer is more soluble in structurally similar solvent

polymers containing polar groups like – OH, - COOH etc., usually dissolve in polar solvents like water, ketone, alcohol etc.,

e.g.,

but these are chemically resistant to non-polar solvents www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

67

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 inaliphatic 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


68

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 dependson crystallinity

crystalline polymers exhibits higher chemical resistance than less crystalline polymers because of denser packing


69

Elasticity

Elasticity of the polymer is mainly because of the uncoiling andrecoiling 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 moleculesshould 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 www.bookspar.com | Website


70

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


71

Molecular Weight of Molecular Weight of

PolymersPolymers

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


72

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 www.bookspar.com | Website


73

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


74

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 definedas 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 themolecules present in the polymer

In computing the number average molecular mass of a polymer, we consider the number fractions


75

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

i

1

nn 1fraction offraction Number


76

i

11

n

Mn 1fraction by on contributiweight Molecular

i

ii

i

44

i

33

i

22

i

11n

n

Mn............................

n

Mn

n

Mn

n

Mn

n

Mn M

Similarly,

Molecular weight contribution by other fractions are

;n

Mn;

n

Mn;

n

Mn

i

33

i

22

i

11

i

ii

n

Mn

Number average molecular mass of the whole polymer is given by

i

iin

n

M n M


77

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 = ni Mi

Weight of fraction 1 = W1= n1M1

ii

1111

Mn

Mn

W

Mn 1fraction offraction weight

1ii

11M

Mn

Mn 1fraction by on contributiweight Molecular

ii

211

Mn

Mnwww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

78

Molecular weight contribution by other fractions are

;Mn

Mn;

Mn

Mn;

Mn

Mn

ii

233

ii

222

ii

211

ii

2ii

Mn

Mn

Weight average molecular mass of the whole polymer is given by

ii

2ii

ii

244

ii

233

ii

222

ii

211

wMn

Mn.................

Mn

Mn

Mn

Mn

Mn

Mn

Mn

Mn M

ii

2ii

wMn

M n M


79

Polymers: Molecular Polymers: Molecular WeightWeight

• number average, Mn

• weight average, Mw

Ni: no. of molecules with degree of polymerization of i

Mi: molecular weight of i


80

• Ratio of Mw to Mn is known as the polydispersity index (PI)

o a measure of the breadth of the molecular weight

o PI = 1 indicates Mw = Mn, i.e. all molecules have equal length (monodisperse)

o PI = 1 is possible for natural proteins whereas synthetic polymers have 1.5 < PI < 5

o At best PI = 1.1 can be attained with special techniques


81

The number-average molecular mass (Mn)is determined by the measurement of colligative properties such as

depression in freezing point

elevation in boiling point

osmotic pressure

lowering of vapour pressure

The weight-average molecular mass (Mw) is determined by

light scattering and

ultra-centrifugal techniques


82

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

Mn= 5119.565

Mw= 10147.56

PDI= 1.982113 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

83

Polymers: Molecular Polymers: Molecular WeightWeight

• Biomedical applications: 25,000<Mn<100,000 and 50,000<Mw<300,000

• Increasing molecular weight increases physical properties; however, decreases processibility


84

TEFLON or FLUON or Polytetrafluoroethylene (PTFE):

Preparation

FC C

FF

Fn

Water emulsion polymerization

peroxide

F

C C

FF

F

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


85

• 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 chainsgives the extreme toughness, high softening point, exceptionally high chemical resistance

• It has high density 2.1 to 2.3 gm/cm3


86

• 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 products where resistance to acid and alkalies are needed

• It is used for coating and impregnating, glass fibers, asbestos fibers (to form belts), filter cloth etc.,

• It is used as catheters, artificial vascular grafts etc.,www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

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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


88

Preparation

Heat- 2n H2O

+n n


89

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 tinwww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

90

• Nylon 6,6 used as sutures

• It is used as a plastic as well as fiber

Uses

• 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


91

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

• Resoleswww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

92

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


93

CH2OH

OH

CH2OH

OH

and

o-methylol phenolp-methylol phenol

Phenol (excess)

OH

formaldehyde

C = O

H

H

+

H+


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These methylol phenols condense rapidly to form Novolacs

CH2OH

OH

CH2OH

OH

or

o-methylol phenolp-methylol phenolOH

OHH2

CH2

COH H2

COH

OH

H2

C

HO

Novolacswww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

95

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


96

OHH2

CH2

COH H2

COH

OH

H2

C

HO

Novolacs (prepolymer) Curing with Formaldehyde or hexamine


97

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

Phenol

OH

Formaldehyde(excess)

C = O

H

H

+

OH--


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CH2OH

OH

o-methylol phenol

CH2OH

OH

p-methylol phenol

CH2OH

OH

CH2OH

CH2OH

OH

CH2OH

HOH2C

di methylol phenol

tri methylol phenol

+ ++

Curing


99

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


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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 copperwww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

101

• These are usable up to 400 °F (204°C)

• The properties can be modified by fillers& reinforcements

• These have the highest compressive strength

• These are machinable

• Phenolics are the resin in plywood

• These tends to be brittle


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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


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Epoxy resins

Preparation

Cl CH2CH2CH

On

CH3

C OHHO

CH3

+

n

CH2 CH2CH

OH

C OO

CH3

CH3

epichlorhydrin bis phenol Alkaline catalyst

60 OC-n HCl


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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


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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

• Their properties can be modified by adding compounds like unsaturated fatty acids or amines and some of the solvents

• They possess excellent electrical resistance

• Epoxy resins stick well to a number of substances including metal and glass


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• No size-change upon cross-linking/hardening

This means they make ideal adhesivesShrinkage causes adhesive failuresAdhesives 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 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

107

• 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


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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


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Elastomers are high polymers, which have elastic properties in excess of 300 %

The elastic deformation in an elastomer arises due tothe 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 www.bookspar.com | Website


110

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


111

As each isoprene unit contains C = C bond, polyisoprene exists in two isomeric forms

viz., cis and trans

Cis-polyisoprene trans-polyisoprenewhere R= CH3

Natural rubber contains the cis isomer while the gutta percha contains the trans isomer


112

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 www.bookspar.com | Website


113

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


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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


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• 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


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A comparative account of the properties of natural and synthetic rubbers

Property Natural rubber Synthetic rubber

Tensilestrength

Low (only 200 kg/cm2) High

Chemicalresistivity

Low – gets oxidizedeven in air

High – not oxidized in air

Action of heat

Cold condition it is hardand brittle, at higher

temp.s soft andsticky

Withstand effect ofheat over a range

of temperature.

With organicsolvents

Swells and dissolves Do not swell and dissolve

Ageing Undergoes quickly Resists ageing

Elasticity On increased stress undergoes permanent

deformation.

Has high elasticity.


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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 www.bookspar.com | Website


118

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

HH

H

H

H

H

H

H

HH

H

H

H

H

H

H

HH

H

H

H

H

C C

C

C C

C C

C

C C C

C C

C

C

C C

C

C

H

H

H

C

H

H

H

H

H

C C

C

C

H

H

H

C

H

H

H

H

H

C C

C

C

H

H

H

C

H

H

H

H

H

+ S +


119

H

HH

H

H

H

H

H

H

HH

H

H

H

H

H

H

HH

H

H

H

H

H

C C

C

C CC C

C

C C CC C

C

C

C C

C

C

H

H

H

C

H

H

H

H

H

C C

C

C

H

H

H

C

H

H

H

H

H

C C

C

C

H

H

H

C

H

H

H

H

H

S S


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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 resiliencei.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


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• 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


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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 suitablefor 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% www.bookspar.com | Website


123

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 www.bookspar.com | Website


124

•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 therubber product

for white colour titanium dioxide

Green chromium oxide

red ferric oxide

Crimson antimony sulphide

yellow lead chromate

---- pigments are added www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

125

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 CH CH CH2

xH2C CH

n

H2C CH CH CH2n x

H2C CH

n+


126

Styrene-butadiene Styrene-butadiene copolymercopolymer

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 shapeswww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

127

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


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Uses

It is used for the manufacture of

• floor tiles

• motor tyres

• shoe soles

• gaskets

• wire and cable insulations

• carpet backing

• adhesives

• tank-lining etc.,


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Silicone rubber

Silicone resins contain alternate silicone – oxygen structure, which has organic radicals attached to silicone atoms

SiO

C

C

H

HH

H

H

H

SiO

C

C

H

HH

H

H

H

O


130

Dimethyl silicone dichloride is bifunctional and can yield very long chain polymer

CH3

CH3

OSi

n

CH3

CH3

Cl ClSin

CH3

CH3

HO OHSin

unstable

Hydrolysis

- HCl

H2O

polymerization

CH3

CH3

OSi( )

unstablewww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS

131

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


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O +

CH3

OSi

CH2H

CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

CH3

OSi

CH2H CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

H2O


133

CH3

OSi

CH2

CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

CH3

OSi

CH2 CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi


134

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 rubbertyre 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


135

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


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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


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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.,

After the removal of fibers, reclaiming agents like petroleum and coal-tar based oils and softeners are added


138

Properties

The reclaimed rubber has

• has lower elasticity

• less tensile strength

• 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


139

Uses

for manufacturing tyres

tubes

automobile floor mats

belts

hoses

battery containers

mountings

shoes, heals etc.,


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DEFINITION: The word polymer is derived from the two greek words polyandmers Polymers are macro...

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