1 ENGINEERING CHEMISTRY UNIT-III by SUDHAKAR REDDY UNIT-III POLYMERS DEFINITION : Polymers are the giant molecules that are built by linking together a large number of small particles. Depending upon the structure, a polymer may be linear (or) branched. Ex. – CH2 – CH2 – CH2--Si – O – Si – O – Si Polyethylene | Linear CH2- Si – O – Si – O – Branched silicon resin Depending upon the type of monomer, they may be homo polymer (made up of same monomer) or co-polymer (made up of different monomers). Ex. – CH2 – CH2 – n Homo polymer Polyethylene TYPES OF POLYMERISATION : Basically there are two types of polymerization. They are (i) Additive (or) Chain growth polymerization. (ii) Condensation (or) Step growth polymerization. ADDITIVE (OR) CHAIN GROWTH POLYMERISATION: This type of reactions attains a product which is exactly multiple of the original mono meric molecule. Such mono meric molecules usually have a double bond. The additive polymerization reactions are initiated by light (or) heat (or) catalyst. MECHANISM: This mechanism involves three steps. (a) Initiation (b) Propagation (c) Termination INITIATION: Homo lytic cleavage of initiator leads to the foundation of free radicals R-O-O-R Heat/hυ 2R ∗ + O 2 PROPAGATION: The readily found radical combines with ethylene, causing opening of double bond and yielding another highly reactive radical. This combines with another ethylene molecule and this process goes on till a very long chain -CH2- is built. R ∗ + CH 2 = CH 2 R-CH 2 − 2 ∗
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ENGINEERING CHEMISTRY UNIT-III by SUDHAKAR REDDY
UNIT-III
POLYMERS DEFINITION: Polymers are the giant molecules that are built by linking together a large number
of small particles.
Depending upon the structure, a polymer may be linear (or) branched.
Ex. – CH2 – CH2 – CH2--Si – O – Si – O – Si
Polyethylene |
Linear CH2- Si – O – Si – O –
Branched silicon resin
Depending upon the type of monomer, they may be homo polymer (made up of same monomer)
or co-polymer (made up of different monomers).
Ex. – CH2 – CH2 – n
Homo polymer
Polyethylene TYPES OF POLYMERISATION:
Basically there are two types of polymerization. They are
(i) Additive (or) Chain growth polymerization.
(ii) Condensation (or) Step growth polymerization.
ADDITIVE (OR) CHAIN GROWTH POLYMERISATION:
This type of reactions attains a product which is exactly multiple of the original mono meric
molecule. Such mono meric molecules usually have a double bond. The additive polymerization
reactions are initiated by light (or) heat (or) catalyst.
MECHANISM:
This mechanism involves three steps.
(a) Initiation
(b) Propagation
(c) Termination
INITIATION: Homo lytic cleavage of initiator leads to the foundation of free radicals
R-O-O-R Heat/hυ 2R∗ + O2
PROPAGATION: The readily found radical combines with ethylene, causing opening of double
bond and yielding another highly reactive radical. This combines with another ethylene
molecule and this process goes on till a very long chain -CH2- is built.
R∗ + CH2 = CH2 R-CH2 − 𝐶𝐻2∗
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ENGINEERING CHEMISTRY UNIT-III by SUDHAKAR REDDY
R-CH2 − 𝐶𝐻2∗+ CH2 = CH2 R-CH2 − CH2 − CH2 − 𝐶𝐻2
∗
TERMINATION: The reaction is terminated by the recombination of final free radicals.
Here ‘n’ and ‘m’ gives the total number of mono meric molecules combined to give polymer.
CONDENSATION (OR) STEP GROWTH POLYMERISATION:
This is a reaction occurring between simple polymer group containing monomers with the
foundation of polymers and elimination of small molecules like H2O, HCl etc.
MECHANISM:
Condensation polymerization involves combination of any two monomers to give dimmers
which in turn combine to give tetramer.
It finally grows into a polymer in a step wise manner.
Molecular weight of the polymer is determined by the number of reacting functional groups
present on the monomer.
Mono functional monomer gives only low molecular weight product. Bi and Poly functional
monomers give high molecular weight condensation polymer.
PLASTICS :
Plastics are the organic material of high molecular weight and which can be molded to the
desired from when subjected to heat and pressure in the presence of catalyst. The properties
are low thermal and electrical conductivities, easy to fabricate, low specific gravity etc,.
Plastics are generally used for making automobiles parts, telephones, electrical instruments,
optical instruments. Plastics having high wear resistance properties can be used for making
gears, bearings etc,.
MERITS:
(i) They have good shock absorption capacity compared with steel.
(ii) Plastics are chemically inert.
(iii) Plastics have high corrosion resistance when compared to metals.
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ENGINEERING CHEMISTRY UNIT-III by SUDHAKAR REDDY
(iv) Plastics can be made according to the order like hard, soft, rigid, tough, brittle, malleable etc,. (v) Fabrication of plastics into desired shape and size is cheap.
(vi) Plastics are dimensionally stable.
(vii) Plastics doesn`t absorb water.
(viii) Thermal co-efficient of expansion of plastics is low.
(ix) Plastics are light in weight having specific gravity from 1 to 2, 4. DEMERITS:
(i) Plastics are soft.
(ii) Plastics have poor ductility.
(iii) Resistance to heat is less.
(iv) Cost of plastics is high.
(v) Plastics can deform under load.
Resins are the basic binding materials which form a major part of plastics. Resins and plastics
are used as anonymous.
CLASSIFICATION OF PLASTICS :
Plastics (or) Resins are classified into two types. They are
(i) Thermo plastic resin
(ii) Thermo set resin
THERMO PLASTIC RESIN THERMO SET RESIN
(i) They are processed by addition polymerization.
(i) These are processed by condensation Polymerization.
(ii) Softness on heating and retaining the same on cooling.
(ii) These are infusible and insoluble mass on heating i.e., heat resistance.
(iii) They are long chain linear polymers without any branched (or) cross linked chain.
(iii) They are branched (or) cross linked polymer.
(iv) On repeated chain heating and cooling, there is no change in chemical nature.
(iv) Some sort of chemical change occurs on heating.
(v) These plastics undergo purely physical process.
(v) These plastics undergo physical as well as Chemical process.
(vi) Water thermo plastics can be recovered E.g.: Teflon, PVC etc,.
(vi) Waste thermo set cannot be recovered E.g.: Bakelite, Nylon, and Polyester.
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ENGINEERING CHEMISTRY UNIT-III by SUDHAKAR REDDY
COMPOUNDING OF PLASTICS:
Compounding of plastics may be defined as the mixing of different materials like plasticizers,
fillers of extenders, lubricants, dyes and pigments to the thermoplastic and thermosetting
plastics to increase their useful properties like strength, toughness etc,.
Many plastics are virtually useless along but are converted into highly serviceable products by
combining them with a variety of additives, stabilizers, by the compounding process. The exact
formulation will depend upon the specific application requirement. The different additives
impart different physical properties which are used to improve the performance of the plastic
materials. Additives are widely used for thermoplastics, thermosets and elasteromers like
phenolic (or) amino resins are useless alone but by the addition of fillers, resins etc., they give a
reversible products. Some of the compounding materials are stabilizers, plasticizers, fillers,
colorants (or) pigments, lubricants and accelerators.
Some of the ingredients used in compounding of plastics are
(1) Plasticizers
(2) Fillers (or) extenders
(3) Dyes and pigments
(4) Lubricants.
PLASTICIZERS:
Plasticizers are substances added to enhance the plasticity of the material and to reduce the
cracking on the surface. Plasticizers are added to increase flexibility and toughness. Plasticizers
also increase the flow property of the plastics. Because of their addition, there is neutralization
of part of the inter molecular forces of attraction between the macro molecules of resin. Hence
greater freedom of movement between the polymeric macro molecules. There by increases the
flexibility and plasticity of the compounded material. Because of the reduction in the inter
molecular forces there is decrease in chemical resistance.
The ester obtain in this format can be polymerized to give the plastics called poly Acrylates. PROPERTIES: They are tough, strong, and have low specific gravity. USES: 1. Air craft’s and cockpit enclosures.
2. The thermoset resin is the phenolic resin.
3. The phenolic resin is similar to Bakelite.
CONDUCTING POLYMERS A Polymer which can be conduct electricity is termed as conducting polymers.
They are classified into two types:
INTRINSICALLY CONDUCTING POLYMER:
The polymer whish have extensive conjugation in the backbone for
conduction are called intrinsically conducting polymer.
These are further two types:
A). CONDUCTING POLYMER HAVING CONJUGATED π-electrons IN THE BACK BONE:
These polymers contain conjugated π-electrons in the back bone. The π-electrons increase the
conductivity. This is because overlapping of conjugated π-electrons results in formation of
valance bond as well as conduction bonds, which extends over the entire polymer molecule.
These two bands are separated by a signification gap.
Thus, electrical conduction could occur only after thermal (or) photolytic activation of
electrons , to give them sufficient energy to jump the gap and reach into the lower level of the
conduction band.
E.g.:- Poly Acetylene, Poly Quinoline etc.,
CONDUCTING POLYMERS
INTRINSICALLY CONDUCTING
POIYMERS
CONDUCTING POLYMERS HAVING CONJUGATED PIE-ELECTRONS IN THE
BACK BONE
DOPING CONDUCTING POLYMERS
EXTRINSICALLY CONDUCTING
POLYMERS
CONDUCTIVE ELEMENT FILLED POLYMERS
BLENDED CONDUCTING
POLYMERS
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ENGINEERING CHEMISTRY UNIT-III by SUDHAKAR REDDY
B). DOPED CONDUCTING PLOYMERS: Intrinsically conducting polymers posses low conductivity, but these posses low ionization
potential and high electron affinity, So these can be easily Oxidized (or) Reduced.
The conductivity of these polymers increased by creating either positive (or) negative charge on the polymers by oxidation (or) reduction.
This technique was called DOPING.
Doping was of two types:
1. P-Doping
2. N-Doping
P-DOPING:
This doping involves treating an intrinsically conducting polymer with a lewis acid , there by
oxidation process takes place and positive charges on the polymer back bone are created.
E.g.:- the p-do pants used are I2, Br2, Cl2, FeCl3, AlCl3
(C2H2)n + 2FeCl3 (C2H2)n+ FeCl4
− + FeCl2
Poly Acetylene
P-Doping also called as “oxidative doping”. N-DOPING:
This doping involves treating an intrinsically conducting polymer with a Lewis base there by
reduction process takes place and negative charges as the polymer back bone are created.
E.g.:- for n-do pant used are Li, Na, Ca tetra butyl Ammonium etc.,
(C2H2)n + B (C2H2)n− 𝐵+
Poly Acetylene Base n-doped Acetylene N-DOPING is also called “Reductive doping”. EXTRINSICALLY CONDUCTING POLYMER:
The polymer whose conductivity is due to the presence of “externally” added are called
extrinsically conducting polymers.
These are of two types: A).CONDUCTIVE ELEMENT FILLED POLYMER:-
It is a resin (or) polymer filled with conducting elements such as carbon-black, metallic fibers ,
metal oxides etc., In this , the polymer acts as the binder to hold the conducting elements
together in the solid entity. This polymer posses reasonably good conductivity. These are low in
cost , light in weight, mechanically durable and strong, easily process able in different forms ,
shapes and sizes.
Generally conducting carbon black is used as filler to its very high surface
area, high porosity and filamentous property.
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ENGINEERING CHEMISTRY UNIT-III by SUDHAKAR REDDY
B).BLENDED CONDUCTING POLYMERS:
It is obtained by blending a conventional polymer with a conducting polymer either by
physical (or) chemical change. Such polymers can be easily processed and posses better
physical, chemical and mechanical properties.
APPLICATIONS OF CONDUCTIVE POLYMERS:
Conducting polymers are found to be in increased use because they are light weight, easy
to process and have good mechanical properties. Some of the important applications of
conducting polymers are:
In Rechargeable Batteries: These batteries are small in size based on percholate doped
ploy acetylene –lithium system. These are about 10-times lighter than conventional lead
storage batteries.
In Optically display device used on polythiophene: Electro chromic system produce
coloured displays with faster switching time and better viewing than conventional liquid
crystals display device (LED).
In wiring in aircrafts and aero space components.
In tele communication systems.
In anti static coating for clothing.
In electromagnetic screening materials.
In electronic device such as transistors and diodes.
In solar cells, drug delivery systems for human body etc.,
In photo voltaic devices. e.g.; - Al/polymer Au photo voltaic cells.
In non-linear optical materials.
In molecular wires and molecular switches.
In Ion-exchange members: The charge residing on the conducting polymers make them
useful as ion-exchange membrane.
POLY ACETYLENE: Polymerization of Acetylene over Ziegler Natta catalyst gives poly acetylene which is mostly
formed in Cis form.
On rising the temperature of this film gives more stable Trans form.
This polymer is infusible, insoluble and tends to become brittle on exposure to air.
Conductivity is modified by doping.
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ENGINEERING CHEMISTRY UNIT-III by SUDHAKAR REDDY
Controlled addition of p-doping agent like Agf5, Br2I2,(or) HclO4 could still enhance the
conductivity.
POLY ANILINE: Reaction of Aniline with Ammonium per sulphate in aqueous Hcl produces poly aniline as a dark blue powder with a conductivity of 55 S 𝑐𝑚−1 DOPING: Doping in semi conductors is achieved by replacing an atom by electron rich (or) electron poor
atom to create p-type and n-type semi conductors. In conducting polymers, doping is achieved
through an oxidation (or) reduction process.
The doping methods are classified into two types:
1. Chemical doping
2. Electro chemical doping
CHEMICAL DOPING:
It involves, a polymer is exposed to an oxidant (Typically Bromine or Iodine) {or} reductant
(far less common, typically involves alkali metals).
E.g.:- Poly phenylene, phenylene sulphide.
ELECTRO CHEMICAL DOPING:
It involves a polymer coated working electrode suspended in an electrolyte solution, in which
the polymer is insoluble along with separated counter and reference electrodes. Electric
potential differences created a charge from the electrolyte to enter the polymer in form of