UNIT – V
POLYMER SCIENCE
Monomer
Monomer is a small molecule which combines with each other to form a giant
molecule called as the polymer.
Some monomers and repeating unit of the polymers are given below
Examples:
Repeating unit of the polymer
Monomer
CH2=CH2
─ (CH2 ─CH2 ) n─
Ethylene
Polyethylene (P.E)
Propylene
Polypropylene (PP)
Polymerization
Polymerisation is a process in which large number of small molecules (called
monomers) combine to give a big molecule (called a polymer) with or without
elimination of small molecules like water.
Example:
CH2=CH2
Ethylene
─ (CH2 ─CH2 ) n─
Polyethylene (P.E)
Functionality
The number of bonding sites or reactive sites or functional groups present in a
monomer molecule is known as its functionality.
Examples:
Ethylene
(CH2=CH2)
functionality=2
Glycerol
functionality=3
Bi – functional monomers mainly form linear (or) straight chain polymer. Tri –
functional monomers form cross-linked polymer.
Tacticity
The orientation of monomeric units or functional groups in a polymer molecule
take place in an orderly (or) disorderly manner with respect to the main chain
known as Tacticity. Tacticity do affect the physical properties of polymer. This
orientation results in three of stereo-regular polymers.
(a) Isotactic Polymer
If the functional groups are arranged on the same side of the main chain,
the polymer is called Isotactic polymer.
(b) Syndiotactic Polymer
If the functional groups are arranged in an alternating fashion, the
polymer is called syndiotactic polymer.
(c) Atactic polymer
If the functional groups are arranged randomly, the polymer is called Atactic
polymer
Degree of polymerization
The number of repeating units (n) in a polymer chain is known as the degree of
polymerisation. Polymers with low degree of polymerisation are known as oligo
polymers. Polymers with high degree of polymerisation are known as high
polymers.
Degree of polymerization = Molecular weight of polymer
Molecular weight of monomer
Classification of polymers:
Polymers can be classified in several criteria
(1) On the basis of chemical composition
(2) On the basis of sources
(3) On the basis of application
1. On the basis of chemical composition:
(i) Homochain polymer:
If the main chain of a polymer is made up of same species of atoms it is called
homochain polymer.
Example:
Polyethylene, Polyvinyl chloride, etc
-C-C-C-C-C-C-C-C-C-C(ii) Heterochain polymer:
If the main chain of a polymer is made up of different atoms it is called
heterochain polymer.
Example:
Terylene, Nylon 66
-C-C-O-C-C-O-C-C-O-C-C-
2.On the basis of sources :
(i) Natural polymers:
Polymers derived from natural materials are called natural
polymers. Example:
Wood, Cotton, Silk, Rubber, etc
(ii) Synthetic polymers:
Polymers synthesized from low molecular weight
monomers are called synthetic polymers.
Example:
Polyethylene, Polyvinyl chloride, Nylon, etc
(iii)Semi synthetic polymer:
Polymers obtained by the partial modification of natural polymers are called as
semi synthetic polymers.
Example:
Cellulose acetate, cellulose nitrate, etc
3. On the basis of application:
(i) Liquid resins:
Adhesives, Sealents comes under this
category Example: Epoxide
(ii) Fibres:
Very long thin filament is called as
Fibre Example: Nylon
(iii) Elastomers:
Polymers having elastic nature come under this category.
Example: Rubber
Copolymerization
Copolymerisation is the joint polymerisation in which two (or) more different
monomers combine to give a polymer. High molecular weight polymers,
obtained by copolymerization, are called copolymers. Copolymerization is
mainly carried out to vary the properties of polymers such as hardness,
strength, rigidity, heat resistance etc. Example:Butadiene and styrene
copolymerize to give Buna-S rubber.
n (CH2=CH─CH=CH2) + nCH2=CH
│
│
C6H5
Butadiene
→ ─( CH2─CH=CH─CH2─CH2─CH)n─
Styrene
C6H5
polybutadiene –co-styrene
S.No Addition /Chain polymerization
Condensation/step
polymerization
The monomer must have at least one The monomer must have two identical
multiple bond.
(or) different functional groups.
Examples:
Examples:
Ethylene: CH2=CH2
Acetylene
i)Glycol
1
CH2─OH
CH2─OH
ii)6-amino hexanoic acid
H2N─(CH2)5─COOH
2
Monomers add on to give a polymer Monomers condense to give a polymer
and no other by product is formed.
and by products such as H2O, CH3OH
are formed.
3
Molecular weight of the polymer is an Molecular weight of the polymer is not
integral multiple of molecular weight an integral multiple of that of the
of monomer.
monomer.
4
High molecular weight polymer is Molecular weight of the polymer rises
formed at once.
steadily throughout the reaction.
5
Longer reaction times give higher Longer reaction times are essential to
yield, but have a little effect on obtain high molecular weight
molecular weight.
Thermoplastics are produced.
6
7
8
Example: Polyethylene.
Homo-chain polymer is obtained.
Thermosetting plastics are produced.
Example:
Bakelite,
Ureaformaldehyde.
Hetero-chain polymer is obtained.
Catalysts used depend on the Catalysts used are usually mineral acids
mechanism, free radical initiators, and bases.
Lewis acids and bases are used.
Free radical mechanism of addition polymerization.
The free radicals produced by the decomposition of compounds called initiators
bring the initiation of the polymer chain growth. The propagation step involves a
continuous and very rapid addition of the monomer units to produce lengthy
growing polymer chains and finally the termination of growing chains are brought
about by coupling, disproportionation or by chain transfer.
Chain Initiation:
Initiation of a chain reaction involves the production of free radicals from an
initiator. Organic peroxides are usually used as initiators. Benzoyl peroxide is a
commonly employed initiator. On heating, benzoyl peroxide forms a pair of free
radicals.
Generally
The free radical undergoes addition reactions with the monomers CH2 = CH X
forming a new free radical (chain initiating species).
The double bond is the site for initiation reaction, which gives rise to new active
centers.
Chain Propagation:
Propagation of the monomer free radical takes place with the addition of each
monomer to the growing chain and long chain radicals result.
(Living polymer)
The Propagation continues till the chain growth is terminated at the free
radical site or till there is no further monomers left for further addition. This is
known as living polymer.
Chain Termination:
Chain termination takes place either by coupling, i.e. by combination of two growing
chains at the free radical site to form a lengthy molecule or by transfer of a hydrogen
atom in a disproportionation reaction or by chain transfer reactions.
Termination by Coupling
Termination by Disproportion:
Cationic Mechanism of polymerization.
In cationic polymerization, the production of carbocation initiates the
polymerization reaction of monomers. Then the initiation is followed by
propagation of the polymer chain growth and finally termination by a proton
transfer.
Chain Initiation:
Common chain initiators used in this type of polymerization are Lewis acids
(AlC13, BF3) which will provide a proton in presence of an active hydrogen
compound.
The proton initiates the process by attacking the monomer unit at the double bond
position forming stable carbocation.
Chain Propagation:
The chain propagation reaction involves further adding up ‘n’ number of
monomer units and simultaneous transfer of the charge to the newly added
monomer unit.
Chain Termination:
The chain termination may occur in two possible ways.
(i) A proton is donated back to the counter ion
,
resulting
in the formation of a double bond at the end of the growing polymer chain and
hence an unsaturated polymer is produced.
Growing Polymer chain
Dead Polymer
This process of donation of a proton and reformation of the catalyst is called
ion-pair precipitation.
(ii) A covalent bond is formed between the carbocation and the counter ion
when the termination takes place by simple coupling.
In both the cases, the regeneration of initiator takes place
Anionic mechanism of polymerization.
In anionic polymerization, the π electrons of the monomers are attacked by a
negatively charged (anion) initiator which initiates the polymerization. The anionic
initiator generates a stable carbanion on reaction with a monomer. This stable
carbanion produced having the active center (carbanion site) propagates the reaction.
The termination is brought about by the addition of an active hydrogen compound
like ammonia.
Chain initiation: Common chain initiators used in anionic polymerization
technique are LiNH2, KNH2, NaNH2, n-butyllithium etc. The negatively charged ion
of the initiator attacks the π electron pair of the monomers, pushing its electrons pair
to the end carbon atom and forming a sigma bond with the monomer unit.
This generates a stable carbanion which propagates the chain by further addition of
monomer units.
Chain Propagation: The stable carbanion produced by the initiators added to n
number of monomer units and carbonium site is transferred to the end carbon atom.
The chain termination in anionic polymerization is not usually a spontaneous
process. In order to bring the termination some active hydrogen compounds like
ammonia or some strongly ionic substances have to be added to the reaction mixture.
If no external agents are added, the anionic polymerization proceeds till all the
monomer are consumed and the carbanion site at the end carbon atom remains
potentially alive. Fresh polymerization process starts if we add more monomer to
this. Studies have shown that polymerization can be restarted in this manner, even
weeks after, by adding fresh monomers. The polymer produced by this technique are
called living polymers.
Plastics
Plastics are high molecular weight organic materials that can be moulded into any
desired shape by the application of heat and pressure in the presence of a catalyst.
Difference between thermoplastics and thermosetting plastics
Sl.No Thermoplastics
Thermosetting plastics
1.
They are generally formed by They are
mainly formed
addition polymerization.
condensation polymerisation
2.
They consist of linear long chain They consist of three dimensional
polymers.
network structures.
3.
All the polymer chains are held All the polymer chains are linked by
together by weak vanderwaals strong covalent bonds.
forces.
4.
They are weak, soft and less They are strong and brittle.
brittle.
5.
They soften on heating and They do not soften on reheating.
harden on cooling.
6.
They can be remoulded.
7.
They have
weights.
8.
They are soluble in organic They are insoluble in organic solvents.
solvents.
low
by
They cannot be remoulded.
molecular They have high molecular weights.
Compression moulding:
This method is applied to both thermoplastics and thermosetting plastics.
Figure shows a typical mould used for compression moulding
The mould is made up of two halves, the upper and the lower halves. The lower half
usually contains a cavity in the shape of the article to be moulded. The upper half has
a projection, which fits into the cavity when the mould is closed. The material to be
moulded is placed in the cavity of the mould. Then the mould is closed carefully
under low pressure.
Finally the mould is heated to 100-2000C and simultaneously high pressure
(100-500 kg/cm2) is applied on the top of the mould. Curing is done either by
heating or cooling. After curing the moulded article is taken out by opening the
mould.
Injection moulding:
This method is mainly applicable to thermoplastics. The powdered plastic material is
fed into a heated cylinder through the hopper. The plastic material melts under the
influence of heat and becomes fluid. The hot fluid is injected at a controlled rate into
the closed mould by means of a piston plunger.
The mould is cooled to allow the hot plastic to cure and becomes rigid. After
curing the mould is opened and the object is ejected. Telephone parts, buckets
etc, are made by this method.
Advantages:
i)
Low mould cost.
ii)
Low finishing cost.
iii)
Low loss of materials.
iv)
High speed production.
Compounding of plastics:
It is the process of incorporating chemical additives to the virgin polymer during
moulding in order to get desired properties.
1. Resins: Resins are the basic binding material and it constitutes the major
portion of plastic. Basically there are two types of resins- thermo plastic and
thermo setting resins.
2. Plasticizers: They are additives which are added to pure polymer in very
small quantities (< 1%) to increase its plasticity and flexibility. Usually they are
relatively small molecules which on entering between the polymer chain imparts
more freedom of movement to the polymer chains. This is achieved by breaking
the intermolecular force of attraction between the chains by the plasticizer
molecules. Common examples of plasticizer are tributylphosphates, triphenyl
phosphate, dibuty1phosphate, dibutyl phthalate, camphor, vegetable oils etc.
3. Fillers or Extenders: Fillers or extenders are inert materials added to the
plastic to increase the bulk and to reduce the cost. It will fill the voids, gaps etc.
in the plastic bulk; hence also give more strength to the plastic. It provides better
hardness, tensile strength, capacity, workability etc, e.g. asbestos, mica, barites
(BaSO4), gypsum, clay CaCO3 etc.
4. Accelerators or Catalysts: These are ingredients added to plastic to
increase the rate of moulding. E.g. H2O2, benzoyl peroxide, zinc oxide, ammonia.
5. Lubricants: Lubricants are added to plastic for easy moulding and to give
better glossy and shining finish to the final products. Lubricants prevent the
plastic materials from sticking to the fabric equipments. E.g. waxes, oils, soaps,
stearates, oleates, silicone oil etc.
6. Stabilizers: To improve thermal stability during moulding as well as for the
products formed, certain substances are added known as stabilizers.
Two types of stabilizers are used:
(i) Opaque moulding compounds e.g. Lead salts and lead chromate, white lead
etc.
(ii) Transparent moulding compounds, e.g. stearates of lead, cadmium and
barium.
7. Pigments: These are chemicals added to plastic to impart pleasing colours as
well as to protect the plastic from degradation. E.g.White-titanium oxide, greenchrome green, black-carbon black, blue-prussian blue, red-ferric oxide, yellowlead chroma
8. Fire Retardants: These are substances added to plastic to
make them fire resistant, e.g. ammonium phosphate, antimony
trioxide, aluminium oxide tri-hydrate etc.
9. Anti Oxidants: These are substances added to prevent oxidation or
degradation of the polymer e.g. diphenylamine, phenols and thio compounds.
IMPORTANT POLYMERS
(1)Bakelite
(2)Polystyrene
(3)Nylon6;6
(4)Polyvinyl chloride (PVC)
(5)Polyethylene
(6)Cellulose nitare
(7)PTFE
(8)Terylene
(1) Bakelite:
Preparation:
Phenol formaldehyde resins are formed by the poly condensation between
phenol and formaldehyde catalyzed by either acids (or) bases. The initial
products are monomethylol phenols, dimethylol phenols and trimethylol
phenols depending upon the concentration of phenol and formaldehyde.
Properties:
(i) It is hard, rigid, infusible and insoluble.
(ii) It has high resistance towards acids salts and many organic solvents
but it is attacked by alkali.
Uses:
(i)
It is used for making telephone parts and T.V cabinets.
(ii) It is used as cation exchanger in water softening
(2) Polystyrene:
Preparation:
It is also known as polyvinyl benzene and it is prepared by the free radical
polymerization of styrene in the presence of benzoyl peroxide catalyst
Properties:
(i) Commercially available polystyrene is atactic in structure.
(ii) It is amorphous in nature.
Uses:
It is used in the manufacture of containers, radio and television cabinets, toys
and many other house hold articles.
(3) Nylon 66
Preparation:
Nylon 66 is obtained by the polymerization of adipic acid with
hexamethylenediamine.
n H2N -( CH2 ) 6 -NH2 + n HOOC- ( CH2 ) 4 - COOH
↓
[ HN -( CH2) 6- NH-CO -( CH2 ) 4 - CO ] n + (2n-1)
H2O Nylon 66
Properties:
(1) It possesses high thermal stability and good abrasion resistance.
(2) It is insoluble in common organic solvents and soluble in phenol and formic
acid.
Uses:
It is used for fibres which are used in making socks, dresses, carpets etc.
(4)Polyvinyl
chloride:
Preparation:
Polyvinyl chloride is obtained by heating water emulsion of vinyl chloride in
presence of benzoyl peroxide or hydrogen peroxide under pressure.
n CH2 CHCl
[ CH2−CH ] n
Cl
Properties:
(1) PVC is colourless, odourless and chemically inert powder.
It is insoluble in inorganic acids and alkalis, but soluble in hot chlorinated
hydrocarbons such as ethylchloride.
Uses:
(1) It is used in the production of pipes, cable insulations, table covers and
rain coats, etc.
(2) It is also used for making sheets, which are employed for tank linings,
light fittings, etc.
LDPE and HDPE.
Low density poly
S.No ethylene(LDPE)
1
2
Prepared under high pressure
using O2 as a catalyst
High density
polyethylene(HDPE)
Prepared under low pressure
using Zeigler-Natta catalyst.
It is completely linear polymer and
It is branched polymer
90℅
and
40 ℅
crystalline
crystalline.
3
Density:0.91-0.92
gms/ml.
Density:0.965 gms/ml.
4
Melting point: 110- 0
125
C.
Melting point: 144-1500C.
At high temperature it is
soluble in
5
It is almost insoluble in all the
solvents.
CCl4, toluene etc.
Preparation: It involves the following two
steps.
Step I: Ethylene gas is liquefied under high pressure (at 1500 atm).
StepII: Liquified ethylene is then pumped into a heated vessel maintained
at 150 to 2500C in the presence of traces of oxygen as a catalyst.
Ethylene polymerises to form LDPE
n CH2 =CH2 → ─ (CH2─CH2─)n
Ethylene Polyethylene (PE)
Uses:
It is used for making films, table cloth, packing material, toys, pipes, detergent
bottles etc.,
CELLULOSE NITRATE
Cellulose nitrate is prepared by reacting cellulose with nitrating mixture i.e.conc. H
NO3 and conc. H2SO4 and the sulphuric acid acts as a dehydrating agent.
H2SO4
Nitro cellulose has different applications depending upon the percentage of
nitrogen content. Nitrocellulose with more than 12.3℅ is widely used as explosives.
With 10℅ to 11℅ nitrogen content it is used as a celluloid plastic and 11 to 12℅ as
nitro lacquers and 12 to 12.5℅ as photographic films.
PTFE
It is prepared by the emulsion polymerization of tetrafluroethylene under
pressure in the presence of benzoyl peroxide as catalyst.
The monomer, tetrafluroethylene can be produced by the dechlorination of
dichloro tetrafluroethane. Teflon is highly tough and possesses high melting point
(350°C) due to the presence of highly electronegative fluorine atom.
Terylene
It is the linear polyester obtained by condensation of terephthalic acid with
ethylene glycol in the presence of sodium methoxide.
Terylene
Properties
PET is a colourless and rigid substance. Due to its structure regularity, it
crystallizes readily. It also has a good tensile and impact strength.
Uses
1) It is mostly used for making synthetic fibers like terylene, dacron etc.
2) It is used for making transparent overhead projectors.
PHYSICAL STRENGTH OF THE POLYMER
Strength :
The strength of the polymer depends on the chemical bond and the intermolecular
force of attraction which binds the polymer chains together. Therefore the strength of
the polymer is decided by its structure whether it is linear, branched or cross linked.
In linear and branched polymer, the chains are held together by intermolecular force.
In cross linked polymers, all chains are held together by chemical bonds. Due to weak
intermolecular forces, the linear or slightly branched polymer chains can slip one
over the other by applying external heat or pressure. Therefore if the polymer has
more slipping power then the strength of the polymer will be less. In cross linked
structure, the slippage of chains are not possible due to strong
covalent bonds which extends in all three dimensions. Hence they are stronger
compared to other polymers.
Among the linear polymers,
(i)
The strength of the polymer increases with increase in the chain length.
(ii)
The presence of polar groups such as chloride, hydroxyl, carboxyl etc. in
the polymers chain increases the intermolecular force of attraction.
Hence linear polymer containing polar groups are stronger than those
containing non-polar groups.
(iii) The presence of bulkier functional groups hinders the slipping of
polymer chains and therefore are stronger than those without bulkier
functional groups.
Examples:
(i) Polyethylene and polyvinyl chloride: In these type of polymers the chains are held
together by means of weak Vanderwaal’s force of attraction. Therefore on applying
either heat or pressure the Vanderwaal’s forces holding the chains breaks and
slipping of one chain over the other becomes possible. Thus the plastic loses its
rigidity and becomes soft and pliable.
(ii) Bakelite : In this type of polymer, the chains are held together by strong covalent
bond. Hence on heating or applying pressure, the chains do not slip over the other
due to its network structure. Therefore they do not get soft, instead it gets charred
due to fission of covalent bonds.
Physical state of a polymer
The arrangement of polymer chain with respect to one another decides its physical
state. Amorphous state of a polymer describes the arrangement of polymer chain in a
random or irregular manner while the crystalline state shows the uniform or regular
arrangement of polymer chains. Both regular and irregular arrangement of polymer
chains within a polymer matrix exists in partially crystalline polymers.
Crystalline polymers have uniform or regular arrangement of polymer chains and
hence, the intermolecular force of attraction will be high. Due to this fact, they have
high softening nature. These polymers have great rigidity, strength and density
compared to amorphous polymers. They are usually brittle, and can be made flexible
by addition of plasticizers. In amorphous polymer, the intermolecular force of
attraction is weak compared to crystalline polymers. Presence of lengthy chains,
bulky side groups or complex branching makes a polymer more amorphous and they
do not crystallize easily.
Examples:
Amorphous – Polystyrene
Crystalline – Polypropylene
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