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WHAT IS A POLYMER?
Polymer is a macro molecule
formed by the union of many
small molecules
called monomers.
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Depending type of monomer, they may be
homopolymer (made up of same monomer(A))A + A + A + A -A-A-A-A-
e.g. :- polythene
polychloroethene
PVC
or
Co-polymer (made up of different monomers)
A + B + A + B -A-B-A-B-
e.g.:- polyamides
polyesters
CLASSIFICATON OF POLYMERS:
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LinearEx. CH2 CH2
(Polyethylene)
BranchedEx. CH2--Si O Si
| |
CH2-- Si O Si
(silicon resin)
DEPENDING UPON THE STRUCTURE,
A POLYMER MAY BE LINEAR OR
BRANCHED .
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MEMBERS OF THE POLYMER FAMILY CAN
BE SEPARATED INTO FOUR DIFFERENTGROUPS DEPENDING ON THEIR
INTEMOLECULAR FORCES.
Elastomers
Fibres
Thermoplastics
Thermosets
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ELASTOMERS
The polymer chains are held together by the
weakest intermolecular forces.
These weak forces permit the polymer to bestretched.
A few-cross links are introduced in between
the chains,which help the polymer to retract to
its original position after tehe force is released
(as in vulcanised rubber).
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FIBRE
Formed by the polymers having high tensile
strength.This can be attributed to the strong
intermolecular forces like hydrogen bonding
which operate in polyamides(nylon-6,6).
These strong forces also lead to close packing
of chains and thus, impart crystalline nature.
As a result , these polymers sharp melting
points.
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ThermoplasticsThermoplastics
The intermolecular forces of attraction are
intermediate between elastomers and fibres.
No cross links between chains.
Weak attractive forces between chains broken by
warming.
Change shape - can be remoulded.
Weak forces reform in new shape when cold .
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ThermosetsThermosets
These polymers are normally made from relatively
low molecular mass semi-fluid polymers which on
heating in a mould become infusible and insolublehard mass.
Extensive cross-linking formed by covalent bonds.
3D network of Bonds prevent chains moving relative
to each other. (bakelite)
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Polymer Structure
Although the fundamental property of bulkpolymers is the degree of polymerization, thephysical structure of the chain is also animportant factor that determines the
macroscopic properties. Configuration refers to the order that is
determined by chemical bonds. Theconfiguration of a polymer cannot be altered
unless chemical bonds are broken andreformed.
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Configuration
The two types of polymer configurations arecis and trans. These structures can not be
changed by physical means (e.g. rotation).
The cis configuration arises when
substituent groups are on the same side of acarbon-carbon double bond. Trans refers to
the substituents on opposite sides of the
double bond.
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Stereoregularity is the term used to describe
the configuration of polymer chains.
Three distinct structures can be obtained. Isotactic is an arrangement where all
substituents are on the same side of the
polymer chain. Asyndiota
ctic polymer chainis composed of alternating groups and
atactic is a random combination of the
groups.
isotactic
sydiotactic
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Chain Structures
The geometric arrangement of the bonds is not theonly way the structure of a polymer can vary. Thereare, however, many ways a branched polymer can bearranged. One of these types is called "star-branching". Star branching results when apolymerization starts with a single monomer and hasbranches radially outward from this point. Polymerswith a high degree of branching are calleddendrimers. Often in these molecules, branchesthemselves have branches. This tends to give themolecule an overall spherical shape in threedimensions.
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A separate kind of chain structure arises when more that
one type of monomer is involved in the synthesis reaction.
These polymers that incorporate more than one kind ofmonomer into their chain are called copolymers. There are
three important types of copolymers.
A random copolymercontains a random arrangement of the
multiple monomers.
A block copolymercontains blocks of monomers of the
same type.
Finally, agraft copolymercontains a main chain polymer
consisting of one type of monomer with branches made up
of other monomers.The following diagram displays the different types of
copolymers.
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Block Copolymer Graft Copolymer Random Copolymer
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Cross-Linking
In addition to the bonds which hold monomers together in apolymer chain, many polymers form bonds betweenneighboring chains. These bonds can be formed directlybetween the neighboring chains, or two chains may bond toa third common molecule. Though not as strong or rigid asthe bonds within the chain, these cross-links have an
important effect on the polymer. Polymers with a highenough degree of cross-linking have "memory." When thepolymer is stretched, the cross-links prevent the individualchains from sliding past each other. The chains maystraighten out, but once the stress is removed they return totheir original position and the object returns to its originalshape.
One example of cross-linking is vulcanization . Invulcanization, a series of cross-links are introduced into anelastomerto give it strength. This technique is commonlyused to strengthen rubber.
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Molar mass distribution In linear polymers the individual polymer chains rarely have the
exact same degree of polymerization and molar mass, and there
is always a distribution around an average value. The molar mass
distribution (or molecular weight distribution) in a polymer
describes the relationship between the number of moles of each
polymer species (Ni) and the molar mass (Mi) of that species. The
molar mass distribution of a polymer may be modified by polymer
fractionation.
Different average values can be defined depending on the
statistical method that is applied. The weighted mean can be
taken with the weight fraction, the mole fraction or the volume
fraction:
1. Number average molar mass or Mn
2. Weight average molar mass or Mw
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Number average molecular weight The numberaverage molecular weight is the ordinary
arithmetic mean or average of the molecular weights of theindividual macromolecules. It is determined by measuringthe molecular weight ofn polymer molecules, summing theweights, and dividing by n .
The number average molecular weight of a polymer canbe determined by gel permeation chromatography,viscometry (Mark-Houwink equation), and all colligativemethods like vapor pressure osmometry or end-groupdetermination.
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Weight average molecular weight The weight average molecular weight is a way of
describing the molecular weight of a polymer. For theweight average molecular weight, this is calculatedby
where Ni is the number of molecules of molecularweight Mi.
The weight average molecular weight can bedetermined by light scattering, small angle neutronscattering (SANS), X-ray scattering, andsedimentation velocity.
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Intuitively, if the weight average molecular
weight is w, and you pick a random
monomer, then the polymer it belongs to will
have a weight ofwon average (for a
homopolymer).
The ratio of the weight average to thenumberaverage is called the polydispersity
index.Some natural polymers are generally
monodispersed,the PDI is unity.Syntheticpolymers always polydispersed, the PDI>1.
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It may be defined as the process of linking or joiningtogether small molecules like monomers to make
large molecules.
Basically there are 2 types of polymerizations.
Additional polymerization or Chain polymerization
Condensation polymerization or Step polymerization
POLYMERISATION:
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Monomers contain C=C bonds(unsaturated
compounds such as alkenes) . Double bond opens
to (link) bond to next monomer molecule.
Chain forms when same or different basic unit isrepeated over and over.
In this polymerization process light, heat and
pressure or catalyst is used to breakdown the
double covalent bonds of monomers.
ADDITIONAL POLYMERIZATION OR
CHAIN POLYMERIZATION:
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May be defined as a reaction occurring betweensimple polar-group-containing monomers with theformation of polymer and elimination of smallmolecules like water, HCl, etc. For example,
hexamethylene diamine and adipic acid condenseto form a polymer, Nylon6:6.
Additional polymerization is a chain reaction
converting of a sequence of three steps. Initiation,propagation and termination.
CONDENSATION OR
STEP POLYMERIZATION
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Initiation step is considered to involve tworeactions. The first is the production of freeradicals , usually , by the dissociation of aninitiator (or catalyst) to yield a pair of radicals R.
I 2R (1)(Initiator) (Free radicals)
The second part of initiation under the addition ofthis radical to the monomer molecule (M) to
produce the chain initiating species .
R + M RM1
A. INITIATION STEP
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Consists of the growth of
R--M1 by successiveadditions of large numbers of monomer molecules
according to equation.
R--M1+ M M2
R--M2+
M M
3R--M3+ M M4 or in general terms
R--Mn + M Mn + 1
[ M CH=CH-- Mn --(CH--CH)n--| |
Y Y ]
B. PROPAGATION STEP:
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At some time, the propagation polymer chain
stops growing and terminates.
R(CH CH)n-- + --CHCH--R R(CH CH)n--CHCH--R
| | | |
Y Y Y Y
C. TERMINATION STEP:
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Stronger attractive forces between chains =
stronger, less flexible polymer. Nylon has strong
hydrogen bonds,so it makes a strong fibre.
Chains able to slide past each other = flexiblepolymer . In polyethene attractive forces are weak
instantaneous dipole - induced dipole,so it will be
flexible .
Chain length If the chain is long,the polymer will
be strong.
BranchingIf the polymer is branched,it will be
harder,but more brittle.
WHAT DECIDES THE PROPERTIES OF A
POLYMER?
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Influences on the Influences on theInfluences on the Influences on the
properties of properties of polymersproperties of properties of polymers:-
Monomer solubilityIf the monomer is If the
monomer is very soluble,the reaction will form
spheres.
Monomer bignessA large monomer will makefragile polymer.
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Areas in polymer wherechains packed in regularway.
Both amorphous andcrystalline areas in samepolymer.
Crystalline - regular chain
structure - no bulky sidegroups.
More crystalline polymer -stronger and less flexible.
Crystalline polymersCrystalline polymers
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Longer chains make stronger
polymers.Critical length needed
before strength increases.
Hydrocarbon polymersaverage of 100 repeating units
necessary but only 40 for
nylons.
Tensile strength measures theforces needed to snap a
polymer.
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Cold drawing is used to increase a polymers strength. Thehandles of plastic carrier bags snap if you fill them full of
tins of beans.
ColdCold--drawingdrawing
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The phase behavior of polymer
solutions and mixtures is more
complex than that of small
molecule mixtures. Whereas
most small molecule solutions
exhibit only an upper critical
solution temperature phase
transition, at which phaseseparation occurs with
cooling, polymer mixtures
commonly exhibit a lower
critical solution temperature
phase transition, at which
phase separation occurs with
heating.
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Thermal Properties of PolymersThermal Properties of Polymers
Polymer Glass Transition : As the temperature of a
polymer drops below Tg(glass transition
temperature),it behaves in an increasingly brittle
manner. As the temperature rises above the Tg, thepolymer becomes more rubber-like. In general,
values of Tg well below room temperature define the
domain of elastomers and values above room
temperature define rigid, structural polymers.
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Comparison between a crystalline material
(1)and an amorphous material (2)is
illustrated in the figure below.
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Tg and Mechanical Properties Response of polymers to the application of a force
is strongly dependent on their temperatures.
Glass is one of the few completely elastic materials
while it is below its Tg.It will remain elastic until it
reaches its breaking point.The Tg of glass occursbetween 510 and 560 degrees C, meaning that it
will always be a brittle solid at room temperature.
In comparison, polyvinyl chloride (PVC)has a Tg of
83 degrees C, making it good, for example, for coldwater pipes,but unsuitable for hot water.PVC also
will always be a brittle solid at room temperature.
SHORE (DUROMETER)
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SHORE (DUROMETER) HARDNESSTEST
Shore hardness is a measure of the resistance of a material to
penetration of a spring loaded needle-like indenter.
Hardness of Polymers (rubbers, plastics) is usually measured byShore scales.
Shore A scale is used for testing soft Elastomers(rubbers) and other
soft polymers.Hardness of hard elastomers and most other polymer materials(Thermoplastics, Thermosets) is measured by Shore D scale.
Shore hardness is tested with an instrument called Durometer.Durometer utilizes an indenter loaded by a calibrated spring.
The measured hardness is determined by the penetration depth ofthe indenter under the load.
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Two different indenter shapes (see the picture below) and two
different spring loads are used for two Shore scales (A and D).
The loading forces of Shore A: 1.812 lb (822 g), Shore D: 10 lb
(4536 g).
Shore hardness value may vary in the range from 0 to 100.
Maximum penetration for each scale is 0.097-0.1 inch (2.5-
2.54 mm). This value corresponds to minimum Shore
hardness: 0. Maximum hardness value 100 corresponds to
zero penetration.
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Practical Uses of polymers
Plastics.Flexibility is determined Plastics by
the monomers.
Clothing.Nylon is present in almost
everything you wear.
Insulators,both thermal and electrical.
Valves,other sealing mechanisms.
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THANK YOU