seminer on polymer

8/8/2019 seminer on polymer

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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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seminer on polymer

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