Polymers MSE9 2

8/12/2019 Polymers MSE9 2

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Polymers

The term polymer implies many "mers" or the building blocks....similar to the

unit cell in metals.A polymer is a chemical compound or mixture of compounds formed by a

process called polymerization, a chemical reaction in which two or more

molecules combine to form larger molecules.

Generally speaking, polymers refer to the intermediate stage before the finalplastic product is produced.

Natural polymers

derived from plants and animals

wood, rubber, cotton

wool, leather and silk

biological polymers

protein, enzymes, starches, cellulose

Synthetic polymers huge expansion since WWII

Historical Classification


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Plastics are derived from organic materials and are in abundance.

Raw materials commonly used in the production of polymers are coal, air,

water, wood, petroleum, limestone, and salt.

Most common material used is petroleum.

These materials contain the basic elements that are used in forming

polymers...carbon, hydrogen, oxygen, nitrogen, chlorine, and fluorine.

Basic Building Blocks


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Hydrocarbon MoleculesHydrocarbon Molecules Most polymers are organic

composed of H and C each C has 4 bonds

each H has 1 bond

bonds are covalent

Bonds between carbons can

single (e.g. ethane)

double (e.g. ethylene or

ethene) triple (e.g. acetylene or

ethyne)

C

H

H

H

H

methane,

simplest

hydrocarbon

form CC

CCCC

=

Single BondSingle Bond


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Unsaturated Hydrocarbon MoleculesUnsaturated Hydrocarbon Molecules

Ethane

C2H6- single bond

Ethylene

C2H4 -double bond

Acetylene C2H2 -triple bond

C C

H H

H H

C CH H

Molecules that have double or triple bonds are termedunsaturated

C

H

H

H

HC

H

H


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Chemistry of Polymer MoleculesChemistry of Polymer Molecules

Unsaturated hydrocarbons may permit the addition of another atom or

group of atoms. Example ethylene C2H4, which is a gas.

C C

H H

H H

R+ C CH H

H H

R-

C C

H H

H H

R- C CH H

H H

+ C C

H H

H H

R- C CH H

H H

R - free radical (unpaired electron) in initiator - catalyst

C C

H H

H H

R- C CH H

H H

R+ C CH H

H H

R- C CH H

H H

- R.termination

initiation

growth


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Structure of Polymer MoleculesStructure of Polymer Molecules

Polymer composed ofmers (repeat unit)

Single unit called

monomer

C C C C C C[ ]mer

C C

H H

H H

monomer

n

Ethylene (C2H

4) - gas

Polyethylene (PE) - solid

polymeric material

carbons are 109 toeach other (tetrahedral

bond angle for sp3

hybridization

zigzag structure

n = the degree of polymerization

Example


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Polymers are gigantic compared to

hydrocarbon molecules

called macromolecules

For most polymers

long, flexible chains with a stringof carbon atoms in the

backbone

remaining electrons can be

involved in side bonding with

atoms or groups of atoms

structural entities are called

mers


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Common PolymersCommon Polymers

C C C C C C

F F F F F F

Mer unit

F F F F F F

C C C C C C

H H H H H H

H Cl H Cl H Cl

Mer unit

C C C C C C

H H H H H H

H CH3 H

Mer unit

CH3 H CH3

Polypropylene (PP)

Polyvinylchloride (PVC)

Polytetrafluoroethylene

(PTFE) Trade name Teflon


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Branched Polymers where side-

branch chains are connected to the

main ones. The chain packingefficiency is reduced compared to

linear polymers lower density.

Cross linked Adjacent linear

chains are joined one to another at

various positions by covalent bonds.

Many rubbers have this structure.

Network Trifuntional mer units

having three active covalent bonds,form three dimensional networks.

Example: epoxy, phenol-

formaldehyde


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IsomersIsomers

Hydrocarbons with the same composition but different

atomic arrangements are called isomers (ex: Butane

and Isobutane - C4H10)

These isomers have different properties (e.g. b.p.)

Two types of isomerism are possible: stereoisomerism

and geometrical isomerism.

C

H

H

H

C

H

H

HC

H

H

C

H

HCH

H

H

C

H

HCH

H

C

H

H H

ButaneIsobutane


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Stereoisomerism

Stereoisomerism: atoms are linked together in the same order, but

can have different spatial arrangement

1 Isotactic configuration: all

side groups R are on the

same side of the chain.

2 Syndiotactic configuration:

side groups R alternate sides

of the chain.

3 Atactic configuration: random

orientations of groups R alongthe chain.


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Geometrical isomerism

Geometrical isomerism: consider two carbon atoms bonded by a

double bond in a chain. H atom or radical R bonded to these two

atoms can be on the same side of the chain (cis structure) or on

opposite sides of the chain (trans structure).

Cis-polyisoprene

Trans-polyisoprene


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Summary: Size Shape -Structure


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Thermoplastic and

Thermosetting Polymers

The response of a polymer tomechanical forces at elevated

temperatures is related to its

dominant molecular structure.

Thermoplast (thermoplastics:

Polymer that soften when heated (and

eventually liquefy) and harden when

cooled processes that are totallyreversible and may be repeated.

Example (polyethylene, most linear

polymers.

Thermoset (thermosetting polymers:

Polymers became permanently hard

when heat is applied and do not

soften upon subsequent heating.

Examples: vulcanized rubbers,

epoxies, phenolics, etc.


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Copolymers (composed

of different mers)

Copolymers: at least two

different types of mers,can differ in the way the

mers are arranged:

Random copolymer

Alternating copolymer

Block copolymer

Graft copolymer

Synthetic rubbers are

copolymers


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Polymer Crystallinity

The crystalline state may exist in

polymeric materials. Atomic

arrangement in polymer crystals is

more complex than in metals or

ceramics (unit cells are typicallylarge and complex).

Polymer molecules are often partially

crystalline (semi-crystalline), with

crystalline regions dispersed within

amorphous material.

Polyethylene


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Crystalline polymers are denser than amorphous polymers, so the

degree of crystallinity can be obtained from the measurement of

density:

c: Density of perfect crystalline polymer

A: Density of completely amorphous polymer

s: Density of partially crystalline polymer that we are analyzing

( )( )

100% xityCrystallinACS

ASC

=


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Polymer Crystals

Thin crystallineplatelets grown from

solution - chains fold

back and forth:

chain-folded model

Polyethylene

The average chain

length is much greater

than the thickness ofthe crystallite


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Spherulites:Aggregates

of lamellar crystallites ~10 nm thick, separated

by amorphous material.

Aggregates

approximately sphericalin shape.

Photomicrographspherulite structure

of polyethylene

M h i l B h i f P l


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Mechanical Behavior of Polymers

The mechanical properties of polymers are specified with many of the

same parameters used for metals. But polymers are highly sensitive tothe rate of deformation (strain rate), the temperature and the

environment.

The stress-strainbehavior can be

brittle (A), plastic (B),

and highly elastic (C)

Deformation shown

by curve C is totally

elastic (rubber-like

elasticity). This classof polymers -

elastomers

A: Brittle Polymer

B: Plastic Polymer

C: Elastomer


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Modulus of elasticity defined as for metals

Ductility (%EL) defined as for metals

Yield strength - For plastic polymers (B), yield strength is defined

by the maximum on curve just after the elastic region (different

from metals)

Tensile strength is defined at the fracture point and can be lowerthan the yield strength (different from metals)

Moduli of elasticity

Polymers: ~ 10 MPa - 4 GPaMetals: ~ 50 - 400 GPa

Tensile strengths

Polymers: ~ 10 - 100 MPa

Metals: 100s - 1000s MPa

Elongation

Polymers: up to 1000 % in

some casesMetals: < 100%

Temperature increase leads to:


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Temperature increase leads to:

Decrease in elastic modulus

Reduction in tensile strength

Increase in ductil ity

polymethyl methacrylate(PMMA)

Viscoelasticity


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Viscoelasticity

Amorphous polymer: glass at low temperatures, rubber atintermediate temperatures, viscous l iquid at high T.

Low temperatures: elastic deformation at small strains ( = E).Deformation is instantaneous when load is applied.

Deformation is reversible.High temperatures: viscous. Deformation is time dependent and

not reversible.

Intermediate temperatures: viscoelastic behavior. Instantaneous

elastic strain followed by viscous time dependent strain.

Viscoelastic behavior determined by rate of strain (elastic for

rapidly applied stress, viscous for slowly applied stress)


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Load is applied at ta and released at t r

Elastic

ViscousViscoelastic

F i T h i f Pl ti


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Forming Techniques for Plastics

Various techniques are employed in the forming of polymericmaterials:

Injection Molding

Compression Molding

Transfer Molding

Rotational Molding

Extrusion

Blow Molding

Blown film extrusion

Thermoforming

Calendaring

Fibering

Foaming

Laminating

C i M ldi


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Process for forming thermosets by applying heat and pressure. A measured amount of thermoset powder, granules or pellets, is fed

into the mold cavity.

Heat softens the material and pressure fills the cavity, then the

material is cured. Heat actually causes the polymer to transform into a highly cross-

linked and networked structure.

Process is of limited use for thermosets because of the cooling time

required of the mold. Typical products include electrical insulators, pot handles, and some

automotive parts.

Compression Molding


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Associated with processing thermoplastics. However, with development of the reciprocating screw type

equipment, thermosets can also be injection molded.

The basic process includes plasticizing, injection, cooling, and

ejection.

Granules are feed from a hopper into to a screw that rotates to feed

the material into a heated chamber to allow the material to change to

a molten state.

The material is then forced through a nozzle into the mold cavity.

A cooling time is necessary to allow the polymer to become solid,

and then is ejected from the mold by mechanical ejector pins.

Injection Molding


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Blown Film Extrusion


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Used to produce thin film hollow tubes.

Somewhat of a combination of extrusion, blown molding and calendaring. As material is extruded, air is forced through the center of a die, causing

the material to expand to the diameter of the mold.

Mold is open at the end, and the material is continuously taken up on

rollers. During the take up process, the walls on the "tube may be seamed welded

and perforated such as the case with garbage bags.

Extrusion


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Continuous flow of molten material is forced through a die.

Shape of the final product is determined by the shape of the dieopening.

Thermoplastic material is fed from a hopper, similar to the

configuration of the screw system in injection molding.

The screw forces the material through a tapered opening in the die.

Heat and friction causes plasticizing to occur, softens the material, and

forces it through the die opening.

Material is cooled by either air or water.

Rate of cooling can be controlled and further forming is possible.

Example, PVC pipe is extruded as electrical conduit. If allow to be

immersed in hot water, the conduit can be bent at 90 degree angles. Products that are extruded include tubing, rods, bars, moldings, sheets

and films.

Extrusion is also used for coating wire and cable.


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Making Fiber


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Making Fiber

Optic Cable

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Polymers MSE9 2