Polymer-(Ramesh- GEC CLT)aroramesh.weebly.com/.../polymer-(ramesh-_gec_clt).pdf · 2018-09-10 · 4.1 Ancient Polymers Originally natural polymers were used: – Wood – Rubber –

POLYMERS

Many Units

Dr. A. R. Ramesh

Assistant Professor1RAMESH-GEC Kozhikode- Polymer

The Structure and Properties of

Polymers

• Also known as

• Bonding +

• Properties

2RAMESH-GEC Kozhikode- Polymer

A polymer is a large molecule (macromolecules) made by

linking together repeating units of small molecules called

monomers

Repeat unit in a polymer is called ‘mer’3RAMESH-GEC Kozhikode- Polymer

4.1 Ancient PolymersOriginally natural

polymers were used:

– Wood

– Rubber

– Cotton

– Wool

– Leather

– SilkOldest known use:

Rubber balls used by Incas

Noah used pitch (a natural polymer) for the ark

Noah's pitch

Genesis 6:14 "...and cover it inside and outside

with pitch."

gum based resins

extracted from pine

trees


Examples of monomers and polymers

Monomer Polymer

HOCH 2CH 2OH

HO CO 2H

CH 2CH 2

CH 2CH 2O

CH 2CH 2O

O C

O

CH2

CH2

CH2

CHClCH

2CH

2

Cl

H2C CH

2

O


Degree of polymerisation: number of repeating unit in the

chain of polymer

Highpolymers: DP > 100

Oligopolymers: DP < 100

Tacticity: Stereochemistry

Atactic: Random

Isotactic: Same Side

Syndiotactic: side groups in alternating fashion


Nomenclature:

Homopolymer: A-A-A-A-A-A-A-A (identical units in linear fashion)

Copolymer: A-B-A-B-A-B-A-B (Different type of monomeric units)

Alternating Copolymer:A-B-A-B-A-B-A-B (alternating fashion)

Random Copolymer: A-B-A-A-A-B-A-B –B (Random fashion)

Block copolymer: A-A-A-A-A-A-B-B-B-B-B (Block of two polymers)

Graft copolymer: A-A-A-A-A-A-A-A-A- (homopolymergrafted in

another homopolymer)


Four Types of Copolymers


9

Block copolymer, example:

Poly(styrene)-block-poly(butadiene)

Random copolymer, example:

Poly(styrene-ran-butadiene)

Graft copolymer,

example:

Poly(styrene)-graft-poly(butadiene)

RAMESH-GEC Kozhikode- Polymer

Classification of polymers

1. Natural & Synthetic : Cotton , silk, wool Polyethylene (PE), PVC,

nylon

2. Organic & Inorganic: PE Glass, silicone

3. Thermoplastic & Themosetting: Soften on heating –linear/branched

(PE, PVC, nylon)

infusible & Insoluble mass on heating – network

(bakelite phenolformaldehyde resin)

4. Plastics, Elastomers, fibres & Liquid resins: Become hard & tough on

heating (PVC)

Vulcanized into rubbery products (natural rubber)

Long filament (nylon)

In liquid form as adhesives (Epoxy adhesivs)10RAMESH-GEC Kozhikode- Polymer

Types of polymerization

1. Addition or chain polymerization

(repeating units and monomers are same)

2. Condensation or step (repeating units and monomers

are not equal, combining two molecules by removing a

small molecule

3. Copolymerization

4. Coordination or Ziegler Natta polymerization


Addition polymerization

• Exact multiple of original monomeric molecule

• No loss of material

• Formed by monomers with double bonds

• Initiated by heat, light, pressure, radiation or catalyst


CH2=CH2 Polyethylene


Mechanism

� Free radical

� Cationic

� Anionic


Radical Polymerization

� Homolytic dissociation of initiator

� Initiator-Thermally unstable compound

� Easily decomposed to free radicals by the action of heat, light or

catalyst

� Eg: Acetyl or benzoyl peroxides 15RAMESH-GEC Kozhikode- Polymer

� Chain initiating species grows by successive addition of

monomer


Chain termination step




Cationic Polymerization

the alkene monomer

reacts with an electrophile

Formation of carbocation

AlCl3 + H2O [AlCl3(OH)]- + H+




Monomers that are best able to undergo cationic polymerization are

those with electron-donating substituents


Anionic Polymerization

Nonterminated chains are called living polymers

The chains remain active until they are killed

KNH2 K+ + NH2-

Bu-/NH2- +

CHAIN INITIATION


+ NH3

+ NH2-

CHAIN TERMINATION


Step-growth polymers OR condensation polymers

made by combining two molecules by removing a small molecule


Nylon 6 is an example of a step-growth polymer formed

by a monomer with two different functional groups


28

Addition Condensation

Example Polystyrene Nylon

Empirical formula No change from monomer.

Changes as byproduct (often water) is given off.

How grows One monomer at a time

Monomer + dimer, hexamer +

octadecamer, etc.

Polydispersity Can be paucidisperse “Most probable”

Molecular weight Wide range: can be very high

Low (except biopolymers)

Synonym Chain growth polymerization

Step growth polymerization

Chain growth Step growth


TABLE 1.1 Comparison of Step-Reaction and

Chain-Reaction Polymerization

Step Reaction Chain Reaction

Growth occurs throughout matrix by

reaction between monomers, oligomers,

and polymers

DPa low to moderate

Monomer consumed rapidly while

molecular weight increases slowly

No initiator needed; same reaction

mechanism throughout

No termination step; end groups still reactive

Polymerization rate decreases steadily as

functional groups consumed

Growth occurs by successive addition of

monomer units to limited number of

growing chains

DP can be very high

Monomer consumed relatively slowly, but

molecular weight increases rapidly

Initiation and propagation mechanisms different

Usually chain-terminating step involved

Polymerizaion rate increases initially as

initiator units generated; remains relatively

constant until monomer depleted

aDP, average degree of polymerization.. 29RAMESH-GEC Kozhikode- Polymer

Copolymerization

Two or more different types of monomers undergo polymerization

together


Coordination or Ziegler Natta polymerization

� Produce stereospecific polymers

� Use Ziegler Natta catalyst

� Complex = {Alkyls + Metal ( group I-III) } + halides of transition

metals (IV-VIII)

� {(C2H5)3Al, (C2H5)2AlCl} + TiCl4



Polymerization Processes

• TWO USEFUL DISTINCTIONS ;

– BETWEEN BATCH AND CONTINUOUS

– AND BETWEEN SINGLE - PHASE AND MULTI -

PHASE

• SINGLE - PHASE

– Bulk or Melt Polymerization

– Solution Polymerization


Polymerization Techniques

These include:

• Bulk Polymerization

• Solution Polymerization

• Suspension Polymerization

• Emulsion Polymerization


Bulk Polymerization

• Bulk polymerization is carried out in the absence of any solvent

or dispersantand is thus the simplest in terms of formulation.

• carried out by adding a soluble initiator to pure monomer in

liquid state.

• The reaction is initiated by heating or exposing to radiation.

• Gives the highest-purity polymer

• This process can be used for many free radical polymerizations

and some step-growth (condensation) polymerization.


• Two stages

• First- Radical initiated thermal polymerization of monomer (heat)

• Cold water circulate

• Second- main reactor, 110-200oc

• 100% complete

• Unreacted monomer volatilize off


Bulk Polymerization

Advantages:

• High yield per reactor volume

• Easy polymer recovery

• The option of casting the polymerisation

mixture into final product form


Bulk Polymerization

Limitations:

• Difficulty in removing the last traces of

monomer

• The problem of dissipating heat produced

during the polymerization

– In practice, heat dissipated during bulk

polymerization can be improved by providing

special baffles


Solution Polymerization

• Definition: A polymerization process in which

the monomers and the polymerization

initiators are dissolved in a inert liquid solvent

at the beginning of the polymerization

reaction. The liquid is usually also a solvent for

the resulting polymer or copolymer.

• Solvent control viscosity increase & heat

transfer


Solution Polymerization

Disadvantages

• Chain transfer to the solvent- difficulty to get high

molecular weight polymers

• Removal of solvent

Advantages

• Polymers used in solution form (adhesives & coatings)

• Preparation of block copolymer


Suspension Polymerization

• Definition: A polymerization process in which the monomer, or

mixture of monomers, is dispersed by mechanical agitation in a

liquid phase, usually water, in which the monomer droplets are

polymerized while they are dispersed by continuous agitation. Used

primarily for PVC polymerization

• If the monomer is insoluble in water, bulk polymerization can be

carried out in suspended droplets, i.e., monomer is mechanically

dispersed.

• The water phase becomes the heat transfer medium.41RAMESH-GEC Kozhikode- Polymer

Suspension Polymerization

• So the heat transfer is very good. In this system, the monomer

must be either

– 1) insoluble in water or

– 2) only slightly soluble in water, so that when it polymerizes it

becomes insoluble in water.

• The behavior inside the droplets is very much like the behavior of

bulk polymerization

• Since the droplets are only 10 to 1000 microns in diameter, more

rapid reaction rates can be tolerated (than would be the case for

bulk polymerization) without boilingthe monomer.42RAMESH-GEC Kozhikode- Polymer

Emulsion Polymerization

� Monomer dispersed as droplets in water containing soap or detergent

� Initiators-water/monomer soluble (persulphate)

� Suspension of high molecular weight polymers obtained

� Solid polymer obtained by coagulating the suspension by adding acid

� Starts with an emulsion incorporating water,

monomer, and surfactant.

� Common- oil-in-water emulsion


Emulsion Polymerization – Schematic



Advantages of emulsion polymerization include:

• High molecular weight polymers can be made at fast

polymerization rates. By contrast, in bulk and solution

free radical polymerization, there is a tradeoff between

molecular weight and polymerization rate.

• The continuous water phase is an excellent conductor

of heat and allows the heat to be removed from the

system, allowing many reaction methods to increase

their rate.



Advantages Continued:

• Since polymer molecules are contained within

the particles, viscosity remains close to that of

water and is not dependent on molecular

weight.

• The final product can be used as is and does

not generally need to be altered or processed.



Disadvantages of emulsion polymerization include:

• For dry (isolated) polymers, water removal is an

energy-intensive process

• Emulsion polymerizations are usually designed to

operate at high conversion of monomer to polymer.

This can result in significant chain transfer to

polymer.


48

Linear polymers can be represented by a

simple sequence such as: A-A-A-A-A .

Polystyrene

Styrene monomer

CH CH2

n

Nylon

Two monomers

make one

repeating unit.**There many different kinds of nylon.

H2N-(CH2)6-NH2

Nylon monomer

HOOCCOOH

Nylon 6,6


49

Polydispersity is the term we use to describe the fact that

not all macromolecules in a given sample have the same

“repeat number” x.

size

#

size

#

size

Polydisperse Monodisperse Paucidisperse

Even in a pure sample, not all molecules will be the same.

Nature often does better than people do.

#


50

The molecular weight of condensation (step growth)

polymers is limited to fairly low values.

Condensations: usually < 50,000 g/mol

Addition: can be quite high

(e.g., 46 x 106 for polystyrene)

Convert that to tons/mol

Nature makes huge polycondensates, but they are usually made in chain growth fashion!

Why?



Structure

Primary bonds (Chemical bond)

Secondary bonds (intermolecular force, weak Van der Waals force) –

Chains held together

� chain length

� high - Strength of van der Waals force increase (Molecular

weight) < 150 atoms

� Low molecular weight – Soft & gummy (sticky gum), brittle at

low temperature

� High Molecular weight – tough & heat resistant

� Polar groups (carboxyl, hydroxyl, Cl, F) increase interaction – nylon,

teflon, polyester52RAMESH-GEC Kozhikode- Polymer

� Slip movement –

� Simple & Uniform shape - easy movement (PE)

� PVC- lumps of Cl – restricted movement – tougher & Stronger

than PE

� Cross linked – covalent bonds in 3D – No movement – Most

strong & tough


• The bonding process.

– When thermoplastic polymers are heated they become

flexible. There are no cross-links and the molecules can

slide over each other.

– Thermosettingpolymers do not soften when heated

because molecules are crosslinked together and remain

rigid.

PVCBakelite


Thermoplastics

• 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.


Thermoplastics

• Those which soften on heating and then harden again on cooling

These are called thermoplastic polymers because they keep their

plastic properties

• These polymer molecules consist of long chains which have only

weak bonds between the chains

• The bonds between the chains are so weak that they can be

broken when the plastic is heated

• The chains can then move around to form a different shape

• The weak bonds reform when it is cooled and the

• thermoplastic material keeps its new shape



Thermosets

• Extensive cross-linking formed by covalent bonds.

• Bonds prevent chains moving relative to each other.

• What will the properties of this type of plastic be like?


Thermosetting

• Those which never soften once they have been moulded

These are called thermosetting polymers because once set into a

shape, that shape cannot be altered

• These polymer molecules consist of long chains which have many

strong chemical bonds between the chains

• The bonds between the chains are so strong that they cannot be

broken when the plastic is heated

• This means that the thermosetting material always keeps its shape



Physical state - Crystallinity

• The polymer chain layout determines a lot of material

properties:

• Amorphous:

• Crystalline:


62

Crystallinity in Polymers• Ordered atomic

arrangements involving

molecular chains

• Crystal structures in terms of

unit cells

• Example shown

– polyethylene unit cell

Adapted from Fig. 4.10, Callister & Rethwisch 3e.

– Polymers can be crystalline (i.e. have

long range order)

– However, given these are large

molecules as compared to atoms/ions

(i.e. metals/ceramics) the crystal

structures/packing will be much more

complexRAMESH-GEC Kozhikode- Polymer

• Polymer crystallinity

– (One of the) differences between small molecules and

polymers

– Small molecules can either totally crystallize or become an

amorphous solid

– Polymers often are only partially crystalline

• Why? Molecules are very large

• Have crystalline regions dispersed within the remaining

amorphous materials

• Polymers are often referred to as semicrystalline


64

Polymers rarely 100% crystalline

• Difficult for all regions of all chains to

become aligned

• Degree of crystallinity

expressed as % crystallinity.-- Some physical properties

depend on % crystallinity.

-- Heat treating causes

crystalline regions to grow

and % crystallinity to

increase.

Adapted from Fig. 14.11, Callister 6e.(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,and J. Wulff, The Structure and Properties of

Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)

crystalline region

amorphousregion


• Polymer crystallinity

– Another way to think about it is that these are two phase

materials (crystalline, amorphous)

– Need to estimate degree of crystallinity – many ways

• One is from the density


– What influences the degree of crystallinity

• Rate of cooling during solidification

• Molecular chemistry – structure matters

– Polyisoprene – hard to crystallize

– Polyethylene –not hard to crystallize

• Linear polymers are easier to crystallize

• Side chains interfere with crystallization

• Stereoisomers – atactic hard to crystallize (why?);

isotactic, syndiotactic – easier to crystallize

• Copolymers – more random; harder to crystallize


Effect of crystallinity on properties

� Density of crystalline regions will be higher

� Less permeable

� Less ease of acid hydrolysis on cellulose

� Less attack of oxygen (PE, Polypropylene)

� High degree of crystallinity - Transparent


4.12 Polymer crystals

– Chain folded-model

• Many polymers crystallize as very thin platelets (or lamellae)

• Idea – the chain folds back and forth within an individual plate

(chain folded model)

4.12 Polymer Crystallinity

• Crystalline regions

– thin platelets with chain folds at faces

– Chain folded structure


Plastic Deformation

• Thermoplastics- structure deforms to plastic stage on the

application of heat & pressure

• Plastic stage – help to get moulded in the desired shape

• Linear polymers – maximum plastic deformation

• Plasticity decreases proportionally with fall in temperature

• Thermoset – cross linking – covalent bond – No chance for slip or

deformation


Mechanical Properties

o Strength measured by stress-strain test

o Amount of stretch for a given stress (applied force) is a measure

of strain

o Stress strain graph – “necking and break point

o Necking – Stress becomes large- uncoiled and line up to a more

orderly arrangement

o Break point – stress becomes so large to break the sample


Tensile Strength

� Defined as the tensile stress requires to stretch the test piece to

the break point

� Expressed in terms of breaking force per unit area of the original

cross sectional area

� Quantifies how much stress the material will endure (tolerate)

before failure

� Ability to withstand stress with getting pulled apart in linear

direction

� Measured by stretching a dumb bell test piece in universal testing

machine

� Increases with polymer chain length & crosslinking


Testing gives information about

� How much force it takes to pull a polymer material apart (tensile

strength at break)

� How far a polymer material will stretch before break (elongation

at break)

� How it deforms as it is gets pulled apart (ratio of tensile stress to

tensile strain


Mechanical Properties of Polymers


Tensile strength

• Mechanical behavior of amorphous and semi-

crystalline polymers is strongly affected by Tg

• In general

• Polymers whose Tg is above the service

temperature are strong, stiff and sometimes

brittle• e.g. Polystyrene (cheap, clear plastic drink cups)

– Polymers whose Tg is below the service temperature

are weaker, less rigid, and more ductile

• Polyethylene (milk jugs)


Tear Resistance (strength)

• Measure of how well a material can withstand the effects of

tearing

• How well a material resists the growth of any cuts when under

tension

• Measured in kN/m

• Sample is held between two holders and a uniform pulling force

applied

• Tear resistance is calculated by dividing the force applied by

thickness of material


Abrasion Resistance

� Defined as the resistance to wear or reciprocal of the abrasion

loss

� Scratch test – material subjects to many scratches (abrasive

wheel) and abrasion determined by loss of weight

� Footwear industry


Resilience

� The physical property of a material that can return to its original

shape or position after deformation that does not exceed its

elastic limit

� Ability of the material to absorb energy when it is deformed

elastically and release that energy upon unloading

� Measure how far the test specimen of the polymer will recover

the original dimension on the release of stress

� Pendulum rebound testing machine


Density

� Density = mass per unit volume (volumetric mass density)

� Relative density or specific gravity – ratio of the density to

the material to the standard material (water)

� Measured to

� Identify a material

� Follow physical changes

� Understand the uniformity of a material among different

sampling units

� Understand the porosity of the material


Glass transition temperature (Tg)

The glass transition temperature (Tg) describes

the temperature at which amorphous polymers

undergo (a second order) phase transition from

a hard brittle, glassy amorphous solid (glassy

state) to a soft, rubbery, viscous amorphous

solid (rubbery/viscoelastic state)


Melting temperature (Tm)

• The (Tm) when applied to polymers

suggests not a solid-liquid phase transition,

but a transition from a crystalline phase to

a solid amorphous phase. Crystalline

melting is only discussed with

thermoplastics, as thermosets will

decompose at high temperatures rather

than melt.


Glass-rubber-liquid

• Amorphous plastics have a complex thermal profile with 3

typical states:

Glassy (Elastic-high modulus)

Leathery

(Elastic-low modulus)

Thermoplastic (uncrosslinked)

Tg Tm

Modulu

s o

f ela

sticity

Temp.

Rubbery Plateau

Elastic at high strain rate

Viscous at low strain rate


How polymers are effected by temperature changes

• Heats solids made of small molecules – melt to form liquid an eventually boil

• Polymers not so simple• E.g. rubber cooled in liquid nitrogen becomes brittle

and can be smashed• It becomes GLASSY• poly(propene) becomes brittle at about -10 C• Structure of many polymers mixture of ordered

areas (crystalline) and random (amorphous)• In glassy state the amorphous regions become

‘frozen’ so cant can’t change shape if it has to move it does so breaking



• If you heat the glassy material, polymer chains reach a temp at which they move relative to each other. This is the glass transistion temperature (Tg)

• When polymer is warmer than this, we see the typical plastic properties we expect-



• On further heating we reach the melting temperature (Tm)

• The crystalline regions break down and polymer becomes a viscous fluid

• These processes are reversible for thermoplastics


Matching polymer properties to needs

• Pure poly(chloroethene)- PVC has a Tg of about 80 C – rigid and quite brittle at room temp

• Used to make drain pipes

• Sometimes called unplasticised PVC or uPVC

• To make it more flexible the Tg needs to be lowered.

• One way of doing this is to copolymerise the chloroethene with a small amount of ethenyl ethanoate



• Introduces different side groups into the polymer chain

• Chains pack together less well – attractive forces are weaker

• Polymer is more flexible because the chains can move over one another more easily


Plasticizers

Are small molecules which occupy position

between polymer chains (like adding

water to mud to make it easy in molding)

1. To increase flexibility, elongation and to

reduce hardness and stiffness.

2. To lower the processing temperature

(energy saving, decomposition

preventing)



• Another way is to use a ‘molecular lubricant’ – a plasticiser

• Allows the PVC chains to slide over each other more easily


Documents

Polymer-(Ramesh- GEC CLT)aroramesh.weebly.com/.../polymer-(ramesh-_gec_clt).pdf · 2018-09-10 · 4.1 Ancient Polymers Originally natural polymers were used: – Wood – Rubber –