Pendahuluan

Plastic Products

• Plastics can be shaped into a wide variety of products: – Molded parts– Extruded sections– Films– Sheets– Insulation coatings on electrical wires– Fibers for textiles

9

Polymer Science and Processing Technology • Successful product design requires a knowledge of:

– the requirements of the final product– the behaviour of polymeric materials– commercial polymer processing technology– relevant cost and market factors.

At the heart of polymer science andtechnology is molecular structure. It dictates not only final product properties,but polymer synthesis and processingmethods.

J.S. Parent 10

Polymer Classification: Terminology

• While we have chosen an applications perspective on polymer classification, many alternate schemes are widely used.

• These are usually composition/property specific, as opposed to applications oriented.

Definisi & Klasifikasi Polimer

Polymers: Introduction

• Polymer: High molecular weight molecule made up of a small repeat unit (monomer).– A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A

• Monomer: Low molecular weight compound that can be connected together to give a polymer

• Oligomer: Short polymer chain• Copolymer: polymer made up of 2 or more monomers

– Random copolymer: A-B-B-A-A-B-A-B-A-B-B-B-A-A-B– Alternating copolymer: A-B-A-B-A-B-A-B-A-B-A-B-A-B– Block copolymer: A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B

14

Polymer

Poly mer many repeat unit (building blocks)

C C C C C C

HHHHHH

HHHHHH

Polyethylene (PE)

ClCl Cl

C C C C C C

HHH

HHHHHH

Poly(vinyl chloride) (PVC)

HH

HHH H

Polypropylene (PP)

C C C C C C

CH3

HH

CH3CH3H

repeatunit

repeatunit

repeatunit

Carbon chain backbone

15

Types of Polymers

• Polymer Classifications– Thermoset: cross-linked polymer that cannot be

melted (tires, rubber bands)– Thermoplastic: Meltable plastic– Elastomers: Polymers that stretch and then return to

their original form: often thermoset polymers– Thermoplastic elastomers: Elastic polymers that can

be melted (soles of tennis shoes)

• Polymer Families– Polyolefins: made from olefin (alkene) monomers– Polyesters, Amides, Urethanes, etc.: monomers linked

by ester, amide, urethane or other functional groups– Natural Polymers: Polysaccharides, DNA, proteins

16

Common PolyolefinsMonomer Polymer

Ethylene

H3CCH3

nRepeat unitPolyethylene

CH3

CH3n

CH3 CH3 CH3 CH3 CH3 CH3CH3Propylene

Polypropylene

PhCH3

n

Ph Ph Ph Ph Ph PhPhStyrene

Polystyrene

ClCH3

n

Cl Cl Cl Cl Cl ClClVinyl Chloride

Poly(vinyl chloride)

F2C CF2

Tetrafluoroethylene

F3C

F2C

CF2

F2C

CF2

F2C

CF2

F2C

CF2

F2C

CF2

F2C

CF2

CF3

nPoly(tetrafluoroethylene): Teflon

17

18

19

Polyesters, Amides, and UrethanesMonomer Polymer

CO2HHO2CHO

OHO O

HO OH2C

H2C O

nTerephthalic acid

Ethyleneglycol

Poly(ethylene terephthalate

H

Ester

HO OH

O O

4H2N NH24

Adipic Acid 1,6-Diaminohexane Nylon 6,6HO N

HNH

H

O O

4 4n

CO2HHO2C

Terephthalic acid

NH2H2N

1,4-Diamino benzene

Kevlar

O

HO

OHN

HN H

n

Amide

HOOH

Ethyleneglycol

H2COCN NCO

4,4-diisocyantophenylmethaneSpandex

H2C

HN

HN

O

HO

O

OH2C

H2C O H

n

Urethane linkage

20

Natural PolymersMonomer Polymer

Isoprenen

Polyisoprene:Natural rubber

O

H

HO

H

HO

H

HOHH OH

OH

Poly(ß-D-glycoside):cellulose

O

H

O

H

HO

H

HOHH OH

OH

H

n

ß-D-glucose

H3N

O

O

RPolyamino acid:protein

H3N

OHN

R1

OHN

Rn+1

O

OH

Rn+2n

Amino Acid

BaseO

OH

OP

O

O

O

oligonucleic acidDNA

NucleotideBase = C, G, T, A

BaseO

O

OP

O

O

O

DNA

DNA

21

Rubbers

Polymer ClassificationPolymers are commonly classified based on their

underlying molecular structure.

Polymers

Elastomers ThermosetsThermoplastics

Crystalline Amorphous

26

Polymer Classification: Thermoplastic/Thermoset

• One of the most practical and useful classification of polymer compounds is based on their ability to be refabricated.

• Thermoplastic: polymers that can be heat-softened in order to process into a desired form.

– Polystyrene, polyethylene– recyclable food containers

• Thermoset: polymers whose individual chains have been chemically crosslinked by covalent bonds and therefore resist heat softening, creep and solvent attack.– Phenol-formaldehyde resins, melamine paints– permanent adhesives, coatings

Thermoplastics and Thermosets• The response of a polymer to mechanical forces at elevated

temperature is related to its dominant molecular structure. • One classification of polymers is according to its behavior and

rising temperature. Thermoplastics and Thermosets are the 2 categories.

• A thermoplastic is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently.

• Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon).

27

• Thermoplastic polymers differ from thermosetting polymers (Bakelite, vulcanized rubber) since thermoplastics can be remelted and remolded.

• Thermosetting plastics when heated, will chemically decompose, so they can not be recycled. Yet, once a thermoset is cured it tends to be stronger than a thermoplastic.

• Typically, linear polymers with minor branched structures (and flexible chains) are thermoplastics. The networked structures are thermosets.

28

Thermoplastics and Thermosets

29

Examples of Thermoplastics

PTFE

More Examples of Thermoplastics

http://www2.dupont.com/Teflon/en_US/index.html

http://en.wikipedia.org/wiki/Teflon

Polymer

Some engineering polymers (1)

Some engineering polymers (2)

Some engineering polymers (3)

Elastomers (1)

Elastomer (2)

Struktur & sintesa Polimer

Polymer Classification: Chain Architecture

• Linear: A linear polymer chain is one without branches. Its actual conformation may not be “line-like”, but varies with chain stiffness, crystallinity and applied stresses.

• Branched: Chains with an appreciable number of side-chains are classified as branched. These side chains may differ in composition from the polymer backbone.

• Crosslinked: A continuous network of polymer chains is a crosslinked condition. In effect, there is just one polymer chain of infinite molecular weight.

• Chain architecture has a dramatic effect on properties such as viscosity, elasticity and temperature stability.

Polymer Classification: Chemical Microstructure

Homopolymers: polymers derived from a single monomer (can be linear, branched or

crosslinked). poly(ethylene), poly(butadiene).

Random copolymers: two monomers randomly distributed in chain. AABAAABBABAABBA poly(acrylonitrile-ran-butadiene)

Alternating copolymers: two monomers incorporated sequentially ABABABABABABABAB poly(styrene-alt-maleic anhydride)

Block copolymers: linear arrangement of blocks of high mol weight AAAAAAAAAAABBBBBBBBBBBBBBBAAAAAAAA polystyrene-block-polybutadiene-block-polystyrene or

poly(styrene-b-butadiene-b-styrene)

Graft copolymers: differing backbone and side-chain monomers poly(isobutylene-graft-butadiene)

43

Copolymerstwo or more monomers

polymerized together • random – A and B randomly

positioned along chain• alternating – A and B alternate

in polymer chain• block – large blocks of A units

alternate with large blocks of B units

• graft – chains of B units grafted onto A backbone

A – B –

random

block

graft

alternating

Copolymers can be used to tailor functionality or generate new phases and behaviors.

Block copolymer, example: Poly(styrene)-block-poly(butadiene)

Random copolymer, example: Poly(styrene-ran-butadiene)

Graft copolymer, example: Poly(styrene)-graft-poly(butadiene)

Molecular Structures for Polymers

• The physical characteristics of a polymer depend also on differences in the structure of the molecular chains (other variables are shape and weight).

• Linear polymers have repeat units joined end to end in single chains. There may be extensive van der Waals and hydrogen bonding between the chains. Examples: polyethylene, PVC, nylon.

46

Branched Cross-Linked NetworkLinear

secondarybonding

Molecular Structures- Branched

• Where side-branch chains have connected to main chains, these are termed branched polymers. Linear structures may have side-branching.

• HDPE – high density polyethylene is primarily a linear polymer with minor branching, while LDPE – low density polyethylene contains numerous short chain branches.

• Greater chain linearity and chain length tend to increase the melting point and improve the physical and mechanical properties of the polymer due to greater crystallinity.

47

Branched Cross-Linked NetworkLinear

Molecular Structures – Cross-linked, Network

• In cross-linked polymers, adjacent linear chains are joined to one another at various positions by covalent bonding of atoms. Examples are the rubber elastic materials.

• Small molecules that form 3 or more active covalent bonds create structures called network polymers. Examples are the epoxies and polyurethanes.

48

Branched Cross-Linked NetworkLinear

secondarybonding

49

Chemistry and Structure of Polyethylene

•Polyethylene is a long-chain hydrocarbon. •Top figure shows repeat unit and chain structures. •Other figure shows zigzag backbone structure.

Tetrahedral arrangement of C-H

50

MOLECULAR WEIGHT• Molecular weight, M: Mass of a mole of chains.

Low M

high M

• Polymers can have various lengths depending on the number of repeat units.

• During the polymerization process not all chains in a polymer grow to the same length, so there is a distribution of molecular weights. There are several ways of defining an average molecular weight.

• The molecular weight distribution in a polymer describes the relationship between the number of moles of each polymer species and the molar mass of that species.

Molecular Weight Distribution

The distribution of molecular weights within a polymer is characterized not by a single, unique average, but defined through a number of different ways.

The number average, Mn,considers the number ofmolecules of each size, Mi, in the sample:

The weight average, Mw, considers the mass ofmolecules of each sizewithin the sample:

i

iin n

MnM

i

iiw w

MwM

52

xi = number fraction of chains in size range i

MOLECULAR WEIGHT DISTRIBUTION

iiw

iin

MwM

MxM

wi = weight fraction of chains in size range i

Mi = mean (middle) molecular weight of size range i

Mn = the number average molecular weight (mass)__

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.

#

MW Requirements of Industrial Polymers

Elastomers amorphous materials operating above Tg physical props derived from chain entanglement, crosslinking

Adhesives range from elastomeric (pressure sensitive) to semi-

crystalline (hot melt) to glassy (epoxy resins)

Plastics broad class of materials whose properties are derived from

an amorphous phase and often from a crystalline phase

Fibres highly crystalline materials physical properties derived from degree of crystallinity

Coatings must be applied as a low viscosity medium and “cure” to

produce satisfactory properties

Composition Distribution

Inclusion of two or more monomers in a material generates a distribution of composition within, and between, polymer chains. Changes to material properties can be pronounced:

Chemical properties: fibre dyeability, hot melt adhesive bonding strength, solvent resistance…

Phase transitions: crystallinity, Tg, Tm, phase separation Mechanical properties: elasticity, modulus, toughness...

“Tailored” polymers can be developed through consideration of: the character of incorporated monomers and,

»polarity of main chain or pendant functionality»potential for crystallization

their sequence distribution within polymer chains.» random chain composition»alternating»block sequencing»graft structure

Random Copolymers

Materials comprised of a random distribution of different monomers are the most widely employed industrial copolymers.

Compositions vary from essentially homopolymers to materials containing equimolar quantities of constituent monomers, depending on the application.

Compare: Poly(acrylonitrile), Poly(butadiene) and Poly(acrylonitrile-co-

butadiene) Poly(ethylene), atactic Poly(propylene) and Poly(ethylene-

co-propylene) Poly(acrylonitrile) and Poly(acrylonitrile-co-methyl acrylate)

Random Copolymer Phase Transitions

Random incorporation of significant amounts of two or more monomers generally:

yields a Tg between the two constituent homopolymers. lowers Tm for crystalline domains, and reduces the extent of

crystallinity.

Shown here is a generic phase diagram for a system of “semi-crystalline” monomers.

Polymer Chain Lengths• Many polymer properties are affected by the length

of the polymer chains. For example, the melting temperature increases with increasing molecular weight.

• At room temp, polymers with very short chains (roughly 100 g/mol) will exist as liquids.

• Those with weights of 1000 g/mol are typically waxy solids and soft resins.

• Solid polymers range between 10,000 and several million g/mol.

• The molecular weight affects the polymer’s properties (examples: elastic modulus & strength). 58

Polymers – Molecular Shape

• Straight (b) and twisted (c) chain segments are generated when the backbone carbon atoms (dark circles) are oriented as in the figure above.

• Chain bending and twisting are possible by rotation of carbon atoms around their chain bonds.

• Some of the polymer mechanical and thermal characteristics are a function of the chain segment rotation in response to applied stresses or thermal vibrations. 59

Polymer Classification: Chemical Class

A popular classification scheme amongst chemists is based on polymer functionality.

Polyesters: poly(ethylene terephthalate) - Dacron

Polyamides: poly(caprolactam) - nylon 6

Urethanes: carbamate linkages through reaction of diisocyanates and diols.

Another (!) classification scheme, again favoured by chemists is based on differences between the polymer and constituent monomer(s).

Condensation polymers: synthesis involves elimination of some small molecule (H2O in the preparation of nylon)

Addition polymer: formed without loss of a small molecule i.e. ethylene polymerization to generate poly(ethylene)

C O

O

N C

OH

N C

O

O

H

Alternating Copolymers

Alternating copolymers are essentially -AB- homopolymers. Condensation polymers are structurally of this class, but are

not considered alternating.

C N

poly(butadiene-ran-acrylonitrile)

C N

poly(butadiene-alt-acrylonitrile)

Alternating Copolymers: Elastomers

Typical stress-strain curves at 0°C for unreinforced rubber vulcanizates:

1: poly(propylene-alt-butadiene) 2: natural rubber: cis-1,4-

polyisoprene

3: synthetic cis-1,4-polyisoprene 4: cis-1,4-polybutadiene

5: poly(styrene-co-butadiene) (25:75)

CHEE 890 J.S. Parent

Segmented Polyurethanes

Schematic morphology of unstretched semicrystalline polyurethane copolymer (segmented block copolymer).

A: hard nylon fibre

B: bicomponent nylon-

spandex

C: mechanical stretch

nylon

D: spandex

E: extruded latex

Graft-Modified Polyolefins

Functionalization of polymers is a cost-effective means of developing specialty materials. Grafting of versatile functional groups to polyolefins can improve blend performance and/or enhance high-temperature properties.

A leading example is the melt grafting of maleic anhydride to polyethylene to generate a material that improves adhesion between polyethylene and nylon phases in toughened nylon parts.

What is the distribution of grafts in the material?

OO O

ROOR

O

O

O

Graft-Modified Materials: Composition Distribution

Silane modified resins are used for enhanced adhesion with siliceous fillers and moisture curing:

Distribution of silane grafts is expected to influence: coupling efficiency to fillers crosslink density of cured articles

Si(OMe)3

Si(OMe)3

ROOR

Polymer Synthesis

There are two major classes of polymer formation mechanisms Addition polymerization: The polymer grows by sequential

addition of monomers to a reactive site»Chain growth is linear»Maximum molecular weight is obtained early in the

reaction Step-Growth polymerization: Monomers react together to

make small oligomers. Small oligomers make bigger ones, and big oligomers react to give polymers.

»Chain growth is exponential»Maximum molecular weight is obtained late in the

reaction

Addition Polymerization

In*A

InitiationIn A* A

Addition Polymerization

Propagation

In*A

InitiationIn A A* A

Addition Polymerization

Propagation

AIn*A

InitiationIn A A A*

Addition Polymerization

Propagation

nAIn A A A A

nA*

A A A A Am

In A A A AnA

*A A A A Am

Combination

*A A A A Am

In A A A AnA

B A A A Am

Disproportionation

TerminationReactive site is consumed

A

In A A A AnA

A*

Chain TransferNew reactive siteis produced

MW kpropagation

k ter mination

MW

% conversion0 100

In*A

InitiationIn A A A A*

7171

Addition (Chain) Polymerization

– Initiation

– Propagation

– Termination

Types of Addition Polymerizations

Ph

Anionic

C3H7 Li C4H9

Ph

Li+ Phn

C4H9

Ph Ph

Li+

n

Ph

Radical

PhCO2•Ph

n

Ph

Cationic

Cl3Al OH2H

PhHOAlCl3

Phn

H

Ph Phn

HOAlCl3

PhCO2

Ph

PhCO2

Ph Phn

Step-Growth Polymerization

Stage 1

Consumptionof monomer

n n

Stage 2

Combinationof small fragments

Stage 3

Reaction of oligomers to give high molecular weight polymer

Step-Growth Polymerization

Because high polymer does not form until the end of the reaction, high molecular weight polymer is not obtained unless high conversion of monomer is achieved.

Xn 11 p

Xn = Degree of polymerizationp = mole fraction monomer conversion

1

10

100

1000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Mole Fraction Conversion (p)

Deg

ree o

f P

oly

meri

zati

on

Nylon-6,6

Cl Cl

O O

4H2N NH24

Adipoyl chloride 1,6-Diaminohexane

Cl NH

NH

H

O O

4 4

NaOH

HO NH

NH

H

O O

4 4n

6 carbondiacid

6 carbondiamine

Nylon-6,6Diamine, NaOH, in H2O

Adipoyl chloridein hexane

Nylon 6,6

Nylon-6,6

Diamine, NaOH, in H2O

Adipoyl chloridein hexane

Nylon 6,6

Since the reactants are in different phases, they can only react at the phase boundary. Once a layer of polymer forms, no more reaction occurs. Removing the polymer allows more reaction to occur.

Some Common Addition Polymers

8181

Condensation (Step) Polymerization

Condensation polymerization (ex. Polyester)

Some Condensation Polymers

85

Ancient Polymers• Naturally occurring polymers (those derived

from plants and animals) have been used for centuries.– Wood – Rubber– Cotton – Wool– Leather – Silk

• Oldest known uses– Rubber balls used by Incas

Cellulose

• Cellulose is a highly abundant organic compound. Extensive hydrogen bonding between the chains causes native celluose to be roughly 70% crystalline. It also raises the melting point (>280°C) to above its combustion temperature.

• Cellulose serves as the principal structural component of green plants and wood.

• Cotton is one of the purest forms of cellulose and has been cultivated since ancient times.

• Cotton also serves (along with treated wood pulp) as the source the industrial production of cellulose-derived materials which were the first "plastic" materials of commercial importance.

Rubber• A variety of plants produce a sap consisting of a colloidal dispersion of

cis-polyisoprene. This milky fluid is especially abundant in the rubber tree (Hevea); it drips when the bark is wounded.

• After collection, the latex is coagulated to obtain the solid rubber. Natural rubber is thermoplastic, with a glass transition temperature of –70°C.

• Raw natural rubber tends to be sticky when warm and brittle when cold, so it was little more than a novelty material when first introduced in Europe around 1770.

• It did not become generally useful until the mid-nineteenth century when Charles Goodyear found that heating it with sulfur — a process he called vulcanization — could greatly improve its properties.

cis-polyisoprene

Sifat-sifat Polimer

Specific Thermoplastic Properties

Thermoset data

Thermoset Properties

Mechanical properties of Polymers

Hooke’s Law

Young's modulus, also known as the tensile modulus or elastic modulus, is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the stress (force per unit area) along an axis to the strain (ratio of deformation over initial length) along that axis in the range of stress in which Hooke's law holds.

Young's modulus, E, can be calculated by dividing the tensile stress by the extensional strain in the elastic (initial, linear) portion of the stress–strain curve: whereE is the Young's modulus (modulus of elasticity)F is the force exerted on an object under tension;A0 is the original cross-sectional area through which the force is applied;ΔL is the amount by which the length of the object changes;L0 is the original length of the object.

• Young's Modulus =Stiffness = E = Slope of Stress/Strain Curve = = E = Slope of Stress/Strain Curve =s / e

• The steeper the stress/strain relationship (the higher Young's Modulus, E), the more stiff the material. Breaking strain and stress are the amounts of strain and stress observed at the breaking point. The more ductile the material, the more strain at breakage; the stronger the material, the higher the breaking stress.

• Ductile materials will deform more than stiff materials before breaking; however, ductile materials are not usually as strong a stiff materials. Compare, for example, a brittle object, say beach glass, and a ductile object, perhaps a rubber chicken. Glass may be strong (high breaking stress) but it is not ductile (low strain (i.e., little deformation) at breaking). A rubber chicken extends quite a bit before it breaks (long live rubber chickens) but can not withstand as much stress.

Stress

• Stress is "force per unit area" - the ratio of applied force F and cross section - defined as "force per area".

• tensile stress - stress that tends to stretch or lengthen the material - acts normal to the stressed area

• compressive stress - stress that tends to compress or shorten the material - acts normal to the stressed area

• shearing stress - stress that tends to shear the material - acts in plane to the stressed area at right-angles to compressive or tensile stress

Strain

Strain is defined as "deformation of a solid due to stress" and can be expressed asε = dl / lo

= σ / E wheredl = change of length (m, in)lo = initial length (m, in)

ε = unit less measure of engineering strainE = Young's modulus (Modulus of Elasticity) (N/m2 (Pa), lb/in2 (psi))Young's modulus can be used to predict the elongation or compression of an object.

• For many materials, Young's modulus is a constant over a range of strains. Such materials are called linear, and are said to obey Hooke's law. Examples of linear materials include steel, carbon fiber, and glass. Rubber and soil (except at very low strains) are non-linear materials.

Modulus of Elasticity (Young's Modulus)Slope of the stress-strain diagram within the elastic zone is called “Modulus of Elasticity” or “Young’s Modulus”. This parameter depends only on the material type.

In this equation E is the “Modulus of Elasticity”,   is the applied stress in psi or ksi within the elastic limit,

and  is the corresponding strain. Since strain does not have any units, E has units of psi or ksi. For structural steel E   29,000 ksi. A smaller modulus of elasticity translates to more a flexible (or less rigid) member.

Thermal properties

Permeability

Polymer MorphologyThe ultimate properties of any polymer (plastic, fiber, or rubber) result from a combination of molecular weight and chemical structure. Polymers require a particular MW, which depends largely on the chemical structure, to have desirable mechanical properties.

Molecular Weight

Mechanical Property

Polymer MorphologyThe mechanical properties result from attractive forces between molecules– dipole-dipole interactions, H-bonding, induction forces, London

forces or ionic bonding, ion-dipole interactions

CO

O

R

CO

O

R+

+

-

-

C

HN

O

R

C

HN

O

R

+

+

-

-

A lower MW polyamide will produce good fiber properties as compared to the polyester H-bonding

H-bondingdipole-dipole

• Hydrogen Bonding– A dipole-dipole interaction for hydrogens bonded

to electronegative elements• Electrostatic Interaction

Polymer Morphology

HO

R

HO

R

HO

R

HO

R

Weak bond ~ 5 kcal mol-1 (c-c ~ 81 kcal mol-1 )Require short bond distance ~ 2.5Å (c-c ~ 1.46Å)

very importantin cellulose

Polymer MorphologyIntermolecular forces drop off very rapidly with distance important polymer molecules be able to pack together closely to achieve maximum cohesive strength.

ex. Natural Rubberunstretched state - molecules are randomly distributed

low modulusstretched state - molecules become aligned, at 600%

elongation high modulus(2000 times higher than unstretched)

unstretched - amorphous / stretched - crystalline

OverviewMorphology is a term that has slightly different meaning depending on

the words with which it is used. In general it has to do with the ‘form’ or ‘structure’ of whatever topic it is used to describe.

For our purposes, we will use it to describe the form or structure of the polymer chains of thermoplastic materials when they are in their frozen or solid state.

For thermoplastic resins, there are two basic morphologies:

AMORPHOUS and SEMI-CRYSTALLINE

AmorphousAmorphous polymers appear random and jumbled when allowed to

cool in a relaxed state. They appear very similarly to their molten state, only the molecules are closer together.

They can be described as being similar to a large pot of spaghetti noodles.

Semi-crystallineA portion of the molecular chains in semi-crystalline polymers tend to

‘fold-up’ into densely packed regions called crystals as the polymer cools.

If more than 35% of the polymer chain will form these crystals – the polymer is classified as semi-crystalline.

Amorphous regions

Semi-crystalline regions

ExampleThink of Semi-crystalline materials like ramen noodles. When in their solid state, they have a compact ordered arrangement

The dense arrangement makes them stiffer and they resist flowing in that state

ExampleAmorphous materials are like cooked ramen noodles in that there is a random arrangement of the molecules and there are no crystals present to prevent the chains from flowing

It is important to remember that both materials have the random, unordered arrangement when molten

Degree of CrystallinityThere are many different factors that can determine the amount of

crystals or degree of crystallinity of a plastic component.• Cooling rate – it takes time for the polymer chains to fold up. If we

cool the polymer more quickly, we form fewer crystals• Additives – some additives can be put into plastics to increase the

degree of crystallinity while others can disrupt the formation of the crystals giving us a lower degree of crystallinity

• Polymer type – different materials can form higher or lower levels of crystallinity depending on their molecular structure

Temperature• As matter heats up, the molecules vibrate faster due to the

addition of the heat energy.• This faster vibration causes the molecules to move

further apart increasing the space or free volume between the molecules

• At some point the molecules get so far apart, they are no longer solid, they behave like a liquid.

• If you continue to heat the matter, the molecules get so far apart they turn into a gas (evaporate)

• With plastic materials, it is very difficult if not impossible to get them to evaporate because of their degree of entanglement.

TemperatureFor most materials, we are concerned with the melting point and boiling point. These are the temperatures at which the matter experiences a ‘change of state’

oSolid to Liquid

oLiquid to Gas

For thermoplastic materials, we are concerned with:

oGlass Transition Temperature

oMelting Temperature

Glass Transition Temperature (Tg)The Tg is important to both morphologies of thermoplastics

In amorphous materials, it is the temperature at which the molecules have enough absorbed enough energy and have moves far enough apart that the material behaves more rubber-like than glass-like.

The material stretches further when pulled

The material absorbs more impact energy without fracturing when struck

When the material does fail, it fails in a ductile manner as opposed to a brittle manner. (If a material fails in a ductile manner, it yields before it fails. In a brittle manner, it fails or ruptures before it yields)

The sample to the left experienced a brittle failure

•The material did not yield before failure

•The material broke like glass

The sample to the right broke in a ductile manner

•The material yielded (stretched) before failure

•The material behaved more like a rubber

Glass Transition Temperature (Tg)

Glass Transition Temperature (Tg)Because a semi-crystalline material has a portion of its chain that remains in an amorphous state, it is also affected by the Tg

- When a S/C material is above its Tg, it can form crystals – once it dips below the Tg crystal formation stops

-The amorphous portions of the chains have enough energy and the molecules are far enough apart, that the molecules can continue to fold up and unfold.

- The crystals are more easily pulled apart- The material is more flexible

Ex. Polyethylene and Polypropylene both have low Tg’s. They are way below room temperature. That is why milk jugs and yogurtcontainers are flexible when you take them out of the refrigerator.

Melt Temperature (Tm)Amorphous materials don’t truly have a Tm. They just continue to soften more until they behave more like a liquid.

The molecules absorb enough energy and move far enough apart (increase the free volume) that the material can flow.

When we refer to the melt temperature for amorphous materials, it is usually the temperature at which we can process it.

Melt Temperature (Tm)For S/C materials, the Tm is the temperature at which the crystals melt.

Once the crystals are melted the material generally flows very easily.

The ideal temperature for growing crystals is approximately 2/3 of the way between the Tg and the Tm.

Not in all cases, but in many, the degradation temperature for S/C materials is not much higher than the melt temperature.

OrientationWhen we talk about orientation in respect to polymer materials, we are talking about the alignment of the molecules.

Think of a polymer molecule like a broken rubber band sitting on your desk.

- As it flows, the molecule straightens out and stretches

Orientation-When it stops flowing, it wants to return to its random state.-As it cools, it will either start to fold up into a crystal or just move closer to the other molecules.

-If we cool the molecule quickly, before it has a chance to relax, we lock in that molecular alignment or orientation.

-When the molecules are aligned – the material is stronger in the direction of the alignment, but weaker transverse to it.

PropertiesYou have already seen that there is a big difference between the two basic morphologies of thermoplastic materials.

These differences cause the different types of materials to experience property differences.

Although the properties are mainly dependent on a specific polymer’s structure, there are tendencies that go with the specific morphologies.

This is why material selection is so important to the plastics field.

If the wrong material is used for a specific application, it can fail and cause damage or worse – personal injury or death.

PropertiesChemical Resistance

Plastic materials are used in virtually every aspect of today’s activities and they come into contact with a wide variety of chemical substances that they need to resist.

As a general rule S/C materials are more resistant to chemical attack than amorphous materials.

It is more difficult for the chemical media to penetrate the dense crystalline structure to damage the polymer chains.

Polyethylene is used to store everything from detergent to mineral spirits to gasoline. Polypropylene is only slightly less chemically

resistant than Polyethylene.

PropertiesChemical Resistance

Of the amorphous materials PVC is probably the best in chemical resistance, mainly due to the large chlorine atom that helps to protect the main polymer chain.

Polycarbonate, Acrylic, Polystyrene and the other styrenics are all very susceptible to chemical attack, especially to mineral spirits and solvents like lacquer and paint thinners, alcohol, and gasoline.

PropertiesOptical Properties

Amorphous materials have a much higher clarity than S/C materials.

The crystals that form in the material diffract the light as it passes through giving the material a translucent to opaque appearance.

If the crystallinity is disrupted by adding a copolymer or other additive or by quenching the material so quickly the crystals don’t have enough time to form, the material may appear somewhat clear.

Amorphous Acrylic more commonly known as Plexiglas and Polycarbonate used in safety glasses and optical lenses are far superior in terms of optical properties.

PropertiesImpact Resistance

The material structure determines the impact resistance, but as a general rule, S/C materials are more brittle than Amorphous.

The chain portions that are folded up in the crystal restrict the polymer chains as they try to move past one another when a force is applied making the S/C materials more brittle.

Polycarbonate is used in safety glasses, but General Purpose Polystyrene (GPPS) is very brittle – both are amorphous, but have different polymer structures.

On the S/C side, Polyethylene is very ductile at room temperature because it is above its Tg, but Nylon and Polyester are brittle at room temperature.

PropertiesViscosity

S/C materials by their very nature flow more easily than Amorphous materials.

The same mechanism that allows the material to fold up into dense crystals allows the polymer chains to slide past one another easily in the melted state.

For this reason materials like Nylon require very tight tolerances on their tooling to prevent melted plastic from leaking out of the cavities causing flash.

Flash

PropertiesWeather Resistance

When it comes to weather resistance, the most damaging aspect of weathering is generally considered to be Ultraviolet light.

The UV light breaks down the chains of the polymers making them more brittle, causing colors to fade or yellow, and causing additives in the polymers to migrate to the surface (chalking).

PropertiesWeather Resistance

Amorphous polymers have better chemical resistance to weathering effects than S/C polymers.

The crystals in the S/C polymers diffract the light so the UV rays spend more time within the polymer structure and do more damage. The clear amorphous polymers allow the damaging radiation to pass through doing less damage.

PropertiesShrinkage

Because they fold up into crystal structures, S/C materials have higher shrinkage rates when compared to Amorphous materials.

In injection molding most amorphous materials will shrink between 0.003-0.007 in/in (0.3-0.7%)

S/C materials shrink differently depending upon the level of crystallinity that they achieve.

Some will shrink over 0.025 in/in depending on processing variables, part thickness, and additives.

Material TypesAmorphous

Polyvinyl Chloride (PVC)

General Purpose Polystyrene (GPPS)

Polycarbonate (PC)

Polymethylmethacrylate (PMMA or Acrylic)

Acrylonitrile Butadiene Styrene (ABS – a terpolymer)

Material TypesSemi-crystalline

Polyethylene (PE, HDPE, LDPE, etc.)

Polypropylene (PP)

Polyamides (PA – Nylon)

Polyesters

Polyethylene Terephthalate (PET)

Polybutylene Terephthalate (PBT)

Polyoxymethylene (POM - Acetal)

Polytetrafluoroethylene (PTFE – Teflon)

Morphologies

• Amorphous structure random, unordered ex. Polystyrene• Crystalline structure regular, order ex. Polyethylene, Polypropylene

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Polymer Microstructure

Polyolefins with side chains have stereocenters on every other carbon

CH3n

CH3 CH3 CH3 CH3 CH3 CH3CH3

With so many stereocenters, the stereochemistry can be complex. There are three main stereochemical classifications for polymers.

Atactic: random orientation

Isotactic: All stereocenters have same orientation

Syndiotactic: Alternating stereochemistry

Capable of crystallizing

155

Why is this important?

• Tacticity affects the physical properties– Atactic polymers will generally be amorphous, soft,

flexible materials– Isotactic and syndiotactic polymers will be more

crystalline, thus harder and less flexible

• Polypropylene (PP) is a good example– Atactic PP is a low melting, gooey material– Isoatactic PP is high melting (176º), crystalline, tough

material that is industrially useful– Syndiotactic PP has similar properties, but is very

clear. It is harder to synthesize

156

Polymer CrystallinityPolymers are 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.

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The effect of temperature on the structure and behavior of thermoplastics.

ThermoplasticsThe crystalline phases of such polymers are characterized by their melting temperature (Tm).Many thermoplastics are completely amorphous and incapable of crystallization, these amorphous polymers (and amorphous phases of semicrystalline polymers) are characterized by their glass transition temperature (Tg).

– the temperature at which they transform abruptly from the glassy state (hard) to the rubbery state (soft).

Thermoplastics

Glass transition temperature (Tg)This transition corresponds to the onset of chain motion

•below the Tg the polymer chains are unable to move and are “frozen” in position.

Both Tg and Tm increase with increasing chain stiffness and increasing forces of intermolecular attraction

ThermosetsThermosets - normally rigid materials - network polymers in which chain motion is greatly restricted by a high degree of crosslinking

As with elastomers, they are intractable once formed and degrade rather than melt upon the application of heat.

ElastomersElastomers - crosslinked rubbery polymers - rubber networks - that can be easily stretched to high extensions (3x to 10x original dimensions) – the rubbery polymer chains become extended upon

deformation but are prevented from permanent flow by crosslinking, and driven by entropy, spring back to their original positions on removal of the stress.

PolysaccharidesThe size of polysaccharide molecules can vary, occurring as polydispersed molecules that have a range of 100 to 100,000 monosaccharide units

– MW 16,000 - 16,000,000 daltons

There are a number of methods used to determine the molecular weight of polysaccharides

– viscosity*, light scattering, ultracentrifugation, osmometry and titration are most common

(*viscosity is routinely used, but is not an absolute method and can be used only in conjunction with one of the other methods)

Molecular Weight DistributionThe simplest, most common molecular weight is the number-average molecular weight (n)

– end-group analysis or colligative properties (b.p. elevation, osmotic pressure, etc)

others commonly used are weight-average molecular weight (w), z-average molecular weight (z) and viscosity-average molecular weight (u)

– light scattering (w), sedimentation equilibrium (z) and solution viscosity (u)

Number-average molecular weight (n)– based on methods of counting the number of

molecules in a given weight of polymer• the total weight of a polymer sample, w, is the sum of

the weights of each molecular species present

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Weight-average molecular weight (w)determination of molecular weight based on size rather than the number of molecules

– the greater the mass, the greater the contribution to the measurement

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Z-average molecular weight (z)some molecular weight determination methods (e.g. sedimentation equilibrium) yield higher molecular weight averages - z

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w = weight fractionM = molecular weightN = number of molecules

Example - a polymer sample consists of 9 molecules of mw 30,000 and 5 molecules of mw 50,000

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Number-average molecular weight (n)

Consider the previous example - 9 molecules of molecular weight 30,000 and 5 molecules of molecular weight 50,000

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Weight-average molecular weight (w)

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zM

Z-average molecular weight (z)

A Typical Molecular Weight Distribution Curve

200 000 400 000 600 000 800 000 1 000 000Mi (g mol-1)

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Molecular Weight of PolymersUnlike small molecules, polymers are typically a mixture of differently sized molecules. Only an average molecular weight can be defined.

• Measuring molecular weight• Size exclusion chromatography• Viscosity

• Measurements of average molecular weight (M.W.)• Number average M.W. (Mn): Total

weight of all chains divided by # of chains

• Weight average M.W. (Mw): Weighted average. Always larger than Mn

• Viscosity average M.W. (Mv): Average determined by viscosity measurements. Closer to Mw than Mn

# o f m o l e c u l e s

Mn

Mw

increasing molecular weight

Mv

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What the Weights MeanMn: This gives you the true average weight

Let's say you had the following polymer sample:2 chains: 1,000,000 Dalton 2,000,0005 chains: 700,000 Dalton 3,500,00010 chains: 400,000 Dalton 4,000,0004 chains: 100,000 Dalton 400,0002 chains: 50,000 Dalton 100,000

10,000,00010,000,000/23 = 435,000 Dalton

1 Dalton = 1 g/mole

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Weight Average Molecular Weight

Mw: Since most of the polymer mass is in the heavier fractions, this gives the average molecular weight of the most abundant polymer fraction by mass.

2,000,00010,000,000

0.201,000,000 200,000

3,500,00010,000,000

0.35700,000 245,000

4,000,00010,000,000

0.40400,000 160,000

400,00010,000,000

0.04 100,000 4,000

100,00010,000,000

0.0150,000 500

Total 609,500

Polymer Solution Viscosity

When a polymer is dissolved in a solvent and then subjected to flow through a narrow capillary it exerts a resistance to that flow. This resistance is very informative. •It provides information on the size of the polymer•Its Flexibility and shape in solution •Its interactions with the solvent it is disolved in.

Polymer Melts

• To shape a thermoplastic polymer it must be heated so that it softens to the consistency of a liquid

• In this form, it is called a polymer melt • Important properties:

– Viscosity– Viscoelasticity

Viscosity of a polymer melt decreases with temperature, thus the fluid becomes thinner at higher temperatures

Figure 13.2 ‑

Viscosity as a function of temperature for selected polymers at a shear rate of 103 s-1

Viscoelasticity

Combination of viscosity and elasticity• Possessed by both polymer solids and polymer

melts• Example: die swell in extrusion, in which the

hot plastic expands when exiting the die opening

Various Fillers