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CHAPTER 4: POLYMER STRUCTURES. Spherulite, rubber specimen. Chain-folded lamellar crystallites, ~10 nm thick, 30,000×. c04cof01. ISSUES TO ADDRESS. • What are the general structural and chemical characteristics of polymer molecules?.

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Chapter 4 polymer structures
CHAPTER 4:POLYMER STRUCTURES

Spherulite, rubber specimen. Chain-folded lamellar crystallites, ~10 nm thick, 30,000×


C04cof01
c04cof01

ISSUES TO ADDRESS...

• What are the general structural and chemical characteristics of polymer molecules?

• What are some of the common polymeric materials, and how do they differ chemically?

• How is the crystalline state in polymers different from that in metals and ceramics ?


4 1 structures of polymers
4.1 Structures of Polymers

  • Introduction and Motivation

    • Polymers are extremely important materials (i.e. plastics)

    • Have been known since ancient times – cellulose, wood, rubber, etc..

    • Biopolymers – proteins, enzymes, DNA …

    • Last ~50 years – tremendous advances in synthetic polymers

    • Just like for metals and ceramics, the properties of polymers

      • Thermal stability

      • Mechanical properties

        Are intimately related to their molecular structure …


4 1 ancient polymers
4.1 Ancient Polymers

Originally natural

polymers were used:

  • Wood

  • Rubber

  • Cotton

  • Wool

  • Leather

  • Silk

Oldest 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


4 2 polymer composition
4.2 Polymer Composition

Most polymers are hydrocarbons

– i.e., made up of H and C

  • Saturated hydrocarbons

    • Each carbon singly bonded to four other atoms

    • Example:

      • Ethane, C2H6


4 2 unsaturated hydrocarbons
4.2 Unsaturated Hydrocarbons

  • Double & triple bonds somewhat unstable

  • Thus, can form new bonds

    • Double bond found in ethylene or ethene - C2H4

    • Triple bond found in acetylene or ethyne - C2H2


4 2 structures of polymers
4.2 Structures of Polymers

  • about hydrocarbons

    • Why? Most polymers are hydrocarbon (e.g. C, H) based

    • Bonding is highly covalent in hydrocarbons

    • Carbon has four electrons that can participate in bonding, hydrogen has only one

    • Saturated versus unsaturated

  • Unsaturated – species contain carbon-carbon double/triple bonds

    • Possible to substitute another atom on the carbon

  • Saturated – carbons have four atoms attached

    • Cannot substitute another atom on the carbon

Saturated

Unsaturated


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4.2 Hydrocarbon Molecules

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Acetylene

Ethyne

Ethylene

Ethene

Hydrocarbons have strong chemical bonds, but interact only weakly with one another (van der Waals’ forces)

(normal) butane

isobutane

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4 2 isomerism
4.2 Isomerism

compounds with same chemical formula can have quite different structures

for example: C8H18

  • normal-octane

Isomerism – compounds of the same chemical composition but different atomic arrangements (i.e. bonding connectivity)

  • 2,4-dimethylhexane




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4.3 Polymer Molecules

Molecules are gigantic

Macromolecules

Repeat units

Monomer


4 3 polymers
4.3 Polymers

  • Polymer molecules

    • what is a polymer?

    • Polymers are molecules (often called macromolecules) formed from a series of building units (monomers) that repeat over and over again

  • polymers can have a range of molecular weights

  • There are many monomers

  • Can make polymers with different monomers, etc..

n is often a very large number!

e.g. can make polyethylene with MW > 100,000! ~3600 mers ~7200 carbons


Chemistry of polymer molecules
Chemistry of polymer molecules

Example: ethylene

  • Gas at STP

  • To polymerize ethylene, typically increase T, P and/or add an initiator

Initiation

Propagation

After many additions of monomer to the growing chain…

R* = initiator; activates the monomer to begin chain growth

  • Initiator: example - benzoyl peroxide


4 4 polymer chemistry
4.4 Polymer chemistry

  • Polymers are chain molecules. They are built up from simple units called monomers.

  • E.g. polyethylene is built from ethylene units: which are assembled into long chains:

Polyethylene or polythene (IUPAC name poly(ethene)) is a thermoplastic commodity heavily used in consumer products (notably the plastic shopping bag). Over 60 million tons of the material are produced worldwide every year.


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Tetrafluoroethylene monomer polymerize to form PTFE or polytetrafluoroethylene

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poly(tetrafluoroethene) or poly(tetrafluoroethylene) (PTFE) is a synthetic fluoropolymer. PTFE is the DuPont brand name Teflon. Melting: 327C

Vinyl chloride monomer leads to poly(vinyl chloride) or PVC

PVC: manufacturing toys, packaging, coating, parts in motor vehicles, office supplies, insulation, adhesive tapes, furniture, etc. Consumers: shoe soles, children's toys, handbags, luggage, seat coverings, etc. Industrial sectors: conveyor belts,

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printing rollers. Electric and electronic equipment: circuit boards, cables, electrical boxes, computer housing.


Chemistry and structure of polyethylene
Chemistry and Structure of Polyethylene polytetrafluoroethylene

Adapted from Fig. 4.1, Callister & Rethwisch 3e.

Note: polyethylene is a long-chain hydrocarbon

- paraffin wax for candles is short polyethylene

• Polymer = many mers

Adapted from Fig. 14.2, Callister 6e.


Polymer chemistry
Polymer chemistry polytetrafluoroethylene

  • In polyethylene (PE) synthesis, the monomer is ethylene

  • Turns out one can use many different monomers

    • Different functional groups/chemical composition – polymers have very different properties!

Monomers


Homopolymer and copolymer
Homopolymer and Copolymer polytetrafluoroethylene

  • Polymer chemistry

    • If formed from one monomer (all the repeat units are the same type) – this is called a homopolymer

    • If formed from multiple types of monomers (all the repeat units are not the same type) – this is called a copolymer

  • Also note – the monomers shown before are referred to as bifunctional

    • Why? The reactive bond that leads to polymerization (the C=C double bond in ethylene) can react with two other units

    • Other monomers react with more than two other units – e.g. trifunctional monomers


The top 10 bulk or commodity
The Top 10 Bulk or Commodity polytetrafluoroethylene


4 5 molecular weight

low polytetrafluoroethyleneM

4.5 MOLECULAR WEIGHT

Molecular weight, M: Mass of a mole of chains.

high M

Not all chains in a polymer are of the same length

i.e., there is a distribution of molecular weights


Molecular weight
Molecular weight polytetrafluoroethylene

  • The properties of a polymer depend on its length

  • synthesis yields polymer distribution of lengths

  • Define “average” molecular weight

  • Two approaches are typically taken

    • Number average molecular weight (Mn)

    • Weight-average molecular weight (Mw)


Molecular weight distribution
MOLECULAR WEIGHT DISTRIBUTION polytetrafluoroethylene

Adapted from Fig. 4.4, Callister & Rethwisch 3e.

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

xi= number fraction of chains in size range i

wi= weight fraction of chains in size range i


Molecular weight1
Molecular weight polytetrafluoroethylene

Are the two different? Yes, one is essentially based on mole fractions, and the other on weight fractions

They will be the same if all the chains are exactly of the same MW! If not Mw > Mn

Get Mn from this

Get Mw from this


Molecular weight2
Molecular weight polytetrafluoroethylene

  • Other ways to define polymer MW

  • Degree of polymerization

    • Represents the average number of mers in a chain. The number and weight average degrees of polymerization are

m is the mer MW in both cases. In the case of a copolymer (something with two or more mer units), m is determined by

fjand mj are the chain fraction and molecular weight of mer j


Example problem 4 1

Number average MW ( polytetrafluoroethyleneMn)

Example Problem 4.1

  • Given the following data determine the

    • Number average MW

    • Number average degree of polymerization

    • Weight average MW

  • How to find Mn?

  • Calculate xiMi

  • Sum these!


C04tf04a
c04tf04a polytetrafluoroethylene

c04tf04a


Example problem 4 11
Example Problem 4.1 polytetrafluoroethylene

  • How to find Mw?

  • Calculate wiMi

  • Sum these!

Number average degree of polymerization

  • (MW of H2C=CHCl is 62.50 g/mol)

Weight average molecular weight (Mw)


C04tf04b
c04tf04b polytetrafluoroethylene

c04tf04b


Degree of polymerization dp
Degree of Polymerization, polytetrafluoroethyleneDP

DP = average number of repeat units per chain

DP = 6

Chain fraction

mol. wt of repeat unit i


4 6 polymers molecular shape
4.6 Polymers – Molecular Shape polytetrafluoroethylene

Molecular Shape (or Conformation) – chain bending and twisting are possible by rotation of carbon atoms around their chain bonds

  • note: not necessary to break chain bonds to alter molecular shape

Adapted from Fig. 4.5, Callister & Rethwisch 3e.

  • C-C bonds are typically 109° (tetrahedral, sp3 carbon)

  • If you have a macromolecule with hundreds of C-C bonds, this will lead to bent chains


Structures of polymers
Structures of Polymers polytetrafluoroethylene

  • Molecular shape

    • Taking this idea further, can also have rotations about bonds

      • Leads to “kinks”, twists

      • “the end-to-end distance of a polymer chain in the solid state (or in solution) is usually much less than the distance of the fully extended chain!

      • This is not even taking into account that you have numerous chains that can become entangled!


  • 4.7 Molecular structure polytetrafluoroethylene

    • Physical properties of polymers depend not only on their molecular weight/shape, but also on the difference in the chain structure

    • Four main structures

      • Linear polymers

      • Branched polymers

      • Crosslinked polymers

      • Network polymers


4 7 molecular structures for polymers

secondary polytetrafluoroethylene

bonding

Linear

B

ranched

Cross-Linked

Network

4.7 Molecular Structures for Polymers

Adapted from Fig. 4.7, Callister & Rethwisch 3e.


Linear polymers polytetrafluoroethylene

  • – polymers in which the mer units are connected end-to-end along the whole length of the chain

  • These types of polymers are often quite flexible

    • Van der waal’s forces and H-bonding are the two main types of interactions between chains

    • Some examples – polyethylene, teflon, PVC, polypropylene


Branched polymers
Branched polymers polytetrafluoroethylene

  • Polymer chains can branch:

  • Or the fibers may aligned parallel, as in fibers and some plastic sheets.

  • chains off the main chain (backbone)

    • This leads to inability of chains to pack very closely together

      • These polymers often have lower densities

  • These branches are usually a result of side-reactions during the polymerization of the main chain

    • Most linear polymers can also be made in branched forms


Crosslinked polymers
Crosslinked polymers polytetrafluoroethylene

  • Molecular structure

    • adjacent chains attached via covalent bonds

      • Carried out during polymerization or by a non-reversible reaction after synthesis (referred to as crosslinking)

      • Materials often behave very differently from linear polymers

      • Many “rubbery” polymers are crosslinked to modify their mechanical properties; in that case it is often called vulcanization

      • Generally, amorphous polymers are weak and cross-linking adds strength: vulcanized rubber is polyisoprene with sulphur cross-links:


Network polymers
Network polymers polytetrafluoroethylene

– polymers that are “trifunctional” instead of bifunctional

  • There are three points on the mer that can react

  • This leads to three-dimensional connectivity of the polymer backbone

    • Highly crosslinked polymers can also be classified as network polymers

    • Examples: epoxies, phenol-formaldehyde polymers


Polymer microstructure
POLYMER MICROSTRUCTURE polytetrafluoroethylene

• Covalent chain configurations and strength:

Direction of increasing strength

Adapted from Fig. 14.7, Callister 6e.

2


4.8 Molecular configurations polytetrafluoroethylene

Classification scheme for the characteristics of polymer molecules

isomerism – different molecular configurations for molecules (polymers) of the same composition

Stereoisomerism

Geometrical Isomerism


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4.8 Molecular Configurations polytetrafluoroethylene

Repeat unit

R = Cl, CH3, etc

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Configurations – to change must break bonds

Stereoisomers are mirror

images – can’t superimpose

without breaking a bond

A

A

C

C

E

E

B

B

D

D

mirror plane


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c04eqf11 polytetrafluoroethylene

Head to-tail

  • Typically the head-to-tail configuration dominates

Head to-head


Structures of polymers1
Structures of Polymers polytetrafluoroethylene

  • Stereoisomerism

    • Denotes when the mers are linked together in the same way (e.g. head-to-tail), but differ in their spatial arrangement

    • This really focuses on the 3D arrangement of the side-chain groups

    • Three configurations most prevalent

      • Isotactic

      • Syndiotactic

      • Atactic


Isotactic
ISOTACTIC polytetrafluoroethylene

  • Stereoisomerism

    • Isotactic polymers

    • All of the R groups are on the same side of the chain

Isotactic configuration

  • Note: All the R groups are head-to-tail

  • All of the R groups are on the same side of the chain

    • Projecting out of the plane of the slide

    • This shows the need for 3D representation to understand stereochemistry!


Syndiotactic
SYNDIOTACTIC polytetrafluoroethylene

  • Stereoisomerism

    • Syndiotactic polymers

    • The R groups occupies alternate sides of the chain

Syndiotactic configuration

  • Note: The R groups are still head-to-tail

  • R groups alternate – one of out of the plane, one into the plane


Atactic
ATACTIC polytetrafluoroethylene

  • Stereoisomerism

    • Atactic polymers

    • The R groups are “random”

Atactic configuration

  • R groups are both into and out of the plane, no real registry

  • Two additional points

    • Cannot readily interconvert between stereoisomers – bonds must be broken

    • Most polymers are a mix of stereoisomers, often one will predominate


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Stereoisomerism—Head-to-tail polytetrafluoroethylene

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isotactic configuration

Syndiotactic conformation

Atactic conformation


Cis trans isomerism
cis/trans Isomerism polytetrafluoroethylene

cis

cis-isoprene (natural rubber)

H atom and CH3 groupon same side of chain

trans

trans-isoprene (gutta percha)

H atom and CH3 group on opposite sides of chain


C04eqf18
c04eqf18 polytetrafluoroethylene

Geometrical Isomerism

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4 9 plastics
4.9 Plastics polytetrafluoroethylene

  • variety of properties due to their rich chemical makeup

  • They are inexpensive to produce, and easy to mold, cast, or machine.

  • Their properties can be expanded even further in composites with other materials.


Glass rubber liquid

9 polytetrafluoroethylene

8

7

6

5

4

3

Temperature

Glass-rubber-liquid

  • Amorphous plastics have a complex thermal profile with 3 typical states:

Glass phase (hard plastic)

Leathery phase

Log(stiffness)Pa

Rubber phase (elastomer)

Liquid


Thermosetting and thermoplastic polymers
Thermosetting and Thermoplastic Polymers polytetrafluoroethylene

Another way to categorize polymers

how do they respond to elevated temperatures?


Thermoplastics polytetrafluoroethylene

  • Thermoplastics –soften when heated, and harden when cooled – process is totally reversible; melt and solidify without chemical change

  • This is due to the reduction of secondary forces between polymer chains as the temperature is increased

    • Most linear polymers and some branched polymers are thermoplastics

    • They support hot-forming methods such as injection-molding and FDM.

.


Thermosets
THERMOSETS polytetrafluoroethylene

  • harden the first time they are heated, and do not soften after subsequent heating

    • During the initial heat treatment, covalent linkages are formed between chains (i.e. the chains become cross-linked)

    • Polymer won’t melt with heating – heat high enough, it will degrade

    • Network/crosslinked polymers are typically thermosets


Thermosets1
THERMOSETS polytetrafluoroethylene

  • irreversibly change when heated are called thermosets.

  • Large cross-linking during change (10 to 50% of mers)

  • which strengthens the polymer (setting). large cross linking

  • Thermosets will not melt, and have good heat resistance.

  • They are often made from multi-part compounds and formed before setting (e.g. epoxy resin).

  • Setting accelerates with heat, or for some polymers with UV light.

vulcanized rubber, epoxies, polyester resin, phenolic resin


4 10 structures of polymers
4.10 Structures of Polymers polytetrafluoroethylene

  • Copolymers

    • Idea – polymer that contains more than one mer unit

    • Why? If polymer A has interesting properties, and polymer B has (different) interesting properties, making a “mixture” of polymer should lead to a superior polymer

“Random” copolymer – exactly what it sounds like

“Alternating” copolymer – ABABABA…


Structures of polymers2
Structures of Polymers polytetrafluoroethylene

  • Copolymers

    • Idea – polymer that contains more than one mer unit

    • Why? If polymer A has interesting properties, and polymer B has (different) interesting properties, making a “mixture” of polymer should lead to a superior polymer

“Block” copolymers. Domains of “pure” mers

“Graft” copolymers. One mer forms backbone, another mer is attached to backbone and is a sidechain (it is “grafted” to the other polymer)


Copolymers
Copolymers polytetrafluoroethylene

Adapted from Fig. 4.9, Callister & Rethwisch 3e.

two 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

alternating

block

graft


Copolymers1
Copolymers polytetrafluoroethylene

  • Polymers often have two different monomers along the chain – they are called copolymers.

  • With three different units, we get a terpolymer. This gives us an enormous design space…


4 11 polymer structure
4.11 Polymer structure polytetrafluoroethylene

  • The polymer chain layout determines a lot of material properties:

  • Amorphous:

  • Crystalline:


Crystallinity in polymers
Crystallinity in Polymers polytetrafluoroethylene

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

  • Ordered atomic arrangements involving molecular chains

  • Crystal structures in terms of unit cells

  • Example shown

    • polyethylene unit cell

  • 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 complex


Structures of polymers3
Structures of Polymers polytetrafluoroethylene

  • 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


Structures of polymers4
Structures of Polymers polytetrafluoroethylene

  • 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


Structures of polymers5
Structures of Polymers polytetrafluoroethylene

4.11 Polymer crystallinity

  • What influences the degree of crystallinity

    • Rate of cooling during solidification

    • Molecular chemistry – structure matters

      • Polyisoprene – hard to crystallize

      • Polyethylene – hard not 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


4 11 polymer crystallinity
4.11 Polymer Crystallinity polytetrafluoroethylene

Polymers rarely 100% crystalline

  • Difficult for all regions of all chains to become aligned

crystalline

region

• Degree of crystallinity expressed as % crystallinity.

-- Some physical properties depend on % crystallinity.

-- Heat treating causes crystalline regions to grow and % crystallinity to increase.

amorphous

region

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.)


4 11 molecular weight crystallinity
4.11 MOLECULAR WEIGHT & CRYSTALLINITY polytetrafluoroethylene

• Molecular weight, Mw: Mass of a mole of chains.

• Tensile strength (TS):

--often increases with Mw.

--Why? Longer chains are entangled (anchored) better.

• % Crystallinity: % of material that is crystalline.

--TS and E often increase

with % crystallinity.

--Annealing causes

crystalline regions

to grow. % crystallinity

increases.

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.)

4


4 12 polymer crystallinity
4.12 Polymer Crystallinity polytetrafluoroethylene

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)

  • Crystalline regions

    • thin platelets with chain folds at faces

    • Chain folded structure


4 12 single crystals
4.12 Single Crystals polytetrafluoroethylene

  • Electron micrograph – multilayered single crystals (chain-folded layers) of polyethylene

  • Single crystals – only for slow and carefully controlled growth rates

Adapted from Fig. 4.11, Callister & Rethwisch 3e.


4 12 semicrystalline polymers
4.12 Semicrystalline Polymers polytetrafluoroethylene

  • Some semicrystalline polymers form spherulite structures

  • Alternating chain-folder crystallites and amorphous regions

  • Spherulite structure for relatively rapid growth rates

Spherulite surface

Adapted from Fig. 4.13, Callister & Rethwisch 3e.


Structures of polymers6
Structures of Polymers polytetrafluoroethylene

  • Polymer crystals

    • More commonly, many polymers that crystallize from a melt form spherulites

      • One way to think of these – the chain folded lamellae have amorphous “tie domains” between them

      • These plates pack into a spherical shape

      • Polymer analogues of grains in polycrystalline metals/ceramics


Photomicrograph spherulites in polyethylene
Photomicrograph – Spherulites in Polyethylene polytetrafluoroethylene

Cross-polarized light used -- a maltese cross appears in each spherulite

Adapted from Fig. 4.14, Callister & Rethwisch 3e.


C04eqf031
c04eqf03 polytetrafluoroethylene

END of chapter 4


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