Chapter 4- Polymer Structures. Chapter 4- Polymer Structures. Polymer Microstructure. Polymer Microstructure ISSUES TO ADDRESS...

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MatSE 280: Introduction to Engineering Materials ©D.D. Johnson 2004, 2006, 2007-08

TEM of spherulite structure in natural rubber(x30,000).

• Chain-folded lamellar crystallites (white lines) ~10nm thick extend radially.

Chapter 4- Polymer Structures

ISSUES TO ADDRESS...

What are the basic

• Classification?

• Monomers and chemical groups?

• Nomenclature?

• Polymerization methods?

• Molecular Weight and Degree of Polymerization?

• Molecular Structures?

• Crystallinity?

• Microstructural features

?

Chapter 4- Polymer Structures

• Poly

mer

= many

mers

Adapted from Fig. 14.2, Callister 6e.

Polymer Microstructure

Polyethylene perspective of molecule

A zig-zag backbone structure with covalent bonds

• Covalent

chain

configurations and strength:

Direction of increasing strength

Polymer Microstructure

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Common Examples

- Textile fibers: polyester, nylon…

- IC packaging materials.

- Resists for photolithography/microfabrication.

- Plastic bottles (polyethylene plastics).

- Adhesives and epoxy.

- High-strength/light-weight fibers: polyamides,

polyurethanes, Kevlar…

- Biopolymers: DNA, proteins, cellulose…

• Thermoplastics: polymers that flow more easily when

squeezed, pushed, stretched, etc. by a load (usually at

elevated T).

– Can be reheated to change shape.

• Thermosets: polymers that flow and can be molded

initially but their shape becomes set upon curing.

– Reheating will result in irreversible change or decomposition.

• Other ways to classify polymers.

– By chemical functionality (e.g. polyacrylates, polyamides, polyethers, polyeurethanes…).

– Vinyl vs. non-vinyl polymers.

– By polymerization methods (radical, anionic, cationic…). – Etc…

Common Classification

Common Chemical Functional Groups

Saturated hydrocarbons (loose H to add atoms)

C C H H H H Ethylene (ethene) C C H H C H H H H Propylene (propene) = 1-butene 2-butene trans cis Acetylene (ethyne) H C CH Unsaturated hydrocarbons (double and triple bonds)

Alcohols

Methyl alcohols

Ethers

Dimethyl Ether

Acids

_{Acetic acid}

Aldehydes

_Formaldehyde

Aromatic

hydrocarbons

Phenol

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Some Common Polymers

C C C N H H H Polyacrylonitrile (PAN) C C H H H X C C H H X H

Vinyl polymers (one or more H’s of ethylene can be substituted)

Common backbone with substitutions

Monomer-based naming: poly________

e.g. ethylene -> polyethylene

if monomer name contains more than one word: poly(_____ ____)

e.g. acrylic acid -> poly(acrylic acid) Monomer name goes here

Monomer name in parentheses

Note: this may lead to polymers with different names but same structure.

C C C C H H H H H H H H … … C C C C H H H H H H H H … … polyethylene polymethylene

Nomenclature

Polymerization Methods

H H

A. Free Radical Polymerization 1. Initiation

Free radical initiator (unpaired electron) C C H H H H monomer C C H H H R H R Radical transferred C C H H σ bonds π bond R H H C H H C R sp2_carbons sp3_carbon

Polymerization Methods

A. Free Radical Polymerization

2. Propagation C C H H H H C C H H H R H C C H H H C H H H C H H R C C H H H H C C H H H C H H H C H H C H H C H H R H H C H H C R H H C C H H

Both carbon atoms will change from sp2_{to sp}3_.

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Polymerization Methods

A. Free Radical Polymerization

3. Termination C C H H H R H C C H H H R H R + C C H H H R H R C C H H H R H + C H H C H H C H H C H H R R

Intentional or unintentional molecules/impurities can also terminate.

Polymerization Methods

B. Stepwise polymerization RCOH O N H2 + RCOH O N H2 _RC_N H O N H2 R_COH O C. Other methods

Anionic polymerization, cationic polymerization, coordination polymerization… RC O N H n HOH + HOH + (n-1) Loses water (condensation)

Proteins (polypeptides have similar composition)

C H C O N H R n Various R groups…

Molecular Weights

Not only are there different structures (molecular arrangements) …… but there can also be a distribution of molecular weights (i.e. number of monomers per polymer molecule).

20 mers 16 mers

10 mers

Average molecular weight =

M

_monomer

15 .

3 M

_monomer

3

10

16

20 =

+

This is what is called numberaverage molecular weight.

MatSE 280: Introduction to Engineering Materials ©D.D. Johnson 2004, 2006, 2007-08 Number average molecular weight:

€ M_n= NjMj j

∑

Nj j

∑

€

=

m

o

N

j

∑

N

j j

∑

€ N_jM_j j

∑

=

Note: Total weight

€

N

_j j

∑

=

Total # of polymer chains Weight average molecular weight:

€

M

w

=

W

j

M

j j

∑

W

j j

∑

=

N

j

M

2j j

∑

N

j

M

j j

∑

€

W

_j

= N

j

M

j In general: € M = NjMα+1j j

∑

NjMαj j

∑

€

M

_n If α = 0 then If α = 1 then

€

M

w Nj = # of polymer chains with length j

Mj = jmo mass of polymer chain with length j

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Molecular Weight:

Different Notations

€

M

_n

=

N

_j

M

_j

j

∑

N

_j

j

∑

€

M

_n

=

x

i

M

i

∑

€

x

i

=

N

i

N

j j

∑

€

M

_w

=

N

_j

M

2 _j

j

∑

N

_j

M

_j

j

∑

€

M

_w

=

w

i

M

i

∑

€

w

_i

=

N

i

M

i

N

_j

M

_j

j

∑

In Lecture Notes In Callister Textbook

MatSE 280: Introduction to Engineering Materials ©D.D. Johnson 2004, 2006, 2007-08 Examples –

Light scattering: larger molecules scatter more light than smaller ones. Infrared absorption properties: larger molecules have more side groups and light absorption (due to vibrational modes of side groups) varies linearly with number of side groups.

Molecular Weights

Why do we care about weight average MW?

-some properties are dependent on MW (larger MW polymer chains can contribute to overall properties more than smaller ones).

Distribution of polymer weights

Polydispersity and Degree of Polymerization

Polydispersity:

€

M

w

M

_n

≥ 1

When polydispersity = 1, system is monodisperse.

Degree of Polymerization:

€

n

_n

=

M

n

m

o

Number avg degree of polymerization

n

w

=

M

w

m

_o

Weight avg degree of polymerization

Compute the number-average degree of polymerization for polypropylene, given that the number-average molecular weight is 1,000,000 g/mol. What is “mer” of PP?

Mer molecular weight of PP is

Example 1

C

₃

H

₆

m

o

=3A

C

+6A

H

=3(12.01 g/mol)+6(1.008 g/mol)

= 42.08 g/mol

Number avg degree of polymerization

n

=

M

_n

m

_o

=

10

6

g /mol

42.08g /mol

= 23,700

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Example 2 (a, b, and c)

A. Calculate the number and weight average degrees of polymerization and polydispersity for a polymer sample with the following distribution.

Avg # of monomers/chain Relative abundance

10 5 100 25 500 50 1000 30 5000 10 50,000 5 nn= Mn mo =m0 m0 jNj j

∑

Nj j

∑

= jNj j

∑

Nj j

∑

=5 *10 + 25 *100 + 50 * 500 + 30 *1000 + 10 * 5000 + 5 * 50000 5 + 25 + 50 + 30 + 10 + 5 = 2860.4 nw= Mw mo = 1 mo ( jmo) 2_N j j

∑

Nj( jmo) j

∑

= j2_N j j

∑

jNj j

∑

=5 *10 2 + 25 *1002 + 50 * 5002 + 30 *10002 + 10 * 50002 + 5 * 500002 5 *10 + 25 *100 + 50 * 500 + 30 *1000 + 10 * 5000 + 5 * 50000 = 35, 800

Note: m0 cancels in all these!

Example 2 (cont.)

B. If the polymer is PMMA, calculate number and weight average

molecular weights.

Mw if monomer is methylmethacrylate (5C, 2O, and 8H)

So m0= 5(12)+2(16)+8(1)= 100 g/mol CH3 | | CO2CH3

€

M

_{n =}

n

_n

m

_{o = 2860.4(100}

g

/

mol

) = 286,040

g

/

mol

M

w =

n

w

m

o = 35,800(100

g

/

mol

) = 3,580,000

g

/

mol

€

M

_w

M

n= 3,580,000 286,040 ~ 12.52 Polydispersity:

Example 2 (cont.)

C. If we add polymer chains with avg # of monomers = 10 such that their relative abundance changes from 5 to 10, what are the new number and weight average degrees of polymerization and polydispersity?

Add 5 more monomers of length 10 …. nn= Mn mo =

∑

jjNj Nj j

∑

=10 * 10 + 25 * 100 + 50 * 500 + 30 * 1000 + 10 * 5000 + 5 * 50000 10 + 25 + 50 + 30 + 10 + 5 = 2750 Mw Mn =3, 580, 000 275000 ~ 13 Polydispersity: € nw =M w mo= j2N j j ∑ jN j j ∑ = 35,800

Note: significant change in number average (3.8 %) but no change in weight average!

MatSE 280: Introduction to Engineering Materials ©D.D. Johnson 2004, 2006, 2007-08 For an asymmetric monomer

T H + T H T H T H T H H T H T T H C H F C H H C F H C H H C H F C H H C H H C F H e.g. poly(vinyl fluoride):

H to T T to T H to H Random arrangement e.g. PMMA C C CH3 C H H C C CH3 C H H O _O C H3 H3C O O C C CH3 C H H C C CH3 C H H O O C H3 O C H3 O H to T _{H to T} H to T

Exclusive H to T arrangement (Why?)

Sequence isomerism

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• Regularity and symmetry of side groups affect properties

• Stereoisomerism: (can add geometric isomerism too)

Polymer Molecular Configurations

Isotactic On one side Syndiotactic Alternating sides Atactic Randomly placed

- Conversion from one stereoisomerism to another is not possible by simple rotation about single chain bond; bonds must be severed first, then reformed!

Polymerize Can it crystallize?Melting T?

• Regularity and symmetry of side groups affect properties

Polymer Geometrical Isomerism

cis

-structure

trans

-structure

with R= CH

₃

to form rubber

Cis-polyisoprene

trans-polyisoprene

H H

-Conversion from one isomerism to another is not possible by simple

rotation about chain bond because double-bond is too rigid! -See Figure 4.8 for taxonomy of polymer structures

Polymer Structural Isomerism

Some polymers contain monomers with more than 1 reactive site

e.g. isoprene C H₂ C C H CH₂ CH₃ trans-isoprene trans-1,4-polyisoprene C H₂ C C H C H₂ CH₃ 1 4 2 trans-1,2-polyisoprene n C H₂ C CH C H₂ C H₃ n 3 3,4-polyisoprene C H2 C H C C H2 CH3 n

Note: there are also cis-1,4- and cis-1,2-polyisoprene

• Covalent

chain

configurations and strength:

Direction of increasing strength

Polymer Microstructure

Van der Waals, H More rigid

Short branching

Long branching

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• Random, Alternating, Blocked, and Grafted

CoPolymers

• Synthetic rubbers are often copolymers. e.g., automobile tires (SBR)

Styrene-Butadiene Rubber random polymer

Molecular Structure

How do crosslinking and branching occur in polymerization?

1. Start with or add in monomers that have more than 2 sites that bond with other monomers, e.g. crosslinking polystyrene with divinyl benzene

… _… stryene polystyrene Control degree of crosslinking by styrene-divinyl benzene ratio + … … styrene

divinyl benzene crosslinked polystyrene Monomers with trifunctional groups lead to network polymers.

Molecular Structure

Branching in polyethylene (back-biting)

C H2 CH2 R C H2 C H2 C H2 C H2 C H₂ C H H R C C H2 CH₂ CH2 C H H H H Same as

Radical moves to a different carbon

(H transfer) R C C H2 CH2 CH2 C H H H H

Polymerization continues from this carbon

Process is difficult to avoid and leads to (highly branched) low-density PE . When there is small degree of branching you get high-density PE.

Example 3

Nitrile rubber copolymer, co-poly(acrylonitrile-butadiene), has

Calculate the ratio of (# of acrylonitrile) to (# of butadiene).

€

M

n

= 106,740g /mol

€

n

_{n = 2000} 3 C = 3 x 12.01 g/mol 3 H = 3 x 1.008 g/mol 1 N = 1 x 14.007 g/mol m0= 53.06 g/mol 4 C = 4 x 12.01 g/mol 6 H = 6 x 1.008 g/mol m₀= 54.09 g/mol 1,4-addition product € mo =M_nn n = 106,740 2000 = 53.57g/mol We need to use an avg. monomer MW: € m_o= f1m1+ f2m2= f1(m1− m2) + m2 € f1=m_m0− m2 1− m2= 53.37 − 54.09 53.06 − 54.09= 0.7 € f_{2 = 1−}f_{1 = 0.3} € f2 f1= 0.7 0.3→ 7 : 3

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• Crosslinking in elastomers is called vulcanization, and is achieved by

irreversible chemical reaction, usually requiring high temperatures.

Vulcanization

• Sulfur compounds are added to form chains that bond adjacent

polymer backbone chains and crosslinks them.

• Unvulcnaized rubber is soft and tacky an poorly resistant to wear.

e.g., cis-isoprene Stress-strain curves

+ (m+n) S

(S)

_n

(S)

_m

Single bonds

Double bonds

• Molecular weight

, M

w

:

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.

crystalline region

amorphous region Adapted from Fig. 14.11, Callister 6e.

Molecular Weight and Crystallinity

Polymer Crystallinity

polyethylene

_{• Some are amorphous.}

• Some are partially crystalline (semi-crystalline). • Why is it difficult to have a 100% crystalline polymer?

%crystallinity =

ρ

c

(

ρ

s

−

ρ

a

)

ρ

s

(

ρ

c

−

ρ

a

)

× 100%

ρs = density of specimen in question ρa = density of totally amorphous polymer ρc = density of totally crystalline polymer

€

%crystallinity =

M

crystalline

M

_total

× 100% =

ρc

V

c

ρs

V

s

× 100% =

ρc

ρs

f

c

× 100%

Volume fraction of crystalline component.

€

Mtotal= Mcrystalline+ Mamophous Ms= Mc+ Ma ρsVs=ρcVc+ρaVa ρs=ρcV_Vc s+ρa V_a Vs = ρcfc+ρafa=ρcfc+ρa(1−fc) = fc(ρc−ρa) + ρa Using definition of volume fractions:

€ f_{c =}Vc V_s € fa =V_Va s

€

f

_c

=

ρ

s

−

ρ

a

ρc

−

ρa

Substituting in fc into the original definition:

%crystallinity =

ρ

c

(

ρ

s

−

ρ

a

)

ρ

s

(

ρ

c

−

ρ

a

)

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Polymer Crystallinity

Degree of crystallinity depends on processing conditions (e.g. cooling rate) and chain configuration.

Cooling rate: during crystallization upon cooling through MP,

polymers become highly viscous. Requires sufficient time for random & entangled chains to become ordered in viscous liquid.

Chemical groups and chain configuration: More Crystalline

Smaller/simper side groups Linear Isotactic or syndiotactic

Less Crystalline Larger/complex side groups

Highly branched Crosslinked, network

Random

Semi-Crystalline Polymers

Fringed micelle model: crystalline region embedded in amorphous region. A single chain of polymer may pass through several crystalline regions as well as intervening amorphous regions.

€

f

_c

=

ρ

s

−

ρ

a

ρ

c

−

ρ

a

Crystalline volume fractions Important

Semi-Crystalline Polymers

Chain-folded model: regularly shaped platelets (~10 – 20 nm thick) sometimes forming multilayers.

Average chain length >> platelet thickness.

Semi-Crystalline Polymers

Spherulites: Spherical shape composed of aggregates of chain-folded crystallites.

Natural rubber

Cross-polarized light through spherulite structure of PE.

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Diblock copolymers

Representative polymer-polymer phase behavior with different architectures:

A) Phase separation with mixed

LINEAR homopolymers. B) Mixed LINEAR homopolymers and

DIBLOCK copolymer gives surfactant-like stabilized state. C) Covalent bond between blocks in

DIBLOCK copolymer give microphase segregation. F. Bates, Science 1991.

• Thermoplastics

:

--little cross linking --ductile --soften w/heating --polyethylene (#2) polypropylene (#5) polycarbonate polystyrene (#6)

• Thermosets

:

--large cross linking (10 to 50% of mers) --hard and brittle

--do NOT soften w/heating --vulcanized rubber, epoxies, polyester resin, phenolic resin

Callister, Fig. 16.9

T

Molecular weight

Tg

Tm

mobile liquid viscous liquid rubber tough plastic partially crystalline solid crystalline solid

Thermoplastics vs Thermosets

Tm: melting over wide range of T depends upon history of sample consequence of lamellar structure thicker lamellae, higher Tm.

Tg: from rubbery to rigid as T lowers

• Packing of “spherical” atoms as in ionic and metallic crystals led to crystalline structures.

• How polymers pack depend on many factors: • long or short, e.g. long (-CH2-)n.

• stiff or flexible, e.g. bendy C-C sp3_.

• smooth or lumpy, e.g., HDPE. • regular or random

• single or branched

• slippery or sticky, e.g. C-H covalent (nonpolar) joined via vdW.

Analogy:

Consider dried (uncooked) spaghetti (crystalline) vs.

cooked and buttered spaghetti (amorphous).

• pile of long “stiff” spaghetti forms a random arrangement. • cut into short pieces and they align easily.

Candle wax more crystalline than PE, even though same

chemical nature.

Packing of Polymers

• Would you expect melting of

nylon 6,6

to be lower than

PE

?

What Are Expected Properties?

€ − N | H − H | C | H                 ₆ − N | H − O || C₋ H | C | H                 ₄ − N | H − O || C₋ € − N | H − H | C | H                 ₆ − N | H − O || C₋ H | C | H                 ₄ − N | H − O || C₋ + + + + + + € − H C H − H C H − € − H C H − H C H − + + + + + + nylon 6,6 polyethylene

a) What is the source of intermolecular cohesion in Nylon vs PE? b) How does the source of linking affect temperature?

Hydrogen bonds Van der Waals bonds

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Which polymer more likely to crystallize? Can it be decided?

What Are Expected Properties?

Linear syndiotactic polyvinyl chloride Linear isotactic polystyrene

• Linear and syndiotactic polyvinyl chloride is more likely to crystallize. • The phenyl side-group for PS is bulkier than the Cl side-group for PVC. • Generally, syndiotactic and isotactic isomers are equally likely to crystallize.

• For linear polymers, crystallization is more easily accomplished as chain alignment is not prevented.

• Crystallization is not favored for polymers that are composed of chemically complex mer structures, e.g. polyisoprene.

What Are Expected Properties?

Linear and highly crosslink

cis-isoprene

• Not possible to decide which might crystallize. Both not likely to do so. • Networked and highly crosslinked structures are near impossible to reorient to favorable alignment.

H

+

+ H20 Networked Phenol-Formaldehyde (Bakelite)

What Are Expected Properties?

alternating Poly(Polystyrene-Ethylene) Copolymer random poly(vinyl chloride-tetra-fluoroethylne) copolymer

• Alternating co-polymer more likely to crystallize than random ones, as they are always more easily crystallized as the chains can align more easily.

MatSE 280: Introduction to Engineering Materials ©D.D. Johnson 2004, 2006, 2007-08 • Soap is a detergent based on animal or vegetable product, some contain petrochemicals

Detergents

grease water detergent

• What properties of soap molecules do you need to remove grease? • “green” end must be “hydrophilic”. Why?

• Opposite end must be hydrocarbon. Why?

Water must be like oxygen (hoard electrons and promote H-bonding)

grease

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Simple polymer: Elmers glue + Borax 

SLIME!

Chemistry Elmer’s glue is similar to “poly (vinyl alcohol)” with formula:

Borax is sodium tetraborate decahydrate (B4Na2O7 • 10 H2O).

The borax actually dissolves to form boric acid, B(OH)3.

This boric acid-borate solution is a buffer with a pH of about 9 (basic). Boric acid will accept a hydroxide OH- from water.

B(OH)3 + 2H2O  B(OH)4- + H3O+ pH=9.2 

OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH

this is a SHORT, n=15 chain of poly(vinyl alcohol)

Hydrolyzed molecule acts in a condensation reaction

with PVA,

crosslinking

it.

Simple polymer: Elmer’s glue + Borax 

SLIME!

Hydrolyzed molecule acts in a condensation reaction with PVA, crosslinking it.