chapter 4 structures of polymers. structures of solid materials general crystalline materials...

Chapter 4

Structures of Polymers

Structures of Solid Materials

general

Crystalline materials- Materials which have

crystal structure.

- dense, ordered packing.

- atoms pack in periodic, 3D arrays @ long range atomic order.

- tend to have higher (max.) bond energies.

- Occurs for: normal solidification conditions.

- Typical of: metals, many ceramics & certain polymers.

met

als

cera

mic

s

poly

mer

s

Noncrystalline materials- Materials which have

amorphous structure.

- non dense, random packing.

- atoms have no periodic arrays @ short range order.

- tend to have lower (min.) bond energies.

- Occurs for: complex structures @ rapid cooling.

- Typical of: polymers, some ceramics & metals.

SC

, B

CC

, F

CC

, H

CP

cry

sta

l s

tru

ctu

res

AX

, AX

2 & A

BX

3 ty

pe

cry

sta

l s

tru

ctu

res

,

Oth

ers

: s

ilic

ate

, g

las

s,

ca

rbo

n c

era

mic

s.

Lin

ea

r, b

ran

ch

ed

, c

ros

s-l

ink

ed

, n

etw

ork

s

tru

ctu

res

(a

s m

ole

cu

lar

ch

ain

s)

Energy

r

typical neighbor bond length

typical neighbor bond energy

Energy

r

typical neighbor bond length

typical neighbor

bond energy

crystal structure

amorphous structure

crystalline SiO2

noncrystalline SiO2

(refer to atomic arrangement in crystalline materials)

(refer to atomic arrangement in noncrystalline materials)


met

als

cera

mic

s

poly

mer

s

SC

, B

CC

, F

CC

, H

CP

cry

sta

l s

tru

ctu

res

Lin

ea

r, b

ran

ch

ed

, c

ros

s-l

ink

ed

, n

etw

ork

Characteristics of polymer molecules

Chemistry (repeat unit

composition)

Size (molecular

weight)

Shape (chain, twisting,

entanglement, etc)

Structure (as polymer molecular

chains)

Cro

ss-li

nke

d

Lin

ea

r

Bra

nch

ed

Ne

two

rk

Isomeric states

Stereo isomers

Geometrical isomers

Iso

tact

ic

Syn

dio

tact

ic

Ata

ctic

cis trans

In general- All polymer are made from organic materials.• natural polymers – plants & animals (organic materials), such as: wood, rubber, cotton, wool, leather, silk etc.• synthetic polymers – R&D from

organic materials, such as: polyethylene, polypropylene, polystyrene, Kevlar, nylon etc.

- Most polymeric materials are…• hydrocarbons (made up of H & C) – organic materials. • composed of very large molecular chains (in 2D array) with organics group (methyl, ethyl, phenyl) @ side group of various atoms (O, Cl, etc).

- 2 classifications:• Thermoplastic polymers.• Thermosetting polymers.

AX

, AX

2 & A

BX

3 ty

pe

cry

sta

l s

tru

ctu

res

,

Oth

ers

: s

ilic

ate

, g

las

s,

ca

rbo

n c

era

mic

s.

Lin

ea

r, b

ran

ch

ed

, c

ros

s-l

ink

ed

, n

etw

ork

s

tru

ctu

res

(a

s m

ole

cu

lar

ch

ain

s)


Pol

ymer

s

cera

mic

s

Met

als

Ch

em

istr

y o

f p

oly

me

r m

ole

cu

les

- polymer is defined as many repeat unit (in terms of molecular chains).• these macromolecules are composed of repeat units.- 2 types of bond exist:• intramolecular bonds: covalent (between C – C).• intermolecular bonds: hydrogen & van der Waals (between C – H @ C – other atoms (O, Cl etc)). - ex: ethane, C2H6

C C C C C C

HHHHHH

HHHHHH

Polyethylene (PE)

repeatunit

ClCl Cl

C C C C C C

HHH

HHHHHH

Poly(vinyl chloride) (PVC)

repeatunit

HH

HHH H

Polypropylene (PP)

C C C C C C

CH3

HH

CH3CH3 H

repeatunit

Adapted from Fig. 4.2, Callister & Rethwisch 3e.

Repeat units:

Smaller structural entities that are repeated along the molecular chain.

C C

H

H HH

HH

Intramolecules Covalent bond (strong)

Intermolecules Hydrogen & van der Waals

bonds (weak)

C2H6composition

(chemical formula)

C C

H

H HH

HH

structure

General terms

Polymer molecules (composition & structure)


Pol

ymer

s

cera

mic

s

Met

als

Ch

em

istr

y o

f p

oly

me

r m

ole

cu

les

Polymer molecules (composition & structure)

- composed of hydrocarbons molecules.• saturated & unsaturated hydrocarbons.

C C

H

H HH

HH

Saturated hydrocarbons

- also known as paraffin molecules/compounds.

- each carbon singly bonded to 4 other atoms.

ex: ethane, C2H6

- others: methane, CH4, propane, C3H8, butane, C4H10, pentane, C5H10 & hexane, C6H14

Unsaturated hydrocarbons

- double & triple bonds between 2 carbon atoms.- unstable & can form new bonds. ex: ethylene or ethene, C2H4

C C

H

H

H

H

ex: acetylene or ethyne, C2H2

C C HH

double bond

triple bond

Other hydrocarbons @ organic groups:

Methyl, CH3

Ethyl, C2H5

Phenyl, C6H5

Alcohols (methyl alcohol), CH3OH @ R – OH

Ethers (dimethyl ether), C2H6OH @ R – OH – R

Acids (acetic acid), CH3O2H

Aldehydes (formaldehyde)

Aromatic hydrocarbons (phenol)

free radical, R from organic groups

Structure of phenyl group


Pol

ymer

s

cera

mic

s

Met

als

Ch

em

istr

y o

f p

oly

me

r m

ole

cu

les

Polymer molecules

- hydrocarbon molecules (compounds) with same composition (chemical formula) can have quite different structures (atomic arrangement). ex: C8H18

Octane (normal)

2,4-dimethylhexane

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3=

H3C CH2 CH3( )6

H3C CH

CH3

CH2 CH

CH2

CH3

CH3

Polymerization

C C

H H

HH

monomer(ethylene)

R +

free radical

R C C

H

H

H

H

initiation

R C C

H

H

H

H

C C

H H

HH

+ R C C

H

H

H

H

C C

H H

H H

propagation

dimer

Isomerism

- reaction between R (catalyst species/free radical) & monomer (from unsaturated hydrocarbon) to form solid polymer materials.

2 steps of polymerization:

Monomer:

Small molecule from which polymer is synthesized.


Pol

ymer

s

cera

mic

s

Met

als

Ch

em

istr

y o

f p

oly

me

r m

ole

cu

les

Polymer molecules

A listing of 10 common polymeric materials

Polymeric materials Structure repeat unit


Pol

ymer

s

cera

mic

s

Met

als

Siz

e o

f p

oly

me

r m

ole

cu

les

Molecular size

Molecular weight distribution

- molecular weight, M: Mass of a mole of chains.

- not all chains in a polymer are of the same length. i.e., there is a distribution of molecular weights.

Low M

high M

short molecule chains

long molecule chains

molecules of #total

polymer of wttotalnM

iiw

iin

MwM

MxM

xi = number fraction of chains in size range i wi = weight fraction of chains in size range i

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


Pol

ymer

s

cera

mic

s

Met

als

Siz

e o

f p

oly

me

r m

ole

cu

les

Molecular size

Molecule weight calculation ex: average mass of a classWhat is the average weight of the students inthis class:a) Based on the number fraction of students in

each mass range?b) Based on the weight fraction of students in

each mass range?

Student Weight

mass (lb)

1 104

2 116

3 140

4 143

5 180

6 182

7 191

8 220

9 225

10 380

answer: The first step is to sort the students into weight ranges. Using 40 lb ranges gives the following table:weight number of mean number weightrange students weight fraction fraction

Ni Wi xi wi

mass (lb) mass (lb)

81-120 2 110 0.2 0.117121-160 2 142 0.2 0.150161-200 3 184 0.3 0.294201-240 2 223 0.2 0.237241-280 0 - 0 0.000281-320 0 - 0 0.000321-360 0 - 0 0.000361-400 1 380 0.1 0.202

Ni NiWi

10 1881total

numbertotal

weight

Calculate the number and weight fraction of students in each weight range as follows:

xi Ni

Ni

wi NiWi

NiWi

x81 120 2

100.2

117.01881

011 x 212081 w

Mn xiMi (0.2 x 110 0.2 x 142 + 0.3 x 184 + 0.2 x 223 + 0.1 x 380) =188 lb

Mw wiMi (0.117 x 110 0.150 x 142 + 0.294 x 184

+ 0.237 x 223 + 0.202 x 380) = 218 lb

Mw wiMi 218 lb


Pol

ymer

s

cera

mic

s

Met

als

Sh

ap

e o

f p

oly

me

r m

ole

cu

les

Molecular shape

- also known as 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.

chain

Twisting

Entanglement


Pol

ymer

s

cera

mic

s

Met

als

Str

uc

ture

of

po

lym

er

mo

lec

ule

s

Molecular chains structures

- shows the structure

of the molecular chains.- all structures

contain van der Waals & hydrogen bonding between the chains.

(1) Linear structures (2) Branched structures

(3) Cross-linked structures (4) Network structures

- polymers are synthesized in which…

• repeat units are joined together end to end in single chains.- flexible & spaghetti like.- common polymers:• Polyethylene• Poly(vinyl chloride)• Polystyrene• Poly(methyl methacrylate)• Nylon• Fluorocarbons


• side-branch chains are connected to

the main chains.- the branches may…• result from side reactions that occur during synthesis.• reduced the packing efficiency & lowered the polymer density. - common polymers:• high-density polyethylene (HDPE)• low-density polyethylene (LDPE)


• adjacent linear chains are joined one

to another at various positions by covalent bonds.- accomplished by additive atoms

that are covalently bonded to the chains.- common polymers:• rubber elastic materials.


• multifunctional monomers forming 3

@ more active covalent bonds make 3D networks.- polymer that is highly cross-linked may also be classified as a network polymer.- these polymers have distinctive mechanical & thermal properties.- common polymers:• epoxies• polyurethanes• phenol-formaldehydeBranched Cross-Linked NetworkLinear

secondarybonding


Pol

ymer

s

cera

mic

s

Met

als

Str

uc

ture

of

po

lym

er

mo

lec

ule

s

Molecular chains structures – isomeric states

Stereo isomers

Geometrical isomers

- shows the molecular configurations for polymers.

- Stereoisomers are mirror images – can’t superimpose without breaking a bond.- ex:

Isotactic configuration

Syndiotactic configuration

Atactic configuration

C CR

HH

HC C

H

H

H

R

or C C

H

H

H

R

EB

A

D

C C

D

A

BE

mirror plane

- Tacticity – stereoregularity or spatial arrangement of R units along chain.

- It can be isotactic, syndiotactic & atactic.

- to change configurations, it must break bonds.

- all R groups on same side of chain.

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

- R groups alternate sides.

- R groups randomly positioned.

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

C C

H

H

H

R

C C

H

H

H

R

C C

H

H

H

R R

H

H

H

CCcis trans

- H atom and CH3 group on same side of chain.

- H atom and CH3 group on opposite sides of chain.

C CHCH3

CH2 CH2

C CCH3

CH2

CH2

H

ex: cis-isoprene ex: trans-isoprene

- either cis @ trans isomerism.


Pol

ymer

s

cera

mic

s

Met

als

Cla

ss

ific

ati

on

of

po

lym

eri

c

ma

teri

als

Polymers classification

Thermoplastic polymers- soften when heated (eventually liquefy) & harden when cooled.• processes that are reversible & repeated.- structures: most linear polymers & some branched polymers with flexible chains.- properties: soft & less dimensional

stability.- common polymers: • polyethylene.• poly(vinyl chloride).• polystyrene.• poly(ethylene terephthalate).

- permanently hard during their formation

& do not soften when heated.• processes that are not reversible & repeated. - structures: most network polymers

& cross-linked polymers.- properties: hard, strong & high dimensional stability.- common polymers:• epoxies.• phenolics.• polyester resins.

Thermosetting polymers- 2 @ more monomers polymerized together. - 4 types:1. random – A and B randomly

positioned along chain.2. alternating – A and B alternate in

polymer chain.3. block – large blocks of A units

alternate with large blocks of B units.

4. graft – chains of B units grafted onto A backbone.

A – B –

random

block

graft

alternating

Copolymers


Pol

ymer

s

cera

mic

s

Met

als

Po

lym

er

cry

sta

llin

ity

Polymer crystallinity

- ordered atomic arrangements involving

molecular chains.- crystal structures in terms of unit

cells. ex: thin platelets with chain folds at faces.

polyethylene unit cell. 10 nm

Adapted from Fig. 4.12, Callister & Rethwisch 3e.

chain folded structure

polyethylene unit cell

- 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 & % crystallinity to increase.

crystalline region

amorphousregion

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.)


Pol

ymer

s

cera

mic

s

Met

als

Po

lym

er

cry

sta

llin

ity

Polymer single crystal- single crystals – only for slow and carefully controlled growth rates.- electron micrograph – multilayered single crystals (chain-folded layers) of polyethylene.

Polymer semi crystal

- some semicrystalline polymers form spherulite structures.- alternating chain-folder crystallites

and amorphous regions.- spherulite structure for relatively

rapid growth rates.

Photomicrograph – Spherulites in polyethylene

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

End of Chapter

4

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