Chapter 14: Polymer Structures - Konkukccdjko.konkuk.ac.kr/upload/sub0503/ch14.pdf · Chapter 14:...

Chapter 14 - 1

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 ?

Chapter 14:Polymer Structures

Chapter 14 - 2

What is a Polymer?

Poly mermany repeat unit

Adapted from Fig. 14.2, Callister & Rethwisch 8e.

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

Chapter 14 - 3

Ancient Polymers• Originally natural polymers were used

– Wood – Rubber

– Cotton – Wool

– Leather – Silk

• Oldest known uses

– Rubber balls used by Incas

– Noah used pitch (a natural polymer) for the ark

“Make for yourself an ark of gopher wood; you shall make the ark with rooms, and shall cover it inside and out with pitch.” (Genesis 6:14)

a black glutinous substance that belongs to the same family as asphalt and bitumen

Chapter 14 - 4

Polymer Classes

Samples of polymer classes, clockwise from top left:

skeins of wool (fiber), polyethylene jug (thermoplastic), rubber bands (elastomer), polycarbonate lenses (thermoset).

- Thermoplastics : polymeric materials that are flexible at high temp. (simply referred to as plastics)

- Thermosets : With certain polymers, heat can cause crosslinks to form irreversibly. The sample hardens so its shape is "set.“ will not soften upon heating.

Chapter 14 - 5

Polymer Composition

Most polymers are hydrocarbons (*organic compounds)– i.e., made up of H and C

*Organic compounds vs Inorganic compounds

1. Organic compounds contain carbon and hydrogen (no metals), while inorganic compounds mostly comprise of metal containing compounds(no carbons)

2. Organic compounds are the result of activities of living beings while inorganic compounds are created either due to natural processes unrelated to any life form or the result of human experimentation in the laboratory.3. Organic compounds are biological and inorganic are mineral

in nature.

4. Inorganic compounds can make salt, while organic cannot.

Chapter 14 - 6

Polymer Composition• Saturated hydrocarbons

– Each carbon singly bonded to four other atoms

– Example:

• Ethane, C2H6

C C

H

H HH

HH

Chapter 14 - 7

Polymer Compositionsp3 hybridization

– According to the valence bond theory, carbon should form two covalent bonds, resulting in a CH2, but very unstable

– A excited 2s electron can be bumped into one of the 2p orbitals.

– 2s and three 2p orbitals fused together to make four, equal energy sp3 hybrid orbitals

Chapter 14 - 8

Polymer Compositionsp3 hybridization

C C

H

H HH

HH

Chapter 14 - 9

Chapter 14 -10

Unsaturated Hydrocarbons

• Double & triple bonds somewhat unstable –can form new bonds

– Double bond found in ethylene or ethene - C2H4

C C

H

H

H

H

A σ bond formed from two sp2 orbitals and a π bond formed from two 2p orbitals together compose a double bond

Unhybridizedsp2 hybridization

Chapter 14 -11

Unsaturated Hydrocarbons– Triple bond found in acetylene or ethyne - C2H2

The C-H bonds are formed by sp-s overlap while the C-C sigma bond is formed by sp-sp overlap. On the other hand, the π bonds are formed by side-to-side overlap of each of the two carbons' two unhybridized p orbitals.

Sp hybridized C atoms(Sp hybrid orbitals grey, p orbitals blue)

Cylindrical electron density distribution

of π electrons

πy bondσ bond

πz bond

C C HH

Hydrogen s orbital

sp hybridization

Chapter 14 -12

Isomerism

• Isomerism

– two compounds with same chemical formula can have quite different structures

for example: C8H18 : 18 different isomers• normal-octane

• 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 CH

CH3

CH2 CH

CH2

CH3

CH3

H3C CH2 CH3( )6

Chapter 14 -

Chapter 14 -14

Polymerization and Polymer Chemistry

• Free radical polymerization

• Initiator: example - benzoyl peroxide

C

H

H

O O C

H

H

C

H

H

O2

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

R= 2

Chapter 14 -15

Chemistry and Structure of Polyethylene


Note: polyethylene is a long-chain hydrocarbon- paraffin wax for candles is short polyethylene

MacromoleculesPolymer

Chapter 14 -16

Bulk or Commodity Polymers

packaging (plastic bag, plastic films, geomembranes, containers including bottles).

1

2

3Pipes, electric cables, flooring, signs, clothing and funiture

Non-stick frying pans(Teflon), Gore-Tex, stenotic arteries

Packaging, clothing, stationery, medical, toys, living hinges and bags

Chapter 14 -17

Bulk or Commodity Polymers (cont)

Packaging(cd cases, styrofoam), containier, bottles, and disposable cutlery and razor

Glass substitute, window, canopy, hard contact lenses,

the earliest commercial synthetic resin,Circuit boards, pool balls, coatings and adhesives

Chapter 14 -18

Bulk or Commodity Polymers (cont)

Ball bearing cages, electroinsulating elements, pipes, carpet, apparel, airbag

Plastic bottles and apparel

Phones, electronic components, CDs, DVDs, bullet-proof glass, cockpit canopy, safety goggles

Chapter 14 -19

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

Low M high M

- Tensile strength often increases with MW; longer chains are anchored better

- Not all chains in a polymer are of the same lengthi.e., there is a distribution of molecular weights

Chapter 14 -20

xi = number fraction of chains in size range i

molecules of #total

polymer of wttotalnM

MOLECULAR WEIGHT DISTRIBUTION

M n xi Mi

M w wi Mi


wi = weight fraction of chains in size range i

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

Chapter 14 -21

Molecular Weight Calculation

Example: average mass of a class

Student Weight

mass (lb)

1 104

2 116

3 140

4 143

5 180

6 182

7 191

8 220

9 225

10 380

What is the averageweight 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?

Chapter 14 -22

Molecular Weight Calculation (cont.)

Solution: 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

number

total weight

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

xi Ni

Ni

wi NiWi

NiWi

For example: for the 81-120 lb range

x81120

2

10 0.2

117.01881

011 x 212081 w

Chapter 14 -23

Molecular Weight Calculation (cont.)

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

weight mean number weightrange weight fraction fraction

Wi xi wi

mass (lb) mass (lb)

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

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

Chapter 14 -24

Degree of Polymerization, DP

DP = average number of repeat units per chain

iimfm

m

:follows as calculated is this copolymers for

unit repeat of weightmolecular average where

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

C C C C

H

H

H

H

H

H

H

H

H( ) DP = 6

mol. wt of repeat unit iChain fraction

m

MDP

n

Chapter 14 -25


Molecular Structures for Polymers

Branched Cross-Linked NetworkLinear

secondarybonding

Rubber elastic materials : vulcanization

LDPEHDPE, PVC, PS, PMMA, Nylon, Fluorocarbon

Epoxide, PU, phenol-formaldehyde

Chapter 14 -26

Polymers – Molecular Shape

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


Chapter 14 -27

Polymers – Molecular Shape

- Extended chain polymer on the left. - A more realistic picture of a polymer

as a random coil on the right.

Chapter 14 -28

Chain End-to-End Distance, r


Random coils and molecular entanglements

Chapter 14 -29

Molecular Configurations for Polymers

Configurations – to change must break bonds

• Stereoisomerism

EB

A

D

C C

D

A

BE

mirror plane

C CR

HH

HC C

H

H

H

R

or C C

H

H

H

R

Stereoisomers are mirror images – can’t superimpose without breaking a bond

Chapter 14 -30

TacticityTacticity – stereoregularity or spatial arrangement of R

units along chain

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

isotactic – all R groups on same side of chain

C C

H

H

H

R

C C

H

H

H

R

C C

H

H

H

R R

H

H

H

CC

syndiotactic – R groups alternate sides

Chapter 14 -31

Tacticity (cont.)

atactic – R groups randomlypositioned

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

Chapter 14 -32

cis/trans Isomerism

H atom and CH3 groupon same side of chain

H atom and CH3 groupon opposite side of chain

Chapter 14 -33

cis/trans Isomerism

common natural rubber, latexelastomer, amorphous

natural rubber, gutta-percha, rigid

Chapter 14 -34

Classification scheme for the characteristics of polymer molecules

Chapter 14 -35

Copolymers

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

block

graft


alternating

Chapter 14 -36

SBR : styrene-butadiene rubberNBR : Nitrile rubber (AN+Butadiene)

Chapter 14 -37

Crystallinity in Polymers

• Ordered atomic arrangements involving molecular chains

• Crystal structures in terms of unit cells

• Ex. polyethylene unit cell

– Some are amorphous

– Some are partially crystalline (semi-crystalline).


Chapter 14 -38

Crystallinity in Polymers

Chapter 14 -39

Polymer Crystallinity

• Crystalline regions

– Thin(10~20nm) platelets with chain folds at faces

– Sometimes forming multilayers

– Chain folded model

– Average chain length >> platelet thickness

10 nm


Adapted from Fig. 14.11, Callister &

Rethwisch 8e.

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

Chapter 14 -40

Polymer Crystallinity (cont.)

Polymers rarely 100% crystalline• Difficult for all regions of all chains to

become aligned

• Degree of crystallinity expressed as % crystallinity.-- Some physical properties

depend on % crystallinity.-- Heat treating causes

crystalline regions to grow and % crystallinity to increase.

Adapted from Fig. 14.11, Callister 6e.(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)

crystalline region

amorphousregion

Fringed micelle model

Chapter 14 -41

Semicrystalline Polymers

Spherulite surface


• Some semicrystallinepolymers form spherulitestructures

• Alternating chain-folded crystallites and amorphous regions

• Spherulite structure for relatively rapid growth rates

Chapter 14 -42

Photomicrograph – Spherulites in Polyethylene


Cross-polarized light used

- a maltese crossappears in each spherulite

- the band or rings results from twisting of the lamellar crystals as they extend like ribbons from the center

Chapter 14 -43

Diffusion in Polymeric Materials

- often characterized in terms of a permeability coefficient (PM)- for the case of steady-state diffusion through a polymer

membrane, Fick’s first law, is modified as

where, ∆P is the difference in pressure of the gas across the membrane and ∆x is the membrane thickness

- for small molecules in nonglassy polymers,

PM = DS

where, D is diffusion coefficient and S is solubility of the diffusing species in the polymer

Chapter 14 -44

• Concept Check : 14.1~ 14.5

• Example problem : 14.1~14.3

• Questions and Problems :14.1-14.8, 14.11-14.23 , 14.25-29

Problems

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