Introduction to Engineering Materials ENGR2000 Chapter 14 ...engineering.armstrong.edu/cameron/ENGR2000_polymers I.pdf · Introduction to Engineering Materials ENGR2000 Chapter 14:

Introduction to Engineering Materials

ENGR2000

Chapter 14: Polymer Structures

Dr. Coates

14.1 Introduction

• Naturally occurring polymers

– Wood, rubber, cotton, wool, leather, silk

• Synthetic polymers

– Plastics, rubbers, fibers

14.2 Hydrocarbon Molecules

• Many organic materials are hydrocarbons

• Composed of hydrogen & carbon

• Covalent bonding

Saturated & Unsaturated hydrocarbons

Saturated

• Molecules with all single

bonds

• No new atoms may be

joined without the

removal of others that are

already bonded

Unsaturated

• Molecules with double or

triple covalent bonds

5

Paraffin Family

14.3 Polymer Molecules

• Polymers are often called macromolecules

• Covalent bonds

• Molecules in the form of long & flexible chains

Mers, polymers & monomers

• Mer units - structural entities which are repeated

along the chain

• Polymer – many mers

• Monomer – a stable molecule from which a

polymer is synthesized

8

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

propagation

Free radical

9

Chemistry and Structure of PolyethyleneAdapted from Fig.

14.1, Callister &

Rethwisch 9e.

Note: polyethylene is a long-chain hydrocarbon

- paraffin wax for candles is short polyethylene

10

Bulk or Commodity Polymers

11

Bulk or Commodity Polymers (cont)

Isomerism

• Isomerism

– two compounds with same chemical formula can

have quite different structures

for example: C8H18

• 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

13

MOLECULAR WEIGHT

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

Low M

high M

Not all chains in a polymer are of the same length

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

14

xi = number fraction of chains in size range i

MOLECULAR WEIGHT DISTRIBUTIONFig. 14.4, Callister & Rethwisch 9e.

wi = weight fraction of chains in size range i

Mi = mean molecular weight of size range i

15

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 average

weight of the students in

this 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?

16

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 weight

range students weight fraction fractionN i W i xi wi

mass (lb) mass (lb)

81-120 2 110 0.2 0.117

121-160 2 142 0.2 0.150

161-200 3 184 0.3 0.294

201-240 2 223 0.2 0.237

241-280 0 - 0 0.000

281-320 0 - 0 0.000

321-360 0 - 0 0.000

361-400 1 380 0.1 0.202

SNi SNiW i

10 1881

total

number

total

weight

Calculate the number and weight

fraction of students in each weight

range as follows:

For example: for the 81-120 lb range

17

Degree of Polymerization, DP

DP = average number of repeat units per chain

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

14.7 Molecular Structure

• Characteristics of a polymer depend on

– Its molecular weight

– Molecular shape

– Structure of molecular chains

Linear Polymers

• Mer units are joined together end to end in single

chains

• Flexible long chains (like a mass of spaghetti)

• Van der Waals & hydrogen bonds between the

chains

• Polyethylene, polyvinyl chloride, nylon, etc.

Branched Polymers

• Side branch chains are connected to the main ones

• Linear polymers form branched polymers when

side reactions that occur during synthesis form the

branches

• Chain packing efficiency reduced->lower density

• Ex. low density polyethylene

Crosslinked Polymers

• Adjacent linear chains are joined one to another at

various positions by covalent bonds

• Crosslinking may be accomplished by additives

• Vulcanized rubber

Network Polymers

• Multifunctional mer units, with three or more

active covalent bonds, form 3-D networks

• Epoxies, phenol-formaldehydes

Questions

• How might the length of the chain affect the likely

phase (hard solid, waxy solid, liquid) at room

temperature? Melting temperature? Why?

• How might molecular weight affect modulus and

strength? Why?

23

14.9 Thermoplastic and Thermosetting

Polymers

• Thermoplastic polymers

– Soften when heated

– Eventually liquefy

– Harden when cooled

– Processes are totally reversible & may be repeated

– Linear & branched polymers

Thermoplastic polymers

• The structure of polyethylene (a) the basic monomer (b)

the double bond in the monomer is opened (c) monomers

are linked together (d) secondary bonds between the

polymer chains

14.9 Thermoplastic and Thermosetting

Polymers

• Thermosetting polymers

– Become permanently hard when heated

– Do not soften or liquefy

– Harder & stronger than thermoplastic polymers

– Cross-linked & network polymers

Thermoset polymers

• The structure of crosslinked rubber (a) double bonds

along the length of the polymer chains (b) formation of

crosslinks (primary bonds) between the chains

14.11 Polymer Crystallinity

• Packing of molecular chains so as to produce an

ordered atomic array

• Linear polymers

– crystallization is easily obtained

– No restrictions to prevent chain alignment

• Crosslinked and network polymers

– amorphous

Practice Problems:

14.3, 14.5, 14.6, 14.13, 14.16