Lecture 11 - Polymers (updated).pdf

1. Polymers

2. Paper or Plastic?
• We live in a plastic society.
• Everything around us is plastic.
• Could you go for a day without
plastic?
• Toothbrush, clothing, food containers,
cooking spatulas, pans, bottled water,
automobile parts, bicycle parts, eye
glasses, iPod, calculator, mouse, computer
parts, printer, stapler, head phones, TV,
clock, flash memory housing, usb
connector, keyboard, shoes, backpack
parts, cell phone, credit cards..

3. Properties
• Light weight
• Corrosive resistance
• Inexpensive
• Formed into variety of shapes
• Some polymers are transparent
• Electrical insulator (Acetals are conductors)
• Low coefficient of friction (Teflon)
• Low strength and stiffness
• Not suitable for high temperature

4. Ancient Polymers
• Naturally occurring polymers (those derived
from plants and animals) have been used for
centuries.
– Wood – Rubber
– Cotton – Wool
– Leather – Silk
• Other natural polymers
– Proteins – Starches
– Cellulose – Enzymes
• Numerous polymers are synthesized

5. Hydrocarbon Molecules
• Many organic materials are hydrocarbons (composed of
hydrogen and carbon).
• Most polymers are made up of H and C.
• The bonds between the hydrocarbon molecules are
covalent.
• Each carbon atom has 4 electrons that may be covalently
bonded, the hydrogen atom has 1 electron for bonding.
• A single covalent bond exists when each of the 2 bonding
atoms contributes one electron (ex: methane, CH4).

6. Saturated Hydrocarbons
 Each carbon has a single
bond to 4 other atoms; the
4 valence electrons are
bonded, the molecule is
stable.
 The covalent bonds in
each molecule are strong,
but only weak hydrogen
and van der Waals bonds
exist between molecules.
 Most of these hydrocarbons have relatively low melting and boiling
points.
 However, boiling temperatures rise with increasing molecular weight.
Paraffin family

7. Melting Point

8. Hydrocarbons at Room Temperature
• Gas
– Methane
– Ethane
– Propane
– Butane
• Plastic
 Liquid  Waxy
20 to 40
Carbons
5 to 19
Carbons
40 or more
Carbons

10. Unsaturated Hydrocarbons
• Double & triple bonds are somewhat unstable –
involve sharing 2 or 3 pairs of electrons, respectively.
They can also form new bonds
– Double bond found in ethylene - C2H4
– Triple bond found in acetylene - C2H2
C C
H
H
H
H
C C H
H

11. Isomerism
• Two compounds with same chemical formula can have
different structures (atomic arrangements).
for example: C8H18
• normal-octane
• 2,4-dimethylhexane
C C C C C C C C
H
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


12. Polymers: Introduction
• Polymer: High molecular weight molecule made up of a
small repeat unit (monomer).
– A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A
• Monomer: Low molecular weight compound that can be
connected together to give a poymer
• Oligomer: Short polymer chain
• Copolymer: polymer made up of 2 or more monomers
– Random copolymer: A-B-B-A-A-B-A-B-A-B-B-B-A-A-B
– Alternating copolymer: A-B-A-B-A-B-A-B-A-B-A-B-A-B
– Block copolymer: A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B

13. Polymer
Poly mer
many repeat unit (building blocks)
C C C C C C
H
H
H
H
H
H
H
H
H
H
H
H
Polyethylene (PE)
Cl
Cl Cl
C C C C C C
H
H
H
H
H
H
H
H
H
Poly(vinyl chloride) (PVC)
H
H
H
H
H H
Polypropylene (PP)
C C C C C C
CH3
H
H
CH3
CH3H
repeat
unit
repeat
unit
repeat
unit
Carbon chain backbone

14. • Free radical polymerization
• Initiator: example – hydrogen peroxide, benzoyl peroxide
C
H
H
O O C
H
H
C
H
H
O
2
C C
H H
H
H
monomer
(ethy lene)
R +
free radic al
R C C
H
H
H
H
initiation
R C C
H
H
H
H
C C
H H
H
H
+ R C C
H
H
H
H
C C
H H
H H
propagation
dimer
R
= 2
Chemistry and Structure of Polyethylene
(initiator monomers)

15. Chemistry and Structure of Polyethylene
• Polyethylene is a long-chain hydrocarbon.
• Top figure shows repeat unit and chain structures.
• Other figure shows zigzag backbone structure.
Tetrahedral
arrangement
of C-H

16. Bulk or Commodity Polymers

17. Bulk or Commodity Polymers (cont)

18. Bulk or Commodity Polymers (cont)

19. Cellulose
• Cellulose is a highly abundant organic compound. Extensive
hydrogen bonding between the chains causes native celluose
to be roughly 70% crystalline. It also raises the melting point
(>280°C) to above its combustion temperature.
• Cellulose serves as the principal structural component of green
plants and wood.
• Cotton is one of the purest forms of cellulose and has been
cultivated since ancient times.
• Cotton also serves (along with treated wood pulp) as the
source the industrial production of cellulose-derived materials
which were the first "plastic" materials of commercial
importance.

20. Rubber
• A variety of plants produce a sap consisting of a colloidal dispersion of
cis-polyisoprene. This milky fluid is especially abundant in the rubber
tree (Hevea); it drips when the bark is wounded.
• After collection, the latex is coagulated to obtain the solid rubber.
Natural rubber is thermoplastic, with a glass transition temperature of –
70°C.
• Raw natural rubber tends to be sticky when warm and brittle when
cold, so it was little more than a novelty material when first introduced
in Europe around 1770.
• It did not become generally useful until the mid-nineteenth century
when Charles Goodyear found that heating it with sulfur — a process
he called vulcanization — could greatly improve its properties.
cis-polyisoprene

21. Types of Polymers
• Polymer Classifications
– Thermoset: cross-linked polymer that cannot be melted
(tires, rubber bands)
– Thermoplastic: Meltable plastic
– Elastomers: Polymers that stretch and then return to
their original form: often thermoset polymers
– Thermoplastic elastomers: Elastic polymers that can be
melted (soles of tennis shoes)
• Polymer Families
– Polyolefins: made from olefin (alkene) monomers
– Polyesters, Amides, Urethanes, etc.: monomers linked
by ester, amide, urethane or other functional groups
– Natural Polymers: Polysaccharides, DNA, proteins

22. Polymer Synthesis
• There are two major classes of polymer formation
mechanisms
– Addition polymerization: The polymer grows by sequential
addition of monomers to a reactive site
• Chain growth is linear
• Maximum molecular weight is obtained early in the reaction
– Step-Growth polymerization: Monomers react together to
make small oligomers. Small oligomers make bigger
ones, and big oligomers react to give polymers.
• Chain growth is exponential
• Maximum molecular weight is obtained late in the reaction

23. Addition (Chain) Polymerization
– Initiation
– Propagation
– Termination

24. Addition Polymerization
In*
A
Initiation
In A* A

25. Addition Polymerization
Propagation
In*
A
Initiation
In A A* A

26. Addition Polymerization
Propagation
A
In*
A
Initiation
In A A A*

27. Addition Polymerization
Propagation
nA
In A A A A
n
A*
A A A A A
m
In A A A A
n
A
*A A A A A
m
Combination
*A A A A A
m
In A A A A
n
A
B A A A A
m
Disproportionation
Termination
Reactive site is consumed
A
In A A A A
n
A
A*
Chain Transfer
New reactive site
is produced
In*
A
Initiation
In A A A A*

28. Types of Addition Polymerizations
Ph
Anionic
C3H7 Li C4H9
Ph
Li+
Ph
n
C4H9
Ph Ph
Li+
n
Ph
Radical
PhCO2•
Ph
n
Ph
Cationic
Cl3Al OH2
H
Ph
HOAlCl3
Ph
n
H
Ph Ph
n
HOAlCl3
PhCO2
Ph
PhCO2
Ph Ph
n

29. Some Common Addition Polymers

30. Low-Density Polyethylene (LDPE)
Chain Length: 1000 - 2000

31. High-Density Polyethylene (HDPE)
Chain Length: 10,000 – 100,000

32. Ultra-high-molecular-weight
polyethylene (UHMWPE)
Helmet
Gears
Joint
Replacement
Chain Length: 2-6 million

33. PVC – (polyvinyl chloride)
Chain Length: 4,000 – 5,000
More Polar  Stronger Bonding

34. Step-Growth Polymerization
Stage 1
Consumption
of monomer
n n
Stage 2
Combination
of small fragments
Stage 3
Reaction of
oligomers to give
high molecular
weight polymer

35. Condensation (Step) Polymerization

36. Nylon-6,6
Cl Cl
O O
4
H2N NH2
4
Adipoyl chloride 1,6-Diaminohexane
Cl N
H
N
H
H
O O
4 4
NaOH
HO N
H
N
H
H
O O
4 4
n
6 carbon
diacid
6 carbon
diamine
Nylon-6,6
Diamine, NaOH, in H2O
Adipoyl chloride
in hexane
Nylon 6,6

37. Nylon-6,6
Diamine, NaOH, in H2O
Adipoyl chloride
in hexane
Nylon 6,6
Since the reactants are in different
phases, they can only react at the
phase boundary. Once a layer of
polymer forms, no more reaction
occurs. Removing the polymer
allows more reaction to occur.

38. Some Condensation Polymers

39. Polyethylene Terephthalate (PETE)
“Polyester”
Chain Length: 4,000 – 8,000
Ester

40. Nylon

41. Kevlar
Strong Network of Covalent
Bonds
And Polar Hydrogen Bonds

42. MOLECULAR WEIGHT
• Molecular weight, M: Mass of a mole of chains.
Low M
high M
• Polymers can have various lengths depending on the
number of repeat units.
• During the polymerization process not all chains in a polymer
grow to the same length, so there is a distribution of
molecular weights. There are several ways of defining an
average molecular weight.
• The molecular weight distribution in a polymer describes the
relationship between the number of moles of each polymer
species and the molar mass of that species.

44. xi = number fraction of chains in size range i
MOLECULAR WEIGHT DISTRIBUTION
i
i
w
i
i
n
M
w
M
M
x
M




wi = weight fraction of chains in size range i
Mi = mean (middle) molecular weight of size range i
Mn = the number average molecular weight (mass)
__

47. Degree of Polymerization, DP
DP = average number of repeat units per chain
weight
molecular
unit
repeat
where 
m
m
M
DP
n

Ex. problem 4.1b,
for PVC: m = 2(carbon) + 3(hydrogen) + 1(Clorine)
(from front of book) = 2(12.011) + 3(1.008) + 1(35.45)
= 62.496 g/mol
DP = 21,150 / 62.496 = 338.42

48. Polymer Chain Lengths
• Many polymer properties are affected by the length of
the polymer chains. For example, the melting
temperature increases with increasing molecular
weight.
• At room temp, polymers with very short chains
(roughly 100 g/mol) will exist as liquids.
• Those with weights of 1000 g/mol are typically waxy
solids and soft resins.
• Solid polymers range between 10,000 and several
million g/mol.
• The molecular weight affects the polymer’s properties
(examples: elastic modulus & strength).

49. What Makes Polymers Unique?
• Really big molecules (macromolecules) like polymers
have very different properties than small molecules
– Chain entanglement: Long polymer
chains get entangled with each other.
• When the polymer is melted, the
chains can flow past each other.
• Below the melting point, the chains
can move, but only slowly. Thus the
plastic is flexible, but cannot be
easily stretched.
• Below the glass transition point, the
chains become locked and the
polymer is rigid

50. Polymers – Molecular Shape
• Straight (b) and twisted (c) chain segments are generated
when the backbone carbon atoms (dark circles) are oriented
as in the figure above.
• Chain bending and twisting are possible by rotation of carbon
atoms around their chain bonds.
• Some of the polymer mechanical and thermal characteristics
are a function of the chain segment rotation in response to
applied stresses or thermal vibrations.

51. Molecular Structures for Polymers
• The physical characteristics of a polymer depend also on
differences in the structure of the molecular chains (other
variables are shape and weight).
• Linear polymers have repeat units joined end to end in
single chains. There may be extensive van der Waals and
hydrogen bonding between the chains. Examples:
polyethylene, PVC, nylon.
Branched Cross-Linked Network
Linear
secondary
bonding

52. Molecular Structures- Branched
• Where side-branch chains have connected to main chains,
these are termed branched polymers. Linear structures
may have side-branching.
• HDPE – high density polyethylene is primarily a linear
polymer with minor branching, while LDPE – low density
polyethylene contains numerous short chain branches.
• Greater chain linearity and chain length tend to increase
the melting point and improve the physical and mechanical
properties of the polymer due to greater crystallinity.
Branched Cross-Linked Network
Linear

53. Molecular Structures – Cross-linked, Network
• In cross-linked polymers, adjacent linear chains are joined
to one another at various positions by covalent bonding of
atoms. Examples are the rubber elastic materials.
• Small molecules that form 3 or more active covalent bonds
create structures called network polymers. Examples are
the epoxies and polyurethanes.
Branched Cross-Linked Network
Linear
secondary
bonding

54. Chain End-to-End Distance, r
• Representation of a single polymer
chain molecule that has numerous
random kinks and coils produced
by chain bond rotations; it is very
similar to a heavily tangled fishing
line.
• ―r‖ is the end to end distance of the
polymer chain which is much
smaller than the total chain length.

55. Physical Properties
Stretch
Linear Polymer
The chains can be stretched, which causes
them to flow past each other. When released,
the polymer will not return to its original form.
Stretch
Cross-Linked Polymer
The cross-links hold the chains together.
When released, the polymer will return to it's
original form.
Relax

56. Molecular Configurations for Polymers
Configurations – to change must break bonds
• Stereoisomerism
E
B
A
D
C C
D
A
B
E
mirror
plane
C C
R
H
H
H
C C
H
H
H
R
or C C
H
H
H
R
Stereoisomers are mirror
images – can’t superimpose
without breaking a bond

57. Tacticity
Tacticity – stereoregularity or spatial arrangement of R
units along chain
C C
H
H
H
R R
H
H
H
C
C
R
H
H
H
C
C
R
H
H
H
C
C
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
C
C
syndiotactic – R groups
alternate sides

58. Tacticity (cont.)
atactic – R groups randomly
positioned
C C
H
H
H
R R
H
H
H
C
C
R
H
H
H
C
C
R
H
H
H
C
C

59. cis/trans Isomerism
C C
H
CH3
CH2 CH2
C C
CH3
CH2
CH2
H
cis
cis-isoprene
(natural rubber)
H atom and CH3 group on
same side of chain
trans
trans-isoprene
(gutta percha)
H atom and CH3 group on
opposite sides of chain

60. Thermoplastics and Thermosets
• The response of a polymer to mechanical forces at elevated
temperature is related to its dominant molecular structure.
• One classification of polymers is according to its behavior
and rising temperature. Thermoplastics and Thermosets are
the 2 categories.
• A thermoplastic is a polymer that turns to a liquid when
heated and freezes to a very glassy state when cooled
sufficiently.
• Most thermoplastics are high-molecular-weight polymers
whose chains associate through weak Van der Waals forces
(polyethylene); stronger dipole-dipole interactions and
hydrogen bonding (nylon).

61. • Thermoplastic polymers differ from thermosetting
polymers (Bakelite, vulcanized rubber) since
thermoplastics can be remelted and remolded.
• Thermosetting plastics when heated, will
chemically decompose, so they can not be
recycled. Yet, once a thermoset is cured it tends
to be stronger than a thermoplastic.
• Typically, linear polymers with minor branched
structures (and flexible chains) are
thermoplastics. The networked structures are
thermosets.
Thermoplastics and Thermosets

62. Examples of Thermoplastics

63. More Examples of Thermoplastics
http://www2.dupont.com/Teflon/en_US/index.html
http://en.wikipedia.org/wiki/Teflon
Polymer

64. Specific Thermoplastic Properties

65. Specific Elastomeric Properties
Elastomers, often referred to as rubber, can be a thermoplastic or a
thermoset depending on the structure. They are excellent for parts
requiring flexiblity, strength and durability: such as automotive and
industrial seals, gaskets and molded goods, roofing and belting, aircraft
and chemical processing seals, food, pharmaceutical and semiconductor
seals, and wire and cable coatings.

66. • Thermoplastics:
--little cross linking
--ductile
--soften with heating
--polyethylene
polypropylene
polycarbonate
polystyrene
• Thermosets:
--large cross linking
(10 to 50% of mers)
--hard and brittle
--do NOT soften with 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
Thermoplastic vs Thermoset

67. 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

68. Types of copolymer
Homopolymer : -A-A-A-A-A-A-A-A-
Random copolymer : -A-B-B-A-B-A-A-B-
Alternating copolymer : -A-B-A-B-A-B-A-B-
Block copolymer : -A-A-A-A-B-B-B-B-
Graft copolymer : -A-A-A-A-A-A-A-A-
B-B-B-B-B-

69. Crystallinity in Polymers
• The crystalline state may exist in
polymeric materials.
• However, since it involves
molecules instead of just atoms or
ions, as with metals or ceramics,
the atomic arrangement will be
more complex for polymers.
• There are ordered atomic
arrangements involving molecular
chains.
• Example shown is a polyethylene
unit cell (orthorhombic).

70. ©2003
Brooks/Cole,
a
division
of
Thomson
Learning,
Inc.
Thomson
Learning
™
is
a
trademark
used
herein
under
license.
The effect of temperature on the structure and
behavior of thermoplastics

71. Thank you