1. CHAPTER 4: POLYMER STRUCTURES
Spherulite, rubber specimen. Chain-folded lamellar crystallites, ~10 nm thick, 30,000×

2. c04cof01
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 ?

3. 4.1 Structures of Polymers
Introduction and Motivation
Polymers are extremely important materials (i.e. plastics)
Have been known since ancient times – cellulose, wood, rubber, etc..
Biopolymers – proteins, enzymes, DNA …
Last ~50 years – tremendous advances in synthetic polymers
Just like for metals and ceramics, the properties of polymers
Thermal stability
Mechanical properties
Are intimately related to their molecular structure …
End of lecture 1

4. 4.1 Ancient Polymers Originally natural polymers were used: Wood
Rubber
Cotton
Wool
Leather
Silk
Oldest known use:
Rubber balls used by Incas
Noah used pitch (a natural polymer) for the ark
Noah's pitch
Genesis 6:14 "...and cover it inside and outside with pitch."
gum based resins extracted from pine trees

5. 4.2 Polymer Composition Most polymers are hydrocarbons
– i.e., made up of H and C
Saturated hydrocarbons
Each carbon singly bonded to four other atoms
Example:
Ethane, C2H6

6. 4.2 Unsaturated Hydrocarbons
Double & triple bonds somewhat unstable
Thus, can form new bonds
Double bond found in ethylene or ethene - C2H4
Triple bond found in acetylene or ethyne - C2H2

7. 4.2 Structures of Polymers
about hydrocarbons
Why? Most polymers are hydrocarbon (e.g. C, H) based
Bonding is highly covalent in hydrocarbons
Carbon has four electrons that can participate in bonding, hydrogen has only one
Saturated versus unsaturated
End of lecture 1
Unsaturated – species contain carbon-carbon double/triple bonds
Possible to substitute another atom on the carbon
Saturated – carbons have four atoms attached
Cannot substitute another atom on the carbon
Unsaturated
Saturated

8. c04eqf02 4.2 Hydrocarbon Molecules Acetylene Ethylene Ethyne Ethene
Hydrocarbons have strong chemical bonds, but interact only weakly with one another (van der Waals’ forces)
(normal) butane
isobutane
c04eqf02

9. 4.2 Isomerism 2,4-dimethylhexane
compounds with same chemical formula can have quite different structures
for example: C8H18
normal-octane 
Isomerism – compounds of the same chemical composition but different atomic arrangements (i.e. bonding connectivity)
2,4-dimethylhexane

10. 4.2

11. c04tf02

12. c04eqf03 4.3 Polymer Molecules Molecules are gigantic Macromolecules
Repeat units
Monomer

13. 4.3 Polymers Polymer molecules what is a polymer?
Polymers are molecules (often called macromolecules) formed from a series of building units (monomers) that repeat over and over again
polymers can have a range of molecular weights
There are many monomers
Can make polymers with different monomers, etc..
End of lecture 1
n is often a very large number!
e.g. can make polyethylene with MW > 100,000! ~3600 mers ~7200 carbons

14. Chemistry of polymer molecules
Example: ethylene
Gas at STP
To polymerize ethylene, typically increase T, P and/or add an initiator
Initiation
End of lecture 1
Propagation
After many additions of monomer to the growing chain…
R* = initiator; activates the monomer to begin chain growth
Initiator: example - benzoyl peroxide

15. 4.4 Polymer chemistry
Polymers are chain molecules. They are built up from simple units called monomers.
E.g. polyethylene is built from ethylene units: which are assembled into long chains:
Polyethylene or polythene (IUPAC name poly(ethene)) is a thermoplastic commodity heavily used in consumer products (notably the plastic shopping bag). Over 60 million tons of the material are produced worldwide every year.

16. Tetrafluoroethylene monomer polymerize to form PTFE or polytetrafluoroethylene
c04eqf08
c04eqf08
poly(tetrafluoroethene) or poly(tetrafluoroethylene) (PTFE) is a synthetic fluoropolymer. PTFE is the DuPont brand name Teflon. Melting: 327C
Vinyl chloride monomer leads to poly(vinyl chloride) or PVC
PVC: manufacturing toys, packaging, coating, parts in motor vehicles, office supplies, insulation, adhesive tapes, furniture, etc. Consumers: shoe soles, children's toys, handbags, luggage, seat coverings, etc. Industrial sectors: conveyor belts,
c04eqf09
printing rollers. Electric and electronic equipment: circuit boards, cables, electrical boxes, computer housing.

17. Chemistry and Structure of Polyethylene
Adapted from Fig. 4.1, Callister & Rethwisch 3e.
Polymer- can have various lengths depending on number of repeat units
Note: polyethylene is a long-chain hydrocarbon
- paraffin wax for candles is short polyethylene
• Polymer = many mers
Adapted from Fig. 14.2, Callister 6e.

18. Polymer chemistry
In polyethylene (PE) synthesis, the monomer is ethylene
Turns out one can use many different monomers
Different functional groups/chemical composition – polymers have very different properties!
Monomers
End of lecture 1

19. Homopolymer and Copolymer
Polymer chemistry
If formed from one monomer (all the repeat units are the same type) – this is called a homopolymer
If formed from multiple types of monomers (all the repeat units are not the same type) – this is called a copolymer
Also note – the monomers shown before are referred to as bifunctional
Why? The reactive bond that leads to polymerization (the C=C double bond in ethylene) can react with two other units
Other monomers react with more than two other units – e.g. trifunctional monomers
End of lecture 1

20. The Top 10 Bulk or Commodity
Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers

21. 4.5 MOLECULAR WEIGHT Molecular weight, M: Mass of a mole of chains.
low M
Simple for small molecules
All the same size
Number of grams/mole
Polymers – distribution of chain sizes
high M
Not all chains in a polymer are of the same length
i.e., there is a distribution of molecular weights

22. Molecular weight The properties of a polymer depend on its length
synthesis yields polymer distribution of lengths
Define “average” molecular weight
Two approaches are typically taken
Number average molecular weight (Mn)
Weight-average molecular weight (Mw)
End of lecture 1

23. MOLECULAR WEIGHT DISTRIBUTION
Adapted from Fig. 4.4, Callister & Rethwisch 3e.
Simple for small molecules
All the same size
Number of grams/mole
Polymers – distribution of chain sizes
Mi = mean (middle) molecular weight of size range i
xi = number fraction of chains in size range i
wi = weight fraction of chains in size range i

24. Molecular weight
Are the two different? Yes, one is essentially based on mole fractions, and the other on weight fractions
They will be the same if all the chains are exactly of the same MW! If not Mw > Mn
End of lecture 1
Get Mn from this
Get Mw from this

25. Molecular weight Other ways to define polymer MW
Degree of polymerization
Represents the average number of mers in a chain. The number and weight average degrees of polymerization are
End of lecture 1
m is the mer MW in both cases. In the case of a copolymer (something with two or more mer units), m is determined by
fj and mj are the chain fraction and molecular weight of mer j

26. Example Problem 4.1 Given the following data determine the
Number average MW
Number average degree of polymerization
Weight average MW
How to find Mn?
Calculate xiMi
Sum these!
Number average MW (Mn)
End of lecture 1

27. c04tf04a
c04tf04a

28. Example Problem 4.1 Number average degree of polymerization
(MW of H2C=CHCl is g/mol)
How to find Mw?
Calculate wiMi
Sum these!
Weight average molecular weight (Mw)
End of lecture 1

29. c04tf04b
c04tf04b

30. Degree of Polymerization, DP
DP = average number of repeat units per chain
DP = 6
Chain fraction
mol. wt of repeat unit i

31. 4.6 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
Adapted from Fig. 4.5, Callister & Rethwisch 3e.
C-C bonds are typically 109° (tetrahedral, sp3 carbon)
If you have a macromolecule with hundreds of C-C bonds, this will lead to bent chains

32. Structures of Polymers
Molecular shape
Taking this idea further, can also have rotations about bonds
Leads to “kinks”, twists
“the end-to-end distance of a polymer chain in the solid state (or in solution) is usually much less than the distance of the fully extended chain!
This is not even taking into account that you have numerous chains that can become entangled!
End of lecture 1

33. 4.7 Molecular structure
Physical properties of polymers depend not only on their molecular weight/shape, but also on the difference in the chain structure
Four main structures
Linear polymers
Branched polymers
Crosslinked polymers
Network polymers

34. 4.7 Molecular Structures for PolymersB
ranched
Cross-Linked
Network
Linear
secondary
bonding
Adapted from Fig. 4.7, Callister & Rethwisch 3e.

35. Linear polymers
– polymers in which the mer units are connected end-to-end along the whole length of the chain
These types of polymers are often quite flexible
Van der waal’s forces and H-bonding are the two main types of interactions between chains
Some examples – polyethylene, teflon, PVC, polypropylene

36. Branched polymers Polymer chains can branch:
Or the fibers may aligned parallel, as in fibers and some plastic sheets.
chains off the main chain (backbone)
This leads to inability of chains to pack very closely together
These polymers often have lower densities
These branches are usually a result of side-reactions during the polymerization of the main chain
Most linear polymers can also be made in branched forms

37. Crosslinked polymers Molecular structure
adjacent chains attached via covalent bonds
Carried out during polymerization or by a non-reversible reaction after synthesis (referred to as crosslinking)
Materials often behave very differently from linear polymers
Many “rubbery” polymers are crosslinked to modify their mechanical properties; in that case it is often called vulcanization
Generally, amorphous polymers are weak and cross-linking adds strength: vulcanized rubber is polyisoprene with sulphur cross-links:
End of lecture 1

38. Network polymers
– polymers that are “trifunctional” instead of bifunctional
There are three points on the mer that can react
This leads to three-dimensional connectivity of the polymer backbone
Highly crosslinked polymers can also be classified as network polymers
Examples: epoxies, phenol-formaldehyde polymers
End of lecture 1

39. POLYMER MICROSTRUCTURE
• Covalent chain configurations and strength:
Direction of increasing strength
Adapted from Fig. 14.7, Callister 6e. 2

40. 4.8 Molecular configurations
Classification scheme for the characteristics of polymer molecules
isomerism – different molecular configurations for molecules (polymers) of the same composition
Stereoisomerism
Geometrical Isomerism
End of lecture 1

41. c04eqf23 4.8 Molecular Configurations Repeat unit R = Cl, CH3, etc
Configurations – to change must break bonds
Stereoisomers are mirror
images – can’t superimpose
without breaking a bond E B A D C
mirror plane

42. c04eqf11 Head to-tail Head to-head
Typically the head-to-tail configuration dominates
Head to-head

43. Structures of Polymers
Stereoisomerism
Denotes when the mers are linked together in the same way (e.g. head-to-tail), but differ in their spatial arrangement
This really focuses on the 3D arrangement of the side-chain groups
Three configurations most prevalent
Isotactic
Syndiotactic
Atactic
End of lecture 1

44. ISOTACTIC Stereoisomerism Isotactic polymers
All of the R groups are on the same side of the chain
Isotactic configuration
Note: All the R groups are head-to-tail
All of the R groups are on the same side of the chain
Projecting out of the plane of the slide
This shows the need for 3D representation to understand stereochemistry!
End of lecture 1

45. SYNDIOTACTIC Stereoisomerism Syndiotactic polymers
The R groups occupies alternate sides of the chain
Syndiotactic configuration
Note: The R groups are still head-to-tail
R groups alternate – one of out of the plane, one into the plane
End of lecture 1

46. ATACTIC Stereoisomerism Atactic polymers The R groups are “random”
Atactic configuration
R groups are both into and out of the plane, no real registry
Two additional points
Cannot readily interconvert between stereoisomers – bonds must be broken
Most polymers are a mix of stereoisomers, often one will predominate
End of lecture 1

47. c04eqf12 Stereoisomerism—Head-to-tail isotactic configuration
Syndiotactic conformation
Atactic conformation

48. cis/trans Isomerism cis trans 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

49. c04eqf18
Geometrical Isomerism
c04eqf18

50. 4.9 Plastics variety of properties due to their rich chemical makeup
They are inexpensive to produce, and easy to mold, cast, or machine.
Their properties can be expanded even further in composites with other materials.

51. Glass phase (hard plastic)
Glass-rubber-liquid
Amorphous plastics have a complex thermal profile with 3 typical states:
Glass phase (hard plastic)
Temperature 3 9 6 7 8 4 5
Leathery phase
Log(stiffness) Pa
Rubber phase (elastomer)
Liquid

52. THERMOPLASTICS Thermosetting and thermoplastic polymers
Another way to categorize polymers – how do they respond to elevated temperatures?
Thermoplastics – these materials soften when heated, and harden when cooled – this process is totally reversible
This is due to the reduction of secondary forces between polymer chains as the temperature is increased
Most linear polymers and some branched polymers are thermoplastics
End of lecture 1

53. THERMOSETS Thermosetting and thermoplastic polymers
Thermosets – these materials harden the first time they are heated, and do not soften after subsequent heating
During the initial heat treatment, covalent linkages are formed between chains (i.e. the chains become cross-linked)
Polymer won’t melt with heating – heat high enough it will degrade
Network/crosslinked polymers are typically thermosets
Polymers which irreversibly change when heated are called thermosets.
Most often, the change involves cross-linking which strengthens the polymer (setting).
Thermosets will not melt, and have good heat resistance.
They are often made from multi-part compounds and formed before setting (e.g. epoxy resin).
Setting accelerates with heat, or for some polymers with UV light.
End of lecture 1

54. Thermoplastics
Polymers which melt and solidify without chemical change are called thermoplastics.
They support hot-forming methods such as injection-molding and FDM.

55. THERMOPLASTICS VS THERMOSETS
--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
Adapted from Fig , Callister 6e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) 3

56. 4.10 Structures of Polymers
Copolymers
Idea – polymer that contains more than one mer unit
Why? If polymer A has interesting properties, and polymer B has (different) interesting properties, making a “mixture” of polymer should lead to a superior polymer
“Random” copolymer – exactly what it sounds like
End of lecture 1
“Alternating” copolymer – ABABABA…

57. Structures of Polymers
Copolymers
Idea – polymer that contains more than one mer unit
Why? If polymer A has interesting properties, and polymer B has (different) interesting properties, making a “mixture” of polymer should lead to a superior polymer
“Block” copolymers. Domains of “pure” mers
End of lecture 1
“Graft” copolymers. One mer forms backbone, another mer is attached to backbone and is a sidechain (it is “grafted” to the other polymer)

58. Copolymers two or more monomers polymerized together
Adapted from Fig. 4.9, Callister & Rethwisch 3e.
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
alternating
block
graft

59. Copolymers
Polymers often have two different monomers along the chain – they are called copolymers.
With three different units, we get a terpolymer. This gives us an enormous design space…

60. 4.11 Polymer structure
The polymer chain layout determines a lot of material properties:
Amorphous:
Crystalline:

61. Crystallinity in Polymers
Adapted from Fig. 4.10, Callister & Rethwisch 3e.
Ordered atomic arrangements involving molecular chains
Crystal structures in terms of unit cells
Example shown
polyethylene unit cell
Polymers can be crystalline (i.e. have long range order)
However, given these are large molecules as compared to atoms/ions (i.e. metals/ceramics) the crystal structures/packing will be much more complex

62. Structures of Polymers
Polymer crystallinity
(One of the) differences between small molecules and polymers
Small molecules can either totally crystallize or become an amorphous solid
Polymers often are only partially crystalline
Why? Molecules are very large
Have crystalline regions dispersed within the remaining amorphous materials
Polymers are often referred to as semicrystalline
End of lecture 1

63. Structures of Polymers
Polymer crystallinity
Another way to think about it is that these are two phase materials (crystalline, amorphous)
Need to estimate degree of crystallinity – many ways
One is from the density
End of lecture 1

64. Structures of Polymers
4.11 Polymer crystallinity
What influences the degree of crystallinity
Rate of cooling during solidification
Molecular chemistry – structure matters
Polyisoprene – hard to crystallize
Polyethylene – hard not to crystallize
Linear polymers are easier to crystallize
Side chains interfere with crystallization
Stereoisomers – atactic hard to crystallize (why?); isotactic, syndiotactic – easier to crystallize
Copolymers – more random; harder to crystallize
End of lecture 1

65. 4.11 Polymer Crystallinity (cont.)
Polymers rarely 100% crystalline
Difficult for all regions of all chains to become aligned
crystalline
region
• Degree of crystallinity expressed as % crystallinity.
-- Some physical properties depend on % crystallinity.
-- Heat treating causes crystalline regions to grow and % crystallinity to increase.
amorphous
region
Adapted from Fig , Callister 6e.
(Fig 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.)

66. 4.11 MOLECULAR WEIGHT & CRYSTALLINITY
• Molecular weight, Mw: 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.
Adapted from Fig , Callister 6e.
(Fig 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.) 4

67. 4.12 Polymer Crystallinity
4.12 Polymer crystals
Chain folded-model
Many polymers crystallize as very thin platelets (or lamellae)
Idea – the chain folds back and forth within an individual plate (chain folded model)
End of lecture 1
Crystalline regions
thin platelets with chain folds at faces
Chain folded structure

68. 4.12 Single Crystals
Electron micrograph – multilayered single crystals (chain-folded layers) of polyethylene
Single crystals – only for slow and carefully controlled growth rates
Adapted from Fig. 4.11, Callister & Rethwisch 3e.

69. 4.12 Semicrystalline Polymers
Some semicrystalline polymers form spherulite structures
Alternating chain-folder crystallites and amorphous regions
Spherulite structure for relatively rapid growth rates
Spherulite surface
Adapted from Fig. 4.13, Callister & Rethwisch 3e.

70. Structures of Polymers
Polymer crystals
More commonly, many polymers that crystallize from a melt form spherulites
One way to think of these – the chain folded lamellae have amorphous “tie domains” between them
These plates pack into a spherical shape
Polymer analogues of grains in polycrystalline metals/ceramics
End of lecture 1

71. Photomicrograph – Spherulites in Polyethylene
Cross-polarized light used -- a maltese cross appears in each spherulite
Adapted from Fig. 4.14, Callister & Rethwisch 3e.

72. c04eqf03
END of chapter 4