1. CHAPTER 4: POLYMER STRUCTURES
ISSUES TO ADDRESS...
• What are the basic microstructural features?
• How do these features influence properties? 1

2. 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

3. Structures of Polymers
Before introducing polymers, let me spend one or two minutes refreshing your memory 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 v unsaturated
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
End of lecture 1
Unsaturated
Saturated

4. Structures of Polymers
Hydrocarbons have strong chemical bonds, but interact only weakly with one another (van der Waals’ forces)
Isomerism – compounds of the same chemical composition but different atomic arrangements (i.e. bonding connectivity)
Chemical formula for both: C4H10
Atomic connectivity different!
Isomerism is important as we will see!
Aside: Read 4.1 – 4.3!
End of lecture 1

5. Structures of Polymers
Polymer molecules
Ok, what is a polymer?
Polymers are molecules (often called macromolecules) formed from a series of building units (monomers) that repeat over and over again
As we will see, polymers can be 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!

6. Structures of Polymers
End of lecture 1

7. Structures of Polymers
Chemistry of polymer molecules (is cool)
Basic idea of how you make polymers …
Stick with ethylene
Gas at STP
To polymerize ethylene, typically increase T, P and/or add an initiator (more on initiators later)
R* is an initiator that is responsible for activating the monomer to begin chain growth (hence the name)
End of lecture 1
After many additions of monomer to the growing chain…

8. Structures of Polymers
Polymer chemistry
stereochemistry
Typical 2D Drawing 3D
End of lecture 1

9. Structures of Polymers
Polymer chemistry
So in polyethylene (or 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

10. Structures of Polymers
Polymer chemistry
Time for nomenclature related to chemical composition
If the polymer is formed from one monomer (i.e. all the repeat units are the same type) – this is called a homopolymer
If the polymer is formed from multiple types of monomers (i.e. 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

11. (And these are just the 10 “more common” polymers)
As you can see, there is a very diverse range of chemistries accessible!
(And these are just the 10 “more common” polymers)
End of lecture 1

12. Structures of Polymers
Molecular weight
As you might imagine (and will see later), the properties of a polymer depend on how many units (mers) form the chain (i.e. polymer chain length)
Nearly all polymer synthesis protocols result in the formation of polymer chains with a distribution of lengths
Need way to quantify this – define an “average” molecular weight
Two approaches are typically taken
Number average molecular weight (Mn)
Weight-average molecular weight (Mw)
End of lecture 1

13. Structures of Polymers
Molecular weight
Number average molecular weight (Mn)
Basic idea: divide the chains into a series of size ranges and determine the number fraction of chains within each size range, then
Hypothetical polymer size distribution based on number fraction of chains
Mi – mean (middle) molecular weight (MW) of size range i
xi – fraction of the total number of chains within the size range
End of lecture 1

14. Structures of Polymers
Molecular weight
Weight average molecular weight (Mw)
Here divide the chains into a series of size ranges and determine the weight fraction of chains within each size range
Hypothetical polymer size distribution based on weight fraction of chains
Mi – mean (middle) molecular weight (MW) of size range i
wi – weight fraction of chains within the size range
End of lecture 1

15. Structures of Polymers
Molecular weight
Are the two different?
Yes, one is essentially based on mole fractions, and the other on weight fractions
They will only be the same if all the chains are exactly the same MW!
Mw > Mn
End of lecture 1
Get Mn from this
Get Mw from this

16. Structures of Polymers
Molecular weight
Polydispersity index (not in your book)
As mentioned above, inevitably there is a distribution of MWs (the sample will have chains of varying length)
Way to quantify this is by the polydispersity of the sample
Ratio of the weight and number average molecular weights (polydispersity index, or PI)
End of lecture 1
For monodisperse polymers (all chains are the exact same MW), PI = 1
Many commercial polymers have PI values over 10!

17. Structures of Polymers
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
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
End of lecture 1
fj and mj are the chain fraction and molecular weight of mer j

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

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

20. Structures of Polymers
Molecular shape
The pictures I have drawn so far of polymers are in a way misleading
How – I have always drawn them as highly elongated (straight) chains – in practice this is almost never the case
Why?
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
End of lecture 1

21. Structures of Polymers
Molecular shape
Taking this idea further, can also have rotations about bonds
Leads to “kinks”, twists
Take home message – 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

22. Structures of Polymers
Molecular structure
Physical properties of polymers depend not only on their molecular weight/shape, but also on the difference in the chain structure
For the purposes of this class, the four main structures we will consider are
Linear polymers
Branched polymers
Crosslinked polymers
Network polymers
End of lecture 1

23. Structures of Polymers
Molecular structure
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
End of lecture 1

24. Structures of Polymers
Molecular structure
Branched polymers – exactly what it sounds like
Have chains off the main chain (backbone)
These are often (though not always!) quite a bit shorter than the main chain
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
End of lecture 1

25. Structures of Polymers
Molecular structure
Crosslinked polymers – polymers where adjacent chains are attached to one another at various positions via covalent bonds
Often carried out during polymerization or by a non-reversible reaction after synthesis (referred to, surprisingly, as crosslinking)
These 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
End of lecture 1
picture

26. Structures of Polymers
Molecular structure
Network polymers – these are polymers that are “trifunctional” instead of bifunctional (see earlier slides)
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

27. Structures of Polymers
Molecular configuration
For polymers with side chain groups (besides Hydrogens), the arrangement of these groups can strongly modify their properties
Consider
Where R is not hydrogen (e.g. –CH3)
There are two “well-defined” arrangements of the R group
Head-to-tail
Head-to-head
Here the R groups alternate
End of lecture 1
Here the R groups are adjacent

28. Structures of Polymers
Molecular configuration
Typically the head-to-tail configuration dominates (why?)
This is a specific example of how mers are connected – as we will see it will have significant influence on polymer properties
More general is the issue of isomerism – different molecular configurations for molecules (polymers) of the same composition
We will focus on two
Stereoisomerism
Geometrical Isomerism
End of lecture 1

29. 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

30. Structures of Polymers
Stereoisomerism
Isotactic polymers
All of the R groups are on the same side of the chain
Isotactic configuration
End of lecture 1
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!

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

32. Structures of Polymers
Stereoisomerism
Atactic polymers
The R groups are “random”
Atactic configuration
End of lecture 1
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

33. Structures of Polymers
Geometrical Isomerism
These typically occur when one has double bonds (cis/trans)
Consider the mer of isoprene
cis configuration
End of lecture 1
The methyl group and hydrogen are on the same side of the C=C double bond
This is referred to as the cis structure
cis-polyisoprene is a natural rubber

34. Structures of Polymers
Geometrical Isomerism
Now consider the other isomer
trans configuration
The methyl group and hydrogen are on opposite sides of the C=C double bond
This is referred to as the trans structure
trans-polyisoprene (gutta percha) has very different physical properties!
Note – cannot simply interconvert between the cis and trans isomers
Why?
End of lecture 1

35. End of lecture 1

36. Structures of Polymers
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

37. Structures of Polymers
Thermosetting 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
End of lecture 1

38. 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…

39. 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)

40. Structures of Polymers
Polymer crystallinity
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
End of lecture 1

41. 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

42. 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

43. Structures of Polymers
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

44. Structures of Polymers
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

45. 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

46. POLYMER MICROSTRUCTURE
• Polymer = many mers
Adapted from Fig. 14.2, Callister 6e.
• Covalent chain configurations and strength:
Direction of increasing strength
Adapted from Fig. 14.7, Callister 6e. 2

47. 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

48. 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

49. ANNOUNCEMENTS Reading: Chapter 4 HW # 3. Due Friday February 9th
3.72; 3.75; 3.81; 3.83; 4.1; 4.2; 4.3; 4.7;
4.10; 4.18; 4.23; 4.27 29