1. Polymers: what they are and how they work?
Michael R. Kessler
School of Mechanical and Materials Engineering
Washington State University

2. What are Polymers Examples of polymers and where they are used.
Often Synthetic Polymers are called Plastic.
The volume of polymers produced is three times larger than that of all metals!
Poly (many) mer (parts)  made up of molecules of very long chains.

3. What is a Polymer?
Poly (many) mer (parts)  made up of molecules of very long chains.
repeat
unit
repeat
unit
repeat
unit C H
Polyethylene (PE) Cl C H
Poly(vinyl chloride) (PVC) H
Polypropylene (PP) C
CH3

4. 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

5. 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

7. Chemistry and Structure of Polyethylene
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

8. Bulk or Commodity Polymers
Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers

9. Bulk or Commodity Polymers (cont)

10. Bulk or Commodity Polymers (cont)

11. Analogy for the Structure of Polymers

12. Properties of Polymers
Glass transition temperature (snake pit analogy)
Strength—measured in maximum stress (force per unit area)
Stiffness—measured in stress per unit strain
Damping—ability to absorb energy
Properties are often temperature
and rate dependent
(Viscoelastic)

13. Properties (Demonstration)
Happy Ball
Sad Ball
Compare the damping properties for two apparently identical polymer balls by bouncing them on the floor.
How do their damping behaviors vary with temperature?
Happy Ball = Neoprene (polychloroprene)
Sad Ball = Norsorex (polynorbornene)

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

15. MOLECULAR WEIGHT DISTRIBUTION
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

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

17. Types of Polymers Thermoplastics Amorphous Semi-crystalline Thermosets
Crystalline Region
Amorphous – Examples include polystyrene, polyvinyl chloride, low density polyethylene (LDPE) …
Semi-crystalline – Examples include high density polyethylene (HDPE), Nylon, Kevlar …
Thermosets – Examples include epoxy, cyanate ester, natural rubber …
Crosslinks

18. Molecular Structures for PolymersB
ranched
Cross-Linked
Network
Linear
secondary
bonding

19. 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

20. Chain End-to-End Distance, r

21. Tacticity
Tacticity – stereoregularity or spatial arrangement of R units along chain
isotactic – all R groups on same side of chain
syndiotactic – R groups alternate sides

22. atactic – R groups randomly positioned
Tacticity (cont.)
atactic – R groups randomly positioned

23. 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
alternating
block
graft

24. Crystallinity in Polymers
Ordered atomic arrangements involving molecular chains
Crystal structures in terms of unit cells
Example shown
polyethylene unit cell

25. Polymer Crystallinity
Crystalline regions
thin platelets with chain folds at faces
Chain folded structure
10 nm

26. 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

27. Semicrystalline Polymers
Some semicrystalline polymers form spherulite structures
Alternating chain-folded crystallites and amorphous regions
Spherulite structure for relatively rapid growth rates
Spherulite surface

28. Photomicrograph – Spherulites in Polyethylene
Cross-polarized light used -- a maltese cross appears in each spherulite
Adapted from Fig , Callister & Rethwisch 8e.