1. Topic 8: Case study: polyethylene & high impact polystyrene
Overview of polymers
Carbon and hydrogen atoms are basic building blocks (in hydrocarbons); other elements include chlorine, fluorine, nitrogen, oxygen, etc.
Backbone of hydrocarbons are C-C covalent bonds: CH4, C2H6, C2H4, C2H2
Name
Boiling points
Methane
-161 C
Stronger
van der Waals
bonds
Ethane
-89 C
Propane
-42 C

2. Poly-”mer”

3. Ethane vs. Ethylene vs. Acetylene
Each carbon has four bonds with neighboring atoms
Single, double & triple bonds
Although polyethylene has single bonds, and looks like a long version of ethane, its starting material is ethylene

4. Polyethylene
Most common polymer found in milk containers, trash cans, dish pans, pipes, grocery bags, etc.
The development of polyethylene is an interesting story: serendipity.
In the 1930’s scientists were curious about the effects of really high pressures on organic gases. Among the gases investigated was ethylene.
Ethylene was placed in a very strong, rigid container and squeezed with 1000 atm pressure.
When the container was opened a “white waxy solid” was found to have formed.
This rather crude experiment led to the investigation of how the polymerization process could be controlled to make a useful product

5. Synthesis of polyethylene
The formation of polyethylene from ethylene occurs by a process called addition polymerization and involves the use of high pressures, high temperatures and catalysts
Catalytic reaction:

6. Synthesis of polyethylene (contd.)
There are also other possibilities: termination of polymerization
Chain branching: hydrogen is replaced by an ethylene mer; chain branching can be controlled by temperature, pressure and catalysts
Short-chain branching produces:
lower density material because of inefficient packing of chains
low crystalline content as crystallinity requires long, straight and parallel chains
weaker materials less bonding between chains in shorter chains

7. Low density polyethylene (LDPE) High density polyethylene (HDPE)
Produced under high pressure (~ 20,000 psi), moderate temperature ( ~ 200 C), oxygen containing catalysts
Under such conditions considerable short-chain branching are produced
Density = 0.92 gm/cc
Tensile strength = 3,000 psi
Young’s modulus = 40,000 psi
Translucent (grocery bags)
Low degree of crystallinity; hence less scattering of light
High density polyethylene (HDPE)
Produced under low pressure (~ 200 psi), low temperature ( ~ C), titanium and aluminum chloride catalysts
Under such conditions very little short-chain branching are produced
Density = 0.96 gm/cc
Tensile strength = 5,500 psi
Young’s modulus = 180,000 psi
Opaque (garbage cans)
High degree of crystallinity; scattering at interfaces between crystalline and amorphous regions
Both types are thermoplastic  softens when heated and hardened when cooled, reversibly  recycleable
In conclusion, materials properties are controlled by processing that in turn controls microstructure even though electronic and atomic structures are the same!

8. Rubber & High Impact Polystyrene (HIPS)
Natural rubber mer:
Natural rubber molecules are not cross-linked but are considerably entangled. Thus, chains can slowly move over each other, leading to substantial stretching if it is deformed slowly. But, it becomes much stronger and brittle if it is deformed very fast. Under this condition, the chains do not have time to disentangle and the material becomes stronger.
“Silly putty” behaves similarly! Time and temperature are important factors. (Balloons behave similarly too!)
Cross-linked rubber (vulcanization using sulfur) does not flow easily, but still can stretch a lot. Vulcanization should be just enough to stop the chains from flowing past each other, but not too much so that they are locked up and cannot straighten and entangle again (coiled again)

9. Variants of natural rubber
Butadiene mer (synthetic)
Very ductile
Natural rubber mer
Soft & ductile
Styrene mer (synthetic)
Brittle
Can we take advantage of these 2 synthetic materials to make a material that is strong, hard and ductile? The answer to this is HIPS
How can we make high impact polystyrene? Blends

10. Polystyrene-polybutadiene blends
Mix polystyrene and rubber physically …
Polystyrene (PS)
crack
Polybutadiene (PB)
… but this produces very little improvement. Cracks propagate in only polystyrene region in such a blend

11. Copolymers
A copolymer is a combination of 2 or more different monomers (or mers) in one chain
Each circle represents a monomer
In this example, we have 2 different types of mers in one chain
(a) Random copolymer
(b) Alternating copolymer
(c) Block copolymer
(d) Graft copolymer

12. Polystyrene-polybutadiene copolymer blends - HIPS
In HIPS, we still have polybutadiene rubber regions in a polystyrene matrix, but now we have styrene-butadiene graft copolymer added to the mix. The graft copolymer acts to join the two components in a much stronger manner because the grafted rubber chain prefers to be in the rubber region while the polystyrene chain wants to be in the polystyrene matrix
Now, any crack that tries to pass around polybutadiene particles has to interact with the network of rubbery branches connecting the PS matrix to the PB particles. The result is the trapping of the crack round the PB particle and the crack spreads in all directions producing crazing of the PS in this area
HIPS becomes very strong, rigid and tough and is used in the dashboard of your car, in the lining of your refrigerator, in computer and telephone housing, etc.
polystyrene
polybutadiene

13. Summary Chain polymerization of polyethylene
High impact polystyrene – polystyrene-polybutadiene copolymer blends
Reading assignment: Ch