1. Day 25: Intro to Polymers
Our approach to this very important class of materials is as follows:
Basic vocabulary and concepts
Simple Polymers and their Properties
Crystallinity in Polymers
Mechanical Behavior of Polymers
Polymer families
Manufacturing issues
We have about 2 weeks to work on this stuff. Please read text!

2. Web Resource
A really good website, comprehensive and more fun to read than the text is

3. Chapter 14 – Polymers What is a polymer? Poly mer C H Cl C H H C CH3
many repeat unit
repeat
unit
repeat
unit
repeat
unit C H
Polyethylene (PE) Cl C H
Polyvinyl chloride (PVC) H
Polypropylene (PP) C
CH3
Adapted from Fig. 14.2, Callister 7e.

4. Basic Definitions
Polymers are huge molecules. They are sometimes found in nature, but nowadays are often produced by chemists and chemical engineers.
Carbon, hydrogen, and other nonmetallic elements are important players.
Covalent (primary) bonding exists within the molecule. Adjacent molecules are bonded with secondary bonds like Vanderwals and hydrogen bond.
Can we make some predictions about density and strength based on the above?

5. Effect of Bonding on Properties
The primary bonds between the chains are strong mostly covalent bonds that you would expect to produce a
Strong
Stiff
high melting temperature material.
The secondary bonds mean that chains can move with respect to each other easily which makes polymers relatively
Weak
low stiffness
low melting temperature

6. Discussion, Polymers (PE) vs. Metals and Ceramics
Polymers are much less dense. This one floats, though not all do. Why?
Lighter atoms, and not as efficiently packed.
Polymers are much weaker. Why?
Heavy dependence on the secondary bond.
Polymers are much more ductile. Why?
Chains can slide past one another. Again, secondary bond is temporary.
Polymers are less stiff. Why?
Way less crystallinity.

7. More Comparisons
The nature of the bonding – shared electrons – causes there to be no free electrons for conducting electricity.
The mechanisms for conducting heat in polymers is also limited.
Hence, these materials are insulators.

8. Three Main Categories Thermoplastics Elastomers Thermosets
Primary bonds along the chains. Secondary bonding between chains.
Elastomers
These are chains that have some kind of strong (often primary bonds) between them. I.e. some “cross-linking”. Elastomers have some other features which will have to be discussed.
Thermosets
These are 3D network solids. Much primary bonding, little secondary.

9. Example of a Thermoplastic Polymer: Polyethylene
Schematic of PE molecule C C
Polyethylene mer
Models of PE
Ethylene

10. Chemistry of Polymers Note: polyethylene is just a long HC
Adapted from Fig. 14.1, Callister 7e.
Polymer- can have various lengths depending on number of repeat units
Note: polyethylene is just a long HC
- paraffin is short polyethylene

11. Effect of Chain Length - PE
Mer has 4 hydrogens and 2 carbons. MW = 28.
LDPE (low density polyethylene) chain has MW of about 200,000 => 7000 mers.
UHMWPE (ultra-high molecular weight PE) has MW between 3,000,000 and 6,000,000 => up to 200,000 mers.
There are a large number of types of PE in between:
Medium Density PE (MDPE)
High Density PE (HDPE)
And many others. PE is a big family, and MW is part of that, but not the whole story.

12. Some Properties COMPARE WITH METALS AND CERAMICS!
Material
Density g/cc
UTS
Ksi
%EL E
ksi
LDPE
0.917 2
600 18
MDPE
0.936
2.5
750 90
HDPE
0.953
4.5
200
240
UHMWPE
0.930 7
350
100
COMPARE WITH METALS AND CERAMICS!
Note the UHMWPE does not have the highest density, but it does have the highest strength.
Note in general how the increase in density and molecular weight goes along with strength increases.

13. Mechanical Properties of Polyethylene
Type 1: (Branched) Low Density of g/cc
Type 2: Medium Density of g/cc
Type 3: High Density of g/cc
Type 4: (Linear) High Density to ultra high density > 0.959

14. Why are longer chains better for Strength and Stiffness?
Picture polymers as cooked spaghetti or boxes of wire or cable.
If the spaghetti or cable is very short, it won’t entangle, but long lengths of cable stirred together will entangle severely.
Entanglement means that strength and stiffness increase.

15. Special Low Versions of Polyethylene Produced through catalyst selection and regulation of reactor conditions
Very Low Density Polyethylene (VLDPE)
Densities between and 0.915
Applications include disposable gloves, shrink packages, vacuum cleaner hoses, tuning, bottles, shrink wrap, diaper film liners, and other health care products
Linear Low Density Polyethylene (LLDPE)
Densities between and 0.930
Contains little if any branching
Properties include good flex life, low warpage, and improved stress-crack resistance
Applications include films for ice, trash, garment, and produce bags

16. Special High Versions of Polyethylene Produced through catalyst selection and regulation of reactor conditions
High Molecular Weight- High Density Polyethylene (HMW- HDPE)
Densities are or greater
MW from 200K to 500 K
Properties include improved toughness, chemical resistance, impact strength, and high abrasion resistance.
High viscosities
Applications include trash liners, grocery bags, industrial pipe, gas tanks, and shipping containers

17. Special High Versions of Polyethylene Produced through catalyst selection and regulation of reactor conditions
Ultra High Molecular Weight Polyethylene (UHMWPE)
Densities are 0.96 or greater
MW from 3M to 6M
Properties include improved high wear resistance, chemical inertness, and low coefficient of friction.
High viscosities result in material not flowing or melting.
Processed similar to PTFE (Teflon)
Ram extrusion and compression molding are used.
Applications include pump parts, seals, surgical implants, pen tips, and butcher-block cutting surfaces.

18. Effect of Side Groups PE had only hydrogen on the sides
What happens when we put different elements or different groups of elements?

19. Polar Side Groups
Polar side groups like Chlorine or Flourine can increase the strength of the secondary bonding.
Bulky side groups like those on polypropylene increase the entanglement like barbed wire increases the entanglement of wire.

20. Polymer
Density g/cc
UTS ksi
%EL
E ksi
PVC
1.35
7.5 45
385 PP
0.950
5.0
150
300 PS
1.05
6.5
1.5
413
MDPE
0.936
2.5
750 90

21. Effect of Backbone The all carbon backbone of PE is very flexible.
The addition of N or O on the backbone can make it stiffer (harder to uncoil and slide past other chains (Nylon, Delrin) )
The existence of ring structures on the chain makes it really stiff.

23. Polymer
Density g/cc
UTS ksi
%EL
E ksi
PVC
1.35
7.5 45
385 PP
0.950
5.0
150
300 PS
1.05
6.5
1.5
413
MDPE
0.936
2.5
750 90
Nylon
1.14 12
15-300
PC (Lexan)
1.2 10
120
345

25. Advantage of Crystallinity
Polymers have a limited ability to crystallize. Some, esp. PE are capable of forming crystalline structures. Over 90% crystalline. Some polymers have 0% crystallinity. They are totally amorphous.
In PE, there is a crystal that forms by chain folding into sheets like this one.

26. Please note that crystallinity is not 100%.
More on Crystallinity
The sheets form blade like structures which tend to grow outward from a common center into a spherical shape. This is called a spherulite.
Please note that crystallinity is not 100%.

27. PE Crystals
The spherulites grow together to give something like a polycrystalline grain. See the micrograph.

28. Effects of Crystallinity
Closer packing means stronger secondary bonds. Chain mobility and sliding is lessened.
Add strength
Adds stiffness, i.e Higher elastic modulus.
Decreases ductility.

29. What factors affect Crystallinity?
Branched chains don’t fold back and forth well. (linear PE is stronger than branched PE)
Bulky Side Groups don’t fold back and forth well (polystyrene is amorphous)
Location of side groups matters

30. Two Types of PS

31. Some Properties of a Syndiotactic PS
Density: 1.11 g/cc
UTS: Ksi
%EL 1.8%
Modulus of Elasticity: 700 Ksi
Just a few remarks about the difference:
Bulky side groups such as phenyl inhibit crystallinity.
Sydndiotactic, ie regular placement on alt. sides promotes crystallinity.
Crystallinity enhances strength and stiffness.

32. Branching
Branching makes it harder for the polymers to lie next to each other and pack efficiently.
Branched, looks like shorter chains!
Strength will be lower and so will density in the branched

33. Improved PS – High Impact PS (HIPS)
We form what is called a “graft copolymer.” This is kind of like an alloy.
Polybutadiene rubber chain
HIPS is strong and tough!!
Atactic polystyrene

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

36. Another Common Polymer: PolyStyrene
Basic Mer
The second most commonly encountered polymer. This is a hard, yet cheap commodity plastic. Yes, it can be foamed.
This is a thermoplastic polymer.
Phenyl group
Typical Properties
Density: 1.05 g/cc
UTS: 6450 psi
%EL: 1.5%
Elastic Modulus: 413 ksi