DAY 25: INTRO TO POLYMERS

1.
2.
3.
4.
5.
6.

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!
WEB RESOURCE

A really good website, comprehensive and more
fun to read than the text is
http://www.pslc.ws/macrog/index.htm
CHAPTER 14 – POLYMERS
What is a polymer?
3
Poly
many
mer
repeat unit
repeat
unit
repeat
unit
H H H H H H
C C C C C C
H H H H H H
H H H H H H
C C C C C C
H Cl H Cl H Cl
Polyethylene (PE)
Polyvinyl chloride (PVC)
repeat
unit
H
C
H
H H
C C
CH3 H
H H
C C
CH3 H
H
C
CH3
Polypropylene (PP)
Adapted from Fig. 14.2, Callister 7e.
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?

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

DISCUSSION, POLYMERS (PE) VS. METALS
AND CERAMICS

Polymers are much less dense. This one floats,
though not all do. Why?


Polymers are much weaker. Why?


Heavy dependence on the secondary bond.
Polymers are much more ductile. Why?


Lighter atoms, and not as efficiently packed.
Chains can slide past one another. Again, secondary
bond is temporary.
Polymers are less stiff. Why?

Way less crystallinity.
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.

THREE MAIN CATEGORIES

Thermoplastics
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 “crosslinking”. Elastomers have some other features which
will have to be discussed.

Thermosets
These are 3D network solids. Much primary bonding,
little secondary.
EXAMPLE OF A THERMOPLASTIC
POLYMER: POLYETHYLENE
Schematic of PE molecule
C
C
Polyethylene mer
Models of PE
Ethylene
CHEMISTRY OF POLYMERS
Adapted from Fig.
14.1, Callister 7e.
10
Note: polyethylene is just a long HC
- paraffin is short polyethylene
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:
1. Medium Density PE (MDPE)
2. High Density PE (HDPE)
3. And many others. PE is a big family, and MW
is part of that, but not the whole story.

SOME PROPERTIES
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.
MECHANICAL PROPERTIES OF POLYETHYLENE
Type 1: (Branched) Low Density of 0.910 - 0.925 g/cc
 Type 2: Medium Density of 0.926 - 0.940 g/cc
 Type 3: High Density of 0.941 - 0.959 g/cc
 Type 4: (Linear) High Density to ultra high density > 0.959

13
Mechanical Properties
Branched Low
Density
Density
0.91- 0.925
Medium
Density
0.926- 0.94
High
Density
0.941-0.95
Linear High Density
0.959-0.965
Crystallinity
30% to 50%
50% to 70%
70% to 80%
80% to 91%
Molecular
Weight
Tensile
Strength, psi
Tensile
Modulus, psi
Tensile
Elongation, %
Impact Strength
10K to 30K
30K to 50K
50K to 250K
250K to 1.5M
600 - 2,300
1,200 - 3,000
3,100 - 5,500
5,000 – 6,000
25K – 41K
38K – 75 K
100% - 650%
100%- 965%
150K – 158
150K – 158 K
K
10% - 1300% 10% - 1300%
No break
1.0 – no
break
D50 – D60
ft-lb/in
Hardness, Shore D44 – D50
0.4 – 4.0
0.4 – 4.0
D60 – D70
D66 – D73
www.csuchico.edu/~jpgreene/itec041/m41_ch06/m41_ch06.ppt
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.

EFFECT OF SIDE GROUPS
PE had only hydrogen on the sides
 What happens when we put different elements or
different groups of elements?

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.
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
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.

22
Polymer
Density UTS
g/cc
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
230-550
PC (Lexan)
1.2
10
120
345
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.

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%.
PE CRYSTALS

The spherulites grow together to give something
like a polycrystalline grain. See the micrograph.
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.

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

TWO TYPES OF PS
SOME PROPERTIES OF A SYNDIOTACTIC PS
Density: 1.11 g/cc
 UTS: 10.5 Ksi
 %EL 1.8%
 Modulus of Elasticity: 700 Ksi


1.
2.
3.
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.
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
IMPROVED PS – HIGH IMPACT PS (HIPS)

We form what is called a “graft copolymer.” This
is kind of like an alloy.
Polybutadiene rubber chain
Atactic
polystyrene
HIPS is strong and
tough!!