day 25: intro to polymers
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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DAY 25: INTRO TO POLYMERS Our approach to this very important class of
materials is as follows:1. Basic vocabulary and concepts2. Simple Polymers and their Properties3. Crystallinity in Polymers4. Mechanical Behavior of Polymers5. Polymer families6. Manufacturing issues
We have about 2 weeks to work on this stuff. Please read text!
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WEB RESOURCE
A really good website, comprehensive and more fun to read than the text is
http://www.pslc.ws/macrog/index.htm
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CHAPTER 14 – POLYMERS
What is a polymer?
Poly mer many repeat unit
Adapted from Fig. 14.2, Callister 7e.
C C C C C C
HHHHHH
HHHHHH
Polyethylene (PE)
ClCl Cl
C C C C C C
HHH
HHHHHH
Polyvinyl chloride (PVC)
HH
HHH H
Polypropylene (PP)
C C C C C C
CH3
HH
CH3CH3H
repeatunit
repeatunit
repeatunit
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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?
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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
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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.
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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.
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THREE MAIN CATEGORIES
ThermoplasticsPrimary bonds along the chains. Secondary bonding between chains.
ElastomersThese 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.
ThermosetsThese are 3D network solids. Much primary bonding, little secondary.
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EXAMPLE OF A THERMOPLASTIC POLYMER: POLYETHYLENE
C C
Ethylene
Polyethylene mer
Schematic of PE molecule
Models of PE
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CHEMISTRY OF POLYMERS
Adapted from Fig. 14.1, Callister 7e.
Note: polyethylene is just a long HC - paraffin is short polyethylene
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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.
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SOME PROPERTIES
Material Density g/cc
UTSKsi
%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.
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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 Mechanical PropertiesBranched LowDensity
MediumDensity
HighDensity
Linear High Density
Density 0.91- 0.925 0.926- 0.94 0.941-0.95 0.959-0.965
Crystallinity 30% to 50% 50% to 70% 70% to 80% 80% to 91%
MolecularWeight
10K to 30K 30K to 50K 50K to 250K 250K to 1.5M
TensileStrength, psi
600 - 2,300 1,200 - 3,000 3,100 - 5,500 5,000 – 6,000
TensileModulus, psi
25K – 41K 38K – 75 K 150K – 158K
150K – 158 K
TensileElongation, %
100% - 650% 100%- 965% 10% - 1300% 10% - 1300%
Impact Strengthft-lb/in
No break 1.0 – nobreak
0.4 – 4.0 0.4 – 4.0
Hardness, Shore D44 – D50 D50 – D60 D60 – D70 D66 – D73
www.csuchico.edu/~jpgreene/itec041/m41_ch06/m41_ch06.ppt
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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.
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EFFECT OF SIDE GROUPS
PE had only hydrogen on the sides What happens when we put different
elements or different groups of elements?
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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.
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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
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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.
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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 15-300 230-550
PC (Lexan) 1.2 10 120 345
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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.
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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%.
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PE CRYSTALS
The spherulites grow together to give something like a polycrystalline grain. See the micrograph.
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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.
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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
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TWO TYPES OF PS
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SOME PROPERTIES OF A SYNDIOTACTIC PS
Density: 1.11 g/cc UTS: 10.5 Ksi %EL 1.8% Modulus of Elasticity: 700 Ksi
Just a few remarks about the difference:1. Bulky side groups such as phenyl inhibit
crystallinity.2. Sydndiotactic, ie regular placement on alt.
sides promotes crystallinity.3. Crystallinity enhances strength and stiffness.
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BRANCHINGBranching makes it harder for the polymers to
lie next to each other and pack efficiently.
Strength will be lower and so will density in
the branched
Branched, looks like shorter
chains!
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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!!