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!

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

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? 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.

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

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.

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

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

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

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.

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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

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

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.

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!

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!!