6.12 Nature of the Bond

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En provenance du cours de Georgia Institute of Technology

Material Behavior

325 notes

Georgia Institute of Technology

Material Behavior

325 notes

Have you ever wondered why ceramics are hard and brittle while metals tend to be ductile? Why some materials conduct heat or electricity while others are insulators? Why adding just a small amount of carbon to iron results in an alloy that is so much stronger than the base metal? In this course, you will learn how a material’s properties are determined by the microstructure of the material, which is in turn determined by composition and the processing that the material has undergone. This is the first of three Coursera courses that mirror the Introduction to Materials Science class that is taken by most engineering undergrads at Georgia Tech. The aim of the course is to help students better understand the engineering materials that are used in the world around them. This first section covers the fundamentals of materials science including atomic structure and bonding, crystal structure, atomic and microscopic defects, and noncrystalline materials such as glasses, rubbers, and polymers.

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Noncrystalline and Semicrystalline Materials [Level of Difficulty: Medium || Student Effort: 2hrs 30mins]

In this module, we discuss materials that are not fully crystalline, such as polymers, rubbers, and glasses. You will learn how the absence of crystallinity affects the behavior of these materials and what factors affect their formation and properties. Lessons include discussions of the microstructure and defects in amorphous materials, partial cystallinity in polymers, and demonstrations of materials exhibiting ductile and brittle behavior at different temperatures.

6.10 Polymers and the Glass Transition Temperature5:44

6.11 Classification of Polymers2:59

6.12 Nature of the Bond6:52

6.13 Molecular Weight Averages7:51

6.14 Chain Architecture4:06

Rencontrer les enseignants

Thomas H. Sanders, Jr.
Regents' Professor
School of Materials Science and Engineering

Welcome back.

In this lesson we're going to describe what we mean by the nature of the bond,

and that is the bond that exists between the polymer chains.

In the first case, we have what we refer to as thermoplastic polymers.

And these polymers actually may have secondary bonding.

And that's a very weak bond that exists between one chain and

another chain if they happen to be lined up.

The other type of connection with respect to chains in a polymer system,

is referred to as a thermoset and it develops a thermoset polymer.

In this case, our association with one chain and

then another is through the process of cross linking.

And developing covalent bonds between the polymers as opposed to secondary bonds

in the case of a thermoplastic.

An example of what occurs with respect to a polymer

chain where we might develop secondary bonding as a result

of the insertion of chlorine on

a molecule, C2H3Cl.

In this particular case our mer unit is shown to the left, and

there's a double bond between the two carbons.

If that double bond is broken, we then have

a saturated mer in which we have only single bonds.

Now because chlorine is an electronegative element, if we happen to be able to put

all the chlorines on one side of the polymer chain as indicated in the visual.

We will also see that by doing this we can align another chain

parallel to it and as a consequence of that electronegative species

what we're going to see is a degree of hydrogen bonding between the chains.

So these secondary bonds are going to help keep the polymer

chains closely packed with respect to one another.

And as a result, what we can do is we can maintain a structure, for

example, if we extrude the polymer, the chains come out along the slip lines of

the extrusion process and they pack in a nice, neat, orderly fashion.

And we have this nice hydrogen bond that exists between the chains.

Now that hydrogen will persist as long as we do not increase the temperature.

When we start increasing the temperature then of course these bonds

that are initially weak become ineffective and

then ultimately what can happen is the polymers will coil.

So, those strongly electronegative

chlorines help create this nice secondary bond between the chains.

Now, when we talk about a material, for example, like natural rubber,

what we can see is, we start out with a mer in which I've indicated

there are double bonds along the polymer back bone.

So we have a couple of double bonds, and if we can break some of those bonds

periodically, what we can do is develop a covalent bond between the two chains.

So as illustrated on the right, we break those one of those bonds and

we do this by putting sulfur into the system.

So in the case of natural rubber, we refer to this process as vulcanization.

So now what we've done is to periodically go through, replace those double bonds

and, as we do that, we begin to develop cross-linking between them.

And so consequently what that does is help develop a rigid

structure as a result of those covalent bonds between the chains.

Another example, a polyester,

as indicated by the molecular diagram on the left and the right.

In the case on the left we have a saturated chain in which we have only

single bonds, and the one on the right, we have a double bond so it's unsaturated.

Now, if we take the polymer to the left, and we connect the mers together,

then what we would do is to produce a thermoplastic polymer.

On the other hand, if we maintain that unsaturated bond, and then we go through

a chemical reaction, we can by virtue of the fact that we break the double bond.

We're able to then create a covalent bond that exists between those two chains,

making the material then, what we refer to as again a thermoset.

Now another type of material Bakelite, and

this a common material with respect to electronic devices.

It's used as a resistor in an electronic device.

And, when you start out with this very complicated structure that's given here

for Bakelite, what can readily happen is, we can develop cross-linking.

With respect to adding together various groupings of this

functional group that we have on the left.

And then eventually, what we wind up doing is to develop

a three-dimensional network of the structure that we refer to as Bakelite.

And one of the things that happens here is that in a structure

where we have the strong cross linkings these types of

materials tend not to be easily recyclable.

In the case of materials that are thermoset what we can

do is to remelt the materials and start over and fabricate them in a new way.

On the other hand when we have these cross link, because of

the strength of that cross linking and the fact that it is a covalent bond

prohibits the process of recycling.

And what will happen is rather than having the polymer melt there'll be

a chemical reaction and the structure will wind up reacting and burning.

And that's associated with the unpleasant smell that you have in

some of your electronic devices when there happens to be a short circuit.

So these are the things that will ultimately control the characteristics of

the polymer.

That is, what's happening between the individual chains.

Thank you.

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