2.10 Weak Bonds

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Из курса от партнера Georgia Institute of Technology

Material Behavior

279 оценки

Georgia Institute of Technology

Material Behavior

279 оценки

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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Atomic Structure and Bonding [Difficulty: Easy || Student Effort: 2hrs]

In this module, we will discuss the structure of the atom, how atoms interact with each other, and how those interactions affect material properties. We will explore how the types of atoms present in a material determine what kind of bonding occurs, what differentiates the three types of primary bonds - metallic, ionic, and covalent, and the implications of the type of bonding on the material microstructure. You will learn how atoms arrange themselves as a natural result of their size and bonding. This knowledge will provide you with a foundation for understanding the relationship between a material's microstructure and its properties.

2.8 Covalent Bonds7:20

2.9 Mixed Bonds7:46

2.10 Weak Bonds6:02

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

We're going to be discussing Weak Bonds in this lesson.

We talked a little bit about it when we were discussing the Development

of Dipoles but we're going to explain it in a little bit more detail.

And what we'll do is we'll use an example of a material or an element like Argon,

where Argon as we described using the Bore model that we developed earlier on.

The electrons are distributed around the positive charge of the nucleus.

Now with that large number of electrons associated

with that argon atom, that isolated atom.

If we bring into proximity another argon atom,

what we can do is over a period of time a statistical

reorganization of the electrons can occur.

And ultimately what can develop is a dipole

that occurs randomly throughout the structure.

And occasionally, it will be plus on one side and minus on the other.

So, what can then happen as a result of the electron redistribution,

we will see that we can begin to develop some secondary bonds.

So even though we think of argon as not being reactive, it can be, and

it can develop some weak interactions that are attributed to the secondary bonds.

But you need to recognize that the bond attraction associated with

these secondary bonds is very weak.

And any thermal energy tends to destroy those bonds.

So, these are temporary and

sometimes people refer to these as temporary dipoles or weak secondary bonds.

Now, again I want to return to the picture of carbon in the chain

associated with a polymer.

Again, one of the things that I want to point out is that we have

an interaxial angle of 109.5 degrees that is associated with

the equal distribution of the carbons around that central carbon atom.

And that's different than the hydrogen-oxygen-hydrogen bond

of 104.5 degrees, and that is attributed to the presence of

the dipoles that result when the charges become redistributed.

So, here are some examples of materials in which we have

replaced the oxygen and we now have sulfur and nitrogen.

And one of the things you can begin to see is as you begin to add increasingly

more electronegative elements, the charge distribution is going to be much greater.

And we're going to have the tendency to have more secondary bonding in

the formation of the structure.

Now if we look in the first two cases we see hydrogen, oxygen and

hydrogen and in the next we see hydrogen, sulfur and oxygen.

And there are a lot of similarities in the compounds that form with oxygen and

sulfur.

But when we look over here at ammonia in terms of the valances that are associated,

we have this pyramidal structure of nitrogen with three hydrogens.

But again, what happens here is because of the distribution and

development of the dipole, we have these weak bonds.

Now what I would like to do is to extend this to a polymeric system.

Now, the simple polymer that I want to use is the building block that's associated

with polyethylene.

And so, we start out with the structure that we have on the left, and

that's the basic building block.

The double bond is broken, it's replaced with a single bond.

And then that represents what we refer to as the Polyethylene Mer.

And those polyethylene mers get put together and

we ultimately form a nice long chain.

One of the things that we can do during processing is to extrude these materials.

And basically when we think about extrusion,

it's like squeezing a toothpaste and watching the toothpaste come out of

the orifice of the toothpaste tube or the dye in the case of an extrusion.

And because of the flow lines that are associated with the process of extrusion,

these polymer chains can line up one on top of the other.

And when that particular situation occurs, what we can do is we

can produce a structure that we refer to as High Density Polyethylene.

If we increase the temperature and those individual polymer chains are allowed

to coil, then we can begin to reduce the density of the polyethylene.

Now if we look at weak bonds in that same material and this time what we're going

to do is we're going to periodically replace hydrogen with chlorine.

And remember that chlorine has an electron negative element.

And when we start replacing with chlorine, that one polymer chain.

And we do the same with another polymer chain.

Then what can happen is, these two chains can come together, and

we can develop a hydrogen bond between the chlorine and the hydrogen,

the chlorine and the hydrogen.

So now what we've done is to develop a weak hydrogen bond between those chains.

And that linear behavior that is associated with the weak hydrogen bond

will persist longer than in the case of having only hydrogens there.

And so we can maintain that higher density structure for

slightly higher temperatures.

However, as the temperature does go up, the hydrogen bonding will begin to

dissipate and as a result, this polymer structure will also begin to coil.

In the next lesson we're going to be developing some basic

ideas in thermodynamics.

Thank you.

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