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کانال خرید و فروش پرنده

کریستالیزاسیون در پلیمرها و DSC

    We're going to divide it by the specific heat of melting, Hc*. The heating rate is temperature increase T per unit time, t. This method has its own page, and it's called differential scanning calorimetry.

    Melting

    Heat may allow crystals to form in a polymer, but too much of it can be their undoing.pslc.gif" />

    We can measure the latent heat of melting by measuring the area of this peak.gif" />

    Of course, not everything you see here will be on every DSC plot. You leave it empty.pslc. When they reach the right temperature, they will have gained enough energy to move into very ordered arrangements, which we call crystals, of course.gif" />

Polyesters are another example.pslc.ws/macrog/images/dsc13.pslc. When this happens, we get a picture like this:

Other atactic polymers like poly(methyl methacrylate) and poly(vinyl chloride) are also amorphous.ws/macrog/images/fiber02.gif" />

As you can see on the lists above, there are two kinds of polystyrene.ws/macrog/images/dsc02.

If you look at the DSC plot you can see a big difference between the glass transition and the other two thermal transitions, crystallization and melting. When this heat is dumped out, it makes the little computer-controlled heater under the sample pan really happy. A good example is nylon. This strong binding holds crystals together.ws

.

Because there is a change in heat capacity, but there is no latent heat involved with the glass transition, we call the glass transition a second order transition. Such folk will just throw their socks in the drawer in one big tangled mess. Ice is a crystal.ws/macrog/images/dsc09. H' is in joules, and the specific heat of melting is usually given in joules per gram, so we're going to get an answer in grams, which we'll call mc.gif" />

For polyethylene, the length the chains will stretch before they fold is about 100 angstroms.jpg" />

Polymers are just like socks in that sometimes they are arranged in a neat orderly manner, like the sock drawer in the top picture. This is because a fiber is really a long crystal. If it's not, it won't.pslc. Because of this change in heat capacity that occurs at the glass transition, we can use DSC to measure a polymer's glass transition temperature.gif" />

This dip tells us a lot of things.gif" />

This means we're now getting more heat flow. If we look at a wide-angle picture of what a lamella looks like, we can see how the crystalline and amorphous portions are arranged.ws/macrog/images/dsc08. Because we have to add energy to the polymer to make it melt, we call melting an endothermic transition.

We're going to talk about the neat and orderly crystalline polymers on this page. This means that when you reach the melting temperature, the polymer's temperature won't rise until all the crystals have melted. But how much of each? DSC can tell us. But because we know the mass of the sample, we can make it simpler. This means we've also got an increase in the heat capacity of our polymer.

But for making fibers, we like our polymers to be as crystalline as possible.

The first thing we have to do is measure the area of that big peak we have for the melting of the polymer. Having extra material means that it will take more heat to keep the temperature of the sample pan increasing at the same rate as the reference pan.ws/macrog/images/stal03.gif" />

The heat flow at a given temperature can tell us something.jpg" />

As you can see, lamella grow like the spokes of a bicycle wheel from a central nucleus. We usually would put this in units such as joules x kelvins x (seconds)-1 x (grams)-1:

And that's how we use DSC to get percent crystallinity.gif" />

This is the switchboard model of a polymer crystalline lamella.gif" />

Of course, it isn't always as neat as this. But polymers with both crystalline and amorphous domains, will show all the features you see above. There is atactic polystyrene, and there is syndiotactic polystyrene. Like this:

As you can also see in the picture, a single polymer chain may be partly in a crystalline lamella, and partly in the amorphous state. Heat flow is heat given off per second, so the area of the peak is given is units of heat x temperature x time-1 x mass-1. If it's regular and orderly, it will pack into crystals easily.pslc.
Differential scanning calorimetry is a technique we use to study what happens to polymers when they're heated. Linear polyethylene is nearly 100% crystalline.pslc. So atactic polystyrene is very amorphous.gif" />

Remember from the glass transition page that when you put a certain amount of heat into something, its temperature will go up by a certain amount, and the amount of heat it takes to get a certain temperature increase is called the heat capacity, or Cp. The kind of crystal we're talking about here is any object in which the molecules are arranged in a regular order and pattern. There's a way we can find out how much of a polymer sample is amorphous and how much is crystalline.

But more importantly, it makes sure that the two separate pans, with their two separate heaters, heat at the same rate as each other.pslc.ws/macrog/images/dsc01. There is a latent heat of melting as well as a latent heat of crystallization.pslc. We get the heat capacity by dividing the heat supplied by the resulting temperature increase. We want to know how much of the polymer was crystalline before we induced more of it to become crystalline.gif" />

How Much Crystallinity?

Remember we said that many polymers contain lots of crystalline material and lots of amorphous material.ws/macrog/images/stal01.

Crystallinity and polymer structure

A polymer's structure affects crystallinity a good deal.ws/macrog/images/stal02.pslc. Want to know more? Then visit the Fiber Page!

Many polymers are a mix of amorphous and crystalline regions, but some are highly crystalline and some are highly amorphous.

In between the crystalline lamellae, there are regions where there is no order to the arrangement of the polymer chains. It's happy because it doesn't have to put out much heat to keep the temperature of the sample pan rising.ws/macrog/images/stal05. The plot will look something like this at first. On the x-axis we plot the temperature. When this happens, we say the polymer is amorphous. A completely crystalline polymer would be too brittle to be used as plastic. This extra heat flow during melting shows up as a big peak on our DSC plot, like this:

Other people don't really care about how neat their sock drawers look. Now if we divide this number by the weight of our sample, mtotal, we get the fraction of the sample that was crystalline, and then of course, the percent crystallinity:

total heat given off during melting Hm, total, and we'll call the heat of the crystallization Hc, total. So the expression becomes simpler:

Of course, being indecisive, the polymer chains will often decide they want to come back into the lamella after wandering around outside for awhile. The specific heat of melting? That's the amount of heat given off by a certain amount, usually one gram, of a polymer. The glass transition is also a thermal transition.

Keywords:
amorphous, crystal, first order transition
glass transition temperature, heat capacity, latent heat
second order transition, thermal transition


Note: Before you read this page, make sure you've read the glass transition page and the polymer crystallinity page.

Crystallinity in Polymers
Why did we just do that? And what does that number H' mean? H' is the heat given off by that part of the polymer sample which was already in the crystalline state before we heated the polymer above the Tc.pslc.

So the heater underneath the sample pan has to work harder than the heater underneath the reference pan.

Specifically what we do is this: We make a plot as the temperature increases. Sometimes part of a chain is included in this crystal, and part of it isn't. In one pan, the sample pan, you put your polymer sample. Their sock drawers look like this:

The polar ester groups make for strong crystals.ws/macrog/images/dsc06. That is to say, we're plotting the heat absorbed by the polymer against temperature.gif" />

Now we have a number of joules per gram. And what are thermal transitions? They're the changes that take place in a polymer when you heat it.ws/macrog/images/dsc07. You can see this in the picture. This means it can pack very easily into crystals.gif" />
Got that? Don't worry.gif" />

Let's say now that we divide the heat flow q/t by the heating rate T/t.

Now we just calculated the total heat given off when the polymer melted. This is because there is no latent heat given off, or absorbed, by the polymer during the glass transition. If we keep heating our polymer past its Tc, eventually we'll reach another thermal transition, one called melting. We end up with heat supplied, divided by the temperature increase.

But they can't always stretch out that straight.ws/macrog/images/dsc05.ws/macrog/images/dsc00. The heating rate is in units of K/s. (Oddly, your mother's good crystal drinking glasses are not crystal at all, as glass is an amorphous solid, that is, a solid in which the molecules have no order or arrangement. If you read the page dealing with polymer crystallinity, you know that many polymers contain both amorphous and crystalline material.ws/macrog/images/stal08. In addition, the aromatic rings like to stack together in an orderly fashion, making the crystal even stronger. They form strong hydrogen bonds.

But atactic styrene has no such order. Let's imagine we're heating a polymer. This means that the little heater under the sample pan is going to have to put a lot of heat into the polymer in order to both melt the crystals and keep the temperature rising at the same rate as that of the reference pan. Both melting and crystallization involve giving off or absorbing heat. Let's look at the polyester we call poly(ethylene terephthalate).

Crystallization

But wait there is more, so much more. In a sample of a crystalline polymer weighing only a few grams, there are many billions of spherulites. We fancy bigshot scientists say that they are in the amorphous state.

Syndiotactic polystyrene is very orderly, with the phenyl groups falling on alternating sides of the chain. Most polymers can only stretch out for a short distance before they fold back on themselves.

Polyethylene is another good example. The computer makes absolutely sure that the heating the rate stays exactly the same throughout the experiment. There is a picture of a stack, called a lamella, right below. Other times there is no order, and the polymer chains just form a big tangled mess, like the socks in the bottom picture.pslc.

But not only do polymers fold like this. The temperature at the lowest point of the dip is usually considered to be the polymer's crystallization temperature, or Tc.

Amorphousness and Crystallinity

Are you wondering about something? If you look at those pictures up there, you can see that some of the polymer is crystalline, and some is not! Yes folks, most crystalline polymers are not entirely crystalline. Because we like you, we're going to tell you that when a polymer chain doesn't wander around outside the crystal, but just folds right back in on itself, like we saw in the first pictures, that is called the adjacent re-entry model.pslc.

So you see, no polymer is completely crystalline. After a certain temperature, our plot will shift upward suddenly, like this:

It's pretty simple, really. With no order, the chains can't pack very well.pslc.

q supplied per unit time, t.gif" />

This page is all about polymer crystals.gif" /> This is the total amount of grams of polymer that were crystalline below the Tc.pslc. When we reach the polymer's melting temperature, or Tm, those polymer crystals begin to fall apart, that is they melt. If we know the latent heat of melting, ΔHm, we can figure out the answer. We use it to study what we call the thermal transitions of a polymer.pslc.ws/macrog/images/stal00.gif" />

Crystallinity and intermolecular forces

Intermolecular forces can be a big help for a polymer if it wants to form crystals. Crystallinity makes a material strong, but it also makes it brittle.pslc. The other one is the reference pan. These chains are called tie molecules.ws/macrog/images/dsc14. The melting of a crystalline polymer is one example.ws/macrog/images/sty02. Here are some of the polymers that tend toward the extremes:

Some Highly Crystalline Polymers: Some Highly Amorphous Polymers:
Polypropylene Poly(methyl methacrylate)
Syndiotactic polystyrene Atactic polystyrene
Nylon Polycarbonate
Kevlar and Nomex Polyisoprene
Polyketones Polybutadiene

Why?

So why is it that some polymers are highly crystalline and some are highly amorphous? There are two important factors, polymer structure and intermolecular forces. Then we saw a big dip when the polymer reached its crystallization temperature. And as you might expect, stereoregular polymers like isotactic polypropylene and polytetrafluoroethylene are highly crystalline. The amorphous regions give a polymer toughness, that is, the ability to bend without breaking.ws/macrog/images/dsc03.ws/macrog/images/dsc12.)

To understand all this talk of crystals and amorphous solids, it helps to go home.pslc.ws/macrog/images/dsc04. It has to put out more heat. This makes picking one discreet Tg kind of tricky, but we usually just take the middle of the incline to be the Tg.ws/macrog/images/dsc11.

How much crystallinity?

DSC can also tell us how much of a polymer is crystalline and how much is amorphous. That's why we subtract the heat given off at crystallization. Polymers also form stacks of these folded chains.

So what kind of arrangements do the polymers like to form?

They like to line up all stretched out, kind of like a neat pile of new boards down at the lumber yard. To put them all together, a whole plot will often look something like this:

Now we're going to subtract the two:

 Reference of this text is: www.

first order transitions. The only thing we do see at the glass transition temperature is a change in the heat capacity of the polymer
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