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جستجو

کانال خرید و فروش پرنده

کریستالیزاسیون در پلیمرها و DSC

This is the switchboard model of a polymer crystalline lamella.pslc. The other one is the reference pan.htm">glass transition page and the polystyrene to understand how this works.pslc.ws/macrog/pvc. When they reach the right temperature, they will have gained enough energy to move into very ordered arrangements, which we call crystals, of course.ws/macrog/pe.ws/macrog/images/dsc03.gif" />

The polar ester groups make for strong crystals. But polymers with both crystalline and amorphous domains, will show all the features you see above. Transitions like melting and crystallization, which do have latent heats, are called first order transitions.pslc.

But more importantly, it makes sure that the two separate pans, with their two separate heaters, heat at the same rate as each other.gif" /> 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.ws/macrog/images/dsc11.pslc. This is because a fiber is really a long crystal. The specific heat of melting? That's the amount of heat given off by a certain amount, usually one gram, of a polymer.gif" />

This dip tells us a lot of things.ws/macrog/ptfe. And as you learned on the glass transition page, polymers have a higher heat capacity above the glass transition temperature than they do below it.pslc. You can see from the picture that the polar amide groups in the backbone chain of nylon(6,6) are strongly attracted to each other. On the y-axis we plot difference in heat output of the two heaters at a given temperature.ws/macrog/images/dsc08. 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.pslc. Let's see what happens when we heat the polymer a little more.

So you see, no polymer is completely crystalline.pslc.ws/macrog/pet.ws/macrog/crystal.

But not only do polymers fold like this. The kind of crystal we're talking about here is any object in which the molecules are arranged in a regular order and pattern.ws/macrog/images/stal02. But because we know the mass of the sample, we can make it simpler. Both melting and crystallization involve giving off or absorbing heat.pslc.

Remember that heat that the polymer gave off when it crystallized? Well when we reach the Tm, it's payback time.pslc. The crystalline portion is in the lamellae, and the amorphous potion is outside the lamellae.


Differential scanning calorimetry is a technique we use to study what happens to polymers when they're heated. When we reach the polymer's melting temperature, or Tm, those polymer crystals begin to fall apart, that is they melt.pslc. Each pan sits on top of a heater.

Polyethylene is another good example.ws/macrog/pmma.ws/macrog/styrene. So a crystalline polymer really has two components: the crystalline portion and the amorphous portion.gif" />

Now we just calculated the total heat given off when the polymer melted. This method has its own page, and it's called

Poly(methyl methacrylate) crystals.gif" />

How Much Crystallinity?

Remember we said that many polymers contain lots of crystalline material and lots of amorphous material. There's a way we can find out how much of a polymer sample is amorphous and how much is crystalline.gif" /> This is the total amount of grams of polymer that were crystalline below the Tc. To put them all together, a whole plot will often look something like this:

Polypropylene

The heat flow at a given temperature can tell us something.ws/macrog/aramid. The crystallization dip and the melting peak will only show up for polymers that can form

This means we're now getting more heat flow.ws/macrog/fiber. When they put their socks away they fold them and stack them very neatly.ws/macrog/images/fiber02. Then finally we saw a big peak when the polymer reached its melting temperature.

So how do we study what happens to a polymer when we heat it? The first step would be to heat it, obviously. We get the heat capacity by dividing the heat supplied by the resulting temperature increase.

Putting It All Together

So let's review now: we saw a step in the plot when the polymer was heated past its glass transition temperature.pslc.

The first thing we have to do is measure the area of that big peak we have for the melting of the polymer.pslc.htm#thertr">thermal transition


Note: Before you read this page, make sure you've read the aramids like Kevlar and Nomex.ws/macrog/images/dsc01. The whole assembly is called a spherulite. Here are some of the polymers that tend toward the extremes:

Some Highly Crystalline Polymers: Some Highly Amorphous Polymers:
amorphous,

Of course, it isn't always as neat as this.pslc.htm#hecap">heat capacity,

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