تمامی مطالب مطابق قوانین جمهوری اسلامی ایران میباشد.درصورت مغایرت از گزارش پست استفاده کنید.

جستجو

کانال خرید و فروش پرنده

کریستالیزاسیون در پلیمرها و DSC

    gif" />

    Of course, not everything you see here will be on every DSC plot. After a certain temperature, our plot will shift upward suddenly, like this:

    Now we just calculated the total heat given off when the polymer melted. You may notice that the change doesn't occur suddenly, but takes place over a temperature range. On the x-axis we plot the temperature. A good example is nylon. If you analyzed a 100% amorphous polymer, like atactic polystyrene, you wouldn't get one of these dips, because such materials don't crystallize. Each pan sits on top of a heater. We're talking about another kind of crystal here. When they reach the right temperature, they will have gained enough energy to move into very ordered arrangements, which we call crystals, of course. But most importantly, this dip tells us that the polymer can in fact crystallize.gif" />

Polyesters are another example.

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. This is because a fiber is really a long crystal.

This page is all about polymer crystals.pslc. We want to know how much of the polymer was crystalline before we induced more of it to become crystalline.gif" />

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

The first thing we have to do is measure the area of that big peak we have for the melting of the polymer.

So you see, no polymer is completely crystalline. There are two pans.

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.ws/macrog/images/stal06.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 other one is the reference pan. No, this page has nothing to do with polymers as used by the new age community. The amorphous regions give a polymer toughness, that is, the ability to bend without breaking. And that's just what we've done in that equation up there.

But they can't always stretch out that straight.ws/macrog/images/dsc13. When this heat is dumped out, it makes the little computer-controlled heater under the sample pan really happy. 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.gif" />

The polar ester groups make for strong crystals.

We're going to talk about the neat and orderly crystalline polymers on this page. This method has its own page, and it's called differential scanning calorimetry. The plot will look something like this at first.

We heat our polymer in a device that looks something like this:

But not only do polymers fold like this.pslc.
Differential scanning calorimetry is a technique we use to study what happens to polymers when they're heated. 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. These chains are called tie molecules. The chains, or parts of chains, that aren't in the crystals have no order to the arrangement of their chains.jpg" />

As you can see, lamella grow like the spokes of a bicycle wheel from a central nucleus.gif" />

The heat flow at a given temperature can tell us something. That's why we subtract the heat given off at crystallization. The temperature at the lowest point of the dip is usually considered to be the polymer's crystallization temperature, or Tc. For the glass transition, there is no dip, and there's no peak, either. In addition, the aromatic rings like to stack together in an orderly fashion, making the crystal even stronger.

But for making fibers, we like our polymers to be as crystalline as possible. There's a way we can find out how much of a polymer sample is amorphous and how much is crystalline.gif" />

As you can see on the lists above, there are two kinds of polystyrene. Above the glass transition, the polymers have a lot of mobility. Ice is a crystal. But how much of each? DSC can tell us. When they put their socks away they fold them and stack them very neatly. Also, we can measure the area of the dip, and that will tell us the latent energy of crystallization for the polymer. On the y-axis we plot difference in heat output of the two heaters at a given temperature.gif" />

Of course, being indecisive, the polymer chains will often decide they want to come back into the lamella after wandering around outside for awhile.pslc.gif" />

Syndiotactic polystyrene is very orderly, with the phenyl groups falling on alternating sides of the chain. The heat flow is going to be shown in units of heat, q supplied per unit time, t.gif" /> And that's how we use DSC to get percent crystallinity. In fact, very few polymers can stretch out perfectly straight, and those are ultra-high molecular weight polyethylene, and aramids like Kevlar and Nomex. To put them all together, a whole plot will often look something like this:

Heat Capacity

We can learn a lot from this plot. We've figured up the heat capacity from the DSC plot. Now our plot is a plot of heat flow per gram of material, versus temperature. If we keep heating our polymer past its Tc, eventually we'll reach another thermal transition, one called melting.

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

Crystallinity in Polymers

 Reference of this text is: www.gif" />

Of course, it isn't always as neat as this. The heating rate is in units of K/s.ws/macrog/images/stal05. And in case you were wondering, we can spot this happening on a DSC plot.

Polyethylene is another good example. We use it to study what we call the thermal transitions of a polymer.gif" />

Now we have a number of joules per gram.ws/macrog/images/dsc05.pslc.pslc.

Crystallinity and polymer structure

A polymer's structure affects crystallinity a good deal. Let's imagine we're heating a polymer. So atactic polystyrene is very amorphous. When we reach the polymer's melting temperature, or Tm, those polymer crystals begin to fall apart, that is they melt. The glass transition is also a thermal transition.ws/macrog/images/dsc14.gif" /> Got that? Don't worry. It helps to look at polystyrene to understand how this works. Completely amorphous polymers won't show any crystallization, or any melting either.

Specifically what we do is this: We make a plot as the temperature increases. When this happens, we say the polymer is amorphous. Neat, huh? Now if we do the same calculation for our dip that we got on the DSC plot for the crystallization of the polymer, we can get the total heat absorbed during the crystallization. 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.

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.pslc. You can see this drop in the heat flow as a big dip in the plot of heat flow versus temperature:

This dip tells us a lot of things. Such folk will just throw their socks in the drawer in one big tangled mess.ws

.ws/macrog/images/sty02.pslc. The polymer sample means there is extra material in the sample pan. 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. 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. The crystallization dip and the melting peak will only show up for polymers that can form crystals. And what are thermal transitions? They're the changes that take place in a polymer when you heat it. So a crystalline polymer really has two components: the crystalline portion and the amorphous portion.gif" />

How Much Crystallinity?

Remember we said that many polymers contain lots of crystalline material and lots of amorphous material.

Crystallization

But wait there is more, so much more. If you read the page dealing with polymer crystallinity, you know that many polymers contain both amorphous and crystalline material. Transitions like melting and crystallization, which do have latent heats, are called first order transitions. When this is the case, we say the polymer is crystalline. Let's see what happens when we heat the polymer a little more.ws/macrog/images/stal08. The heating rate is temperature increase T per unit time, t. 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.ws/macrog/images/dsc06.

The Glass Transition Temperature

Of course, we can learn a lot more than just a polymer's heat capacity with DSC. If you're making plastics, this is a good thing. Polymers also form stacks of these folded chains.

When polymers fall into these crystalline arrangements, they give off heat.pslc. Then we saw a big dip when the polymer reached its crystallization temperature.ws/macrog/images/dsc02.gif" />

This means we're now getting more heat flow.pslc.

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. Both melting and crystallization involve giving off or absorbing heat. When the polymer crystals melt, they must absorb heat in order to do so. We're going to divide it by the specific heat of melting, Hc*.ws/macrog/images/dsc07. 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.

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.ws/macrog/images/stal07.gif" />

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. The specific heat of melting? That's the amount of heat given off by a certain amount, usually one gram, of a polymer. 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. 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/stal05. Other times there is no order, and the polymer chains just form a big tangled mess, like the socks in the bottom picture. This extra heat flow during melting shows up as a big peak on our DSC plot, like this:

This is the total amount of grams of polymer that were crystalline below the Tc.pslc.

Melting

Heat may allow crystals to form in a polymer, but too much of it can be their undoing. We end up with heat supplied, divided by the temperature increase.pslc.pslc.pslc.pslc. Is everyone following me?

Now with our magic number H' we can figure up the percent crystallinity. When this happens, we get a picture like this:

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. You see, some people are very neat and orderly.pslc. 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.gif" /> It's pretty simple, really. We'll call the heat total heat given off during melting Hm, total, and we'll call the heat of the crystallization Hc, total.pslc. With no order, the chains can't pack very well.gif" />

Crystallinity and intermolecular forces

Intermolecular forces can be a big help for a polymer if it wants to form crystals.

Other atactic polymers like poly(methyl methacrylate) and poly(vinyl chloride) are also amorphous. The kind of crystal we're talking about here is any object in which the molecules are arranged in a regular order and pattern. The only thing we do see at the glass transition temperature is a change in the heat capacity of the polymer. And of course, we usually take the temperature at the top of the peak to be the polymer's melting temperature, Tm. When we start heating our two pans, the computer will plot the difference in heat output of the two heaters against temperature. If it's not, it won't. In ice all the water molecules are arranged in a specific manner.pslc. This means that when you reach the melting temperature, the polymer's temperature won't rise until all the crystals have melted.

To understand all this talk of crystals and amorphous solids, it helps to go home. There is atactic polystyrene, and there is syndiotactic polystyrene. We usually would put this in units such as joules x kelvins x (seconds)-1 x (grams)-1:

Other people don't really care about how neat their sock drawers look. They're kind of like passengers trying to get comfortable in airline seats, and never quite succeeding, because they can move around more. We just multiply this by the mass of the sample:

Polymers are just like socks in that sometimes they are arranged in a neat orderly manner, like the sock drawer in the top picture.

In between the crystalline lamellae, there are regions where there is no order to the arrangement of the polymer chains.pslc. In one pan, the sample pan, you put your polymer sample.pslc.

We can measure the latent heat of melting by measuring the area of this peak.

Remember that heat that the polymer gave off when it crystallized? Well when we reach the Tm, it's payback time.ws/macrog/images/stal10. We usually divide the area by the heating rate of our dsc experiment. The phenyl groups come on any which side of the chain they please.

Also, because the polymer gives off heat when it crystallizes, we call crystallization an exothermic transition. So the computer turns on the heaters, and tells it to heat the two pans at a specific rate, usually something like 10 oC per minute.

So how do we study what happens to a polymer when we heat it? The first step would be to heat it, obviously.ws/macrog/images/stal01. They form strong hydrogen bonds.

But atactic styrene has no such order. One is very crystalline, and one is very amorphous.gif" />

This is the switchboard model of a polymer crystalline lamella.ws/macrog/images/fiber02.ws/macrog/images/dsc03.ws/macrog/images/stal00. Most polymers can only stretch out for a short distance before they fold back on themselves.gif" />

For polyethylene, the length the chains will stretch before they fold is about 100 angstroms. So is table salt, sodium chloride. Then you tell the nifty computer to turn on the heaters.pslc. It gets simpler.pslc.pslc. But polymers with both crystalline and amorphous domains, will show all the features you see above. So the expression becomes simpler:

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. This is because there is no latent heat given off, or absorbed, by the polymer during the glass transition. They wiggle and squirm, and never stay in one position for very long. Sometimes part of a chain is included in this crystal, and part of it isn't.ws/macrog/images/stal09. It can be crystalline or amorphous. Their sock drawers look like this:

گزارش پست ]
منبع
برچسب ها :

, , , , , , , , , , , , , , ,

آمار امروز جمعه 29 دي 1396

  • تعداد وبلاگ :55617
  • تعداد مطالب :212350
  • بازدید امروز :266791
  • بازدید داخلی :66392
  • کاربران حاضر :139
  • رباتهای جستجوگر:127
  • همه حاضرین :266

تگ های برتر امروز

تگ های برتر