4 & 5 - Glass Transition Polymer engineering

8/4/2019 4 & 5 - Glass Transition Polymer engineering

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MM538 Dr. Kausar Ali SyedPolymer Engineering Lecture No 4 & 5

July 21 & Sept 15, 2010

Have you ever left a plastic bucket or some other plastic object outside during the winter, and found that it

cracks or breaks more easily than it would in the summer time? What you experienced was the phenomenon

known as the glass transition. This transition is something that only happens to polymers, and is one of thethings that make polymers unique. The glass transition is pretty much what it sounds like. There is a certain

temperature(different for each polymer) called the glass transition temperature, or Tg for short. When the

polymer is cooled below this temperature, it becomes hard and brittle, like glass. Some polymers are used

above their glass transition temperatures, and some are used below. Hard plastics like polystyrene and poly(methyl methacrylate), are used below their glass transition temperatures; that is in their glassy state. Their

Tg's are well above room temperature, both at around 100oC. Rubber, elastomer like polyisoprene

andpolyisobutylene, are used above their Tg's, that is, in the rubbery state, where they are soft and flexible.

Amorphous and Crystalline Polymers

The glass transition is not the same thing as melting. Melting is a transition which occurs in crystalline

polymers. Melting happens when the polymer chains fall out of their crystal structures, and become a

disordered liquid. The glass transition is a transition which happens to amorphous polymers; that is,

polymers whose chains are not arranged in ordered crystals, but are just strewn around in any old fashion,even though they are in the solid state.

But even crystalline polymers will have some amorphous portion. This portion usually makes up 40-70% of

the polymer sample. This is why the same sample of a polymer can have both a glass transition temperatureanda melting temperature. But you should know that the amorphous portion undergoes the glass transition

only, and the crystalline portion undergoes melting only.

When the temperature is warm, the polymer chains can move around easily. So, when you take a piece of the

polymer and bend it, the molecules, being in motion already, have no trouble moving into new positions torelieve the stress you have placed on them. But if you try to bend sample of a polymer below its Tg, the

polymer chains won't be able to move into new positions to relieve the stress which you have placed on

them. So, one of two things will happen. Either (A) the chains are strong enough to resist the force you

apply, and the sample won't bend; or (B) the force you apply will be too much for the motionless polymerchains to resist, and being unable to move around to relieve the stress, the polymer sample will break or

shatter in your hands.

This change in mobility with temperature happens because the phenomenon we call "heat" is really a form of

kinetic energy; that is, the energy of objects in motion. It is actually an effect of random motion of

molecules, whether they are polymer molecules or small molecules. Things are "hot" when their moleculeshave lots of kinetic energy and move around very fast. Things are "cold" when their molecules lack kinetic

energy and move around slowly, or not at all.


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Now the exact temperature at which the polymer chains undergo this big change in mobility depends on thestructure of the polymer. To see how a small change in structure can mean a big change in Tg, take a look at

the difference between poly(methyl acrylate) and poly(methyl methacrylate).

Acrylate and Methacrylate

One might not think that this little methyl group would make a whole lot of difference in the behavior and

properties of the polymer, but it does. Poly(methyl acrylate) is a white rubber at room temperature, butpoly(methyl methacrylate) is a strong, hard, and clear plastic.

As it turns out, how soft or hard a polymer is at a given temperature is determined by what we call chainmobility, that is, how well the polymer chains wiggle past and around each other. The more they can move,

the softer the polymer is. and slithering would become quite difficult.

Those extra methyl groups will hinder the movement or any slithering of the poly(methyl methacrylate)

chains. Poly(methyl acrylate), on the other hand, without that extra methyl group getting in the way, they can

slither all they want. If the polymer chains can slither and wiggle past and around each other easily, thewhole mass of them will be able to flow more easily. So, a polymer which can move around easily will be

soft, and one which can't will be hard, to put it simply.

When you walk down the street, you are undergoing translational motion. While polymers are not incapable

of such motion, mostly they are not undergoing translational motion. But they are still moving around,

wiggling this way and that. To be sure, by the time we get down to the glass transition temperature, it isalready too cold for the polymer molecules, tangled up in each other as they are, to move any distance in one

direction. The motion that allows a polymer above its glass transition temperature to be pliable is not usually

translational motion, but what is known in the business as long-range segmental motion. While the polymer

chain as a whole may not be going anywhere, segments of the chain can wiggle around, swing to and fro, andturn like a giant corkscrew. When the temperature drops below the Tg, the long-range segmental motion

grinds to a halt. When this long-range motion ceases, the glass transition occurs, and the polymer changes

from being soft and pliable to being hard and brittle.

Messing Around with the Glass Transition

Sometimes, a polymer has a Tg that is higher than we'd like. That's ok, we just put something in it called a

plasticizer. This is a small molecule which will get in between the polymer chains, and space them out from

each other. We call this increasing the free volume. When this happens they can slide past each other more

easily. When they slide past each other more easily, they can move around at lower temperatures than they


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would without the plasticizer. In this way, the Tg of a polymer can be lowered, to make a polymer morepliable, and easier to work with.

If you're wondering what kind of small molecule we're talking about, here are some that are used asplasticizers:

Have you ever smelled "that new car smell"? that smell is the p lasticizer evaporating from the plastic parts

on the inside of your car. After many years, if enough of it evaporates, your dashboard will no longer be

plasticized. The Tg of the polymers in your dashboard will rise above room temperature, and the dashboardwill become brittle and crack.

The Glass Transition vs. Melting

It's tempting to think of the glass transition as a kind of melting of the polymer. But this is an inaccurate way

of looking at things. There are a lot of important differences between the glass transition and melting. Like I

said earlier, melting is something that happens to a crystalline polymer, while the glass transition happensonly to polymers in the amorphous state. A given polymer will often have both amorphous and crystalline

domains within it, so the same sample can often show a melting point anda Tg. But the chains that melt are

not the chains that undergo the glass transition.

There is another big difference between melting and the glass transition. When you heat a crystalline

polymer at a constant rate, the temperature will increase at a constant rate. The heat amount of heat requiredto raise the temperature of one gram of the polymer one degree Celsius is called the heat capacity.

Now the temperature will continue to increase until the polymer reaches its melting point. When thishappens, the temperature will hold steady for awhile, even though you're adding heat to the polymer. It will

hold steady until the polymer has completely melted. Then the temperature of the polymer will begin to

increase once again. The temperature rising stops because melting requires energy. All the energy you add toa crystalline polymer at its melting point goes into melting, and none of it goes into raising the temperature.

This heat is called the latent heat of melting. (The word latentmeans hidden.)

Now once the polymer has melted, the temperature begins to rise again, but now it rises at a slower rate. The

molten polymer has a higher heat capacity than the solid crystalline polymer, so it can absorb more heat with

a smaller increase in temperature.

So, two things happen when a crystalline polymer melts: It absorbs a certain amount of heat, the latent heat

of melting, and it undergoes a change in its heat capacity. Any change brought about by heat, whether it ismelting or freezing, or boiling or condensation, which has a change in heat capacity, and a latent heat

involved, is called afirst order transition.
http://pslc.ws/macrog/plastz.htmhttp://pslc.ws/macrog/plastz.htm


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But when you heat an amorphous polymer to its Tg, something different happens. First you heat it, and thetemperature goes up. It goes up at a rate determined by the polymer's heat capacity, just like before. Only

something funny happens when you reach the Tg. The temperature doesn't stop rising. There is no latent heat

of glass transition. The temperature keeps going up.

But the temperature doesn't go up at the same rate above the Tg as below it. The polymer does undergo an

increase in its heat capacity when it undergoes the glass transition. Because the glass transition involves

change in heat capacity, but it doesn't involve a latent heat, this transition is called a second order transition.

It may help to look at some nifty pictures. The plots show the amount of heat added to the polymer on the y-axis and the temperature that you'd get with a given amount of heat on thex-axis.

The plot on the left shows what happens when you heat a 100% crystalline polymer. You can look at it and

see that it's discontinuous. See that break? That's the melting temperature. At that break, a lot of heat is addedwithout any temperature increase at all. That's the latent heat of melting. We see the slope getting steeper onthe high side of the break. The slope of this kind of plot is equal to the heat capacity, so this increase in

steepness corresponds to our increase in heat capacity above the melting point.

But in the plot on the right, which shows what happens to a 100% amorphous polymer when you heat it, we

don't have a break. The only change we see at the glass transition temperature is an increase in the slope,

which means, of course, that we have an increase in heat capacity. We can see a heat capacity change at the

Tg, but no break, like we do in the plot for the crystalline polymer. As mentioned earlier, there is no latent

heat involved with the glass transition.

And this is the difference between a first order transition like melting, and a second order transition like theglass transition.

THE GLASS TRANSITION TEMPRATURE (explained in a different way)

Most noncrystalline solids are either rubbers or glasses. A modern working definition of aglass is a material

that lacks long range order and is below the temperature at which atomic and molecular rearrangement can

occur on a time scale similar to that of the experiment , In contrast, a rubber is an amorphous solid for which

molecular rearrangements can occur on the time scale of the experiment. Hence, glass and rubbers are

structurally similar with the chief difference between the two being the ability to rearrange molecularly. For


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example, compare the room temperature response of a piece of glass and rubber band to a blow from a

hammer. The glass will break (i.e., it is brittle) while the rubber band will stretch to absorb the energy. When

a rubber band is imersed in liquid nitrogen, however, it becomes a brittle glass. For any amorphous solid, the

critical temperature that separates glassy behavior from rubbery behaviour, on the time scale of the

experiment, is known as theglass transition temperature.

The glass transition temperature is a characteristic of all non crystalline materials regardless of whether they

are organic polymers, metals, or inorganic oxide glasses. It is most clearly demonstration by considering the

density or volume changes associated with heating or cooling a material. Consider the processes that may

occur during cooling of a material from the melt, as shown in figure (a). In this figure, the specific volume,

which is the inverse of the mass density (in other words, volume per unit mass), is plotted as a function of

temperature. As the temperature is lowered, the specific volume of the liquid decreases. The slop of the line,

normalized to the sample volume, V, is called the volumetric (or bulk) thermal expansion coefficient, v:

(V1V0) / V0 = v (T2 T1) or v = (I/V0)(V/T) or v = (I/V)(dV/dT)

This expression is analogous to the definition of the linear thermal expansion coefficient , th. It can be

shown that for many materials v ~ 3 th.

When the cooling rate is low, the sample may crystallize at a fixed temperature called the melting point, T m.

A sudden reduction in volume occurs at this temperature as a result of the change in atomic packing from

that of the liquid to that of the crystalline solid. Below Tm, the specific volume once again decreases

approximately linearly with temperature reflecting, the thermal expansion coefficient of the solid, which is

roughly 1/3 that of the liquid for many materials.

All materials tend to crystallize when cooled below Tm because the energy of a collection of atoms packed

in a crystalline array is lower than that of any other arrangements of the atoms. The crystal represents the

lowest - energy state and therefore the most stable state of the material. Crystal formation occurs over aperiod of time because the establishment of long range order requires atomic arrangements by diffusion. In

most materials, therefore it is possible to avoid crystal formation by cooling at a sufficiently high rate so as

to suppress the diffusion necessary to establish LRO in the crystal. In this case, as shown in Fig. (a), the

volume of the collection of atoms continues to decrease with the slope characteristic of the liquid below the

melting temperature, forming asupercooled liquid.

Since for most materials vliq

> vsol

. If the supercooled liquid continued to change its volume at the rate

characteristic of the liquid, it would eventually have a specific volume less than that of the corresponding

crystal at the same temperature.

Since it is not thermodynamically possible for the more loosely packed supercooled liquid to have a specific

volume lower than that of the crystal, the slope of the curve for the supercooled liquid must eventually

decrease to at least of the crystal. As shown in Fig. (b), the temperature at which this slope change occurs is

the glass transition temperature, Tg. Since several physical property changes are associated with T g, this

phenomenon is thoroughly characterized and studied in amorphous and semi-crystalline polymers.


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Many polymers are semicrystalline, containing both amorphous and crystalline regions. As shown in Fig (c),

a semi-crystalline polymer exhibits two critical temperatures, Tm and Tg. The crystalline regions undergo a

discontinuous change in volume at Tm, and the amorphous region cause a slope change at Tg.

Amorphous materials at temperatures below Tg are called glasses and exhibit brittle behavior. Between the

glassy and liquid states, amorphous materials are essentially supercooled liquids. In the specific case ofpolymers, the supercooled liquid state is called rubber. Supercooled liquids, including polymers and oxides

(but not metals), exhibit viscous behavior. This type of deformation will be discussed in our next lecture.. In

addition, the elastic modulus of amorphous solids decreases by several orders of magnitude as the

temperature increases above Tg.

Our definition of Tg included the phrase on the time scale of the experiment. What influence does time have

on the mechanical behavior of amorphous solids?

The key, once again, is to recognize that molecular motion requires time. If a load is applied slowly to a

sample, at a temperature near Tg, there may be sufficient time for molecular motion. In the contrast, a rapidloading rate may not allow sufficient time for significant motion regardless of the temperature. Therefore,

since the loading rate influences the time available for molecular motion, it also influences the effective glass

transition temperature of the solid. During a high-loading-rate experiment (i.e., when the time for molecular

motion is very short), the effective Tg is higher than it would be in a low-loading-rate (slow) experiment.

Those of you familiar with the childrens toy Silly Putty may have experienced the influence of loading rate

on Tg. The Tg for this material is near room temperature. If the material is pulled rapidly it breaks in two, but

if it is loaded more slowly it can be pulled down to a very fine diameter. In this example the high loading rate


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raises the effective Tg of the material above room temperature, and therefore the material behaves in a brittle

manner. In contrast, at the low loading rate the effective glass transition temperature is below room

temperature and the material does not exhibit brittle behavior.

Example Problem

Do you think the glass transition temperature for ordinary window glass is above or below room

temperature? What about the glass transition temperature for the polymer in a rubber band?

Solution

Our everyday experiences with window glass tell us that it is a brittle solid at all common temperatures.

Since glasses exhibit brittle behavior when the temperature is below Tg, we can conclude that the glass

transition temperature for window glass must be well above room temperature. In contrast, a rubber band is

not brittle at room temperature. Therefore, we can conclude that the Tg for this polymer is below room

temperature.

Factors Influencing Tg

We know at this point that some polymers have high Tg's, and some have low Tg's. The question we haven't

bothered to ask yet is this: why? What makes one polymer glass transition at 100oC and another at 500

oC?

The very simple answer is this: How easily the chains move? A polymer chain that can move around fairly

easily will have a very low Tg, while one that doesn't move so well will have a high one. In other words, the

value of Tg depends on the mobility of the polymer chain - the more immobile the chain, the higher the valueof Tg. This makes sense. The more easily a polymer can move, the less heat it takes for the chains to

commence wiggling and break out of the rigid glassy state and into the soft rubbery state.

In particular, anything that restricts rotational motion within the chain should raise Tg. A polymer chain that

can move easily will change from a glass to a rubber at a low temperature. If the polymer chains don't move

as easily, then it will require a relatively high temperature to change the compound into a rubbery form.

So then now another question arises..

What makes one polymer move more easily than another?

There are several things that affect the mobility of a polymer chain.

Backbone Flexibility

Pendant Groups Part I: Fish Hooks and Boat Anchors

Pendant Groups Part II: Elbow Room

Backbone Flexibility (Chain Stiffness)

This is the biggest and most important one to remember. The flexibility of a polymer chain depends on thegroups present in the polymer backbone. The more flexible the backbone chain is, the better the polymer will

move, and the lower its Tg will be.


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Let's look at some examples. The most dramatic one is that of silicones. Let's take a look at one calledpolydimethylsiloxane.

This backbone is so flexible that polydimethylsiloxane has a Tg way down at -127oC! This chain is so

flexible that it's a liquid at room temperature, and it's even used to thicken shampoos and conditioners.

Now we'll look at another extreme, poly(phenylene sulfone).

This polymer's backbone is just plain stiff. It's so rigid that it doesn't have a T+! You can heat this thing toover 500 oC and it will still stay in the glassy state. It will decompose from all the heat before it lets itself

undergo a glass transition! In order to make a polymer that's at all processable we have to put some flexible

groups in the backbone chain. Ether groups work nicely.

Polymers like this are called poly(ether sulfone), and those flexible ether groups bring the Tg of this one

down to a more manageable 190oC.

Groups such as the following are called stiffening groups because they decrease the flexibility of the

polymer chain.

Stiffening groups in the polymer chain reduce the flexibility of the chain and raise the value of T g .

As an example, poly(ethylene terephalate) is a stiffer molecule than poly(ethylene adipate) because abenzene ring is not as flexible as a chain of CH2 groups.


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Tg = 69oC

Tg = -70oC

Pendant Groups Part I:

Fish Hooks and Boat Anchors

A group attached to the polymer backbone and present in the repeating unit is called a pendant group.

Pendant groups have a big effect on chain mobility. Even a small pendant group can act as a fish hook that

will catch on any nearby molecule when the polymer chain tries to move like corkscrew. Pendant groups alsocatch on each other when chains try to slither past each other.

Examples of pendant groups are the methyl group in polypropylene and the benzene ring in polystyrene.

polypropylene polystyrene

Polymers with pendant groups still are designated as linear polymers. The presence of pendant groupsmodifies the properties of a polymer.

The influence of pendant groups on the glass transition temperature is somewhat more complicated.

1. Bulky pendant groups, such as a benzene ring, can catch on neighboring chains like a "fish hook" and

restrict rotational freedom. This increases Tg.

Atactic

PolypropyleneTg = - 20C


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Atactic

PolystyreneTg = 100C

2. Flexible pendant groups, such as aliphatic chains, tend to limit how close chains can pack. This increasesrotational motion and lowers Tg.

Poly(methyl methacrylate) Tg = 105oC

Poly(butyl methacrylate) Tg = 20oC

One of the best pendant groups for getting a high Tg is the big bulky adamantyl group. An adamantyl group

is derived from a compound called adamantane.

A big group like this does more than just act like a hook that catches on nearby molecules and keeps thepolymer from moving. It's a downright boat anchor. Not only does it get caught on nearby polymer chains,its sheer mass is such a load for its polymer chain to move that it makes the polymer chain move much more

slowly. To see how much this affects the Tg, just take a look at two poly(ether ketones), one with an

adamantane pendant group and one without.
http://pslc.ws/macrog/adam.htmhttp://pslc.ws/macrog/adam.htmhttp://pslc.ws/macrog/adam.htmhttp://pslc.ws/macrog/adam.htmhttp://pslc.ws/macrog/adam.htm


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The Tg of the polymer on the top is already decent at 119oC, but the adamantyl group raises even higher, to

225 oC.

Pendant Groups Part II: Elbow Room

But big bulky pendant groups can lower the Tg, too. You see, the big pendant groups limit how closely the

polymer chains can pack together. The further they are from each other, the more easily they can move

around. This lowers the Tg, in the same way a plasticizer does. The fancy way to say that there ismoreroombetween the polymer chains is to say there is more free volume in the polymer. The more free volume, the

lower the Tg generally. We can see this with a series of methacrylate polymers:

You can see a big drop each time we make that pendant alkyl chain one carbon longer. We start out at 120oC

for poly (methyl methacrylate), but by the time we get to poly(butyl methacrylate) the Tg has dropped to only20oC, pretty close to room temperature.

Intermolecular Forces

The intermolecular forces for polymers are the same as for small molecules. Because polymer molecules areso large, though, the magnitude of their intermolecular forces can vastly exceed those between smallmolecules.

The presence of strong intermolecular forces is one of the main factors leading to the unique physicalproperties of polymers.

Dipole-dipole forces result from the attraction between polar groups, such as those in polyesters and vinylpolymers with chlorine pendant groups.

Hydrogen bonding can take place when the polymer molecule contains -OH or -NH groups. Hydrogenbonding is the strongest of the intermolecular forces. Polymers such as poly (vinyl alcohol) and polyamides

are hydrogen bonded.

Stronger intermolecular forces lead to a higher Tg. PVC has stronger intermolecular forces than

polypropylene because of the dipole-dipole forces from the C-Cl bond.


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Atactic Polypropylene Tg = -20oC

Atactic Poly(vinyl chloride) Tg = 81oC

Cross-Linking

The presence of cross links between chains restricts rotational motion and raises Tg.

Plasticizers

Plasticizers are low molecular weight compounds added to plastics to increase their flexibility and

workability. They weaken the intermolecular forces between the polymer chains and decrease Tg. Plasticizers

often are added to semicrystalline polymers to lower the value of Tg below room temperature. In this casethe amorphous phase of the polymer will be rubbery at normal temperatures, reducing the brittleness of the

material.

Plasticizers are added to the plastic used for automobile upholstery. In older automobiles, the plasticizer may

be distilled from the upholstery during hot weather so that it becomes brittle over time.

Some plasticizers have been identified as major health and environmental problems. Before 1977,

Polychlorinated biphenyls (PCBs) were used as plasticizers in paints and plastics. Because they are toxic and

possible endocrine disruptors, PCBs no longer are used.

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4 & 5 - Glass Transition Polymer engineering