CHAPTER 4 - Transition Phenomena

8/11/2019 CHAPTER 4 - Transition Phenomena

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Chapter 4

Transition Phenomena


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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

theglass transiti on.

This transition is something that only happens to

polymers, and is one of the things which 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 tr ansiti on temper atur e, orTgfor short.


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When the polymer is cooled below this temperature,

it becomeshard and brittle, like glass. Some

polymers are used above their glass transition

temperatures, and some are used below. Hard

plastics likepolystyreneandpoly(methyl

methacrylate), are used below their glass transition

temperatures; that is in their glassy state. TheirTg'sare well above room temperature, both at around

100 oC. Rubberelastomerslikepolyisopreneand

polyisobutylene, are used above theirTg's, that is, in

therubbery state, where they are soft and flexible.


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Rubber and elastomers

Rubber is a member of an important group ofpolymers calledelastomers. Elastomers areamorphous polymersthat have the ability tostretch and then return to their original shape at

temperatures above Tg. This property is importantin applications such as gaskets and O-rings, so thedevelopment of synthetic elastomers that canfunction under harsh or demanding conditionsremains a practical goal. At temperatures below Tgelastomers become rigid glassy solids and lose allelasticity.


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Rubber and elastomers

An example of this caused the space shuttle

Challenger disaster. The heat and chemical

resistant O-rings used to seal sections of the

solid booster rockets had an unfortunately

high Tgnear 0 C. The unexpectedly low

temperatures on the morning of the launch

were below this Tg, allowing hot rocket

gases to escape the seals.


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Unstretched rubber and stretched rubber


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Amorphous and Crystalline Polymers

We have to make something clear at this point.The glass transition is not the same thing asmelting. Melting is a transition which occurs in

crystalline polymers. Melting happens whenthe polymer chains fall out of their crystalstructures, and become adisordered liquid.The glass transition is a transition whichhappens toamorphouspolymers; that is,

polymers whose chains are not arranged inordered crystals.


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But even crystalline polymers will have a some

amorphous portion. This portion usually makesup40-70%of the polymer sample. This is why

the same sample of a polymer can have both a

glass transition temperatureandamelting

temperature.

But you should know thatthe amorphous

portion undergoes the glass transitiononl y, and

the crystalline portion undergoes meltingonl y.


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The Snake Pit

Now, to understand just why polymers with no order tothem arehard and bri ttle below a cer tain temper ature,andsoft and pli able above it, it can help to think of apolymer in the amorphous state as a big room full of

slithering snakes. Each snake is a polymer chain. Nowas you may remember,snakes are cold blooded animals,so all their body heat has to come from theirsurroundings. When it's warm, the snakes are happy,and can go on about their business of slithering and

sliding with no trouble at all.They will move all aboutrandomly,over and around each other, and they slitherhither and thither, just having a great time, or as gooda time as snakes ever have.


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Translational motion?

There is a difference between polymers and snakes thatwe probably should discuss at this point. An individualsnake is not only wiggling around, but actually movingfrom one side of the room to the other. This is called

translational motion. When you walk down the street,presuming you're not like most Americans who neverwalk anywhere, you are undergoing translationalmotion. While polymers are not incapable of suchmotion,mostly they are not undergoing translational

motion.But they are still moving around, wiggling thisway and that, much like little kids in church.


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To be sure, by the time we get down to the glass

transition temperature, it is already too cold for thepolymer 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-r ange 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, and turn like a giant corkscrew.


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Try This!

Want to have some fun? First, bring some liquid

nitrogen to class. Then put some in a styrofoam cup,

and drop in some household objects made from

polymers, like rubber bands or plastic wrap. The

liquid nitrogen, being nippy as it is, will cool the

objects below their glass transition temperatures. Try

to bend your rubber band (hold it with a pair of pliers,

because you could get frostbite if you try to touch it

with your fingers) and it will shatter The rubber band

will shatter because it is below its glass transition

temperature.


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First-order transition

When a low-molecular-weight crystalline

solid melts or when a liquid boils, change in

volume and enthalpy, as well as otherprimary thermodynamics properties, take

place at constant temperature. Such phase

changes are termed first order transitions

and are true thermodynamic changes ofstate


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First order transition

,Gm

mm

P

mm

T

GS

T

G

VP


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Second order transition

,Gm

,

2,

2

2

2

p mm m

p p

m mm T

TT

CG S

T T T

G V V KP P


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Fusion behavior of some linear polymers

The equilibrium crystalline

melting point for polymers is

defined as that temperature at

which the last of crystallites

melt The actual value of Tm is

subject to a strong hysteresis

effect

Tm depends on the melt

history of the polymer asreflected in crystallinity and

size distribution


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The glass transition

As amorphous material is heated from

below a certain characteristic temperature,

the specific volume increases at a fixed rate.At the glass transition temperatureTg, this

rate increases, and there is a discontinuity in

the volume expansion curve


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Specific volume of polyvinyl acetate as a function

of temperature

Amorphous polymersexhibit a change fromglasslike behavior

belowTgto soft,rubbery behavior asthe T is raised aboveTg

Demonstration on the

properties of liquid airor N2


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Specific volume-temperature curves for

semicrystalline polymer

A:liquid region

B:liquid with some

elastic response

C:Rubbery region D:glassy region

E:crystallites in a

rubbery matrix

F:crystallites in aglassy matrix


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Tg & Tm

Tm

is the temperature at which crystalline

domains lose their structure, or melt. As

crystallinity increases, so does Tm. T

gis the temperature below which

amorphous domains lose the structural

mobility of the polymer chains and become

rigid glasses.


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Polymer LDPE HDPE PP PVC PS PAN PTFE PMMA Rubber

Tm

(C)

110 130 175 180 175 >200 330 180 30

Tg(C)

_110 _110 _20 80 90 95 _110 105 _70

Tmand Tgvalues for some common addition

polymers are listed below


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Mechanical properties

Er(t, T) = S(t, T)/ (0)

Relaxation tensile modulus for

viscoelastic bodies at low strains

S(t, T)is the stress, function of

time and temperature (0)is the constant tensile strain

applied at time zero

If either time or temperature is held

constant, one can measure a

modulus that is either a function of

temperature or time, respectively


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Er(10) versus temperature for crystalline

isotactic polystyrene

Its general characteristics are

shared by all linear

amorphous polymers and

their crystalline homologues

Five regions of viscoelasticbehavior(see next slide)


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Transition temperatures and Engineering-

use temperatures

Elastomers:

Amorphous structural polymers:

Tough, leatherlike polymers: Highly crystalline and oriented(fibrous)

polymers:

Semicrystalline polymers:

(see textbook, pp.88)


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Transition temperatures and Engineering-

use temperatures(II)4. Highly crystalline and oriented (fibrous) polymers: These materials must

be used at temperatures substantially below Tm (on the order of 100),

since changes in crystal structure can occur above Tgas Tm is approached-

In some applications,the glass transition is not important,for it repre-

sents a minor transition for these highly crystalline materials.Typicalfibrous materials like nylon and poly(1,4,ethylene terephthalate)with

Tmon the order of 275 must be used at temperatures below l75

5. Semicrysfalline polymers:At about 50 percent crystallinity, these poly-

mers can be used at temperatures between Tgand Tmwhere the material

exhibits moderate rigidity and a high degree of toughness, a somewhat

anahgous to a reinforced rubber. The classical example of this situation

is branched (low-density)polyethylene, which is used under ambient

conditions with Tg= -120 and Tm=115.


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The Principle of corresponding

temperature

Two linear amorphous

polymers of the same

molecular weight

distribution areapproximately equivalent

at corresponding

temperature

If we useTgas the criticalT, a reduced T can be

defined asTr= T/Tg


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Molecular motion and transitional

phenomena

Motion of chain ends,loops

and segments in the glass

transition zone

Below Tg: there isrelatively little molecular

movement, the chain

segments are frozen in the

fixed positions. There is

little opportunity fordiffusional rearrangement

of segmental position


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Molecular Motion

In therubbery plateau region, theshort-rangesegmental motionsare very rapid, molecular motionis still restricted by chain entanglement, there are nopermanent changes in molecular conformation.

Inrubbery flow region,molecular slipis becomingimportant as the degree of entanglement decreases,but some elasticity is still retained.

In the liquid flow region,the slip of entire moleculesis the dominant mode of motion, large changes in

conformation occur, and little elasticity is exhibited.


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The effect of composition on polymeric

transitional phenomena

The relationship between polymer

properties and polymer composition

Transitional properties of a polymer areamong the most important factors in

determining its utility

Homopolymer systems and copolymer and

polyblend systems


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Homopolymer systems

Boyer-Beaman systems For linear and semicrystalline

homopolymers, composition

affects both glass and crystal

melting transitions in the same

manner

The ratio Tg/Tm(K) ranged

from0.5(for symmetrical

polymers, eg. PE,

polybutadiene) ~0.75(for

unsymmetrical polymers, eg.

PS, polyisoprene)


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Homopolymer systems

Boyer-Beaman systems


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External plasticization

Plasticizationrefers to theprocess of making a materialmore susceptible to plasticflow

In polymer molding andextrusion operations,

plasticization is achieved byraising T.

Increased deformability can

also be achieved by theaddition of low MW organiccompounds, referred to asexternal plasticizers

Not only is Tg reduced but also the T

range of the transition region is broadened

to a max width at an intermediate content


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Relative lowering of melting point and glass-

transition temperature by plasticization

Tg and Tm vary withplasticizer content inpoly(vinyl chloride)

Notice how Tg drops

more rapidly than doesTm

Plasticizer content isoften limited to about

40% because oflimited compatibility


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Molecular weightGlass transition of polystyrene fractions vs M-1 and MW

The Tg is markedlydependent on MW

This dependence is result ofthe relatively greatercontribution of chain-end

segments As No. of chain ends

increases (Mn decreases),the available free volume asa whole increases relatively

faster with T, and the glasstransition occurs at lower T

Tg= Tg

K/Mn


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Effect of copolymer/heterogeneity

Comparison of the transition

properties of homogeneous and

heterogeneous copolymers of vinyl

chloride and methyl acrylate

Broadening of the transition zone

results with increasing heterogeneity

because chains of different

composition exhibit different

transition temperatures

It is enhanced if the homopolymers

are incompatible


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Variation in glass transition temperature

with copolymer composition For a series of homogeneous

copolymers or compatiblepolyblends, the values of Tg willusually fall between(but not aboveor below)those of the parenthomopolymers in some smoothlyvarying fashion

When the monomers differconsiderably in their chemicalnature, values of Tg may fall well

below those of either of thecorresponding homopolymers

Copolymers that exhibit thisbehavior are from methylmethacrylate-acrylonitrile, styrene-methyl methacrylate


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Semicompatible copolymer and polyblend

systems Block and graft copolymers and

polyblends in which the

homopolymer portions show varying

degrees of compatibility

In these semicompatible systems, one

of the components will tend todistribute as droplets in a continuous

matrix of the other component

If one component is completely

incompatible in the other, two glass

transitions will be observed

An immiscible blend formed froma mixture of PS and styrene-

butadiene copolymer


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Er vs. Temperature for polyblends of polystyrene and a

30/70 butadiene-styrene copolymer

If all segments in either block or graft

copolymers are fully compatible, the

resulting behavior will be analogous

to that of homogeneous copolymers

and compatible polyblends

The most useful transition propertiesare obtained in systems that are on the

borderline of compatibility and

incompatibility- that is, in

semicompatible systems

The most interesting curves are the

ones in which we observe extension

of the leatherlike behavior over awider temperature range


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Effect of cis and trans isomerism of Tgand Tm


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Cis-Trans

Cis-polyisoprene

Nature rubber

Trans-polyisoprene


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Effect of chain substituent length on Tgfor

acrylate and methacrylate polymers

Tg for the number of carbon atoms in n-alkyl acrylate and methacrylate polymers

Tg decreases with increasing length of the side chain until the length reaches 8

carbons for the acrylate series and 12 carbons for the methacrylate series.

The increase observes thereafter is due to crystallization of the side chains. At low

T, these side-chain crystallites bind the polymer into a firm, waxy structure.


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Acrylate and Methacrylate

One might not think that this little methyl group would make a whole lotof difference in the behavior and properties of the polymer, but it does.

Poly(methyl acrylate) is awhite rubberat room temperature, but

poly(methyl methacrylate) is a strong, hard, andclear plastic.


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We can see this with a series ofmethacrylatepolymers:


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Effect of chain-substituent branching on

Tg for acrylate polymers

Symmetry also affect transition temperatures.

Polyisobutylene( 2CH3groups close to the backbone should tend tostiffen the chain) has higher Tg than isotactic polypropylene(Tm = 165oCand Tg =10oC)?

Polyisobutylene is a rubber that crystallizes at 44oCand Tg = -71oC. This

discrepancy is due to that PP has a tighter helix than polyisobuylene,which produces a stiffer chain- 3 units in 1 turn compared to 8 units in 5turns


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If the polymer chain is drawn in a zig-zag fashion,as shown above, each of the substituent groups (Z)

will necessarily be located above or below theplane defined by the carbon chain. Consequentlywe can identify three configurational isomers ofsuch polymers. If all the substituents lie on oneside of the chain the configuration is called

isotactic. If the substituents alternate from oneside to another in a regular manner theconfiguration is termedsyndiotactic. Finally, arandom arangement of substituent groups isreferred to asatactic. Examples of these

configurations are shown here.


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Many common and useful polymers, such as polystyrene, polyacrylonitrile and

poly(vinyl chloride) are atactic as normally prepared. Customized catalysts that

effect stereoregular polymerization of polypropylene and some other monomers

have been developed, and the improved properties associated with the increased

crystallinity of these products has made this an important field of investigation.The following values of T

ghave been reported.

Polymer Tgatactic T

gisotactic T

gsyndiotactic

PP

20 C 0 C

8 C

PMMA

100 C 130 C 120 C


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Measuring the Tg

Differential scanning calorimetryis a technique

we use to study what happens to polymers

when they're heated. We use it to study what

we call the thermal transitionsof a polymer.

And what are thermal transitions? They're the

changes that take place in a polymer when you

heat it. The melting of a crystalline polymer is

one example. Theglass transitionis also a

thermal transition.


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It pretty simple, really. In the most popular DSC design,

two pans sit on a pair of identically positioned platformsconnected to a furnace by a common heat flow path. In

one pan, you put your polymer sample. The other one is

the reference pan. You leave it empty. You then tell the

nifty computer to turn on the furnace. So the computer

turns on the furnace, and tells it to heat the two pans at a

specific rate, usually something like10oC per minute.

The computer makes absolutely sure that the heating

rate stays exactly the same throughout the experiment.

But more importantly,it makes sure that the two

separate pans heat at the same rate as each other.


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The plot will look something like this at first.

The heat flow at a given temperature can tell us something.

The heat flow is going to be shown in units of heat, q

supplied per unit time, t.The heating rate is temperatureincrease

Tper unit time,t.


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GLASS TRANSITION

measured temperatures


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This meansheat is being absorbed by the sample. It also

means that we havea change (increase) in its heat capacity.

Thi s happens because the polymer h as just gone through the

glass transiti on. And as you learned on the glass transition

page, polymershave a higher heat capacity above the glass

transition temperature than they do below it. Because of

this change in heat capacity that occurs at the glass

transition, we can use DSC to measure a polymer's glasstransition temperature. You may notice that the change

doesn' t occur suddenl y, but takes place over a temper atur e

range.This makes picking one discreet Tg kind of tricky,

but we usually just take the middle of the incline to be the

Tg.


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Melting

Heat may allow crystals to form in a polymer, but too

much of it can be their undoing. If we keep heating our

polymer past itsTc, eventually we'll reach another

thermal transition, one called melting. When we reach

the polymer's melting temperature, or Tm, thosepolymer crystals begin to fall apart, that is they melt.

The chains come out of their ordered arrangements, and

begin to move around freely. And in case you were

wondering, we can spot this happening on a DSC plot.


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Remember the heat that the polymer gave off when it

crystallized? Well when we reach theTm, it's

payback time. There is a latent heat of melti ngas wellas a latent heat of crystallization. When the polymer

crystals melt, they must absorb heat in order to do so.

Remember melti ng is a fi r st order transition. This

means that when you reach the melting temperature,

the polymer's temperature won't rise until all thecrystals have melted. This also means that the

furnace is going to have to put additional heat into

the polymer in order to melt both the crystalsan d

keep the temperature rising at the same rate as that

of the reference pan. This extra heat flow during

melting shows up as a large dip in our DSC plot as

heat is absorbed by the polymer.


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It looks like this:

We can measure the heat of melting by measuring the area of this

dip. And of course, we usually take the temperature at the apex of

the dip to be the point where the polymer is completely melted.Because we have to add energy to the polymer to make it melt, we

call melting anendothermictransition.

Putting It All Together


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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.

Then we saw a big peak when the polymer reached its

crystallization temperature. Then finally we saw a big dip

when the polymer reached its melting temperature. To put

them all together, a whole plot will often look something

like this:


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Polymer Transitions

Three states of matter: solids, liquids, and Gases; twotransitions connect them: melting and boiling(or freezingand condensing)

Differential scanning calorimetry heating trace ofquenched copolymer showing a glass transition and acrystallization transition, and a melting transition


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Of course, not everything you see here will be on every DSC

plot. The crystallization peak and the melting dip will only

show up for polymers that can formcrystals. Completely

amorphous polymers won't show any crystallization, or any

melting either. But polymers with both crystalline and

amorphous domains, will show all the features you see above.

If you look at the DSC plot you can see a big difference

between the glass transition and the other two thermaltransitions, crystallization and melting. For the glass

transition,there is no peak, andthere' s no dip, either. This is

because there isno latent heat given off, or absorbed, by the

polymer during the glass transition. Both melting and

crystallization involve absorbing or giving off heat. The onlything we do see at the glass transition temperature is a change

in the heat capacity of the polymer.


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Because there is a change in heat capacity,

but there is no latent heat involved with the

glass transition, we call the glass transition

asecond order transition. Transitions like

melting and crystallization, which do have

latent heats, are calledf i r st order transitions.


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Messing Around with the Glass Transition

Sometimes, a polymer has aTg that is higher than we'dlike. That's ok, we just put something in it called aplasticizer. This is a small molecule which will get inbetween the polymer chains, and space them out from

each other. We call this increasing thef ree volume.When this happens they can slide past each other moreeasily. When they slide past each other more easily,they can move around at lower temperatures than theywould without the plasticizer. In this way, the Tg of apolymer can be lowered, to make a polymer more

pliable, and easier to work with.


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Th Gl T iti M lti


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The Glass Transition vs. Melting

It's tempting to think of the glass transition as a kind ofmelting 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 transitionhappens only to polymers in the amorphous state.A

given polymer wil l often h ave both amor phous and

cr ystall ine domains within it, so the same sample can

often show a melting point and a Tg. But the chains that

melt are not the chains that undergo the glasstransition.


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Latent heat of melting

The temperature rising stops because melting

requires energy. All the energy you add to a

crystalline polymer at its melting point goes into

melting, and none of it goes into raising the

temperature. This heat is called the

l tent he t of

melting

. (The word

l tent

means 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.


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What Becomes the High Tg Polymer?

Ok, we know at this point that some polymers have highTg's, and

some have low Tg's. The question we haven't bothered to ask yet is

this:why?What makes one polymer glass transish at 100 oC 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

lowTg, while one that doesn't move so well will have a high one.

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.

So then I suppose we've brought ourselves to another question...


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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 aTg! You can heat

this thing to over 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 groupswork nicely.


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Poly(ether sulfones)

Polymers like this are calledpoly(ether sulfones), andthose flexible ether groups bring theTg of this one

down to a more manageable 190 oC.

CHAPTER 4 - Transition Phenomena

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