What is the Glass Transition Temperature, Tg? 1

[RichGeoffroy]

We’ll often hear someone talking about the Glass Transition Temperature (Tg) of a polymer, but so what? What does it mean, and why should I care about it?

The glass transition temperature is a function of chain flexibility. The glass transition occurs when there is enough vibrational (thermal) energy in the system to create sufficient free-volume to permit sequences of 6-10 main-chain carbons to move together as a unit. At this point, the mechanical behavior of the polymer changes from rigid and brittle to tough and leathery --- the behavior we define as “plastic behavior”.

Actually, the glass transition temperature is more important in plastics applications than is the melting point, because it tells us a lot about how the polymer behaves under ambient conditions. The melting temperature is often referred to as the “first-order transition” --- that’s where the polymer changes state from solid to liquid. Technically, only crystalline polymers have a true melting point; that’s the temperature at which the crystallites melt and the total mass of plastic becomes amorphous. Amorphous polymers do not have a true melting point, however, they do have a first-order transition where their mechanical behavior transitions from a rubbery nature to viscous rubbery flow.

All polymers have some temperature at which their physical properties are rigid and glassy material, similar to crystal polystyrene (Tg = 100[sup]o[/sup]C). Take polypropylene (Tg = 0[sup]o[/sup]C) down to -40[sup]o[/sup]C and try to break it, you’ll see what I mean. In its glassy state, the mechanical behavior of the polymer is relatively stable. The material is very hard and brittle, and the properties don’t change significantly with temperature --- modulus remains high and impact strength is almost nil. However, as the temperature rises, there will be a point where the behavior of the polymer will fairly rapidly change from glassy to a very tough and leathery behavior. This change in behavior is evidenced by a sharp decline in modulus (stiffness), or increase in impact strength as the ambient temperature is increased. This region is termed the glass transition region. The temperature at the midpoint of the transition from glassy to rubbery, the glass transition region, is defined as the glass transition temperature, Tg.

If the ambient temperature is elevated further, the material behavior becomes similar to a rubber. In this region, called the rubbery plateau, the low modulus and high impact strength again become less significantly affected by temperature. However, at some point the material becomes so soft that it will flow under very low pressure, this is the final transition to viscous rubbery flow. This is considered the “melting” temperature of the polymer, or the first-order transition temperature.

So what does all of this mean? Basically if a polymer’s glass transition temperature is well above (say, 50[sup]o[/sup]C above) ambient room temperature, the material will behave like a brittle glassy polymer --- it’ll be stiff with low impact resistance. Conversely, if the Tg is well below room temperature, the material is what is commonly termed a rubber or elastomer --- soft and easily stretched; and those materials whose Tg is reasonably close to the ambient temperature will exhibit plastic material behavior --- strong and tough with good impact resistance.

In applications that can experience temperature extremes, it is important to know what the potential exposure temperatures are and how they will affect the mechanical behavior of the material. In the earlier example of polypropylene, a tough plastic in room-temperature applications, we saw that it turns glassy and brittle at low temperatures, while at elevated temperatures, the material becomes soft and easily deformed under low loads --- rubber-like. This change in properties is simply the effect of temperature on the mechanical behavior of the material as it proceeds from well below its Tg, through its glass transition and into the rubbery plateau. At even higher temperatures, the crystallites will melt and the material will flow under moderate pressure --- the transition which occurs in the plasticating unit --- which allows us to fabricate parts from a material which we call PLASTIC.




Rich Geoffroy
Polymer Services Group
POLYSERV@aol.com

 

[patprimmer]

Well done again.

I always understood that a second order transition resulted from increased kinetic energy in the molecule, but I never associated a smallish range in the number of carbons in the backbone moving as a unit, nor that number being quite similar for all polymer types, but rather particular bonds between chains were broken as an activation energy was reached. I considered these bonds varied considerably from polymer to polymer, and depended on the geometry of the bonds re spacing on the chain and orientation to the adjacent chain, hence the differences in the properties of nylons 12, 6, 6.6 and 4.6 for instance.

It is many many years since I studied this, so maybe I have forgotten something, or maybe the explanation has been revised since the 60s

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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[gambro]

Well done!

Actually, the "real world of polymers" is even more complex with regard to Tg. For example :
1. Certain additives (plasticizers) can increase chain mobility and thus lower Tg
2. For PA, water molecules introduced in the structure (conditioning) act in a similar fashion to give toughness
3. Some high Tg materials are not brittle but tough, for example Polycarbonate, this being due to the molecular structure
4. Copolymerization can change Tg (manufacture of thermoplastic elastomers with various degrees of softness)
5. The connection Tg <--> brittleness is much more complicated for semi-cristalline plastics such as PP, which depends on degree of crystallinity, crystallite size and other factors

As a friend of mine once said : Plastics are like women : you can love them but never completely understand them.

 

[Latexman]

My $0.02 - a latex paint made with a copolymer with too low a Tg will pick up dirt too easily (too tacky) compared to one with a little higher Tg. That's one reason why paints are somewhat regional.


Good luck,
Latexman

 

[25362]

Hans-Georg Elias in his "An Introduction to Plastics" (ISBN 1-56081-784-4 VCH, New York) brings an interesting Chapter 5., titled Thermal Properties, dealing, among others, on glass transition temperature peculiarities.

 

[11211]

Another interesting book that deals with the explanation of the glass transition temperature in great detail is called "Metastable Liquids" by Pable Debenedetti

 

[11211]

What is your opinion on the glass transition temperature? Is it a dynamic or a static property?

 

[RichGeoffroy]

In an e-mail response to this thread, an Engineering-Tips reader wrote:

Hi;

I'm from Turkey studying in Textile engineering department in Uludað University in Bursa.

I know the difference between melting point and glass transition point but I wonder why some polymers have two glass transition point in textile terms (for example we consider polyester has two glass transition point). Do you have any information about this subject.

Thank you...

Oguzkesimci:

To my knowledge, polymers have only a single glass transition temperature. This second-order, or ?-transition is the temperature at which the mechanical behavior of the polymer changes from that of a brittle glass to more of a tough, leathery behavior. However, some polymers exhibit third-order and even fourth-order transitions.

Polycarbonate, for instance, has a third-order or ?-transition at about -90°C, whereas, the glass transition temperature is reported to be around 150°C. With a high glass transition temperature, polycarbonate should behave as a brittle glass at room temperature, similar to other amorphous polymers like PS or PMMA. Some researchers have accredited PC’s remarkable toughness to the presence of the ?-transition, although little is understood about the mechanism.

I guess we still have a lot to learn about polymers.



Rich Geoffroy
Polymer Services Group
POLYSERV@cox.net

 

[RichGeoffroy]

Clarification:

The second-order, or glass transition temperature, is also referred to as the “alpha”-transition, while a third-order transition is referred to as a “beta”-transition.

For those who may not remember, the first-order transition is the crystalline melting point for the polymer.



Rich Geoffroy
Polymer Services Group
POLYSERV@cox.net

 

[gambro]

As a short comment : Sometimes blends of two polymers exhibit two glass transition points, one for each of the blend partners.

 

[ajamnia]

This has been quite informative for me. I believe I understand Tg. What confuses me is how I can relate Tg to Vicat transition temperature. Any comments?

 

[RichGeoffroy]

ajamnia:

Vicat and Heat Deflection Temperature are attempts to measure the deflection temperature of a polymer under load. These values are close to the Tg for amorphous polymers, but are generally no where near the Tg for a crystalline polymer, which generally exhibits a Tg well below room temperature.

Deflection Temperature under Load has more to do with the actual level of modulus at a temperature, rather than whether or not the material is beyond its Tg. However, impact and elongation tend to change much more significantly as a polymer moves through its glass transition temperature. Other properties like refractive index and coefficient of linear thermal expansion also change significantly at Tg.



Rich Geoffroy
Polymer Services Group
POLYSERV@cox.net