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Glass Transition Temperature: Definition, How It Works, Factors, and Advantages

This article explains what the glass transition temperature (Tg) is and how it works, and why it is important in industries that deal with glassy and polymeric materials. The article also discusses the factors that affect Tg and the methods used to measure it.

Xomety X
By Team Xometry
May 6, 2023
 14 min read
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The term "glass transition temperature" (Tg) refers to the temperature at which an amorphous solid material transforms from a hard, brittle state to a softer, more malleable state that resembles a supercooled liquid. This phenomenon occurs because the bonds between molecular chains become less rigid and more mobile. Anyone who wishes to process or manufacture glassy and polymeric materials must understand Tg properties. 

Many industrial, technological, and scientific fields need materials with measured, stable, and controlled Tg values. Molecular weight, chemical makeup, thermal background, and environmental factors all impact Tg. The design, optimization, and performance of materials can benefit significantly from an understanding of Tg and its underlying principles. This article will define glass transition temperature and explain how it works, its implications, and the benefits of making it work for you. 

What Is a Glass Transition Temperature?

The glass transition temperature is the point below which a polymer or other non-crystalline material changes from a rigid, glassy state to a rubbery or viscous state. This physical change is brought on by the fact that polymer chains become more mobile as the temperature rises. Tg can be measured using methods like differential scanning calorimetry and dynamic mechanical analysis. It is a vital property for comprehending the behavior and processing of polymers.

Is Tg the Same As Glass Transition Temperature?

Yes, Tg is the accepted shorthand symbol for glass transition temperature, so they are interchangeable terms. Tg is the temperature at which an amorphous material changes from a rigid, glassy state to a more flexible, rubbery state. The physical characteristics of the material, such as its viscosity and thermal expansion coefficient, change during this transition. The scientific and engineering communities generally accept the abbreviation "Tg" for this term.

What Are the Units Used for Glass Transition Temperature?

Glass transition temperature can be given in degrees Celsius (°C), degrees Fahrenheit (°F), and Kelvin (K). This temperature represents the point at which heating or cooling causes an amorphous material to change between a hard, glassy state and a rubbery, viscous state.

How Does the Glass Transition Temperature Work?

At the glass transition temperature, the energy contained in the polymer chains begins to match or exceed the energy contained in their intermolecular bonds. This results in an increase in molecular mobility and a gradual shift from a rigid to a more flexible state. Below Tg, the substance is in a glassy state with a highly ordered, rigid, and brittle structure. The material's flexibility and molecular mobility increase once Tg is reached, enabling it to deform under stress or flow under shear. The transition is dictated by internal energy within the molecular chains; when they become more mobile, they alter the polymer’s overall physical characteristics.

What is the Importance of Glass Transition Temperature?

Tg is significant because of how it affects the mechanical, thermal, and processing characteristics of polymers. The Tg value, for instance, can determine the temperature range over which a polymer will be considered stable and retain its mechanical properties. A polymer will be hard and brittle below Tg and soft and deformable above Tg. Tg has an impact on polymer processing as well because it specifies the range of temperatures where the item’s shape can be altered without major changes to its overall form.The glass transition temperature can also have an impact on a product's shelf life since polymers can deteriorate or change shape if stored at temperatures above Tg.

What Is the Importance of Tg in 3D Printing?

The temperature at which any material is 3D-printed and cooled can have an impact on its dimensional stability, stiffness, and toughness. Both sides of the glass transition temperature can affect prints. If they’re too cool, the layers may crack or fail to bind together. If they’re too hot, the part may warp, deform, or simply collapse under its own weight. PLA materials, for instance, can soften and deform at relatively low temperatures because of a low Tg of about 55–60°C. By contrast, ABS has a Tg of about 105°C, which makes it more resistant to warping and deformation during printing. For more information, see our guide on Everything You Need to Know About 3D Printing.

How Does the Tg of 3D Printing Material Affect Its Printability?

The range of temperatures at which a 3D printing material can be extruded, melted, and fused into layers depends on its Tg (glass transition temperature). The material will not flow properly if the temperature is too low, producing prints that are brittle and weak. If it is too high, the material will deteriorate and produce surfaces with poor adhesion.

How Does the 3D Printing Cooling Rate Impact Tg?

The cooling rate during 3D printing can significantly influence the glass transition temperature of printed parts. Tg tends to rise with slower cooling rates and fall with faster cooling rates. This is because slower cooling rates give molecular chains more time to align and create stronger bonds, increasing Tg. If the material cools quickly, on the other hand, the molecules don’t have time to reach that optimal alignment, so they have weaker bonds. Depending on the specific printing material and printing conditions, the precise effect of the cooling rate on Tg can change.

Can Tg Predict 3D Printed Part Stability and Durability?

No, Tg cannot predict a 3D-printed part’s stability and durability, although it is a significant material property. Other properties, plus printing parameters and design considerations also influence the printed item’s performance. However, there are techniques and tools that can help you predict the strength and resilience of 3D-printed parts. For instance, finite element analysis (FEA) can simulate the mechanical behavior of a 3D printed part under a variety of conditions such as static and dynamic loads, temperature and humidity changes, and exposure to various environments. You can get insights into the stress and strain distribution within the part, identify potential failure modes, and optimize the design and material selection for improved stability and durability by analyzing the FEA results.

Is It Possible To Change a 3D Printing Material's Tg Chemically?

Yes, by changing the chemistry, you can alter the Tg of a material used for 3D printing. A change in its chemical makeup or the simple addition of chemical agents will change its properties. For example, you can increase a PLA print’s glass transition temperature by adding additives, but doing so may also compromise its strength and infusibility.

What Are the Different Methods of Measuring Glass Transition Temperature?

The different methods of measuring glass transition temperature are listed below:

  1. Fourier Transform Infrared Spectroscopy (FTIR): FTIR measures changes in molecular vibrations that take place close to Tg.
  2. Thermomechanical Analysis (TMA): Tg is calculated from the plot of thermal expansion or compression of the material versus temperature. A change in the curve’s slope indicates the Tg point. TMA measures the dimensional changes of a sample as it is heated or cooled.
  3. Differential Scanning Calorimetry (DSC): DSC measures the glass transition by providing data on the energy absorbed or released during the transition.
  4. Dynamic Vapor Sorption (DVS): DVS calculates the change in sorption behavior, or the polymer's ability to absorb water vapor as a function of temperature.
  5. Dynamic Mechanical Analysis (DMA): The DMA process deforms the polymer sinusoidally in order to measure its mechanical properties. The material’s Tg appears at the peak of its storage modulus.
  6. Dielectric Analysis (DEA): This is a way to measure electrical properties in a polymer as a function of temperature. A significant increase in permittivity and the occurrence of the peak in dielectric loss are both associated with the glass transition. 

How Does Tg Relate to the Behavior and Properties of Polymeric Materials?

The glass transition temperature, also known as the critical temperature, is the point at which polymeric materials change from a glassy state to a rubbery state. This transition alters the polymer's stiffness, ductility, and conductivity, along with other thermal, electrical, and mechanical properties. The processing conditions and end-use applications of polymeric materials are largely dependent on the Tg value. The Tg value is a key factor in the design and selection of materials because it influences their stability, durability, and shelf life.

What Type of Polymeric Materials Are Measured for Glass Transition Temperature?

Polymeric materials fall under the following three categories, but glass transition temperature does not apply to all of them. 

1. Crystalline Polymers

Since crystalline polymers do not have clearly defined Tg, these measurements do not apply to them. Crystalline polymers are characterized by other properties instead. 

2. Semi-Crystalline Polymers

Semi-crystalline polymers are substances that contain both disordered amorphous and ordered crystalline regions. Differential scanning calorimetry (DSC) or dynamic mechanical analysis (DMA) is used to measure the glass transition temperature of such polymers. The semi-crystalline polymer polyethylene, with a Tg of about -100°C, is one common example.

3. Amorphous Polymers

Amorphous polymeric materials change from a hard, brittle state to a rubbery, flexible state at a temperature known as the glass transition temperature. Dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), differential scanning calorimetry (DSC), and other methods can be employed to identify this value. In polystyrene, for instance, that temperature happens to be about 100 °C.

What Are the Different Factors That Are Affecting the Tg Value of a Material?

The following factors can have an impact on a material's Tg value:

1. Pressure

High pressure can raise a material's Tg value by compressing the polymer chains and increasing their rigidity.

2. Moisture

Moisture content can influence a material's Tg value because it can plasticize the polymer. Water molecules can interrupt intermolecular bonding force and add volume between polymer chains, ultimately having the effect of lowering the Tg value.

3. Composition

The Tg value of a material can vary depending on its composition. For instance, a material may be made from two different homopolymers. The Tg value range for the overall structure may be wider than that of a material made only of homopolymer A or homopolymer B. The addition of various fillers or additives can also impact a material’s Tg value.

4. Molecular Weight

In general, polymers with higher molecular weights also have higher Tg values because their intermolecular forces are stronger and require more energy to break.

5. Chemical Structure

A polymer's Tg value may vary depending on its chemical composition. Because they have more free volume and are less densely packed, polymers with flexible backbones have lower Tg values.

6. Plasticizers

Plasticizers increase the free volume between polymer chains and reduce intermolecular forces. This also has the effect of lowering the Tg value.

7. Crystallinity

Highly crystalline polymers have more rigid, ordered structures and higher Tg values.

8. Polar Groups

The intermolecular interactions between the polymer chains can be significantly changed if the monomer is modified with polar groups. In particular, the type and quantity of polar groups incorporated into the polymer can be changed to alter the Tg value.

9. Chemical Cross-linking

Crosslinking can raise a polymer’s Tg value because it restricts molecular motion and makes the polymer more rigid.

10. Thermal History

The thermal history of a polymer can alter its Tg value. A polymer's Tg value, for example, might be lower if it cools quickly than if it cools slowly.

How Does Tg Impact the Durability and Longevity of Polymer Products?

Tg has an impact on the toughness and longevity of polymer products. Polymers with higher Tg values tend to last longer because they are more resistant to creep deformation and have better mechanical properties at higher temperatures.

What Are the Advantages of a Tg?

The glass transition temperature (Tg) is an integral property of amorphous materials. There are several reasons you’d want to know it:

  1. Predicting the Mechanical Properties: A material's properties can be predicted based on temperature. The material is rubbery and flexible above Tg but stiff and brittle below Tg.
  2. Stability: Tg also indicates the thermal stability of the material. Higher-Tg materials can withstand higher temperatures before experiencing significant structural changes and are more resistant to thermal degradation
  3. Processing: Tg is used to identify the ideal processing temperature range to prevent structural changes in the material and maintain the desired properties.
  4. Shelf-life: The Tg can also be used to estimate a material's shelf life. Lower-Tg materials are more susceptible to structural changes over time, which results in shorter shelf life.

What Are the Disadvantages of a Tg?

Materials with a glass transition temperature have many beneficial uses in materials science and engineering, but they also have their own drawbacks. Here are a few examples:

  1. Limited Applicability: The Tg concept only applies to materials that experience a glass transition cannot be used to explain how substances like metals and ceramics behave.
  2. Sensitivity to Sample Preparation: A material's thermal history and preparation can have an impact on its Tg. It can be challenging to compare data from different sources because different Tg measurement techniques can produce inconsistent results.
  3. Dependence on Composition: The overall Tg can depend on the molecular weight of a polymer or the number of additives in a glass. This implies that Tg might not always be a reliable predictor of a material's behavior.
  4. Not a Complete Characterization: Tg is just one of many attributes that characterize a material's behavior. It does not give a comprehensive picture of its mechanical or thermal characteristics.

Is Tg a Stiffness Indicator for a Material?

No, a material's glass transition temperature is not a direct indicator of stiffness, but it can be linked to stiffness. The glass transition temperature is the point at which material changes from a glassy to a rubbery state, affecting its mechanical properties.

Is Tg a Temperature at Which a Material Changes From a Solid to a Liquid State?

No, the temperature at which a substance transforms from a solid to a liquid state is not the same as the Tg. Rather, Tg is the temperature at which a material changes from a glassy state to a rubbery state, which is indicated by a change in mechanical properties like stiffness and viscosity.

Does Tg Have an Impact on How Polymers Are Manufactured and Processed?

Yes, the Tg has a significant effect on the production and processing of polymers. The Tg establishes a polymer's processing temperature range and affects how the substance behaves during cooling and solidification, which in turn affects its mechanical and physical characteristics. Various processing techniques and environmental factors can also affect the Tg of a polymer.

Can a Material's Glass Transition Temperature Be Changed as It Is Being Synthesized?

Yes, changing the chemical makeup or processing conditions during synthesis changes a material's glass transition temperature. For instance, adding plasticizers can lower Tg while boosting cross-linking can raise it.

What Is the Difference Between Glass Transition Temperature and Melting Temperature?

The temperature at which a material goes through a reversible transition from a hard, brittle state to a more rubbery, flexible state is known as the glass transition temperature. The temperature at which a material changes from a solid to a liquid state is known as the melting point (Tm). The flexible material behavior is not the same as liquid behavior.

Summary

This article presented glass transition temperature, explained what it is, and discussed its factors and advantages. To learn more about glass transition temperature, contact a Xometry representative.

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Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.