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# Glass transition temperature (Tg) - A210

Glass transition temperature (Tg)
Foundational knowledge article
Document Type Article
Document Identifier 210
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Material

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Prerequisites

## Introduction

Glass transition represents the temperature region where a polymer behaviour transitions from a hard-glassy material, to that of a soft rubbery material. At this temperature point (more correctly over a temperature range or region), a sudden loss in mechanical stiffness occurs.

## Scope

This page defines polymer glass transition temperature (Tg) and explains its significance to composites processing. Typical values for glass transition of various polymer matrices are provided, along with a brief introduction to measurement techniques.

## Significance

The glass transition temperature (Tg) of a polymer is critical for defining its operating temperature limits. At temperatures above Tg, a polymer experiences a sudden drop in its mechanical stiffness.

Typically, when mechanical stiffness is desired, a polymer’s service temperature should be below its Tg. However, in applications where flexibility is desired, the polymer should be in service about its Tg temperature. An example of this latter case is utilizing the flexibility provided by rubbers and elastomers.

## Prerequisites

Recommended documents to review before, or in parallel with this document:

## Definition

Example of the effects of softening when polymer is at a temperature above its glass transition temperature (Tg).

Polymers are classified as glassy when their molecular backbones exhibit the inability to move and remain “frozen” in crumpled immobile conformations. Below the glass transition temperature (Tg), upon heating only thermally induced expansion between the molecules occurs. Above Tg, amorphous regions within the polymer observe liquid-like “flow” with the molecular chains gaining rapid ability to move freely, while crystalline regions remain locked in the glassy state configuration.

Specific volume change with temperature - thermoset polymers.

As an outcome of this behavioral transition, a polymer exhibits a sudden change in its specific volume to temperature response (thermal expansion) at its Tg transition. Mechanically, a softening drop in mechanical stiffness occurs. This substantial drop in mechanical stiffness is shown in the example picture, showing a polymer below and above its glass transition temperature.

Young's Modulus drop at the glass transition point.

At temperatures above Tg, a substantial reduction in Young’s Modulus (E) is observed between the stiff glassy state and the softened rubbery state. To ensure in service mechanical stiffness of the polymer, the operational temperature should be below Tg. However, there are situations where having the service temperature above the polymer’s Tg to obtain rubbery behaviour is intended. Elastomers are the classic example, where their rubbery behaviour is desired and the material is used above its Tg.

### Typical Property Values

Polymer Glass transition temperature Source
(°C) (°F)
Thermoset polymers
Epoxy 150-265 302-509 [1]
Phenolic 127-159 261-318 [1]
Polymide 320-330 608-626 [1]
BMI 294-300 561-572 [1]
Cyanate 230-265 446-509 [1]
Thermoplastic polymers
Polyamide-imide (PAI) 275 527 [1]
Polyaryl ethers 220-260 428-500 [1]
Polyether sulphone (PES) 220 428 [1]
Polyether-imide (PEI) 210 410 [1]
Polyarylene sulfide (PAS) 200-210 392-410 [1]
Polyetherether ketone (PEEK) 140-145 284-293 [1]
Polyphenylene sulfide (PPS) 85-95 185-203 [1]
Polyarylene ketone 200-210 392-410 [1]
Polyimide (PI) 250-280 482-536 [1]

## Measurement

The glass transition temperature (Tg) of a polymer can be measured using several different laboratory techniques. Each method relies on a different measurement principle, resulting in slight differences of determined Tg between the methods.

Common laboratory techniques to measure Tg include:

• Differential Scanning Calorimetry (DSC) – measures heat flow changes
• Dynamic Mechanical Analysis (DMA) – measures behavioural change in mechanical stiffness
• Thermal Mechanical Analysis (TMA) – measures volume changes
Characterization Method Example Data Measurement Notes (How to) KPC Method Document
Differential Scanning Calorimetry (DSC)
Example of how glass transition temperature (Tg) is determined by differential scanning calorimetry (DSC) heat flow.
A heat flow change is observed in the region of Tg. An advantage to DSC is that only a small sample size is required (milligrams). Method document coming soon.
Dynamic Mechanical Analysis (DMA)
Example of how glass transition temperature (Tg) is determined by dynamic mechanical analysis (DMA).
At Tg, the storage modulus (G' or E') drops by several orders of magnitude. Two properties are normally reported; tosional (G'), bending (E') stiffness and loss tangent (tan $$\delta$$). Method document coming soon.
Thermal Mechanical Analysis (TMA)
Example of how glass transition temperature (Tg) is determined by thermal mechanical analysis (TMA).
Slope thermal expansion vs. temperature experiences changes at Tg. Method document coming soon.

## Related pages

Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents
Case Studies
Perspectives Articles

## References

1. [Ref] Pilato, Louis A.; Michno, Michael J. (1994). Advanced Composite Materials. Springer-Verlag Berlin Heidelberg. doi:10.1007/978-3-662-35356-1. ISBN 978-3-540-57563-4.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)

## Welcome

Welcome to the CKN Knowledge in Practice Centre (KPC). The KPC is a resource for learning and applying scientific knowledge to the practice of composites manufacturing. As you navigate around the KPC, refer back to the information on this right-hand pane as a resource for understanding the intricacies of composites processing and why the KPC is laid out in the way that it is. The following video explains the KPC approach:

## Understanding Composites Processing

The Knowledge in Practice Centre (KPC) is centered around a structured method of thinking about composite material manufacturing. From the top down, the heirarchy consists of:

The way that the material, shape, tooling & consumables and equipment (abbreviated as MSTE) interact with each other during a process step is critical to the outcome of the manufacturing step, and ultimately critical to the quality of the finished part. The interactions between MSTE during a process step can be numerous and complex, but the Knowledge in Practice Centre aims to make you aware of these interactions, understand how one parameter affects another, and understand how to analyze the problem using a systems based approach. Using this approach, the factory can then be developed with a complete understanding and control of all interactions.

## Interrelationship of Function, Shape, Material & Process

Design for manufacturing is critical to ensuring the producibility of a part. Trouble arises when it is considered too late or not at all in the design process. Conversely, process design (controlling the interactions between shape, material, tooling & consumables and equipment to achieve a desired outcome) must always consider the shape and material of the part. Ashby has developed and popularized the approach linking design (function) to the choice of material and shape, which influence the process selected and vice versa, as shown below:

Within the Knowledge in Practice Centre the same methodology is applied but the process is more fully defined by also explicitly calling out the equipment and tooling & consumables. Note that in common usage, a process which consists of many steps can be arbitrarily defined by just one step, e.g. "spray-up". Though convenient, this can be misleading.

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