Glass Transition Temperature

The glass transition temperature (Tg) is a phenomenon of amorphous polymers. At this temperature, polymers undergo a transition from glassy to rubbery state. Tg is an important feature of polymer behavior. It marks a region of dramatic changes in the physical and mechanical properties.

 

• Below Tg: Due to lack of mobility, the polymers are hard and brittle like glass.
• Above Tg: Due to some mobility, the polymers are soft and flexible like rubber.

Glass transition temperature curve showing polymer transition

The units of glass transition temperature are:
 

• Degree Celsius (°C)
• Degree Fahrenheit (°F)
• Kelvin (K)

The value depends on the mobility of the polymer chain. The Tg for most synthetic polymers lies between 170K to 500K.

 

Polymers are made up of long chains of molecules. Tg depends on the chemical structure of the polymer defined by its crystallinity. They may be amorphous, crystalline or semi-crystalline. 

 

Amorphous polymers

Amorphous polymers have a random molecular structure. At Tg, they take the glassy-state properties like brittleness, stiffness, and rigidity (upon cooling). They have a lower Tg than semi-crystalline polymers. This is because their polymeric chains are physically entangled and have spaces between them or at the chain ends. This space is known as free volume which helps polymer chains to move at low temperatures. Higher the free volume lower the glass transition temperature.

They do not have a sharp melting point. As temperature rises the amorphous material softens. These materials are more sensitive to stress failure due to the presence of hydrocarbons. Examples of amorphous polymers are PC, PMMA, PVC, ABS, and GPPS.


 

Crystalline polymers

Crystalline polymers have a highly ordered molecular structure. They do not soften as the temperature rises, but rather have a defined and narrow melting point (Tm). This melting point is generally above the upper range of amorphous thermoplastics. Examples of crystalline polymers include polyolefins, PEEK, PET, POM, etc.


 

Semi-crystalline polymers

Semi-crystalline polymers have a combination of random and ordered structures. These ordered structures are crystals that restrict the movement of polymer chains resulting in higher Tg. 

Note: Tg is the property of amorphous polymers and the amorphous part of a semi-crystalline solid. 


 

Polymers with high Tg

Some polymers are used below their Tg (in glassy state) like:
 

• Polystyrene
• Poly(methyl methacrylate)

These polymers are hard and brittle. Their Tg's are higher than room temperature. Learn more about brittle transition temperature » 


 

Polymers with low Tg

Some polymers are used above their Tg (in rubbery state), for example, rubber elastomers like: 
 

• Polyisoprene
• Polyisobutylene

They are soft and flexible in nature. Their Tg's are less than room temperature. 

 

At the molecular level, the chains in amorphous regions of the polymer gain enough thermal energy to begin sliding past one another at a noticeable rate. The temperature where the entire chain movement occurs is called the melting point. It is greater than the Tg. 

 

• Glass Transition is a property of the amorphous region while melting is the property of the crystalline region.
• Below Tg, there exists a disordered amorphous solid where chain motion is frozen and molecules start wiggling around above Tg. The more immobile the chain, the higher the value of Tg.
• While, below Tm it is an ordered crystalline solid which becomes disordered melt above Tm.

The operating temperature of polymers is defined by transition temperatures. 

A Heat Versus Temperature Plot for a Crystalline Polymer (L) and Amorphous Polymer (R)
(Source: PSLC)

Glass transition temperature is an important property used to change the physical properties of polymers.
 

• Increasing Tg improves handling characters, solubility, and reproducibility in the dissolution of solids.
• Changes in physical properties such as hardness and elasticity.
• Changes in volume, percent elongation to break, and Young's modulus of solids.
• Used for quality control, research, and development.

Chemical structure

Addition of plasticizers

The plasticizers increase the free volume between polymer chains, spacing them apart. The polymer chains slide past each other at lower temperatures resulting in a decrease in Tg. 


 

Water or moisture content

An increase in moisture content forms hydrogen bonds between the polymeric chains. These bonds increase the distance between the chain structures. This results in an increase in free volume and decreases Tg. 


 

Effect of entropy and enthalpy

The value of entropy is high for amorphous material and low for crystalline material. If entropy is high, then Tg is also high. 


 

Pressure and free volume

An increase in pressure of the surrounding leads to a decrease in free volume and ultimately high Tg. 


 

Other factors governing Tg

Other factors have a significant impact on the glass transition temperature of polymers. These include:
 

• Branching,
• Alkyl chain length,
• Bond interaction,
• Flexibility of polymer chain,
• Film thickness, etc.
   

DSC, DTA and DMA are by far the most dominant techniques used for glass transition temperature measurements.

 

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) is a thermo-analytical technique using differential scanning calorimeter. It monitors the difference in heat flow between the sample and reference against time or temperature. It also programs the temperature change of the sample in a specified atmosphere. 

DSC determines the thermal properties of the polymer. It applies to amorphous sections of polymers that are stable. These materials do not undergo decomposition or sublimation in the glass transition region. 

The test standards used to find Glass Transition Temperature of resins via DSC include:
 

• ASTM E1356-08(2014) – Standard Test Method for Assignment of the Glass Transition Temperatures by Differential Scanning Calorimetry
• ASTM D3418-15 – Standard Test Method for Transition Temperatures, Enthalpies of Fusion and Crystallization of Polymers by Differential Scanning Calorimetry
• ASTM D6604-00(2017) – Standard Practice for Glass Transition Temperatures of Hydrocarbon Resins by Differential Scanning Calorimetry
• ISO 11357-1:2016 – Plastics — Differential scanning calorimetry (DSC)
  • Part 1: General principles
  • Part 2: Determination of glass transition temperature and step height

Differential Thermal Analysis (DTA)

Differential Thermal Analysis (DTA) is a popular thermal analysis technique. It is often employed for measuring Tg of the material. This test method is similar to differential scanning calorimetry (DSC). The technique involves an inert reference material.
 

• The material under analysis in DTA undergoes various heating and cooling (thermal) cycles.
• It determines the temperature difference between the reference and the sample. It maintains the same temperature throughout the heat cycles for reference and sample. This ensures that the testing environment is uniform.

Measurement Principles of DTA (Source: Hitachi High-Tech Corporation)
Where Graph (a) Shows the Temperature Change of the Furnace, Reference and Sample Against Time
Graph (b) Shows the Temperature Difference (ΔT) Against Time Detected with the Differential Thermocouple

The test standards used to determine Glass Transition Temperature of resins via DTA include:
 

• ASTM E794-06(2018) – Standard Test Method for Melting and Crystallization Temperatures by Thermal Analysis

Dynamic Mechanical Analysis (DMA)

Dynamic Mechanical Analysis (DMA) uses a dynamic mechanical analyzer to measure the stiffness of materials as a function of temperature, humidity, dissolution media or frequency. 

Typical DMA Analysis Graph

In this technique, a mechanical stress is applied to the sample and the resultant strain is measured by the instrument. These parameters are used to evaluate glass transitions, degree of crystallinity, and stiffness behavior of the sample. 

The test standards used to determine Glass Transition Temperature of resins via DMA include: 
 

• ASTM E1640-13 – Standard Test Method for Assignment of the Glass Transition Temperature by Dynamic Mechanical Analysis

Several other methods to determine Tg include:
 

• Specific heat measurements
• Thermo mechanical analysis
• Thermal expansion measurement
• Micro-heat-transfer measurement
• Isothermal compressibility
• Heat capacity determination

Get Inspired: Learn how to combine data from multiple tools DSC, TGA, DMA, FTIR for Optimal Material Analysis