What is the Glass Transition Temperature of Plastics?

Hello people; the physical properties of any polymer have a direct impact on its applicability. Let’s take a look at the Tg of plastic materials.

What is the Glass Transition Temperature of Plastics

When an amorphous polymer is heated, the temperature at which the polymer structure turns “Sticky liquid or rubbery” is known as Glass Transition Temperature or Tg. It is also stated as a temperature at which amorphous polymer starts showing glassy-state properties such as stiffness, rigidness, and brittleness after cooling.

The temperature at which the polymers are heated depends on their chemical structure and thus can be used to identify polymers. For example, a Tg is only exhibited by an amorphous polymer. On the other hand, crystalline polymer shows both Tm and Tg since they contain an amorphous fragment.

The value of glass transition temperature depends on the mobility of the polymer chain, and most synthetic polymer ranges between 170,000 to 500,000.

The transition from glass to the rubber-like state is an integral feature of the polymer attribute, signifying a region of drastic changes in the physical properties such as elasticity and hardness.

Talking about Tg, the most notable changes are volume, hardness, elongation to break, and Young Modulus of solids.

Hard and rigid plastics like polystyrene and poly(methyl methacrylate) are used below their glass transition temperatures, meaning that when they are in their glassy state, their Tg values are both at around 100 °C (212 °F).

On the contrary, Rubber elastomers like polyisobutylene and polyisoprene are used above their Tg when they are in their rubbery state. This is because they’re soft and flexible in that state, and crosslinking intercepts free for their molecules, thus providing rubber with a fixed shape at room temperature.

The prime application of determining a polymer’s Tg is quality control and research and development. Apart from that, it is also used as an integral tool for modifying plastic materials’ physical properties.

The glass transition temp. For the most mainstream thermoplastics is as follows:

The Tg of polypropylene is -20 and -10.

The Tg of polyethylene is -110 as both minimum and maximum values.

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How to Measure Glass Transition Temperature?

The most common test method to determine the glass transition temperature of plastics is ASTM E1356. The test method assesses the Tg of materials using differential thermal analysis or differential scanning calorimetry.

The test methods are only applicable for amorphous material and crystalline materials that have partial amorphous regions that are stable and do through decomposition in the glass transition region. For both methods, DTA and DSC, their peaks are related to endothermic and exothermic transitions with thermal input and show phase changes.

• Talking about DTA, the difference between the sample and referenced material is observed against temperature or time. While the rise or fall in temprature of the sample in the particular atmosphere is programmed.
• In DSC, the difference in heat flow to a sample and reference material is observed against time or temperature. Thus, the rise or fall in temperature is programmed in a specific atmosphere.

Apart from DTA and DSC, there are several other methods to determine the glass transition temperature of plastic material, such as:

• Thermomechanical analysis 
• Thermal expansion measurement 
• Micro-heat transfer measurement 
• Heat capacity 
• Isothermal compressibility 
• Specific heat measurements

The Difference Between Amorphous and Crystalline Polymers 

Polymers (plastics, elastomers, and rubber) are large macromolecules formed by combining many smaller units called monomers. The molecules can be both amorphous and crystalline.

Glass Transition Temperature Vs. Melting Temperature 

The Factors Affecting Tg 

Chemical Structure:

Molecular Weight – In straight-chain polymers, if there is an increase in molecule weight, it leads to a decrease in chain-end concentration, thereby decreasing free volume at the end group region and increase in Tg.

Polar Groups – Attendance of polar groups increases intermolecular forces, interchain interaction, and cohesion, decreasing the free volume and increasing Tg.

Molecular Structure – Placing a large, inflexible side group increases the Tg of the material due to increased mobility.

Chemical cross-linking – an increase in the cross-linking decreases mobility leading to a decrease in free volume and an increase in Tg.

Water and Moisture Content: 

An increase in moisture content leads to the creation of hydrogen bonds with polymeric chains increasing the distance between them. And thus, increasing the free volume and decreasing Tg. Thus HDPE glass transition temperature is -110, whereas PVC glass transition temperature is 60 in its rigid form.

Addition of Plasticizers:

Adding plasticizers will increase the free volume in the polymer structure. That makes it easier for the polymer chains to slide past each other, resulting in polymer chains moving around at lower temperatures resulting in a decrease in the Tg of a polymer.

Entropy and Enthalpy Effects:

Usually, the value of entropy is higher in amorphous material compared to crystalline material. Therefore, if the entropy value is high, then the value of Tg is also high.

Pressure and Free Volume:

As the pressure increase in the surroundings, it will lead to a decrease in free volume and, ultimately, high Tg.

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Glass Transition Temperature of Mainstream Plastics 

Note – All the data mentioned above in the table is well researched by our team and comes from reliable sources. However, the information is shared strictly for knowledge purposes. Before working with any material, manufacturers should contact their material suppliers for detailed and accurate information about ht material they’re going to work with.

FAQs 

Below are the frequently asked questions on polypropylene glass transition temperature, PVC glass transition temperature, and HDPE glass transition temperature.

How does cooling affect glass transition temperature?

The glass transition temperature will increase or lower viscosities as the cooling rate increases.

What is the meaning of transition temperature?

The temperature at which there is a sudden change of physical properties, such as a change of phase or crystalline structure or at which a substance becomes superconducting, is called transition temperature.

Do metals have glass transition temperatures?

Like plastics and glasses, metals also have glass transition temperatures, but unlike glasses and plastics, many metals crystallize almost immediately after the glass transition temperature is passed.

What is a rule of thumb relating Tg to TM?

The connection between melting and glass transition is already implied by the well-established rule of thumb that Tg ≈ (2/3)Tm.

Why does polyethylene have a lower Tg than polystyrene?

Polyethylene has TG because of its high flexibility and absence of side groups. In addition, PE is semi-crystalline due to its relatively simple and consistent structure.

The Conclusion 

That was all I had to say about the glass transition temperature of polymers. Always have a keen eye on the difference between Tg and Tm because it’s crucial. Talking about the testing methods, whether it’s DTA or DSC, choose wisely to have the best results in the quality control department.

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Have a wonderful day.