Glass Transition Temperature of Polymers: A Complete Guide
Glass transition is a commonly observed behavior of polymers that is useful for many applications. However, this transition takes place with many considerations in place, and understanding the glass transition temperature of polymers is significant.
If you wish to learn more about this subject, the guide below can help you.
What is Glass Transition Temperature of Polymers?
Glass transition temperature (Tg) is the one at which the polymers transform from a glass and brittle state to a state of rubber. If you are wondering about what is Tg, then Tg is a very critical feature of the behavior of the polymer.
It gives the region where the changes take place in the glass in terms of its physical as well as mechanical properties.
Which Type of Polymers Undergo Glass Transition?
The three different types of polymers that undergo a glass transition are discussed in the section below.
1. Amorphous Polymers
Amorphous polymers are composed of a molecular state which is random. When at Tg, the amorphous polymers take the state of glass, which makes them stiff, rigid, and brittle.
When they have a lower tg, they have space between the molecules, which allows movement at low temperatures.
When the temperature rises, the materials tend to get softer in amorphous polymers. Some of the amorphous polymers are PMMA, ABS, PVC, etc.
2. Crystalline Polymers
The crystalline polymers have a molecular structure that is highly ordered. They do not get soft when there is a rise in temperatura and have a narrow punto de fusión.
Such a melting point is usually above the amorphous polymer's upper range. Some of the crystalline polymers are POM, PEEK, and PET.
3. Semi-crystalline Polymers
Semi-crystalline polymers have a mix of ordered and random structures. The order structures limit the movement of polymer chains, which increases the Tg.
Pros and Cons of Polymers
The pros and cons of polymers are given below:
Pros
- Polymers are lightweight and, hence, are suitable for weight-sensitive applications.
- The polymers offer design flexibility and hence can be shaped with versatility even in complex geometries.
- They are corrosion-resistant and do not even deteriorate due to moisture or chemical exposure.
- Polymers have excellent electrical insulation properties and, hence, are suitable for wiring and electronic applications.
Contras
- Polymers have low strength, which limits their use in high-stress applications.
- Polymers are heat-sensitive as they usually have low melting points and may not be suitable for environments with high temperatures.
- The use of polymers in harsh conditions or UV rays, in the long run, may cause them to degrade.
- Certain polymers are sensitive to some chemicals, which cause them to dissolve or crack.
What are the Examples of Polymers with High or Low Tg?
Polymers with High Tg
Polymers like polymethyl methacrylate and polystyrene have high Tg, and they are brittle and hard.
Polymers with Low Tg
Polymers like polyisobutylene and polyisoprene have their Tg lower than room temperature; hence, they are soft and flexible.
What is the Difference Between Tg and Tm?
Tg is the glass transition temperature whereas melting point is represented by Tm.
- Tg defines the way the amorphous region of the semi-crystalline plastic goes from a glass-like state to a rubber-like state.
- Tm is the temperature at which the crystalline regions in the semi-crystalline plastic turn from solid to liquid state.
What are the Factors Affecting Tg?
Some of the factors affecting the Tg are as follows:
Chemical Structure
- In a polymer consisting of a straight chain, the increase in molecular weight decreases the concentration on the end of the chain, and hence the Tg increases.
- If the molecular structure is bulky, a decrease in mobility is observed, which increases the Tg of the material.
- In cases where there is an increase in cross-linking, the mobility of the polymer decreases, which increases the Tg.
- In case polar groups are present, the intermolecular forces are high, which decreases the free volume and results in high Tg.
Addition of Plasticizers
The addition of plasticizers increases the space between the chains of polymers, which creates space. Such increases allow the chains to slide past, and hence, there is a decrease in Tg.
Water or Moisture Content
Where there is a high moisture content in the polymeric chains, then a decrease in performance appears, which decreases Tg.
Effect of Entropy and Enthalpy
In amorphous polymers, the entropy value is high, and in crystalline polymers, it is low; in case the entropy value is high, Tg is even higher.
Pressure and Free Volume
Due to the increase in pressure, the free volume decreases, and hence the Tg is high.
Other Factors Governing Tg
Other factors which impact the glass transition of polymers are as follows:
- The length of the alkyl chain
- Thickness of film
- Interaction of bond
- Flexibility in the polymer chain
- Branching of chains
What are the Methods to Determine Tg?
The three methods of determining the Tg are as follows:
1. Differential Scanning Calorimetry (DSC)
DSC is a thermo-analytical method that uses a calorimeter with differential scanning. It monitors the change in the heat flow between reference and sample compared to the temperature or time.
It also determines the polymer’s thermal properties and is used for the amorphous regions of stable polymers.
2. Differential Thermal Analysis (DTA)
DTA is also a thermal analysis method. It measures the material’s Tg. The material goes through multiple cycles of heating and cooling, and a difference is determined between the sample and reference temperature.
The temperature is maintained through the different heat cycles, ensuring that the testing environment has constant conditions.
3. Dynamic Mechanical Analysis (DMA)
DMA analyzes the material stiffness using a mechanical analyzer. The mechanical stress applied on the sample and the instrument measures the strain. It helps to evaluate the glass transition, behavior of stiffness and also the crystallinity degree.
Conclusión
Glass transition temperature is a technical mechanism and understanding it is very crucial for choosing the polymers for different applications.
You can get in touch with us at DEK if you are planning to have components manufactured using different polymers and need high quality.
Preguntas frecuentes
What are the units of glass transition temperature?
The glass transition temperature units are Fahrenheit, Celsius, and Kelvin.
Why is it important to identify the Tg of polymers?
Tg determines the region where the change in physical and mechanical properties of a polymer takes place.
What happens to a polymer at the glass transition temperature?
At the glass transition temperature, the polymer changes its state from brittle to rubber state.
