Glass Transition Temperature (Tɡ): Definition, Significance and Factors Affecting

What is glass transition temperature?

When plastic or rubber is cooled up to a certain temperature, it becomes so hard and brittle that it breaks into pieces on the application of stress. So, the temperature below which the polymer becomes hard, brittle, and glassy and above which it is softener and flexible is known as glass transition temperature (Tɡ).

Glass transition temperature (Tɡ)
The glass transition is a property of only the amorphous region of a semi-crystalline solid whereas the crystalline portion remains crystalline during the glass transition.

It is important to note that the transition does not occur suddenly at a unique temperature but rather over a range of temperatures. The temperature in the middle of the inclined region is taken as the Tɡ.

Significance of glass transition temperature (Tg)

  1. Glass transition temperature (Tɡ) is used as a measure for evaluating the flexibility of a polymer and the type of response the polymeric material would exhibit to mechanical stress.
  2. Glass transition temperature (Tɡ) tells us that, above it, a polymer will be soft and flexible and exhibit viscoelasticity (Delayed elastic response) and below it, a polymer will be hard and brittle and will possess dimension stability.
  3. The glass transition temperature (Tɡ) value along with the Tm value indicates the temperature region at which a polymeric material transforms from a rigid state to a soft viscous state. This helps in choosing the right processing temperature which is the temperature region in which the material can be converted into the finished products through different processing techniques such as molding calendaring and extrusion.

Factors affecting glass transition temperature (Tg)

1. Chain flexibility and rigidity

Stiffening groups in the polymer chain reduce the flexibility of the chain and increase the value of glass transition temperature (Tɡ). In other words, we can say that the greater the intrinsic chain flexibility, the smaller will be the value of glass transition temperature (Tɡ).

2. Intermolecular Forces

Stronger intermolecular forces due to dipole forces, H-bonding, etc decreases the mobility of the chain which increases the value of Tɡ. For example, Tɡ of PVC is greater than Tɡ of polypropylene because PVC has stronger intermolecular forces than polypropylene because of the dipole-dipole forces from the C-Cl bond.

3. Molecular weight

Glass transition temperature (Tɡ) is directly proportional to the molecular weight of the polymer that is when the molecular weight of the polymer increases, Tɡ also increases.

4. Pendant Groups

Pendant groups have a considerable amount of effect on chain mobility. Bulky pendant groups like benzene can catch on neighboring chains like a "fish hook" and restrict rotational freedom which increases the glass transition temperature (Tɡ).

For example, a Big group like the adamantyl group will not only act as a "fish hook" but also it will act as a "downright boat anchor" that will increase the Tɡ.

Tɡ of poly(ether ketone) with an adamantyl group is 225 ℃ which is greater than the Tɡ of poly(ether ketone) that does not have an adamantyl 
group.

5. Cross-linking

The greater the degree of cross-linking, the higher will be the Tɡ because the presence of cross-links between chains restricts rotational motion.

6. Plasticizers 

Plasticizers are low molecular weight compounds that are added to plastics to increase their flexibility and workability. This weakens the intermolecular forces between the polymer chains and increases the mobility thus decreasing the glass transition temperature (Tɡ).

7. Copolymerization

Random copolymers have lower Tɡ since it tends to promote the disorder, reduce molecular packing, and reduce the inter-chain forces of attraction.

Determination of Glass transition temperature (Tg)

Glass transition temperature (Tɡ) can be determined by various methods like thermal analysis, Dynamic mechanical behavior, Broad-line NMR, Dielectric loss method, and Differential scanning calorimetry (DSC).

The differential Scanning calorimetry (DSC) method is the most common method to calculate glass transition temperature (Tɡ).

Popular Posts

Conducting Polymers: Definition, Examples, Properties and Applications

Image
What are Conducting Polymers? As the name suggests organic polymers that conduct electricity are known as conducting polymers. They are also known as intrinsically conducting polymers (ICPs) and they have alternating single and double bonds along the polymer backbone (conjugated bonds) or that are composed of aromatic rings such as Phenylene, naphthalene, anthracene, pyrrole, and thiophene which are connected through carbon-carbon single bonds. Examples: Polyacetylene, Polypyrrole, Polyaniline, etc What is the reason behind conducting nature of these polymers? Conducting polymers comes in two forms that are doped conducting polymers and non-doped conducting polymers. The conductivity of non-doped conjugated polymers is due to the existence of a conductivity band similar to a metal. In a conjugated polymer, three of the four valence electrons form strong sigma bonds through sp² hybridization where electrons are strongly localized. The remaining unpaired electron of each carbon atom re
Read more

Crime Scene: Definition, Types and Characteristics

Image
What is a Crime Scene ? A place where the crime is committed or where the maximum physical evidence related to crime is found is known as a crime scene. A crime scene is a starting point of the investigation which provides information about the suspect and the victim. This helps to reconstruct the crime and fast resolution of the case. It is noted that the crime scene is not limited to a single place but may extend to a wider area depending upon the nature of the crime committed. For example, In a murder case where murder is done at one place and the body is disposed on another place. In this case, we have two crime scenes that give information about the crime. Types of Crime Scene Based on evidence found on the crime scene: 1. Primary Crime Scene The crime scene where the actual crime occurred or where more usable pieces of evidence were found is known as the primary crime scene. For example, A murder scene, theft, assault, etc. 2. Secondary Crime Scene The crime scene which is some
Read more

Documentation of the Crime Scene: Step by Step

Image
What is documentation of the crime scene and Why it is done? After securing the crime scene, the next most important step is documentation of the crime scene because documentation permanently records the crime scene and its physical evidence. Documentation should be done without any wastage of time as the scene and its evidence may get altered over time. The documentation should be done innovatively and the originality of the crime scene should be maintained and documentation should not have pauses or breaks, it should remain constant till the whole crime scene is documented. There are four main tasks of documentation that is note-making, photography, videography , and sketching . It should be noted that all four tasks are necessary and none is a substitute for another. For example, notes are not substitutes for photography, video is not a substitute for sketching, etc. Following are the sequence for crime scene documentation: 1. Note making Documentation of crime scene in the f
Read more