Glass Transition Temperature (Tg)
Glass transition temperature (Tg) is defined as the temperature at which the internal energy of polymer molecules increases to such an extent that the chain segments of the polymer molecules are just ready to leave their lattice position. At this temperature, the chain segments start moving fast one another even if an infinitesimally small strain is applied to the polymer.
In simple way, the temperature at which a polymer changes from a hard, glassy state to a soft, leathery state, or vice versa is called glass transition temperature. It is directly related to a strength of material and mechanical properties. The glass transition temperature is a thermal property of amorphous as well as semi-crystalline region while crystalline region exhibit a Tm (melt temperature). The value of Tg depends on the mobility of the polymer chain, and for most synthetic polymers its value lies between 170K to 500K.
Below the glass transition temperature, the chain segments of the polymer are frozen on the lattice sites. The polymers in this conditions is hard, invariably brittle and hence, breakable like glass.
Above the glass transition temperature, the segments of the polymer begin to exhibit difussional motion as do the molecules of a liquid.
At temperature sufficiently higher than Tg, the polymer is present in the molten state. In this state there is exchange of bonds between the atoms.
The crossed linked inorganic polymers show a wide range in their glass transition temperatures. In this temperature range, they change from rigid solids to leathery solids, from leathery solids to rubbery solids and finally to highly viscous liquids.
The glass transition temperature of a linear polymer is fairly sharp. This is because the movement of chain segments from one site to another does not involve the exchange of bonds as happened in the case of crossed linked inorganic polymers. The chain in a linear polymers are held together by weak Van der Waal forces which are overcome by the supply of even a small amount of energy. Therefore, the energy required for the movement of chains in such polymers is small. Consequently, the glass transition temperature of linear polymer is lower than of a cross linked polymer in which the movement of chain segments requires the breaking of some bonds.
Glass transition temperature of a polymer can be affected by its chemical structure, molecular weight, degree of crystallinity, and the presence of plasticizers or additives. Amorphous polymers generally have lower Tg values compared to semi-crystalline polymers, which exhibit both Tg and a melting temperature (Tm) due to their crystalline regions
| Parameters and Tg Values | |
|---|---|
| Parameters | Value |
| Higher Molecular weight | Higher Tg |
| Lower Molecular weight | Lower Tg |
| More Crosslinking | Higher Tg |
| Presence of Plasticizers | Lower Tg |
| Bulky Side Groups | Higher Tg |
| Flexible Side Groups | Lower Tg |
| Stronger Intermolecular Forces | Higher Tg |
| Stiffer Chains | Higher Tg |
| More Flexible Chains | Lower Tg |
| More Free Volume | Lower Tg |
Glass transition temperature is measured by using Differential Scanning Calorimetry (DSC) or Dynamic Mechanical Analysis (DMA), where changes in heat capacity, thermal expansion, or mechanical properties are observed as the temperature changes.
Test Your Knowledge
Glass Transition Temperature MCQs
1. What is the primary characteristic of a polymer below its glass transition temperature (Tg)?
A. The polymer is in a molten state.
B. The chain segments exhibit diffusional motion.
C. The polymer is hard, brittle, and breakable like glass.
D. There is an exchange of bonds between atoms.
View Answer
Option C is correct answer.
Below the glass transition temperature, the chain segments of the polymer are frozen on the lattice sites. The polymers in this condition are hard, invariably brittle and hence, breakable like glass.
2. How is the glass transition temperature (Tg) defined in terms of internal energy and chain segments?
A. The temperature at which chain segments become completely immobile.
B. The temperature where internal energy is minimized, leading to a highly crystalline state.
C. The temperature at which internal energy allows chain segments to just leave their lattice position and move with an infinitesimally small strain.
D. The temperature where the polymer melts and turns into a gas.
View Answer
Option C is correct answer.
Glass transition temperature (Tg) is defined as the temperature at which the internal energy of polymer molecules increases to such an extent that the chain segments of the polymer molecules are just ready to leave their lattice position.
3. Which type of polymer region exhibits a melting temperature (Tm) in addition to a glass transition temperature (Tg)?
A. Amorphous region
B. Crystalline region
C. Semi-crystalline region
D. All polymer regions
View Answer
Option C is correct answer.
The glass transition temperature is a thermal property of amorphous as well as semi-crystalline region while crystalline region exhibit a Tm (melt temperature).
4. For most synthetic polymers, what is the range for their glass transition temperature (Tg)?
A. 0K to 100K
B. 170K to 500K
C. 500K to 1000K
D. Above 1000K
View Answer
Option B is correct answer.
For most synthetic polymers its value lies between 170K to 500K.
5. What is the effect of increasing the molecular weight of a polymer on its glass transition temperature (Tg)?
A. Tg decreases
B. Tg remains unchanged
C. Tg increases
D. Tg becomes irrelevant
View Answer
Option C is correct answer.
Higher Molecular weight leads to Higher Tg.
6. Why do linear polymers generally have a lower glass transition temperature (Tg) compared to cross-linked polymers?
A. Linear polymers have stronger intermolecular forces.
B. Movement of chain segments in linear polymers requires breaking of bonds.
C. Linear polymer chains are held by weak Van der Waal forces, requiring less energy for movement.
D. Cross-linked polymers have more free volume.
View Answer
Option C is correct answer.
The chain in a linear polymers are held together by weak Van der Waal forces which are overcome by the supply of even a small amount of energy. Therefore, the energy required for the movement of chains in such polymers is small. Consequently, the glass transition temperature of linear polymer is lower than of a cross linked polymer.
7. Which of the following parameters would lead to a lower glass transition temperature (Tg)?
A. More crosslinking
B. Bulky side groups
C. Presence of plasticizers
D. Stiffer chains
View Answer
Option C is correct answer.
Presence of Plasticizers leads to Lower Tg.
8. What happens to the polymer at temperatures sufficiently higher than Tg?
A. It becomes hard and brittle.
B. It changes from a leathery solid to a rigid solid.
C. It is present in the molten state with exchange of bonds.
D. Its chain segments are frozen on lattice sites.
View Answer
Option C is correct answer.
At temperature sufficiently higher than Tg, the polymer is present in the molten state. In this state there is exchange of bonds between the atoms.
9. Which of the following techniques is commonly used to measure glass transition temperature (Tg)?
A. X-ray Diffraction (XRD)
B. Fourier-Transform Infrared Spectroscopy (FTIR)
C. Differential Scanning Calorimetry (DSC)
D. Nuclear Magnetic Resonance (NMR)
View Answer
Option C is correct answer.
Glass transition temperature is measured by using Differential Scanning Calorimetry (DSC) or Dynamic Mechanical Analysis (DMA).
10. How does the degree of crystallinity affect the glass transition temperature (Tg) of a polymer?
A. Amorphous polymers generally have higher Tg values.
B. Semi-crystalline polymers exhibit only Tg, not Tm.
C. Amorphous polymers generally have lower Tg values.
D. Crystallinity has no effect on Tg.
View Answer
Option C is correct answer.
Amorphous polymers generally have lower Tg values compared to semi-crystalline polymers.