Difference Between Glass Transition Temperature and Melting Temperature

Main Difference – Glass Transition Temperature vs Melting Temperature

Glass transition temperature and melting temperature are two chemical terms that are often confusing. Glass transition temperature is discussed under polymer chemistry because this transition can be observed in polymer compounds. But melting temperature can be observed in any compound. The main difference between glass transition temperature and melting temperature is that glass transition temperature describes the transition of a glass state into a rubbery state whereas melting temperature describes the transition of a solid phase into a liquid phase.

Key Areas Covered

1. What is Glass Transition Temperature
      – Definition, Factors Affecting the Temperature
2. What is Melting Temperature
      – Definition, Factors Affecting the Temperature
3. What is the Difference Between Glass Transition Temperature and Melting Temperature
      – Comparison of Key Differences

Key Terms: Amorphous, Crystalline, Freezing Point, Glass Transition Temperature, Melting Temperature, Polymer, Semi-Crystalline, Thermosetting Polymers

What is Glass Transition Temperature

Glass transition temperature is the temperature at which a hard glassy state of an amorphous material is converted to a rubbery state. This term is discussed regarding polymer compounds since polymers, especially thermosetting polymers, can undergo this glass transition. The short term for glass transition temperature is tg.

The glassy state of a thermosetting polymer is very hard and rigid. The rubbery state is very viscous and pliable. Pure crystalline polymers do not have a glass transition temperature. Only amorphous polymers and semi-crystalline polymers show this property. Pure amorphous polymers have only the glass transition temperature.

Factors Affecting Glass Transition Temperature

• Chemical structure of the polymer – main structure, pendant groups, crosslinking, polymer chain polarity, etc. Presence of bulky pendant groups increases the tg because bulky groups cause the increase of amorphous nature. Crosslinking increases tg because crosslinks restrict the rotational motion of polymer chains.
• Molecular weight of compound – glass transition temperature is directly proportional to the molecular weight.
• Plasticizers – these are the compounds added to polymer material to improve the properties. Plasticizers increase the tg due to the reduction of cohesive forces between polymer chains. It increases the amorphous nature of the polymer.
• Flexibility – flexibility is inversely proportional to the tg of the compound.

Figure 01: Glass Transition Temperature

Each polymer with an amorphous structure has its own unique glass transition temperature. Different glass transition temperatures of different polymers allow them to be used for suitable applications depending on this. For example, a rigid material with a lower glass transition temperature is suitable for high-temperature applications.

What is Melting Temperature

Melting temperature is the temperature at which a solid material is converted into its liquid form. In other words, this is the temperature that causes a solid to melt.  Here a phase transition of matter occurs. In this melting temperature or the melting point of a substance, the solid phase and the liquid phase exist in equilibrium.

Figure 2: Melting Point

The melting temperature can also refer to the freezing point. This is because when the temperature of a liquid gradually decreases, the liquid is converted into its solid phase at the same temperature. But sometimes they can be different from each other because the solid formation can happen via different crystal patterns.

At the melting temperature of a substance, the entropy increases since the tightly packed molecules of that solid substance are released. The melting temperature is highly dependent on the pressure. Therefore, the melting point of a substance is given at a specific pressure, i.e., standard pressure.

Figure 3: Phase Diagram of Water

Factors Affecting Melting Temperature of a Substance

• Pressure – The pressure has a direct effect on the melting temperature. Higher the pressure, higher the melting temperature.
• Chemical bonding – In compounds having strong chemical bonds between molecules, the melting temperatures are higher.
• Shape and size of molecules – Substances with smaller molecules melt easily. The shape of molecules affects the packing of molecules inside a substance. So the shape also affects the melting temperature.

Difference Between Glass Transition Temperature and Melting Temperature

Definition

Glass Transition Temperature: Glass transition temperature is the temperature at which a hard glassy state of an amorphous material is converted to a rubbery state.

Melting Temperature: Melting temperature is the temperature at which a solid material is converted into its liquid form.

Transition

Glass Transition Temperature: Glass transition temperature describes the transition of a glass state into a rubber state.

Melting Temperature: Melting temperature describes the transition of a solid phase into a liquid phase (phase transition).

Substances

Glass Transition Temperature: Glass transition temperature can be observed in amorphous and semi-crystalline compounds.

Melting Temperature: Melting temperature can be observed in crystalline substances.

Factors 

Glass Transition Temperature: Glass transition temperature depends mainly on the chemical structure of the substance.

Melting Temperature: Melting temperature depends mainly on the chemical bonding of molecules in the substance and the external pressure.

Conclusion

Glass transition temperature can be observed in amorphous and semi-crystalline polymer compounds. Melting temperature can be observed in crystalline compounds. But the main difference between glass transition temperature and melting temperature is that glass transition temperature describes the transition of a glass state into a rubbery state whereas melting temperature describes the transition of a solid phase into a liquid phase.

References:

1. “Glass transition temperature Tg.” AdhesiveandGlue.com, Available here.
2. “What is a Glass Transition Temperature? – Definition from Corrosionpedia.” Corrosionpedia, Available here 
3. “Melting point.” Wikipedia, Wikimedia Foundation, 11 Nov. 2017, Available here.

Image Courtesy:

1. “Melting Ice Cubes” by jar [o] (CC BY 2.0) via Flickr
2. “Phase Heat Diagram” By NipplesMeCool at English Wikibooks – Transferred from en.wikibooks to Commons., (Public Domain) via Commons Wikimedia