Main Difference – Polyamide vs Polyimide

Polyamide and polyimide are two different polymer compounds. These compounds are made out of monomers, hence they consist of repeating units. They have different mechanical properties based on their structures. Polyamides can be found as synthetic polyamides and natural polyamides. Polyamides are made via the polymerization of diamines and dicarboxylic acid monomers. Polyimides are formed from either the reaction between dianhydride and diamine or, the reaction between dianhydride and diisocyanate. The main difference between polyamide and polyimide is that the monomers used for polyamide manufacture are diamines and dicarboxylic acids whereas the monomers used for polyimide manufacture are either dianhydride and diamine or dianhydride and diisocyanate.

Key Areas Covered

1. What is a Polyamide
– Definition, Different Polyamides, Step-Growth Polymerization
2. What is a Polyimide
– Definition, Different Types, Properties
3. What is the Difference Between Polyamide and Polyimide
– Comparison of Key Differences

Key Terms: Amide, Amine, Kapton, Kevlar, Nylon, Polymerization, Polyamide, Polyimide

What is a Polyamide

A polyamide is a synthetic polymer made by the linkage of an amino group of one molecule and a carboxylic acid group of another. Therefore, a polyamide is a polymer composed of repeating amide linkages (-CO-NH-). Polyamides are either synthetic or natural.

Natural Polyamides

• Proteins
• Wool
• Silk

Synthetic Polyamides

• Nylon
• Aramids (Kevlar)
• carbamide-methanal

The most common and widely used synthetic polyamides are nylon forms. These forms are named based on the number of carbon atoms present in the carboxyl group and the amine group. Ex: in nylon 6, there are 6 carbon atoms in the carboxyl group. In nylon 6,6, there are 6 carbon atoms in the carboxyl group and 6 carbon atoms in the amine group. Synthetic polyamides are formed via step-growth polymerization or solid-phase synthesis.

Step-growth Polymerization for Nylon 6 Production:

In nylon manufacture, the polymerization occurs between an amine group and a terminal carbonyl group. In other words, the monomers for this polymerization are amines and carboxyl acids. Both types of monomers should have two functional groups per monomer for the polymerization to occur.

Figure 1: Reaction between Carbonyl Group and Amine Group

The reaction between these two groups creates a bond between C and N atoms. This occurs with the elimination of a hydroxyl group from the carbonyl group and a hydrogen atom from amine group. Therefore the byproduct of this polymerization is a water molecule.

What is a Polyimide

Polyimides are incredibly strong polymers made from imide monomers. These polymers have many applications due to their high thermal and chemical resistance. Kapton is a classic example for polyimides. The monomers used in this polymerization are pyromellitic dianhydride and 4,4′-oxydianiline.

Classification of Polyimides

• Based on the Main Chain Composition
  • aliphatic
  • semi-aromatic
  • aromatic
• Based on the Type of Interactions Between Polymer Chains
  • thermoplastic
  • thermosetting

Polyimides can be synthesized by several methods. The most common methods among them are the reaction between dianhydride and diamine, and the reaction between dianhydride and diisocyanate.

Figure 2: Polyimide Formation

Among these polyimides, thermosetting polyimides have thermal stability, good chemical resistance, and mechanical properties, etc. These polyimides have a characteristic yellow color. They are well resistant to flame combustion, hence there is no need to mix them with flame retardants.

Difference Between Polyamide and Polyimide

Definition

Polyamide: A polyamide is a synthetic polymer made by the linkage of an amino group of one molecule and a carboxylic acid group of another.

Polyimide: Polyimides are incredibly strong polymers made from imide monomers.

Linkage

Polyamide: Polyamides have repeating amide linkages.

Polyimide: Polyimides have repeating imide linkages.

Monomers

Polyamide: The monomers for polyamide manufacture are diamines and dicarboxylic acids.

Polyimide: The monomers for polyimide manufacture are either dianhydride and diamine or dianhydride and diisocyanate.

Examples

Polyamide: Most common synthetic polyamides are nylon and Kevlar. Natural polyamides include proteins, silk and wool.

Polyimide: A common example for a polyimide is kapton.

Conclusion

Polyamides and polyimides are two types of compounds that often get confused due to their similar names. But they are very different from each other in chemical properties as well in mechanical properties. The main difference between polyamide and polyimide is that the monomers used for polyamide manufacture are diamines and dicarboxylic acids whereas the monomers used for polyimide manufacture are either dianhydride and diamine or dianhydride and diisocyanate.

Reference:

1. Lazonby, John. “Polyamides.” The Essential Chemical Industry online, Available here.
2. “Polyimide.” Wikipedia, Wikimedia Foundation, 30 Jan. 2018, Available here.
3. “Polyimides.” The Polymer Science Learning Center, Available here.

Image Courtesy:

1. By The original uploader was LukeSurl at English Wikipedia Later versions were uploaded by DMacks at en.wikipedia. – Transferred from en.wikipedia to Commons (Public Domain) via Commons Wikimedia
2. “Polyimide Formation (schematic) V1” By Jü – Own work (CC BY-SA 4.0) via Commons Wikimedia

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