Lecture 6: Polyamides, Polyimides

What are engineering plastics?

Plastics with better mechanical or thermal properties than commodity plastics. Examples include PA, PC, POM, PET

What is the key structural group in polyamides?

The amide group –CONH–, which enables strong intermolecular hydrogen bonding.

Mention 2 different synthesis methods of Polyamides

1. Step-growth reaction with diamines and dicarboxylic acids (PA 6.6)

2. Ring-opening polymerization of a lactam ring (PA 6)

How does the –CONH– group affect polyamide properties?

It increases:

• interchain interactions

• melting temperature (Tm)

• mechanical strength

• resistance to swelling and solvents.

How does the distance between amide groups affect properties?

Shorter distance → more hydrogen bonding → higher:

• density

• stiffness (because its getting more difficult to separate the chains)

• strength (because its getting more difficult to separate the chains)

• Tm (because its getting more difficult to separate the chains)

• creep resistance (because its getting more difficult to separate the chains)

• Water absorption (because its getting mooore polar)

Why do polyamides with an even number of methylene groups have higher Tm?

Even numbers allow better chain packing and more regular hydrogen bonding, giving higher crystallinity and Tm.

Why do polyamides absorb water?

Because the amide groups are polar and hydrogen-bond with water.

How does water affect polyamide properties?

Water acts as a plasticizer, which:

• lowers Tg

• reduces strength and modulus

• increases impact strength.

What effect does copolymerization have on polyamides?

It reduces crystallinity and increases the amorphous fraction, giving more flexible and transparent materials.

Why are heat stabilizers added to PA?

To prevent thermal oxidation during processing and use

Why are nucleating agents used in polyamides?

They:

• improve mechanical properties

• speed up crystallization

• enable faster injection molding.

Why are glass fibers added to polyamides?

To increase:

• strength

• stiffness

• dimensional stability

• reduce moisture uptake.

What must be considered when processing PA?

• water absorption

• high melting temperature

• oxidation because of aliphatic CH2 groups

• crystallization rate

• drying before processing.

What are common applications of polyamides?

• fibers (textiles)

• automotive components

What are aramids?

Aromatic polyamides with very high strength and heat resistance.

Examples:

• Kevlar

• Nomex

Why is Kevlar extremely strong?

Because of:

• rigid aromatic backbone

• strong hydrogen bonding

• highly oriented chains.

What properties characterize polyimides?

• very high thermal stability

• solvent resistance

• excellent electrical insulation

• flame resistance.

Why are polyimides difficult to process?

Because their rigid aromatic structure gives very high Tg and thermal stability.

Why are modified polyimides developed?

To improve processability while retaining thermal stability, like Polyamide- imide (PAI)

What are the main advantages of polyamides?

• high strength

• high wear resistance

• good creep resistance

• good chemical durability.

What are the main disadvantages of polyamides?

• moisture absorption

• sensitivity to UV

• limited thermal stability because of aliphatic CH2 chains

• Expensive