Fiber Properties

Step 1: Recall the key properties of polyamide fibers (e.g., Nylon). Nylons are known for their exceptional strength, toughness, and abrasion resistance.

Step 2: Evaluate each stated feature.
- A. Good dimensional stability: Polyamides are thermoplastic and can be heat-set to provide excellent dimensional stability. They don't shrink or stretch easily after being properly heat-set. This is a correct feature.
- B. High wet modulus: Modulus is a measure of stiffness or resistance to extension. Polyamides retain a significant portion of their stiffness when wet compared to fibers like rayon. This is a correct feature.
- C. High resistance to alkali: The amide linkage (-CO-NH-) in polyamides is stable and resistant to attack by alkalis. They can be scoured and washed in alkaline solutions without significant damage. This is a correct feature. (Note: They have poor resistance to strong acids).
- D. Low strength and high elongation: This is incorrect. Polyamides are known for their high strength (tenacity) and controlled elongation. They are among the strongest synthetic fibers.

Step 3: Combine the correct features. The outstanding features are A, B, and C. Feature D is incorrect. Therefore, the correct option is (B).

Step 1: Identify the key properties and applications of each fiber.
- A. Silk fibre: Silk has a unique triangular cross-section that reflects light like a prism, giving it a deep, rich luster that changes with the viewing angle. This is known as directional lustre.
- B. Wool fibre: The crimpy, bulky nature of wool fibers allows them to trap a large amount of air. Trapped air is an excellent insulator, which is why wool is known for its warmth and is used for thermal insulation in blankets and winter clothing.
- C. Nomex fibre: Nomex is a brand name for a meta-aramid fiber. Its primary characteristic is its excellent thermal stability and inherent flame resistance. It does not melt or drip and chars at high temperatures. It is used in protective clothing for firefighters and race car drivers. This corresponds to fire retardant.
- D. Kevlar fibre: Kevlar is a brand name for a para-aramid fiber. It has extremely high tensile strength and modulus for its weight. This exceptional strength and toughness make it ideal for use in ballistic protection, such as bulletproof vests.

Step 2: Match the fibers in List I with their applications in List II.
- A (Silk) II (Directional lustre).
- B (Wool) IV (Thermal insulation).
- C (Nomex) I (Fire retardant).
- D (Kevlar) III (Bulletproof).

Step 3: Assemble the combination. The correct matching is A-II, B-IV, C-I, D-III. This corresponds to option (C).

Step 1: Understand crease recovery. Crease recovery (or wrinkle resistance) is the ability of a fiber to return to its original shape after being bent or crushed. Cotton is known for its poor crease recovery; it wrinkles easily.

Step 2: Analyze the molecular structure of cotton. Cotton is composed of cellulose, which consists of long, linear polymer chains. These chains are highly crystalline, meaning they are packed tightly together in an ordered arrangement. The forces holding these chains together are hydrogen bonds.

Step 3: Relate the structure to the property. When cotton is creased (especially in the presence of moisture), the hydrogen bonds between the cellulose chains break. The chains slip past one another into a new, wrinkled position. In this new position, they form new hydrogen bonds, locking the wrinkle in place. Because the hydrogen bonds are relatively weak and there are no strong cross-links (like in wool) to pull the chains back, the fiber does not recover its original shape.

Step 4: Evaluate the options.
- (A) Strong lateral linkages: Incorrect. If cotton had strong cross-links, it would have good crease recovery.
- (B) Flexible polymer chain: The cellulose chain itself is quite rigid due to its ring structure.
- (C) Weak lateral linkages: Correct. The hydrogen bonds are weak enough to break and reform easily, allowing the chains to slip and set into a creased configuration.
- (D) Breakage of polymer chains: This would be permanent fiber damage, not creasing.