ME Polymers and Composites 2 — Questions and Answers
Question 1: Which type of polymerization involves the repeated addition of monomers to an active chain end without byproduct formation?
- Condensation polymerization
- Addition (chain-growth) polymerization (Correct answer)
- Ring-opening polymerization
- Ionic polymerization
Correct answer: Addition (chain-growth) polymerization
Addition polymerization occurs when monomers with double bonds (e.g., ethylene) add sequentially to a growing chain end initiated by a free radical, cation, or anion.
Question 2: The degree of crystallinity in a semicrystalline polymer affects which property most directly?
- Glass transition temperature only
- Density, stiffness, and optical clarity (Correct answer)
- Electrical conductivity
- Thermal expansion direction
Correct answer: Density, stiffness, and optical clarity
Higher crystallinity increases density, stiffness (modulus), and opacity, while reducing impact toughness and transparency in semicrystalline polymers.
Question 3: In particulate composite materials, randomly distributed particles provide:
- Highly anisotropic properties
- Isotropic property improvements (Correct answer)
- No improvement in stiffness
- Directional electrical conductivity
Correct answer: Isotropic property improvements
Random particle distribution makes particulate composites isotropic, meaning properties are the same in all directions, unlike fiber composites which are anisotropic.
Question 4: The critical fiber length in a short-fiber composite is important because fibers shorter than this length:
- Cause delamination
- Cannot be loaded to their full tensile strength (Correct answer)
- React chemically with the matrix
- Increase composite density
Correct answer: Cannot be loaded to their full tensile strength
Below the critical fiber length, shear stress transfer from matrix to fiber is insufficient to load the fiber to its full tensile strength, reducing reinforcement efficiency.
Question 5: Kevlar (aramid) fibers are particularly valued for which combination of properties?
- High modulus and high density
- High tensile strength, low density, and energy absorption (Correct answer)
- High electrical conductivity and flexibility
- High thermal conductivity and hardness
Correct answer: High tensile strength, low density, and energy absorption
Aramid fibers like Kevlar have excellent tensile strength, low density, and high toughness, making them ideal for ballistic protection and lightweight structural applications.
Question 6: Interlaminar shear strength (ILSS) in laminated composites is critical because failure in this mode occurs as:
- Fiber fracture perpendicular to the load
- Delamination between plies due to shear (Correct answer)
- Matrix microcracking within plies
- Debonding at fiber ends only
Correct answer: Delamination between plies due to shear
ILSS governs delamination resistance; low ILSS allows plies to separate under shear loading, which is a common and catastrophic failure mode in laminated composites.
ME Polymers and Composites 2
Which type of polymerization involves the repeated addition of monomers to an active chain end without byproduct formation?