Polymers and Composites Flashcards

Which type of polymerization involves the repeated addition of monomers to an active chain end without byproduct formation?

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.

The degree of crystallinity in a semicrystalline polymer affects which property most directly?

Answer: Density, stiffness, and optical clarity

Higher crystallinity increases density, stiffness (modulus), and opacity, while reducing impact toughness and transparency in semicrystalline polymers.

In particulate composite materials, randomly distributed particles provide:

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.

The critical fiber length in a short-fiber composite is important because fibers shorter than this length:

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.

Kevlar (aramid) fibers are particularly valued for which combination of properties?

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.

Interlaminar shear strength (ILSS) in laminated composites is critical because failure in this mode occurs as:

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.