Composite Materials MCQ with answer and solution

Concept

Polymer structure and remoldability

Formula

NONE

Step-by-step solution

1. Polymeric materials are classified by their thermal behavior into thermoplastics and thermosets. 2. Thermoplastics have linear or branched molecular backbones linked only by secondary van der Waals bonds that separate upon heating, allowing remolding. 3. Thermosetting polymers form rigid 3D covalent cross-linked networks that permanently harden and degrade upon subsequent heating. 4. Therefore, the ability to be remolded upon heating is the distinguishing characteristic of thermoplastics. Answer: A

Explanation

Thermoplastics are composed of linear or branched polymer chains held together by weak secondary intermolecular forces that dissociate upon heating. This allows thermoplastics to be repeatedly softened, reshaped, and solidified upon cooling. In contrast, thermosetting plastics undergo irreversible cross-linking during curing and cannot be remolded.

Why the other options are wrong

B) Densities overlap across polymer types and do not serve as a distinguishing classification criterion. C) A cross-linked structure is exclusive to thermosetting polymers. D) Both conventional thermoplastics and thermosets are non-conductive electrical insulators.

Shortcut

Thermoplastic = Reversible melting and remolding; Thermoset = Permanent irreversible setting.

Common mistake

Incorrectly associating cross-linking with thermoplastics instead of recognizing it as the defining feature of thermosets.