Problem 9 - Entrance Test

Thermoplastics (like polyethylene, polypropylene) are characterized by linear or branched polymer chains held together by relatively weak intermolecular forces, allowing them to soften upon heating and harden upon cooling, making them recyclable. Thermosetting plastics (like Bakelite, melamine) form extensive three-dimensional cross-linked networks during curing, which makes them rigid and prevents them from being remelted or reformed once hardened. Option D is incorrect. Thermosetting plastics are generally much stronger, more rigid, and more brittle than thermoplastics because of their extensive cross-linking, which restricts chain movement significantly. Thermoplastics vary in strength and rigidity; some are quite strong, but generally, thermosets excel in these properties, especially at high temperatures. The 'higher degree of crystallinity' refers more to specific thermoplastics and doesn't make them inherently stronger or more rigid than a highly cross-linked thermoset.