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Thermoplastics: Linear/branched chains held by intermolecular forces (van der Waals, H-bonds, dipole-dipole). Heating weakens these forces → softens → can be remolded. Cooling restores rigidity. Reversible → recyclable. Examples: PE, PP, PVC, PS, nylon-6,6, Terylene, PMMA. Thermosetting: 3D covalent cross-linked network formed during curing (heating under pressure). Once set, cannot soften — further heating causes decomposition. Irreversible → NOT recyclable by melting. Examples: Bakelite (phenol-HCHO), melamine resin, urea-HCHO, epoxy resin, vulcanized rubber (lightly cross-linked = elastomer, not rigid thermoset). Elastomers: Special category — lightly cross-linked, can stretch 500-1000%, snap back. Tg well below room temperature. Examples: vulcanized rubber, Buna-S, neoprene, silicone rubber. Fibers: Very strong intermolecular forces → high tensile strength → can be drawn into threads. Examples: nylon, Terylene, Orlon. JEE trap: nylon-6,6 is a strong fiber but it's a THERMOPLASTIC (not thermosetting) — it can be remelted.