Polyethylene terephthalate is a semicrystalline thermoplastic polyester built of terephthalate units alternating with ethylene glycol residues ^[1]. Each repeat unit carries a rigid aromatic ring and a flexible glycol segment, so the chain is stiff enough to crystallise yet mobile above the glass transition near 78 degrees Celsius ^[2]. Crystallinity develops between roughly 130 and 240 degrees Celsius and governs density, barrier behaviour and clarity, which is why amorphous and crystallised grades differ in the same properties ^[1]. Melting near 255 degrees Celsius bounds the processing window, and the ester linkage is the point at which hydrolysis and transesterification act ^[3]. Copolymers that replace the glycol or the diacid segment shift both transitions and are distinguished from the homopolymer ^[2].