a semicrystalline polymer
Because a polymer's tangled chains can never all line up, a crystallizable polymer freezes into a two-in-one material: tiny crystals embedded in a sea of disordered chains, with individual molecules threading from one region into the other. This mixed state — part ordered, part disordered, coexisting in the same solid — is what we mean by a semicrystalline polymer. It is not a crystal with a few flaws and not an amorphous solid with a few crystals; it is a genuine composite of two phases interwoven at the scale of nanometres.
The standard picture is a stack of thin chain-folded crystalline lamellae (each perhaps 10-20 nm thick) separated by thin amorphous layers of tangled chain. The crucial feature is that a single long chain does not stay in one place: it can run through a crystalline lamella, emerge into the amorphous layer, wander a while, and enter the next lamella, so chains physically stitch the crystals together. The chains that bridge from one crystal to the next across an amorphous gap are called tie molecules, and they carry load between the hard crystals. Whether the amorphous layer is rubbery or glassy depends on temperature: above the glass transition (Tg) it is soft and mobile; below Tg it is a rigid glass. So a semicrystalline polymer really has two structural transition temperatures — a glass transition for its amorphous part and a melting point for its crystalline part.
This two-phase architecture is why semicrystalline plastics are so useful: the stiff crystalline lamellae provide strength, modulus, and resistance to heat and chemicals, while the amorphous ties and their entanglements provide toughness, impact resistance, and the ability to stretch without shattering. Get the balance right and you have polyethylene, polypropylene, PET, and nylon — the workhorse plastics of fibres, bottles, films, and engineering parts. It also explains their milky, translucent look: the crystalline and amorphous regions have different densities and refractive indices, so light scatters at their boundaries and at the spherulites the lamellae build.
Stretch a piece of semicrystalline polyethylene and it necks and turns white before it breaks: the crystalline lamellae are being pulled apart and re-oriented while the tie molecules bridging them stretch taut and take the load. That cooperation of hard crystals and load-bearing amorphous ties — impossible in a single-phase material — is exactly what makes these plastics both stiff and tough.
A semicrystalline polymer is a two-phase solid: crystalline lamellae plus amorphous tie regions, stitched by shared chains.
Semicrystalline is not amorphous-with-defects nor crystal-with-defects: it is two genuine coexisting phases. That is why such a polymer shows both a glass transition (amorphous part) and a melting point (crystalline part), at different temperatures.