the melting temperature

If the glass transition is where a polymer's tangled amorphous regions soften, the melting temperature is where its neatly ordered crystalline regions come apart. Written Tm, it is the polymer's version of ice melting into water — but spread over a range rather than happening at one crisp temperature, because a polymer's little crystals are not all the same thickness or perfection. Above Tm a semicrystalline polymer has lost all its crystalline order and flows as a true melt, which is why Tm is the temperature you must reach to injection-mould or extrude it.

Physically, Tm is where enough thermal energy is available to pull the chain-folded crystallites apart so the ordered stacks dissolve into a mobile liquid. It always sits above the glass transition (Tm is greater than Tg), and its value depends on how strongly the chains attract one another and how stiff the backbone is. Real numbers: high-density polyethylene melts around 130 degrees C, polypropylene around 165 degrees C, PET around 255 degrees C, and nylon-6,6 around 265 degrees C — nylon is high because its chains hold together with strong hydrogen bonds. The melting is a range, not a point, because thinner and more defective crystals melt at lower temperatures than thick, perfect ones.

It matters because Tm sets two practical limits at once: the processing temperature (you must heat above Tm to melt and shape the plastic) and roughly the maximum service temperature (near Tm the crystals soften and the part loses strength). One honest and important clarification: only semicrystalline polymers have a true Tm. A fully amorphous polymer like polystyrene or PMMA (acrylic) has no crystals to melt, so it has no melting point — it simply softens through its Tg and gradually flows. As a rough rule of thumb for polymers that have both, Tg in kelvin is often about one-half to two-thirds of Tm in kelvin.