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Chemistry · L5 · Tg vs Tm: Two Different Transitions

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Why a fully amorphous polymer shows only a glass transition while a semicrystalline polymer shows both a Tg and a true first-order Tm, with a leathery region in between where crystallites act as physical cross-links.

by @openstemUpdated Chemistry
Heat a polymer sampleAmorphous orsemicrystalline?Only Tg observed —no crystalline fraction to meltBoth Tg and Tm observedBelow Tg: rigid glassAbove Tg: rubberyBelow Tg: amorphous regionsfrozen, crystallites rigidBetween Tg and Tm: leathery —crystallites act as physical cross-linksAt Tm: crystallites melt(first-order, latent heat)Above Tm: fully viscous meltFully amorphous (e.g. atactic PS, PMMA)Semicrystalline (e.g. HDPE, PET)

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