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Polymers and composites

Which statement about the glass transition temperature Tg and melting point Tm of a crystalline (semicrystalline) polymer is correct?

  1. ATg is higher than Tm
  2. BTg is the temperature at which chain motion in the amorphous regions freezes or is released, and it is lower than Tm
  3. CTg is the temperature at which the crystalline regions melt, and amorphous polymers, which have no crystals, have no Tg
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Answer: B. Tg is the temperature at which chain motion in the amorphous regions freezes or is released, and it is lower than Tm

A crystalline polymer consists of crystalline and amorphous regions. Tg is the temperature at which segmental motion of the chains in the amorphous regions freezes or is released; below Tg the polymer is hard and glassy, above it rubbery. Tm is the temperature at which the crystalline regions melt; Tg is always lower than Tm, and for many polymers Tg/Tm (as a ratio of absolute temperatures) is about 0.5–0.8. PET, for example, has a Tg of about 70–80 °C and a Tm of about 260 °C. It is Tm that represents melting of the crystalline regions; amorphous polymers such as polystyrene have no Tm, but they do have a Tg. Polymers such as polyethylene, whose Tg is below room temperature, are flexible because their amorphous regions are rubbery at room temperature.

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