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Polymers and composites

When a plasticizer is added to poly(vinyl chloride) (PVC), what happens to the glass transition temperature Tg and the properties of the material?

  1. ATg rises and the material becomes hard and brittle at room temperature
  2. BTg falls and the material becomes soft at room temperature
  3. CTg is unchanged and only the melting point of the crystals falls
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Answer: B. Tg falls and the material becomes soft at room temperature

A plasticizer works its way between the chains, weakening interchain interactions and increasing free volume, so segmental motion becomes easier and Tg falls. Rigid PVC has a Tg of about 80 °C, but adding plasticizer brings Tg below room temperature, giving the flexible PVC used for hoses and wire insulation. Raising Tg to make a polymer harder is done by introducing crosslinks or stiff aromatic rings, or by increasing the molecular weight. PVC has low crystallinity, and the plasticizer acts mainly by lowering the Tg of the amorphous regions, so it is not that only the melting point falls. When the plasticizer migrates to the surface over time, the material hardens or stains its surroundings.

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