Glass Transition, Crystallinity and Entanglement in Polymers · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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When plastics turn stiff or soft

Science · Chemistry · ages 23-24
Name ______________________   Date ____________

  1. A polymer's DSC heating trace has a heat-capacity baseline step near 65 C and a heat-release peak near 110 C. Which feature marks Tg?

    • The heat-release peak near 110 C, because every thermal peak marks Tg
    • The heat-capacity baseline step near 65 C
    • Both features together, because Tg requires a step followed by a peak

  2. What changes in a polymer's amorphous regions as it warms through Tg?

    • Segment motion becomes faster and those regions soften
    • Covalent chains break into shorter molecules throughout the sample
    • Every crystalline region melts as soon as the glass transition starts

  3. For comparable linear-polymer melts at the same temperature, zero-shear viscosity in the Newtonian regime generally rises more steeply with molar mass above Mc.

    Circle one:   True   False

  4. Two linear-polymer melts have the same chemistry, similarly narrow molar-mass distributions and the same temperature. Compare their zero-shear viscosities in the Newtonian regime. One is below Mc and one well above Mc. What is expected?

    • The higher-molar-mass melt generally has greater zero-shear viscosity
    • The higher-molar-mass melt has lower zero-shear viscosity because entanglements speed relaxation
    • The melts must have identical zero-shear viscosity because their repeat-unit chemistry is the same

  5. Polymer A has Tg = 10 C. Polymer B has bulky side groups and measured Tg = 80 C. At 25 C, which polymer has glassy amorphous regions on the measurement timescale?

    • A, because being above its measured Tg makes its amorphous regions glassy
    • Both, because the same test temperature gives them identical segment mobility
    • B, because 25 C is below its measured Tg

  6. Why is a polymer's glass transition different from first-order equilibrium melting?

    • It breaks covalent bonds throughout the chains instead of changing segment mobility
    • It is a kinetic mobility change without the latent heat of melting
    • It melts only small crystals while every amorphous segment remains immobile

  7. A polar backbone must always give a higher Tg than a nonpolar backbone, regardless of flexibility, packing or other structural differences.

    Circle one:   True   False

  8. A partly crystalline polymer's heating scan shows a Tg step and a melting peak, but no separate crystallisation peak. Which interpretation is possible?

    • The missing crystallisation peak proves the sample contained no crystals before heating
    • A melting peak requires new crystals to form earlier in that same heating scan
    • Existing crystals can melt without a separate crystallisation peak
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Answer key

For grown-ups. Fold this page away before handing over the rest.

When plastics turn stiff or soft W1-mt_vGEYO5ILqt-s1

  1. The heat-capacity baseline step near 65 C · Tg usually produces a baseline step through a heat-capacity change. The separate heat-release peak is consistent with crystallisation, not an additional requirement for Tg.
  2. Segment motion becomes faster and those regions soften · Tg concerns segment mobility in amorphous regions. It does not mean chain breakdown or melting of crystalline regions.
  3. True · The statement concerns the low-rate Newtonian plateau. In that regime, entanglements give a steeper molar-mass dependence above Mc, not an abrupt viscosity jump.
  4. The higher-molar-mass melt generally has greater zero-shear viscosity · Under these comparable conditions, the higher-molar-mass melt generally has greater zero-shear viscosity. Entanglements slow relaxation; the comparison does not use viscosities measured during finite-rate shear thinning.
  5. B, because 25 C is below its measured Tg · 25 C is above A's Tg but below B's Tg. B therefore has glassy amorphous regions. The measured values, rather than side-group bulk alone, support this answer.
  6. It is a kinetic mobility change without the latent heat of melting · Tg is tied to segment motion and observation timescale. It has a heat-capacity change, not the latent heat associated with equilibrium melting.
  7. False · Polar interactions can raise Tg when they restrict segment motion, but flexibility, packing and other structural differences also matter. Polarity alone gives no universal ranking.
  8. Existing crystals can melt without a separate crystallisation peak · The sample can contain crystals before heating begins. They can melt without enough new crystallisation during the scan to produce a separate detectable peak.
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