Polymer Liquid & Solid State

Small Molecules

• The equilibrium condition of the solid state is often crystalline, where the molecules experience vibrations of small amplitude about their equilibrium positions in a regular array.
• During melting, thermal energy induces translational motion. The system has random close packing, usually with a slightly smaller density in the melt than in the crystal. The melt retains a fairly well-defined average number of nearest neighbors.
• With sufficient thermal energy, the melt vaporizes, yielding a system of much lower density.
• Melting and vaporization are first-order phase transitions. Such transitions are accompanied by a discontinuity in thermodynamic quantities defined by a first derivative of the free energy, such as

  V = (d G/d P)_T

  The isobaric V vs. T curve shows discontinuities at two temperatures. These two discontinuities are observed at the same temperature upon heating and cooling, if the temperature changes are slow. Both transitions involve a latent heat.

• Fast cooling may result in different behavior than slow cooling, if the stable solid phase is crystalline.
  • Fast cooling may allow insufficient time for crystallization. The system is trapped in an amorphous state, a glass, below the glass transition temperature, T_g.
  • There is no discontinuity in V at T_g. There is, however, a discontinuity in properties that depend on the second derivative of the free energy, implying that the change may be a second order phase transition. Alternatively, the system may form a metastable liquid, where motion is so slow as to by undetectable on the "human" time scale.
  • The system is not in equilibrium below T_g, because its properties depend on age and prior history. The glass can be converted to the crystal by heating above T_g, followed by slow cooling.

Polymers vs. Small Molecules

• The gas is inaccessible to polymers. The thermal energy required for separation of the large molecules exceeds the energy required for the rupture of covalent bonds. Therefore the polymer experiences thermal degradation before vaporization.
• Non-crystalline polymer:
  • Slightly above T_g, it looks like a solid, but is deformable
  • Further above T_g, it behaves more like a liquid, but with some elasticity
  • A few covalent crosslinks will produce rubber-like elasticity, where the system can experience a large deformation in response to a stress, and then return to its initial state when the stress is removed.
• Crystallizable polymer: Melting may yield a partially ordered liquid crystalline state, where some order persists.

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