Glass Transition Temperature

The glass transition temperature (Tg) is one of the most important thermophysical properties of amorphous polymers. It is sometimes called the “melting point of amorphous materials” and as unscientific as this sounds, it is an appropiate description for the glass transition. In the highly viscous region above the Tg, polymeric materials are soft and rubbery, wheras below the Tg, polymers are hard and brittle. However, there is one important difference between glass transition and melting; melting is a true first-order phase transition, whereas glassification (vitrification) is only a pseudo-second-order transition, that is, melting causes a discontinuity in the first derivative of the Gibbs free energy (volume, entropy), whereas glassification causes a (pseudo) discontinuity in the second derivative of the Gibbs free energy (e.g. heat capacity, expansion coefficient, etc.).

The glass transition is a complex process that is affected by a number of factors, including heating rate, ageing history, morphology, molecular weight. In fact, the true nature of the glass transition is not well understood. Several theories have been developed over the years to explain the glass transition. The theories can be divided into kinetic and equilibrium theories. The kinetic theories consider the glass transition as a dynamic process. Vitrification or glassification is caused by "freezing" the movements of chain segments (kinetic units). Starting at very low temperatures, the first (solid-state) transition occurs when localized bond movements (bending and stretching of bonds) and side chain movements can occur. This is the so called gamma transition (Tγ). As the temperature increases, other localized motions that involve whole side chain and localized group movements are activated and the material starts to develop some toughness. This transition is called beta transition (Tβ). As heating further continues, the Tg is reached. In this region, large scale coordinated motions of the polymer chains occur and a dramatic change in properties is observed.

Another theory treats the glass transition as a true second-order thermodynamic transition. The ideal equilibrium state can, of course, not be reached because it requires an infinite time. The first equilibrium theory of the glass transition was developed by Gibbs and DiMarzio (1955 - 1958). They estimated the changes in conformational entropy with increasing temperature and postulated that the conformational entropy becomes zero when a thermodynamic second-order transition is reached. Below this temperature, all conformations are essentially "frozen".

 

Glass Transition of some Polymers

Polymer Tg (K)a,b
Polypropylene (PP) 260 (239)
Polyethylene (HDPE)* 148* (148, 186, 238)
Polymethylacrylate (PMA) 283 (285)
Polypropyleneglycol (PPG) 198 (205)
Polyvinylacetate (PVA) 303 (316)
Polystyrene (PS) 373 (363)
Polytetrafluoroethylene (PTFE) 390 (384)
Poly(Bisphenol A carbonate) (PC) 447 (435)
Poly(ethylene terephthalate) (PET) 345 (346)

a Sigma-Aldrich
b The Tg's in parenthesis have been calculated with the GIM module of the software 3Ps-Tg from Triton Road.

 

References
  1. JH. Gibbs, E.A.DiMarzio J. Chem. Phys. 28, 373 (1955), 28, 807 (1958)
  2. E.A.DiMarzio, JH. Gibbs, J. of Polymer Sci. 1A: 1417 - 1428 (1963)

 

  • Summary

    Glass Transition

    is a pseudo-second-order-Ehrenfest transition. It is characterized by a discontinuity in the second derivative of the Gibbs free energy (volume, entropy).

  • A glass is characterized by the absence of long-range order (frozen liquid).

  • The glass transition is a complex process that is affected by a number of factors including heating rate, ageing history, morphology, and molecular weight.

  • Below the glass transition temperature, polymers are hard and relative brittle and above it, they are soft and rubbery.

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