Different Phases in Polymeric Materials:

Unlike metals and ceramics, the solid phase of polymers are characterized by four major phases: Glassy Phase, Transition Phase, Elastomeric Phase and Rubbery Flow region. Each phase has distinct mechanical properties related to the microstructure of the Polymer. However, the extent up to which these phases are expressed varies from polymer to polymer. The figure below shows various phases:

When the temperature is well below the glass transition temperature, you will obtain the glassy-phase of the polymer. Depending on the cooling rate, for some polymers you may also find a semi-crystalline and semi-amorphous phase. Often, the polymers in the glassy state, show good mechanical properties like higher modulus, strength and hardness. The polymer in this stage shows a high resistance to flow. The modulus of elasticity at this stage may be defined by the following relationship.

ε_p = f(σ/E1)+(1-f)((σ/E2)
E= σ/ε = {f/E1 + (1-f)/E2)-1
where f is the fraction of stiff covalent bond

f=1 : E= E1 = 1000 GPa (Diamond)
f=0: E=E2 = 1 GPa (Paraffin wax)
f=0.5 to 0.75 E-3-8 GPa

As the polymer is heated from the glassy phase, the polymeric chains start to slide against each other gaining their mobility. This continues over a range of temperature until it reaches a plateau again known as the elastomeric phase.

The transition stage from amorphous to elastomeric phase is defined as the glass transition phase. The temperature corresponding to the centre of the transition region is defined as the glass-transition temperature or Tg.

For the polymeric materials, especially for the thermo-plastic polymers, Tg plays a crucial role as the mechanical properties like elastic modulus and strength sharply fall below this temperature. Also, the material shows a highly lossy behaviour which is exploited in designing damping materials.
In this region E at different Temperature could be superimposed by shifting data from a log – log plot of E vs time from any temperature

WLF Equation: Log aT = C1(T-Tr)/(C2 + T – Tr)

One of the first pieces of data anyone looks at while evaluating a potential new material for Printed Wiring Board (PWB) applications is its Glass Transition Temperature (Tg) Tg's of materials used run from 1150 C (FR-4) to about 2500 C (for high-end polyimide materials) with an almost unbroken continuum in between based on various classes of resins, cross-linking agents and blending ratios.

At this stage let us briefly discuss about different procedures to measure the Tg of a polymer.