Glass transition:

Another significant parameter determined from through DTA is glass transition (Tg), a second order transition caused through relaxation   of chain segment within the amorphous portion of a polymer. The first evidence that a glass transition could be detected by DTA was provided in the year of 1957, where a transition at 28 °C was shown but not interpreted. As such this transition is not associated with latent heat but rather a sudden change in specific heat to bring a liquid into the glassy state. The essential condition is to cool it rapidly to approximately two third of its melting point.

T_g/T_m =0.66

The glass transition of a  polymer  can be obtained  using  number average molecular mass M ranging  from  8 × 10³ to  3 × 10 ⁶   from expression:

T_g = [(96.5 +/- 1.0) - (2.8 +/- 0.1)] × 10 ³ / M

The deduction about the slope of the glass transition as depicted by DTA curve:

a)         The slope of the curve ?T surface temperature should be sigmoidal.

b)         The maximum value of ?T at the glass transition temperature should be linearly dependent on the heating rate.

c)         The inflection point T_g should rise in temperature with the heating rate. It has been shown that glass transition is associated with a sudden shift in base line.

The Tg depends on heating rate, volume fraction and molar mass. The DTA results provide important information about polymerization reaction, mainly about heat of polymerization, degree of curing, effect of catalysts, decomposition reaction and radiation effects.