TABLE OF CONTENTS The glass transition temperature ( T g) is defined as the temperature at which a material loses its glasslike, more rigid properties and becomes rubbery and more flexible in nature. Practical definitions of T g differ considerably between different methods, therefore, specification of T g requires an indication of the method used.
Amorphous polymers when heated above T g pass from the hard to the soft state. During this process, relaxation of any internal stress occurs. At the T g many physical properties change abruptly, including Young's and shear moduli, specific heat, coefficient of expansion, and dielectric constant. For hard polymeric materials this temperature corresponds to the highest working temperature; for elastomers, it represents the lowest working temperature. Several methods exist for determining T g. These include differential thermal analysis (DTA), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), dilatometry, dynamic mechanical analysis (DMA), and nuclear magnetic resonance (NMR) spectroscopic methods. Each method requires interpretation to determine T g. For this reason exact agreement is frequently not obtained between results obtained by different methods.
As discussed previously, T g is shown by a change in the expansion coefficient and the heat capacity as a sample material is heated or cooled through this transition region.
Since DSC measures heat capacity directly, rapidly, and accurately, it is an ideal technique for the determination of T g. The calorimeter accepts polymers in any form (powder, pellet,...
