Phase Transitions in Amorphous and Crystalline Polymers

When a polymer melt is being cooled, it often partially crystallizes. But this melt crystallization sometimes cannot take place because of a lack of 

In heat fiux DSCs on heating, the glass transition is observed as a jump in the baseline (i.e., heat flow signal) pointing downward, while in the Perkin-Elmer power compensation DSCs the glass transition is a jump in the baseline pointing upward, although lately a number of manufacturers have provided an option to select the direction of the endoterm-exotherm. 

Sometimes it is stiU debated whether the glass transition is a purely kinetic transition or a second-order thermodynamic transition (van Krevelen 2003). On one hand, it is true that the crystallization process for a number of (atactic) polymers would not take place even at infinite time, and this transition possesses the characteristics of a second-order thermodynamic transition (at least formally, in the Ehrenfest sense see definition of the phase transition in Section 2.2). But the absence of crystallization does not prove that the glass transition is a thermodynamic second-order transition, and it is also true that the glass transition does not occur as a definite sharp transition as would be required by equilibrium thermodynamics. Therefore, the glass transition must be considered a kinetic transition. 

The glass transition temperature is extremely important from a practical perspective for many polymers it determines the highest use temperature, while at the same time it defines the lowest possible processing temperature. DSC is a suitable technique for measuring the characteristics of the glass 

For semicrystalline polymers the ratio TJT varies between 1.5 and 2.0, with several exceptions like PTFE (Wunderlich 1960,1990).This empirical rule also holds for many small-molecular-mass substances, such as ethanol, water, and Si02 (Wunderlich 1990). 

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