A Kinetic Investigation of Thermal Shrinkage of Aromatic Polymers by Thermomechanical Analysis

The kinetics of thermal shrinkage of the stretched polypyromellitimide (PI) films, Du Pont Kapton H, were investigated by thermomechanical analysis (TMA) at a constant rate of heating. The two-stage model of extended polymers has been applied to analyze the TMA data. The activation energy of the contraction reaction could be obtained from TMA curves at various heating rates. The one-step shrinkage of the 12.5% stretched PI film gives an activation energy of 10 kcal/mole for the first shrinking. From the 30% stretched sample, the second activation of contraction is 25 kcal/mole, and from 40% sample the third one is 33 kcal/mole. These three contraction reactions are observed successively for the TMA curves for highly stretched samples, and correspond to the various kinds of molecular motion of this special rigid polymer structure, which are also observed in the dynamic mechanical and dielectric properties of the same polymer.

Polypyromellitimide (PI) is a typical thermally stable polymer due to the rigid aromatic and heterocyclic ring structures in its backbone chain. This polymer is able to be cold-drawn in the glassy state, and it shrinks markedly on heating. The thermal shrinkage was investigated thermoanalytically by thermomechanical analysis (TMA) at a uniform heating rate. The activation energies of thermal shrinkage in three stages are estimated by this method.