Stages of Structural Ordering Leading to Stress Induced Crystallization of PEEK Films: A Mechano-Optical Study on Deformation, Relaxation and Retraction

Influence of rubbery state large uniaxial deformation followed by relaxation and retraction stages on the mechano-optical behavior of cast amorphous poly(ether ether ketone) films was investigated. At early stage of stretching the birefringence remains linearly proportional to stress. In this linear optical regime, the C material remains amorphous. Beyond a critical stress level, a deviation from linearity is observed where a small fraction of the material exhibit extreme preferred orientation with substantial translational disorder resembling nematic-like order. The onset of this deviation also coincides with the onset of strain hardening. Stretching beyond this transition leads to stress induced crystallization. Films stretched to deformation levels well below the onset of strain hardening, remain amorphous before and after the relaxation stages. The entropy reduction gained through deformation of the polymer chains is not sufficient to form crystalline order at short time scales. Once the initial deformation increased beyond the onset of strain hardening, a long-range network of connected crystallites establishes itself.