Effect of Physical Aging on the Bending Recovery of PEEK and PI Films

The relationship between the viscoelasticity and bending recovery responses of PI and PEEK films were studied as well as the effect of physical aging treatment on these properties. The residual elastic modulus (E-r) and degree of aging of the films were measured from the stress relaxation-recovery experiment using DMA and the enthalpy relaxation endotherms near glass transition temperature using DSC, respectively. The instantaneous bending recovery ratio (BR+) of the films was measured from the change in radius of curvature before and after the bending under the condition of 85 degrees C/85%RH/24 hr. The DMA test was also performed at 85 degrees C in order to evaluate the progression of the stress relaxation during bending. The E-r and BR+ values showed good correlation, and higher contributions of elastic response or longer delay of the stress relaxation led to improved bending recovery. Films with BR+ values close to zero can be obtained if the conditions of E-r <0.05 are satisfied. A physical aging method was proven to be effective for controlling the viscoelastic response of the films; it contributes to the retardation of stress relaxation. The optimum physical aging temperatures were 200 degrees C for PI and 130 degrees C for PEEK, respectively. Both films achieved a sufficient aging effect with the treatment time of 24 hours.