Non-Linear Viscoelastic Behavior of Poly-Ether-Ether-Ketone (PEEK) and PEEK-Based Composites

It has been recognized for some time that, immediately upon cooling to temperatures below their glass transition, polymers are characterized by departure from the equilibrium chain conformation. The viscoelastic properties of the amorphous and semicrystalline PEEK and of a PEEK-based composite (APC2) have been analyzed as a function of the aging time and related free volume relaxation at temperatures close to the glass transition region. The effect of thermal treatments on the three systems is presented. The presence of a second phase, namely crystallite and fibers, determines a loss of the mobility of the polymeric amorphous chains and consequently a decrease of the rate of the free volume relaxation. Creep tests approaching "zero stress" were performed on samples subjected to different thermal histories.