Thermally stimulated current studies on molecular relaxation and blow moulding of poly(ethylene terephthalate)

The thermally stimulated current (TSC) technique has been used to investigate molecular relaxation in poly(ethylene terephthalate) (PET) films unstretched and biaxially stretched at 90 and 95 degrees C. Unstretched PET films show two peaks at 77 and 90 degrees C corresponding to alpha and rho relaxation processes, respectively. The alpha relaxation is associated with the main glass transition of the material. The rho peak with lower intensity is attributable to permanent dipoles. Both biaxially stretched samples show one TSC peak at 95 degrees C, supposed to correspond to rho relaxation. The disappearance of the alpha peak, accompanied by the displacement of the rho peak to higher temperature, is the result of the higher thermal stability of the permanent dipoles, which is strongly influenced by the stiffening of amorphous parts and the crystallization by stretching. In both stretched samples, the continuous distribution of pre-exponential factors over activation energies observed might correspond to a single relaxation mode. The kinematics of stretching PET has been discussed in terms of activation energy and temperature dependence of relaxation time. (C) 1999 Society of Chemical Industry.