Temperature dependence of the elastic modulus of crystalline regions of Poly(ethylene terephthalate)

The temperature dependence of the elastic modulus E1 of crystalline regions of poly(ethylene terephthalate) (PET) in the direction parallel to the chain axis has been investigated by x-ray diffraction. The axial chain contraction coefficient is constant from 25°C to 200°C, and correspondingly the value of E1 (108 GPa) remain unchanged up to 215°C. This is in contrast with the temperature dependence of E1 values of various polymers previously reported. It is considered for PET that the axial thermal molecular motions which cause the decrement of E1 do not take place in the crystalline regions, and that the force constant of bond stretching and that of bond angle bending are constant over both the temperature and the stress range studied. It is reasonable to conclude that the mechanical structure of drawn PET is composed of the series model.