Cryogenic properties of several copolyesters

Copolyesters of polyethylene terephthalate(PET) and polyethylene-2,6-naphthalene dicarboxylate(PEN) with composition of PET/PEN= 100/0, 95/5, 90/10, 85/15, 70/30, 50/50, 30/70, 10/90, 0/100 were prepared. The mechanical properties of uniaxial-drawn films were examined at 83 K and 296 K. PET/PEN=90/10 copolymer film possessed especially excellent cryogenic properties. It was found to withstand elongations in excess of 40% at stress levels of about 400 MPa at 83 K for PET/PEN=90/10 film uniaxial-drawn 5 times. Differential scanning calorimetry(DSC) curves of samples before and after tensile test at 83 K were compared for PET/PEN=90/10 film uniaxial-drawn 5 times. After tensile test at 83 K, the peak position of cold-crystallization shifted and the peak area between the curve and a baseline decreased, indicating that crystallization is allowed to take place during tensile test at 83 K. Dielectric loss tangent was measured in the temperature range from 18 K to the glass transition temperature. The relaxation below 100 K of PET and its copolymers was observed to be dependent on the morphology of samples. On the basis of the results obtained, relationship between the structure of polymers and their cryogenic properties has been discussed