Effects of processing temperature and stretch ratio on the mechanical properties of uniaxially and biaxially stretched poly(ethylene naphthalate) (PEN) films were investigated by using Instron tensile tester, wide-angle X-ray diffractometer, differential scanning calorimeter, and density measurement. The base films (Sun-Kyung Synthetic Fiber Co.) were uniaxially and biaxially stretched with experimental variables of stretching temperature (145-165℃) and stretch ratio (1x-4x for uniaxial stretching, 1x1-3.5x3.5 for biaxial stretching). The tensile modulus and strength of uniaxially and biaxially stretched films had the highest value when the film was stretched at 145℃. Above this temperature, the tensile modulus and strength decreased rather due to both low crystallinity and low orientation of stretched film. Below 145℃ tthe film could not be stretched. Therefore, 145℃ is the optimum temperature at which the highest crystallinity induced by the stress-hardening effect gave a rise to the highest tensile modulus and strength. Tensile modulus, stresses at yield and break, and crystallinity increased with the stretch ratio.
