Crystal structure, morphology, orientation, and mechanical properties of biaxially oriented polyamide 6 films

Investigations on crystal structure, orientation and mechanical properties of extrusion cast and biaxially stretched polyamide 6 films were carried out by using differential scanning calorimetry, wide angle X-ray diffraction, birefringence and tensile testing. The measurements were taken at three stages (1) after extrusion film casting, (2) after film stretching, (3) after solution annealing. Aging unstretched films, which had about 5% crystallinity, at room condition (22–23 °C and 22–31%RH, relative humidity) caused significant elevations in glass transition temperature (Tg) cold crystallization temperature (Tc) and crystallinity. The melting point (Tm) remained almost constant throughout the aging. The room temperature crystal form was investigated. Poorly defined β (or pleated α) crystals were found in the aged films, when they were either unstretched or stretched at low temperature. By increasing the stretch temperature from 40 to 180 °C, the poorly defined β (or pleated α) unit cell from low temperature stretching became similar to the well known α unit cell. Annealing the films, which had been stretched at 65 °C, in boiling 20% formic acid solution perfected the crystal transformation to the well defined α form. We developed a pseudo-orthorhombic unit cell in order to calculate biaxial orientation factors of the crystalline phase. The mechanical properties were successfully correlated with out-of-plane birefringences.