Orientation Effects in Biaxially Stretched Polypropylene Film

Synopsis

Unoriented, quenched sheet of polypropylene has been reheated and biaxially stretched sequentially in two perpendicular directions on a pantograph stretcher for stretch ratios up to 6×6 over a twelve degree stretch temperature range. Orientation of (040) in the film plane, called population 1 crystals, predominates at and above 4×4. At 5×5 a second crystal orientation occurs which is orientation of (110) in the film plane called population 2 crystals, found also in similarly prepared polyethylene. A method of 040 peak resolution enables calculation of the fraction of population 2 crystals with stretch ratio and temperature. A proposed mechanism of formation of population 2 crystals is intralamellar slip along (110 ). Correlation of the amount of population 2 crystals with the area under the stress-strain curve obtained during stretching, as well as line breadth measurements, support such a mechanism. The generation of population 2 crystals occurs concurrent with no significant change in c-axis orientation which orients in the film plane below 4×4. The significance of biaxial over uniaxial stretching to detect this structural change is emphasized.