Determination of the crystalline and noncrystalline molecular orientation in oriented polypropylene by infrared spectroscopy

The application of orientation to enhance the properties of a wide range of macromolecular materials has stimulated an interest in the characterization of the effects of such orientation processes. In semicrystalline polymers, this requires the characterization of the orientation in both the crystalline and noncrystalline phases. An infrared dichroism method was developed to simultaneously measure the orientation in the crystalline and noncrystalline phases of polypropylene. The method was applied to a series of melt‐crystallized and gel‐crystallized polypropylene specimens drawn up to a maximum of about 50 ×. The Hermans orientation functions of each phase were determined. The crystalline orientation was found to increase rapidly and approach perfect orientation at relatively low draw ratio (i.e., > 10 × ). However, the noncrystalline orientation was found to increase much more slowly up to about 50 × draw ratio. The Youngs modulus was found to increase nearly linearly up to about 50 × and to correlate with the increase in the noncrystalline‐orientation function and not with the crystalline‐orientation function.