Structural and residual stress analysis of constrained and free uniaxially oriented polypropylene films

Herein, we study comparatively the crystal structure, orientation, and residual stress of polypropylene (PP) films processed by uniaxial constant width (UCW) and uniaxial free width (UFW) stretching modes, respectively. Compared with UFW PP film, UCW PP film exhibits broad lamellar thickness distribution with thinner average lamellar thickness, and accordingly lower melting temperature region. UCW PP film also has larger lateral lamellar size with high orientation along stretching direction. In addition, a preferential orientation of b-axis perpendicular to the surface exists in the UCW PP film, while a cylindrical symmetry around the stretching direction is in UFW PP film. A new single tilt X-ray diffraction method is utilized to measure the residual stress of PP films. The direction of maximum principal tensile residual stress is approximately along stretching direction for both films, while tensile residual stress occurs in UCW PP film and compressive residual stress in UFW PP film in transverse direction. Good correlation of the direction of maximum residual stress with crystal orientation is found in both PP films.