Effect of strain level on stress relaxation and recovery behaviors of isotactic biaxially oriented polypropylene films

Abstract

The long-term recovery process and the changes in the uniaxial tensile properties and in the structure of isotactic biaxially oriented polypropylene (i-boPP) films after pre-extended at strain levels ranging from 1% up to 130% at room conditions were examined by using tensile testing, Fourier transform infrared spectroscopy/attenuated total reflectance, atomic force microscopy, and X-ray diffraction methods. It is significantly observed that the pre-extended i-boPP films at strain levels from 1% to 6% recovered completely back to their initial lengths and tensile properties. However, the i-boPP films showed a very slow recovery process and obtained very high remaining deformations changing with time which indicates irreversible structural processes after they were extended at higher strain levels. In order to predict the remaining deformation or length and the characteristics of the recovery process at any time, the linear equation of strain with respect to log time was proposed. The reasons for the changes in the tensile properties, the morphology, and the structure of the pre-extended i-boPP films were examined in detail. (c) 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42948.