Robust, transparent films of propylene−ethylene copolymer through isotropic-orientation transition at low temperature accelerated by adjustment of ethylene contents

Abstract

Transparently oriented films with high performance are fabricated by uniaxial stretching of propylene−ethylene copolymers (coPPs) at a low processing temperature. The coPP with ethylene content of 4 wt% (coPP4) can reduce processing temperature by approximately 50 °C compared to polypropylene (PP) and reach a tensile strength about 259.2 MPa, which is higher than that of PP about 215.9 MPa. As both crystallinity and orientation of coPP4@7.5 (draw ratio value followed by “@“) is lower than that of PP@4.5, this abnormal phenomenon is ascribed to the fibrillar crystals existing in coPP4@7.5 revealed by small angle X-ray scattering (SAXS) and scanning electronic microscopy (SEM). Structure analysis attests to that the tensile properties of the films are closely related to the hierarchically crystalline morphology especially in the fibrillar crystals. During the stretching process, it is intriguing that an appropriate amount of ethylene contents can accelerate the formation and perfection of fibrillar crystals originated from the deformation of coPPs spherulites, which are favorable for the enhancement of tensile strength. Interestingly, there is a significant linear decrease (R2 = 0.98) of the ultimate molecule orientation of stretched films with the increase of ethylene content. Furthermore, the stretching processing has little effect on crystallinity and long period for coPPs, which is distinctly different from the case of PP stretching. Meanwhile, it is found that ethylene segments lead to formation of lots of γ-form except for coPP16, which tends to transform to α-form during stretching of coPPs.