Elongational crystallization of isotactic polypropylene forms nano-oriented crystals with ultra-high performance

Although the large ‘melt elongation’ of polymers should significantly affect crystallization that controls the structure and properties of solids, it has been difficult due to technical reasons. In this paper, we succeeded in bulky elongational crystallization of isotactic polypropylene (iPP) by compressing the supercooled melt. The crystallization behavior and structure were observed by means of optical microscope, transmission electron microscopy (TEM) and X-ray scattering. When the elongational strain rate () surpassed a critical value ( * =2 × 10 2 s −1 ), the crystallization behavior, structure and physical properties underwent a discontinuous change. The crystallization rate increased by a factor as large as 10 6 , the crystallite lateral size decreased by a factor of 10 −3 to 20–30 nm and the morphology changed from well-known spherulites to a novel morphology of ‘nano-oriented crystals (NOCs)’. These NOCs showed a high crystallinity (nearly equal to unity). The above results indicate that the polymer melt changed from isotropic to ‘oriented melt’ which accelerated nucleation and growth rates and induced NOC formation. NOCs showed ultra-high performance such as high tensile strength at break (≅2.1 × 10 2 MPa), comparable to that of metals, and a high thermal resistance (≅176 °C). NOCs will be useful in a wide variety of applications and will contribute to efforts to construct a sustainable society by enabling the development of lightweight, recyclable materials.