Oriented re-crystallization of polypropylene through partial melting and its dramatic influence on mechanical properties

A promising method for toughening isotactic polypropylene (iPP) without loss of stiffness is to enhance molecular orientation during processing. However, conventional injection molding cannot impart remarkable molecular orientation to the molded bars as a result of retarded lamellar growth upon rapid solidification and thus has little contribution to the toughness enhancement. Herein, we demonstrated that molecular orientation in the conventional injection molded iPP bars has been notably enhanced through oriented re-crystallization after partial melting at 175 °C. As a result, about 3.5 times increase in the toughness, in parallel with apparent strength enhancement, has been gained in these re-crystallized bars. Our findings offer a simple and economic approach to produce tough iPP and maybe other semi-crystalline polymers in practice.