Visco- and plastoelastic fracture of nanoporous polymer sheets

We study the dependence of the fracture surface energy on the pulling velocity for nanoporous polypropylene (PP) sheets to identify two components: static and dynamic components. We show that these terms can be interpreted, respectively, as plastoelastic and viscoelastic components, as has been shown for soft polyethylene (PE) foams in previous work. Considering significant differences in the pore size, volume fraction, and Young’s modulus of the present PP and previous PE sheets, the present results suggest a universal physical mechanism for the fracture of porous polymer sheets. The simple physical interpretation emerging from the mechanism could be useful for developing tough polymers. Equivalence of Griffith's energy balance in fracture mechanics to a stress criterion is also discussed and demonstrated using the present experimental data