Multiaxial Reinforcement of Cross-Linked Isotactic Poly(propylene) upon Uniaxial Stretching

Isotactic poly(propylene) (iPP) is one of a few polymers that show homoepitaxy, i.e., the formed crystals act as nuclei for so-called daughter crystals that are formed nearly perpendicular to the backbones of the primary crystallized mother crystals. Lightly cross-linked isotactic poly(propylene) (x-iPP) offers the opportunity to form nearly all mother crystals in stretching direction and simultaneously allow formation of daughter crystals. Since crystals in polymers act as reinforcement along chain direction, this unique behavior allows multiaxial reinforcement in iPP induced only by stretching in one direction. In this study the influence of applied uniaxial strain is explored on the resulting multiaxial crystal orientations and Young's moduli parallel, perpendicular as well as under an angle of 45 degrees to the stretching direction of the sample. It is shown that the occurring multiaxial orientations strongly depend on applied strain during crystallization and cause significantly improved Young's moduli parallel as well as perpendicular to the prior stretching direction while that under an angle of 45 degrees is slightly decreasing. The here described technique to obtain multiaxially oriented morphologies is not restricted to thin films but can be efficiently applied also to bulk samples.