Unfolding of Isotactic Polypropylene under Uniaxial Stretching

Despite numerous investigations on polymer processing, understanding the deformation mechanisms of semicrystalline polymer under uniaxial stretching is still challenging. In this work, $^{\mathrm{13}}$C-$^{\mathrm{13}}$C Double Quantum (DQ) NMR was applied to trace the structural evolution of $^{\mathrm{13}}$C-labeled isotactic polypropylene ($i$PP) chains inside the crystallites stretched to engineering strain (e) of 21 at 100 \textdegree C. DQ NMR based on spatial proximity of $^{\mathrm{13}}$C labeled nuclei proved conformational changes from the folded chains to the locally extended chains induced by stretching. By combining experimental findings with literature results on molecular dynamics, it was concluded that transportation of the crystalline chains plays a critical role to achieve large deformability of $i$PP.