[Yuanfei Lin, Polymer] Stress-induced Microphase Separation of Interlamellar Amorphous Phase in Hard-elastic Isotactic Polypropylene Film

The initial nonlinear linear mechanical behavior in hard elastic isotactic polypropylene films is studied systematically by PhD Yuanfei Lin. This work has been published on Polymer (10.1016/j.polymer.2018.06.009).

Lamellar stacks, arranged alternatively with hard crystal and soft amorphous nano-layers, are the basic building units to determine the intrinsic mechanical properties of semi-crystalline polymeric materials. The mechanical instability of hard-elastic isotactic polypropylene (iPP) films with highly parallel lamellar stacks is studied with in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS/WAXS) techniques during cyclic tensile deformation. Unexpectedly, the micro-strain, deduced from the relative variation of lamellar periodicity, shows an accelerated increase at the onset of instability and reaches values larger than the corresponding macro-strain after yield, during which no irreversible plastic destruction of crystal is observed. Combining the unpredictable increase of long period and other structural information obtained with in-situ SAXS/WAXS measurements, we propose that stress-induced microphase separation of interlamellar amorphous phase is responsible for the yield behavior and the hyperelasticity of hard-elastic iPP films, which stems from the heterogeneous distribution of tie chain/trapped entanglement in interlamellar amorphous nano-layers. This reversible stress-induced non-equilibrium phase transition of interlamellar amorphous phaseis different from current plastic deformation models with crystal destruction in semi-crystalline polymers but in line with the nearly 100% elastic recovery ratio of hard-elastic films.

This work is supported by the National Natural Science Foundation of China (51633009, 51325301, and 51227801).