Flow-Induced Crystallization of Polymers during Multi-Axial Deformation*

Flow- or stretch-induced crystallization (FIC or SIC) is believed to be mainly responsible for the excellent mechanical properties of polymers during real service conditions. With the development of synchrotron radiation X-ray scattering, time resolution has been increased from minutes to millisecond, the in-situ tracing of the structural evolution of polymers under complex external fields becomes possible. Recently, the FIC behaviors of polymers (i.e. natural rubber and polyethylene) during multi-axial deformation like biaxial stretching, film blowing and balloon blowing have been systematically studied by our group with a series of custom-built experimental devices combined with synchrotron radiation X-ray scattering techniques. Considering the phenomenon of frustrating SIC for natural rubber during biaxial stretching, here we proposed a new model for SIC based on the results of theoretical calculation, which decouples the free energy contributions of chain orientation from that of conformational entropy reduction. Furthermore, based on molecular dynamics simulation techniques which provide great advantages in exploring molecular level information, the nucleation and growth mechanism during biaxial deformation were acquired, which not only verifies the above model, endows it with molecular origin but also proposes the probability of application of this model for the crystallization of different polymer systems and other flow fields.