Pressure Quench of Flow-Induced Crystallization Precursors

We developed a novel protocol to study the mutual influence of shear flow and pressure on crystallization of polymers. Here, we have applied this protocol, named pressure quench, to polyethylene with a bimodal molecular weight distribution. With pressure quench, the undercooling, required to initiate crystallization of flow-induced precursors generated at high temperature, is obtained by increasing pressure, i.e., leaving the specimen isothermal. We find that pressure enhances the effect of shear. In particular, results show that the pressure quench effectively lightens up shear-induced precursors which otherwise are not observable, even with high-resolution synchrotron X-ray scattering. A pressure quench in combination with SAXS and WAXD gives insight into the early stages of crystallization. In this paper we focus on the use of WAXD since it provides all the information required to demonstrate our main issues. We conclude that precursors with different stability can be formed during shear and, with annealing, the least stable ones relax back to the melt. Finally, it is demonstrated that when pressure is released after crystallization, an inverse quench takes place and crystalline structures partially melt, similar to an increase of the temperature.