Effects of annealing stress field on the structure and properties of polypropylene hollow fiber membranes made by stretching

Abstract

By adopting three types of annealing stress fields, this work focused on investigating their effects on the microstructural evolution of polypropylene (PP) hollow fiber precursors obtained by the melt spinning process. PP hollow fiber membranes were then prepared by stretching the precursors at designed stretching ratios. The microstructure and properties of the precursors and membranes were characterized by differential scanning calorimetry (DSC), two-dimensional small-angle X-ray scattering (2D-SAXS), scanning electron microscopy (SEM), optical microscopy, pure water permeability, solute rejection, etc. In terms of the results, the effects of the stress on the development of crystal structure and rigid amorphous fraction (RAF) in the precursors were further discussed. Their roles in controlling the structure of the membranes were also studied in detail. It was indicated that stress acting on the precursors influenced the development of shish-kebab crystal structure and RAF during the annealing process. Fine crystalline lamellae structures in the RAF of the precursors were obtained when no stress was applied. Corresponding membranes made by stretching had better structure and properties, which mainly depended on the development of crystalline lamellae in the transition zone. Thus this study might provide a new insight for controlling the structure of membranes.