Study On The Rheological Properties And Phase Separation Behaviors Of Polymer Membrane Materials

Poly(vinylidene fluoride)(PVDF) is one of the widely used fluorine-containing membrane materials, due to its good mechanical strength, thermal and chemical stability, radiation resistance and process ability. PVDF hollow fiber membranes are usually prepared by two methods: thermally induced phase separation (TIPS) and solution phase inversion methods with complex processes and many influential factors. However, the melt spinning-stretching (MS) technique does not come up against these problems. Consequently, MS technique is a promising method. Rheological data are of great interest not only in quality control, but also in designing suitable processing conditions for MS technique.Rheological behavior at a wide range of shear rates(10^-25-×10³s^-1) for poly(vinylidene fluoride)(PVDF) melt was investigated by means of high pressure capillary rheometer and advanced rheological extension system (ARES). The influences of some factors, such as molecular weights, temperature and shear rates, on apparent viscosity, non-Newtonian exponent and flow activation energy of PVDF melt were systematically discussed. It was found that PVDF melt was Newtonian fluid at very low shear rates. In addition, the melting viscosity of PVDF melt was almost independent of shear rates. However, it was non-Newtonian fluid at higher shear rates, and its melting viscosity decreased with the increase of shear rates. The results showed that increasing temperature and decreasing shear rates could lead to an increase of non-Newtonian exponent of PVDF melt. The significant decrease of its apparent viscosity with the increase of molecular weight indicated that the effect of molecular weight on apparent viscosity decreased when increasing shear rate. Flow activation energy of PVDF melt decreased with the increase of shear rates, which was accordant with the sensitivity of apparent viscosity on temperature. Consequently, for PVDF with high molecular weight, its viscosity could be modulated by controlling of shear rates and temperature to improve its process ability. In addition, the comparison between the molecular weights determined by dynamic rheological measurements and by gel-permeation chromatography (GPC) showed agreement to some extent. So therheological technique appeared to be a powerful tool for the molecular characterization of polymeric materials, especially for the samples which are difficult to be solved or indissoluble in common solvents.The dynamic rheological properties of poly(vinylidene fluoride)(PVDF) samples of different molecular weights were studied using advanced rheological extension system (ARES) with oscillatory parallel plates. Information on the rheological properties of such materials at melt temperature is of interest as this can lead to an improved understanding of polymer behavior in processing and fabrication technologies. Shift factors derived from time-temperature superposition showed good adherence to the Arrhenius equation with a flow activation energy E for viscous flow found to be almost 70kJ/mol, essentially independent of PVDF molecular weight suggesting the unit of flow is a collection of chain segments rather than the whole chain itself.Due to its excellent wear resistance, impact resistance, corrode resistance, self-lubrication and so on, ultra high molecular weight polyethylene (UHMWPE) was called as supernatural plastics. And microporous UHMWPE is a new type of functional material based on UHMWPE, it can be used for heterogeneous phase separation filtration and so on in chemical, medical and energy field.The thermally induced phase separation mechanism of liquid paraffin/UHMWPE system was investigated by small angle laser light scattering measure system and differential scanning calorimeter. In addition, the morphology characterization was performed by scanning electron microscope. It was found that the thermally induced phase separation mechanism of liquid paraffin/UHMWPE was solid-liquid phase separation due to UHMWPE crystallization. Moreover, the mechanism was independent of the molecular weight of UHMWPE. However, the microstructure of microporous UHMWPE depended on the content and the molecular weight of UHMWPE. The micropore diameter of liquid paraffin/UHMWPE system decreased and the micropore homogeneousness increased with the increase of the content of UHMWPE. The results showed that there was larger micropore diameter in the system containing larger molecular weight UHMWPE with the same content, the crystalline temperature was also higher.