| 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 processability. PVDF hollow fiber membranes are usually prepared by two methods: thermally induced phase separation (TIPS) and solution phase inversion methods. Up to date, PVDF hollow fiber membranes made by melt spinning-stretching (MS) method have not been reported. In this paper, we aimed at studies on the rheological behavior for PVDF with high molecular weight, controlling of crystal structure and the transformation under temperature and stress field, effects of melt spinning and stretching process on the skin layer and microporous structure of PVDF hollow fiber membrane prepared by MS method, the relationship between the tensile modulus and stretching rate, etc. As a result, PVDF hollow fiber membranes were successfully prepared by MS method.Rheological behavior for PVDF melt was investigated by means of high pressure capillary rheometer and advanced rheological extension system (ARES). The influences of some factors including molecular weight,temperature and shear rate 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, that was accordant with the sensitivity of apparent viscosity on temperature. Consequently, for PVDF with high molecular weight, its viscosity can be modulated by controlling of shear rates and temperature to improve its processability.PVDF samples prepared by high pressure capillary rheometer at different shear rateswere characterized by SAXD. It was indicated that the stacked lamellar structure normal to the fiber axis existed in the samples, the long period of the crystal lamellae was about 15.2nm. At 500s"1 shear rate, the diffraction intensity was relative low as shown by SAXD patern. When the shear rate was in the range of 500-1580s"', the diffraction intensity would increase with the increasing shear rate. Above that, the diffraction intensity would not be changed.The effects of quenching on the phase structure of vinylidene fluoride (VDF) segments in PVDF homopolymer and poly(vinylidene fluoride -co- hexafluoro- propylene) (PVDF-HFP) copolymer and their blends with poly(methyl methacrylate)(PMMA) were discussed. The phase transformation of VDF segments was affected by quenching temperature, film thickness and PMMA contents in the blends. Lower quenching temperature and the blending with PMMA were in favor of the p phase transformation. However, thicker films were unfavorable to the transformation. When the film was thicker than 8um, the a phase would become the predominant crystals in the films. The reason might be that the quenching temperature in the thicker films was quite different on the surface and in the inner of the films upon quenching from the melt. The quenching temperature in the inner of the films would be higher than that on the surface, which would result in the a phase in the bulk and the (3 phase on the surface. As a result, the a phase would become the predominant results with little amount of P phase on the surface. The critical quenching temperature of P crystal phase in PVDF-HFP film was near 40°C, which was higher than that of PVDF film.PVDF fibers made by melting spinning method were characterized by SAXS,WAXS,SEM and tensile experiment. It was found that the stacked lamellar structure arranged in the direction normal to the fiber axis existed in the fibers and its long period was about 13.4nm. The tensile experiment also showed that initial modulus of PVDF fibers was 3.5GPa, and the elastic recovery of the fiber was 85% at 50% room-temperature extension. These phenomena indicated that PVDF fibers had the characteristics of hard elasticity. A developed Clark piled-lamellae structure model was used to explain the mechanism of the hard elasticity of PVDF fibers. Moreover, the abnormal phenomenon of initial modulus of PVDF fibers decreasing with the increase of stretching rates was also explained by thismodel.The P phase transformation of stretched PVDF hard elastic fibers changed distinctly with the stretching temperature and approached a maximum value near 70 °C. A "bimodal crystalline model" was used to explain the a—>p transformation, it was suggested that the p phase transformation was related with the reorganization of the microcrystallites and paracrystals existed at the lamellar surface, while not the stacked crystalline lamellae. SAXS and WAXS were also used to study the crystal phase transformation. It was indicated that the stacked crystalline lamellae in stretched PVDF fibers were not rearranged and still remained the c-axis orientation. The long perids of stretched PVDF hard elastic fibers increased from 19.04nm to 39.75nm. In addition, the P crystal transformation also increased with the stretching rate.Based on the above studies, microporous PVDF hollow fiber membranes were prepared successfully by MS method. The membranes had excellent mechanical properties, its break stress reached 213MPa. The membranes also had uniform micropores, its average pore size and porosity were 23.6nm and 59.56% respectively, as measured by a mercury porosimeter. The N2 permeation of the membranes was 1.01><10~5cm3/cm2- s ? cmHg ,and it increased with the increasing of spin-draw ratio of the fibers.
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