On the Structure of Porous Poly(vinylidene fluoride) Membrane Prepared by Phase Inversion from Water-NMP-PVDF System
A PVDF membrane was prepared by isothermal immersion precipitation of a PVDF/NMP solution from pure water bath. FESEM imaging of the formed membrane revealed its porous structure, featuring tight skin, macrovoids, and cellular pores. These structural characteristics were common to membranes that were formed by precipitation of amorphous polymers from harsh nonsolvents. Evidence of crystallization in the present PVDF membrane could be observed on the surfaces of the various pores in the form of branching dendrites.
Preparation and characterization of highly hydrophobic poly(vinylidene fluoride) – Clay nanocomposite nanofiber membranes (PVDF–clay NNMs) for desalination using direct contact membrane distillation
Abstract Electrospun nanofiber membranes consisting of poly(vinylidene fluoride) (PVDF) blended with clay nanocomposites were prepared and tested in this paper for direct contact membrane distillation (DCMD) applications. Various compositions of PVDF–clay nanocomposite nanofiber membranes (NNMs) were prepared and characterized by water contact angle, Fourier transform infrared spectroscopy, differential scanning calorimetry, scanning electron microscopy and investigated for DCMD. The incorporation of clay nanocomposites increase the hydrophobicity of the membranes, which increase as the concentration of clay nanocomposite increases in the mixture. The highest water contact angle achieved was 154.20 ± 3.04°. The melting point of the PVDF–clay electrospun nanofiber membrane increases with the increasing concentration of clay indicating that the clay particles influence the crystallization process of the nanocomposite membrane. The PVDF–clay NNMs showed improved performance in DCMD applications and provide a way to prevent pore wetting in DCMD process.
Comparative study of in vitro flow characteristics between a human aortic valve and a designed aortic valve and six corresponding types of prosthetic heart valves.
A model of a pulsatile cardiovascular system is described including an electronic control, a hydraulic driving unit, a volume storage, a peripheral resistance and measuring equipment. Six different artificial heart valves, a designed aortic valve (Aachen pockett valve II) and a human aortic valve have been tested in the outflow position of the pump. Three of them are presently being used in clinical applications. Measurements of pressure and volume flow have been obtained for different frequencies n, pump volumes Vp and time ratios between pressure and suction phase. From the experimental results a comparative evaluation was deducted for different working conditions. In comparison to natural human aortic valve (NK) large volume flow is verified by the Aachen pocket valve II (AT), the Gott-Dagget value (GD), the Starr-Edwards valve (SE) and the Lillehei-Kaster valve (LK). The Gott-Daggett valve has the smallest back flow volume. The Smeloff-Cutter and in particular the Wada-Cutter valve have the largest regurgitation. This is to be expected since the Cutter valves are purposely so designed that they do not close tightly in order to avoid blood sedimentation. The human aortic valve, the Gott-Daggett valve and the Aachen pocket valve demonstrate the most favourable opening-closing characteristics. The human aortic valve achieves a negligible pressure loss, followed by the Björk-Shiley and the Lillehei-Kaster valve.