Preparation And Surface Properties Of Acrylonitrile-based Copolymer Membranes

Polymer separation membranes have been increasingly applied in various fields, such as biomedical devices contacting with body or body fluid, membrane reactors immobilized with enzyme, and traditional ultra-filtration process in which biofouling is expected to be as low as possible. It is generally accepted that the surface properties of membrane, especially its biocompatibility, play an important role in such applications. By combining the merits of polyacrylonitrile (PAN), which include the superior membrane/fiber-forming property and physicochemical stability, with the excellent hydrophilicity and biocompatibility of poly(N-vinyl-2-pyrrolidone) (PVP), poly[acrylonitrile-co-(N-vinyl-2-pyrrolidone)] (PANCNVP) was synthesized to prepare separation membrane with desired surface properties in this work. The related experiments and results are summarized as follow:PANCNVP was synthesized through water phase precipitation copolymerization (WPPCP) using NaClO₃-Na₂S₂O₅ as initiator. Effects of initiator system and its concentration, ratio of oxidant to reducer, monomer concentration, temperature and time of reaction on the copolymerization were studied. Thus an optimized parameter for the synthesis of PANCNVP was obtained. It was found that persulfate could induce the crosslinking of NVP units. Comparisons with the corresponding solution polymerization indicated that the WPPCP process was competitive in the aspects of yield, conversion of NVP, molecular weight of the resultant polymer as well as post-treatments.Evaluations of the biocompatibility of PANCNVP membrane were performed by a series of experiments such as protein adsorption, platelets adhesion, recalcification time, macrophage adhesion and culture of endothelial cells. Results confirmed the excellent biocompatibility for the PANCNVP membrane, which would be attributed to the NVP moieties. For example, the ability of anti-platelet adhesion was greatly enhanced. About 320 platelets per area unit adhered onto the PAN membrane surface while only about 20 on the PANCNVP31 membrane surface. Besides. based on the morphology changes of the adhered platelets, the former was of a highly activated status while the latter was normal or at the early stage of activation.Structures of water molecules in the PANCNVP membrane was explored to elucidatethe origin of biocompatibility of NVP moieties through water swelling experiments, thermal analysis, Fourier transform infrared (FT-IR) spectroscopy and two-dimensional (2D) correlation analysis as well as quantum chemistry calculations. Results of water swelling and thermal analysis confirmed that, the water uptake of membranes increased sharply with the content of NVP, and the amount of free water increased a half while that of bound water increased up to three times. It was found from the FT-IR spectra at transmission mode and attenuated total reflection (ATR) mode that, the vibration peaks induced by hydrogen-bonding in PAN system were around 3600 cm^-1; however, the absorption bands from PANCNVP system located in the range of 3700-3200 cm^-1. It was also found that the further 2D IR analysis of time-resolved FT-IR spectra resolved the overlapped spectra to some extent and provided the sequence of group changes. Furthermore, the ability of NVP moieties to bind water molecules tightly was further confirmed by calculations.The preparation, structures, performance and post-treatments of PAN-based asymmetric membranes were systematically investigated. Firstly, we examined the leaching of PVP from the PAN/PVP blending membranes. Based on the results of water contact angle measurements, FT-IR/ATR, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy, it was concluded that water can leach out the added PVP from the membranes during membrane formation or permeation process, which may deteriorate membrane performance. PANCNVP membranes were prepared and the morphologies were carefully studied. Filtration of bovine serum albumin (BSA) solution was carried out to determine the anti-biofouling ability of the PANCNVP membranes. Positively, biocompatible PANCNVP endowed its separation membrane with good anti-biofouling ability. In order to further modulate the structure and performance, the membranes were post-treated with persulfate, which can induce the crosslinking of PVP. It was found that the post-treatments by persulfate can greatly increase the flux with only slight decrease in the rejection of BSA. Possible mechanism of the post-treatments was also proposed and evidenced by the FT-IR/ATR spectra.Nanofibrous membrane with both high surface area to volume and large porosity was fabricated by electrospinning technique. Parameters including molecular weight of polymer as well as concentration, viscosity and velocity of polymer solution were optimized, and their effects on the morphology and diameter of nanofibers were explored. Water swelling behaviors of the nanofibrous membranes and structures of water moleculesinside were carefully studied and compared with those of dense membranes. Results indicated remarkable overshoot for nanofibrous membranes. Composite membranes covered with random or aligned nanofibers were also prepared by modified electrospinning procedure. Behaviors of blood platelets on the composite membrane surfaces showed that, nanofibers evidently promoted the activation, adhesion and even alignment of platelets, regardless of the chemistry of nanofibers or underlying membranes.Nanofibrous membrane is a latent carrier for enzyme immobilization due to its above-mentioned characteristics. Nanofibrous membranes filled with or without carbon nanotubes (CNT) were prepared from copolymers containing NVP moieties or porphyrin pendants. Immobilization of catalase was carefully studied, including the amount of immobilized catalase, the activity, the kinetic parameters and the stability. It was found that, the extremely high surface area to volume caused large amount of immobilized enzyme while the large porosity and good connectivity of pores diminished the diffusion resistance of substrate. Therefore, the enzyme immobilized on nanofibrous membranes achieved high efficiency for catalysis. Besides, the filling of CNT greatly enhanced the retention activity of the immobilized enzyme; and the introduce of NVP moieties and porphyrin pendants also increased the retention activity when compared with PAN sample.