Uv Preparation Of Temperature-sensitive Ultrafiltration Membrane

Ultraviolet photografting polymerization is widely utilized for the functional modification of membranes. Thermal-sensitive membrane, which can alter the permeability automatically in response to environmental temperature change, can be obtained by grafting temperature-responsive polymer onto membrane. Such intelligent membrane can meet much more complex separation needs, and therefore have widely potential application.A new method for the preparation of thermal-sensitive membrane is proposed in this research. Poly(N-isopropylacrylamide) (PNIPAAm) grafted membrane is obtained by introducing NIPAAm onto the surface and in the pores of photosensitive ultrafiltration membrane via UV photografting polymerization. Firstly, polyacrylonitrile (PAN) is photofunctionalized by free radical polymerization with a photo-sensitive monomer N,N-diethylaminodithiocarbamoyl methylstyrene (DTCS), and the copolymer P(AN-co-DTCS) exhibits photosensitivity to UV light. Then, Photografting modification of the membrane prepared by photofunctional copolymer P(AN-co-DTCS) can be carried out without any initiator, as the group of N,N-diethyldithiocarbamate (DC) can be initiated to generate free radicals for the grafting polymerization of NIPAAm. The acrylamide band I in FTIR spectra confirms that PNIPAAm is successfully grafted onto the photo-sensitive membrane with the absence of initiator.The influence of the grafting polymerization conditions on the grafting yield is investigated. Extending grafting time and increasing temperature favor the increase of grafting yield. The maximum grafting yield is observed when the monomer concentration is 0.4mol/L. When the graft polymerization temperature is over 35℃, which is above the lower critical solution temperature (LCST) of PNIPAAm, the hydrophobic PNIPAAm chains tangle together and suppresses the diffusion of monomer, which results in the decrease of the polymerization rate. The SEM photographs of the PNIPAAm grafted membrane show that, the PNIPAAm chains exist both on the surface and in the pores of the membrane and leading to the density of pores on the surface layer. Comparing to the ungrafted membrane, AFM images clearly show that the surface of the PNIPAAm grafted membrane is rougher. At room temperature, the surface contact angle of the PNIPAAm grafted membrane decreases with the increase of grafting yield, which reveals that the surface wettability of the membrane is effectively improved after grafting.The swelling/deswelling behavior of the PNIPAAm-g-P(AN-co-DTCS) membranes as a function of temperature is studied. The result indicates that, at the vicinity of the LCST, a slight temperature change will induce a sharp and rapid swelling-deswelling phase transition of the PNIPAAm grafted chains. Consequently, the grafted PNIPAAm chains will act as a sensor to regulate the permeability of the PNIPAAm grafted membrane and inducing remarkable water flux change in response to temperature change. Moreover, PNIPAAm grafted membranes with variable grafting yield show different thermal-controlled permeability. Accordingly, a flux mechanism for grafted membranes with different grafting yield is proposed to explain the thermal-responsive permeability.Polyethylene glycols (PEG) and Bovine serum albumin (BSA) with different molecular weights are used as the model compounds to evaluate the thermal-dependent permeability of the grafted membrane. The solute permeability is influenced by the temperature of the solution, the grafting yield of NIPAAm and the molecule size of the solute. It is found that PNIPAAm grafted membrane with a grafting yield of 5.35% shows a better thermal-controllability of solution flux and rejection for BSA solution. In the range of 25-45℃, the antifouling property of the grafted membrane is enhanced as a result of the hydrophilicity of the PNIPAAm grafted chains.