Nanofiltration membrane prepared from polyacrylonitrile ultrafiltration membrane by low-temperature plasma: 2. Grafting of styrene in vapor phase

Ar low-temperature plasma treatment and subsequent monomer grafting in vapor phase were employed to prepare hydrophobic nanofiltration (NF) membranes from polyacrylonitrile (PAN) ultrafiltration (UF) membranes. FTIR-ATR spectroscopy, XPS analysis and contact angle measurement were used to characterize the chemical and physical changes of PAN UF membranes modified by plasma-induced graft polymerization in vapor phase. It was revealed from FTIR spectra and XPS spectra that hydrophobic monomer, styrene, was successfully grafted onto PAN UF membrane surface. XPS analysis also indicated that the N/C atomic ratio decreases with increasing of grafting reaction time. But the O/C atomic ratios of the membranes grafted for less than 40 min are higher than that of virgin membrane. The enrichment of surface oxygen was observed, which might result from the formation of oxygen-containing groups after the residual active sites were exposed to air. The contact angle measurement results showed the grafted styrene groups caused a great increase in hydrophobicity of membrane surface grafted sufficiently. The filtration performances including permeation flux, J, and rejection, R, were determined with a mixture containing dewaxed oil (M_w ≈ 450 g/mol) and dewaxing solvent (toluene and methyl ethyl ketone) at 20°C. The experimental results demonstrated the prepared hydrophobic NF membranes could reject the dewaxed oil up to 72.8%, which implies that these NF membranes can be used to recover the dewaxing solvent from the dewaxed lube oil filtrates.