J. Phys. Chem. C 2007, 111, 16219-16226
16219
Compacted Nanoscale Blocks To Build Skin Layers of Reverse Osmosis and Nanofiltration Membranes: A Revelation from Small-Angle Neutron Scattering Puyam S. Singh*,† and Vinod K. Aswal‡ RO Membrane DiVision, Central Salt & Marine Chemicals Research Institute (C.S.I.R.), G.B. Marg, BhaVnagar 364002, India, and Solid State Physics DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India ReceiVed: May 8, 2007; In Final Form: August 17, 2007
The nanoscale structures of selective polyamide skin layers of reverse osmosis (RO) and nanofiltration (NF) membranes, based on trimesoyl chloride-m-phenylenediamine and trimesoyl chloride-piperazine, respectively, were studied by small-angle neutron scattering. In both the membranes, polysulfone films of about 40 µm thickness were used as supports over which thin polyamide layers of about 0.2 µm thickness were coated by interfacial polymerization. The small-angle neutron scattering profiles of the composite films were largely dominated by polysulfone scattering on account of larger material amount, and the weak scattering intensities of polyamides were only treated to generate initial approximations, which suggest highly compacted polyamides blocks of sizes well above 100 Å. For measurements of good polyamides scattering data, the neat RO and NF polyamides were coated on glass plates and then collected in water. Various suspended RO and NF polyamide solutions containing various amounts of H2O, D2O, and H2SO4 were prepared for good neutron scattering contrasts and dilute solution preparations. Clear scattering from compacted nanometer-sized blocks of about 120-140 Å in radii was observed for both RO and NF polyamide, with the compaction of nanoblocks loosened by the solvation effect of H2SO4. From their scattering signatures, the stability of these nanoblocks in water at temperatures up to 70 °C and in water of high salinity was revealed.
Introduction The thin film composite polyamide reverse osmosis (RO) and nanofiltration (NF) membranes, both comprised of a top skin polyamide layer coated over a polysulfone support base layer are commercially very successful membranes for water desalination and are increasingly used for wastewater treatment, separation of organics from aqueous streams, and a variety of other separations in chemical and food industry.1 The RO membrane can separate electrolytes of monovalent ions of which sizes are less than 10 Å, while the NF membrane, which has average pore size about 10 Å, can separate a mixture of monovalent and divalent electrolytes. The active polyamide skin layers of membranes, which are principally responsible for permeability and selectivity, are generally formed by interfacial polymerization between m-phenylenediamine dissolved in water and trimesoyl chloride monomer dissolved in organic solution for formation of the RO membrane or, between piperazine dissolved in water and trimesoyl chloride monomer dissolved in organic solution for the NF membrane. The polyamide molecules are assembled as a thin film at the liquid-liquid interface within a few seconds of reaction. The thickness of the film reaches about 2000 Å in less than 1 min of reaction. The present work was undertaken to study the primary building units that create thin films by recording scattering signatures from the films as well as from the disrupted polyamide chain networks of the films. As the polyamides formed are highly cross-linked structures, not completely soluble in organic solvents, and polyamide chain got broken in case the polyamides * Author for correspondence. † Central Salt & Marine Chemicals Research Institute. ‡ Bhabha Atomic Research Centre.
are treated with strong acidic solvents such as H2SO4, understanding of the size and structure of the macromolecular units of polymers has been difficult. In addition, the rapid structural building processes of the membrane implicate the roles played by preparative conditions and factors in the determination of membrane structure and performance. It is now well-accepted that the polyamide membrane film building process is of a very complex nature2-4 and that the early simple model of polymer film growth5 is not the appropriate model. Characterization of these thin films using techniques such as atomic force microscopy (AFM),7-9 scanning electron microscopy (SEM),10 streaming potential measurements,8,9,11,12 X-ray photoelectron spectroscopy (XPS),13,14 attenuated total reflectance infrared (ATRIR),15,16 electron spin resonance (ESR),17 nuclear magnetic resonance (NMR)14,18 and computer simulation19 has been performed to probe only surfaces, film thickness, or molecular structure. The internal features at nanometer length scales, which are equally important for understanding membrane transport processes, have been largely ignored. Recently, Freger20 smartly demonstrated the nanoscale heterogeneity of these films by transmission electron microscopy (TEM) observation of film samples after selective staining of amino groups and carboxyl groups of polymer networks by sodium tungstate and uranyl nitrate, respectively. Small-angle scattering technique is another excellent tool to characterize the size and shape of nanoparticles.21 This technique can provide structural understanding at nanometer length scale as well as microstructure of the polymers. While both small-angle X-ray and neutron scattering (SAXS/SANS) can give similar resolutions and probe matter at the same length scale, neutrons are more penetrating and particularly more suited for low electron density polymer
10.1021/jp073506d CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007
16220 J. Phys. Chem. C, Vol. 111, No. 44, 2007 compounds. SANS is undertaken with thermal neutrons by interaction with nuclei, and scattering intensity from hydrogencontaining polymer suspended in deuterated solvent such as D2O is strong because there is a large difference in neutron scattering lengths between H and D. By comparison, SAXS is sensitive to electron density only. The technique of small-angle scattering has been used22 to an extent for characterizing the polyamide membrane mentioned above using X-rays over the scattering wave vector range 0.0006-0.04 Å-1, corresponding to the length scale range of about 150-10 500 Å. The scattering study, however, has been limited as it was performed on the polyamide films at very small angles only after isolating the solid polyamide films from the base layers. Therefore, SAXS profiles were dominated by scattering from the fractal surfaces of large structures, and information on the smaller primary structural building units was not clear. Nevertheless, the early scattering result indicated that polyamide film is constituted by close packing of particles. The work reported here is an extension of the earlier scattering study of the membranes to probes the structures at nanoscales using neutrons instead of X-rays and at scattering ranges not studied earlier. The SANS studies of the polyamides membranes were performed over the scattering wave vector range 0.0180.35 Å-1, corresponding to length scale range of about 20350 Å, by taking measurements from both RO and NF polyamides in dilute solution system after collecting the immediately prepared thin films in water containing large amounts of D2O. The scattering experiments were also performed on the polysulfone support film as well as on the polyamide-polysulfone composite films of RO and NF membranes. In order to achieve more detailed information on how the polyamide networks are built into thin films, the scattering data from the polymer networks after disrupting the connecting network with various amount of H2SO4 were also recorded. Measurements from the suspended polymer water solution at higher temperature or in water solution of high salinity were further performed to check the polymer stability in these conditions. Experimental Section a. Materials. Polysulfone casting solution was prepared by dissolving 15 wt % Udel P-3500 PS (supplied by the Solvay) in A.R. grade N,N-dimethylformamide. The resultant polymer solution was cast from a water bath consisting 4 wt % dimethylformamide and 0.1 wt % sodium lauryl sulfate surfactant using a phase inversion process. The resulting polysulfone layer was washed thoroughly with distilled water and has a thickness of about 40 µm. The composite RO polyamide membrane was prepared by initially immersing the polysulfone film in a 2% (w/v) solution of m-phenylenediamine (Lancaster Chemical Co.) in water, followed by immersing into an n-hexane solution of 0.1% (w/v) trimesoyl chloride (Aldrich), which resulted in lamination of a 0.2 µm skin layer of polyamide over the surface of the polysulfone support. For the preparation of the composite NF polyamide membrane, piperazine instead of m-phenylenediamine was used, and the resulting skin layer is rather thinner,
