Fabrication of self-assembled nanoporous organic coatings for membranes technology

Aim of the present work is the preparation of organic membranes with narrow pore size distribution and high porosity to be applied in the field of microfluidic analyses and as selective gates in microchip-based technologies for separation, detection and dosing of molecular or ionic species, charged nanoparticles or biomolecules. Highly selective membranes with controlled porosity can be obtained by coating appropriate supports with nanoporous thin films. The capability of block copolymers to spontaneously arrange into well-defined ordered structures with nanoscopic size has proven to be particularly advantageous for templating nanoporous membranes and recently block copolymer-derived membranes have been proposed as highly selective layers in composite membranes. The main advantages of block copolymer-templated membranes over other types of polymer membranes are their narrow pore size distribution, high void fraction and smooth surfaces, resulting in superior selectivity, high fluxes and fouling resistance. Pore size can be easily varied to optimize size selectivity, and additional separation mechanisms can be introduced by tuning the membrane surface properties through functionalization of the pore walls. In this contribution, we describe the fabrication procedure of nanoporous membranes with uniform pore size and high pore density obtained by spin-coating polystyrene-block-poly(ethylene-oxide) (PS-b-PEO) solutions on commercial silicon nitride microsieves (5μm of pores diameters). To better control the formation of well-defined nanostructures, a supramolecular approach was applied, based on the complexation by hydrogen-bonding of the PEO block with small molecules bearing hydroxyl groups, i.e. resorcinol. The addition of resorcinol in different PEO/resorcinol ratios was investigated and the effect on the alignment of PEO domains (and consequently on the final porosity) was confirmed. From PS-b-PEO/resorcinol mixtures, ordered films with normally oriented PEO/resorcinol cylindrical domains were obtained. These nanostructured films were subjected to a previous UV treatment, to improve their mechanical properties by crosslinking the PS matrix, and then to a selective wet treatement with iso-propanol, to remove resorcinol and create an hydrophilic porous system in the PEO domains. Composite membranes consisting of large-pore silicon nitride support and a continuous polymeric nanoporous layer were fabricated and tested as gates for controlled transport of chemicals.