An innovative approach for hydrophobic sulfonated polyethersulfone substrates

An innovative approach for hydrophobic sulfonated polyethersulfone substrates V. Sabatini, S. Checchia, G. Soliveri, H. Farina, M. A. Ortenzi, G. Di Silvestro Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano Self-cleaning materials can be a very advantageous solution for covering solar cells, whose service life and efficiency can be enhanced by preventing accumulation of dust and filth on cell covers that can cause the loss of incident light through scattering or light absorption. The use of polymeric materials, such as Polyethersulfone (PES) or sulfonated Polyethersulfone (SPES), carrying sulfonic acid groups directly on the polymeric chain, can be very advantageous in this field when hydrophilic/hydrophobic properties can be modulated: within this framework, the use of Ionic Liquids (I.Ls.) combined with SPES can be a way to create tailor-made hydrophobic materials for solar cells covering. PES and SPES are totally amorphous polymers widely used as substitutes for heavy and fragile glasses, due to their thermal, optical and mechanical properties. In fact, PES and SPES are characterized by excellent thermal and UV resistance (e.g. very high glass transition temperature that can reach 230°C and low thermal expansion coefficient), optical features (refractive index: n=1,63 at λ= 589,3 nm), chemical stability, oxidation resistance, as well as by good mechanical properties and easy processability, i.e. for the production of films or layers. SPES in particular has received great attention in the last decade due to its effectiveness for a variety of separation processes, such as ion exchange, reverse osmosis and others. Sulfonation degree of PES is notoriously difficult to control due to the electron-withdrawing sulfone moiety that deactivates adjacent aromatic rings for electrophilic substitution; this problem can be overcome by co-polymerizing sulfonated monomers, thereby better controlling the macromolecular structure and the sulfonation degree of the polymer. I.Ls., a class of molten salts, are interesting candidates for the development of hydrophobic materials. In fact, I.Ls. have excellent thermal stability and their physical-chemical properties can be modulated changing the nature of the cation or anion. Modulating cationic apolar groups can dramatically influence the tendency of I.Ls. towards efficient ion packing and, in turn, hydrophobic features, e.g. the longer the alkyl chains, the more hydrophobic the salt. In this study, SPESs with different sulfonation degree were successfully synthesized using a sulfonated monomer, 2,5-dihydroxybenzene-1-sulfonate potassium salt; despite the hydrophilic properties of SPES, due to the presence of sulfonated comonomer, its wetting properties can be modulated thanks to an innovative synthesis with I.Ls., through an ionic exchange reaction between the potassium cation of sulfonated hydroquinone comonomer of SPES and the cation of I.L.s, that can be substituted with different apolar groups. After the addition of I.Ls., the hydrophobic properties increase as the quantity of sulfonated groups increase due to the higher number of potassium ions available for substitution with I.L. cations. In particular, 1-Ethyl- 3 Methyl- Imidazolinium- ethyl sulfate (MEIM) and 1,3 Dibutyl- Imidazolinium- Dibutyl phosphate (BBIM) were synthesized and used; polymeric films based on SPES and I.Ls. were prepared as innovative solution for self-cleaning materials that could, in a future, be used for covering solar cells. Conventional and innovative characterization techniques were used: Wide Angle X-Ray Scattering (WAXS) and synchrotron radiation, allowed to study SPES structure both indirectly and directly; the spatial arrangement of sulfonic groups modified with I.Ls., the wetting properties and the morphology of SPES films were studied by static contact angle measurement and Scanning Electron Microscopy (SEM).