From the functionalization of polyelectrolytes to the development of a versatile approach to the synthesis of polyelectrolyte multilayer films with enhanced stability

A general approach to side-chain allyl-functionalization of three different polyelectrolytes (PEs), namely poly(allylamine hydrochloride) (PAH.HCl), branched polyethyleneimine (PEI) and poly(sodium 4-styrenesulfonate) (PSS), is reported for the first time. This functionalization does not affect the electrolytic properties of the newly functionalized PEs (named PEs-ene), as confirmed by the effective formation of PE-ene multilayer (PEM) films. The stepwise construction of these films is monitored using different techniques, including QCM, SEM, XPS, and WCA measurements. The incorporation of allyl functional groups into the PE side-chains allows for the stabilization of the resulting PEM films via thiolene UV photo-crosslinking in the presence of a water-soluble dithiol crosslinker. To overcome the problem of film delamination, the covalent crosslinking occurs not only between different layers of PEM films but also with the substrates preliminarily functionalized with allyl functional groups either via sulfur-gold chemistry or via chemical reduction of aryldiazonium salts using two newly synthesized anilinium derivatives. The stability of the resulting crosslinked PEM films in a strongly alkaline solution (pH = 14) is validated on gold substrates under QCM conditions (from 30 min to 3 h), while XPS and WCA measurements are used for evidencing such a stability under strongly acidic conditions (pH = 1) or at high salt concentration (saturated NaCl solution) on a modified anion-exchange membrane during a longer study time, i.e. 10 days. Finally, this stability study also gives evidence for a greater crosslinking efficiency of PEG-diSH, namely a macromolecular crosslinker, over dithiothreitol (DTT), a small molecule. The versatility and effectiveness of the approach presented here are expected to find widespread interest in different fields of emerging applications.