Fabrication of microporous thin films from polyelectrolyte multilayers

Abstract

A simple process has been developed to create large area, highly uniform microporous thin films. Multilayers of weak polyelectrolytes were assembled onto silicon substrates by the sequential adsorption of poly(acrylic acid) and poly(allylamine) from aqueous solution. These multilayers were then immersed briefly into acidic solution (p^H ≈ 2.4) to effect a substantial and irreversible transformation of the film morphology. The resulting microporous structures are 2-3 times the thickness of the original films, possess a correspondingly reduced relative density of ½ to 1/3, and are stable against further rearrangement under ambient conditions. In addition, the microporous films may undergo a secondary reorganization in neutral water, leading to a morphology with more discrete throughpores. A mechanism is proposed for these transformations based on interchain ionic bond breakage and reformation in this highly protonating environment, leading to an insoluble precipitate on the substrate which undergoes spinodal decomposition with the solvent. FTIR (Fourier transform infrared spectroscopy) analysis supports the underlying chemical basis of this pH-induced phase separation, and AFM (atomic force microscopy), in situ ellipsometry, and SEM (scanning electron microscopy) have been used to monitor the morphological changes. The unique combination of properties exhibited by these microporous films makes them potential candidates for microelectronic and biomaterial applications.