Fabrication of microporous thin films from polyelectrolyte multilayers

A simple process has been developed to create large area, highly uniform mi croporous thin films. Multilayers of weak polyelectrolytes were assembled o nto silicon substrates by the sequential adsorption of poly(acrylic acid) a nd poly(allylamine) from aqueous solution. These multilayers were then imme rsed briefly into acidic solution (pH approximate to 2.4) to effect a subst antial and irreversible transformation of the film morphology. The resultin g microporous structures are 2-3 times the thickness of the original films, possess a correspondingly reduced relative density of 1/2 to 1/3, and are stable against further rearrangement under ambient; conditions. In addition , the microporous films may undergo a secondary reorganization in neutral w ater, leading to a morphology with more discrete throughpores. A mechanism is proposed for these transformations based on interchain ionic bond breaka ge and reformation in this highly protonating environment;, leading to an i nsoluble precipitate on the substrate which undergoes spinodal decompositio n 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 (scannin g 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 applica tions.