pH-dependent growth and morphology of multilayer dendrimer/poly(acrylic acid) films

Because hyperbranched poly(amidoamine) dendrimers are relatively well-defined polycations with a high density of charge, they provide unique materials for controlled synthesis of films by alternating electrostatic adsorption of polycations and polyanions. Thicknesses of generation 4 (G4) dendrimer/poly(acrylic acid) (PAA) "bilayers" can be tuned from < 10 to > 2000 Angstrom simply by varying deposition pH, while generation 8 (G8) dendrimer/PAA bilayers can be as thick as 4000 Angstrom. The highest thicknesses occur when using deposition pH values at which PAA and dendrimers are only partially charged, and the thickest films show high surface roughnesses, which may in part account for the rapid film growth. Deposition of dendrimer/ PAA films on porous alumina supports yields highly gas-permeable membranes. This is in sharp contrast to the case of poly(allylamine hydrochloride)/PAA films, which allow little gas flux. Dendrimer/PAA films can also be induced to form microporous coatings through pH- and salt-induced structural changes, and pore diameters in these structures can be varied from 0.02 to 0.4 mum.