Structures of free-standing vertical thin films of hydrophobically modified poly(sodium acrylate)s

Our investigation of free-standing vertical films of hydrophobically modifi ed poly(acrylic acid) sodium salt by the X-ray reflectivity technique shows that the organization of these films strongly depends on the molecular wei ght and hydrophilic-hydrophobic monomer unit ratio of the copolymers. Long polymer chains with a low amount of hydrophobes form films composed of two layers of coils facing each other. In contrast, short polymer chains with a higher degree of hydrophobic modification form films with a well-defined s emiordered structure whose properties are closely linked to the properties of the corresponding solution. We show that this structure is of electrosta tic origin. Finally, the comparison of films made of the copolymers in thei r charged and neutral forms shows that the dominant force within the two ty pes of film is respectively the counterion osmotic pressure and the energy of adsorption of the hydrophobic monomers.