Factors influencing the interdiffusion of weak polycations in multilayers

One promising aspect of the electrostatic multilayer assembly techniques is the ability to consistently and predictably create controlled heterostructures that may be of interest for active devices, designed biomaterials, membranes, or other composite thin film Structures. This promise is mitigated by the challenge of controlling diffusion and exchange processes that can take place in certain layer-by-layer assembled film systems and cause unanticipated or unwanted materials distributions and in extreme cases completely disrupt the assembly. To further understanding toward prediction and control of these processes, we investigate a series of polyamines and their interdiffusion and exchange within preassembled multilayer films to explore the role of polyion degree of ionization, hydropliobicity/hydrophilicity of the backbone, basicity of amine groups, and polyion topology in a polyelectrolyte interdiffusion and exchange process. Interdiffusion of these polyamines will be examined within a polyhexylviologen (PXV)/poly(acrylic acid) multilayer model system in which conditions favor exchange of the polyamine with PXV, a strong polycation containing quarternary ammonium groups along the backbone. Four amine-containing polycations were examined: linear and branched polyethylene imine (LPEI and BPEI), polyamidoamine (PAMAM) dendrimer, and poly(allylamine hydrochloride) (PAH). It was found that fully charged polycations in dilute aqueous Solution are unable to diffuse through the multilayer film whereas partially charged polycations have the necessary mobility. Remarkably, despite strong differences in the nature of the polycation, as in the case of PAH, LPEI, and BPEI, for every polyamine there existed the same critical degree of ionization in solution below which interdiffusion was possible, which was seen to be near 70% in these exchange experiments with PXV. Only for the highly branched PAMAM dendrimer was this value different; the critical degree of ionization for PAMAM was observed to be 55%. Kinetics of the interdiffusion were significantly impacted by the polyion degree of ionization and molecular weight.