Characterization of pH-induced changes in the morphology of polyelectrolyte multilayers assembled from poly(allylamine) and low molecular weight poly(acrylic acid)

We report characterization of pH-dependent behavior in polyelectrolyte multilayers (PEMs) fabricated from poly(allylamine) (PAH) and low molecular weight poly(acrylic acid) (PAA) synthesized by living/controlled polymerization. Exposure of these films to solutions of low pH (e.g. pH 2.0-3.2) resulted in transformations from films that were smooth and uniform to films with porous morphologies, as characterized by scanning electron microscopy (SEM). We observed large differences in both the extent of this transformation and the sizes of the pores that resulted compared to films fabricated using higher molecular weight PAA used in past studies. Whereas transformations reported in past studies generally lead to pores with sizes in the range of 0.3-2 μm, we observed larger-scale transformations and films with cell-like internal structures comprised of networks of closed pores, interconnected pores, and through-pores with sizes as large as 10-15 μm depending on pH and the manner in which the films were incubated. Films fabricated using fluorescently end-labeled samples of PAA permitted real-time imaging of changes in internal structure using confocal microscopy (LSCM). The results of these studies also revealed large differences in the nature of these transformations when films were placed in contact with surfaces as opposed to when dipped into aqueous solutions. Our results reveal approaches that can be used to fabricate films with large pores (e.g., pores with sizes on the order of 10-15 μm) and suggest methods that could potentially be used to generate PEMs having controlled gradients in pore size.