Modulating Polyelectrolyte Multilayer Growth and Nanoscale Surface Patterning via an Alternating pH-Stack Assembly Strategy

Advanced materials with tunable wettability and nanoscale surface patterning are vital for both fundamental studies and application-driven technologies. This work investigates how pH, ionic strength, and layer-by-layer (LbL) deposition strategies influence the structural evolution and surface features of branched polyethylenimine (BPEI)/poly(styrenesulfonate) (PSS) multilayers. The LbL strategy was employed using both conventional single-pH deposition and an alternating pH-stack assembly (ApHSA) approach to investigate how interdiffusion and electrostatic modulation drive structural and surface changes. At 0.4 M NaCl, films assembled at pH 5 exhibited granular nodules that coalesced into a vermicular network accompanied by linear growth and hydrophilic surfaces. In contrast, films deposited at pH 9 showed exponential growth, evolving from fine clusters to smooth hydrophobic surfaces. Water contact angles varied widely from 47° (pH 5, 1 M NaCl, 5 bilayers) to 103° (pH 9, 0.4 M NaCl, 10 bilayers), indicating a strong sensitivity to deposition conditions. The ApHSA protocol, applied at 0.4 M NaCl, produced sigmoidal growth curves and surface morphologies that transitioned from vermicular patterns to discrete aggregates over successive stack units. This progression reflects enhanced lateral and vertical interdiffusion enabled by optimized ionic screening. Concurrently, wettability shifted smoothly from hydrophobic to hydrophilic with increasing stack units. Preliminary transport measurements revealed that ApHSA multilayers exhibit improved methanol blocking while maintaining moderate proton conductivity, reflecting a more selective ionic architecture. Although the overall performance is not yet superior to that of benchmark Nafion membranes, these results demonstrate that pH-programmed charge modulation can balance transport selectivity and morphological control. These findings demonstrate that simple modulation of the deposition pH enables programmable control over multilayer architecture and surface properties using standard polyelectrolytes. The ApHSA strategy offers a scalable and versatile platform for engineering multifunctional polymer coatings with tailored interfacial characteristics.