Humidity-Controlled Growth of Self-Healing Poly(ethylenimine)/Poly(acrylic Acid) (PEI/PAA) Multilayer Films

Functional polymer coatings with tunable electrical and surface properties are central to applications in organic electronics, sensing, and practical coatings, where durability and self-healing (SH) capability are also critical for reliable operation. A good candidate for these applications is Poly(ethylenimine)/poly(acrylic acid) (PEI/PAA) layer-by-layer (LbL) films with intrinsic SH ability, whose properties are strongly influenced by the amount of water retained within the multilayer structure during growth. In this paper, we investigate PEI/PAA multilayers deposited under controlled relative humidity (RH = 50%, 70%, 90%) to clarify the role of water in film formation and performance. Films deposited at 50% and 70% RH exhibit capacitances of approximately 1.06 nF and 0.70 nF, respectively, while those grown at 90% RH reach approximately 5 nF. Electrical transport shows a pronounced negative differential resistance (NDR) peak at around 1.3 V, strongest in the 90% RH film. Through terahertz time-domain spectroscopy (THz-TDS), we independently confirm hydration differences through changes in the dielectric constant and, consequently, the refractive index, which manifest as relative delays of the transmitted ultrashort terahertz pulses. Surface characterization reveals a increase in roughness, from 1.8 ± 0.2 μm to 14.8 ± 3.1 μm, accompanied by enhanced hydrophobicity (contact angle rising from 68° to 90°). Only the 90% RH film exhibits complete self-healing within 10 min. Overall, humidity-controlled deposition provides a simple, scalable strategy to tune dielectric response, surface functionality, and SH performance in weak polyelectrolyte multilayers. Such control enables the design of reproducible, multifunctional coatings for next-generation soft-electronic and sensing applications.