Polydopamine-based chemistry has
been employed for various surface
modifications attributed to the advantages of universality, versatility,
and simplicity. Co-deposition of polydopamine (PDA) with polyethyleneimine
(PEI) has then been proposed to realize one-step fabrication of functional
coatings with improved morphology uniformity, surface hydrophilicity,
and chemical stability. Herein, we report the co-deposition kinetics
related to the solution composition with different dopamine/PEI ratios,
PEI molecular weights, dopamine/PEI concentrations, and the substrate
surface with varying chemistry and wettability. The addition of PEI
to dopamine solution suppresses the precipitation of PDA aggregates,
resulting in an expanded time window of steady co-deposition compared
with that of PDA deposition. Low-molecular-weight PEI at low concentration
accelerates the co-deposition process, while high-molecular-weight
PEI and high concentration of either PEI or dopamine/PEI are detrimental
to the co-deposition efficiency. Meanwhile, the surface morphology
and chemical composition of the co-deposition coatings can be regulated
by the solution conditions during co-deposition. Moreover, obvious
deviations in the co-deposition rate and the amount of substrates
bearing various functional groups, such as alkyl, phenyl, hydroxyl,
and carboxyl, are revealed, which are quite different from PDA deposition.
The initial adsorption rates further reflect the change in interactions
between the aggregates and these substrates caused by PEI, which follows
the sequence of carboxyl > hydroxyl > alkyl > phenyl. These
results
provide deep insights into the PDA/PEI co-deposition process on various
substrates.
Thin-film composite (TFC) nanofiltration membranes are prepared via interfacial polymerization with a polyphenol coating as an interlayer for the thin and smooth polyamide selective layer. The polyphenol interlayer is simply fabricated by the codeposition of tannic acid and diethylenetriamine without changing the surface morphology of the polysulfone ultrafiltration substrate. An interfacial polymerization is conducted from piperazidine and trimesoyl chloride on the polyphenol interlayer to construct the polyamide selective layer. The as-prepared TFC nanofiltration membranes show nearly tripled fold of water permeation flux as compared with those prepared at the same condition without an interlayer. They also exhibit a high rejection to NaSO (>98%) because the thin and defect-free polyamide selective layer is formed on the polyphenol interlayer. These nanofiltration properties have high reproducibility, which means the TFC nanofiltration membranes are suitable for scale-up industrial applications.
A mussel-inspired interlayer of polydopamine (PDA)/polyethylenimine (PEI) is codeposited on the ultrafiltration substrate to tune the interfacial polymerization of piperazine and trimesoyl chloride for the preparation of thin-film composite (TFC) nanofiltration membranes (NFMs). This hydrophilic interlayer results in an efficient adsorption of piperazine solution in the substrate pores. The solution height increases with the PDA/PEI codeposition time from 45 to 135 min due to the capillary effect of the substrate pores. The prepared TFC NFMs are characterized with thin and smooth polyamide selective layers by ATR/IR, XPS, FESEM, AFM, zeta potential, and water contact angle measurements. Their water permeation flux measured in a cross-flow process increases to two times as compared with those TFC NFMs without the mussel-inspired interlayer. These TFC NFMs also show a high rejection of 97% to NaSO and an salt rejection order of NaSO ≈ MgSO > MgCl > NaCl.
Mussel-inspired chemistry, particularly the versatile coating capability of polydopamine (PDA), has received much research interest as a promising strategy for fabricating functional coatings in numerous fields. However, the understanding of deposition mechanisms and adhesion behaviors of PDA on different substrates still remains incomplete, significantly limiting the related fundamental research and its practical applications. In this work, a colloidal probe atomic force spectroscopy technique was employed to quantify the interaction forces and adhesion between the PDA coatings and the substrate surfaces with different wettabilities. The surface force measurements and thermodynamic analysis of interaction energy indicate that the surface wettability has a significant influence on the adhesion, deposition behaviors, and morphologies of PDA coatings. Compared with the hydrophilic surfaces, the hydrophobic surfaces exhibit stronger adhesion with the PDA coatings. Furthermore, for the first time, this work demonstrates that ethanol has the capability of effectively displacing the trapped air/vapor layer or the so-called "hydrophobic depletion layer" on the hydrophobic substrate to allow the intimate contact between PDA and the substrate, thus enhancing the adhesion and facilitating the PDA deposition. This work provides new insights into the fundamental PDA deposition mechanism as well as the design and development of versatile mussel-inspired coatings on the substrates of varying hydrophobicity.
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