Regulated Layer-by-Layer Assembly of Polyelectrolyte Multilayers in Microchannel Devices

In a recent paper published in the journal Composites Part B, researchers developed a new barrier film to effectively inhibit the penetration of organic solvents that deteriorates the device performance in microfluidic systems such as inkjet printing or lab-on-a-chip devices.

Study: Highly penetrant organic solvent-resistant layer-by-layer assembled ultra-thin barrier coating for confined microchannel devices. Image Credit: Gorodenkoff/Shutterstock.com

They deposited barrier layers inside microchannels using a surface charge-regulated layer-by-layer stacking of polyelectrolyte multilayers. A highly negatively charged PEM surface of -64.2 mV with inner pores less than 0.5 nm resulted in excellent barrier performance.

Uses of Organic Solvent-Resistant Barrier Films

Organic solvents in microfluidic systems deteriorate the device performance and lifetime due to channel blockage and swelling deformation. In the case of organic electronic device manufacturing, pre-deposited organic layers are susceptible to the organic solvent present in the newly deposited organic layers during the solution-based layer stacking and integration process.

Furthermore, the swelling deformation of the polymeric microchannels in inkjet printing, food/drug packaging, organic solvent nanofiltration (OSN), and lab-on-a-chip devices are inhibited by using such organic solvent barrier films (OSBFs).

Layer-by-layer (LbL) stacking has been used in many applications owing to its excellent ability to smoothly and precisely form a highly complex and multifunctional nanoscale coating film on any type of surface incorporating various deposition techniques such as dipping, spin coating, spraying, and microfluidic assembly.

The LbL-stacked polyelectrolytes multilayers (PEM) films consisting of oppositely charged polyelectrolyte chains, and their stacking conditions, such as zeta potential of the polyelectrolytes, salt concentration, and pH condition are of great importance for controlling the internal porosity and surface properties of the LbL-stacked thin films.

The PEM films synthesized with poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (PSS) are efficient organic solvent barriers due to the presence of much smaller internal pore size, the Donnan repulsion effect between a net negative surface charge of PEM surface and negatively charged organic solvents, and crosslinking promoted by glutaraldehyde (GA) or dialdehyde starch.

About the Study

In this study, first positively charged PAH, relatively more negatively charged PSS, and poly( acrylic acid) PAA polyelectrolyte solutions were prepared by dissolving the polymer in deionized (DI) water containing sodium chloride (NaCl).

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After that, plasma treatment induced a negatively charged surface on the substrate followed by spin coating of positive PAH and negative PSS successively on it until the desired bilayer thickness was achieved. Finally, GA crosslinking was performed on the PAH/PSS film by immersing it in a GA solution. Similarly, PAH/PAA film was synthesized to make a comparison with PAH/PSS film.

Subsequently, wrinkling-based metrology for precisely evaluating the barrier property of the multilayered films was used. A poly(dimethylsiloxane) (PDMS) microchannel was fabricated by embedding an ultrahigh-molecular-weight polyethylene fiber with a diameter of 200 μm in the film followed by thermally curing the matrix and smoothly removing the fiber to form a wavy-patterned single microchannel.

Finally, PAH/PSS and PAH/PAA solutions were flowed into the PDMS microchannels to form a GA-crosslinked PAH/PSS barrier layer around the inner wall of the microchannel followed by pouring various organic solvents to analyze the barrier performance of each crosslinked microchannel separately.

Observations

The  PEM film of PAH/PSS demonstrated unsatisfactory barrier performance as it had weak surface charges and large intrinsic pores, which caused the stacked film to be susceptible to erosion by organic solvents. The subsequent GA crosslinking process reduced the inner pore size by densifying the PEM film and improved the negative surface charge by promoting the formation of imine bonds between the ammonium groups in the PAH layer.

Thus, the zeta potential of the PAH/PSS film increased from -44.5 to -64.2 mV, which further strengthened the repulsion of the negatively charged organic solvent molecules from the barrier surface.

Moreover, GA crosslinking was confirmed by distinctly developed surface wrinkles. Also, Fourier transformation infrared spectroscopy (FT-IR) confirmed that GA crosslinking enhanced the barrier performance of the LbL-stacked PEM films owing to the conversion of the primary ammonium groups in PAH to imine bonds.

An uncoated-barrier PDMS microchannel swollen by organic solvents was evident when the microchannel diameter decreased from 208 to 191 μm with a subsequent decrease in the flow rate of the solution from 20.63 to 17.41 μL/s. However, a GA-crosslinked PAH/PSS PEM film demonstrated a negligible change in diameter.

Conclusions

The researchers prepared LbL stacked bilayer PEM films using PAH, PSS, and PAA with different combinations, followed by GA crosslinking as a barrier coating against organic solvents. GA-crosslinked PAH/PSS barrier layer exhibited excellent barrier performance in PDMS microchannels against organic solvents and proved to be a promising candidate for applications in microfluidic systems.

Reference

Hong, S., Yoo, S., Choi, G., Lee, J., Choi, Y., Kim, M., Kim, E., Kwon, S., Kim, D., Park, J., Yoo, P., Highly penetrant organic solvent-resistant layer-by-layer assembled ultra-thin barrier coating for confined microchannel devices, Composites Part B: Engineering, Volume 230, 2022, 109537, ISSN 1359-8368, https://www.sciencedirect.com/science/article/pii/S1359836821009033

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Bismay Prakash Rout

Written by

Bismay Prakash Rout

Bismay is a technical writer based in Bhubaneshwar, India. His academic background is in Engineering and he has extensive experience in content writing, journal reviewing, mechanical designing. Bismay holds a Masters in Materials Engineering and BE in Mechanical Engineering and is passionate about science & technology and engineering. Outside of work, he enjoys online gaming and cooking.

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