(7fr) Applying CVD Polymers in Membrane Separation, Biomedical Devices and Soft Electronics

Research Interests:

My current and future research interest lies in the chemical vapor deposition (CVD) of multifunctional polymer thin films, including microporous polymers, zwitterionic polymers, polymer based ion gels and conducting polymers, and their applications in gas separation, water desalination/purification, biomedical devices and soft electronics. Two CVD techniques, initiated CVD (iCVD) and oxidative CVD (oCVD) are chosen for their solvent-free nature. Due to this unique feature, iCVD and oCVD are distinct from solution-based or melt-processing methods for being able to directly form ultrathin (<10 nm), large area, defect-free and conformal polymer coatings on almost any substrates from vapor phase monomers in one step.

1. Previous and Current Research

1.1. Fabricating ultrathin polymer membranes for gas separations: Achieving both high flux and high gas selectivity is the ultimate goal in the membrane gas separation community. Reducing the thickness layer should increase the gas flux due to the inverse relationship between the membrane thickness and the gas flux. However, there is known challenge to achieve sub-100 nm pinhole-free gas-selective layer via conventional phase inversion technique. In this regard, three unique techniques have been employed to fabricate pinhole free sub-100 nm gas-selective layer. The use of these techniques also expands the library of gas-selective polymer membranes. These studies were carried out under the supervision of Prof. Karen Gleason at MIT and Prof. Steven Regen at Lehigh University.

1) Sub-100 nm gas-selective microporous polymer films synthesized via CVD.

Microporous polymers represent a class of most promising membrane materials for gas separation. However, two representative types of microporous polymers, polymers of intrinsic microporosity (PIMs) and hyper-crosslinked polymers (HCPs) both have their drawbacks. PIMs are subject to physical aging at low thickness due to their linear structure, while the highly crosslinked HCPs are difficult to form thin films. Therefore, an initiated plasma enhanced CVD technique was employed to solve this dilemma. The choice of monomer was a porphyrin derivative Zinc-tetraphenylporphyrin (ZnTPP). Porphyrin and its derivatives are promising monomers for synthesizing microporous polymer films due to their robust and rigid macrocycle structure. Nevertheless, there is no successful example of porphyrin derived polymeric membranes by conventional solution-based techniques. In contrast, ZnTPP derived ultrathin (<100 nm) and large area (175 cm²) microporous polymer films were successfully synthesized via the iPECVD technique. These films displayed high selectivities for multiple gas pairs, including CO₂/N₂, CO₂/CH₄, O₂/N₂ etc. Gas-selective ultrathin films derived from CoTPP, MnTPP and TPP were also successfully synthesized by the same technique. Furthermore, such films can be crosslinked by adding co-monomer to the vacuum system to further enhance the film stability. This research represents the first effort on fabricating highly gas-selective CVD polymer films.

2) Sub-10 nm Langmuir-Blodgett (LB) Bilayer as Highly H₂ Selective Membranes.

LB technique is known for its ability to deposit ultrathin films at nanometer scale; however, the defect (e.g., pinhole) control has been a persistent issue, which prevents the technique from applying in the (defect-sensitive) gas separation field. To overcome this difficulty, a strategy combining the organic synthesis and LB deposition was employed. In short, a series of charge bearing calix[n]arene based surfactants and polymeric surfactants have been synthesized and used as the building blocks of the LB bilayers. The defects were then removed by inserting polyelectrolyte in between the bilayer of surfactants to ionically crosslink the surfactant building blocks. Extremely high H₂/CO₂ selectivity (ca. 200) was achieved for a 7 nm LB bilayer by optimizing the surfactant/polyelectrolyte combination and the depositing condition. The intrinsic H₂/CO₂ separation performance of the 7 nm LB bilayer reached Robeson’s Upper bound, which is the best performance reported for a 7 nm polymer membrane.

3) Sub-20 nm Polyelectrolyte Multilayers (PEMs) as Highly CO₂ Selective Membranes.

Polyelectrolyte Layer-by-Layer (LbL) deposition is known for depositing defect-free nanometer thick ultrathin films of different compositions. By combining the polyelectrolyte synthesis and the LbL technique, a series of sub-20 nm PEMs were fabricated to study their gas separation performances. Ultrahigh CO₂/N₂ selectivity (up to 120) was observed for a 14 nm PEMs membrane. Further study revealed that by optimizing the size of the polyelectrolyte side group, the permeance of CO₂ can be doubled while maintaining the CO₂/N₂ selectivity above 100. The research demonstrated the feasibility of applying polyelectrolyte LbL technique in the field of gas separation.

1.2. Fabricating Ultrathin Ion Gels via Initiated CVD (iCVD) and Their Application as Thin Film Transistor (TFT) Gate Insulator.

Ion gel is considered as a potential gate insulator replacement for oxides or polymer thin films in TFTs operated at low voltage, due to its large capacity (>1 μF/cm²). However, due to the inherent slow polarization speed, current ion gel membranes suffer from significant capacity loss at higher operation frequency (e.g., > 20 KHz). Thinning down the membrane is considered to be the best strategy to overcome this weakness, but was not possible to obtain membranes at < 1 µm by solution-based techniques. In contrast, by employing an iCVD technique, a series of ultrathin crosslinked polymers were deposited as the base membranes, followed by injecting ionic liquid into the based membranes to form ion gel membranes with thicknesses from 20 nm to 2000nm. A clear thickness dependency of the ion gel capacity at high frequency is observed. By reducing the thickness of Ion gel membranes below 300 nm, the capacity of the ion gel can be maintained at 1 to 2 μF/cm² from 10 Hz to 1MHz. This capacity value is 1 order higher than the best polymer film insulator reported to date (Moon et al., Nat. Mater. 2015, 14, 628). At the same time, the capacity value at higher frequency (20 KHz to 1MHz) is far superior than the best performance reported to date for solution-processed ion gel membranes. (Cho et al., Nat. Mater. 2008, 7, 900). The study was carried out under the supervision of Prof. Karen Gleason at MIT.

1.3. Surface modification via Initiated CVD (iCVD) for applications in water desalination.

Biolfouling and scaling are two persistent issues in the field of water desalination. Biofouling affects membrane desalination, e.g., reverse osmosis (RO) desalination, while mineral scaling affects mostly the thermal based desalination processes, e.g., multi-stage flash distillation (MSF). To this regard, both ultrathin amphiphilic polymer and zwitterionic polymer films have been deposited onto RO membranes to render surfaces with desired fouling-resistant properties. As for scale control in the heat exchange tubes, the research has focused on the design of a new depositing technique to deposit on the internal wall of the heat exchange tube. UV lamp instead of conventional hot filament is chosen for better heat management capability. The use of UV lamp also simplified the deposition process by avoiding the use of initiator, because the UV can trigger the self-polymerization of the monomers. The studies were carried out under the supervision of Prof. Karen Gleason at MIT.

2. Future Research Plans:

2.1. Fabricating ultrathin microporous polymer membranes and their composite membranes with metal organic framework (MOF) via iCVD and oCVD, for gas separation, and potential applications in water purification, energy storage, catalysis and sensors.

2.2. Fabricating ultra-stable fouling-resistant zwitterionic polymers via iCVD, for their applications in water desalination/purification, oil/water separation and biomedical devices (e.g., electrode for bio-implants, endoscope etc.)

2.3. Fabricating ion gels via iCVD and conducting polymers via oCVD for their applications in soft electronics and in the field of renewable energy.

Teaching Interests:

I would like to teach polymer science and engineering core classes, including polymer chemistry, polymer physics, polymer science and engineering, polymer laboratory, plastics engineering etc. I can also teach some of the core courses in Chemical Engineering, Materials Science and Engineering and Chemistry. For instances, transport process, thermodynamics and kinetics, Laboratory Chemistry, Introduction to Chemical Engineering, Inorganic Chemistry, physical chemistry, organic chemistry and analytical chemistry etc.

I am also happy to design and teach new classes, such as CVD polymers, membrane separation processes, metallic/conducting polymers etc.

Selected Publications:

1. Andong Liu^†, Minghui Wang^†, Stefan Schroeder, Junjie Zhao & Karen K. Gleason, “Nanoscale Pinhole-Free Ion-Gel Film Enabled by Initiated Chemical Vapor Deposition: A Promising Material as Gate Insulator for Low-Voltage High-Speed Thin Film Transistors”, (†Equally contributed, manuscript finished and to be submitted, patent application in process)
2. Minghui Wang, Peter Kovacik & Karen K. Gleason, “Conductive and Fouling-Resistant Zwitterionic Polymeric Films Synthesized by Chemical Vapor Deposition”, Manuscript finished and to be submitted, MIT disclosure form filed, MIT Case 19732J)
3. Nicolas D. Boscher^†, Minghui Wang^†, Alberto Perrota, Katja Heinze, Mariadriana Creatore & Karen K. Gleason, “Metal Organic Covalent Network Chemical Vapor Deposition for Gas Separation”, Advanced Materials, 2016, 28, 7479-7485. (†Equally contributed. Related Patent Application, US20170158809A1)
4. Minghui Wang, Nicolas D. Boscher, Katja Heinze & Karen K. Gleason, “Gas Selective Ultrathin Organic Covalent Networks Synthesized by iPECVD: Does the Central Metal Ion Matter?” Advanced Functional Materials, 2017, DOI:10.1002/adfm.201606652.
5. Minghui Wang, Junjie Zhao, Xiaoxue Wang, Andong Liu & Karen K. Gleason, “Recent Progress on Submicron Gas-selective Polymeric Membranes”, Journal of Materials Chemistry A, 2017, DOI: 10.1039/C7TA01862B
6. Minghui Wang^†, Xiaoxue Wang^†, Priya Moni^†, Andong Liu, Do Han Kim, Won Jun Jo, Hossein Sojoudi & Karen K. Gleason, “CVD Polymers for Devices and Device Fabrication”, Advanced Materials, 2017, 29, 1604606. (†Equally contributed)
7. Minghui Wang, Vaclav Janout & Steven L. Regen, “Gas Transport Across Hyperthin Membranes”, Accounts of Chemical Research, 2013, 46 (12), 2743-2754.
8. Minghui Wang, Song Yi, Vaclav Janout & Steven L. Regen, “A 7 nm Thick Polymeric Membrane With a H₂/CO₂ Selectivity of 200 That Reaches the Upper Bound”, Chemistry of Materials, 2013, 25 (19), 3785-3787.
9. Minghui Wang, Vaclav Janout & Steven L. Regen, “Unexpected Barrier Properties of Structurally Matched and Unmatched Polyelectrolyte Multilayers”, Chemical Communications, 2013, 49, 3576-3578.
10. Minghui Wang, Vaclav Janout & Steven L. Regen, “Hyper-thin Organic Membranes with Exceptional H₂/CO₂ Permeation Selectivity: Importance of Ionic Crosslinking and Self-healing”, Chemical Communications, 2011, 47, 2387-2389.