@article{ko_truong_woo_dickey_hsiao_genzer_2021, title={Counterpropagating Gradients of Antibacterial and Antifouling Polymer Brushes}, volume={12}, ISSN={["1526-4602"]}, url={https://doi.org/10.1021/acs.biomac.1c01386}, DOI={10.1021/acs.biomac.1c01386}, abstractNote={We report on the formation of counterpropagating density gradients in poly([2-dimethylaminoethyl] methacrylate) (PDMAEMA) brushes featuring spatially varying quaternized and betainized units. Starting with PDMAEMA brushes with constant grafting density and degree of polymerization, we first generate a density gradient of quaternized units by directional vapor reaction involving methyl iodide. The unreacted DMAEMA units are then betainized through gaseous-phase betainization with 1,3-propanesultone. The gas reaction of PDMAEMA with 1,3-propanesultone eliminates the formation of byproducts present during the liquid-phase modification. We use the counterpropagating density gradients of quaternized and betainized PDMAEMA brushes in antibacterial and antifouling studies. Completely quaternized and betainized brushes exhibit antibacterial and antifouling behaviors. Samples containing 12% of quaternized and 85% of betainized units act simultaneously as antibacterial and antifouling surfaces.}, journal={BIOMACROMOLECULES}, publisher={American Chemical Society (ACS)}, author={Ko, Yeongun and Truong, Vi Khanh and Woo, Sun Young and Dickey, Michael D. and Hsiao, Lilian and Genzer, Jan}, year={2021}, month={Dec} }
 @article{liu_islam_ku_boyd_zhong_urbas_smith_derov_nguyen_kim_et al._2021, title={Novel computational design of high refractive index nanocomposites and effective refractive index tuning based on nanoparticle morphology effect}, volume={223}, ISSN={["1879-1069"]}, url={https://doi.org/10.1016/j.compositesb.2021.109128}, DOI={10.1016/j.compositesb.2021.109128}, abstractNote={This study introduces a method to predict the refractive index (RI) of nanocomposites with the Finite Elements Analysis (FEA) based on the Fabry-Pérot interference. The efficacy was verified by comparing the estimated composites’ RI with the available data in the literature. In the experimental verification, the FEA-based prediction showed closer results with the measurement as compared to the effective medium approximation (EMA) approaches, which are prevalently used to predict the physical properties of nanocomposites. Due to the modeling capability, the FEA-method could investigate the effect of the nanoparticle morphology (particle size, shape, and orientation) and distribution. Large particle size, particle agglomeration in high electric-field amplitude region, and particle elongation along the light oscillating direction are found to be the major factors to enhance the RI of composites. The underlying mechanism of RI changing is attributed to the light scattering by embedded nanoparticles, which provides one potential real-time RI tuning schematic.}, journal={COMPOSITES PART B-ENGINEERING}, publisher={Elsevier BV}, author={Liu, Sipan and Islam, Md Didarul and Ku, Zahyun and Boyd, Darryl A. and Zhong, Yaxu and Urbas, Augustine M. and Smith, Evan and Derov, John and Nguyen, Vinh Q. and Kim, Woohong and et al.}, year={2021}, month={Oct} }
 @article{ko_christau_klitzing_genzer_2020, title={Charge Density Gradients of Polymer Thin Film by Gaseous Phase Quaternization}, volume={9}, ISSN={["2161-1653"]}, DOI={10.1021/acsmacrolett.9b00930}, abstractNote={We report on the rapid formation of charge density gradients in polymer films by exposing poly([2-dimethylaminoethyl] methacrylate) (PDMAEMA) films resting on flat silica substrates to methyl iodide (i.e., MI, also known as iodomethane) vapors. We adjust the charge gradient by varying the MI concentration in solution and the process time. The thickness of the parent PDMAEMA film does not affect the diffusion of MI through and the reaction kinetics in the films. Instead, the diffusion of MI through the gaseous phase constitutes the limiting step in the overall process.}, number={2}, journal={ACS MACRO LETTERS}, author={Ko, Yeongun and Christau, Stephanie and Klitzing, Regine and Genzer, Jan}, year={2020}, month={Feb}, pages={158–162} }
 @article{miles_ko_genzer_2020, title={Dependence of deposition method on the molecular structure and stability of organosilanes revealed from degrafting by tetrabutylammonium fluoride}, volume={22}, ISSN={["1463-9084"]}, DOI={10.1039/c9cp05221f}, abstractNote={We probe the structure of self-assembled monolayers (SAMs) comprising organosilanes deposited on flat silica-based surfaces prepared by liquid and vapor deposition by removing the organosilane molecules gradually from the underlying substrate via tetrabutylammonium fluoride (TBAF). Removal of organosilanes from the surface involves the cleavage of all pertinent Si-O bonds that anchor the organosilane molecules to the SAM, i.e., direct organosilane-surface linkages and in-plane crosslinks between neighboring organosilanes. We gain insight into the organosilane structure and stability by monitoring the organosilane density as a function of exposure time to TBAF. Degrafting of trifunctional chloro- and methoxy-alkylsilanes deposited from solution yields similar degrafting kinetics. We observe fast degrafting for organosilane SAMs deposited from the vapor phase, indicating that SAMs prepared in this manner form more loosely packed arrays, with less in-plane connectivity, compared to their solution-deposited counterparts. Bulkier, fluorinated silanes form more stable SAMs due to their ability to readily align and form a network with few aggregates and a relatively high fraction of surface bonds. The addition of a polymer brush to an anchored organosilane molecule demonstrates that increased bond tension accelerates the degrafting process despite the increased diffusion resistance.}, number={2}, journal={PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, author={Miles, Jason and Ko, Yeongun and Genzer, Jan}, year={2020}, month={Jan}, pages={658–666} }
 @article{islam_kim_ko_ku_boyd_smith_nguyen_myers_baker_kim_et al._2020, title={Design of High Efficient Mid‐Wavelength Infrared Polarizer on ORMOCHALC Polymer}, volume={305}, ISSN={1438-7492 1439-2054}, url={http://dx.doi.org/10.1002/mame.202000033}, DOI={10.1002/mame.202000033}, abstractNote={Abstract While an organically modified chalcogenide (ORMOCHALC) can be used to fabricate a polymeric mid‐wavelength infrared (MWIR) polarizer with competitive extinction ratio compared to the commercial wire‐grid polarizers, which are composed of fragile inorganic materials, there is still a knowledge gap regarding the systematic design process to obtain high transmission efficiency and extinction ratio. To this end, a computational parameter study for design optimization is conducted with the geometric parameters of the bilayer grating ORMOCHALC polarizer. The computational study shows that the Fabry–Pérot cavity is the primary mechanism that determines the transmission behaviors and the extinction ratio. A bilayer grating design, guided by the parameter study, is realized through the thermal nanoimprint and metal deposition processes. The extinction ratios measured with the Fourier‐transform infrared are 245, 304, and 351 at the wavelength of 3, 4, and 5 µm, respectively. Compared to the state‐of‐the‐art of the polymeric MWIR linear polarizers, the extinction ratio is improved by 1.4 times, and the transmission efficiency is increased by 2.5 times. Theoretical analysis with the multiple‐layer model based on the transfer matrix method predicts a matched transmission behavior with the experiment and a full‐wave electromagnetic simulation.}, number={5}, journal={Macromolecular Materials and Engineering}, publisher={Wiley}, author={Islam, Md Didarul and Kim, Jun Oh and Ko, Yeongun and Ku, Zahyun and Boyd, Darryl A. and Smith, Evan M. and Nguyen, Vinh Q. and Myers, Jason D. and Baker, Colin C. and Kim, Woohong and et al.}, year={2020}, month={Mar}, pages={2000033} }
 @article{islam_liu_boyd_zhong_nahid_henry_taussig_ko_nguyen_myers_et al._2020, title={Enhanced mid-wavelength infrared refractive index of organically modified chalcogenide (ORMOCHALC) polymer nanocomposites with thermomechanical stability}, volume={108}, ISSN={["1873-1252"]}, url={http://dx.doi.org/10.1016/j.optmat.2020.110197}, DOI={10.1016/j.optmat.2020.110197}, abstractNote={Abstract Organically modified chalcogenide (ORMOCHALC) polymers have proven to be alternatives to the conventional inorganic materials for mid-wavelength infrared (MWIR, λ = 3–5 μm) optical components. While the refractive index of ORMOCHALC can be reinforced by the content of chalcogenides such as sulfur (S) and selenium (Se), the increased portion of the S or Se deteriorate the thermomechanical stabilities. As a remedy, this study utilizes ZnS nanoparticles to reinforce both optical and thermomechanical properties of the sulfur-based ORMOCHALC polymer, poly(S-random-1,3-diisopropenylbenzene). The refractive index n and extinction coefficient k of the nanocomposites were characterized by Infrared Variable Angle Spectroscopic Ellipsometry (IR-VASE). The results show a significant increment in the refractive index of Δn = 6.58% at the wavelength of 4 μm by adding 20 wt% ZnS (or 7.29 vol%) in the ORMOCHALC polymer. The low extinction coefficient of the nanocomposites (}, journal={OPTICAL MATERIALS}, author={Islam, Md Didarul and Liu, Sipan and Boyd, Darryl A. and Zhong, Yaxu and Nahid, Masrur Morshed and Henry, Reece and Taussig, Laine and Ko, Yeongun and Nguyen, Vinh Q. and Myers, Jason D. and et al.}, year={2020}, month={Oct} }
 @article{ko_miles_genzer_2019, title={Determining Water Sorption and Desorption in Thin Hydrophilic Polymer Films by Thermal Treatment}, volume={1}, ISSN={["2637-6105"]}, DOI={10.1021/acsapm.9b00607}, abstractNote={Knowledge of dry thickness of polymer films is required to determine the areal density of polymeric grafts and the degree of swelling of polymer networks and surface-anchored polymer assemblies. Because hydrophilic polymer films absorb water at ambient conditions and retain it, it is challenging to establish accurate dry thickness in such systems. Here we report on determining water uptake by chargeable/charged polymer films by monitoring the coefficient of thermal expansion (CTE) and thermo-optic coefficient (TOC) using ellipsometry. Knowing accurate amount of moisture in polymer films is needed for numerous applications, including, humidity and temperature sensors, polymer nanoreactors, lubricating coatings, antibacterial surfaces, and many others.}, number={9}, journal={ACS APPLIED POLYMER MATERIALS}, author={Ko, Yeongun and Miles, Jason R. and Genzer, Jan}, year={2019}, month={Sep}, pages={2495–2502} }
 @article{ma_lin_kim_ko_kim_oh_sun_gorman_voinov_smirnov_et al._2019, title={Liquid Metal Nanoparticles as Initiators for Radical Polymerization of Vinyl Monomers}, volume={8}, ISSN={["2161-1653"]}, url={https://doi.org/10.1021/acsmacrolett.9b00783}, DOI={10.1021/acsmacrolett.9b00783}, abstractNote={Sonication of gallium or gallium-based liquid metals in an aqueous solution of vinyl monomers leads to rapid free radical polymerization (FRP), without the need for conventional molecular initiators. Under ambient conditions, a passivating native oxide separates these metals from solution and renders the metal effectively inert. However, sonication generates liquid metal nanoparticles (LMNPs) of ∼100 nm diameter and thereby increases the surface area of the metal. The exposed metal initiates polymerization, which proceeds via a FRP mechanism and yields high molecular weight polymers that can form physical gels. Spin trapping EPR reveals the generation of free radicals. Time-of-flight secondary ion mass spectrometry measurements confirm direct polymer bonding to gallium, verifying the formation of surface-anchored polymer grafts. The grafted polymers can modify the interfacial properties, that is, the preference of the metal particles to disperse in aqueous versus organic phases. The polymer can also be degrafted and isolated from the particles using strong acid or base. The concept of physically disrupting passivated metal surfaces offers new routes for surface-initiated polymerization and has implications for surface modification, reduction reactions, and fabrication of mechanically responsive materials.}, number={11}, journal={ACS MACRO LETTERS}, publisher={American Chemical Society (ACS)}, author={Ma, Jinwoo and Lin, Yiliang and Kim, Yong-Woo and Ko, Yeongun and Kim, Jongbeom and Oh, Kyu Hwan and Sun, Jeong-Yun and Gorman, Christopher B. and Voinov, Maxim A. and Smirnov, Alex I. and et al.}, year={2019}, month={Nov}, pages={1522–1527} }
 @article{li_lin_dai_ko_genzer_2019, title={Mechanochemical Degrafting of a Surface-Tethered Poly(acrylic acid) Brush Promoted Etching of Its Underlying Silicon Substrate}, volume={35}, ISSN={["0743-7463"]}, DOI={10.1021/acs.langmuir.9b02610}, abstractNote={The stability of surface-tethered polyelectrolyte brushes has been investigated during the past few years. We have previously reported on degrafting of poly(acrylic acid) (PAA) polymer brushes from flat silicone substrates. Here we present detailed study on the effects of NaCl concentration and the grafting density and molecular weight on the stability of PAA brushes during incubation in 0.1 M ethanolamine buffer (pH 9.0) solutions. Without NaCl in the buffer solution, the PAA brushes remain intact. Adding NaCl facilitates etching of the substrate due to accelerating dissolution of the top silica layer and promoting degrafting of the PAA chains. The PAA grafting density and molecular weight play an important role in substrate etching by affecting the penetration barrier and local concentration of the etchants. We also tested the stability of self-assembled monolayers (SAMs) made of hydrophobic alkyltrichlorosilanes anchored on silicon substrates. The results demonstrated that the SAMs were too thin to protect the substrates from etching, in contrast to thick poly(methyl methacrylate) (PMMA) brushes. Our findings suggest that both polymer brushes (especially polyelectrolyte brushes) and SAMs anchored to silicon substrates may undergo erosion/etching on the substrates in basic environments, which compromises their stability and therefore jeopardizes their applications in coating, biosensing, etc.}, number={42}, journal={LANGMUIR}, author={Li, Yuanchao and Lin, Yiliang and Dai, Yunkai and Ko, Yeongun and Genzer, Jan}, year={2019}, month={Oct}, pages={13693–13699} }
 @article{ko_genzer_2019, title={Spontaneous Degrafting of Weak and Strong Polycationic Brushes in Aqueous Buffer Solutions}, volume={52}, ISSN={["1520-5835"]}, DOI={10.1021/acs.macromol.9b01362}, abstractNote={Polymers grafted to substrates have traditionally been considered stable because of the covalent bonds that hold the polymers attached to the substrate. However, several recent reports have indicated that grafted polymers may detach from substrates under specific conditions. In this work, we report on a systematic study of polymer degrafting involving polycationic brushes with different degrees of quaternization (DQ, mol %), which have been incubated in three different buffer solutions (pH 4, 7.4, and 9) with the same ionic strength of 0.05 M. We have varied the molecular weight (MW) and grafting density (σ) of the polymer brushes using a combinatorial setup to examine the effect of MW, σ, and DQ on polymer degrafting. Furthermore, we explored the effect of the bonding environment at the base of the initiator (mono- vs. tri-functional) of the grafted polymer layer at the substrate on the overall stability of polymer brushes on the substrate. The two major findings in this paper are (1) degrafting of polyc...}, number={16}, journal={MACROMOLECULES}, author={Ko, Yeongun and Genzer, Jan}, year={2019}, month={Aug}, pages={6192–6200} }
 @article{patil_miles_ko_datta_rao_kiserow_genzer_2018, title={Kinetic Study of Degrafting Poly(methyl methacrylate) Brushes from Flat Substrates by Tetrabutylammonium Fluoride}, volume={51}, ISSN={["1520-5835"]}, DOI={10.1021/acs.macromol.8b01832}, abstractNote={Polymer degrafting is a process in which surface-attached polymer brushes are removed from the substrate by breaking a chemical bond in proximity to the substrate. This paper provides insight into the kinetics of degrafting poly(methyl methacrylate) (PMMA) brushes using tetrabutylammonium fluoride (TBAF) and demonstrates how the process can be modeled using a series of degrafting reactions. The trichlorosilane-based polymerization initiator utilized here to synthesize PMMA grafts by surface-initiated atom transfer radical polymerization anchors to the silica substrate by up to three potential attachment points. During the degrafting sequence this anchoring reduces to two and one chemical bond and finally results in complete liberation of the PMMA macromolecule from the substrate. We investigate the effect of TBAF concentration, the initial grafting density of PMMA grafts on the substrate, and TBAF exposure time on degrafting of PMMA by monitoring the instantaneous areal grafting density of PMMA on the sub...}, number={24}, journal={MACROMOLECULES}, author={Patil, Rohan and Miles, Jason and Ko, Yeongun and Datta, Preeta and Rao, Balaji M. and Kiserow, Douglas and Genzer, Jan}, year={2018}, month={Dec}, pages={10237–10245} }
 @article{li_lin_ko_kiserow_genzer_2018, title={Visualization of Mechanochemically-Assisted Degrafting of Surface-Tethered Poly(Acrylic Acid) Brushes}, volume={7}, ISSN={["2161-1653"]}, DOI={10.1021/acsmacrolett.8b00241}, abstractNote={We report visualization of mechanochemically assisted degrafting of surface-tethered poly(acrylic acid) (PAA) brushes in a basic aqueous buffer at nanometer to micrometer length scale by monitoring changes in local etching of silicon substrates. PAA brushes were prepared by surface-initiated atom transfer radical polymerization and incubated in 0.1 M ethanolamine buffer (pH 9.0) with 0.5 M NaCl. Morphological changes of the underlying substrates were monitored by scanning electron microscopy and atomic force microscopy. The appearance of regular-shaped pits indicated etching of the substrate, and both their number and size grew with increasing incubation time. We compared the etching behaviors for PAA, poly(methyl methacrylate) (PMMA), and poly(poly(ethylene glycol) methacrylate) (PPEGMA) brushes grafted on silicon substrates. After incubation for 7 days, the substrate of PMMA brush remained intact. In PAA brush systems, we detected the formation of a few large pits whose size grew in time. Many pits showed up on the substrate of PPEGMA brush but with substantially smaller size compared to PAA. Our findings suggest that hydrophobicity and stability of the grafted polymers play an important role in the morphological changes of the underlying silicon substrates under given incubation conditions.}, number={6}, journal={ACS MACRO LETTERS}, author={Li, Yuanchao and Lin, Yiliang and Ko, Yeongun and Kiserow, Douglas and Genzer, Jan}, year={2018}, month={Jun}, pages={609–613} }
 @article{miles_schlenker_ko_patil_rao_genzer_2017, title={Design and Fabrication of Wettability Gradients with Tunable Profiles through Degrafting Organosilane Layers from Silica Surfaces by Tetrabutylammonium Fluoride}, volume={33}, ISSN={["0743-7463"]}, DOI={10.1021/acs.langmuir.7b02961}, abstractNote={Surface-bound wettability gradients allow for a high-throughput approach to evaluate surface interactions for many biological and chemical processes. Here we describe the fabrication of surface wettability gradients on flat surfaces by a simple, two-step procedure that permits precise tuning of the gradient profile. This process involves the deposition of homogeneous silane SAMs followed by the formation of a surface coverage gradient through the selective removal of silanes from the substrate. Removal of silanes from the surface is achieved by using tetrabutylammonium fluoride which selectively cleaves the Si-O bonds at the headgroup of the silane. The kinetics of degrafting has been modeled by using a series of first order rate equations, based on the number of attachment points broken to remove a silane from the surface. Degrafting of monofunctional silanes exhibits a single exponential decay in surface coverage; however, there is a delay in degrafting of trifunctional silanes due to the presence of multiple attachment points. The effects of degrafting temperature and time are examined in detail and demonstrate the ability to reliably and precisely control the gradient profile on the surface. We observe a relatively homogeneous coverage of silane (i.e., without the presence of islands or holes) throughout the degrafting process, providing a much more uniform surface when compared to additive approaches of gradient formation. Linear gradients were formed on the substrates to demonstrate the reproducibility and tuneability of this subtractive approach.}, number={51}, journal={LANGMUIR}, author={Miles, Jason and Schlenker, Spencer and Ko, Yeongun and Patil, Rohan and Rao, Balaji M. and Genzer, Jan}, year={2017}, month={Dec}, pages={14556–14564} }