Partially Cured Photopolymer with Gradient Bingham Plastic

Letter pubs.acs.org/macroletters

Partially Cured Photopolymer with Gradient Bingham Plastic Behaviors as a Versatile Deformable Material Rhokyun Kwak,† Hyun-Ha Park,‡ Hangil Ko,‡ Minho Seong,‡ Moon Kyu Kwak,§ and Hoon Eui Jeong*,‡ †

Center for BioMicrosystems, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, Ulsan, 44919, Republic of Korea § Department of Mechanical Engineering, Kyungpook National University, Daegu, 41566, Republic of Korea ‡

S Supporting Information *

ABSTRACT: We present rheological and mechanical behaviors of a partially cured photopolymer. When an ultraviolet (UV)-curable resin is exposed to UV light in atmospheric conditions, a partially cured layer is formed on the top of the resin owing to inhibitory effects of oxygen. Interestingly, such a partially cured resin behaves like a Bingham plastic with a yield stress, being a rigid solid at low shear stress and a viscous liquid at high stress. Unlike typical Bingham plastic materials, however, deformation rate saturation is observed with an increase in applied stress, which is attributed to the gradient in the degree of photopolymerization of the resin (termed “gradient Bingham plastic”). This gradient Bingham plastic can be utilized for the robust fabrication of diverse 3D, multiscale structures.

U

observed. The physical mechanism of Bingham plastics is relatively well established; when a liquid contains particles or large molecules, there exist some physical interactions between them, resulting in the formation of a weak, solid structure. Accordingly, a certain amount of external pressure is required to break such a pseudo network; above the critical pressure, the particles would start to flow like a viscous liquid. Here, we show that partially cured polyurethane acrylate (PUA) in free-radical polymerization behaves like a Bingham plastic whose yield stress is determined by the degree of conversion during photopolymerization. A set of rheological analyses and imprinting tests of the partially cured PUA together with a theoretical analysis demonstrate that a gradient in the degree of photopolymerization exists in the partially cured resin. Thus, the partially cured polymer presented here can be referred to as “gradient Bingham plastic” for brevity. We demonstrate that this gradient Bingham plastic can be used as a versatile and robust deformable material for a variety of 3D, multiscale structures. Figure 1 presents a model system of a UV-molding process, showing the oxygen permeation, diffusion, and consumption in the system. The UV-molding technique is one of the most representative and widely used micro/nanofabrication techniques utilizing UV-curable resins. During the UV-molding process, oxygen trapped in the mold cavity or that has

ltraviolet (UV)-curable polymers have been utilized extensively owing to their unique benefits such as rapid polymerization, insolubility in organic solvents, and high resistance to heat and mechanical shocks.1−3 While traditional use of photopolymers has been limited to photoresists, coatings, and adhesives, a range of new applications is actively being pursued in the fields of micro- and nanofabrication.4−13 It is well-known that when a UV-curable resin is exposed to UV light in air in a micro/nanofabrication process, an oxygeninduced partial curing layer is formed on its surface owing to inhibitory effects of oxygen.14−16 Recently, this partially cured layer has been exploited to generate multiscale structures for a variety of applications in various fields including biomimetics17−19 and microfluidics.20−27 Despite recent interest, however, detailed studies on the rheological and mechanical behaviors of the partially cured layer have not been conducted. Furthermore, a method for the precise control of the rheological and mechanical properties of the partially cured layer has not been fully explored. Direct clues for solving these issues would allow for more versatile applications of photopolymers in the fields of micro-, nano-, and 3D fabrication. Viscoplastic materials show rate-dependent, inelastic deformation when a load is applied.28,29 Such a deformation is purely plastic, meaning that the material undergoes an unrecoverable shape transformation even after the load is removed. Among viscoplastic materials, Bingham plastics are a class of materials that exhibit solid-like behavior at low stress and a liquid-like viscous flow at high stress.29−31 Here, a yield stress, defined as the critical stress that initiates material flow, is typically © XXXX American Chemical Society

Received: March 29, 2017 Accepted: May 2, 2017

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DOI: 10.1021/acsmacrolett.7b00233 ACS Macro Lett. 2017, 6, 561−565

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ACS Macro Letters

Figure 1. Model system of a UV-molding process in which a mold with a cavity is placed on a UV-curable resin coated on a substrate. Upon subsequent UV exposure, a partially cured resin with a tacky surface can be generated as a result of competition between diffusion of the permeated or trapped oxygen and consumption of the oxygen in the resin.

permeated through the mold diffuses into the resin and inhibits UV curing by scavenging radicals generated from the photoinitiator by the UV light. As a result, the top of the resin, where the oxygen concentration is high, remains tacky and undercured, while the resin beneath the surface is cured completely, resulting in partially cured photopolymers. By utilizing the inhibitory effect of oxygen with proper UV curing conditions, a partially cured polymer with required rheological and mechanical properties can be obtained. To study the rheological properties of partially cured photopolymers, partially cured PUA was prepared using the UV-molding process (see Supporting Information (SI) and Figure S1 for experimental details). Then, rheological properties of the partially cured PUA film were measured using an advanced rheometric expansion system (ARES, Rheometric Scientific). The rheological properties of noncured liquid PUA were also measured for comparison. Figure 2a,b shows the results of the measurement of shear stress as a function of shear rate for the liquid PUA and the partially cured PUA (20 s UV exposure, UV intensity: ∼10 mW cm−2), respectively (see Figure S2 for more experimental results). As shown in Figure 2a, the liquid PUA showed a linear increase in shear rate for any applied shear stress, which is typical behavior of a Newtonian fluid. The overall constant dynamic viscosity of the liquid PUA (at shear rate > 1 s−1) also supports its Newtonian characteristic (Figure 2c). In contrast, completely different rheological behaviors were observed for the partially cured PUA film. As shown in Figure 2b, no shear rate was observed until a shear stress of ∼353.8 Pa was applied. Furthermore, the dynamic viscosity at a low shear rate was higher than that of the liquid PUA overall by 4 orders of magnitude. It also decreased monotonically to ∼241.3 Pa·s at a high shear rate of 10 s−1 (Figure 2d). These rheological behaviors indicate that the partially cured PUA has solid-like properties under the critical stress, while it behaves as a liquid above this stress, which is consistent with the characteristics of Bingham plastic materials. To confirm the Bingham plastic behaviors of the partially cured PUA, its measurement results were compared with rheological models for yield stress fluids: Bingham, Herschel-Bulkley, and Casson (Figure 2b).29 The Bingham model represents the ideal case of plastic flow, which predicts that the fluid flows like a liquid with a constant plastic viscosity (μ′) as soon as the Bingham yield stress is exceeded (τ

Figure 2. Rheological properties of liquid PUA and partially cured PUA. Shear stress vs shear rate relationships for (a) liquid PUA and (b) partially cured PUA (20 s). Dynamic viscosity vs shear rate relationships for (c) liquid PUA and (d) partially cured PUA (20 s). In (b), fitting curves for each model are as follows: y = 557.9 + 226.1x (R2 = 0.78, Bingham), y = 353.8 + 693.7x0.41 (R2 = 0.82, HerschelBulkley), and y0.5 = 3640.5 + (104.3x)0.5 (R2 = 0.92, Casson).

= τBingham yield stress + μ′γ̇). Here, τ is the shear stress and γ̇ is the shear rate. However, most viscoplastic fluids do not show linear flow behavior in practice. The Herschel-Bulkley model can be used for nonlinear viscoplastic flow behavior (τ = τHB + kHB|γ̇|n−1γ̇). Here, kHB and n are constants, equivalent to the power law parameter of plastic viscosity (μ′ ≈ γ̇n−1).29,31 This model is useful for correlating experimental parameters, but three model parameters must be determined. The simpler twoparameter model, that is, the Casson model, allows us to determine the parameters by quick fitting of the experimental data. With the Casson critical stress, τC and the constant viscosity at the infinite shear limit, τ∞, the relationship between 1/2 shear rate and shear stress is defined as τ1/2 = τ1/2 C + (η∞γ̇) . The two parameters (n and τHB) for the Herschel-Bulkley model were determined from the power fitting curve for the measurement result of shear stress as a function of shear rate for the partially cured PUA shown in Figure 2d (η′ ∼ γ̇−0.59 and, therefore, n = 0.41). The yield stress for the Herschel-Bulkley model was set as an experimental value, that is, τHB = 353.8 Pa (at γ = 0.05 s−1, Figure 2b). As shown in Figure 2b, the Herschel-Bulkley and Casson models give better correlations with the overall experimental data than the ideal Bingham model does. The rheometric measurements together with the comparisons with existing yield stress fluid models confirmed 562

DOI: 10.1021/acsmacrolett.7b00233 ACS Macro Lett. 2017, 6, 561−565

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ACS Macro Letters that the partially cured PUA exhibited Bingham plastic behaviors. The viscoplastic properties of the partially cured PUA were further investigated by imprinting with microscale wedge patterns on a partially cured, micropatterned PUA (see SI and Figure S3 for experimental details). This test was performed to investigate the gradient deformation behaviors of partially cured PUA formed using different UV exposure times. Figure 3 shows the relationship between imprinting

∂CO2 ∂t

=D

∂ 2CO2 ∂x 2

− kCO2

(1)

where CO2 is the oxygen concentration in the UV resin, D is the diffusion constant of oxygen, and k is the reaction constant of oxygen consumption. For simplicity, we assumed that the oxygen consumption may be approximated to be a first-order kinetic reaction, which usually describes complex, coupling reactions of photoinitiation and polymer chain reactions.14,32,33 Accordingly, two boundary conditions and one initial condition are required to solve the above second-order differential equation: C′O2 = (1 − P′)exp( −kt ) + P′, x = 0

δC′O2 δx

(2)

=0 x=L

(3)

C′O2, i = 1, t = 0

(4)

where C′O2 is the normalized oxygen concentration divided by C O 2 ,i [C′ O 2 = C O 2 /C O 2 ,i ], which is the initial oxygen concentration. P′[P/(kCO2,i)] is the dimensionless parameter that describes the relative contribution of oxygen permeation to consumption, where P is the concentration of oxygen that permeated through the porous mold. Here, P is set to be constant for a given permeability, mold thickness, and pressure drop inside and outside of the mold. L is the depth of the cured polymer resin. Figure 4a shows three-dimensional plots of the oxygen concentration ratio within the resin as a function of time and depth of the resin for different values of P′. As shown in the figure, the oxygen concentration ratio is heavily dependent on the three parameters. It decreases with increases in time and depth of the resin and with decreases in values of P′. Provided that the curing was complete for C′O2 < 0.01, this oxygen concentration gradient along the resin depth means that there is essentially a gradient in the photopolymerization reaction along the depth direction, which is the key reason for the gradient Bingham behaviors of the partially cured PUA.32 By utilizing the plot shown in Figure 4a, one can determine optimal UV curing conditions to obtain cross-linked UV resin with a desired degree of photopolymerization. Figure 4b shows the oxygen concentration ratio as a function of resin depth and values of P′ for a fixed UV exposure time of 10 s. As shown in the figure, the oxygen concentration ratio decreased exponentially with the resin depth. The concentration ratio is also a function of the values of P′. Provided that the curing was complete for C′O2 < 0.01, as mentioned above, the thickness of the partially cured layer is ∼8.5 μm for a P′ of 1. The thickness of the partially cured layer decreased with the decrease in the P′ values. Figure 4c shows the oxygen concentration ratio with resin depth for different times at a fixed P′ of 0.01. As shown, the oxygen concentration ratio and the thickness of the partially cured PUA layer can also be modulated with different times for fixed P′ values. The partially cured photopolymers with gradient Bingham plastic properties have potential as a versatile and robust deformable material for complex multiscale 3D structures, as the relatively undercured part of the gradient Bingham plastic can be engraved with micro- or nanostructures by applying a

Figure 3. Maximum imprinting depth of microwedge patterns over the partially cured underlying microline structures as a function of applied imprinting pressure after different UV exposure times. A UV radiation system (λ = 320−500 nm, Minuta Tech., Korea) was used for photopolymerization with a high-energy mercury lamp with an intensity of 100 mW cm−2 at a distance of 15 cm.

depth, imprinting pressure, and UV curing time (see Figure S4 for imprinting test results in N2 environment). Considering the analogy to the shear stress vs shear rate relationship of the rheological analysis of viscoplastic fluids, several interesting findings were observed. First, the imprinting depth increased with the imprinting pressure. However, the imprinting depth remained at zero until the imprinting pressure exceeded a critical value. This indicates that the partially cured PUA layer behaves as a Bingham plastic fluid with a yield stress. The intercept of each graph at zero imprinting depth for samples exposed to UV light for different times increased with the UV curing time, indicating that the yield stress of the partially cured PUA increased with the UV curing time. Second, unlike typical Bingham plastics, the indentation depth stopped increasing at over a certain imprinting pressure. This suggests that the photopolymerized PUA is not spatially homogeneous and a gradient in the degree of polymerization is present in the depth direction. Third, the slope of each graph increased with the depth. This indicates that the viscosity deeper in the partially cured layer is higher than that close to the surface, demonstrating that there is a gradient in the rheological and mechanical properties along the thickness of the partially cured PUA. All of these results confirm that the partially cured photopolymer showed “gradient Bingham plastic” behaviors. To gain an understanding of the partial curing kinetics, we derived a kinetic model to predict the oxygen concentration in the PUA resin. The one-dimensional mass balance for oxygen concentration is written as 563

DOI: 10.1021/acsmacrolett.7b00233 ACS Macro Lett. 2017, 6, 561−565

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Figure 5. (a) Photograph of multiscale PUA structures in which 750 nm pillars are uniformly formed all over the “UNIST” logo. (b) Photograph of multiscale PUA structures in which 750 nm pillars are formed only on the flambeau mark, whereas the remaining area was covered with 150 nm pillars with antireflective properties. (c) SEM image showing 750 nm pillars formed on the underlying structure, which corresponds to the red box in (a) and (b). (d) SEM image showing 150 nm pillars formed on the underlying structure, which corresponds to the green box in (b).

concentration gradient in the resin that appeared during the UV curing process. By controlling the oxygen concentration and the UV curing conditions, a partially cured photopolymer with desired rheological and mechanical properties can be obtained in a precise and controllable manner, and it can be used as a robust and versatile deformable material for a variety of 3D, multiscale structures.

Figure 4. (a) Oxygen concentration in the PUA resin as a function of time and resin depth for different mold permeabilities (P′). (b) Oxygen concentration as a function of resin depth for different P′ at 10 s. (c) Oxygen concentration as a function of resin depth for different times at a fixed value of P′ (0.01).

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ASSOCIATED CONTENT

S Supporting Information *

stress above a yield stress, whereas the cured part can act as a robust supporting structure that does not deform upon application of stress. To demonstrate the unique applicability of the partially cured photopolymers, we fabricated multiscale PUA structures in which nanostructures were selectively formed on a macroscale “UNIST” logo (Figures 5 and S5). These two-leveled structures could be formed using a sequential, two-step molding process from the partially cured PUA resin with gradient Bingham plastic properties in the vertical direction (see SI and Figure S5 for experimental details). Furthermore, more-leveled (e.g., three-leveled) or different types of multiscale structures could be generated by exploiting these partially cured photopolymers with gradient Bingham plastic behaviors (Figure S6). In summary, we have presented viscoplastic behaviors of a partially cured photopolymer. A set of rheological analyses, imprinting tests, and theoretical studies demonstrated that the partially cured photopolymer behaves as a Bingham plastic with a yield stress. However, unlike typical Bingham plastics, the partially cured PUA film showed deformation saturation with an increase in applied stress, exhibiting gradient Bingham plastic properties, which were caused by the oxygen

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00233. Experimental details and supporting figures (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +82-52-217-2339. ORCID

Moon Kyu Kwak: 0000-0002-8902-7685 Hoon Eui Jeong: 0000-0002-1413-3774 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Midcareer Researchers Supporting Program through the National Research Foundation of Korea (NRF; 2016R1A2B2014044). R.K. was supported by the institutional program in the Korea Institute of Science and Technology (2E26840). 564

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R.; Rolland, J. P.; Ermoshkin, A.; Samulski, E. T.; DeSimone, J. M. Continuous liquid interface production of 3D objects. Science 2015, 347, 1349−1352. (24) Vitale, A.; Quaglio, M.; Chiodoni, A.; Bejtka, K.; Cocuzza, M.; Pirri, C. F.; Bongiovanni, R. Oxygen-Inhibition Lithography for the Fabrication of Multipolymeric Structures. Adv. Mater. 2015, 27, 4560− 4565. (25) Guo, X.; Chan-Park, M. B.; Yoon, S. F.; Chun, J. H.; Hua, L.; Sze, N. S. K. UV embossed polymeric chip for protein separation and identification based on capillary isoelectric focusing and MALDI-TOFMS. Anal. Chem. 2006, 78, 3249−3256. (26) Hwang, S. Y.; Kwon, K. W.; Jang, K. J.; Park, M. C.; Lee, J. S.; Suh, K. Y. Adhesion Assays of Endothelial Cells on Nanopatterned Surfaces within a Microfluidic Channel. Anal. Chem. 2010, 82, 3016− 3022. (27) Sikanen, T.; Aura, S.; Heikkila, L.; Kotiaho, T.; Franssila, S.; Kostiainen, R. Hybrid Ceramic Polymers: New, Nonbiofouling, and Optically Transparent Materials for Microfluidics. Anal. Chem. 2010, 82, 3874−3882. (28) Dzuy, N. Q.; Boger, D. V. Yield Stress Measurement for Concentrated Suspensions. J. Rheol. 1983, 27, 321−349. (29) Nguyen, Q. D.; Boger, D. V. Measuring the Flow Properties of Yield Stress Fluids. Annu. Rev. Fluid Mech. 1992, 24, 47−88. (30) Papanastasiou, T. C. Flows of Materials with Yield. J. Rheol. 1987, 31, 385−404. (31) Huilgol, R. R.; You, Z. Application of the augmented Lagrangian method to steady pipe flows of Bingham, Casson and Herschel-Bulkley fluids. J. Non-Newtonian Fluid Mech. 2005, 128, 126−143. (32) O’Brien, A. K.; Bowman, C. N. Impact of oxygen on photopolymerization kinetics and polymer structure. Macromolecules 2006, 39, 2501−2506. (33) Dendukuri, D.; Panda, P.; Haghgooie, R.; Kim, J. M.; Hatton, T. A.; Doyle, P. S. Modeling of Oxygen-Inhibited Free Radical Photopolymerization in a PDMS Microfluidic Device. Macromolecules 2008, 41, 8547−8556.

REFERENCES

(1) Decker, C. The use of UV irradiation in polymerization. Polym. Int. 1998, 45, 133−141. (2) Xia, Y. N.; Whitesides, G. M. Soft lithography. Angew. Chem., Int. Ed. 1998, 37, 550−575. (3) Rolland, J. P.; Hagberg, E. C.; Denison, G. M.; Carter, K. R.; DeSimone, J. M. High-resolution soft lithography: Enabling materials for nanotechnologies. Angew. Chem., Int. Ed. 2004, 43, 5796−5799. (4) Abdelhedi-Miladi, I.; Montarnal, D.; Obadia, M. M.; Ben Romdhane, H.; Drockenmuller, E. UV-Patterning of Ion Conducting Negative Tone Photoresists Using Azide-Functionalized Poly(Ionic Liquid)s. ACS Macro Lett. 2014, 3, 1187−1190. (5) Schultz, A. R.; Lambert, P. M.; Chartrain, N. A.; Ruohoniemi, D. M.; Zhang, Z. Y.; Jangu, C.; Zhang, M.; Williams, C. B.; Long, T. E. 3D Printing Phosphonium Ionic Liquid Networks with Mask Projection Microstereolithography. ACS Macro Lett. 2014, 3, 1205−1209. (6) Kirschbaum, S.; Landfester, K.; Taden, A. Unique Curing Properties through Living Polymerization in Crosslinking Materials: Polyurethane Photopolymers from Vinyl Ether Building Blocks. Angew. Chem., Int. Ed. 2015, 54, 5789−5792. (7) Nguyen, K. T.; West, J. L. Photopolymerizable hydrogels for tissue engineering applications. Biomaterials 2002, 23, 4307−4314. (8) Choi, S. J.; Suh, K. Y.; Lee, H. H. Direct UV-replica molding of biomimetic hierarchical structure for selective wetting. J. Am. Chem. Soc. 2008, 130, 6312. (9) Ito, S.; Tanaka, Y.; Yoshikawa, H.; Ishibashi, Y.; Miyasaka, H.; Masuhara, H. Confinement of Photopolymerization and Solidification with Radiation Pressure. J. Am. Chem. Soc. 2011, 133, 14472−14475. (10) Kang, D.; Pikhitsa, P. V.; Choi, Y. W.; Lee, C.; Shin, S. S.; Piao, L. F.; Park, B.; Suh, K. Y.; Kim, T. I.; Choi, M. Ultrasensitive mechanical crack-based sensor inspired by the spider sensory system. Nature 2014, 516, 222−226. (11) Pang, C.; Lee, G. Y.; Kim, T. I.; Kim, S. M.; Kim, H. N.; Ahn, S. H.; Suh, K. Y. A flexible and highly sensitive strain-gauge sensor using reversible interlocking of nanofibres. Nat. Mater. 2012, 11, 795−801. (12) Jeong, H. E.; Lee, J. K.; Kim, H. N.; Moon, S. H.; Suh, K. Y. A nontransferring dry adhesive with hierarchical polymer nanohairs. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 5639−5644. (13) Jeong, K. H.; Kim, J.; Lee, L. P. Biologically inspired artificial compound eyes. Science 2006, 312, 557−561. (14) Decker, C.; Jenkins, A. D. Kinetic Approach of O-2 Inhibition in Ultraviolet-Induced and Laser-Induced Polymerizations. Macromolecules 1985, 18, 1241−1244. (15) Andrzejewska, E. Photopolymerization kinetics of multifunctional monomers. Prog. Polym. Sci. 2001, 26, 605−665. (16) Lee, T. Y.; Guymon, C. A.; Jonsson, E. S.; Hoyle, C. E. The effect of monomer structure on oxygen inhibition of (meth)acrylates photopolymerization. Polymer 2004, 45, 6155−6162. (17) Jeong, H. E.; Kwak, R.; Kim, J. K.; Suh, K. Y. Generation and Self-Replication of Monolithic, Dual-Scale Polymer Structures by TwoStep Capillary-Force Lithography. Small 2008, 4, 1913−1918. (18) Jeong, H. E.; Kwak, R.; Khademhosseini, A.; Suh, K. Y. UVassisted capillary force lithography for engineering biomimetic multiscale hierarchical structures: From lotus leaf to gecko foot hairs. Nanoscale 2009, 1, 331−338. (19) Kwak, R.; Jeong, H. E.; Suh, K. Y. Fabrication of Monolithic Bridge Structures by Vacuum-Assisted Capillary-Force Lithography. Small 2009, 5, 790−794. (20) Dendukuri, D.; Pregibon, D. C.; Collins, J.; Hatton, T. A.; Doyle, P. S. Continuous-flow lithography for high-throughput microparticle synthesis. Nat. Mater. 2006, 5, 365−369. (21) Dendukuri, D.; Gu, S. S.; Pregibon, D. C.; Hatton, T. A.; Doyle, P. S. Stop-flow lithography in a microfluidic device. Lab Chip 2007, 7, 818−828. (22) Pregibon, D. C.; Toner, M.; Doyle, P. S. Multifunctional encoded particles for high-throughput biomolecule analysis. Science 2007, 315, 1393−1396. (23) Tumbleston, J. R.; Shirvanyants, D.; Ermoshkin, N.; Janusziewicz, R.; Johnson, A. R.; Kelly, D.; Chen, K.; Pinschmidt, 565

DOI: 10.1021/acsmacrolett.7b00233 ACS Macro Lett. 2017, 6, 561−565