Biobased Polymer Coating Using Catechol Derivative Urushiol

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Bio-based Polymer Coating using Catechol Derivatives Urushiol Hirohmi Watanabe, Aya Fujimoto, Jin Nishida, Tomoyuki Ohishi, and Atsushi Takahara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00484 • Publication Date (Web): 13 Apr 2016 Downloaded from http://pubs.acs.org on April 19, 2016

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Bio-based Polymer Coating using Catechol Derivatives Urushiol Hirohmi Watanabe*, Aya Fujimoto, Jin Nishida, Tomoyuki Ohishi†, and Atsushi Takahara* Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

ABSTRACT: We have investigated the mechanism of the superior mechanical robustness of coated thin films of the catechol derivative, urushiol. We synthesized hydrogenated urushiol (hurushiol) by hydrogenating the double bonds in the long alkyl side chain of urushiol, and the physical properties of thin films of mixtures of urushiol and h-urushiol were evaluated. Atomic force microscopy observations revealed that these coated thin films have a homogeneous surface with no phase separation regardless of the h-urushiol content, arising from the similarity of the chemical structures. The films showed good adhesive properties because the adhesion originates from the catechol structure. In contrast, curing time depended strongly on the h-urushiol content. The curing of the h-urushiol thin film took 12 h, whereas the urushiol thin film was cured within 10 min. Moreover, the strain-induced elastic buckling instability for mechanical measurements test and bulge test confirmed that the increase in the h-urushiol content decreased the mechanical strength. Because the double bonds in the urushiol side chain contribute to forming the highly

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cross-linked structure, the lack of double bonds in h-urushiol resulted in the slow curing and low mechanical strength. Interestingly, the mechanical robustness started to increase over 80 mol % h-urushiol. The saturated long alkyl side chain of h-urushiol faced the surface, and the regular structure of the uniform side chain may improve the mechanical properties of the coated film. Our results will help to develop biomimetic catechol-based coatings.

INTRODUCTION Urushi (oriental lacquer) is a traditional natural curing resin obtained from the sap of the lacquer tree (Toxicodendron vernicifluum). The main component of urushi sap is urushiol (60– 65% in the sap), a mixture of structurally similar catechol derivatives that have unsaturated double bonds in their long alkyl side chain.1,2 Urushi is cured in two steps. The first step is the oxidative reaction of the catechol moiety of urushiol to form oligomers. This step proceeds at room temperature under high humidity, and laccase in the sap (0.1–1.0%) or added iron catalyzes this reaction.3 The second step is the aerobic oxidation of the double bond in the side chain to form a highly cross-linked structure. Although the reaction takes several hours or days, heating accelerates the reaction. A thin film of urushiol and iron(II) acetate mixture was rapidly cured at 100 °C for 10 min.3 Many other catecholic and phenolic derivatives are found in nature. For example, 3,4dihydroxyphenylalanine (DOPA) is a catechol-bearing amino acid that is found in mussel adhesive protein. Because DOPA shows superior adhesive properties, many researches have focused on mimicking of the structure to produce functional materials.4-8 We have found that urushiol provides many advantages over other natural catechol or phenolic derivatives such as


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DOPA. The greatest advantage is the superior mechanical properties of urushiol that is necessary for coatings. Nanometer-thick urushiol thin films showed superior robustness, whereas the corresponding DOPA thin film was brittle.9 The robustness of urushiol thin film is quite interesting because urushiol has lower catechol density than DOPA. In this study, we investigated the robustness of urushiol thin films using a model compound. The robustness may arise from the flexibility of the long alkyl side chain of urushiol. Actually, Lee and co-workers reported the fabrication of robust poly(dopamine) thin films by adding poly(ethyleneimine) as a flexible segment.10 The other factor is the highly cross-linked network structure that is formed by the oxidative polymerization of the catechol moiety and the subsequent addition reaction of the double bond in the side chain. It is quite important to understand which was the main factor in the robustness. Therefore, h-urushiol with a saturated long alkyl side chain was newly synthesized by hydrogenating urushiol and used as a model compound. Because h-urushiol does not contain any double bonds in the side chain, the oxidative polymerization only takes placed similar to DOPA. As a result, the thin film cannot form highly cross-linked networks. In contrast, the flexibility is still imparted because long alkyl side chain is remained intact even after the hydrogenation. It is clear that h-urushiol may give some critical information about the superior robustness of urushiol thin films. The mechanical properties of these films were evaluated by the strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) test and the bulge test. Moreover, adhesion and solvent tolerance tests were also performed to confirm the difference of film properties between urushiol and h-urushiol thin films. These results help advance the practical use of urushiol as a catechol derivative with superior mechanical properties, and are important for the design of artificial catechol derivatives that mimic the superior physical and chemical properties of urushiol.


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EXPERIMENTAL Chemicals. Urushiol used in this study was extracted from raw urushi according to the literature.11 Raw urushi prepared from sap collected from Rhus vernicifera in Japan was purchased from Kanwa-do Urushi Club Co., Ltd., Shiga, Japan. Caution: Uncured urushiol can cause an allergic skin rash on contact and should be handled carefully. All solvents in this study were purchased from Kanto Chemicals and used without further purification. Hydrogenation of urushiol. Urushiol (5.0 g, 16.1 mmol) was dissolved in ethanol (20 mL). Palladium-activated carbon ethylenediamine complex (1.0 g, Pd 3.5% to 6.5%, Wako, Tokyo, Japan) was added to the solution. The solution was stirred at room temperature for 24 h under atmospheric hydrogen pressure using an H2 balloon. h-Urushiol was obtained as a white powder after vacuum filtration and recrystallization from n-hexane (yield: 45%). Preparation of thin films. Urushiol and h-urushiol was mixed in the desired ratio, dissolved in ethanol, and iron(II) acetate was added to the solution. The iron(II) acetate concentration was 0.5 equivalents with respect to the total amount of the mixture. The solution was diluted with propylene glycol monomethyl ether acetate (PGMEA) as necessary. The solution was homogenized with an ultrasonic cell disruptor (KS-Branson Sonifier, Branson Ultrasonics Co., Danbury, CT) prior to spin coating. The solution was directly spin coated on substrates such as Si, glass and gold substrates at 3000 rpm to form a thin film. The film was cured at 100 °C. To transfer the thin film to other substrates, the mix was spin coated on a vacuum UV (VUV)treated Si wafer coated with a 1.0-µm-thick layer of poly(sodium styrenesulfonate) (PSSNa; Mw


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= 8.1 × 103). After the urushiol film was cured, the sample was immersed in water to dissolve the PSSNa layer and detach the thin film from the Si substrate. Then, thin film was placed on another substrates such as poly(dimethylsiloxane) (PDMS) and a cupper plate with a circular hole to measure mechanical properties. The detailed procedure is given in the Supporting Information (Figure S1). Measurements. 1H-NMR spectra were recorded on a spectrometer (AV-400, Bruker Co., Billerica, MA, 1H 400.13 MHz). The solvent was deuterated chloroform and tetramethylsilane was the internal standard used for calibrating the chemical shift. Ultraviolet-visible (UV-Vis) absorption spectra were recorded on an UV–Vis spectrophotometer (UV-3500PC, Shimadzu, Co., Kyoto, Japan). Thermogravimetric analysis (TGA) was carried out on a thermal analysis instrument (Pyris 1, PerkinElmer Inc., Waltham, MA) under ambient atmosphere at a heating rate of 20 °C min−1. Atomic force microscopy (AFM; AFM5500, Agilent Technologies, Inc., Santa Clara, CA) was performed in non-contact AC mode with a rectangular 160 µm cantilever (OMCL-AC160TS-W2, Olympus Co., Tokyo, Japan). The spring constant of the cantilever was 42 N m-1. Static water contact angle (CA) measurements were performed by using a contact angle measurement system (DSA10, Krüss GmbH, Hamburg, Germany) equipped with an automatic liquid dispenser and a CCD camera at 23.5 °C and humidity of 60%. X-ray photoelectron spectroscopy (XPS; XPS-APEX, Physical Electronics GmbH., Ismaning, Germany) was performed with a monochromatic Al−Kα X-ray source at a power of 150 W and a pressure of 1 × 10−6 Pa. The take-off angle of the X-rays was fixed as 45° for the measurements. The mass of the thin films was determined by quartz crystal microbalance measurements (Initium Affinix QN, ULVAC Inc., Kanagawa, Japan) with 27 MHz gold electrodes. The thin films were spin coated directly on the surface of the electrodes. The Young’s modulus of the


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material was determined by the SIEBIMM test at room temperature. The ultimate tensile strength (σ) and ultimate elongation (ε) were determined at room temperature by the bulge test. Details of the SIEBIMM test and bulge test are given in the Supporting Information.

RESULTS AND DISCUSSION Characteristics of h-urushiol. h-Urushiol was characterized by 1H-NMR spectroscopy (Figure S2). The signals for the aromatic (catecholic) proton and the methyl proton at the end of the side chain were at 6.7 and 0.87 ppm, respectively. A signal for the methylene side chain was visible at 1.2–1.4 ppm, similar to urushiol. However, signals of the olefinic proton at 4.9–6.3 ppm that were present in urushiol almost disappeared after the hydrogenation, indicating the successful hydrogenation of double bonds in the urushiol side chain. The hydrogenation ratio of h-urushiol was determined as 95% from the integration ratio of protons in the 1H-NMR spectra (see Supporting Information). Isolating pure h-urushiol was difficult; even after recrystallization from n-hexane several times h-urushiol contained traces of partially hydrogenated urushiols because of the similarity of the structures. h-Urushiol was obtained as a white powder, whereas urushiol is a liquid at room temperature. Although urushiol side chains are a mixture of monoenes, dienes, and trienes at different positions, h-urushiol has a uniform saturated hydrocarbon side chain. The regularity of h-urushiol with a long alkyl chain increased the melting point of the material. Ultraviolet–visible (UV–vis) absorption measurements revealed that h-urushiol has an absorption peak at around 275 nm that is typical for catechol compounds (Figure S3). When hurushiol was mixed with iron(II) acetate, a broad absorption peak at 350–900 nm characteristic


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of metal-catecholate complexes appeared. Therefore, the catechol structure remained intact, even after hydrogenation. The thermal stability of urushiol and h-urushiol was evaluated by TGA (Figure S4). h-Urushiol decomposed in a single stage, whereas urushiol decomposed in two main stages. Both h-urushiol and urushiol showed mass loss from 230–310 °C, indicating the decomposition of the monomers. The mass loss of urushiol over 310 °C may arise from the decomposition of thermally cross-linked urushiol. The yield of char residue (3% against total mass) from urushiol is also characteristic of highly cross-linked material. In contrast, h-urushiol did not produce char residue. Because h-urushiol does not contain double bonds in the side chain, it does not form a highly cross-linked structure. The 5% mass loss occurred at 235 and 234 °C for urushiol and h-urushiol, respectively. Fabrication of h-urushiol thin films. Thin films of mixtures of urushiol and h-urushiol were fabricated by spin coating and heating. Films were only obtained with iron(II) acetate. The formation of metal-catecholate complexes increased the viscosity of the spin-coating solution to form highly viscous liquid films.3 The coated films were cured at 100 °C. The curing time increased with the increase in the h-urushiol content. The pure urushiol thin film became tackfree within 10 min, whereas the 1:1 urushiol and h-urushiol film (50 mol % h-urushiol) took 180 min to be tack-free, as confirmed by a pencil test. An h-urushiol thin film needed 12 h to cure. The increase in curing time may arise from the lack of double bounds in h-urushiol. The double bond in the side chain of urushiol is convenient for the material processing of thermosets. The surface morphology and thickness of samples were evaluated by AFM observations (Figure 1). The root mean square (RMS) surface roughness of 50 mol % h-urushiol and pure hurushiol thin films were 0.37 and 0.34 nm, respectively, in a 5 × 5 µm area, indicating that a smooth surface similar to the urushiol thin film (0.24 nm RMS) was obtained. The flat surface


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arose from the compatibility of urushiol and h-urushiol because of their similar structures. The thickness of the coated thin films was evaluated after curing. Although the concentration of the total amount of the mix was same with PGMEA as the solvent, the film thickness gradually decreased as the h-urushiol content increased (Figure S5). The oligomeric compounds in extracted urushiol may increase the viscosity of the spin-coating solution to form a thick film. Spin coating a 100 mM h-urushiol solution at 3000 rpm for 60 s produced a 38-nm-thick film.

Figure 1. AFM images of coated films on a VUV-treated Si substrate (5 × 5 µm area). (a) 50 mol % h-urushiol thin film and (b) h-urushiol thin film. The static water contact angle (CA) of coated samples was evaluated. Figure 2 shows the change in CA as a function of the h-urushiol content. The CA value gradually increased with the amount of h-urushiol. The difference in CA values reached about 25° between urushiol and hurushiol thin films. Because the surface morphologies of these samples were similar, the difference arose from the difference in chemical components on the surface.12 The polar component of the surface energy was almost zero in h-urushiol thin film (Table S1). Therefore, the hydrocarbon groups were concentrated at the sample surface. In our previous work, we demonstrated that the superhydrophobicity of poly(fluoroalkyl acrylate) thin films arose from the


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aggregated morphology of the long perfluoro side-chain at the surface.13 Both urushiol and hurushiol have a long side-chain in the structure. However, urushiol did not form a regular structure because of the mixture of side chains, and thus the film was amorphous. Only the hurushiol side chains were regular and faced the sample surface.

Figure 2. Contact angle of thin films as a function of the mole fraction of h-urushiol. XPS measurements of urushiol and h-urushiol were conducted to determine the chemical species present at the film surface. Samples on VUV-treated Si substrates showed only C1s, O1s, and Fe2p3 peaks at 285, 532, and 712 eV, respectively (Figure S6). However, the atomic ratio was slightly different (Table S2). The concentration of carbon was high in the h-urushiol thin film and the concentration of oxygen was low, consistent with the concentration of the h-urushiol hydrocarbon side chain at the thin film surface. To evaluate solvent tolerance, the h-urushiol thin film on a Si substrate was immersed in PGMEA, which was the solvent used for spin coating. No change in surface morphology was observed by AFM even after immersion for 24 h (Figure S7). The coated thin film also remained intact in various solvents, such as water, ethanol, THF, and chloroform, as confirmed by CA


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measurements. Therefore, the h-urushiol thin film had sufficient solvent tolerance even though the film did not have a densely cross-linked structure. Subsequently, the adhesive properties of the h-urushiol thin film were evaluated by ASTMD3359 standard tape test methods.14 The sample surface was scratched with a razor blade in a crosshatched pattern, and the adhesion was evaluated from 0B (poor) to 5B (good) after tape (3M Scotch Tape #810) was applied and removed. The h-urushiol thin film strongly adhered to various substrates, such as Si wafer, glass, polyethylene, polystyrene, and copper, similar to the urushiol thin film (Table S3). The strong adhesion of urushiol arises from the catechol structure, and the h-urushiol thin film retains these properties. Mechanical properties of thin films. The compressive Young’s modulus of thin films was determined by the SIEBIMM test.14-16 Figure 3 shows the relationship between the h-urushiol ratio and the Young’s modulus of thin films. Young’s modulus gradually decreased as the hurushiol content increased and reached a minimum of 0.43 ± 0.04 GPa, almost half of the urushiol thin film, at 70 mol % h-urushiol. The Young’s modulus started to increase with the increase in the h-urushiol content. The Young’s modulus of the h-urushiol thin film reached 0.69 ± 0.03 GPa. The complicated behavior may arise from two conflicting factors. The first is the difference in the cross-linking density of the materials. Because a highly cross-linked structure is formed by the thermal reaction of urushiol, the increase in the h-urushiol content decreases the cross-linking density, resulting in a lower Young’s modulus. The second is the difference in the uniformity of the side chain. When the h-urushiol content of the thin film was sufficiently high, the saturated long alkyl side chain of h-urushiol formed a regular structure and faced the film surface, as shown by the CA and XPS measurements. These regulated structure is generally


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shows high packing density13, and it may contribute to increase Young’s modulus. This is why the Young’s modulus was recovered at a high h-urushiol content.

Figure 3. Young’s modulus and specific stiffness as a function of the mole fraction of hurushiol. The mechanical properties of thin films were also evaluated by bulge tests.14,17,18 Figure 4 shows the ultimate tensile strength (σ) and ultimate elongation (ε) of the thin films as a function of h-urushiol content. The σ and ε values decreased reaching a minimum at 70 mol % h-urushiol,

Figure 4. Ultimate tensile strength (■) and ultimate elongation (○) as a function of h-urushiol mole fraction.


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and then starting to increase, similar to Young’s modulus. The complicated behavior can also be explained by competition between the cross-linking density and the uniformity of the side chain.

CONCLUSIONS h-Urushiol was synthesized by hydrogenation of the urushiol side chain to investigate the physical characteristics of urushiol derivatives. h-Urushiol formed metal-catecholate complexes with iron(II) acetate, and the thin film had superior adhesive properties similar to urushiol. Because these characteristics originated from the catechol structure, they were not affected by the hydrogenation. Moreover, coated thin films of mixtures of h-urushiol and urushiol had a homogeneous surface regardless of the h-urushiol content because of the similarity of the chemical structure of these two compounds. In contrast, the mechanical properties depended strongly on the h-urushiol content. The bulge test revealed that the ultimate tensile strength and the ultimate elongation decreased with the increase in the h-urushiol content. The Young’s modulus determined by the SIEBIMM test also decreased with the increase in the h-urushiol content. Because h-urushiol does not contain double bonds in the side chain, the increase in the h-urushiol content led to low cross-linking density, decreasing the mechanical strength. However, the mechanical strength started to increase when the h-urushiol content was high. hUrushiol has a single structure unlike urushiol, and the regular side chain of h-urushiol faced the surface as shown by the CA and XPS measurements. Thus, the regular side chain structure may increase the mechanical strength of the thin film. Traditional products coated with urushi, such as Japanese lacquerware (urushi ware), are highly prized in East Asia. Various decorative techniques, such as maki-e, in which gold powder


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is applied to urushi ware, are used to create beautiful artistic effects. Urushiware is also used in practical products because of its excellent physical properties and chemical resistance. Synthetic paints sometimes deteriorate even under normal environmental conditions, whereas urushiware can retain its glossy surface for more than a thousand years. The highly cross-linked structure of the coating imparts this robustness. Antibacterial, anti-fouling, and thermal insulating properties are also granted by the coating. Thus, urushi is a promising natural curing resin. However, urushi has several disadvantages for large-scale use. One is the product yield. Although the lacquer tree can be cultivated, the amount of sap that can be collected is limited. In addition, skin contact with uncured urushiol can cause lacquer poisoning, which is why the use of urushi coatings is restricted, unlike synthetic paints. For use on a large scale, artificial urushi that overcomes these problems must be designed. Our results may help to design artificial urushiol that contains the following structures, which are desirable for fabricating a robust, tough artificial urushi coating. (1) Catechol backbone: the structure allows curing through oxidative polymerization. Various characteristics, such as adhesive, antibacterial, and anti-fouling properties, originate from this structure. The catechol structure will form the fundamental backbone of artificial urushi. (2) Polymerizable group other than catechol: the polymerization of these groups produces a highly cross-linked structure forming a robust material. (3) Uniform long alkyl side chain: a long alkyl side chain makes the material flexible. The DOPA thin film shattered into small pieces, even though it was highly cross-linked. Moreover, a highly packed structure may increase the robustness of the material. Lacquer poisoning is mild when the catechol structure is disubstituted.19 Therefore, separating the polymerizable group and


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the long alkyl side chain is desirable, unlike urushiol, which is a monosubstituted catechol derivative with a long polymerizable alkyl side chain. Artificial urushi that contains these three structures is expected to form superior bioinspired coating materials.

ASSOCIATED CONTENT Supporting Information. Supporting information contains (1) Transfer of thin films, (2) Determination of mechanical properties, (3) 1H-NMR spectra of materials, (4) Ultraviolet– visible (UV–vis) absorption spectra of materials in ethanol, (5) TGA curves of materials, (6) Film thickness of samples, (7) surface energy of thin films, (8) XPS spectrum of urushiol and hurushiol thin films on quartz plate, (9) Solvent tolerance of thin films, and (10) adhesive test. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Hirohmi WATANABE: e-mail: [email protected] *Atsushi TAKAHARA: e-mail: [email protected] Present Addresses † T. O. is currently at Seikei University. Author Contributions


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H. Watanabe and A. Takahara contributed equally to this work.

ACKNOWLEDGMENT This study was partially supported by a ImPACT (Impulsing Paradigm Change through Disruptive Technologies) Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

REFERENCES (1) Kumanotani, J. Japanese Lacquer-A Super Durable Coating (Proposed Structure and Expanded Application). In Polymer Applications of Renewable-Resource Materials; Carraher, C., Jr., Sperling, L. H., Eds.; Springer US: 1983; Vol. 17, p 225-248. (2) Majima, R. The Main Element Of Japans. (I. Announcement.) - Urushiol And Urushiol Dimethyl Ether. Ber. Dtsch. Chem. Ges. 1909, 42, 1418-1423. (3) Watanabe, H.; Fujimoto, A.; Takahara, A. Characterization Of Catechol-Containing Natural Thermosetting Polymer “Urushiol” Thin Film. J. Polym. Sci. A Polym. Chem. 2013, 51, 3688-3692. (4) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318, 426-430. (5) Kohri, M.; Shinoda, Y.; Kohma, H.; Nannichi, Y.; Yamauchi, M.; Yagai, S.; Kojima, T.; Taniguchi, T.; Kishikawa, K. Facile Synthesis of Free-Standing Polymer Brush Films Based on a Colorless Polydopamine Thin Layer. Macromol. Rapid Commun. 2013, 34, 1220-1224. (6) Sileika, T. S.; Barrett, D. G.; Zhang, R.; Lau, K. H. A.; Messersmith, P. B. Colorless Multifunctional Coatings Inspired by Polyphenols Found in Tea, Chocolate, and Wine. Angew. Chem., Int. Ed. 2013, 52, 10766-10770. (7) Yabu, H.; Saito, Y.; Shimomura, M.; Matsuo, Y. Thermal Nanoimprint Lithography Of Polymer Films On Non-Adhesive Substrates By Using Mussel-Inspired Adhesive Polymer Layers. J. Mater. Chem. C 2013, 1, 1558-1561. (8) Manolakis, I.; Noordover, B. A. J.; Vendamme, R.; Eevers, W. Novel L-DOPA-Derived Poly(ester amide)s: Monomers, Polymers, and the First L-DOPA-Functionalized Biobased Adhesive Tape. Macromol. Rapid Commun. 2014, 35, 71-76. (9) Watanabe, H.; Fujimoto, A.; Takahara, A. Surface Functionalization by Decal-like Transfer of Thermally Cross-Linked Urushiol Thin Films. ACS Appl. Mater. Interfaces 2014, 6, 18517-18524. (10) Hong, S.; Schaber, C. F.; Dening, K.; Appel, E.; Gorb, S. N.; Lee, H. Air/Water Interfacial Formation of Freestanding, Stimuli-Responsive, Self-Healing Catecholamine JanusFaced Microfilms. Adv. Mater. 2014, 26, 7581-7587.


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(11) Kumanotani, J. Urushi (Oriental Lacquer) - A Natural Aesthetic Durable And FuturePromising Coating. Prog. Org. Coat. 1995, 26, 163-195. (12) Shafrin, E. G.; Zisman, W. A. Constitutive Relations In The Wetting Of Low Energy Surfaces And The Theory Of The Retraction Method Of Preparing Monolayers. J. Phys. Chem. A 1960, 64, 519-524. (13) Honda, K.; Morita, M.; Otsuka, H.; Takahara, A. Molecular Aggregation Structure and Surface Properties of Poly(fluoroalkyl acrylate) Thin Films. Macromolecules 2005, 38, 56995705. (14) Watanabe, H.; Fujimoto, A.; Yamamoto, R.; Nishida, J.; Kobayashi, M.; Takahara, A. Scaffold for Growing Dense Polymer Brushes from a Versatile Substrate. ACS Appl. Mater. Interfaces 2014, 6, 3648-3653. (15) Stafford, C. M.; Harrison, C.; Beers, K. L.; Karim, A.; Amis, E. J.; Vanlandingham, M. R.; Kim, H. C.; Volksen, W.; Miller, R. D.; Simonyi, E. E. A Buckling-Based Metrology For Measuring The Elastic Moduli Of Polymeric Thin Films. Nat. Mater. 2004, 3, 545-550. (16) Chung, J. Y.; Nolte, A. J.; Stafford, C. M. Surface Wrinkling: A Versatile Platform for Measuring Thin-Film Properties. Adv. Mater. 2011, 23, 349-368. (17) Tsakalakos, T. The bulge test: A comparison of the theory and experiment for isotropic and anisotropic films. Thin Solid Films 1981, 75, 293-305. (18) Small, M. K.; Nix, W. D. Analysis Of The Accuracy Of The Bulge Test In Determining The Mechanical Properties Of Thin Films. J. Mater. Res. 1992, 7, 1553-1563. (19) Kawai, K.; Nakagawa, M.; Zhang, X.-M.; Kawai, K.; Ikeda, Y.; Yasuno, H.; Miyakoshi, T.; Sato, A.; Konishi, H. Evaluation Of Structure-Activity Relationships For The Allergenicities Of Urushiol Analogs By Using Patch Tests And Lymphocyte Stimulation Tests In Humans Environmental Dermatology 1996, 3, 73-81.


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Langmuir

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Biobased Polymer Coating Using Catechol Derivative Urushiol

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