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Click Chemistry Engineered Hyperbranched Polyurethane−Urea for Functional Coating Applications Sasidhar Kantheti, Ramanuj Narayan, and Kothapalli VSN Raju* Polymers & Functional Materials Division, Indian Institute of Chemical Technology, Hyderabad 500007, Andhra Pradesh, India ABSTRACT: This article reports a facile route for the synthesis of different generations of 1,2,3-triazole-rich hyperbranched polyether polyols using click chemistry and extends their application for the development of moisture-cured polyurethane−urea coatings. Initially, three generations of chlorinated hyperbranched polyether polyols were synthesized by ring-opening polymerization of epichlorohydrin with trimethylolpropane in different mole ratios. These chlorinated polyols on continuous treatments with sodium azide and propargyl alcohol result in three generations of 1,2,3-triazole-rich hyperbranched polyether polyols. Furthermore, these different generations of polyols were reacted with 1-isocyanato-4-[(4-isocyanatocyclohexyl) methyl] cyclohexane (H12MDI) at OH:NCO ratio of 1:1.2 to obtain −NCO-terminated triazole-rich hyperbranched polyurethanes, which were subjected to curing under atmospheric moisture to obtain hyperbranched polyurethane−urea coatings. The coatings showed considerable enhancement in thermal stability, glass transition temperature, and corrosion resistance properties with an increase in generation number. All three generations of coating films show excellent resistance toward various bacterial and fungal attacks. exceptional yield, high tolerance of functional groups,8 simple reaction conditions, insensitivity to solvents and reaction medium, high chemo selectivity with no byproducts, chemical inertness, and ease of product isolation.9 The contribution of each 1,2,3-triazole ring to the enthalpy of formation is 168 kJ mol−1. Hence, their inclusion in the polymer matrix improves the overall thermal and moisture resistance of the polymer hybrid, which could be utilized for the development of high-enthalpy modifiers in energetic binders10 and heatresistant polymeric resins in advanced composite materials.11 The large dipole moment of 1,2,3-triazole modulates N-2 and N3 nitrogen atoms present in the triazole ring as good H-bond acceptors.12−14 This hydrogen-bonded triazole acts as a biologically active site which shields the material from various bacterial and fungal attacks, which can find its application in antibacterial coatings and biomedical applications.7,15,16 The present work focuses on synthesis of 1,2,3-triazole-rich hyperbranched polyether polyols. In the first step, three generations of chlorinated hyperbranched polyether polyols were synthesized by ring-opening polymerization of epichlorohydrin with trimethylolpropane in 3:1, 6:1, and 9:1 ratios, namely, G-1-Cl, G-2-Cl, and G-3-Cl, respectively. In the second step, these chlorinated polyols were converted to azideterminated hyperbranched polyols (G-1-N3, G-2-N3, and G-3N3) by treatment with sodium azide. Finally, azide-terminated hyperbranched polyether polyols were treated with propargyl alcohol in the presence of Cu(I) catalyst to get three generations of 1,2,3-triazole-rich hyperbranched polyether polyols (G-1polyol, G-2-polyol, and G-3-polyol). These polyols were then subjected to reaction with H12MDI in OH/NCO ratio of 1:1.2 to

1. INTRODUCTION Hyperbranched polymers (HBPs) and dendrimers are a group of polymers garlanded with heavily branched structures and a large number of reactive end functional groups.1 In the past decade, several research groups have focused on HBPs because of their unique physicochemical properties, which present potential applications in coatings, additives, drug and gene delivery, macromolecular building blocks, nanotechnology, and supramolecular science.2 Among all the HBPs, hyperbranched polyether polyols in particular arrest the major attention because of their inherent bioinert ether scaffold and highly dense peripheral functional groups, which have been exploited in biomedical applications ranging from drug delivery to surface coatings.3 Unlike linear polymers, these polymers show lower viscosities at higher molecular weights, which is credited to lesser molecular entanglements in HBPs. For coating and resin applications this type of behavior is interesting in terms of environmental issues in order to maintain lower volatile organic compounds (VOCs) in coating formulations.4 Most of the polyether polyols are used in the development of polyurethane coatings; however, other end uses range from synthetic lubricants to surface-active agents.5 Hyperbranched polyether polyols are generally synthesized by ring-opening polymerization of epoxides (like glycidol, epichlorohydrin, etc.) or oxetanes. However, polymerization of Epoxides (or oxiranes) is convenient in comparison with that of their oxetane counterpart because of the high ring strain of the three-membered ring.6 Incorporation of aromatic groups, heterocyclic rings, or some inorganic groups into the HBP structure bestows additional properties on polyurethane coatings.7 Thus, nitrogen-rich aromatic compounds like triazoles and tetrazoles could be a dependable choice for integration with polyurethane matrix. Since the discovery of Cu(I) catalyzed azide−alkyne 1,3-dipolar cycloaddition reaction for the formation of 1,2,3-triazoles by K Barry Sharpless in 2001, research on these materials has become much prevalent. This has been attributed to the product’s © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8357

February April 18, April 28, April 28,

14, 2014 2014 2014 2014

dx.doi.org/10.1021/ie500627x | Ind. Eng. Chem. Res. 2014, 53, 8357−8365

Industrial & Engineering Chemistry Research

Article

Figure 1. FT-IR images of (a) chlorinated hyperbranched polyols, (b) azidated hyperbranched polyols, (c) 1,2,3-triazole containing hyperbranched polyols, and (d) hyperbranched polyurethanes.

get −NCO-terminated polyurethanes. The excess NCO present was cured with atmospheric moisture to get the moisture-cured polyurethane−urea to obtain G-1-PU, G-2-PU, and G-3-PU, respectively.

(Kratos Analytical, Manchester, U.K.) equipped with a pulsed nitrogen laser (λmax, 337 nm; pulse width, 3 ns). Ions were accelerated into the analyzer at a voltage of 20 kV. The thermal stability of the different HBPU coatings was studied using TGA Q500 Universal TA Instruments (UK) and differential scanning calorimetry (PerkinElmer TA DSC Q100, U.S.) at a constant heating rate of 10 °C min−1 in nitrogen atmosphere. The electrochemical polarization study of the coatings on a mild steel panel was conducted by a three-electrode method on an IM6ex (ZAHNER Elektrik, Germany) instrument using a 3.5% NaCl solution. The salt spray tests were carried out in a salt mist chamber following ASTM B 177-94 standard. A 3.5 wt % NaCl solution was atomized by compressed air in the chamber containing the specimen. The antibacterial activity analysis of the different coating films was performed on both Gram-positive and Gram-negative bacterial strains. In the present work, the Staphylococcus spp., Bacillus spp., and Pseudomonas spp. were used as Gram-positive bacterial strain and Escherichia coli as Gram-negative. The 24 h aged active bacterial cultures were poured in the Luria Agar medium and allowed to solidify. After solidification of the medium, polymer samples (films) with 2 × 2 cm2 (approx) were embedded in the medium and incubated at 37 °C for 24 h. The polymer samples were washed and dried with double-distilled water before the experiment. 2.3. Synthesis of Chlorinated Hyperbranched Polyols (G-1-Cl, G-2-Cl, and G-3-Cl). These polyols were prepared according to the literature.17 For G-1-Cl, trimethylolpropane

2. EXPERIMENTAL DETAILS 2.1. Materials. Trimethylol propane, epichlorohydrin, sodium azide, propargyl alcohol, boron trifluoride dietherate, tetra butyl ammonium iodide, sodium ascorbate, 1-isocyanato-4[(4-isocyanatocyclohexyl) methyl] cyclohexane (H12MDI), and dibutyl tin dilaurate (DBTDL) were purchased from Aldrich chemicals (Milwaukee, WI, U.S.). Copper sulfate pentahydrate, acetonitrile, N,N-dimethylformamide, and t-butyl alcohol were purchased from SD Fine Chemicals (Mumbai, India). All the chemicals were used for reaction process without further purification. 2.2. Characterization Methods. The 1H NMR and 13C NMR study of the synthesized polyether polyols was done in VARIAN-200 and BRUKER-300 MHz spectrometers by taking tetramethylsilane (TMS) as the standard at room temperature and dissolving in DMSO-d6 or CDCl3 solvent. Fourier transform IR spectra (FT-IR) of all the samples, which were coated on dry KBr discs, were obtained on a Thermo Nicolet Nexus 670 spectrometer. Each sample was scanned 16 times with resolution of 4 cm−1 in the range 400−4000 cm−1. Matrix-assisted laser desorption ionization (MALDI) mass spectra of the final hyperbranched polyethers were recorded using a Kompact MALDI SEQ laser desorption time-of-flight mass spectrometer 8358



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Figure 2. 1H NMR images of (a) chlorinated hyperbranched polyols, (b) azidated hyperbranched polyols, and (c) 1,2,3-triazole containing hyperbranched polyols and 13C NMR of (d) chlorinated hyperbranched polyols, (e) azidated hyperbranched polyols, and (f) 1,2,3-triazole containing hyperbranched polyols.

Scheme 1. Synthetic Route for Different Generations of Triazole-Rich Hyperbranched Polyols

After complete addition, the reaction mixture was stirred at 70 °C for 12 h. The same procedure was repeated for the synthesis of G2-Cl and G-3-Cl; however, the TMP and epichlorohydrin were taken in 1:6 and 1:9 mol ratios, respectively. FT-IR overlay picture is shown in Figure 1a, and 1H NMR and 13 C NMR

(2gm, 14.9 mmol, 1 equiv) was taken into single-neck roundbottom flask equipped with nitrogen inlet and magnetic stir bar. Initially TMP was heated at 56 °C to melt. Then the heat was removed and 0.01 mL of BF3 etherate was added as a catalyst. To this reaction mixture epichlorohydrin (4.13 g, 44.7 mmol, 3 equiv) was added dropwise for a period of 20 min under stirring. 8359



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Scheme 2. Synthesis of Triazole-Rich Hyperbranched Polyurethane−Urea (G-1-PU, G-2-PU, and G-3-PU)

Figure 3. MALDI-TOF spectrum of G-1-polyol, G-2-polyol, and G-3-polyol.

C NMR (125 MHz, CDCl3, δ): 7.39 (−CH3), 23.12 (−CH2−), 42.93(−C−CH2−CH3), 53.27 (−CH2−N3), 65.52 (−CH2−O−), 72.61 (−CH−OH), 78.86 (−CH2O−C−CH2− N3). 2.5. Procedure for Click Reaction to Form 1,2,3Triazole Containing Hyperbranched Polyols. The azide, G-1-N3 (5 g, 1 equiv), and excess quantity of propargyl alcohol (3 g, 4.5 equiv) were dissolved in 1:1 mixture of t-butyl alcohol and water solvent mixture in a round-bottom flask. To the reaction mixture, 5 mol % CuSO4 and 10 mol % sodium ascorbate solutions (in distilled water) were added. The reaction mixture was stirred for 24 h at room temperature. The solvents were then evaporated, and the residue was dissolved in acetone. The acetone solution was dried over sodium sulfate and the G-1polyol was collected after evaporating the solvent. The same procedure was employed to synthesize G-2 and G-3 polyols. FTIR overlay picture of the polyols is shown in Figure 1c, and 1H NMR and 13C NMR overlay pictures are given in panels c and f of Figure 2, respectively. Various steps involved in the synthesis of triazole-rich hyperbranched polyols are shown in Scheme 1. FT-IR (KBr, cm−1): 3352.68 (O−H str), 2887.11 (C−H str), 1459.27 (C−O str), 1100−1015 (C−O−C str), 1650−1630 (triazole str + moisture). 1 H NMR (500 MHz, DMSO-d6, δ): 0.82 (t, 3H, J = 7.1 Hz, −CH3), 1.31 (q, 2H, J = 7.3 Hz, −CH2−), 3.34 (m, 6H, −C− CH2−O−), 3.67 (m, 9H, −O−CH2−CHOH and −CH−OH), 3.93 (s, 6H, triazole(C)−CH2−OH), 4.67 (d, 6H, triazole(N)− CH2−), 7.61 (s, 3H, triazole-H). 13 C NMR (125 MHz, CDCl3, δ): 6.97 (−CH3), 22.27 (−CH2−), 42.60 (−C−CH2−CH3), 52.63 (triazole (N)− CH2−), 55.46 (triazole(C)−CH2−OH), 68.33−71.80 (−C− CH2−O− and −CH−OH), 123.13 (triazole (C)−CH2OH), 147.44 (triazole (C)). 2.6. Synthesis of Hyperbranched Polyurethane Coating Free Films. The polyether polyol (G-1-polyol, G-2-polyol, 13

overlay pictures are given in panels a and d of Figure 2, respectively. FT-IR (KBr, cm−1): 3432.78 (O−H str), 2956.04 (asym −CH3 str), 2917.58 (asym −CH2 str), 2879.12 (sym −CH3 str), 1451.57 (C−O str), 1100−1015 (C−O−C str), 749.75 (C−Cl str). 1 H NMR (500 MHz, CDCl3, δ): 0.85 (t, 3H, J = 7.55 Hz, −CH3), 1.37 (q, 2H, J = 7.54 Hz, −CH2−), 3.39 (s, 6H, −CH2− O−), 3.59 (m, 6H, −CH2−Cl), 3.72 (m, 6H, −CH−CH2−O−), 3.98 (m, 3H, HC−O−). 13 C NMR (125 MHz, CDCl3, δ): 7.55 (−CH3), 23.12 (−CH2−), 43.34 (−C−CH2−CH3), 45.75 (−CH2−Cl), 70.00 (−CH−OH), 72.12 (−CH2−O−), 78.98 (−C−CH2Cl). 2.4. Synthesis of Azidated Hyperbranched Polyols (G1-N3, G-2-N3, G-3-N3). The above synthesized chlorinated hyperbranched polyol G-1-Cl (5 g, 1 equiv) was dissolved in 100 mL of acetonitrile and water mixture (8:1). To this, excess sodium azide (7.2 g, 7.5 equiv) was added portionwise. The resulting reaction mixture was stirred for 36 h at 80 °C. After the completion of the reaction, the solvent was evaporated. The product was extracted in chloroform and washed with brine solution twice. The organic layer was collected and dried over sodium sulfate. The solvent was evaporated to obtain G-1-N3. The synthesis of G-2-N3 and G-3-N3 were similar to that of G1N3. FT-IR overlay (Figure 1b), 1H NMR (Figure 2b), and 13C NMR (Figure 2e) confirm the formation of compounds. FT-IR (KBr, cm−1): 3432.78 (O−H str), 2956.04 (asym −CH3 str), 2917.58 (asym −CH2 str), 2879.12 (sym −CH3 str), 2102.53 (N3 str), 1451.57 (C−O str), 1100−1015 (C−O−C str). 1 H NMR (500 MHz, CDCl3, δ): 0.82 (t, 3H, J = 7.55 Hz, −CH3), 1.31(q, 2H, J = 7.55 Hz, −CH2−), 3.34 (m, 6H, −CH2− N3), 3.45 (m, 6H, −C−CH2−O−), 3.5−3.7 (m, 6H, −CH− CH−O−), 3.92 (m, 3H, HC−OH). 8360



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Table 1. Thermal Stability and Glass Transition Temperature (Tg) Data of Different Hyperbranched Polyols and Polyurethane− Urea (from TGA, DSC) sample code G-1Polyol G-2Polyol G-3Polyol G-1-PU G-2-PU G-3-PU

onset decomposition temperature (T1ON) (°C)

10% wt loss temperature (Td10) (°C)

50% wt loss temperature (Td50) (°C)

% wt remaining at 300 °C

% wt remaining at 400 °C

195.50

204.85

276.69

31.48

10.90

−

229.31

247.65

298.85

48.38

10.88

−

249.57

239.24

311.85

63.50

10.22

−

235.89 242.61 259.12

223.07 246.98 249.43

347.12 353.02 362.22

76.32 77.22 81.60

26.11 27.64 29.40

64.61 68.67 70.53

Tg

Figure 4. (a) TGA profile of three generations of hyperbranched polyols. (b) TGA profile of three generations of hyperbranched polyurethanes. (c) DTG profile of three generations of hyperbranched polyols. (d) DTG profile of three generations of hyperbranched polyols.

Figure 5. (a) DSC profile of three generations of hyperbranched polyurethanes. (b) Tensile properties of different generations of hyperbranched polyurethanes.

75 °C to prepare the prepolymer resin (Scheme 2). Prior to casting, one drop of DBTDL (catalyst) and Tegostab (surfactant) were added to the prepolymer resin. These prepolymer resins were casted on a tin foil supported by a glass plate through a manual driven square applicator. The excess −NCO present in the polyurethane films were moisture cured at

or G-3-polyol, 2 g) was dissolved in 5 g of DMF and cellosolve acetate mixture by heating in an isomantle. This dissolved polyether was added dropwise through a dropping funnel to the flask containing the H12MDI in 1:1.2 OH/NCO ratios at 60 °C. The addition was continued for 15−20 min. Once the addition was complete, the reaction mixture was stirred for another 4 h at 8361



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30 °C and laboratory humidity condition (25−30%) for 15 days. The films casted on tin foil (G-1-PU, G-2-PU, and G-3-PU) were recovered after amalgamation and cleaning. FT-IR spectroscopy was used for structure−property correlation study, whereas the coating properties were evaluated by TGA, universal testing matchine (UTM), DSC, antibacterial studies, and salt spray studies.

Table 4. Antimicrobial Activities of Different Hyperbranched Polyurethane−Urea Coatingsa sample code G-1PU G-2PU G-3PU

3. RESULTS AND DISCUSSION 3.1. FT-IR Analysis. In the present investigation, FT-IR experiments were performed to confirm the formation of

sample code

max. stress (N/mm )

elongation (%)

G-1-PU G-2-PU G-3-PU

44.29 48.08 51.97

16.41 14.58 10.85

coating thickness (μm)

Ecorr (mV)

Icorr (A/cm2)

polarization resistance (Rp) (kOhm cm2)

−

−476.6

3.86 × 10−6

8.85

0.14

63

−398.8

251.39

0.02

G-2-PU

67

−357.4

500 × 10−9 227 × 10−9

392.18

0.009

Pseudomonas aeruginosa

Staphylococcus aureus

+

++

−

+

+

+

++

−

+

++

+

++

−

+

++

cm−1 (asymmetric ring deformation) and also the appearance of ether stretching at 1100−1000 cm−1. Next, the presence of the peak at 2100 cm−1 and also the absence of C−Cl stretching peak at 750 cm−1 supports the formation of azidated hyperbranched polyols. The click reaction between azidated hyperbranched polyols and propargyl alcohol was asserted by the disappearance of the azide peak at 2100 cm−1 in the final FT-IR spectra of G-1, G-2, and G-3 polyols. The FT-IR overlay of all the samples are shown in Figure 1. The structures of the as prepared polyols were further supported from 1H, 13C NMR, and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) analysis. Furthermore, the formation of hyperbranched polyurethanes, i.e., the reaction between hyperbranched polyols such as G-1, G2, and G-3 polyols with H12MDI was also confirmed by the disappearance of the free −NCO peak at 2270 cm−1, which indicates the complete moisture curing of the polyurethane free films (Figure 1d). Different absorption bands analogous to the urethane linkages were shown in the FT-IR spectra of polyurethanes. The observed absorption bands at 3050−3700 cm−1 (−NH stretch), 2800−3000 cm−1 (−CH stretch consisting of asymmetric −CH3 stretch at 2957 cm−1, asymmetric −CH2 stretch at 2920 cm−1, symmetric −CH3 stretch at 2872 cm−1, and symmetric −CH2 stretch at 2851 cm−1), 1600−1800 cm−1 (−CO stretching of amide I), 1500−1600 cm−1 (amide II stretch consisting of a mixture of peaks δN−H, νC−N, and νC−C), 1630 cm−1 (aromatic triazole ring stretching), 1394 cm−1 (δCH2

corrosion rate (CR) (mm/yr)

bare mild steel panel G-1-PU

Candida albicans

“+” indicates showing of antimicrobial activity of the coating film. “−” indicates the negative test result of the coating film.

Table 3. Electrochemical Parameters Measured from the Tafel Plots of Different Hyperbranched Polyurethane−Urea Coatings and Bare Metal sample code

Escherichia coli

a

Table 2. Tensile Properties of Different Generations of Hyperbranched Polyurethane−Urea 2

Bacillus subtilis

chlorinated hyperbranched polyols, azidated hyperbranched polyols, final triazole-rich polyols, and corresponding hyperbranched polyurethane coating free films. The formation of chlorinated polyols was established by disappearance of the epoxide peaks around 3050 cm−1 (C−H str of epoxide) and 900

Figure 6. Fog test results and the polarization curves of different generations of hyperbranched polyurethane coatings along with the bare metal evaluated by the Tafel method in a 5% NaCl solution. 8362



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Figure 7. Antimicrobial activity of Hyperbranched polyurethane coating films on different bacteria and fungus.

symm/assym), 1215−1350 cm−1 (amide III consisting of νC−N, −NH bending, and C−Cα), 1254 cm−1 (δOH free), 766 cm−1 (amide IV due to −NH out of plane vibration), and 720 cm−1 (−CH2 rocking) indicates the formation of hyperbranched polyurethane coatings. 3.2. 1H and 13C NMR and MALDI Analysis. The structural configurations of the various intermediates involved in the synthesis of triazole-rich hyperbranched polyols were affirmed by 1 H NMR and 13C NMR analysis. The formation of the chlorinated hyperbranched polyols was confirmed by the disappearance of peaks related to epoxide at δ 2.6, δ 2.8, and δ 3.2 in 1H NMR and peaks at δ 51.35 and δ 46.89 in 13C NMR followed by the appearance of new peaks related to ether linkage at δ 3.75 and δ 3.98 in 1H NMR and δ 72.21 and δ 70.13 in 13C NMR. The azidated hyperbranched polyols formation was established by the absence of peaks at δ 3.59 (−CH2−Cl) in 1H NMR and δ 45.75 (−CH2−Cl) in 13C NMR and the presence of peaks at δ 3.34 (−CH2−N3) in 1H NMR and δ 53.27 (−CH2− N3) in 13C NMR. Finally, the triazole-rich hyperbranched polyols were characterized by the presence of the peaks corresponding to 1,2,3-triazole ring at δ 7.61 in 1H NMR and peak at δ 147.44 in 13 C NMR. Figure 2 shows 1H NMR and 13C NMR spectra of different samples. The molecular weights of the three generations of 1,2,3triazole-rich polyether polyols (G-1, G-2, and G-3 polyols) were analyzed from MALDI-TOF analysis. Figure 3 represents the MALDI-TOF spectra of three generations of hyperbranched polyols. It is observed that the molecular weight of the polyol increases with generation number. From the MALDI plots, there is wide range of molecular weight distribution present in the final hyperbranched polyols, but the molecular weight corresponding to the highly intense peak in the spectrum has good agreement with theoretical molecular weight. The molecular weights were observed as salts of sodium or hydrogen ions. Theoretically, the molecular weight of the final hyperbranched polyols can be determined by the following equation.

M = 134.17 + n(155.10) + M Na+/H+ where n = 3 or 6 or 9

3.3. TGA, DTG, and DSC Analysis. The relative thermal stability of three generations of hyperbranched polyols and corresponding polyurethanes were evaluated by thermogravimetric analysis (TGA) in N2 environment. The characteristic thermal decomposition temperatures, such as onset decomposition temperature TON, temperature corresponding to 10% or 50% weight loss (Td10, Td50), and percent weight remaining at 300 and 400 °C, for three generations of polyols and their corresponding hyperbranched polyurethanes were recorded (Table 1). It is observed that the thermal stability of the hyperbranched polyols and corresponding polyurethanes increases with generation number. From the DTG (differential thermogravimetric analysis) thermogram analysis of hyperbranched polyols, it is seen that the degradation takes place in one step and the maximum decomposition temperature (Td max) increases with generation number. Nevertheless, in the case of hyperbranched polyurethanes, the degradation takes place in three steps. The degradation around 150−200 °C is due to the presence of adsorbed moisture and solvents. The degradation at 300−400 °C corresponds to urethane−urea, and the degradation at 450 °C corresponds to 1,2,3-triazole units (Figure 4).18 The glass transition temperature of the hyperbranched polyurethanes was determined by differential scanning calorimetry; it was observed that glass transition temperature also increases with generation number of the polyols (Figure 5a). The increasing thermal stability with generation number in the case of polyols and corresponding polyurethanes could be due to two factors. First, with the increase in generation number, the number of stable triazole units increases. Second, end hydroxyl groups in the polyols and urethane segments in polyurethanes enhance the cross-linking density with increasing generation number. The formation of cross-linked network is ascribed to the hydrogen bondings in the polymer matrix. 3.4. UTM Analysis. The tensile behavior of different generations of hyperbranched polyurethanes studied by UTM instrument and the corresponding data is reported in Table 2. This data showcases an increase in the tensile strength and decrease in the percent elongation at break with the generation number of the hyperbranched polyurethanes. For instance, the tensile strength and percent elongation at break for three

M = M trimethylolpropane + n(M epichlorohydrin + M sodium azide − M sodium chloride + M propargyl alcohol) + M Na+/H+

(1)

M = 134.17 + n(92.52 + 65.0 − 58.44 + 56.01) + M Na+/H+ 8363



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that 1,2,3-triazole has a large dipole moment and that the N-2 and N-3 nitrogen atoms of triazole ring become excellent Hbond acceptors.12−14 In addition, Wei et al.24 revealed that hydrogen-bonded triazole is a biologically active species. Thus, in the current study we expect the antimicrobial nature of polyurethane coating may be due to the formation of intermolecular hydrogen bonding between triazole and −NH groups of urethane linkages.

generation polyurethanes (G-1-PU, G-2-PU, and G-3-PU) are 44.29, 48.08, and 51.97 N/mm2 and 16.41, 14.58, and 10.85%, respectively. These data suggest that the tensile modulus follows a trend similar to that of thermal stability. With the generation number of polyol, number of cross-linking units increases in the polyurethane matrix, which leads to increase in the mechanical property. 3.5. Electrochemical and Fog Test Results. The electrochemical test is a technique used to study the corrosion performance of coated metals exposed to aqueous environments.19,20 The mild steel panels (2 × 2 cm2) were first subjected to chemical cleaning and surface preparation steps to eliminate trace amounts of surface oxides. The panels were washed with acetone, ethanol, and doubly distilled water to remove dust particles and surface oils. The coatings were applied on mild steel panels with the help of a spin coater, and the panels were dried for 24 h. The corrosion resistance of various polyurethane coatings was evaluated by the Tafel method in a 3.5% NaCl solution. Initially, the coated panels were immersed in 3.5% NaCl solution for 30 min. Polarization resistance Rp for various samples was evaluated from Tafel plots, according to the Stearn−Geary equation (eq 2),21 whereas corrosion rate CR (in millimeters per year) was calculated using eq 3.22,23 Rp =

ba × bc 2.303(ba + bc)Icorr

(2)

CR =

M Icorr n×F×d

(3)

4. CONCLUSIONS The present work reports a facile route for synthesizing 1,2,3triazole-rich hyperbranched polyether polyols by using azide− alkyne click chemistry. This was followed by the formation of eco-friendly moisture-cured hyperbranched polyurethane−urea for functional coating applications. The work is a typical example of the suitability of click chemistry in coating applications. The as-prepared polyurethane coatings provide an interesting arena for research in polymers to explore various industrial products. The present work could extend its use in diverse marine environments where high corrosion resistivity is expected. Apart from this, the films are antimicrobial in nature with improved thermal and mechanical stability, which extends their use in versatile biological applications. Nevertheless the coating is not just limited to these strategies; a typical expansion of the work could be association with various other incorporated materials of interest to tune the obtained coating for a particular usage. We hope that the present work showcases an advancement of 1,2,3triazoles in the development of coating materials, which can open new avenues in various scientific fields.

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The Ecorr and Icorr values for bare metal, G-1-PU, and G-2-PU were recorded from the Tafel polarization curves (Table 3). Ecorr value corresponds to the corrosion resistance potential of coating, whereas Icorr corresponds to the corrosion current. For instance, the Ecorr and Icorr values of bare metal, G-1-PU, and G-2PU are −476.6, −398.8, and −357.4 mV and 3.86 × 10−6, 500 × 10−9, and 227 × 10−9 A/cm2, respectively. The decrease in Icorr value correlates to enhancement in corrosion resistance of coating. The same could be derived from the depleting Ecorr value. A similar improvement in corrosion resistance was obtained from a salt fog test using 5% NaCl solution for 200 h. The observed results suggest that the increase in generation number improves the coating resistance toward corrosion in a salt water environment. Again, this improvement with generation number is mainly due to the formation of highly cross-linked urethane networks and the presence of chemically inert triazole units, which results in a strong barrier on the mild steel surface and restricts the penetration of corrosive species. Figure 6 represents images of coated mild steel panels before and after 200 h of fog test along with potentiodynamic polarization curves (Tafel plots). 3.6. Antimicrobial Properties. The antibacterial activities were measured based on the formation of inhibition zone loss of bacterial growth beneath and surroundings of the films placed on Luria−Bertani agar medium (Table 4). In a similar way, the antifungal activities of the different hyperbranched polyurethane films were also determined by the above-mentioned method using fungal strains. The colonies of various bacteria were grown on Czapek−Dox medium. The Czapek−Dox plates consist of fungal cultures, and the embedded films were incubated for 4−5 days at ambient temperatures of 28 ± 4 °C; loss of bacterial moss surrounds beneath the film was monitored. All the polyurethane coating free films show excellent activity on all bacterial stains, except for Candida albicans (Figure 7). Earlier reports revealed

AUTHOR INFORMATION

Corresponding Author

*Phone/fax: (+91) 40 27193991. E-mail: drkvsnraju@gmail. com, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research work was supported by CSIR, India under Intel-Coat Project (CSC-0114). The authors would like to extend their thanks to the Director, CSIR- IICT for permitting the work to be carried out.

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REFERENCES

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