Environmentally friendly polyurethane dispersion derived from dimer

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Environmentally friendly polyurethane dispersion derived from dimer acid and citric acid Soumen Chandra, and Niranjan Karak ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03474 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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ACS Sustainable Chemistry & Engineering

Environmentally friendly polyurethane dispersion derived from dimer acid and citric acid Soumen Chandra and Niranjan Karak* Advanced Polymer and Nanomaterial Laboratory, Department of Chemical Sciences, Tezpur University, Napaam 784028, Assam, India *Email : [email protected] KEYWORDS Bio-based polyurethane, water dispersion, self-healing, UV-resistance, sustainable.

ABSTRACT Sustainable polyurethane dispersion with smart properties has enormous potential in the polymer industries due to its unique and environmentally friendly characteristics. Biodegradable water dispersion thermoplastic polyurethanes (WBPUs) with self-healing capability were synthesized, and they were water-dispersible after neutralization with triethylamine (TEA). The bio-based macroglycol and chain extender were synthesized via the catalyst-based esterification reactions of dimer acid (DA),

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poly(ethylene glycol) (PEG-200), citric acid (CA) and glycerol. Three different compositions of WBPUs were synthesized by reacting poly(ε-caprolactone) diol (PCL, M.W~2000 g/mol) and bio-based macroglycol with aliphatic and aromatic diisocyanates in the presence of various amounts of branch generating unit. FT-IR, 1HNMR,

13CNMR,

GPC, and XRD analyses of WBPUs were conducted to study the physicochemical structure, crystallinity and molecular weight. The WBPUs exhibited high elongation at break (678.9-485.3%), toughness (51.49-32.6MJ m-3), tensile strength (9.26-7.82 MPa), scratch hardness (3.5-2 kg), gloss at 60° (25.4-15.6°), impact resistance (8.95-8.33 kJ/m) and chemical resistance. Furthermore, these WBPUs possessed microwave assisted self-healing and UV-aging characteristics, and they were found to be biodegradable.

INTRODUCTION Polyurethanes are one of the most promising members of polymer family, due to their wide spectrum of applications. They are frequently employed in many industrial products like smart textiles, coating materials, adhesives, laminates, paints, plasticizers


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etc.1,2 Owing to their excellent properties and versatile fields of applications, several solvent-borne polyurethanes have been synthesized from petroleum-based nonbiodegradable raw materials,3 which poses a great challenge towards the environment and human health. To address these challenges, the scientific community is leaning towards manufacturing polyurethanes from bio-based resources, which are more environmentally friendly.4 Thus recently, researchers are more focusing on the synthesis of bio-based waterborne polyurethanes from natural renewable resources. Use of natural renewable or bio-based resources as the precursors including chain extender offers many advantages like easy availability, inexpensiveness, as well as environmentally benign to waterborne polyurethanes. Moreover, the use of renewable resources with their bio-origin impression in the molecular chains imparts more biodegradability to such polyurethanes. State of art literature cites copious examples of such bio-based polyurethanes. Karimi et al. reported bio-based polyurethane films from canola oil5, while Kalita and Karak synthesized castor oil-modified polyurethanes by an A2+ B3 approach with several advantages over their linear analogs.6 Furthermore, solvent-borne polyurethane systems suffer from large content of volatile organic


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compounds (VOCs), which are considered as the potent pollutants to environment.7 In this context, the design of waterborne polyurethane is emerging as a suitable ecofriendly approach towards polyurethane applications, reducing the release of VOCs.1 For example, Liu et al. synthesized polyurethane by an organic solvent-free and selfcatalytic process.8 Díez-García et al. manufactured waterborne poly(urethane-urea) by using corn-based renewable carbon (100%) poly(trimethylene ether glycol) and poly(ethylene oxide).9 Wan and Chen synthesized waterborne polyurethane using biobased starting materials and fabricated polyurethane/graphene oxide nanocomposite to improve mechanical properties of the polymer.10 From all of these reports, however, it is cleared that the mechanical properties of such bio-based waterborne polyurethanes are poor. Hence, over the last couples of decades, polymer industries displayed copious attention towards the development of polyurethane dispersions, and their mechanical and thermal properties have been widely studied.11 In this case also, in order to improve properties like tensile strength, hardness, elongation at break and high alkali resistance, several approaches including physical blending with acrylic and vinyl polymers,12 formation of interpenetrating networks,13 hybrid system by crosslinking or grafting of


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vinyl or acrylic monomer with polyurethane moieties etc. have been utilized.14 Furthermore, many external and internal emulsifiers such as ionic segments containing diols including carboxylate and sulfonic group-containing diols15 are also reported to produce

polyurethane

dispersions.

Dimethylolpropionic

acid

(DMPA),

and

dimethylolbutanoic acid (DMBA) are the typical commercially available emulsifiers that are used for this purpose. The main reason behind the use of DMPA and DMBA as emulsifiers in polyurethane dispersion manufacturing is to enhance the mechanical and thermal properties of the synthesized polyurethane films.16 Even, a non-ionic segment like poly(ethylene oxide) moiety was also incorporated in polyurethane chain to create its water dispersion.17 In this context, Alvarez et al. reported polyurethanes/acrylate dispersion using DMPA as an internal emulsifier18. Here, the dispersion contains a hydrophilic part and plays a crucial role in dispersion by providing surface charge to disperse hydrophobic polyurethane segments in water. On the other hand, Xiao et al. reported waterborne polyurethane with sodium 2,4-diaminobenzenesulfonate chain extender, where they have prepared the polymer by an organic solvent-free approach.19


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In the present study, therefore, water dispersion thermoplastic polyurethane (WBPU) was synthesized using bio-based precursors like citric acid and glycerol along with dimer acid and PEG for the first time, by a facile approach. Here, a glycerol modified citric acid ester polyol was synthesized as a multipurpose moiety, acting as a branching unit, chain extender and internal emulsifier to achieve water dispersion of the synthesized polyurethane. Different compositions of WBPUs were synthesized by varying the weight percentages of the branching chain extender unit (7.2, 4.7 and 3.5%). The water dispersions of the synthesized polyurethanes were achieved by using trimethylamine, as a neutralizing agent. The synthesized polyurethanes were characterized by different spectroscopic and analytical techniques. Various properties including, physical, mechanical, chemical, thermal etc. along with self-healing and UVaging capabilities were evaluated. The biodegradation of the films was also studied against P. aeruginosa and B. subtilis bacterial strains.

EXPERIMENTAL SECTION Materials


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Dimer acid (Mn ~570 g/mol) and poly(ε-caprolactone) diol (PCL Mn ~2000 g/mol) were purchased from Sigma-Aldrich, USA and used without further purification. Butanediol (BD), poly(ethylene glycol) (PEG-200) and glycerol were acquired from Merck, India and were dried for 12 h under vacuum prior to use. Diisocyanates including isophorone diisocyanate (IPDI, Sigma-Aldrich, USA) and 2, 4/2, 6-toluene diisocyanate (TDI, Merck, India) were used as received. Oxalic acid (Rankem, India), sodium hydroxide (Rankem, India), isopropyl alcohol (SRL, Mumbai), butyl amine (Sigma-Aldrich, USA), bromophenol blue (indicator at pH 3.5~ 4.6), and para-toluene sulfonic acid (p-TSA, SRL, Mumbai) were used without any further purification. All the solvents including xylene (Merck, India), and tetrahydrofuran (Merck, India) were dried by distillation and kept with 4A type molecular sieve before use. Synthesis of bio-based macroglycol Bio-based macroglycol was obtained by stoichiometric controlled reaction between dimer acid (DA) and PEG-200 in presence of catalytic amount p-TSA at elevated temperature. Briefly, 2.85 g (5 mM) of dimer acid and 2 g (10 mM) of PEG-200 with 0.014 g of p-TSA (0.5 wt % with respect to dimer acid) were taken in a three-neck round


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bottom flask attached with a nitrogen gas inlet, a magnetic stirrer and a Teflon septum. The reaction was continued at 150 °C for 2 h in a temperature controlled oil bath. The water release during esterification was simultaneously removed by purging nitrogen gas to the reaction mixture. Finally, completion of the reaction was determined by measurement of acid value. After, 2 h of reaction, the acid value was reached to 25 mg of KOH/g, which indicates that the reaction was almost completed. The product obtained is named as DAPEG. Synthesis of bio-based chain extender Similarly, bio-based chain extender was synthesized by the esterification reaction between citric acid and glycerol in presence of catalytic amount p-TSA. Briefly, 2.3 g (12 mM) citric acid, 2.2 g (24 mM) glycerol and 0.02 g p-TSA (0.5 wt % with respect to citric acid) were taken in a three-neck round bottom flask with the similar arrangement as above. The reaction was continued at 140 °C for 1.5 h and generated water molecules were removed by purging nitrogen gas in the reaction mixture. After, 2 h of reaction, the acid value was reached to 165 mg of KOH/g. From this acid value measurement, the completion of the reaction was supported. The product is encoded as CAG.


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Synthesis of water dispersion thermoplastic polyurethane (WBPU) from bio-based macroglycol and chain extender Bio-based WBPUs were synthesized by pre-polymerization technique. Briefly, 0.69 g (0.75 mM) of DAPEG and 4.5 g (2.25 mM) of PCL were taken in a three-neck round bottom flask with the same arrangement like the above esterification reaction. Firstly, the reaction mixture was heated at 80 °C for half an hour to melt the solid PCL under slow mechanical stirring. After, melting PCL, 1.035 g (6 mM) of TDI was added dropwise to the reaction mixture by using a syringe at room temperature with the continuous flow of nitrogen gas. The reaction was allowed to proceed for 4 h at 85-90 °C to achieve appropriate viscosity without gel formation, which indicates that prepolymer was formed successfully. Then the temperature of the reaction mixture was brought to room temperature. After cooling the reaction, 0.289 g (3.15 mM) of 1, 4butanediol and 0.576 g (1.65 mM) of CAG were added to the reaction mixture with 2 mL of DMF. The reaction was left for 0.5 h at 250 rpm speed for proper mixing of 1, 4butanediol and CAG. Successively, 0.96 g (4.27 mM) of IPDI was added dropwise to the reaction mixture at room temperature. Then 2 mL of xylene was added to maintain


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the solid content of 70-80%. Then the reaction was continued at 100-110 °C for 5 h until the reaction mixture was reached to maximum viscosity without gel formation. The presence of a small amount of xylene was removed by purging continuous nitrogen gas into the reaction mixture. The completion of the reaction was confirmed by the butyl amine test, and FTIR spectral analysis of the product indicating no absorption at 2270 cm-1. In the final step, water dispersion of the synthesized polyurethane was obtained by treating the product with triethylamine followed by slow addition of water. Other two WBPUs were synthesized under the same reaction conditions but by varying composition of branching unit, CAG and butane diol. The compositions of all three WBPUs are tabulated in Table 1. Preparation of polyurethane film All the synthesized polyurethane were uniformly cast on metal steel plates (150 mm × 50 mm × 5 mm ) for the impact study, and on glass slides (80 mm × 30 mm × 10 mm) for gloss and scratch resistance tests. Simultaneously, synthesized polyurethanes were cast in Teflon sheets and dried in an open atmosphere for 48 h followed by vacuum


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drying at an elevated temperature to remove the volatiles. Finally, polyurethane films were kept in a desiccator for further analysis after peeling them from the Teflon sheet. Characterization To determine the functionality of the synthesized bio-based macroglycol, chain extender and polyurethane, FT-IR analyses were conducted by a Nicolet (Madison, U.S.A.) FT-IR impact 410 spectrophotometer over the range of 4000-400 cm-1, using KBr pellets.13C-NMR and 1H-NMR spectra of the chain extender, bio-based macroglycol and the synthesized polyurethane were conducted by a 400 MHz NMR spectrometer (JEOL, Japan) using DMSO-d6 as the solvent with TMS as the internal standard. The tensile strength and elongation at break of all the WBPU films were determined with the help of a Universal Testing Machine (UTM) using model WDW-10 (JINAN, CHINA), with a 1.0 kN load cell and a crosshead speed of 50 mm/min. Scratch hardness of all three polyurethane film was measured by a hardness tester, model number 705 (Sheen Instrument Ltd., U.K.) and a travel speed was maintained at 50 mm/s. Simultaneously, the impact resistance was determined by impact resistance tester (S.C. Dey & Co, Kolkata, 1 m is the maximum height) using the standard falling weight method. Thermal


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property of all three polymers was checked with the TGA-4000 (PerkinElmer, USA) at a heating rate of 5 °C/min and nitrogen flow rate of 30 mL/min within the temperature range of 32-720 °C. Successively, differential scanning calorimetry (DSC) analysis was conducted by the DSC 6000 instrument (Perkin Elmer, USA) at a nitrogen gas flow rate of 30 mL/min and heating rate of 10 °C/min within the temperature range of −40 to +150 °C. A D8 Focus XRD machine (Bruker AXS, Germany) equipped with a Cu Kα radiation source (λ = 1.54 Å) was used to record the XRD patterns of all the polyurethanes in the rage of 2θ = 10-80°. Inter planer distance (d) of PCL moiety was determined by the following equation [1]. d= nλ/2sinθ................ [1] Here, n is an integer (equal to 1), while λ and θ correspond to the wavelength of the incident X-ray beam and the incident angle of X-ray beam, respectively. The specific gravity of the synthesized WBPUs was measured by the standard method. The gloss values were evaluated by a mini gloss meter (Sheen Instrument Ltd., U.K.). The weight average molecular weight (Mw), number average molecular weight (Mn) and polydispersity index (PDI) of the polymers and the intermediates were estimated by gel


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permeation chromatography (GPC, Waters, U.K.) using linear polystyrene as the standard. Biodegradation study The biodegradation study was carried out following McFarland turbidity method using

Pseudomonas aeruginosa as a gram negative bacterial strain and Bacillus subtilis as a gram positive bacterial strain for a period of four weeks.20 These two different bacterial strains were collected from a laboratory of Molecular Biology and Biotechnology Department, Tezpur University. The medium used to grow the bacterial strain contains following salts 2 g of (NH4)2SO4, 4.75 g of KH2PO4, 2 g of Na2HPO4, 0.5 mg of CaCl2·2H2O, 1.2 g of MgSO4·7H2O, 100 mg of MnSO4·5H2O, 10 mg of H3BO3·5H2O, 70 mg of ZnSO4·7H2O, 100 mg of CuSO4·7H2O, 1 mg of FeSO4·7H2O, and 10 mg of MoO3, which was prepared in 1 L demineralized water. The prepared salt solution was sterilized by an autoclave at 120 °C temperature and pressure of 103.5 kPa for 30 minutes to achieve bacteria free solution. Both the bacterial strains were cultured in the prepared 10 mL medium solution and incubated at 37 °C for 48 h. Successively, all three synthesized polyurethane films were cut into small pieces (1 cm × 2 cm). These


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pieces of WBPU films were sterilized by exposing them to UV light of wavelength 254 nm and immersed in the bacterial solutions. Simultaneously, the solutions were incubated at 37 °C. Bacterial growth in the polymer films was determined by optical density measurement at 600 nm within one week interval up to four weeks. The above experiment was repeated for three times and values were reported as the average results of three experiments. Chemical resistance test Chemical resistance studies were conducted by immersing small pieces of polyurethane films into different chemical media including acidic solution (pH~4-5), basic solution (pH~8-9), 10% NaCl solution, 20% ethanol solution and tap water for a period of one month. Chemical resistance of three different polyurethane films was determined by measuring the weight loss after exposure of the films in different media. Self-healing ability test Self-healing ability of the three WBPUs was tested by microwave irradiation (300 W). To study self-healing ability, each of the three WBPU films was cut by a razor blade in the transverse direction and individually irradiated to microwave irradiation of 300 W.


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The healing efficiency of the films was measured by taking tensile strength before and after healing of the films under the same conditions. The healing efficiency was calculated using the given formula. Healing efficiency of polymer films = [(Tensile strength after healed)/Tensile strength before healed)] × 100 -------------(2) UV-Aging test UV-aging test was performed by following an artificial method. UV-light attached chamber was used to exposue the synthesized polyurethane films for 168 h. Polymer film with size (1 cm × 3 cm) was subjected to irradiate by the UV-light of 8 W with a wavelength of 265 nm. The effect of UV aging was measured by estimating tensile strength before and after UV-light treatment of the polyurethane films.

RESULT AND DISCUSSION Synthesis and characterization of DAPEG Poly(ethylene glycol) modified ester of dimer acid was synthesized by a catalyst based stoichiometric controlled esterification reaction between dimer acid and glycerol at elevated temperature (Scheme 1). The completion of the reaction was confirmed by


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the measurement of the acid value of the product compared to the free dimer acid. The molecular weight of DAPEG was found to be 1152 g/mol, which is close to its theoretical value. The solubility of the synthesized DAPEG ester was determined in organic polar and nonpolar solvents. From the solubility measurement, it was observed that DAPEG was highly soluble in the polar organic solvents like THF, DMSO, DMF and acetone but insoluble in the non-polar solvents, which indicates that DAPEG contains lots of polar functional groups. The obtained result is also found to be in consistent with the FT-IR data. Simultaneously, the esterification of dimer acid was confirmed by FT-IR, 1H-NMR and

13C-NMR

spectral analyses. The details result obtained from characterizations of

DAPEG are described below. FT-IR analysis of DAPEG The functionality of synthesized DAPEG was identified through FT-IR analysis and compared with FT-IR spectrum of free dimer acid as shown in Figure 1(a). The characteristic broad absorption peak at 3425 cm-1 corresponds to O-H stretching frequency of carboxylic acid group of dimer acid was observed in the spectrum, but this O-H stretching frequency was shifted to 3440 cm-1 for DAPEG. Furthermore, due to


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esterification, C=O stretching frequency of dimer acid at 1709 cm-1 was slightly shifted towards higher frequency region for DAPEG and the peak was observed at 1734 cm-1. Simultaneously, in FT-IR spectrum of dimer acid, the absorption peaks at 1419 cm-1and 937 cm-1 were observed due to in-plane and out of plane vibrations of O-H bond of carboxylic acid group, but these peaks were almost disappeared in FT-IR spectrum of DAPEG, which confirmed that the desired ester was successfully formed21. Furthermore, in FT-IR spectrum of DAPEG, peaks at 1190 cm-1 corresponds to stretching frequency of C-O-C, but in FT-IR spectrum of dimer acid, this absorption peak was absent, which reveals that ester bond was successfully formed after esterification reaction between dimer acid and PEG-200.22 The peaks at 1460 cm-1 and 1240 cm-1 in the FT-IR spectrum of DAPEG correspond to the stretching frequency for C-C and C-O bonds, respectively.23 Successively, the chemical structures of DAPEG was further characterized by 1H-NMR and 13C-NMR spectral analyses. 1H-NMR

and 13C-NMR spectral analyses of DAPEG

Chemical structure of DAPEG was confirmed by 1H-NMR analysis, as shown in Figure S1 of Supporting information (SI). In 1H-NMR spectrum of DAPEG, peaks at 0.77(a)


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ppm and 1.16(b) ppm correspond to the terminal methyl protons (-CH3) and methylene protons (-CH2) of DAPEG molecule, respectively.24 Simultaneously, peak at 1.44(c) ppm is credited to the protons for the cyclic ring of dimer acid. In 1H-NMR spectrum of DAPEG, peaks at 2.16 (d) ppm and 2.07 (e) ppm represent α and  protons of dimer acid with respect to the ester functional group. Similarly, peak at 4.04 (f) ppm corresponds to the α proton of PEG moieties with respect to the ester functional group. Peaks in the region 3.52-3.37 (g) ppm are assigned to the rest protons of PEG-200. Simultaneously,

13C-NMR

spectral analysis was carried out to identify the chemical

structure of DAPEG, as shown in Figure S1 of SI. The peak for terminal methyl carbon was observed at 14.1 ppm (a). Peaks in the range from 19.8 to 29.8 (b) ppm represent all the methylene carbons of DAPEG moiety. The peak at 31.9 ppm (c) reveals the presence of α carbon of dimer acid with respect to the ester functional group. Simultaneously, peak at 63.3 ppm (d) corresponds to the carbon of PEG, which is attached with ester functional group. Successively, 60.3 (e), 68.7 (f), 70.3 (g) and 72.8 (h) peaks are due to the presence of PEG moiety25. Small peaks at 174.5 (i) and 172.8 (j) ppm were observed in the spectrum for two different carbonyl carbons.


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Synthesis and characterization of CAG Glycerol modified ester of citric acid was synthesized by catalyst based esterification reaction between primary carboxylic acid groups of citric acid and primary hydroxyl groups of glycerol at elevated temperature (Scheme 1). The completion of the reaction was confirmed by measurement of the acid value of the product by comparing the same of free citric acid. The molecular weight of CAG was found to be 350 g/mol, which is close to its theoretical value. Simultaneously, the solubility of CAG was evaluated in many organic polar and nonpolar solvents. From the solubility measurement, it was observed that the synthesized CAG was mainly soluble in the high polar organic solvents including DMSO, DMF and water but less soluble or completely insoluble in the less polar solvents like THF, acetone, hexane and chloroform. The high solubility of CAG in the polar solvents is due to the presence of free carboxylic acid group in CAG, which is also confirmed from the FT-IR analysis. Simultaneously, the esterification of dimer acid was confirmed by FT-IR, 1H-NMR and

13C-NMR

analyses. The details result

obtained from characterizations of CAG are described below. FT-IR analysis of CAG


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The functionality of the synthesized CAG was confirmed by FT-IR analysis and compared with FT-IR spectrum of free citric acid to determine the complete esterification reaction, as shown in Figure 1(b). The characteristics broad absorption peak at 3496 cm-1 was observed due to the presence of hydroxyl group of carboxylic acid functionality of citric acid but this peak was slightly shifted after esterification and the peak was observed at 3488 cm-1. In FT-IR spectrum of free citric acid, two C=O peaks at 1750 cm-1 and 1709 cm-1 are assigned for free C=O and hydrogen bonded C=O of carboxylic acid group, but these C=O peaks were merged and slightly shifted after the esterification reaction. The C=O peak of CAG was observed at 1742 cm-1.26 Successively, in FT-IR spectrum of citric acid, two small absorption peaks at 1427 cm-1 and 937 cm-1 observed are due to in-plane and out of plane vibrations of O-H bond of carboxylic acid group of citric acid but these peaks are totally absent in the FT-IR spectrum of citric acid, the observed data indicates that citric acid is converted to the corresponding ester.27 Successively, in FT-IR spectrum of CAG, peak at 1195 cm-1 corresponds to the stretching frequency of C-O-C, but in FT-IR spectrum of citric acid, this absorption peak was absent, which indicates that the ester bond is successfully


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formed after esterification reaction between citric acid and glycerol. The peaks at 1427 cm-1 and 937 cm-1 in the FT-IR spectrum of CAG correspond to the stretching frequency for C-C and C-O bonds, respectively. Successively, to identify the chemical structure of CAG, it was further characterized by 1H-NMR and 13C-NMR spectral analyses. 1H-NMR

and 13C-NMR spectral analyses of CAG

Chemical structure of CAG was confirmed by 1H-NMR spectral analysis, as shown in Figure S2 of SI. In 1H-NMR spectrum of CAG, peak at 3.58 (a) ppm corresponds to the terminal protons of hydroxyl group attracted with the glycerol unit. The peaks at 3.84 ppm (b), 3.97 ppm (c) and 4.66 ppm (d) indicate the presence of   and  protons of glycerol unit with respect to the ester functional group. Simultaneously, peak at 2.72 (e) ppm is attributed to the methylene protons for citric acid unit. In the 1H-NMR spectrum of CAG, peak at 7.46 (f) ppm represents the proton for free carboxylic acid group of citric acid28. Simultaneously,

13C-NMR

spectral analysis was carried out to identify the

chemical structure of CAG, as shown in Figure S2 of SI. Peak for terminal methyl carbon, due to the glycerol unit is appearing at 63.66 ppm (a). The peaks in the position of 66.1 (c) and 69.8 (b) ppm represent to the  and  carbons of glycerol unit with


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respect to the ester functional group. Peak at 39.5 ppm (e) reveals the position for α carbon of citric acid, which is attached with the ester functional group. Simultaneously, the peak at 72.7 ppm (f) corresponds to the peak for  carbon of citric acid unit with respect to the ester functional group. Successively, the peaks at 170.3 ppm (d) and 174.4 ppm (g) are due to the presence of two different carbonyl carbons, one for carbonyl carbon of free carboxylic acid and other for carbonyl carbon of ester functional groups of the product. Synthesis and characterization of polyurethane dispersion All the WBPUs were synthesized by two steps one pot pre-polymerization technique using PCL as the main macroglycol, DAPEG as a biodegradable macroglycol, and CAG as the bio-based chain extender, along with TDI used as an aromatic diisocyanate and IPDI as an aliphatic diisocyanate as shown in Scheme 2. In the first step, per-polymer was obtained by taking the bio-based macroglycol, DAPEG and the synthetic macroglycol PCL using TDI as the aromatic diisocyanate. Finally, the pre-polymer was treated with the chain extender and IPDI at an elevated temperature until the viscous product was formed without gel formation. Here, IPDI helps to progress the reaction by


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stepwise manner with the terminated free hydroxyl group of the chains. This is due to the fact that IPDI contains two different isocyanate groups in two different chemical environments and hence possesses different reactivities. Therefore, it reacts with diol terminated reactant in step by step manner, thus gel formation tendency in the second step is reduced, and desired polyurethane was obtained without gel formation. The water dispersion of three polymers was obtained after TEA treatment only, may due to salt formation and hence increase of polarity. FTIR spectral analysis of WBPU Chemical functional groups of the synthesized polyurethanes are confirmed by FT-IR spectral analysis, as shown in Figure 1(c). In the FT-IR spectra of all three polyurethanes, the absence of isocyanate peak in the region 2250-2270 cm-1 indicates the completion of the urethane reaction. The broad peaks at 3440 cm-1 and 3245 cm1are

assigned for O-H and N-H stretching vibrations of the polymers29. Again, two small

peaks at 1742 cm-1 and 1637 cm-1 correspond to the presence of two types of carbonyl peak in the polymer matrix; one for C=O stretching vibration of the ester and other for C=O stretching vibration of the amide linkages.30 The small peak at 1460 cm-1 is


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assigned for the bending vibration of N-H bond. In FT-IR spectra of all WBPUs, a small broadband in the range of 977 - 1211 cm-1 corresponds to the presence of O-C=O and C-N bonds. The presence of all such bonds confirmed that urethane linkage was successfully formed in the polymer. Simultaneously, peaks at 2925 cm-1 and 2852 cm-1 correspond to the asymmetric and symmetric stretching vibrations of sp3 hybridized C-H bond, while C-H bending vibration was observed at 1400-1460 cm-1. Successively, 1HNMR and

13C-NMR

spectral analyses of all WBPUs were further conducted to identify

the actual structure of the synthesized polyurethanes. 1H-NMR

and 13C-NMR spectral analyses of WBPU

1H-NMR

spectrum of the representative WBPU-1 is shown in Figure 2(a). From the

structure of the polymer, three types of chemical environments are expected for the terminal methyl protons in the polymeric chains. In the 1H-NMR spectrum of WBPU-1, three small peaks at 0.83 (a), 0.88 (b) and 0.94 ppm (c) were observed that correspond to the terminal methyl protons of dimer acid and IPDI moieties. Therefore, the result is consistent with our expected chemical structure of polyurethane (Scheme 2).31 Successively, methylene protons of WBPU-1 were found at 1.25 ppm (d). Peaks at 2.22


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and 1.40 ppm correspond to the α and  protons of the dimer acid moiety with respect to the ester linkage. In the 1H-NMR spectrum of WBPU-1, a small peak observed at 2.00 (i) ppm reveals for the cyclic protons of the dimer acid moiety. Simultaneously, peak at 2.46 ppm (g) is for protons of the terminal methyl group of aromatic TDI unit. Three small peaks at 4.58 (q), 4.41 (p) and 2.85 ppm (k) in 1H-NMR spectrum of WBPU1 are due to the presence of glycerol moiety in the polymer matrix. For the methylene group of PEG generates the peaks at 3.54 (j), while 3.45 ppm (l) indicates the presence of PEG and butane-diol moieties in the polymer chains. The peak obtained at 2.46 ppm (f) corresponds to the methylene proton of the citric acid unit. Simultaneously, peaks at 4.06 ppm (n) and 3.96 ppm (o) are assigned for the presence of methylene protons of PCL moiety. The peaks in the range of 6.99-7.89 ppm (s) are appearing due to the presence of aromatic moiety. The result indicates that TDI effectively incorporated in the polyurethane chains. In 1H-NMR spectrum, the peaks in the range of 8.92-8.67 ppm (t) are generated due to the protons attached with the nitrogen of the aliphatic isocyanate32, while the proton attached with the nitrogen of the aromatic isocyanate generates the peak in the range of 9.57-9.44 ppm (u). 13C-NMR spectrum of WBPU-1 is


25


shown in Figure 2(b). In the

13C-NMR

Page 26 of 55

spectrum of WBPU, two small peaks at 17.57

ppm (a) and 17.92 ppm (b) are assigned for the terminal methyl carbons of dimer acid, IPDI and TDI units. The peaks in the range 28.34-24.61 ppm (c) are due to the methylene carbons of dimer acid unit. Simultaneously, the peak for other carbons of IPDI was obtained at 33.85 ppm (e). From the glycerol moiety in the polymer chain, peaks at 70.28 ppm (j), 68.83 ppm (k) and 70.28 ppm (l) were observed, which correspond to three different chemical environments of carbon atoms that are present in the glycerol moiety. Consecutively, carbons in the aromatic ring of TDI moiety generate peaks in the range of 137.10-137.81 ppm. The peak in the range of 154.79-154.09 ppm (o) obtained is due to the carbonyl carbon in the urethane linkage, while the carbonyl carbon of the ester linkage of DAPEG and CAG generates peak at 173.16 ppm (p).33 The degree of branching (DB) of all the WBPU are calculated from

13C-NMR

spectral

analyses. Here, CAG was taken as the branch generating moiety. Therefore, five small peaks

obtained are due to five carbon atoms with penta, tetra, tri, di, and mono-

substituted units in the CAG moiety. Thus five peaks were obtained for five substituents labelled as T, L, D1, D2, D3 for the terminal, linear and three different dendritic units of


26

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the CAG moiety (Figure S3 of SI). Therefore, the degree of branching was calculated using the following equation by the integration of the peak areas (T, L, D1, D2 and D3). DB = (D1+ D2+ D3 + T)/ (D1+ D2+ D3 + L + T)............. [2] Degree of branching for three WBPUs varies from 0.88 to 0.72, the result was very close to the earlier reported similar type polyurethanes34. XRD analysis of WBPU XRD analysis of all three polyurethanes was conducted to study the crystal structure of the polymers, as shown in Figure 1(d). Two distinct peaks were observed in all three polymers at 2θ~21.3° and 23.7° correspond to d spacing of 0.419 nm and 0.381 nm, respectively. These two peaks are due to the presence of semi-crystalline PCL moieties in the polymer chains and correspond to [110] and [200] planes.35 Successively, the degree of crystallinity in all three polymers was calculated by integrating the peak areas of two distinct sharp peaks. The result reveals that though the amount of PCL is same in three cases but with the increase of branching unit in the polymer matrix, the degree of crystallinity decreases. This is due to the fact that higher dendritic architecture destroys the crystalline arrangement of the polyurethane chains.7 Degree of crystallinity in three


27


Page 28 of 55

polyurethanes, WBPU-1, WBPU-2 and WBPU-3 were found to be 8.8%, 13.4% and 17.4%, respectively. Physical property of WBPU The values of physical properties of the three WBPUs are tabulated in Table 2. The synthesized all WBPUs are soluble in most of the polar solvents like DMSO, DMF and THF but insoluble in nonpolar and halogenated solvents including hexane, xylene and chloroform. The high solubility of WBPU in the polar solvents demonstrated that polymer surface contains lots of polar functionalities such as ester group, free carboxylic acid, hydroxyl group and urethane linkage, which force to the polymer chains to be dispersed in the polar solvent. The images of water dispersibility of different WBPUs (WBPU-1, WBPU-2 and WBPU-3) are provided in Figure S4 of SI. These representative photos show that higher bio-based chain extender containing polyurethane, WBPU-1 has marginal better water dispersibility due to slightly higher polarity than WBPU-3, which contains relatively less amount of bio-based chain extender. But as the difference in the amount of such chain extender is not very high (Table 1), hence the water dispersibility is also not varied too much. Simultaneously, the specific gravity of all three polyurethanes was measured, and the value varies in the range of 1.15 - 1.22, which is very close to the specific gravity of


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earlier reported polyurethanes.35 Among the three polymers, WBPU-1 shows the lowest specific gravity due to the highest branching of the polymer chains and hence possesses the lowest crystallinity. Consecutively, molecular weights of all three WBPUs were determined by GPC study, and the results of weight average and number average molecular weights (g/mol) for WBPU-1, WBPU-2 and WBPU-3 were found to be Mw~ 34041, 30313 and 25666, and Mn~24722, 22230 and 18248 respectively. The polydispersity index (PDI) value was also determined and the values vary in between 1.36-1.406. Therefore, the synthesized all the polyurethanes are expected to exhibit very well defined property. The molecular weights obtained from GPC analysis for the all the three polyurethanes are quite high as linear polystyrene was used as the standard and hence these values are higher in comparison to earlier reported polyurethane dispersion.33 Further, molecular weight is found to be varied with degree of branching and the increasing branching results effective increase in molecular weight of the polymers. The viscosity values of the polyurethane solutions were also relatively high due to the high molecular weight of the polymer chains. Mechanical properties of WBPU


29


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Mechanical properties of all three WBPUs were studies by UTM instrument using a relatively slow rate of external force. This slow rate of external force generates high relaxation time to achieve high elongation. Tensile strength, elongation at break and toughness of the three polyuretrhane strips are calculated from their respective stressstrain profiles, which are obtained from UTM instrument, as shown in Figure 3. The values of tensile strength, elongation at break and toughness for all the polyurethanes are also tabulated in Table 2. From the stress-strain profiles, it is noticed that all the polymers exhibited moderate tensile strength and high elongation at break. The mechanical properties of such polyurethanes highly depends on their molecular mass, amount and composition of soft and hard segments, flexibility and rigidity of the chain molecules, physical or virtual cross-linking, entanglement of chains, orientation of segments, amount of rigid moieties present, hydrogen bonding, polar-polar interactions, van der Waals forces etc.36 Based on these parameters the tensile strength and elongation are compared among the three polyurethanes. From the result, it is cleared that WBPU-1 exhibited the highest elongation at break as compared to the other two polyurethanes. The highest elongation of WBPU-1 reflected the highest flexibility, which


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may be due to the highest molecular weight. Simultaneously, other factors also strongly influence the flexibility of polymer including amorphousness and crystallinity of the soft segment.20 Higher molecular weight generates higher chain entanglement and higher free volume that reflect higher flexibility of the polymer. Simultaneously, the additional secondary force is also helping for the full extension of the polymer chain, thus relatively high elongation at break was observed in all three cases. Successively, the rigidity of the polymer strongly influences by the intermolecular attraction, chain entanglement, crystallinity of the polymer matrix, the presence of aromatic moiety in TDI and fused ring in IPDI moiety.37 The good strength and high elongation result in relatively high toughness. The impact resistance of all WBPUs was also determined and found to be very high (larger than the maximum limit of instrument used) due to the high strength and elongation. Similarly, the scratch hardness of the polyurethanes was also compared with each other. The values of scratch hardness for the three polymers are tabulated in Table 2. From the result, it is observed that with increasing molecular weight, tensile strength and elongation values of scratch hardness increases effectively. While static hardness, measured by durometer is the measurement of rigidity and softness of the


31


Page 32 of 55

polymer. The values obtained from the experiment are incorporated in Table 2. The values of hardness in all three polymers are found almost the same but slightly less value is obtained in case of WBPU-1 due to low crystallinity as compared to the other polymer compositions. Thermal property of WBPU Thermal properties of all WBPUs were studies by TGA and DSC analyses to determine thermal stability and melting temperature, as shown in Figure 4(a). Generally, the thermal stability of polyurethanes depends on several factors including the ratio of hard to soft segment that is present in the polymer matrix, molecular weight of the synthesized polymer, the degree of urethane linkage, the presence of physical crosslinking, interactions in polymer chains, the presence of aromatic moieties etc. Here, two steps thermal degradation patterns for all the synthesized polymers are demonstrated by TGA thermograms and their corresponding DTG curves, which are very close to the previously reported results.6 The first step corresponds to the degradation of less stable thermo-labile moieties like the aliphatic chains of DAPEG and CAG moieties. On the other hand, the second step of degradation was observed due to


32

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the presence of relatively more thermally stable moieties such as urethane linkage, ester linkage, and cyclo-aliphatic and aromatic portions of DAPEG, IPDI and TDI moieties. Two steps of degradation are merged due to the presence of strong intermolecular secondary interactions. From the DTG curves of all WBPUs, it is seen that the thermal degradation starts from the temperature of 263-272 °C but a rapid weight loss was observed in the range of 368-460 °C. The details of the degradation data are tabulated in Table S1 of SI. Successively, DSC analysis was used as a useful analytical tool to determine the melting temperature of the synthesized polyurethanes. DSC analysis of all WBPUs was conducted in the range of -40 °C to 150 °C and compared with each other. Just to show the differences including melting temperatures; DSC curves from 25 to 75 ºC only shown in Figure 4(b). The melting temperature of the soft segment is determined from the DSC curves and the results are tabulated in Table S1 of SI. From the analyses of DSC curves, it is seen that the melting temperature increases with the increase of molecular weight and degree of branching of WBPU. The enthalpy peaks of the polymers may be explained by the combined effect of rigidity and crystallinity of the structure.


33


Page 34 of 55

Biodegradation of WBPU Biodegradation of three different compositions based WBPU films was studied by the exposure of two types of bacteria strains namely pseudomonas aeruginosa as a gram negative bacteria and bacillus subtilis as a gram positive bacteria for a period of four weeks. In this study, the optical density of the bacteria suspension in both the cases was plotted against exposure time, as shown in Figure 5. From the plot, it is observed that optical density steadily increases with the increase of exposure time. This is also supported by the measurement of weight losses of WBPU-1, WBPU-2 and WBPU-3 films after 4 weeks of exposure against P. aeruginosa (a) and B. Subtilis (b) bacterial strains (Figure S5 of SI). The rate of biodegradation of WBPU strip depends on several factors including the presence of bio-degradable part or percentage of bio-content, amount of ester functional groups and the ratio of hydrophobic and hydrophilic part in the polymer matrix.38 Due to different in cell wall structures, P. aeruginosa exhibited effective binding efficiency towards the polyurethane chains and produced higher degradation capability than B. subtilis. Successively, P. aeruginosa possessed higher bio-surfactant activity and cell surface hydrophobicity than B. subtilis, therefore former


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exhibited higher degradation capability compared to the later.39 Bio-degradation mainly occurs during cleavage of the main polymer chain into small molecular fragments. The synthesized polyurethanes contain lots of hydrolyzable ester and urethane linkages, which are mainly responsible for bio-degradation of them. From the bio-degradation experiment, it was observed that WBPU-1 possesses higher bio-content thus it exhibits higher bio-degradation capability compare to the other WBPUs. Similar, result was also achieved against the B. Subtilis bacterial strain. Chemical resistance of WBPU All the polyurethane films exhibited excellent chemical resistance in different chemical environments including acidic medium (pH~4-5), basic medium (pH~8-9), 20% EtOH solution, 10% NaCl solution and tap water. The amounts of weight loss of the films after exposure of one month are tabulated in the Table S2 of SI. From the results of the experiment, it is cleared that weight losses of the polyurethane films are comparable with respect to other reported polyurethanes.31 All the polymer films exhibited good chemical resistance due to the presence of strong hydrogen bonding and van der Waals forces between the polymer chains, which prevent the polymer chains to degrade under


35


Page 36 of 55

the exposed chemical media. From the experiment of chemical resistance, it was observed that all the films showed relatively poor chemical resistance towards alkali solution, this is due to the presence of easily hydrolyzable ester functional group of DAPEG and CAG molecules. Successively, polyurethane linkages are generally hydrolyzed in the presence of a basic solution, which reduces the chemical resistance towards alkali solution. Self-healing property Self-healing ability of the three polyurethane films was determined under microwave irradiation and the time required to complete healing was also determined. Microscopic images of the fracture films with the healed films during microwave irradiation are shown in Figure 6. Simultaneously, healing times required to complete healing were compared for the different polyurethane films. Basically, healing process in the fracture surface involves several steps such as surface rearrangement, surface approach, wetting, diffusion and randomization.40 After exposure of the microwave irradiation, some polar groups that are present in the polymer matrix absorbed the radiation and started to oscillate their dipoles. During this oscillation process, heat is generated and


36

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the temperature reaches to above the glass transition temperature (Tg) of the soft segment of polyurethane, which leads to a rapid Brownian motion of soft segment resulting diffusion of the molecular chain at the same time the hard segment of the polyurethane maintain the structural integrity of the polymer strip.41 This increase of motion of the chain molecules is due to high microwave absorbing capacity of this highly polar polyurethanes. As the healing process achieved by rearrangement and diffusion of the soft segment, thus healing is occurred again and again to heal the fracture surface. The microscopic images of the fracture, intermediate and healed surfaces are also compared. From the images of before and after healing, it is cleared that all the films are completely healed after exposure of microwave irradiation. In addition, it is also cleared that WBPU-1 exhibited the lowest healing time, while WBPU-3 took the highest healing time. The result reveals that with increasing amount of CAG and branching healing efficiency increased effectively due to the presence of high polar functionalities. The healing efficiency for three polymers is calculated after 2nd cycle of healing process and the histogram for healing efficiency are shown in Figure S6 of SI. From the histogram, it can be concluded that healing efficiency is almost the same in both cases.


37


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Here it is pertinent to mention that this is not a plastic flow as the temperature (35 °C) generated during the exposure of 300 W microwave irradiation for the specified time period, is much less than the melting point (>45 °C) of the polyurethanes. This is happened due to the micro-Brownian motion of the molecular chains of the soft segment (above its glass transition temperature) as explained above. It is also found that many other thermoplastic types of polyurethane with low microwave absorbing capacity cannot heal by this process34. UV aging property WBPU films were subjected to UV-aging test to identify weather resistance ability of the synthesized polyurethanes. Actual weather resistance ability is not so much essay to test and it is a time-consuming process, thus artificial weather conditions are adapted to determine the weather resistance property of the polymer films. The slight decrement of the tensile strength and elongation at break values of the films was observed after treatment of UV radiation. This indicates that after exposure of UV- light, some UVactive moieties effectively absorbed the radiation and break the polymer chains, which slightly reduces the strength and elongation of the polyurethane films.42 The retention of


38

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tensile strength and elongation at break values are compared for all three polyurethanes after UV- light exposure, as given in Table S3 of SI. From the results, it is seen that WBPU-1 exhibited the highest UV-resistance capability due to the highest degree of branching and confined geometry of the polymer chains among others.

CONCLUSION From this study, it may be concluded that water dispersion of bio-based thermoplastic polyurethane can be achieved by neutralizing the citric acid-based emulsifier as one of the multifunctional reactants. The structure of such polyurethanes can also be confirmed by FTIR and 1HNMR and

13C-NMR

spectral analyses. Again, high molecular

weight and balance of rigidity and flexibility of the polymer chains executed high mechanical strength, flexibility and impact resistance. The prominent self-healing capability of the synthesized polyurethane is the result of high microwave absorption capacity. Most importantly, high UV-resistance, chemical resistance and biodegradable capability of the synthesized polyurethane are real assets for their practical applications


39


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in their potential fields. Therefore, the studied bio-based polyurethane can offer a sustainable material with environmentally benign attributes.

ASSOCIATED CONTENT Supporting Information. 1H-NMR

and

13C-NMR

spectra of DAPEG and CAG, designation of L, T, D1, D2, D3

peaks for WBPU-1 in NMR spectrum, digital images of water dispersion of polymers, weight losses of WBPU-1, WBPU-2 and WBPU-3 after 4 weeks of biodegradation, healing efficiency histogram up to second cycle, Tables for comparative study for thermal property, weight losses in chemical resistance, retention (%) of mechanical properties of UV-aged WBPUs.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Telephone: +91-3712-267009, FAX: +91-3712267006


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT Authors would like to acknowledge HRDG-CSIR for financial support through project Grant No. 22(0759)/17/EMR-II dated 10th October 2017.

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Figure 1. (a) FT-IR spectra of dimer acid, and DAPEG; (b) FT-IR spectra of citric acid, and CAG; (c) FT-IR spectra and (d) XRD spectra of WBPUs


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Figure 2. (a) 1H-NMR and (b) 13C-NMR spectra of WBPU-1


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Figure 3. Stress-Strain profiles of the synthesized WBPUs

Figure 4. (a) TGA thermograms (inset DTG curves); and (b) DSC curves of all WBPUs


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Figure 5. Bacterial growth curves of P. aeruginosa (a) and B. Subtilis (b) on WBPU films

Figure 6. Optical images of mechanically damaged films of WBPU-1 (a-d) (after exposure of 090 s microwave irradiation); WBPU-2 (e-h) (after exposure of 0-120 s microwave irradiation); WBPU-3 (i-l) (after exposure of 0-150 s microwave irradiation)


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Scheme 1. Synthetic paths for bio-based macroglycol (DAPEG) and chain extender (CAG)


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Scheme 2. Schematic representation of synthetic pathway for polyurethane


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Table 1. Composition of the synthesized WBPUs. Composition

WBPU-1

WBPU-2

WBPU-3

PCL (mM)

2.25

2.25

2.25

DAPEG (mM)

0.75

0.75

0.75

BD (mM)

3.15

4.46

5.22

CAG (mM)

1.65

1.125

0.825

TDI (mM)

6

6

6

IPDI (mM)

4.27

4.27

4.27

Total (g)

8.05

7.96

7.93

Hard segment (wt %)

35.4

34.8

34.3

Soft segment (wt %)

65.7

65.2

65.7

Bio-based content (wt %)

12.4

10.1

8.9

Branching unit (wt %)

7.2

4.7

3.5

-NCO/-OH ratio

1

1

1


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Table 2: Comparative study for physical and mechanical properties of WBPUs

Properties

WBPU-1

WBPU-2

WBPU-3

Tensile strength (MPa)

8.36

7.82

9.26

Elongation at break (%)

678.9

525.3

485.3

Toughness (MJ m-3)

51.49

32.6

Scratch hardness (kg)

3.5

3.0

2.0

Impact resistance (kJ m−1)

8.95

7.64

8.33

Hardness (Shore A)

32.6

47.51

51.49

Specific gravity

1.15

1.22

1.25

Viscosity (dL/g) at 33°C

0.32

0.31

0.29

Gloss value at 60°

15.6

25.4

20.4

39.23


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For Table of content

Synopsis Bio-degradable smart water dispersible polyurethane with self-healing, UV-aging and biodegradability was reported.


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Environmentally friendly polyurethane dispersion derived from dimer

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