Clean synthesis and characterization of green nanostructured polymeric thin films from endogenous Mg (II) ions coordinated methylolated-Cashew nutshell liquid

Journal of Cleaner Production 238 (2019) 117716

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Clean synthesis and characterization of green nanostructured polymeric thin films from endogenous Mg (II) ions coordinated methylolated-Cashew nutshell liquid Fahmina Zafar a, *, Shabnam Khan a, Anujit Ghosal a, b, Mudsser Azam c, d, Eram Sharmin e, Qazi Mohd Rizwanul Haq c, Nahid Nishat a, ** a

Inorganic Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi, 110025, India School of Life Sciences, Beijing Institute of Technology, Beijing, China Microbiology Research Laboratory, Department of Biosciences, Jamia Millia Islamia, New Delhi, 110025, India d Zoonosis Research Center, School of Medicine, Wonkwang University, South Korea e Department of Pharmaceutical Chemistry, College of Pharmacy, Umm Al-Qura University, PO Box 715, 21955, Makkah Al-Mukarramah, Saudi Arabia b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 December 2018 Received in revised form 16 July 2019 Accepted 19 July 2019 Available online 20 July 2019

Efficient utilization of renewable feedstocks has been considered pivotal in sustainable development, controlling global warming, and as an alternative to petrochemical precursors. Here, we have presented a facile one-pot synthesis method for the fabrication of bacterial resistant, free-standing polymeric films and coatings by utilizing agro-waste based precursor material. The conversion of cashew nut shell liquid (CNSL) into a value-added product for this application was achieved through coordination of methylolated-CNSL and Mg (II) ions (Mg(II)CNSL). The resulting metal-organic framework (MOF) was then cured by following a greener approach with zero toxic/residue production through reacting Mg(II) CNSL with aliphatic amine [Mg(II)CNSL-FA]. The formulation mechanism of nanostructured Mg(II)CNSLFA via the ring-opening reaction of benzoxazine, its polymerization with hydroxyl groups in CNSL, and coordination of Mg(II) ions were elucidated by FTIR, ATR, XRD, SEM/TEM, and DSC studies. The performance (flexibility, swelling behaviour, and water contact angle values), morphology, and thermal stability of Mg(II)CNSL-FA films were also compared (virgin ligand, CNSL-FA). The potential application of the film as an antibacterial material has been further tested against various bacterial strains (E.coli, P. aeruginosa, S. aureus, and B. subtilis). Overall, the research work is expected to broaden the utilization of CNSL, an agro-waste via cleaner production of flexible, free-standing, metal ions coordinated antibacterial film with possible application in surface coatings, adsorption, and water purification. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Prof. Jiri Jaromir Klemes Keywords: Agro by-product Cashew nut shell liquid Bio-coordination polymers Thin film TGA Antibacterial

1. Introduction The increasing environmental concerns and restricted availability of petrochemical resources have motivated researchers to find alternative resources for sustainable development. The cleaner production of materials through cost-efficient and clean synthesis strategies without producing any toxic residues, using environmentally benign non-toxic catalysts, and solvents, is still a challenge. Primarily if the product is intended for direct human use

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Zafar), [email protected] (N. Nishat). https://doi.org/10.1016/j.jclepro.2019.117716 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

such as toys, packaging films, containers, and others, thereby, new and facile greener approaches are in demand. Biomaterials with advanced properties can meet the needs of the present-day without compromising the ability of future growth (Li et al., 2019). Among various biomaterials, agro-wastes are considered very attractive as well as profitable for the production of advanced green materials such as reactive precursors, surfactants, fire retardants, biopolymers and coordination polymers. Conventional coordination polymers (CPs) or metal-organic frameworks (MOFs) are generally produced via self-assembly of metal ion nodes with organic ligands (mostly petroleum-based). The porosity, large inner surface area, multiple possible coordination modes, and topologies with versatile architectures make them promising for different applications (Kempahanumakkagari et al., 2018). However, the

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involvement of multi-step synthesis, petro-based precursors, volatile organic solvents, and limited solubility restrict their more extensive application, particularly in the field of biomedical such as drug delivery, intracellular imaging, wound healing, and antimicrobial agents (Li et al., 2018; Horcajada et al., 2008). Therefore, the use of more appropriate organic ligands based on biomaterials can broaden the application of such CPs. CPs synthesized using natural derivatives (Bio-CPs or Bio-MOFs) such as cyclodextrins (Hartlieb et al., 2017), porphyrin (Zhao et al., 2018), amino acids (Imaz et al., 2009), phenolic derivatives (Cooper et al., 2015) as ligands with different metal ions have been reported and can be utilized for biomedical applications (Imaz et al., 2011). However, the significance of Bio-CPs can be further augmented by the use of renewable resource based or, industrial by-products or wastes based non-toxic, biodegradable, biocompatible and costeffective precursors (Khan et al., 2018; Zafar et al., 2016) and endogenous cations such as Ca2þ, Mg2þ, Zn2þ, Cu2þ/þetc. The use of Mg2þ ions, which are in abundance, cheap, lightweight, and non-toxic, with many similarities to the transition metal ions, can play an essential role for antibacterial applications (Biswas et al., 2016). An appropriate facile methodology combining the value-added properties of biomaterials derived from a waste material and metallic ions following most of the rules of green chemistry would be an accomplishment. Cashew nut shell liquid (CNSL) is a model agro by-product, biodegradable, renewable, and natural resource for phenolic lipid. The long hydrocarbon chain (C15) at the meta position of phenolic groups with varying degree of saturation and unsaturation renders it as an interesting reactive precursor. The chemical transformation converting them into tailor-made advanced materials such as epoxies (Atta et al., 2017), polyesters (Mwangi and Mbugua, 2013), phenalkamines (Wazarkar et al., 2017), polyols (Shrestha et al., 2018), benzoxazines (Amarnath et al., 2018), Schiff base (Isac Sobana Raj et al., 2011), novolac (Khan et al., 2018), gels (Vemula and John, 2008), metalloporphyrins (Prakash Rao et al., 2019), coordination polymers (Khan et al., 2016), and others have been reported with practical applications (Balachandran et al., 2013). Cardanol has been previously used for environmentally benign synthesis of flame retardants (Jia et al., 2019), PVC plasticizers (Greco et al., 2017), free-standing films (Khan et al., 2018) and others (Zheng et al., 2018). Therefore, to explore the application of bio-based CPs as antibacterial agents, CNSL and endogenous Mg2þ cation have been used as constitutive building blocks. Further, no literature has reportedly used the naturally occurring phenolic lipids (CNSL) based CPs. These materials are biologically and environmentally compatible, nanostructured, porous, less/non-toxic, and biologically active as antibacterial agents. Considering its abundant availability and worldwide production (Zafar et al., 2016), the utilization of CSNL would scale up the use of agro-waste at bulk. It would inevitably affect the present world today for a better, cleaner, and sustainable tomorrow. The research work outlined in this paper is a state-of-the-art approach with intentions to develop a new area of research in amorphous and porous Bio-CPs. The work is quite different from the research of other scientific groups on the development and functional studies of petro-based CPs and nano CPs. The present work describes cleaner production of nanostructured bio-CPs [Mg(II)CNSL-FA] containing endogenous divalent cation (Mg2þ) and phenolic lipid moieties from technical CNSL for antibacterial application. Freestanding Mg(II)CNSL-FA films/coatings were prepared via methylolation reaction of technical CNSL (Col, 86%) with formaldehyde (F) in presence of citric acid (CA) to form CNSL-F followed by metallation [Mg (II) acetate] and amination using aliphatic amine (AA) {hexamethylene tetramine (HMTA),10% optimum percentage}.

2. Experimental section 2.1. Synthesis of CNSL-F CNSL-F has been prepared by following a reported method with some modifications (Khan et al., 2016). The mole ratio of CNSL and F was maintained to be 1:0.7 with CA (1% based on CNSL) as a catalyst. Briefly, 1 mol of CNSL and half of the amount of alcoholic catalyst solution (1%) were taken in a 250 mL three-necked conical flask fitted with a condenser, thermometer, and mechanical stirrer at 100 ± 5  C. A mixture of 0.7 mol of F and the remaining half amount of the alcoholic catalyst solution was added to the reaction mixture maintained at 120 ± 5  C, in a dropwise manner using a burette, in 1 h. The pH of the reaction mixture was 6.0e6.5 after complete addition. The progress of the reaction was monitored periodically with the help of thin layer chromatography (TLC), pH, and finally confirmed by FTIR. The reaction was stopped and cooled at room temperature when pH dropped to 4. The content of the flask was transferred to a sample bottle for further analysis. CNSL-F produced dark brown viscous liquid (Yield ¼ 80.54%).

2.2. Synthesis of Mg(II)CNSL-FA Mg(II)CNSL-FA was synthesized by mixing CNSL-F and Mg(II)Ac in 2:1 ratio with 10 wt % AA (concerning CNSL-F) in triplicates. The synthesis was carried out via in-situ two-step reaction: (i) In a 250 mL three-necked conical flask fitted with air condenser and a mechanical stirrer, CNSL-F was stirred continuously until the temperature reached to 80 ± 5  C. An alcoholic solution of Mg(II)Ac was added dropwise using a burette within 15e20 min. After complete addition of Mg(II)Ac, the temperature of the reaction mixture was raised and set at 100 ± 5  C for 30 min with continuous stirring. (ii) After the stipulated time, the temperature of the reaction was lowered to 80 ± 5  C, and methanolic solution of HMTA (10% optimum amount) was added dropwise to it for 15 min by using burette with continuous stirring. After complete addition of HMTA temperature was again raised and maintained at 100 ± 5  C. The reaction was periodically monitored by TLC and FTIR (Yield ¼ 90 ± 5%). The synthesis of CNSL-FA was carried out by adopting the aforementioned similar method, excluding the addition of Mg(II) Ac. The reaction was monitored periodically by TLC and FTIR (Yield ¼ 89.5 ± 5%).

2.3. Preparation of free-standing thin film of Mg(II)CNSL-FA Free-standing film-of Mg(II)CNSL-FA was prepared by thermal polymerization process as described in our previously reported paper (Khan et al., 2018) by the following steps: (i) preparation of solutions with optimum viscosity: 70% (w/v) solution of Mg(II) CNSL-FA in xylene, (ii) pouring the solution over Teflon sheets, (iii) keeping it undisturbed at ambient temperature for 2 h followed by placing it in air oven at 80  C for additional 2 h to remove the residual solvent, (v) and subsequent curing of film in an oven in ambient atmosphere (air) at different temperatures 100  Ce180  C for 10 h at interval of 2 h. After cooling it to room temperature, the reddish-brown coloured transparent film was obtained. Finally, the film was cut into appropriate dimensions (size 0.5 cm  0.5 cm) for thermal stability analysis in an air oven, (1 cm  1 cm), swelling behaviour and (0.25 cm  0.25 cm) for antibacterial testing. CNSL-FA free-standing film was also prepared, as a reference, by adopting the method as mentioned above.

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2.4. Coating preparation Coatings of CNSL-FA and Mg(II)CNSL-FA were prepared on commercially available carbon steel strips (CS) (composition 2.87 wt % C and 97.13 wt % Fe). Before material application, CS (standard size 70 mm  25 mm  1 mm) strips were successively polished with SiC paper of fine grades (120e150 G), thoroughly washed with double distilled water, degreased with methanol and acetone and kept for drying at ambient temperature. 70% w/v solutions of CNSL-FA and Mg(II)CNSL-FA were applied on prepared CS by brush and kept undisturbed for drying at ambient temperature for 2 h, followed by curing in air oven from 80  C to 180  C for definite time period (same as discussed in preparation of freestanding film).

2.5. Characterization Standard laboratory method of TLC was used to monitor the progress of the reaction. Viscosity measurements were done on a digital viscometer instrument to determine the viscosity of the synthesized products. IR Affinity-1 CE spectrometer (Shimadzu corporation analytical and measuring instrument division, Kyoto, Japan) was used to record FTIR spectra in the mid operating range (4000-500 cm1). To check acidic and basic condition of the system, pH meter (digital) (Hanna instrument, range 0.0e14.0 and accuracy 0.1pH@20oC) was used. The tested samples were applied between Zinc Selenide window with a 0.05 mm thick Teflon spacer, and all the spectra were recorded averaging 40 scans with data spacing of 4 cm1. FTIR spectral data acquisition was achieved by using IR solution software. Attenuated Total Reflectance, ATR, TENSOR 37 spectrophotometer, Germany, was used to obtain ATR spectra of the cured film (mid operating range: 4000-600 cm1). The tested film was placed onto Universal Diamond ATR top plate, and data acquisition was carried out with the use of Opus-Spectroscopic software. The elcometer instrument, model 345 NT, Manchester, UK, was used to measure the thickness of the film. 1/8 inch conical mandrel bend test was used to determine the flexibility of the film. X-ray diffractometer (Ultima IV model, Rigaku cooperation, Japan) fitted with Cu K-alpha radiation (k ¼ 1.5406) was used to record wide-angle powder XRD patterns of materials with an angle range 2 q equal to 10 to 60 and a scan rate of 1 /min. Optical microscope, Leitz, Wetzlar, Germany, was used to study the surface morphology of materials. SEM and EDX, model FEI Quanta 200F-Oxford EDS system IE 250 X Max 80, of the sample was done after gold coating to analyze the morphology of the sample and its surface composition. TEM was performed with TECNAI 200 kV TEM, FEI, Electron Optics, equipped with digital imaging, and 35 mm photography system (All India Institute of Medical Sciences, New Delhi, India) to find out the size and shape of material. Contact angles were measured by Drop snake 2.1, Contact angle measurement, using Image J software (1.46 version) with the greyscale image of water drop of 1 mL volume on the surface of CNSL-FA/Mg(II)CNSL-FA film. Thermal stability of the films (size 0.5 cm  0.5 cm) of CNSL-FA and Mg(II)CNSL-FA was initially checked at laboratory in an air oven. The films were taken, and their initial weights were noted. These films were kept in the oven at an initial temperature of 180  C to final temperature 300  C for 20 minutes at each temperature. After each successive increment in temperature, the final weight of the film was taken after cooling it to room temperature with the use of desiccators followed with calculation of % weight loss. A graph between % weight loss and the temperature was plotted to determine thermal stability (in the air) of synthesized films. The % weight loss in determining thermal stability can be calculated as follows:

Weight loss% ¼

3

Final weight  Initial weight  100 Initial weight

TGA and DSC also analyzed the thermal behaviour of materials under N2 atmosphere at a heating rate of 10  C/min (Mettler Toledo AG, Analytical CH-8603, Schwerzenbach, Switzerland). Integral procedural decomposition temperature (IPDT) of the cured materials was calculated from TGA thermogram by reported method (Doyle, 1961). This method correlates to the volatile parts of the polymeric material and is used to access the inherent thermal stability of polymeric materials. IPDT accounts for the whole shape of the curve in a single number by measuring the area under the curve and calculated by using the following formula IPDT (oC) ¼ A* K*(Tf -Ti) þ Ti. Where, A*{equal to (S1 þ S2)/(S1 þ S2 þ S3)} is the area ratio of the total experimental curve defined by the total TGA thermogram, K* equal to (S1 þ S2)/S1, Ti - the initial experimental temperature and Tf - the final experimental temperature (Table S3) (Vyazovkin and Sbirrazzuoli, 2006). 2.6. Swelling behaviour To measure the water absorption or swelling of metal incorporated polymeric films, samples of 1 cm  1 cm in dimension with known weight were dipped in water for 24 h at room temperature. These samples were kept in a vacuum desiccator for 24 h before immersion. The wet sample was gently wiped and dried with the aid of air-dryer and reweighed. The water absorption (%) was calculated according to the equation:

%Water adsorption ¼

Wafter  Wdry  100 Wdry

Where Wafter is the weight of the sample after dipping in water and Wdry is the dry weight of the sample before dipping in water. 2.7. Antibacterial activity Antibacterial activity of CNSL, CNSL-F, CNSL-FA and Mg(II)CNSLFA materials along with free-standing films of latter two resins were tested against gram-negative bacteria [Escherichia coli, MTCC 443 and Pseudomonas aeruginosa MTCC 2453] and gram-positive bacteria [Staphylococcus aureus, MTCC 902 and Bacillus subtilis MTCC 736] with respect to ampicillin as standard drug. The bacterial cultures were procured from Microbial Type Culture Collection, IMTECH Chandigarh, India. The antibacterial activity of CNSL, CNSL-F, CNSL-FA, and Mg(II) CNSL-FA materials was carried out by using Kirby Bauer disk diffusion method (Fahmina Zafar et al., 2016). In brief, MuellerHinton agar (HiMedia India) plates were inoculated by 50 ml of 0.5 McFarland unit grown test organism to get a uniform lawn of culture. Circular wells of 6 mm diameter were made using sterile borer at an appropriate distance. Stock solutions were prepared at a concentration of 500 mg/mL for CNSL and CNSL-F and a concentration of 86.95 mg/mL for CNSL-FA and Mg(II)CNSL-FA. Tests were performed for three different dilutions as A, B, and C (Table S4). The wells in agar plates were loaded with 50 mL of each dilution for CNSL, CNSL-F, CNSL-FA, and CNSL-FA-Mg(II). The parafilm sealed plates were incubated at 37  C for 14e18 h. After incubation, the zone of growth inhibition (mm), an indication of the antibacterial activity of test samples, was measured. The antimicrobial activity of developed CNSL-FA and Mg(II) CNSL-FA films, growth inhibition studies were performed against the aforementioned bacterial strains (Khan et al., 2018). The films were cut into uniform square pieces of 0.25 cm  0.25 cm size and

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sterilized by autoclaving. Sterilized Luria broth (LB) (200 mL) was dispensed into each well of 96-well microtiter plates (ThermoFisher Scientific IL), and one piece of film was placed in each well under aseptic condition. Secondary culture of the bacterial isolate was diluted to the concentration of 106 cells per mL. Each well containing LB and film was inoculated with 10 mL of diluted culture. Well containing LB without culture was used as negative control and well containing LB and culture, but no film was used as a positive control. All reactions were performed in triplicate. The 96 well-microtiter plates were sealed using a parafilm and incubated at 37  C for 14e18 h. After incubation, film pieces were removed from the well, and optical density (OD) was measured at 600 nm to determine the bacterial growth in each well. The percentage (%) inhibition was calculated using the formula:

(Anastas and Eghbali, 2010; Hens et al., 2018).

Percent (%) inhibition ¼ [(OD control e OD sample)/ OD control] x 100

3.3. Spectral analysis

3. Results and discussion In general, the commercial feasibility of synthesis processes often remains inefficient due to expensive raw materials, final product being insufficiently pure, use of costly toxic catalysts or solvents, and overall cumbersome, time-consuming process with uncertain degradation fate after service. Utilizing bio-resources such as CNSL rather than fossil-derived feedstocks may overcome the persisting environmental and health concerns. Therefore, the significance of this research work utilizing CNSL can be well realized considering the following viewpoints: (a) potential of bioresource as well as an agricultural by-product with costeffectiveness, (b) citric acid as a non-toxic metabolic catalyst (FDA approved), (c) overall cleaner production of CPs by simple, singlestep, solvent-free, in-situ approach without the generation of any side products or toxic residues, and (d) finally, complying with the principles of “Green Chemistry” (Tang et al., 2008). 3.1. Synthesis The synthesis was achieved by a single-pot condensation reaction (120  C) in between CNSL and F, which introduced methylol group (-CH2OH) at ortho/para positions in aromatic ring during the formation of CNSL-F followed by formation of benzoxazine intermediate (CNSL-FA) via amination reaction (between phenolic OH and HMTA) at 100  C (Scheme 1a). During the formation of Mg(II) CNSL-FA, Mg(II)Ac was first to get reacted to attach with methylol groups at ortho position followed by reaction with HMTA at 100  C (Scheme 1b). This reaction mechanism of formation was predicted due to observed “O-Mg-O” formation (FTIR) instead of “ArCH2-OCH2Ar” linkages via a condensation reaction between hydroxyl (-CH2OH) and acetate [Mg(II)Ac] groups with the liberation of acetic acid as a by-product. The addition of -CH2OH at ortho position of aromatic ring, formation of benzoxazine intermediate along with the introduction of O-Mg-O linkages in Mg(II)CNSL-FA was confirmed by comparative FTIR analysis of CNSL, CNSL-F, CNSL-FA, and Mg(II)CNSL-FA. The overall synthesis complies with cleaner production strategies, utilizing bio-based agro-by-product or waste (Principle 5, 7, and 12) with no side products/generation of toxic wastes. The facile, effective, and environmentally benign chemical approach of “Green Chemistry” (Principle 3) without any side derivatives (Principle 8) and use of safe catalyst from human health perspectives (Principle 9) further promotes this work. The utilization of “Green Chemistry” principles paves way for cleaner production of advanced materials

3.2. Physical characterization The preliminary confirmation of reaction progress was identified by TLC. It was carried out on silica gel plates as stationary phase and petroleum: diethyl ether (7:3 ratio) was used as mobile phase for the same. Rf values were measured and tabulated in Table S1. Rf values of CNSL-F, CNSL-FA, and Mg(II)CNSL-FA were different with successively lower values compared to CNSL. The observation can be correlated to the relative formation of CNSL-FA and Mg(II)CNSLFA. The higher degree of condensation was measured by digital viscometer, indicating the increase in viscosity of CNSL-FA and Mg(II)CNSL-FA compared to CNSL.

The structure of the final material [Mg(II)CNSL-FA] was confirmed by comparing the FTIR spectrum of CNSL and CNSL-F along with the comparison with CNSL-FA spectrum. The spectral bands (cm1) observed in CNSL, CNSL-F, CNSL-FA, and Mg(II)CNSLFA were attributed to the following functional groups: CNSL (Fig. 1a): 3354 (eOH y, intermolecular hydrogen-bonded), 3078 (Ar C¼C-H y), 3010 (C¼C-H y), 2927(asymm CH2/CH3 y) and 2854 (symm CH2/CH3 y), 1612, 1598 (ArC¼C y, merged with C¼C y), 1589 (C¼C y), 1456 (C-H dof CH2/CH3), 1346 (O-H in-plane d), 1267 and 1155 (phenolic C-O y), 1074 (Phenolic/tert. C-OH, y), 1048 (very weak Ar¼C-H bending, in-plane and out of plane d), 998 (ArC¼C), 912 and 875 (¼C-H d monosubstituted alkene), 779 and 695 (Ar¼CH deformation of 3 adjacent hydrogen atoms and C-H out of plane d). CNSL-F (Fig. 1b): 3327 (eOH y, intermolecular hydrogenbonded), 3078 (Ar C¼C-H y), 3010 (C¼C-H y), 2927 (asymm CH2/ CH3 y) and 2854 (symm CH2/CH3 y), 1612, 1598 (Ar C¼C y, merged with C¼C y), 1589 (C¼C y), 1456 (C-H d of CH2/CH3), 1346 (O-H inplane d), 1267 and 1155 (phenolic C-O y), 1074 (Phenolic/tert. C-OH, y), 998 (Ar C¼C), 911 and 874 (¼C-H d monosubstituted alkene), 780 and 695 (Ar¼C-H deformation of 3 adjacent hydrogen atoms), 1090 cm-1 (C-O y of CH2OH). CNSL-FA (Fig. 1c): 3077.66 (Ar C¼C-H y), 3009 (C¼C-H y), 2925.16 (asym CH2/CH3 y) and 2852.65 (symm CH2/CH3 y), 1614e1632 (Ar C¼C y, merged with C¼C y), 1582 (C¼C y), 1458.39 (C-H d of CH2/CH3), 1367 (O-H in-plane d), 911 and 868 (¼C-H d monosubstituted alkene), 1350e1374 (C-N ʋ), 12401248 cm1(asymm. C-O-C ʋ in aryl alkyl ether), 1191, 1165 (C-N-C ʋ), 1110 cm1 (asymm. C-O-C ʋ in dialkyl ether), 1003 (symm. C-O-C ʋ), 969, 978 (N-CH2-O or N-C-O ʋ). Mg(II)CNSL-FA (Fig. 1d): 3076.78 (Ar C¼C-H y), 3009.24 (C¼C-H y), 2926.71 (asym CH2/CH3 y) and 2853.72 (symm CH2/CH3 y), 1602 (Ar C¼C y, merged with C¼C y), 1587 (C¼C y), 1454.13 (C-H d of CH2/CH3), 1367 (O-H in-plane d), 911.38 and 873 (¼C-H d monosubstituted alkene), 1461.47 (Tetra substituted ring), 1350e1374 (C-N ʋ), 1238.22 cm1 (asymm. C-O-C ʋ in aryl alkyl ether), 1155 (C-N-C ʋ), 1003e1015 (symm. C-O-Mg ʋ), 955- 941 (N-CH2-O or N-C-O ʋ) and 510e622(O-Mg-O ʋ). These spectral results revealed that the spectrum of CNSL-F was identical to the spectrum of CNSL to some extent. The peak at 3354 cm1 in CNSL spectrum confirmed the presence of eOH str. which gets broadened with slight shift to 3327 cm1 in CNSL-F and gets further consumed in CNSL-FA and Mg(II)CNSL-FA. It is related to the fact that the latter two reactions occurred via consumption of -OH groups. New peaks appeared at 1090 cm1 (C-O y of CH2OH), 780 cm1 and 695 cm1 (Ar¼C-H deformation of three adjacent hydrogen atoms) in CNSL-F corresponding to the ortho substitution of eCH2OH group at the aromatic ring. However, the peak corresponding to CH2OH gets disappeared in the spectra of CNSL-FA and Mg(II)CNSL-FA. Besides, the presence of the characteristic peaks of benzoxazine rings such as C-N ʋ, asymm. C-O-C ʋ in aryl alkyl ether,

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Scheme 1. Synthesis of CNSL-FA (a) and Mg(II)CNSL-FA (b) resins.

Fig. 1. FTIR spectra of CNSL (a), CNSL-F (b), CNSL-FA(c) and Mg(II)CNSL-FA (d).

C-N-C ʋ, asymm. C-O-C ʋ in dialkyl ether, symm. C-O-C ʋ, N-CH2-O or N-C-O ʋ correspond to the involvement of benzoxazine intermediate with the reaction of HMTA in both cases. Closer examination of the spectrum of Mg(II)CNSL-FA revealed some shifting of benzoxazine ring bands along with the appearance of two new bands at 510 cm1e 622 cm1 as compared to the spectrum of CNSL-FA. The shifting and appearance of new bands can be attributed to the formation of Mg-O linkage along with the electrostatic interactions of Mg(II) with donor atoms of the polymeric chain.

3.4. Film curing Curing or film formation of CNSL-FA and Mg(II)CNSL-FA was carried out in successive steps at different time intervals and temperature ranging from 100  C to 180  C. The films when cured directly at 180  C developed pores and cracks on the surface. CNSLFA and Mg(II)CNSL-FA films produced were pore and crack-free, thin (thickness 100e120 mm), and flexible (passed 1/8 inch bend

test). Bend test of the prepared films revealed their flexible nature, which could be related to the presence of long alkyl chains in the precursor. The tentative two-step curing scheme has been shown in Fig. 2 (Khan et al., 2018). The first step, a physical process (at room temperature and 80  C for 30 min in the oven), involved solvent evaporation leading to chain entanglement. The second step occurred at a higher temperature, involving crosslinking between polymeric chains within the available functional groups. The latter was a thermally activated chemical process. The said mechanism was confirmed by comparing ATR-FTIR spectra of films (Fig. 3) with respective FTIR spectrum of the uncured sample. The reappearance of phenolic eOH stretching at 3360-3364 cm1 along with decrease in C-N stretching peak value can be related to the presence of N-alkylated moieties in the cured thin films. Closer examination of spectra revealed the disappearance of characteristic peaks of benzoxazine and tetra-substituted benzene ring. These results supported the opening of benzoxazine ring at higher temperature leading to crosslinking and formation of free-standing thin films. 3.5. Physico-mechanical performance The physicomechanical tests of CNSL-FA and Mg(II)CNSL-FA coatings were performed and tabulated in Table S2. Scratch hardness values increased from CNSL-FA to Mg(II)CNSL-FA coatings since nanorods of hybrid Mg(II)CNSL-FA interact electrostatically with the polymeric matrix and fill the empty space of the matrix as also observed in TEM images (Fig. 5). Also, homogeneously dispersed nanorods of hybrid Mg(II)CNSL-FA act as rigid obstacles, restricting the movement of the tip of scratch tester, consequently enhancing the scratch hardness (Harb et al., 2016). Similarly, the increase in gloss value from CNSL-FA to Mg(II)CNSL-FA can be attributed to the increase in crosslinking density with Mg(II), which results in increase in reflection of light by Mg(II)CNSL-FA coating surface. The adhesion with coatings was confirmed by cross-hatch adhesion test, as after performing the test not a single square got peeled off. All coatings passed the impact resistance (150 lb/inch) and bend test (1/8 inch conical Mandrel). The impact resistance of coatings induces crack healing properties through controlled chain

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Fig. 2. Curing of CNSL-FA and Mg(II)CNSL-FA resins.

flexibility. The bend test showed good flexibility of coatings (no cracks/rupture occurred on the surface after the test). The bending ability or flexibility of coatings is due to the long alkyl chain of CNSL moiety, present within polymeric backbone that acts as an internal plasticizer (De and Karak, 2013). The physicomechanical analysis exhibited improvement in performance as compared to those of the reported work (Khan et al., 2018). The incorporation of Mg(II) in polymer leads to an increase in scratch hardness and gloss values for Mg(II)CNSL-FA coatings. The enhanced coating performance of Mg(II)CNSL-FA can be attributed to the presence of flexible long alkyl chains of CNSL having strong covalent linkages along with electrostatic interactions with nanorods of hybrid Mg(II)CNSL-FA and the polymeric matrix through donor or polar groups. 3.6. DSC The coordination with Mg(II) ions was also identified by DSC thermogram of CNSL-FA and Mg(II)CNSL-FA (Fig. 4). A single glass transition temperature (Tg) at 48  C compared to two Tg (48  C and 72  C) and melting temperature (Tm) at 150  C in Mg(II)CNSL-FA indicated the formation of a new hybrid material in the latter.

The broadening of endotherm ranging from 80 to 200  C can be related to the molecular rearrangement along with the evaporation of entrapped solvent/moisture molecules (Ahmad et al., 2012; Khan et al., 2018). The presence of Tg along with Tm correlates to the semi-crystalline nature of Mg(II)CNSL-FA, which was also supported by XRD analysis. The variation in number of exotherms, two in case of CNSL-FA {temperature range 180e255  C (peak maxima at 210  C) and 255e400  C (peak maxima 345  C)} and three exotherms in case of Mg(II)CNSL-FA {200e248  C (peak maxima 221  C), 248e365  C (peak maxima at 340  C), and 365e381  C (peak maxima at 374  C)} also concluded the formulation of a coordinated product. Last two exotherms of Mg(II)CNSL-FA (from 248 to 381  C) were observed in temperature range of single peak of CNSL-FA at 255e400  C. The first exotherm peak in both cases may be due to oligomerization reaction leading to two different types of dimers formation, through the unsaturation of side chain (both from internal monoene, diene, and vinylic bond or triene) (Rodrigues et al., 2006). The second exotherm peak in both cases could be correlated to the thermally activated ring-opening polymerization of oxazine rings followed by crosslinking of polymeric chains by removal of ammonia and formaldehyde (Li and Yan,

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sharp peak. It shows a broad peak or large hump at 2q ¼ 20 , which can be attributed to the inclusion of carbonized phase in organic frame and amorphous nature of the matrix. However, in case of Mg(II)CNSL-FA, along with the broad peak at 2q ¼ 20 , additional diffractions at 2q ¼ 21.3 , 25.9 , 27.3 , and 69.7 confirmed degree of crystalline nature. All these diffractions can be indexed with XRD diffractions for Mg-O4 (JCPDS: 27e0759) with rhombohedral lattice planes [003], [113], [300], and [443], respectively. The initial peak at 2q ¼ 11.5 has also been indexed with Mg-O linkage of magnesium hydroxide methoxide or isopropoxide (JCPDS: 22e1788, or 31e1769). Thus, we can predict the formation of a hexa-dented complex, with Mg at the centre of organic ligands (Scheme 1). The same has also been schematically represented in our proposed reaction pattern. Hence, the formation of semi-crystalline metalorganic frame (CNSL-FA) with Mg(II) at centre surrounded by oxygen groups of organic moieties, Mg(II)CNSL-FA, can be predicted by XRD analysis of the material. The semi-crystalline nature and formation of O-Mg-O linkages were also well supported by DSC as well as FTIR analysis, respectively. 3.8. Optical micrograph Fig. 3. ATR spectra of CNSL-FA (a) and Mg(II)CNSL-FA (b).

The given optical micrographs (Fig. S2 a and b) revealed distinctly that Mg(II) are arranged in a definite pattern within the matrix as compared to CNSL-FA. Mg(II) containing film showed a fixed pattern, which repeated itself over the whole film. From Fig. S2b, it can also be seen that every metal has four neighbouring metal ions separated by polymeric spacings. 3.9. TEM

Fig. 4. DSC thermogram of CNSL-FA and Mg(II)CNSL-FA.

2015). TGA results (Fig. 7) also supported the same information, as around 5e10% weight loss was observed at this temperature range of 180e400  C. Moreover, no melting temperature was observed which highlighted the amorphous behaviour of CNSL-FA. Mg(II) CNSL-FA showed a higher starting or ring-opening polymerization temperature (222  C) and narrow exothermic peak as compared to CNSL-FA (210  C). The presence of Mg(II) within the framework increased electrostatic interactions leading to increased rigidity and decreased the mobility of molecules during polymerization reaction. 3.7. XRD (X-ray diffraction) analysis XRD of CNSL-FA and Mg(II)CNSL-FA have been shown in Fig. S1. It reveals the inclusion of Mg (II) ions in organic matrix (CNSL-FA) bringing some crystallinity or directionality to the resulting (Mg(II) CNSL-FA) organic-inorganic hybrid (as indicated by the melting temperature in DSC). XRD pattern of CNSL-FA does not show any

Transmission electron microscopy of Mg(II)CNSL-FA highlighted the utility of CNSL-FA as matrix or organic framework which acts as template material for the formulation of CP. The three-dimensional (3D) array of Mg(II)CNSL-FA appeared through stacking or interlacing of one dimensional (1D) nanorods of Mg(II)CNSL-FA within CNSL-FA. The difference in colour, CNSL-FA, organic layer (light grey layer) and Mg(II)CNSL-FA, organic-inorganic hybrid, crystalline CP (dark black nanospindles and nanospheres), is evident in TEM images (Fig. 5). Fig. 5(A) indicated the initiation of nanorod formation, i.e. nucleation of nano spherical Mg(II)CNSL-FA in CNSL-FA matrix to acquire nanorod/spindle shape CPs. The literature revealed that a particular concentration of metallic ions and organic template/reducing layer would result in formation of elongated 1D hybrid nanostructures. Fig. 5(B) showed magnified view of synthesized nanorods of size ca. 24.7 nm  3.4 nm, having an aspect ratio of 14.8 engulfed inside polymeric CNSL-FA. Fig. 5 (C and D) presented the interlacing of polymeric hybrid Mg(II)CNSL-FA at low and high magnifications, respectively. On an average, at a distance of ca. 50 nm, circular knots with high-density inorganic content (metallic nodes) are considered to be primary nucleation centres of interconnected nanochannels. These channels acted as template for growth of 1D nanorods of Mg(II)CNSL-FA. However, the SAED pattern and fringes of nanorod cannot be acquired due to polymeric engulfment of the system. 3.10. FE SEM SEM micrograph (Fig. 6 A and B, at low and high magnifications) of freestanding film of CNSL-FA and Mg(II)CNSL-FA helped to analyze the 2D surface along with morphological changes within. Higher magnification images revealed surface channels in CNSL-FA, which could be nanovoids acting as hosts for incoming Mg(II) ions. Fig. 6C showed white lining throughout the surface of Mg(II)CNSLFA, which may have resulted from filling of nanovoids present in

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Fig. 5. TEM of Mg(II)CNSL-FA at 200 kV at 100 nm with magnifications X29000 (A) 50 nm with X43000 magnification (B), 100 nm with X19500 (C) and 50 nm with X43000 (D).

CNSL-FA. A closer look at the circular nucleus or joints showed a slightly elevated surface with small frills over it (Fig. 6D and E). It is interesting to note a uniform distance between these elevated surfaces similar to what was observed in case of predicted initial nucleation points (Fig. 5 C and D). However, an exact correlation is not possible. Fig. 6 (F and G) also revealed rod-like structure of Mg(II)CNSL-FA encapsulated inside polymeric covering, where Mg(II)CNSL-FA became the base over which polymeric branches extruded throughout its length. Thus, the dispersion of nanorods and formation of CP imparted the ability to form a free-standing film. The surface roughness of film was evident throughout the SEM micrographs (Fig. 6G). Along with variation in colour, specific surface morphologies were observed in the film. SEM-EDX analysis (Fig. S3) of films confirmed the presence of elements, i.e. inorganic (Mg) and organic (C and O) in Mg(II)CNSL-FA. 3.11. Thermal stability of films The results obtained through thermal stability (in air) study (Fig. S4) indicated that 5 wt % weight loss was observed at 240  C and 280  C, respectively, for CNSL-FA and Mg(II)CNSL-FA. This weight loss correlated to the trapped solvent and water of the film. At 300  C, wt loss of 8.63% in CNSL-FA and 5.70%, in Mg(II)CNSL-FA was observed. It revealed that Mg(II)CNSL-FA film had better thermal stability than CNSL-FA film in an air atmosphere. Fig. 7 showed TGA curves of CNSL-FA and Mg(II)CNSL-FA in nitrogen atmosphere. The TGA curve presented a two-step degradation process for both films. The 1st step of degradation was observed at 90e320  C (slight degradation) while the second degradation with a steep weight loss was observed in the temperature range of 320e520  C. 1st step of degradation was correlated to the opening of benzoxazine rings and polymerization process followed by crosslinking of polymeric chain with evaporation of ammonia as by-product (Khan et al., 2018). 2nd step of degradation, similar in both systems, corroborated to the scission of methylene linkages in long alkyl chain. The steep decline in weight at second stage

(5e10 wt %) was observed at 360e400  C and 270e370  C in case of CNSL-FA and Mg(II)CNSL-FA, respectively. This loss was proportional to the length of an alkyl chain: the longer the length of an alkyl chain, the steeper decline in weight loss (Khan et al., 2018). IPDT of CNSL-FA and Mg(II)CNSL-FA was estimated (Table S3). IPDT accounts for the whole shape of the TGA curve in a single number, which is calculated by measuring the area under the curve. It has been correlated to the volatile parts of the materials and used for estimation of inherent thermal stability of the materials (Laxmi et al., 2018). IPDTs of CNSL-FA and Mg(II)CNSL-FA were estimated to be 586.27  C and 690.41  C, respectively. These results revealed that the inherent thermal stability of Mg(II)CNSL-FA is much higher than CNSL-FA. The higher stability of Mg(II)CNSL-FA can be related to the presence of Mg(II) metal ions in CPs matrix, which resulted in a crosslinked hybrid structure of metal ions and CNSL-FA chains. Hence, we can say that Mg(II)CNSL-FA film can be used for general application purpose up to a temperature of 300  C without any significant loss in its properties. The limited oxygen index (LOI) values of CNSL-FA and Mg(II) CNSL-FA were also calculated from char yield obtained in TGA analysis (Fig. 7) using the equation proposed by Van Krevelan and Hofytzer [LOI ¼ 17.5 þ 0.4 (char yield)] (Van Krevelen et al., 2009). LOI value indicated flammability of polymers. Minimum LOI of a flame retardant material is 20.9% that is related to the percentage of oxygen in air. On the basis of LOI, polymers can be categorized as (i) easily burning polymers, LOI < 20.9%, (ii) slow-burning polymers, LOI ¼ 21e28%, (iii) self-extinguishing polymers, LOI ¼ 28e100%, and (iv) non-flammable polymers, LOI > 100 (Amarnath et al., 2018). LOI values were calculated as 21.5 and 24.7 for CNSL-FA and Mg(II)CNSL-FA, respectively. Higher LOI value in latter case can be related to the role of magnesium in flame retardancy and potential of Mg(II)CNSL-FA as flame retardant polymeric film. Cured films of Mg(II)CNSL-FA associated with eO-Mg-O- linkages on decomposition (endothermic) formed MgO and water. The gaseous water phase would envelop the flame, thereby excluding

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Fig. 6. SEM of (A) CNSL-FA at 1 mm with magnification 25.41 KX, (B) CNSL-FA at 100 nm with 150.24 KX magnification, (C) Mg(II)CNSL-FA at 200 nm with 97.39 KX, (D) Mg(II)CNSLFA at 10 mm with magnification1.15 KX, (E) Mg(II)CNSL-FA at 2 mm with magnification 6.87 KX, (F) Mg(II)CNSL-FA at 2 mm with magnification 14.53 KX and (G) Mg(II)CNSL-FA at 200 nm with magnification 65.60 KX.

oxygen and diluting flammable gases (Kim, 2003). The decomposed MgO is alkaline, non-toxic, and reduces corrosive gases which exist during polymer decompositions (Hornsby and Watson, 1990). 3.12. Contact angles measurements Contact angle results (Fig. S5) showed that both CNSL-FA and Mg(II)CNSL-FA films were hydrophobic. The inclusion of Mg(II) in CNSL-FA generated air pockets with one-dimensional nanorod structure with an organic framework. Surface roughness and air pockets with a three-dimensional array of Mg(II)CNSL-FA film lead to a shift in water contact angle value from 96 to 101. In other words, the surface wettability or hydrophobic nature of the resulting Mg(II)CNSL-FA film improved marginally over CNSL-FA film. 3.13. Water absorption

Fig. 7. TGA thermogram of CNSL-FA and Mg(II)CNSL-FA.

Swelling water curve for hydrophobic Mg(II)CNSL-FA and CNSLFA films have been reported in Fig. 8. The results revealed new behaviour of films that they both were water-swellable regardless of their hydrophobic nature. The curve showed that the presence of

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Fig. 8. % water absorption of the films at different time intervals.

metal ion in Mg(II)CNSL-FA significantly increased the absorption capability of CNSL-FA films. The rate of water absorption was higher up to first 5 days after which it got stabilized in next 20 days of immersion in both cases. The swelling ability of these hydrophobic films might be attributed to the 3D cross-linked polymeric matrix. The structural changes, nanostructured surface with certain level of flexibility within the material along with possible interactions (hydrogen bonding) between Mg(II) ions and water molecules may have been responsible for significant enhancement of water absorption of Mg(II)CNSL-FA films (Francolini et al., 2010). 3.14. Antibacterial activity

Mg(II)CNSL-FA was tested against S. aureus, B. subtilis, E. coli and P. aeruginosa (Table S5, Fig. 9) by agar diffusion method. Ampicillin was used as a control. The increased antimicrobial effect of test compounds was observed at dilution A. The uncured Mg(II)CNSLFA showed moderate activity against S. aureus and E. coli (ZOI: 13 mm) whereas good activity against B. subtilis (ZOI: 25 mm) at least concentration (21.74 mg/mL). CNSL-FA ligand did not show any activity either at least concentration (21.74 mg/mL) or at higher concentration (65.25 mg/mL). CNSL and CNSL-F showed moderate activity (ZOI: 11 mm) against E. coli at concentration 125 mg/mL that can be related to the absence of functionality like hydroxyl group present in CNSL while the bioactivity of uncured Mg(II)CNSLFA can be related to Mg (II) ions. CNSL-FA and Mg(II)CNSL-FA cured films were also analyzed for their antibacterial activity using growth inhibition assay against gram-positive and gram-negative strains. CNSL-FA films showed percent inhibition of 12.16%, 16.67%, and 50% against E. coli, S. aureus, and B. subtilis, respectively, while an increased percent inhibition (i.e. 58.42%, 25%, and 67%, respectively) was observed in case of Mg(II)CNSL-FA films. However, CNSL-FA as well as Mg(II) CNSL-FA films were found to be inactive against P. aeruginosa (Fig. 10). Comparatively, the increased activity of Mg(II)CNSL-FA in comparison to CNSL-FA (virgin polymer) could be presumably attributed to the disruption of membrane integrity due to the presence of metal ion in the entity (Table S6) (Zielecka et al., 2011). The mechanism of action could be tentatively correlated to the electrostatic interactions occurring between the positively charged centre in Mg(II)CNSL-FA with negatively charged lipids on bacterial surface (Sharmin et al., 2013). Mg(II) metal ions present in the films induced their lethal action by first adhering themselves to lipid layer and to peptidoglycan cell wall in E. coli and S. aureus, which leads to cell death (Fig. 11). Such antimicrobial active materials like Mg(II)CNSL-FA (with certain advancements as per the requirement) would pave their future use as an efficient material for the application of free-standing films and coatings on medical devices, walls, and hospital surfaces.

Antibacterial activity of CNSL, CNSL-F, uncured CNSL-FA and

Fig. 9. Zone of inhibition for Ampicillin (a) and Mg(II)CNSL-FA (b) against B. subtilis (MTCC 736), S aurious (MTCC 902) and E. coli (MTCC 443).

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Fig. 10. % of inhibition against different bacterial strains.

stability, and antibacterial activity. The advancement of properties with superior thermal stability (~300  C) and hydrophobic (CA101 ) nature can be explained due to the formation of semicrystalline three-dimensional array structure with the stacking of nanorods (24.7 nm  3.4 nm, the aspect ratio of 14.8) coordinated with Mg (II) ions. Apart from antibacterial application these films have potential to be utilized as sustainable flame retardant surface covering material. Acknowledgements Dr F. Zafar is grateful to the Department of Science & Technology, New Delhi, India for project under the Women Scientists Scheme (WOS) for Research in Basic/Applied Sciences (Ref. No. SR/ WOS-A/CS-97/2016). S. Khan acknowledges University Grant Commission for financial support as SRF with Ref No. F1-17.1/201415/MANF-2014-15-MUS-UTT-36965/(SA-III/Website. Authors are also thankful to the Head, Department of Chemistry, Jamia Millia Islamia (JMI), for providing facilities to carry out the research work and CIF, Centre for Interdisciplinary Research in Basic Sciences, JMI for FTIR-ATR spectral analysis.

Fig. 11. Schematic representation for antibacterial mechanism of Mg(II)CNSL-FA.

4. Conclusions The present work deals with the development of technical CNSL (agro-waste) and endogenous Mg (II) ions based transparent freestanding sustainable coordination polymeric films. Most importantly, the synthesis strategy is following cleaner production concept. Cleaner production is a continuous preventive integrated strategy applied to products and processes for economic, health, and environmental benefits, promoting the use of bioresources or biowastes. The bacterial resistant, volatile organic component free films were fabricated by innovative, widely applicable, direct, rapid, economical, and eco-friendly green approach with great potential to replace the petro-based CP in broad range of applications. Periodic pH and TLC measurements helped in timing the reaction progress. FT-IR spectra confirmed the chemical formulation of the material supported by ATR and DSC studies. The impact of coordination polymerization governed various properties such as viscosity, hydrophobicity, flexibility, crystalline nature, thermal

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