Functionalized graphene tagged polyurethanes for corrosion inhibitor

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Functionalized graphene tagged polyurethanes for corrosion inhibitor and sustained drug delivery Dinesh K. Patel, Sudipta Senapati, Punita Mourya, Madan M Singh, Vinod Kumar Aswal, Biswajit Ray, and Pralay Maiti ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.7b00342 • Publication Date (Web): 04 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Biomaterials Science & Engineering

Functionalized graphene tagged polyurethanes for corrosion inhibitor and sustained drug delivery

Dinesh K. Patel1, Sudipta Senapati1, Punita Mourya2, Madan M. Singh2, Vinod K. Aswal3, Biswajit Ray4 and Pralay Maiti1*

1

School of Materials Science and Technology, Indian Institute of Technology (Banaras Hindu

University), Varanasi 221005, India. 2

Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi

221005, India. 3

Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085,

India. 4

Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005,

India.

* Corresponding author: [email protected] (P. Maiti)

ACS Paragon Plus Environment


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Abstract: Surface functionalization of graphene oxide with sulfonate group and subsequent grafting with polyurethane chains leads to the significant improvement in the properties of polymer and modified graphene as a filler. Modification of graphene oxide is revealed through spectroscopy while grafting of polymer chain over sulfonated graphene is confirmed through 1H-NMR and other techniques. Higher order of self-assembly phenomena is observed in nanohybrids as compared to pure polymer through greater interaction between polymer chain and sulfonated graphene. Significant improvement in corrosion inhibition phenomena is observed using nanohybrids at low concentration as compared to pure polymer indicating its superior efficiency as a corrosion inhibitor. Nanohybrids also exhibit better biocompatible nature in lower concentration of filler with considerable sustained release of drug vis-à-vis pure polymer suggest its potential to use as a biomaterial for tissue engineering applications.

Keywords: Surface modification, self-assembly, corrosion inhibitor, drug delivery and biocompatibility.


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Introduction: Blocky polyurethane elastomer is extensively used in biomedical area due to its biocompatible and other versatile nature.1 It is synthesized using a polyol having polyether or / polyester group, a diisocyanate and chain extender. Polyurethane contains urethane linkages in its chain and the properties of these polymers can be modified by changing the chemical composition, nature and ratio of its components during the process of synthesis. Properties of polymer can also be tuned by incorporating suitable filler in the matrix. Polymer with polyether polyol exhibits the greater enzymatic degradation resistance than polyester based material and extensively used in the preparation of medical scaffold for applications in longer duration.2 In recent years, polymeric materials are extensively used in fuel cell application due to high energy conversion, low green house effect and high power density over the fossil fuels but due to the lack of aqueous solubility and electrical conductivity some modification is required to overcome these problems.3,4 Dispersion of polyurethane in aqueous medium can be achieved by inserting the polar group such carboxylate or sulfonate moieties in their polymer chain which also facilitates the electrical conductivity of polyurethane.5 These grafted ions in polyurethane create an alternative possibility to use in gel electrolyte in place of PVDF and its copolymer.6,7 Among various fuel cells, proton exchange membrane fuel cell (PEMFC) has drawn the tremendous attention due to its high efficiency as well as minimum environmental impact.8-10 But due the rapid performance loss restrict it for commercial applications. This problem can be resolved by utilizing the stable carbon based nanomaterial. Among various carbon nanomaterials (fullerenes and carbon nanotubes) graphite has drawn the tremendous attention due to its extraordinary mechanical, thermal and electrical properties.11 Moreover, due to inert and hydrophobic nature of graphite restrict it to use as metal nanocatalyst support materials.12-13 Oxidation of graphite improves its hydrophilicity and reactivity which facilitates good dispersion in aqueous medium



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as well as deposition on metal surface. However, loss in conductivity occurs due to defect which disturbs the conjugating structure of graphite. Conductivity can be achieved by reducing the graphene oxide by various reducing agents. Sulfonation of graphene oxide using sulfonating agent cause the insertion of hydrophilic group as well as simultaneous reduction which improve the hydrophilic property and conductivity, respectively.14 This sulfonated moiety facilitates better deposition on metal surface which improve the adhesive performance for possible corrosion inhibitor. In this article, we have modified the graphene oxide into sulfonated graphene oxide followed by the insertion of amine moiety in sulfonated graphene for further grafting with polyurethane and evaluate the effect of these modifications on the properties of polyurethane for possible drug delivery vehicle and corrosion inhibiter. Surface functionalization of graphene oxide is confirmed through spectroscopic techniques like FTIR, UV-visible, XRD and 1H-NMR measurement. Various polyurethane functionalized graphene nanohybrids are prepared through chemical grafting the amine group of modified graphene with prepolymer of polyurethane chains. Homogenous dispersion is observed which is considered as the key factors for improvement in various properties of nanohybrids as compared to pure polymer due to better interaction between polymer matrix and functionalized graphene sheet. Self-assembly phenomenon is also facilitated in presence of functionalized graphene through greater interactions. Nanohybrids exhibit superior corrosion inhibition at very low concentration. Functionalized nanohybrid has been used as drug delivery vehicle with sustained release and greater biocompatible nature of the nanohybrids as compared to pure polymer indicates the developed nanohybrids have the potential to use as efficient biomaterial as well as corrosion inhibitor.


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Experimental: Materials: The following chemicals were used as received, poly(tetramethylene glycol) (PTMG) (Sigma, USA, number-average molecular weight (Mn) = 2900 g.mol-1), 1,6-hexamethylene diisocyanate (HMDI) and 1,4 butanediol (BD) (Sigma, USA). Ammonium Sulphate (Himedia), potassium permagnate, phosphoric acid (Sigma) and concentrated sulphuric acid were used as received. Dibutyltindilaurate (DBTDL) from Himedia was used as catalyst and dimethyl formamide (DMF) as solvent was purchased from Merck, India. Synthesis of graphene oxide: Graphene oxide synthesis of was done through modified Hammer’s method.15 Graphite flake (3.0 g, 1 wt. equiv.) was added to 9:1 ratio of concentrated H2SO4 and H3PO4 (360:40 ml) followed by addition of KMnO4 (18.0 g, 6 wt. equiv.) slowly. Then the reaction temperature was increased up to 50 oC with continuous stirring for overnight. 30% H2O2 was added along with ice water to cool the reaction mixture and centrifuged at 4000 rpm. Repeated washing of the solid product was done using the dilute HCl and ethanol. Finally, the sample was washed with distilled water until the media reached pH ~7. The solid sample was dried at 70 oC for two days under reduced pressure. Synthesis of sulfonated graphene oxide: Sulfonated graphene oxide was synthesized of done using ammonium sulphate as reported by He et al.14 Ammonium sulphate and graphene oxide was dispersed in distilled water in the mass ratio of 1:5 followed by the ultrasonication for 45 min. The mixture was completely dried and was heated at 245 oC for 1h. (NH4)2SO4 is believed to be decomposed at 245 oC and gives SO3 moiety to react with carbon through its surface hydrogen atom to form –SO3H moiety in graphene sheet.16



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Synthesis of amine functionalized sulfonated graphene oxide: Amine functionalized sulfonated graphene oxide was prepared using ammonia solution as reported by Lai et al.17 500 mg of the graphene oxide was taken in ethylene glycol followed by ultra sonication for half an hour. Additional ammonia solution was added and kept the reaction mixture for 12h at 180 oC with stirring. Color of the reaction mixture was altered from slight yellow to dark brown. Obtained brown solid material after the filtration was repeatedly washed of the material was performed using distilled water. Solid material was dried at 70 oC for 48h under reduced pressure. Synthesis of polyurethane and its nanohybrids: Pure polyurethane with butane diol as a chain extender (BD-PU) was synthesized in 2 steps; (1) prepolymer synthesis using PTMG and HMDI, (2) subsequent addition of chain extender (BD). DMF as solvent and catalyst (DBTDL: 0.1 ml of 1 wt.% toluene solution) were used to regulate the reaction rate and complete the polymerization process within 24 h under stirring at 70 oC. The molar ratio of PTMG : BD : HMDI was kept 1: 4: 5, respectively. The polymer flakes were obtained by pouring the solution in deionized water slowly and dried at 70 oC for two days under reduced pressure. Content of hard segment in polymer was maintained at 30% by using a predetermined amount of the reactant as mentioned. Polyurethane sulfonated graphene nanohybrids were synthesized by the addition of different weight of sulfonated graphene oxide in prepolymer solution and continued the chain extended reaction for two days. Scheme I represents the preparation of functionalized sulfonated graphene and chemically tagged graphene chain called nanohybrids where graphene sheet becomes a part of polymer chain. The following are the two different reaction steps showing the synthesis of pure PU and its nanohybrid. The number after SN indicates the percentage of sulphonated graphene in the nanohybrid.


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Structural analysis: X-ray diffraction was performed using a diffractometer (Rigaku miniflex 600X-ray) fitted with a graphite monochromator using Cu Kα source of wavelength of 0.154 nm. 40 kV and 20 mA was applied to run the generator. Thin film of the samples were placed on a quartz holder and were scanned at a rate of 1o/ min. Small-angle neutron scattering (SANS) experiments were performed on a spectrometer at a reactor (Dhruva) under Bhabha Atomic Research Centre, Mumbai, India. SANS experiment was conducted in the scattering vector (q) within the range of 0.17 ≤ q ≤ 3.5 nm-1. Background contribution was corrected from the scattering data of the samples. OrnsteinZernike model was fitted disjointedly in lower range of q. The characteristic length (Λc) was calculated using the equation Λc= 2π / qm, where, qm is the scattering vector q equivalent to the peak position in scattering pattern. The temperature of all the experiment was kept steady at 30 o

C.

1

H-NMR:

Bruker spectrometer was used to record the 1H NMR spectra of the samples using d6-DMSO as solvent. Tetramethylsilane was used as standard chemical to record the chemical shift of the samples in parts per million. Spectroscopic measurement: FTIR spectrum was measured in reflectance mode at 30 oC from 400 to 4000 cm-1 using a FTIR (Nicolet 6700) with the resolution of 4 cm-1. The UV-visible studies were made in the range of 200 to 800 nm in the reflectance mode using solid samples using JascoV-650 spectrophotometer. Thermal studies:



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The melting point (Tm) and the heat of fusion (∆H) of the samples were measured using differential scanning calorimeter (Mettler 832) within a temperature window of -30 to 200 oC at a scan rate of 10o/ min. The peak temperature and heat of fusions were obtained / calculated from the endotherms from the area under the curves. The thermal stability of polymer / nanohybrids was studied through thermogravimetric analyzer (TGA, Mettler-Toledo) in the temperature window of 40 to 600 oC. All the experiment was performed at a heating rate of 20 o/ min in nitrogen atmosphere. Morphological investigation: Dispersion in modified graphene in polyurethane matrix was viewed through TEM (FEI Technai) operating at a high voltage of 200 kV. Optical microscope (OM) and Atomic force microscope (AFM) are used to correlate the morphology of PU and its nanohybrids. AFM was done using a NT-MDT multimode AFM, Russia, controlled by a Solver scanning probe microscope regulator. Semi-contact mode was employed with the tip mounted on 100 µm long, single beam cantilever with a resonant frequency in the window of 240 - 255 kHz, and the corresponding spring constant of 11.5 N/m. Surface morphology of thin film in optical range was studied through optical microscope. For optical measurements, ~50 µm lean samples were used, prepared using solvent casting method. Coupon preparation: Mild steel samples with the percentage composition: Carbon, 0.253; Manganese, 0.03; Silicon, 0.12; Phosphorus, 0.013; Sulfur, 0.024; Chromium, 0.012; iron, remainder were used as test specimens for gravimetric and electrochemical measurements. Specimens were mechanically polished with different grades of emery papers starting from coarse (320) to very fine (2000), washed with distilled water, degreased with acetone and dried prior to each experiment. The


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corrosive solution (0.5 M H2SO4) was prepared by using analytical reagent (AR) grade sulfuric acid with double distilled water. For determination of corrosion resistance of sulfonated graphene, PU and its nanohybrid dissolved in DMF by ultra sonication and it was mixed in corrosive solution to study corrosion protection of mild steel. Gravimetric Measurement: For gravimetric measurement, 10, 20, 60, 100, 120, 150 and 200 ppm of sulfonated graphene, PU and its nanohybrid were used in 150 ml of 0.5M H2SO4 solution. The specimens, which were accurately weighed before immersion in the presence and absence of inhibitor, were retrieved after 6 h at 35oC. Then, the mild steel specimens were washed with water and then with acetone, before finally dried in a desiccator. The specimens were reweighed, and differences in weights before and after immersion were recorded as the weight loss. The corrosion rate (CR) in mg cm-2 h-1 was measured from the following equation:

CR =

∆W s×t

…….. (1)

where, ∆W is average weight loss, t is the immersion time, and s the total surface area of the specimen. The corrosion rate values were further used for predicting the inhibition efficiency (%IE), as follows % IE =

C R0 − C R × 100 C R0

(2)

0 where, C R and C R are the corrosion rates of mild steel specimen in absence and presence of an

inhibitor, respectively. Electrochemical measurement:



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These experiments were performed in a conventional three electrode glass cell assembly using mild steel as working electrode, platinum as counter electrode and a silver/silver chloride as reference electrode. All the electrochemical experiments were done using CHI (Model 604A) Electrochemical Analyzer. The working electrode with 1 cm2 exposed surface area was immersed in the corrosive solution to attain the open circuit potential (OCP). The potential versus current density curves were recorded from −250 to +250 mV with respect to OCP at a scan rate of 0.5 mV s-1. Linear Tafel segments of the anodic and cathodic curves were extrapolated to obtain corrosion potentials (Ecorr) and corrosion current densities (Icorr). Inhibition efficiency was measured by using respective Icorr values in place of corrosion rates (CR) in equation (2). The EIS measurements were performed in the frequency window of 100 KHz to 0.01 Hz with amplitude of 5 mV peak-to-peak using AC signals at OCP. The electrodes were placed for half an hour in the solution before performing the impedance measurements. The Nyquist illustration of the impedance data were analyzed with Zsimpwin software. The charge transfer resistance (Rct) values were calculated from the difference in the impedance at low and high frequency. The inhibition efficiency was measured by using the following equation; % IE = where, Rct

and

× 100 ………………

(3)

R0ct are the charge transfer resistance in presence and absence of inhibitor,

respectively. Cell culture: Culture of HeLa (Human cervical cancer cell line) cells was performed using Dulbecco's Modified Eagle Medium (DMEM) having 10% heat inactivated fetal bovine serum, 100 U/ ml


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penicillin and 100 µg/ml streptomycin. The temperature of the culture media was kept at physiological temperature (37 oC) in a CO2 incubator with 5% CO2 supply. Cell viability: Biocompatibility of pure polyurethane / nanohybrids was verified on HeLa cell line through cell viability test. The percentage of viable (live) cells was calculated through MTT (3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. Briefly, cells grown to 80–90% confluence were placed onto the 96-well culture plates in 0.1 ml of DMEM. All the treatments in triplicate were done for MTT assay. Pure polymer and its nanohybrids (~ 8mg) were added into the wells and incubated for 1, 3 and 5 days intervals at 37 °C. After incubation, media were replaced with fresh medium having 0.5 mg/ml MTT to each well and were incubated at 37 °C for four hours to produce formazan. The formazan was dissolved in DMSO and the absorbance was taken through a spectrophotometer at fixed wavelength of 570 nm. The % cell viability was measured using the formula:

% Cell viability = × 100 where, S is the optical density of the sample representing HeLa cells treated with the sample and C is the optical density of ‘control’ i.e. HeLa cells incubated in medium alone. Fluorescence imaging: Influence of pure polyurethane / nanohybrids on cell proliferation behavior was studied using fluorescence microscopic technique after 24h of incubation. HeLa cells were seeded in a 96 well plate as described earlier and were grown up to 80 - 90 % confluence. Cells were treated with pure polyurethane and its nanohybrids and were incubated at 37 oC in a CO2 incubator. Cells in 96 well plates were stained with 100 µg / ml acridine orange and 100 µg / ml ethidium bromide. The images were captured through Leica microscope.



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Cell adhesion: For this assay, approximately 80-90% confluence cells were used in 96 well plates and were seeded on the various sample surfaces (~10 mg) and incubated in a CO2 incubator for 4 h to examine comparatively the cell adhesion on PU and its nanohybrids. The specimen was washed with PBS after 4 h of incubation to remove the unattached cells and then the attached cells were fixed with ice-cold 4% paraformaldehyde for 30 min. Cell permeabilization was performed using 20% methanol for 20 min. Attached cells were stained with 0.5% crystal violet solution (aq.) (SRL, Mumbai, India) for half an hour. Excess stains were then removed through successive gentle washes using deionized water followed by the elution of residual crystal violet with 10% acetic acid. Absorbance was taken using the Microplate reader at 580 nm. The optical density was linked directly with the number of cells attached. Drug assay and release: Standard stock solution of anticancerous, dexamethasone (1 mg/ml) was prepared first. Standard curve was illustrated after collecting absorbance data using a UV-visible spectrophotometer considering the absorbance at 242 nm in the concentration window of 1-100 µg/ml. In vitro release studies were performed in PBS buffer at pH~ 7.4. Drug loaded polyurethane (5 wt.% of drug with respect to polymer weight) and its nanohybrids were prepared using solution route followed by evaporation of solvent. 50 ml of released medium was added on to sample in an incubator shaker kept under 100 rpm at 37 oC. Predetermined aliquots were taken from the release medium at fixed time interval and similar quantity was replaced with fresh buffer. The amount/concentration of drug in the specimen was calculated by taking the absorbance at 242 nm.


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Results and discussions: Evidence of graphene functionalization: FTIR spectra of sulfonated graphene oxide followed by the functionalization through amine moiety are given in supplementary Figure S1a. Appearance of the peaks at 1152 and 1077 cm-1 are associated to S=O stretching vibration indicating the presence of sulfonic moiety in modified graphene.14 Attenuated in the peak intensity of carbonyl group (>C=O, 1739 cm-1) indicates the simultaneous reduction during sulfonation. Amine functionalization was done as reported by Lai et al.17 Appearance of the absorption peak at 1575 cm-1 clearly indicates the presence of –(N–H) moiety in the ring. Disappearance of the absorption peaks at 1398 cm-1 (C-OH) and 1739 cm-1 (– C=O) indicate the considerable dehydration as well as decarboxylation occurred during the amine functionalization. UV-visible spectra of sulfonated species followed by amine functionalized graphene are provided in supplementary Figure S1b. Pure graphene oxide exhibits two absorption peaks at 235 and 260 nm for π→π* and n→π*, respectively.18 This peak has shifted to 252 and 255 nm for sulfonated graphene and sulfonated graphene with amine moiety, respectively, indicating the reduction of graphene oxide and reformation of graphene electronic configuration as observed in FTIR spectra.19,20 1H-NMR spectra of graphene oxide and sulfonated graphene oxide is given in supplementary Figure S1c. 1H-NMR lines in graphene oxide at 7.6 and 4.4 ppm is due to the chemical shift of aromatic naphthelic hydrogen and hydroxyl group.17 Appearance of a peak line at δ~8.9 ppm indicates the presence of sulfonyl group in sulfonated graphene oxide. XRD patterns of graphene oxide and sulfonated graphene oxide is presented in supplementary Figure S1d. Graphene oxide exhibits two diffraction peaks at 10.6o (001) due to insertion of oxygenated groups in the ring and 24.7o (002) peak for graphitic sheet structure corresponding to the d-spacing of 0.84 and 0.37 nm, respectively.21 A weak peak



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at 14.6o appears after sulfonation with decreasing of d-spacing from 0.84→0.61 nm indicating the removal of some oxygenated moiety during the sulfonation. Another very broad diffraction peak at 24.9o corresponding to the d-spacing of 0.36 nm suggests the reduction of graphene oxide.22 However, disappearance of peak at 14.6o in sulfonated graphene with amine moiety and other very broad peaks at 24.9o indicate the reduction of modified graphene oxide towards the graphitic structure which is also verified through FTIR and UV-visible spectroscopy. Particle size as determined through DLS also supports the larger dimension of functionalized graphene as expected supplementary Figure S1e.

Grafting of polyurethane on functionalized graphene and their Interactions: Shorter polyurethane chain with isocyanate end group was reacted with amine functionality in graphene to graft polymer on chemically modified graphene as confirmed through 1H-NMR measurement. Pure polyurethane and nanohybrids exhibit a NMR peak at δ~7.0 ppm indicate the presence of –(N–H) moiety of urethane linkage.23 Appearances of new peak in lower field region at δ~8.2 ppm in nanohybrids clearly indicate the grafting of prepolymer chain to modified graphene.24 Further, a peak at δ~9.8 ppm in nanohybrids suggest the presence of –SO3H moiety on graphene as discussed whose intensity increase for higher content of modified graphene Figure 1a. FTIR spectra of pure polyurethane and its indicated nanohybrids are shown in Figure 1b. Pure polyurethane exhibits a >N–H stretching peak at 3316 cm-1 which shift towards higher wavenumber at 3329 cm-1 in nanohybrids suggesting strong interaction between the sulfonated graphene sheet and PU chains. Pure polyurethane exhibits a strong absorption peak at ~1681 cm-1 and a weak peak at ~1717 cm-1 indicating the presence of hydrogen bonded and free >C=O moiety in polymer chain, respectively.25 The intensity of


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hydrogen bonded >C=O moiety has reduced considerably and has shifted towards higher frequency region of 1684 cm-1 in nanohybrids also suggest greater interaction between polymer chain and modified graphene sheet. On the other hand, intensity of free >C=O peak becomes prominent in nanohybrids further consolidating the minimized interaction within urethane linkages rather increasing interaction occurs between graphene sheet and polymer.26 The >S=O peak varies from 1170 to 1190 cm-1 depending on the chemical environment.14 In PU-SN1, the said peak appears at 1170 cm-1 while the peak has been shifted to 1208 cm-1 in PU-SN2 and PUSN4 presumably due to greater interaction between sulfonyl group with PU chain. Interaction between polymer chain and graphene sheet is also verified through UV-visible spectroscopy presented in Figure 1c. Pure polyurethane exhibits two absorption peak at 250 and 285 nm for π→π* and n→π*, respectively.27 Prominent red shifting of π→π* and n→π* from 250→256 nm and 285→293 nm in nanohybrids also confirms good interaction between modified graphene and polymer chain possibly through dipolar interaction. Fluorescence emission spectra of pure PU and its indicated nanohybrids, excited at 285 nm, also support the interactive system as presented in Figure 1d. Nanohybrids exhibit the broad emission band originated from π*→π transition and shifted towards higher wavelength (466→477 nm from pure PU to PU-SN4) due to the electronic redistribution through the greater interaction between polymer chain and sulfonated graphene.24 Enhancement in the emission spectra at ~470 nm in nanohybrids is due to the greater interaction between polymer chain and sulfonated graphene which facilitates the electronic redistribution over the entire moieties. Lower side emission band is originated from the coupling of π*→π and n→π transitions.28 Very high fluorescence in nanohybrid is presumably due to functionalization in the graphene sheet followed by its wrapping up by polymer chain. However, NMR measurements confirm the grafting of polyurethane with functionalized graphene and



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other spectroscopic studies clarify the greater interaction between graphene and PU as opposed to interaction between urethane linkages in pure PU. Now, it is important to understand the consequential of the better interaction in PU grafted graphene. Dispersion, morphology and thermal behavior: Properties of hybrid material depend upon the dispersion of nanofiller into matrix. Dispersion of functionalized graphene in the polyurethane matrix is observed through TEM bright field images (Figure 2a). Homogenous dispersion is observed in nanohybrids even at higher content of functionalized graphene presumably due to better interaction between the components (Supplementary Figure S2a). Surface morphology of nanohybrids is significantly influence by the dispersion and interaction behavior of nanofiller in matrix as evident from the SEM image of nanohybrids with 1 wt.% of sulfonated graphene (Figure 2b). Sulfonated graphene exhibits the grain surface morphology whose extent enhances in higher percentage of filler. Pure polyurethane exhibits the flake like surface morphology suggesting the crystalline nature. More ordered domain nature is observed in AFM micrograph with the dimension of ~310 nm due to the greater interaction between sulfonated graphene and polymer chain (Figure 2c). Optical image of nanohybrids also exhibits strip like structure suggesting the conglomerated morphology with the size of ~2.5µm originated through the interaction between the polymer chain and modified graphene Figure 2d. Thermal degradation behaviour of pure PU and its different nanohybrids has been studies through thermogravimetric analyzer (TGA) and has been presented in Figure 3a. Pure PU and its nanohybrids exhibit two step degradation process of hard segment at lower temperature region followed by the soft segments at higher temperature zone.29 Enhancement in the thermal stability is observed for nanohybrid having the lower percentage (1 wt.%) of modified graphene


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caused by the disruption of hydrogen bonded network structure in nanohybrid in presence of graphene as opposed to pure PU, where self-assembly is discussed below. Further, functionalized graphene create the barrier effect for heat conductivity and diffusivity of volatile decomposition product.30 Melting behavior of pure PU and its nanohybrids has been studied using differential scanning calorimeter and are presented in Figure 3b. Pure PU and nanohybrids exhibit two melting peaks (lower and higher temperature region) suggesting the presence of segmented structure in chain. Lower endothermic peak is due the soft segment of polymer chain originated by the presence of polyol moiety, whereas higher endothermic peak is due to hard segment of polymer chain.31 Enhancement in melting temperature (both soft and hard segments) is observed in nanohybrids presumably due to induced crystallinity especially for hard segment on top of graphene layers arising from better interactions. Change in the heat of fusion (∆H) and melting temperature (Tm) as a function of graphene content is presented in supplementary Figure S3a. Enhancement in the heat of fusion obtained from the area under the melting curve at lower content of graphene suggests induced crystallization in nanohybrid. Further, variation of crystallinity in pure polymer and its nanohybrids is also supported from XRD measurement and is given in Supplementary Figure S3b. Pure polyurethane exhibits sharp peaks at ~22.1o and ~23.9o indicating the crystalline nature due to the predominant hydrogen bonded interaction between the urethane moieties of neighboring polymer chains while the intensity of the peak corresponding to θ~22.1o gets enhanced in nanohybrids primarily due to induced crystallization in presence of modified graphene. Graphene induced self-assembly in polyurethane: X-ray diffraction patterns of pure polyurethane and its indicated nanohybrids at lower angle are shown in Figure 4a. Pure polyurethane exhibits a diffraction peak at 5.7o with



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corresponding d-spacing of 1.54 nm indicating the presence of molecular sheet structure which provide the Bragg’s reflection.31 This is to mention that the molecular sheet structure is formed by the hydrogen bonding in the hard segment. Shifting of peak position towards higher angle (lower d-spacing; ~1.40 - 1.52 nm) in nanohybrids indicate even compact packing of the molecular sheet in presence of graphene sheet through better interactions in nanohybrids. Further, the presence of hump in small angle scattering patterns (SANS) at q ~ 0.42 nm-1 with the corresponding characteristic length, Λc (= 2π/qm) of 15, 14.6, 11 and 10 nm for pure PU, PUSN1, PU-SN2 and PU-SN4, respectively, indicate greater assembly of hydrogen bonded sheet having one order higher in magnitude Figure 4b. Systematic decrease in characteristic length in nanohybrids with increasing graphene content as compared to pure polyurethane indicates that the lesser number of molecular sheets are required to assemble for the formation of domain structure in nanohybrids. Lowering in characteristic length (Λc) in polyurethane graphene nanohybrids is also reported in our previous work.26 SANS patterns are best fitted in OrnsteinZernike model (Equ. 1). I (q) = I (0) / (1+ξ2q2)……………….(4) where, I(q) is the scattered intensity, I(0) is scattered intensity at zero wavevector, ξ is the correlation length and q is the wavevector. The correlation length (ξ) of pure PU, PU-SN1, PUSN2 and PU-SN4 are 0.60, 1.37, 3.35 and 3.75 nm, respectively, suggest the larger blob size in presence of graphene. It is noteworthy to mention that correlation length (ξ) is the lateral dimension of domain structure as obtained from SANS studies and the incorporation of graphene increases the size of blob in nanohybrids as compared to pure PU due to the presence of the graphene sheet presumably within the molecular sheets.32 Higher order self-assembly is revealed through AFM measurement with the domain structure of dimension ~240 nm in pure PU which


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increase to ~305 nm in nanohybrids Figure 4c, once more formed by the extensive hydrogen bonding between the blobs as observed through SANS measurement. Optical images also support the higher order self-assembled structure with the dimension of ~3.1 µm crystalline zone in pure polymer and the size is more or similar in nanohybrids Figure 4d. The reason for the formation of such self-assembled structure in pure aliphatic polyurethane is well reported and it is due to the extensive hydrogen bonding interaction between urethane moieties whereas in nanohybrids, this is happened through the polar interaction between electron rich moiety of modified graphene sheet and dipole of urethane linkages.26,31 Formation of nanostructure and step by step self-assembled is, however, confirmed through the XRD measurement (1.54nm) with gradual increasing the molecular stack size in SANS (15 nm) which form further larger crystalline cluster of size ~305 nm as observed in AFM image and subsequently more crystalline domain in (~3.1 µm) in optical images. This self-assembly of various dimensions should have an impact on the properties of PU and its nanohybrids and their variation in size in presence of various amount of graphene may lead to the following improvement in the delivery of drug and other applications like corrosion inhibitor.

Drug delivery and biological responses using nanohybrid: One of the important applications of biomaterial is whether they deliver drug at a controlled rate to achieve the maximum therapeutic result. In vitro drug release studies of anticancerous dexamethasone loaded drug in pure polymer and its nanohybrids are done in phosphate buffer solution using absorption technique33 (pH ~7.4) at 37 oC and are shown in Figure 5a. Initially pure PU shows the burst release followed by gradual release of ~54% up to 10 days whereas sustained drug release has been observed in nanohybrids starting from the



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beginning. Slow release of drug in nanohybrids is presumably due to the barrier effect of the two dimensional sulfonated graphene as well as the larger number of inhomogeinities due to self assembly as discussed earlier. However, the release rate has further decrease drastically with increasing the modified graphene content as compared to pure PU. Release kinetics of loaded drug are best fitted using Korsmeyer- Peppas model (r2=0.99) leading to the exponent (n) values of 0.46, 0.54 and 0.56 for pure PU, PU-SN1 and PU-SN4, respectively, showing almost Fickian behavior Figure 5b. Several steps such as penetrations of liquids into matrix, dissolution of the drug or diffusion process play importance role in overall drug delivery. Any of these processes may act as a rate determining steps.34 Release rate constant (k), correlation coefficient (r2) and diffusion release exponent (n) obtained using different mathematical models for drug loaded in PU and its nanohybrids are given in Supplementary Table S1. Increasing values of exponent for nanohybrids vis-à-vis pure PU further suggest the delayed diffusion of loaded drug due to overall barrier effect either in presence of sulfonated graphene sheet or inhomogeinities caused by self assembly. A model has been proposed based on the release kinetics where only hard segments in pure PU are responsible for barrier effect against the added effect of two dimensional filler in nanohybrids Figure 5c. Further, greater amount of filler causes more hindrance toward diffusion resulting more delayed diffusion in specimen like PU-SN4 against PU-SN1. Therefore, it is possible to tune the release rate of the loaded drug by using suitable amount of modified graphene in polymer matrix. An efficient drug delivery vehicle should be biocompatible in nature for its actual implementation in biomedical fields. Biological responses of pure polyurethane and its nanohybrids are evaluated through MTT assay using the HeLa cell lines at different time intervals and have been presented in Figure 6a. Cells treated only using culture medium are


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treated as control. Cell viability of nanohybrids with lower concentration (1-2 wt.%) of sulfonated garphene are more or less same with pure polyurethane at different time (1 - 5 days) interval indicating biocompatible nature of the nanohybrid at par with pure PU. This is to mention that polyurethane is considered to be a biocompatible polymer.35 However, a slight decrease in cell viability is observed in nanohybrid with higher percentage graphene (PU-SN4) indicating some cytotoxic property. Corr et al. studied the effect of the sulfonated graphene on Hep3B cell lines and reported drastic decrease of cell viability up to 20% at 100 µg/ml concentration and complete necrosis of cells at 72 h and considered higher insolubility of formazan in presence of sulphonated graphene as the reason behind it.36 However, our studies indicate nanohybrid with 4wt % of sulfonated graphene (i.e.~331 µg/ml; three times the concentration reported earlier) but still showing the 60% cell viability after 5 days interval suggesting that grafting of the biocompatible polyurethane chains over sulfonated graphene sheet through amine moiety induced sufficient biocompatibility in nanohybrids as compared pure functionalized graphene. Cell viability data is further supported by the Fluorescence image of cell proliferation using acridine orange and ethydine bromide dye Figure 6b. Cells are healthier on the nanohybrid having the low content (1-2 wt %) of sulfonated graphene as compared to pure polyurethane and other nanohybrid. Furthermore, acceleration in cell (human mesenchymal stem cells) differentiation on the surface of graphene film has also been reported earlier.37 However, the toxicity of functionalized graphene has been overcome by grafting the graphene using biocompatible polyurethane in this study. Polyurethane, a well known biocompatible material, is frequently used in medical arena as a transplant material. Biocompatibility of pure polyurethane and its nanohybrids have been evaluated in terms of cell adhesion on HeLa cell lines Figure 6c. Nanohybrids having lower



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content of sulfonated graphene (1 wt.%) exhibits better cell adhesion property as compared to pure polyurethane while the cell adhesion slightly decrease in PU-SN4 nanohybrid indicating certain compromise of the filler concentration in nanohybrids and it is suggested that up to 2% of functionalized graphene is best for its utilization in biomedical applications. Integrated Modulation Contrast (IMC) image of HeLa cell cultured on pure polyurethane and its nanohybrids also support the better cell adhesion of nanohybrid as compared to pure PU Figure 6d. Cells are more properly adhered on nanohybrids having 1-2wt % of sulfonated graphene as well as pure polyurethane. Slight alternation in cell morphology is observed in nanohybrids(PUSN4).36 In this context, it is suggested that 1 wt.% of functionalized graphene is best suited for the preparation of nanohybrid for biological application as it shows better biocompatibility.

Graphene nanohybrid as corrosion inhibitor: The functionality in chemically modified graphene nanohybrid should have versatile applications as it contains ionic group and at the same time attached with long chain polymer which may have certain affinity towards metal/conductor to cover up and protect it from corrosion. The weight loss of mild steel samples in 0.5M H2SO4 in absence and presence of sulfonated graphene, pure PU and 1% nanohybrid (PU-SN1) are determined after 6h of immersion at 35 oC. Variation of %IE with different concentration of sulfonated graphene, pure PU and its nanohybrids are given in Figure 7a. Gravimetric parameters as well as inhibition efficiency values in the absence and presence of different concentrations of investigated inhibitor at 308K is given in Supplementary Table S2. Massive increase in %IE with concentration is observed in case of nanohybrids as compared to pure PU at lower concentration region (10 ppm, ~65% IE) which further increases with increasing concentration of nanohybrids. The inhibitive


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property of nanohybrids is attributed to the presence of the more active centre (ionic sulfonate group in graphene ring) which facilitates the adsorbability on the metal surface area and thereby prevents the corrosion process.38 The increase in inhibition efficiency with its concentration is due to the availability of larger number of molecule which cover greater surface area of the corroding metal. Poly(acrylic acid) exhibits the maximum 75.5% IE at 200 ppm whereas, nanohybrid (PU-SN1) show the 79 % IE at just 100 ppm level indicating the developed hybrid material has a better potential to be used as a corrosion inhibitor.39 Effect of nanohybrids on the kinetic behavior of the partial cathodic and anodic reactions is determined through potentiodynamic polarization measurements. Tafel polarization curves for mild steel in 0.5M H2SO4 solution having 10 and 120 ppm of sulfonated graphene, pure polymer and nanohybrid with blank (0.5M H2SO4) is presented in Figure 7b. The potentiodynamic polarization parameters including the corrosion current density (Icorr), corrosion potential (Ecorr), anodic Tafel slope (βa), cathodic Tafel slope (βc) and corrosion efficiency are given in Table 1. Shifting of the corrosion current density towards lower values from 1860→704 µA cm-2 for blank to nanohybrid at 10 ppm concentration suggests a significant decrease in corrosion current density primarily due to wide adsorption of the materials over the surface preventing hydrogen evolution.40 Corrosion inhibition efficiency obtained by potentiodynamic polarization measurement is presented in Figure 7c and its trend is very similar to the efficiency observed through gravimetric measurements. However, the inhibition efficiency of the nanohybrid is much higher than pure polymer and functionalized graphene for the entire concentration zone. Functionalized graphene show lower efficiency because less coverage of the surface by the nanoparticle while this surface coverage is greater using polymer showing better efficiency. On the other hand, nanohybrid having both ionic species and long chain chain polymer has the



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highest inhibition efficiency and 150 ppm shows ~ 85% inhibition efficiency. Functionalized graphene is in suspension while polymer and nanohybrid are soluble in corrosive solution; this might be reason behind less surface coverage and correspondingly lower inhibition efficiency of functionalized graphene. Impedance parameter of the mild steel/sulphuric acid interface surface in absence and presence of various concentrations of functionalized graphene, pure polymer and nanohybrids are given in Table 2. It is observed that at each concentration charge transfer resistance (Rct) values in nanohybrids are higher as compared to functionalized graphene and pure polymer resulting in lower CPE values. This increase in Rct and consequent decrease in CPE can be explained on the basis of decrease of local dielectric constant or increasing the thickness of the interface double layer denoting the significant adsorption of materials over the metal/solution interface.41 Nyquist plots at 10 and 120 ppm concentrations of functionalized graphene, pure polymer and nanohybrids with blank are shown in Figure 7d. Nyquist plots show the semicircular nature suggesting the single charge transfer phenomena during the dissolution.42 The slightly depressed nature of semi-circle Nyquist plots is due the roughness and other inhomogeneties of solid electrodes.43 Slight inductive loop at low frequency is attributed to the relaxation process caused by adsorption of species like FeSO4 or inhibitor species on the electrode surface.44 There is small deviation from the origin and the deviation of Z/ - Z// in the Nyquist plot from the origin is due to the solution resistant (Rs) part of the circuit. Increase in the diameter of semi-circle was observed at higher concentration (120 ppm) suggesting the higher charge transfer resistance and this is more pronounced in nanohybrid as compared to that in pure polymer. Values of interfacial impedance parameters are analyzed in terms of equivalent circuit


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diagram presented in Supplementary Figure S4.45 The impedance of the CPE can be represented as: Z CPE = Y0

−1

(iω )− n

(5)

where, Y0 is a proportionality factor and ‘n’ has the meaning of phase shift. The value of ‘n’ represents the deviation from the ideal behavior and it lies between 0 and 1. Chi square (goodness of fit) values indicate a good fitting to the proposed circuit.46 Inhibition efficiency obtained by electrochemical impedance spectroscopy correlates well with gravimetric measurement and potentiodynamic polarization results. To understand the mechanism of the corrosion inhibition, it is necessary to know the adsorption nature of the materials over the mild steel. Inhibition efficiency obtained from gravimetric measurements is used for calculation of the extent of surface coverage (Ɵ) over the mild steel, given in Supplementary Table S2. The adsorption process follows the Langmuir adsorption isotherm. Kads.Cinh =

Ɵ Ɵ

(6)

where, Kads represents the equilibrium constant and Cinh is the molar concentration of the inhibitor. Nature of the curve and slope are obtained by plotting Cinh/Ɵ vs. Cinh. The linear plot indicates the Langmuir adsorption nature of the inhibitor. Figure 7e shows the Langmuir adsorption isotherm plot for pure polyurethane, functionalized graphene and nanohybrids. The standard free energy (∆G0ads) for pure polymer, modified graphene and nanohybrid is calculated by using following relationship. ∆Goads = -RT ln (55.5 Kads)


(7)


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where, numeric value 55.5 is the molar concentration of the water and T is the experimental temperature. The obtained values of ∆Goads are found to be -34.658, -33.67 and -38.10 kJmole-1 for pure polyurethane, modified graphene and nanohybrids, respectively. Negative value of ∆Goads clearly indicates the spontaneous adsorption of the material over the mild steel surface. Langmuir adsorption is considered as the best possible fitting of the data points, other models such as Freundlich and Temkin model also show good fit but the value of regression coefficient for those fitting of data points are poor. Higher values of linear regression parameters clearly indicate the Langmuir model is the best possible fitting in adsorption isotherm. Other fitting curves are given in supplementary Figure S5 and table S3. Developed nanohybrid with ionic group in its two dimensional filler along with chemically tagged polymer chain with graphene has several advantages which include delivery of cancerous drug in a sustained manner along with potential uses as corrosion inhibitor showing multiple applications of the nanohybrid/nanobiohybrid.

Conclusion: Functionalization (sulfonation) of graphene oxide followed by the insertion of amine moiety in the graphene is revealed through 1H-NMR, FTIR and UV-visible spectroscopic techniques. Polyurethane is chemically tagged with the sulfonated graphene using the chain extension through amine group. Chemical grafting of polyurethane is confirmed through various spectroscopic methods. Very high fluorescence intensity is observed in nanohybrid essentially due to functionalization in graphene followed by its wrapping with polymer chains. Self assembly behavior has changed considerably in presence of graphene which reduce the predominant hydrogen bonding in pure PU rather secondary interaction develop between


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modified graphene and PU chain. Nanohybrid exhibits better corrosion inhibition as compared to pure polymer even at low concentration through selective surface coverage following the Langmuir adsorption isotherm. Further, modified graphene in nanohybrid releases the cancerous drug from the matrix in well sustained manner leading to develop better drug delivery vehicle for cancer treatment. Cell viability and fluorescence imaging of cells also indicate the nanohybrid as better biomaterial suitable for its use in biomedical application in tissue engineering drug delivery.

Acknowledgments: The author (D.K. Patel) gratefully acknowledges the financial support from CSIR/ UGC (1706/2012 (i) EU-V) New Delhi in form of fellowship. Author also acknowledges the Science and Engineering Research Board (Grant No: R&D/SERB/LT/SMST/16/17/06) (SERB) & Council for Scientific and Industrial Research, (Grant No. 02(0074)/12/EMR-II) New Delhi for financial supports. Authors gratefully acknowledge Prof. N. Misra, School of Biomedical Engineering IIT (BHU), Varanasi for providing the spectrophotometer facility. Author(s) also acknowledges the Central Instrumentation Facility Centre (CIFC) IIT-BHU, Varanasi for 1H-NMR, SEM and AFM characterization.

Supplementary Information: Figure S1: Describe the characterization of functionalized graphene oxide, whereas Figure S2 contains the TEM and SEM image of nanohybrid. Figure S3 includes the plot of heat of fusion and melting temperature vs. graphene content and XRD patterns of the samples. Table S1 represent drug release kinetics. Table S2 Contains gravimetric parameters as well as inhibition



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efficiency and Figure S4 represent best fitted equivalent circuit. Figure S5 indicates Freundlich and Temkin adsorption isotherm fitting and Table S3 shows the linear regression parameter

values of different models.

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Scheme 1 (a) Reaction scheme showing the modification of graphene oxide and subsequent grafting using prepolymer chain with sulfonated graphene sheet; (b) cartoon showing chemical tagging of polyurethane with sulfonated graphene sheet.


Page 35 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1: (a) 1H-NMR spectra of pure PU and its indicated nanohybrids showing evidence of sulphonation and grafting; (b) FTIR spectra of pure PU and its indicated nanohybrids showing the shifting peaks because of interactions; (c) UV-visible spectra of pure PU and its indicated nanohybrids; and (d) Fluorescence emission spectra of pure PU and its indicated nanohybrids excited at 285 nm with sulfonated graphene.



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Page 36 of 44

Figure 2: (a) Bright field TEM image (scale bar = 100 nm); (b) surface morphology of nanohybrid through SEM; (c) AFM micrograph of indicated nanohybrid (1µm ×1µm); and (d) Optical image of indicated nanohybrids (Scale bar = 20 µm).


Page 37 of 44

a. 1.0

b. PU-SN1 0 291 C

0

(285 C)PU

7.9

0.5

PU-SN1 13.3

0.1

2.5

PU

5.3

0.0

1.7

PU-SN2

Heat Flow / a.u.

Weight loss

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(∆H, j/g)

100

200

300 0

400

500

0

50

100

150

200

0

T/ C

T/ C

Figure 3: Thermal behavior of PU and its indicated nanohybrids, (a) TGA curves of pure polyurethane and its nanohybrids; and (b) DSC thermograph of polyurethane and its indicated nanohybrids. The numbers indicate the heat of fusion of the respective peaks.



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Page 38 of 44

Figure 4: (a) X-ray diffraction patterns of PU and its indicated nanohybrids. Vertical lines

indicate the peak position with interplanar spacing; (b) Small angle neutron scattering patterns; I (q) vs q (wavevector) plot of PU and its indicated nanohybrids. Inset figure shows the Ornstein–Zernike fitting for the calculation of correlation length, ξ; (c) AFM micrographs of PU and its indicated nanohybrids (5µm ×5µm) obtained through tapping mode; and (d) Optical images of PU and its indicated nanohybrids (Scale bar = 20 µm).


Page 39 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5: (a) Sustained drug release profile of PU and its indicated nanohybrids; (b) Fitting of

data points with Korsemeyer–Peppas model for understanding the mechanism of drug release; and (c) schematic model of drug release kinetics. The blue line represents the polymer chains, whereas plates indicate the graphene sheets. Arrows show the probable path for drug diffusion.



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Page 40 of 44

Figure 6: Behavior of pure PU and its indicated nanohybrids towards cell biology, (a) Cell

viability of pure PU and its indicated nanohybrids with time interval of 1, 3 and 5 days; (b) Fluorescence microscopic image of cells treated with PU and its indicated nanohybrids after 1 day of cell proliferation (Mag.: 20×); (c) Cell adhesion on pure PU and its indicated nanohybrids after 1 day of cell incubation; and (d) Integrated modulation contrast (IMC) image of HeLa cell cultured on pure polyurethane and its nanohybrids after 1 day of cell incubation.


Page 41 of 44

b. -2

a. 100 PU-SN1

120 ppm

-3 Blank GNS PU PU-SN1

PU

75

50

log (i/A)

IE/%

-4 GNS

-5 -2

10 ppm

-3

25

Blank GNS PU PU-SN1

-4 0

0

50

100

150

200

-5

C/ppm

-0.60 c. 100

-0.55

-0.50 -0.45 Potential / V

d. -10 120 ppm

PU-SN1

-6

Z''/ohm

-4 50

GNS

25

0

-2 0 -6

10 ppm

Blank GNS PU PU-SN1

-4

0

50

100

150

-2

200

C/ppm

0

0

5

10

15

20

25

30

Z'/ohm

15

PU

Cinh/θ

e.

25

-0.40 Blank GNS PU PU-SN1

-8

PU

75

IE/%

20

GNS

10

5

Cinh/θ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

0 0

3

6

9

12

Cinh x 10-6 /M

PU-SN1

10 5 0

0

3

6

9

12

15

Cinh x 10-6 /M

Figure 7: (a) Inhibition efficiency of sulfonated graphene, pure polyurethane and it indicated

nanohybrid using Gravimetric method; (b) Potentiodynamic polarization behavior of mild steel in 0.5M H2SO4 for the blank, sulfonated graphene, pure polyurethane and its indicated nanohybrid; (c) Inhibition efficiency measured through polarization method; (d) Nyquist plot for mild steel for blank, sulfonated graphene, pure polyurethane and its indicated nanohybrid at 10 and 120 ppm; and (e) Langmuir adsorption isotherm for corrosion inhibition using sulfonated graphene, pure polyurethane (inset) and its indicated nanohybrid.



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Page 42 of 44

Table 1: Polarization parameters as well as inhibition efficiency values in the absence and

presence of different concentrations of investigated inhibitor at 35 oC. Potentiodynamic polarization Inhibitor Blank GNS

PU

PU-SN1

Cinh Ppm 00 10 20 60 100 120 150 200 10 20 60 100 120 150 200 10 20 60 100 120 150 200

-Ecorr mV 472 476 450 489 557 475 481 552 481 550 490 487 491 489 486 507 488 485 478 490 498 510

icorr µA cm-2 1860 1561 1384 1288 1166 1145 1133 1076 1133 1070 901 755 572 553 429 704 550 475 425 405 247 241

βa mV dec-1 164 135 177 166 115 164 162 214 162 214 126 159 131 172 185 123 173 127 198 120 186 187

-βc mV dec-1 183 176 194 193 128 153 177 206 177 206 186 192 174 199 220 165 199 162 227 149 197 217


IE % --16.0 25.6 30.7 37.3 38.4 39.1 42.1 39.1 42.4 51.5 59.4 69.2 70.3 77.0 62.1 70.6 74.5 77.2 78.1 86.7 87.0

Page 43 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Electrochemical parameters as well as inhibition efficiency values in the absence and

presence of different concentrations of investigated inhibitor at 35 oC.

Inhibitor

Blank GNS

PU

PU-SN1

Cinh ppm

Rs Ω cm2

00 10 20 60 100 120 150 200 10 20 60 100 120 150 200 10 20 60 100 120 150 200

2.5 2.7 2.7 2.6 2.2 2.5 2.2 2.4 2.4 2.3 2.4 2.3 2.5 2.4 2.5 2.4 2.2 2.4 2.0 2.4 1.9 2.1

Rct Ω m2 4.61 5.58 6.50 6.84 7.23 7.35 7.63 8.06 7.62 8.07 9.39 10.51 14.66 16.11 22.60 12.78 16.01 16.80 19.01 22.40 28.68 35.50

EIS CPE Y0 µ Ω sn cm-2 265.0 255.0 228.0 226.6 207.5 202.4 191.1 132.8 291.1 272.8 250.0 191.0 176.7 120.0 107.0 324.2 290.0 278.0 285.7 220.0 174.3 104.7

n

Chi square

IE %

0.909 0.926 0.969 0.956 0.952 0.917 0.918 0.878 0.918 0.888 0.887 0.896 0.909 0.840 0.903 0.954 0.894 0.864 0.916 0.903 0.907 0.896

0.0027 0.0008 0.0018 0.0187 0.0069 0.0012 0.0010 0.0060 0.0011 0.0061 0.0011 0.0042 0.0185 0.0067 0.0007 0.0099 0.0060 0.0049 0.0114 0.0007 0.0108 0.0048

--17.4 29.0 32.6 36.2 37.3 39.5 42.8 39.5 42.8 51.0 56.1 68.5 71.4 79.6 64.0 71.2 72.6 75.7 79.4 84.0 87.0



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Page 44 of 44

Table of Content (TOC) Functionalized graphene tagged polyurethanes for corrosion inhibitor and sustained drug delivery

Dinesh K. Patel, Sudipta Senapati, Punita Mourya, Madan M. Singh, Vinod K. Aswal, Biswajit Ray and Pralay Maiti

Sulfonation of graphene oxide followed by subsequent grafting of prepolymer chain with amine functionality exhibits the significant improvement in corrosion inhibition efficiency at low concentration. Self-assembly phenomena facilitate in presence of functionalized graphene with lesser number of molecular sheets are required to form the patterned structure. Nanohybrids exhibit biocompatible nature with sustained drug release suitable for its use as drug delivery vehicle.


Functionalized graphene tagged polyurethanes for corrosion inhibitor

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