Strategic Formulation of Graphene Oxide Sheets for Flexible Monoliths

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Strategic Formulation of Graphene Oxide Sheets for Flexible Monoliths and Robust Polymeric Coatings that Embedded with Durable Bio-inspired Wettability Avijit Das, Jumi Deka, Adil Majeed Rather, Bibhas K Bhunia, Partha Pratim Saikia, Biman B. Mandal, Kalyan Raidongia, and Uttam Manna ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14028 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

Strategic Formulation of Graphene Oxide Sheets for Flexible Monoliths and Robust Polymeric Coatings that Embedded with Durable Bio-inspired Wettability** By Avijit Dasa, Jumi Dekaa, Adil M. Rathera, Bibhas K. Bhuniab Partha Pratim Saikiac, Biman B. Mandalb, Kalyan Raidongiaa, Uttam Mannaa* We dedicate this work to Prof. David M. Lynn, University of Wisconsin-Madison on his birth day October 25

[*] Dr. Uttam Manna a Department of Chemistry, Indian Institute of Technology-Guwahati, Kamrup, Assam 781039, India E-mail: [email protected] b Biomaterial and Tissue Engineering Laboratory, Department of Biosciences and Bioengineering, Indian Institute of Technology Guwahati, Assam-781035, India c Department of Chemistry, N N Saikia College, Titabar, Jorhat, Assam, India

Keywords: Reactive Interface, Asymmetric Wettability, Superhydrophobicity, oil/water Separation, Polymeric Material

ABSTRACT

Artificial bio-inspired superhydrophobicity—that are generally developed through appropriate optimization of chemistry and hierarchical topography, is being recognized for its immense prospective applications related to environment and healthcare. Nevertheless, the weak interfacial interactions that associated in the fabrication of such special interfaces—often provide delicate bio-mimicked wettability, and the embedded antifouling property collapses on exposure to harsh and complex aqueous phases, and also after regular physical deformations including bending, creasing etc. Eventually, such materials with potential antifouling property became less relevant for practical application. Here, a facile, catalyst-free and robust 1,4 conjugate addition reaction has been strategically exploited for appropriate covalent integration of modified

1 ACS Paragon Plus Environment


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graphene oxide to develop polymeric materials with 1) tunable mechanical property and 2) durable anti-fouling property—which is capable of performing both in air and under oil. Further, this approach provided a facile basis for 3) engineering the superhydrophobic monolith into arbitrary free-standing shapes, and 4) decorating various flexible (metal, synthetic plastic etc.) and rigid (glass, wood etc.) substrates with thick and durable three dimensional superhydrophobic coatings. The synthesized superhydrophobic monoliths and polymeric coatings with controlled mechanical property are appropriate to withstand different physical insults, including twisting, creasing and even physical erosion of the material without compromising the embedded anti-wetting property. The materials are also equally resistant to various harsh chemical environments, and the embedded antifouling property remained unperturbed even after continuous exposure to extremes of pH (pH 1 and pH 11), artificial sea water for a minimum of 30 days. These flexible and formable free-standing monoliths and stable polymeric coatings that are extremely water repellent both in air and under oil, are of utmost importance owing to their suitability in practical circumstances and robust nature.

1. INTRODUCTION Special and extreme wettability of liquid on solid surface—which is primarily discovered in living objects (e.g.; lotus leaf, rose petal, fish scale etc.) in nature, is composed of essential chemistry and topography, and such bio-inspired artificial interfaces are being recognized as a general avenue for developing functional materials in context of various environment, energy and healthcare related applications.1-6 In this regard, extremely water repellent superhydrophobic materials (having advancing contact angles, θAdv, ≥150° and contact angle hysteresis, θHys, ≤10°)1-2 are particularly well characterized and are highly suitable for wide variety of applications, including protein crystallization, guided water transfer, harvesting of liquid water 2 ACS Paragon Plus Environment

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from mist, controlled small molecule release and synthesis of anti-bio-fouling coating etc.7-13 However, the conventional thin superhydrophobic interfaces are generally fabricated by adopting a thin layer of inert low surface energy coating on the appropriately customized hierarchical topography (either hydrophilic and/or hydrophobic).14-15 In general, the appropriate co-existence of low surface energy and hierarchical features, provided the essential metastable trapped air for extreme heterogeneous wettability of liquid on solid surface. However, slight perturbations in any of these physical and/or chemical parameters during the practically relevant physical manipulations (including bending, creasing, compressing, scratching etc,) and severe chemical exposures are known to damage such interfacial special wettability.16-17 In the recent past, some elegant designs including mechanically durable coatings, repairable coatings, and self-healable coatings were introduced to achieve durable anti-wetting property.5,18-25 These approaches provided comparatively durable—but thin superhydrophobic coating, mostly through either a complex/tedious fabrication process or additional postfabrication intervention.19-25 For example, the maintenance of the anti-wetting property in practical scenarios through the concept of restoration of anti-wetting property back to the material by either significant post-fabrication interventions or treatment with appropriate stimuli (i.e.; humidity, temperature, pH etc.) are likely to impose additional complexity during the process of healing of anti-wettability in practical scenario as controlling the optimum conditions for the essential stimuli would be challenging and complex. Further, the coexistence of appropriate topography and essential chemistry—that is necessary for heterogeneous wettability, is maintained only up to a few nanometres across the thickness in such reported thin superhydrophobic coatings. Thus, these thin anti-fouling coatings with insufficient metastable



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trapped air would be highly liable to compromise the embedded superhydrophobicity in practical scenarios.26 The work reported here was motivated by the reported literature on ‘internally’superhydrophobic materials—in which the anti-wetting property exists both at the surface and in the interior of the material.26-31 The metastable air, which confers the anti-fouling property to the material, was trapped across the thickness of the material, unlike the conventional and thin superhydrophobic coatings.26,30 The coexistence of a) appropriate topography (or porosity) and b) essential low surface energy environment, is primarily required across the thickness (including the interior and the surface of the material), to achieve such ‘internal’-superhydrophobicity. The fabrication of materials with such specific chemical and physical requirements is inherently difficult and challenging.26-31 Such a new class of materials have emerged recently with very few promising proof-of-concept studies. Nevertheless, facile and scalable designs are desirable to synthesize such ‘internally’-superhydrophobic materials—without demanding any sophisticated instrument and laborious process. Further, the single and simple synthetic approach, which would be appropriate to provide both 1) highly-flexible and shapeable free-standing monolith, and 2) substrate independent durable and thick bulk-superhydrophobic coatings, is of potential interest in diverse fundamental and applied contexts. In our past designs, 1,4-conjugate addition reaction between amine and acrylate groups was used in synthesis of ‘chemically-reactive’ nanocomplex—which was further appropriately integrated in the polymeric thin film coating (multilayers) fibrous

substrates37

to

develop

bio-inspired

35-36

and covalently immobilized on

interfaces

(including

underwater

superoleophobicity35-36 and superhydrophobicity37). Here, we have extended this facile and catalyst-free chemical approach to synthesize an amine-reactive, formable and highly flexible


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polymeric monolith (without using any porous external matrix) and substrate-independent polymeric coating through appropriate covalent integration of small amount of aminographeneoxide (AGO) with branched polyethyleneimine (BPEI) and dipentaerythritol penta/hexa-acrylate (5Acl) mixture at ambient conditions (Scheme 1). This current chemical approach is highly capable of tailoring both the mechanical property and liquid wettability in the synthesized material and provided a simple avenue to develop: a) highly flexible, scalable and self-standing monoliths—which could be molded into desired shapes and b) durable and thick polymeric coatings—that could be deposited on various flexible and rigid substrates. The synthesized materials, having extreme water repellency both in air and under oil, remained unperturbed, even after severe physical insults, including removal of the top surface of the material, and after prolonged (30 days) exposure to chemically complex aqueous phases (including extremes of pH, sea water, river water, etc.). Examples of such durable and simple chemical approach that has the ability to control mechanical property in the polymeric material, and is capable of providing both free-standing superhydrophobic monolith and substrate independent thick anti-fouling coating with impeccable durability, and has potential to synthesize other smart materials (i.e.; durable Janus membrane having extreme wettability at two opposite interfaces), is unprecedented in literature. This facile synthetic approach could be useful for several fundamental and applied contexts. 2. RESULTS and DISCUSSIONS In recent past, the catalyst-free and fast reactivity between primary amine-containing synthetic polymer (BPEI) and small molecules (5Acl) having multiple acrylate groups, has been exploited in synthesis of functional and thin polymeric coatings.32-38 In past, the strategic use of this facile chemistry provided simple basis to control both the topography and chemistry of the polymeric



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coatings.34-38 Here, in the current design, two-dimensional flexible graphene oxide (GO)-sheets, which are well recognized for their exceptional mechanical properties,39 were strategically functionalized and incorporated in the porous and ‘reactive’ polymeric-material through facile and scalable 1,4-conjugate addition reaction as shown in Scheme 1. This direct covalent incorporation of such flexible GO-sheets was anticipated to have the ability to modulate the mechanical property in the synthesized polymeric materials. In the current demonstration, the synthesized self-standing materials are capable of surviving various physical manipulations, including bending, twisting, creasing etc., without sacrificing the embedded anti-fouling property in the material. In recent past, few durable superhydrophobic coatings were synthesized through strategic incorporation of carbon nanotubes and natural nanomaterials.40-41 Here, in our design, we have covalently incorporated two-dimensional nanomaterial for synthesising durable superhydrophobic polymeric material with tailored mechanical property. So, the GO-sheets were first decorated with primary-amine groups (Nitrogen 7%, material characterized with Energy Dispersive X-ray spectroscopy (EDS), Raman Spectra, FTIR and FESEM; Figure S1) by adopting a standard approach (see supporting information for details), prior to incorporating them in the gel-material through a 1,4-conjugate addition reaction. The synthesized AGO was found to be chemically reactive with 5Acl molecules as shown in Figure S2. Then, we started by mixing 1mL of 5Acl (132.5mg/mL) and 0.125mL of BPEI (50mg/mL) in methanol at 200C temp. This composition was unable to provide gel-material (Figure 1A), however, the addition of AGO (4.25µg/mL) resulted in the polymeric gel-material within 1h (Figure 1A, labeled as PGM1). This result indicated that the added-AGO expedites and controls the gelation process, likely through 1,4-conjugate addition reaction (Scheme 1D) between the grafted primary amine groups (from AGO and BPEI) and acrylate groups (from 5Acl). Additional control experiments


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were designed to understand the role of grafted amines on AGO-sheets behind this polymericgelation process. In cases where, pristine GO (300.77µg/mL) sheets (lacking primary amine groups) were added, we observed a mild yellowish transparent solution (labelled as GO; Figure 1A) without any further physical changes in the solution, even after a day. This experiment unambiguously revealed the active participation of grafted amines from AGO in synthesis of the polymeric gel-material. On the addition of AGO (final concentration of AGO in solution is 4.25µg/mL) in the mixture of BPEI/5Acl in methanol, the colorless mixture slowly transitioned into a faint turbid solution (after 20 min), to a milky solution (after 30 min), and eventually after 1h, a gel-material (labelled as PGM1; Figure 1B and Figure S3A-F) was formed. However, this appearance of turbidity in the BPEI/5Acl mixture occurred more rapidly upon increasing the amount of AGO in the reaction mixture. For example, a highly turbid solution was noticed within 10 min, after increasing the AGO concentration to 30.77 µg/mL in the reaction mixture of BPEI/5Acl, under otherwise identical conditions, whereas other mixtures (labelled as PGM1, PGM2, PGM3) having lesser amount of AGO-doping (4.25µg/mL, 8.16µg/mL and 15.09µg/mL) appeared as clear solutions (Figure S3A-B)—but eventually became turbid over time (Figure S3C-E). Further, DLS study confirmed the existence of polymeric-nano-complexes in the AGOdoped mixtures of BPEI/5Acl; these nano-complexes grew over time and aggregated to form bigger particles that eventually settled down from the solution in methanol, and thus allowed to monitor (Figure 1C) the gelation process in the early stage of the reaction. Both the formation and growth of the nano-complexes were delayed when smaller amounts of AGO were used. No growth of nano-complexes was observed in the mixture of BPEI/5Acl in absence of AGOdoping. Thus, this study revealed that the AGO-doping not only initiates the gelation process, but it also controls the rate of gelation processes.



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The existence of AGO-sheets in the polymeric gels was further confirmed by inspecting the Raman spectra of both the AGO and AGO-doped polymeric-material (PGM4)—the characteristic D (at 1344cm-1) and G (at 1600cm-1) band signatures for AGO were observed in the polymericmaterial. Further, the existence of ‘reactive’ functional moieties in the polymeric-materials were characterized by FTIR spectral analysis. Interestingly, we observed characteristic two peaks (Figure 1D) at 1735cm-1and 1409cm-1 corresponding to the carbonyl stretch and symmetric deformation of the C-H bond for the β carbon of the vinyl groups, respectively, revealing the existence of residual acrylate groups in the polymeric-materials irrespective of AGO doping amount. Further, the post chemical modification of the material by exploiting this residual acrylate functional groups with strategically selected amine-containing hydrophobic small molecules, provided a basis to adopt extreme water fouling property in the material. The highest AGO-doped polymeric-material (PGM4), displayed extreme liquid water repellency both in air (Figure 1E-F) and under oil (Figure 1G-H) with advancing contact angle above 160°, once it was post functionalized with primary amine containing hydrophobic small molecule (decylamine). The FTIR spectral analysis confirmed the successful post chemical modification of the material—the characteristic peak at 1409cm-1 which denotes that the residual acrylate groups were almost exhausted in the polymeric-material on treatment with decylamine (Figure 1I). Further, a 5µL aqueous droplet dropped on this synthesized superhydrophobic material from a 7mm height was observed to bounce twice before its settlement on the surface (Figure S4A-F), and the droplet could readily roll off from the surface on tilting the superhydrophobic material at 3° and streams of water bounced away from the surface after hitting the surface of the material (Figure S4G-H). This is consistent with the Cassie-Baxter (CB) state in the superhydrophobic


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material and the existence of the meta-stable trapped air layer in the material is evident from the presence of shiny reflection of light at water/material interface (Figure S4I-J). This facile chemical approach also provided an exceptional control on the modulation of mechanical property in the polymeric material. The polymeric material (PGM1) with low content of AGO found to be tough, and the compressive stress and strain at yield were measured to be ~347 kPa and 41 %, respectively, however, the material (PGM4) with more AGO sheets was found to be highly elastic (Figure 2A).

Further, the compressive modulus was gradually

decreased from 1237.56 ± 129.40 kPa (PGM1) to 15.57±2.93 kPa (PGM4) with increasing the content of modified graphene oxide in the polymeric sample (Table 1 and Figure 2). Thus, the polymeric-materials became more-elastic on increasing covalent integration of AGO sheets through 1,4-conjugate addition reaction. Further, the synthesized polymeric material was repetitively deformed for 1000 times with compressive strain of 80 %. The material remained non-adhesive superhydrophobic with advancing water contact angle above 156° and contact angle hysteresis below 10° even after successive deformations of the same polymeric material for 750 times. However, the material became highly adhesive superhydrophobic with contact angle hysteresis of 28° after extending this consecutive deformation for 1000 times as shown in Figure S5. The AGO-doped free-standing polymeric-materials (4.6cm X 1.2cm) were manually bent and creased (Figure 2B-C)—and as expected, the outcomes of these physical manipulations on the material were noticed to be abruptly different: 1) the lowest AGO-doped material (PGM1) was broken into two pieces from the creased-region of the material (Figure 2D); a very similar phenomenon was also observed for PGM2 (data not shown here) as well, and a significant crack around creased-region were observed in moderately AGO-doped materials (PGM3, Figure 2E). However, a highly AGO-doping material (PGM4) remained physically intact with no apparent



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change around the creased-region of this material (Figure 2F). Thus, the result here clearly indicated that the amount of added-AGO in the polymeric-material plays an important role in improving the mechanical property of the materials—the material becomes more flexible and capable of surviving harsh physical manipulations on increasing the AGO-doping level. Further, this polymeric-material (PGM4) can be manually rolled and twisted without any noticeable physical damage to the material (Figure 2G-H). Importantly, the anti-wetting property of the material remained intact even after these various physical manipulations and the water droplets were beaded with high water contact angle (θAdv ~164°, Figure 2I) on the material. Further, the anti-wetting property on other polymeric-materials (lower AGO-doped) was examined in detail. All other synthesized polymeric-materials (PGM1, PGM2, PGM3) were efficient in displaying the extreme liquid water repellency (Figure S6) (having θAdv> 150° and θHys < 4°), after post chemical-functionalization with decylamine, irrespective of the mechanical property in the materials (Figure 3A). This result suggests that the AGO-doping level in the synthesized polymeric-materials has little effect on controlling the topography of the polymeric material as confirmed by studying the morphology of these synthesized polymeric-materials under field emission scanning electron microscope (FESEM). The morphology of the polymericmaterials was apparently similar in low magnification irrespective of AGO-doping level in the materials as shown in Figure 3B,D,F—they are highly porous with randomly arranged granular domains, and the topography in each material is appropriate enough to display desired antiwetting property. However, in the high magnification, the granular domains in the material were observed to be more diffused and larger in nature upon increasing the AGO-doping amount in the materials (Figure 3 C,E,G and Figure S7). Doped AGO-sheets in the material might be folded around these polymeric-granular structures during the gelation process, and any phase separated


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domain, populated with AGO-sheets, was not observed in the material. Moreover, the increase in diffused inter-connected structure in PGM4 is most likely because of more complexation of AGO-sheets with polymeric-granules in the material and this diffused inter-connections among the granular structures through flexible AGO-sheets is hypothesized to be the key component in achieving desired physical flexibility in the materials. Therefore, this AGO-doped material could provide a basis to develop highly-flexible self-standing superhydrophobic material—which can be moulded into arbitrary shapes and sizes (Figure 3H-J) including the structure of vessels that are present in guava leaf (Figure 3H). Moreover, the properties of the material allow it to be machined, e.g., to make an array of wells (Figure 3J) that could be useful in restricting the motion of aqueous droplets on extremely non-adhesive anti-fouling surface and would eventually facilitate various prospective demonstrations, including developing biosensor for protein detection, tissue engineering, protein crystallization, high-throughput drug sensing etc. Then, the highly AGO-doped polymeric-material (PGM4) was sliced into multiple parts to arbitrarily expose the interior of the material (Figure 4A-B), and the morphology of the interior was examined with the FESEM study, which revealed the existence of similar topography what was observed on the surface of the material (PGM4, Figure 3G)—the diffused granular domains are randomly arranged (Figure 4C) in the interior of the material as well. Further, the antiwetting property was examined beyond the surface of the material, where the interior of the material was exposed again by slicing the material into multiple pieces and freshly exposed material/air interfaces displayed extreme water repellency with θAdv>150° and θHys< 5°) (Figure 3B, Figure S8J). Then, several standard experiments were adopted to investigate the physical and chemical durability of the anti-wetting property in the material. First, sand paper (5cm X 1.5cm) was



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manually rubbed back and forth on the polymeric-material (3cm X 1.5cm) 20 times with an applied load of 100g, resulting in physical erosion of the topmost layer, and exposed the interior of the polymeric monoliths. However, the material displayed anti-fouling property (with θAdv>150°, Figure 4D,E) even after this substantial physical damage. Very similar to the surface, a porous morphology with arbitrary arrangement of granular domains was observed in this damaged material under FESEM study (Figure 4F). Second, 150 g of sand was poured on the material from 20 cm height and the anti-wetting property was found to remain intact as noted in Table 2 and Figure S6C, without causing any noticeable physical damage to the material. Third, the material was exposed to adhesive tape, to cleave the material randomly, to examine the antiwetting property on arbitrarily fractured material. As expected, the anti-wetting property remained unaffected even after this harsh physical insult (Figure S8F-G). Finally, material was scratched (4mm wide and 1mm depth) with a knife—but the anti-wetting property of the material was found to be unperturbed (Figure S8H-I). Next, the chemical durability of the anti-wetting property in the material was examined by exposing the synthesized material to several different chemically harsh environments, including extremes of pH, simulated sea water, river water, cationic (DTAB) and anionic (SDS) surfactants-contaminated (0.1mM) water and BSA-protein contaminated water. However, the material displayed extreme repellency to these chemically harsh and complex aqueous media (table 2 and Figure S9), moreover the anti-wetting property of the material remained mostly unaffected even after longer exposure (30 days) of this material in such chemically-complex aqueous media (Figure S10). Thus, this material can successfully withstand various severe physical and chemical insults. Further, this highly durable superhydrophobic monolith was exploited in oil/water separation. While this synthesized polymeric material extremely repelled


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water droplet, the droplet of motor oil was beaded on the material with contact angle of 0° (Figure S11). This extremely selective oil and water wettability was used in removing floating oils. As a proof of concept demonstration, a piece of superhydrophobic monolith was brought in contact with floating oil (motor oil) droplet on air/water interface as shown in Figure 4G-J, where oil phase was selectively collected by the superhydrophobic material, even in presence of harsh chemical contaminants in aqueous phase. Moreover, this polymeric material was also capable of removing other floating oils (e.g., silicone oil, vegetable oil, kerosene oil) on air/water interface as shown in Figure S12A-O. The synthesized polymeric material was efficient in absorbing these oils (Figure S12P) above 500 (wt.) %, irrespective of the chemical nature of the used oils. Next, the AGO incorporated mixture of BPEI/5Acl—before its transformation to semi-solid gelmaterial, was used in coating various rigid (glass and wood) and flexible (filter paper, Al-foil, plastic film) substrates. In this current demonstration, we have selected same composition of AGO, BPEI and 5Acl, which was used in developing PGM4 (flexible superhydrophobic monolith), and after mixing of these components in methanol, the colorless solution was found to be milky turbid within 15 minutes. This highly turbid solution was uniformly deposited on the selected substrates, and after air-drying, a crack- and peel-free coating was observed on each substrate irrespective of the chemical (metal, synthetic polymer, glass, natural polymer) compositions and the physical (smooth, fibrous) nature of the selected substrates as shown in Figure 5. Further, the coated substrates were treated with strategically selected decylamine molecules, and the surface of each coated substrate were noticed to be extreme water repellent with advancing water contact angle above 160° and contact angle hysteresis of 5° as shown in Figure 5A-H (superhydrophobic- glass, wood, plastic, Al-foil), T-U (superhydrophobic filter



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paper). Next, a coated rigid substrate—that is superhydrophobic glass substrate (Figure 5I-M) was exposed to a standard adhesive tape pulling test, to examine the stability of the polymeric coating on the rigid and smooth glass surface. So, the coated glass substrate was brought in contact with the adhesive tape surface with an applied load (200 g; Figure 5I), and then peeled off from the surface of polymeric coating. During this process, the top surface of the polymeric coating was transferred to the adhesive surface (Figure 5L), and the freshly exposed interiors of the polymeric coating that were attached onto the surfaces of both the adhesive tape and the substrate (glass), were also capable of displaying superhydrophobicity with advancing CA above 160° and CA hysteresis below 5° as shown in Figure 5J-M. However, a complete detachment of the coating was never observed even after successive (5 times) adhesive tape test, which supported the existence of both 1) the stable polymeric coating on the substrate and 2) the bulksuperhydrophobicity in the deposited polymeric coating. Next, a piece of flexible plastic was chosen as substrate in order to examine the durability of the superhydrophobic coating against other physical manipulations including bending, twisting, rolling and creasing, and both the antiwetting property and the integrity of the material remained unaffected—and no peeling or cracking was visible during these exercises as shown in Figure 5N-R. Next, we have selected fibrous and inherently hydrophilic filter paper (Figure 5S) to demonstrate the synthesis of Janus membrane with extreme asymmetric wettability (Figure 5T-V)—which is useful in separation of emulsion droplet, bubble aeration, bulk oil/water separation, drug delivery, wound dressing and guided liquid movement etc.42-44 Selectively, one side of the filter paper was strategically coated, and the surface was capable of displaying superhydrophobicity (Figure 5T-U) with water advancing contact angle of 162.4°, after appropriate post chemical modification. However, during this process, the intrinsic water wettability on the other side of the filter paper remained


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intact with water contact angle of 0° and eventually provided Janus membrane (Figure 5V) with extremes of wettability (superhydrophilicity and superhydrophobicity).

3. CONCLUSION In conclusion, the current report exploited a simple and robust 1,4 conjugate addition reaction between amine and acrylate groups for strategic and covalent integration of modified grapheneoxide—which is well recognized for its impeccable mechanical property. The optimum incorporation of AGO-sheets in the covalently cross-linked ‘amine-reactive’ gel, provided a highly flexible and durable polymeric materials—which were appropriate to display extreme water repellency both in air and under oil. Further, this semi-solid gel-material allowed to mold in desired shapes—which could be useful in various prospective applications including protein detection, tissue engineering, protein crystallization, high-throughput drug sensing etc., and the same reaction mixture (before gelation) can be exploited in coating various relevant rigid (glass, wood) and flexible (plastic film, Al-foil, filter paper) substrates, irrespective of their chemical composition, and the antifouling property and integrity of material remained intact even after performing different physical manipulations including bending, creasing, tweeting and even the physical erosion of the top surface of the material. The strategic selection of substrate and appropriate deposition of polymeric coating further provided a Janus membrane with extremely asymmetric wettability. Thus, the current design provided a general and simple avenue to develop 1) flexible & shapeable polymeric monoliths, 2) durable polymeric coating on wide range of relevant substrates and 3) Janus membrane with extreme asymmetric wettability, which would be relevant in demonstrating several important prospective applications (related to energy and healthcare) of this bio-inspired antifouling property, even at practical settings.



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Experimental section: Materials: Branched polyethyleneimine (PEI, MW~25000), Dipentaerythritol penta-/hexaacrylate (5-Acl, Mw~ 524.21g/mol), propylamine (98%), hexylamine (99%), octylamine (99%), decylamine (95%), bovine serum albumin (≥96%), sodium dodecyl sulfate (SDS, ≥99%), dodecyltrimethylammonium bromide (DTAB) (99%) were purchased from Sigma Aldrich, Bangalore, India. Butylamine was obtained from Spectrochem Pvt Ltd., Mumbai, India. Pentylamine and heptylamine were purchased from Alfa Aesar, India. Rhodamine 6G was purchased from Labo Chemie, Mumbai, India. Conc. Hydrochloric acid and Potassium Permanganate were obtained from Fischer Scientific, Mumbai, India. Graphite powder was purchased from Asbury Carbon. Sodium hydroxide, Methanol, Conc. Sulphuric acid, Conc. Hydrochloric acid, Hydrogen peroxide, Nitric Acid, Hydrazine hydrate, Ammonium hydroxide were purchased from Merck Specialties Private Limited, India. THF was obtained from RANKEM, Maharashtra, India. Sandpaper (grit no. 400) was purchased from Million International, India. Glass slide (Boroleb, India), aluminum foil (Parekh Aluminex Ltd. India), Adhesive tape (Jonson tape Ltd. India) were acquired from different sources. Sand was collected from a contraction site at IIT-Guwahati, Assam. This sand was used in experimental demonstrations after thorough washing with water. General considerations: Dynamic light scattering (DLS) measurements were taken with Zetasizer Nano ZS90 (model no ZEN3690) instrument. All the FTIR data were collected using PerkinElmer instrument, where samples were first embedded in KBr pellet following standard protocol. Liquid water contact angles on the materials were estimated using KRUSS Drop Shape Analyser-DSA25 instrument at ambient temperature and 3 µL deionized (DI) water droplets were used for dynamic contact angle measurements. SEM images were obtained using Carl 16 ACS Paragon Plus Environment

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Zeiss field emission scanning electron microscope (FESEM). All non-conductive polymeric samples were gold sputtered under vacuum to achieve a thin layer of conductive gold coating on the polymeric samples. Raman spectra were acquired using Laser Micro Raman System (Horiba Jobin Vyon, Model LabRam HR). All digital pictures were taken with canon power shot SX420 IS digital camera. Synthesis of Graphene oxide (GO): GO sheets were prepared by following the modified Hummers’ method.45 First, graphite (1g) powder was added to 50 mL of concentrated sulphuric acid, and whole system was chilled at 0 °C using an ice bath. Then, KMnO4 (6g) was slowly added to the solution keeping the temperature of the reaction mixture below 10°C. The reaction mixture was then transferred to a water bath, where the temperature was maintained at 35 °C for 2 hours, and then, the reaction mixture was diluted with 100 ml of DI water. The sudden rise in the temperature during the dilution process was controlled by submerging the reaction vessel in an ice bath. Then, 8 mL of 30 % hydrogen peroxide solution was added to the dilute solution to reduce unreacted KMnO4. As prepared GO sample was thoroughly washed by adopting a twostep washing method, where HCl and acetone solvent were consecutively used. Synthesis and Characterization of Amino-grapheneoxide (AGO): AGO sheets were synthesized from the synthesized GO sheets by adopting two conjugative reactions: 1) nitration followed by 2) reduction. For the nitration reaction on GO sheets, the dried GO powder (50 mg) was mixed with 100 mL of 50 % nitric acid and kept it for 12 hours at room temperature with continuous agitation. Nitro GO obtained as such was then washed with acetone and dried under vacuum before re-dispersing it in 50:50 ethanol-water mixture (0.1 mg/mL) for the reduction reaction. The reduction was carried out by heating the dispersion at 70°C after the addition of 150 µl of ammonium hydroxide and 50 µl of hydrazine hydrate under continuous magnetic stirring. As



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prepared AGO was characterized with Field Emission Scanning electron microscope (FESEM), energy dispersive x-ray (EDX) analysis, and Fourier transform infrared (FTIR) and Raman spectroscopy. The FESEM image in Figure S1 reveals the existence of two-dimensional sheetlike morphology of amino graphene with lateral dimensions extending from 2 to 8 µm2. The EDX analysis was carried out at different AGO sheets yielded an approximate composition of the sheets to be 63:30:7 for carbon, oxygen, and nitrogen atoms, respectively. The FTIR spectra of GO and amino graphene samples were compared in Figure S1, where both the spectra confirmed the presence of O-H groups and (sp2) C=C groups—the characteristic peaks are observed at around 3350 cm-1 and 1625 cm-1, respectively. However, the appearance of new peaks at 1380 cm-1 and 1237 cm-1 in the AGO sample, corresponding to N-H in-plane bending vibration, and C-N stretching, revealed the presence of amine groups in the sheets. The hump at 3230 cm-1 can also be assigned to –NH2 stretching vibration of amino-graphene oxide sheets. Figure S1 further compared the Raman spectra of GO with that of AGO. Both the spectra show prominent D and G band at 1330 cm-1 and 1595 cm-1, respectively. The ratio of ID/IG was found to be decreased from 0.8 with GO to 0.72 of AGO, suggesting removal of some of the defects during the process of functionalization. These synthesized AGO sheets in ethanol was directly used in the fabrication of superhydrophobic polymeric material (details are accounted in following section). Preparation of superhydrophobic polymeric gel material: First, the solution of 5Acl (1.325 g) and BPEI (0.5 g) in methanol were prepared in separate glass vials by dissolving each of them in 10 mL methanol. The, solutions (1 mL) of 5Acl and BPEI (0.125mL) in methanol were mixed together in glass vials. Then, the mixtures were doped with different amount of amino graphene oxide (4.25µg/mL, 8.16µg/mL, 15.09µg/mL and 30.77µg/mL: denote as PGM1, PGM2, PGM3


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and PGM4 respectively) in separate glass vials as shown in Figure S3A (last four glass vial from left) and kept under agitation for 1 h. Depending on the amount of added AGO sheets in the mixture, colorless solutions are appeared as turbid liquids after certain time, and eventually after 1 h, it transformed to gel material (as shown in Figure S3). Then, this semi-solid gel was taken out and it was manipulated into different shapes like dolphin, leaf and array of wells with the help of appropriate negative replica (doh cutter with appropriate shape was used for giving the shape of dolphin and array of wells to the gel material and PDMS mold having negative replica of guava leaf was used for preparing the polymeric material with similar shape and structure of guava leaf). Post Chemical Modifications: To adopt the desired superhydrophobicity, the gel materials were washed with THF for 30 minutes and followed by treated with n-decylamine (37.47mg/mL) in THF for 6h. After this treatment, the polymeric material was washed again with THF thoroughly and dried in open air. After drying, these materials (PGM1, PGM2, PGM3 and PGM4) were characterized with FTIR, FESEM, Raman spectroscopy and contact angle measurements. Moreover, to explore

this material further in revealing the effect of various chemical

modifications on the water wettability, the polymeric material (PGM4) was sliced into seven different parts using a sharp-edged razor blade (Gillette) and each of them were washed with THF prior to being treated with solution of propylamine (34.23mg/mL), butylamine (35.23mg/mL), pentylamine (35.95mg/mL), hexylamine (36.6mg/mL), heptylamine (37mg/mL), octylamine(37.23mg/mL) and decylamine (37.47mg/mL) respectively, following the reported protocol (A. M. Rather, U. Manna, Chem. Mater. 2016, 28, 8689). After this treatment, each of them was washed thoroughly with THF and dried them in open air. After drying, we measured water contact angle and contact angle hysteresis for each material.



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Coating on different substrate: Before complete gelation (after 15 min), the highly turbid polymeric solution (PGM4, 3.25 mL) was spread uniformly over area of 8 cm2 on desired substrates (al foil, plastic, glass) with microscopic glass slide. After air drying, the coated (thickness of 0.5 mm) substrates were placed in decylamine solution in THF for overnight, and then washed thoroughly with THF to remove unreacted and excess decylamine molecules from the coating. Then the materials were allowed to dry in air. The material was covered with al-foil to suppress the rate of drying process. Physical Manipulations of polymeric materials: The n-decylamine treated free standing polymeric materials (PGM1, PGM2, PGM3 and PGM4) were exposed to various physical manipulations. First, all of them were bent manually and then creased completely with moderate manual pressure as shown in Figure2B-C—which resulted in a broken (PGM1 and PGM2) and cracked (PGM3) material from the region of creasing whereas the polymeric material (PGM4) with higher AGO doping can withstand such physical manipulation without any visual change in the material. Addition to this, the material (PGM4) can be rolled and twisted without any further damage to the material. Moreover, the anti-wetting property of the material remained unaffected, even after this treatment, irrespective of the amount of AGO doping in the material. Estimation of Mechanical Property: After the synthesis of polymeric materials (PGM1, PGM2, PGM3, PGM4), the mechanical property was investigated using a Universal Testing Machine (Instron 5944, U.S.A) equipped with a 100 N load cell. All the tests were recorded at ambient condition. All the polymeric samples were cut cylindrically with a diameter of 1.5 cm and thickness of 1.5 cm prior to examine the mechanical property. Physical and Chemical Durability of the Anti-Wetting Property: The superhydrophobicity property remained intact with the synthesized polymeric material even after exposing the


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material in severe physical insults including sand drop test, sand paper abrasion, scratch test, adhesive tape test etc. Detailed processes are documented in following sections: Sand Paper Abrasion Test: A dried polymeric material (3cm X 1.5cm) was abraded twenty times with 400 grit sand paper (5cm X 1.5cm) under 100g load. First, a sand paper was immobilized on a glass slide using adhesive tape and was placed on the polymeric material (which is immobilized on another glass slide), and then, a weight of 100 g was applied (Figure S8A) prior to rub the sand paper manually across the surface of the polymeric material back and forth for multiple times. After this process, colored deionized water droplets was placed on abraded surface for visual inspection of the anti-wetting property, further contact angle measurements were carried out following standard procedure, moreover, the morphology of the abraded surface was examined with FESEM study. Sand Drop Test: The polymeric material (PGM4) was first fixed on a glass slide using adhesive tape, and then 150 g of sand grains were poured from a height of 20 cm on the material using a glass funnel as shown in Figure S8C. After post treatment, the anti-wetting property of the material was investigated by examining the water contact angle and contact angle hysteresis. Scratch Test: Superficial scratches (1mm depth and 4.5mm wide) were made manually on the surface of polymeric material with a sharp knife, subsequently colored water droplets were beaded on scratched area of the material to investigate the anti-wetting property of the material. Adhesive Tape Test: A double sided tape was used here in the demonstration of random fracture of the material, where one adhesive surface of the tape was placed on glass slide prior to adhering the polymeric material on another surface of the same adhesive tape. Then, a 200g load was applied on the polymeric material to improve the contact of the material against the adhesive surface and then, the polymeric material was removed manually from the adhesive tape surface.



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Chemical Durability Test: The decylamine treated polymeric material was retained their antiwetting property mostly intact even after exposing this material to wide range of harsh chemical conditions. The polymeric materials were exposed to various chemically harsh aqueous phases including simulated sea water (solution of 0.226g MgCl2, 0.325g MgSO4, 2.673g NaCl and 0.112g CaCl2 in 100 mL deionized water) droplet, 0.01M NaOH ( pH =12), 0.1M HCl (pH =1), river water (Bhambhaputra river, Assam, India), 10w% BSA protein solution, sodium dodecyl sulphate (0.1mM) and dodecyltrimethylammonium bromide (0.1mM) contaminated aqueous solutions, and the impacts of these harsh chemicals on the anti-wetting property were investigated by measuring the contact angles.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Figure S1-S12 accounting the characterization of AGO, reactivity of AGO with BPEI, the gelation of AGO doped BPEI/5Acl mixtures, the bouncing of water droplet on the superhydophobic interface, the effect of successive mechanical compression on the water wettability of the material, the post functionalization of AGO incorporated polymeric materials, the size of granular domains, the physical abrasions, the exposures of various chemicals, the effect of prolonged exposures of chemically harsh medium on the wettability, the wettability of both oil and water, the absorption based separation of various floating oils by the superhydrophobic material.

Acknowledgements


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We acknowledge financial support from Science and Engineering Research Board (YSS/2015/000818), Department of Science and Technology, Government of India and from Indian Institute of Technology-Guwahati (startup grant). We thank CIF and Department of Chemistry, Indian Institute of Technology-Guwahati, for their generous assistances in executing various experiments and for the infrastructures. We thank Prof. David M. Lynn for the invaluable discussions and suggestions on this research work. We thank Dr. Devasish Chowdhury and Institute of Advanced Study in Science and Technology (IASST) for kind help in FESEM characterization of some samples. Mr. Avijit Das thanks Council of Scientific & Industrial Research (CSIR, India) for the junior research fellowship.



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Scheme 1. A) Chemical structures of branched Poly(ethylene imine) (PEI) and Dipentaerythritol Pentaacrylate (5Acl) molecule and Schematic representation of amino graphene oxide (AGO). B, C) Demonstrating the synthesis of ‘reactive’ polymeric gel from AGO doped solution of BPEI and 5Acl in methanol. D) Schematic representing the 1,4-Conjugate addition reaction between Acrylate and primary amine groups.

Fig. 1. A-B) Digital images of the reaction solutions of AGO incorporated BPEI/5Acl mixtures (PGM; last four vials from left to right; PGM1, PGM2, PGM3 and PGM4) after 30 minutes (A) 24 ACS Paragon Plus Environment

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and 60 minutes (B). No gel-material was formed both in absence (first vial from left) and in presence (second vial from left; 300.77µg/mL) of graphene oxide (GO) C) DLS study illustrating the growth of the nano-complex in AGO-doped mixtures of BPEI/5Acl. No such growth of nano-complex was noticed in BPEI/5Acl mixture in absence of AGO-doping. D) FTIR spectra of AGO-doped (PGM1, PGM2, PGM3 and PGM4) gel-materials. E-H) Digital (E, G) and water contact angle images (F,H) of beaded water droplet on decylamine-treated polymeric material in air (E-F) and under hexane (model oil, G-H). I) FTIR spectra of the material before (black) and after (red) post-modification with decylamine.



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Fig. 2. A) The compressive stress-strain curve of AGO embedded polymeric materials (PGM1, PGM2, PGM3, PGM4). B-F) Digital images illustrating the process of manual bending (B) and creasing (C) of AGO-doped, self-standing materials (PGM1, PGM3, PGM4) and the effect (D-F, scale bar: 1 cm) of these treatments on these materials after releasing the manual pressure. G-H) Digital images of rolled (G) and twisted (H) self-sanding superhydrophobic film and (I) contact angle image after application of physical manipulations.

Table 1 accounts the change in mechanical properties in AGO integrated polymeric materials (PGM1, PGM2, PGM3, PGM4).


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Fig. 3. A) Accounts the advancing water contact angle (θAdv.; black line) and water contact angle hysteresis (θHys. grey line) on various AGO-doped polymeric-materials after decylaminetreatment. B-G) FESEM image of AGO-doped materials (PGM1, PGM3 and PGM4) in low magnification (B,D,F scale: 10µm) and in high magnification (C,E,G scale: 5 µm). H-J) Digital images (scale: 1 cm) of superhydrophobic material that were molded in structures of guava leaf (H), dolphin (I), array of wells (J).



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Fig. 4. A-B) Digital images (scale: 1cm) of beaded red-aqueous droplets on the decylaminetreated polymeric-material (PGM4) before (A) and after arbitrary slicing (five pieces, B) of the material. C) FESEM image of the interior of the material (PGM4; scale: 10µm). D-E) Digital image (D; scale: 1cm) and contact angle (E) of beaded water droplets on the sand-paper treated polymeric-material (PGM4) and F) FESEM image of the material (PGM4, scale: 10µm). G-J) Illustrating the selective separation of floating oil droplet from water/air interfaces by the spongy superhydrophobic monoliths.


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Table-2. accounts the effects of various physical and chemical treatments on the anti-wetting property of the material.



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Fig. 5. A-H) Digital images (A, C, E, G; scale bar: 1cm) and contact angle images (B, D, F, H) of beaded water droplet on polymeric gel (post-modified with decylamine) coated glass (A-B), wood (C-D), plastic (E-F), Al-foil (G-H). I-M) Digital images (I-J, L; scale bar: 1cm) and contact angle images are depicting the adhesive tape test, where polymeric coating on glass substrate was brought in contact with adhesive tape surface and applied a load of 200 g (I) for 10 minutes, and then the adhesive tape was peeled off from the polymeric coating, which resulted in transfer of top surface of polymer coating to adhesive surface (L), however, both the freshly exposed interior of the polymeric coating on glass substrate (J-K) and adhesive tape (L-M) are appropriate to repel beaded water droplet extremely. N-Q) Digital images of the various physical manipulations of coated-plastic including bending (N), twisting (O), rolling (P), creasing (Q). R) Digital images (scale bar: 1cm) of beaded water droplet on the coated plastic, after performing the physical manipulations. S-U) Digital image (S-T, scale: 1cm) and θAdv (U) of beaded water (red color aids visual inspection) droplet on bare (S) and coated (T-U) filter paper. V) Digital image (scale: 1cm) of coated filter paper that is with extreme asymmetric (superhydrophilic/superhydrophobic) wettability at two air solid interfaces, where the coated solid/air interface of filter paper displayed superhydrophobicity and bare air/solid interface was with native superhydrophilicity.


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Strategic Formulation of Graphene Oxide Sheets for Flexible Monoliths

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