Noncovalently Functionalized Tungsten Disulfide Nanosheets for

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Non-covalently functionalized tungsten disulfide nanosheets for enhanced mechanical and thermal properties of epoxy nanocomposites Megha Sahu, Lakshmi Narashimhan, Om Prakash, and Ashok M Raichur ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2017 Downloaded from http://pubs.acs.org on April 5, 2017

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ACS Applied Materials & Interfaces

Non-covalently functionalized tungsten disulfide nanosheets for enhanced mechanical and thermal properties of epoxy nanocomposites Megha Sahu†, Lakshmi Narashimhan†, Om Prakash‡, Ashok M. Raichur§†* †

Department of Materials Engineering, Indian Institute of Science, Bengaluru, 560012,

Karnataka, India. ‡Boeing International Corporation India Private Limited, RMZ Infinity, Tower D, 5th Floor, Old Madras Road, Bengaluru, 560001, Karnataka, India. §

Nanotechnology and Water Sustainability Research Unit, University of South Africa, The

Science Campus, Florida Park, 1710 Roodepoort, Johannesburg, South Africa. *

Corresponding author. Email: [email protected] Fax: +91-80-23600472; Tel: +91-80-

22933238

1. Abstract In the present study, noncovalently functionalized tungsten disulfide (WS2) nanosheets were used as toughening agent for epoxy nanocomposites. WS2 was modified with branched polyethyleneimine (PEI) to increase the degree of interaction of nanosheets with epoxy matrix, prevent restacking and agglomeration of the sheets in the epoxy matrix. The functionalization of WS2

sheets

was

confirmed

through

Fourier

transform

infrared

spectroscopy

and

thermogravimetric analysis. The exfoliation of the bulk WS2 was confirmed through X-ray

1 ACS Paragon Plus Environment


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diffraction and various microscopic techniques. The epoxy nanocomposites containing up to 1 wt% of WS2-PEI nanosheets were fabricated. The nanocomposites containing WS2-PEI nanosheets showed remarkable improvement in fracture toughness (KIC). KIC increased from 0.94 MPa m-1/2 to 1.72 MPa m-1/2 for WS2-PEI nanosheets loading as low as 0.25 wt%. Compressive and flexural properties also showed significant improvement as incorporation of 0.25 wt% of WS2-PEI nanosheets resulted in 43% and 65% increase in compressive and flexural strength of epoxy nanocomposites respectively, compared to neat epoxy. Thermal stability and thermo-mechanical properties of WS2-PEI modified epoxy also showed significant improvement. The simultaneous improvement in mechanical and thermal properties could be attributed to good dispersion of WS2-PEI nanosheets in the matrix, intrinsic high strength and thermal properties of the nanosheets and improved interaction of the WS2 nanosheets with the epoxy matrix owing to the presence of PEI molecules on the surface of the WS2 nanosheets. Keywords: Polymer nanocomposites, layered tungsten disulfide, fracture toughness, toughening of epoxy, 2D transition metal dichalcogenides, layered nanomaterials.

2. Introduction Epoxy polymers are known for their excellent mechanical strength, good electrical insulation, low shrinkage, high heat and chemical resistance. Tetrafunctional epoxy polymers are one of the most important thermosetting polymers. These are used as matrix for fiber reinforced composites in aerospace industries. However, the poor resistance to crack propagation in the presence of defects requires the use of reinforcing agents to prevent failure of the structure at low loads. Several approaches have been developed in the past to improve the fracture properties of epoxy polymers such as incorporation of rubber particles1-2, thermoplastic particles3-4 and inorganic


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nanoparticles5-6. However, improvement in mechanical properties is achieved at the cost of weight of the final composite as these fillers improve the mechanical properties of epoxy composites at high loading levels (5-30 wt%). With the continued need of light weight advanced epoxy nanocomposites for aerospace industries, need for toughening agents capable of increasing mechanical and thermal properties at low weight percent is also increasing. In the last decade, 2D layered materials such as graphene and its derivatives have attracted a lot of attention in the field of polymer nanocomposites. This is due to their unique structure and outstanding mechanical properties. Graphene has advantages over other carbon based materials such carbon nanotubes7 and fullerenes8 because of its superior mechanical properties9 and enormous surface area10. Hence, graphene has been explored as a reinforcing agent for various polymers11-17. However, the electrical conductivity of graphene limits its application as a reinforcing agent for polymer nanocomposites where unaltered electrical insulation properties are required. Recently graphene analogues such as molybdenum disulfide (MoS2), tungsten disulfide (WS2) and hexagonal boron nitride have attracted increased attention in the field of polymer research owing to their graphene like properties such as high thermal and mechanical properties18-19. However, unlike graphene, these non-carbon 2D layered materials are semiconducting in nature and are thus suitable for fabrication of electrically insulating polymer nanocomposites. WS2 is a member of the family of transition metal dichalcogenides (TMD). Like other TMD materials, WS2 has layered structure with W layer sandwiched between two layers of S atoms. Each layer of WS2 is stacked together by van der Waal’s forces. 2D WS2 nanosheets have attracted considerable attention in the field of nanoelectronics due to the possibility of tuning the band gap by varying the number of WS2 layers. Recently, WS2 nanosheets have been explored in the field of light emitting transistors20, photodetectors21, photovoltaics22 and solar cells23. WS2



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nanosheets have been explored as potential reinforcing agents for thermoplastic polymers such as polyurethane24 and poly(vinyl alcohol)25. Until now most of the studies have focused on use of few layered MoS226-29 and hexagonal BN30-33 for enhanced thermal and mechanical properties of polymers. However, WS2 in its closed cage structures such as fullerene like nanoparticles34 and inorganic nanotubes35 has been explored extensively for improving thermal and mechanical properties of epoxy polymers. A number of studies have reported improvement in the mechanical and thermal properties of epoxy polymers with loading of WS2 nanoparticles or nanotubes below 1 wt%34, 36-39. However, 2D WS2 nanosheets have superior properties compared to WS2 fullerene like and nanotubes form. The Young’s modulus of exfoliated WS2 (272 ± 18 GPa)19 is considerably higher than that of bulk WS2 (150 GPa)40 and WS2 nanotubes (150-170 GPa)41. Monolayered WS2 sheets have been shown to have high intrinsic strength with 2D modulus of 177 ± 12 N/m19, which is half of that of graphene (349 ± 12 N/m).9, 42 Flexible and strong 2D WS2 nanosheets hold great potential as reinforcing agent for epoxy nanocomposites with unaltered insulating properties. Various methods have been reported to obtain monolayered or few layered WS2 such as metal ion intercalation assisted sonication43, chemical vapor deposition route22,

44-47

, sulfurization

process48-49 and liquid exfoliation50. However, WS2 is extremely chemically inert51 and hence the nanosheets obtained from these methods have to be further modified to impart affinity toward the epoxy matrix. Cheng et al.52 modified the surface of WS2 nanosheets with PEG molecules for better therapeutic effects in animals for photothermal therapy. Similarly, the functionalization of WS2 sheets is expected to improve the interfacial interaction of WS2 nanosheets with epoxy matrix. The incorporation of surface functionalized WS2 nanosheets as reinforcing agents for epoxy polymers is yet to be explored.


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To the best of our knowledge, till date there are no reports on the use of pristine or noncovalently modified WS2 nanosheets as toughening agents for epoxy polymers. Here, we report surface modified WS2 nanosheets as toughening agent for epoxy polymers. We have simultaneously exfoliated and modified the surface of WS2 nanosheets with amine rich polymer PEI. PEI has branched structure which reduces the agglomeration of WS2 nanosheets in the polymer matrix and increases the interaction of WS2 nanosheets with epoxy matrix. The amine groups of PEI are capable of forming chemical bonds with epoxy groups of the epoxy polymer. The aim of the work is to increase the affinity of the WS2 sheets towards the epoxy matrix and prevent the agglomeration of the sheets to achieve uniform distribution for better matrix-filler interaction for improved mechanical and thermal properties of epoxy polymers.

3. Experimental 3.1 Materials Powdered tungsten disulfide (WS2) and branched polyethyleneimine (Mw = 25000, PEI) were purchased

from

Sigma-Aldrich

methylenebisbenzenamine

(TGDDM,

Co.

(USA).

Araldite

Epoxy,

MY

721)

N,N,N',N'-tetraglycidyl-4,4'and

curing

agent

4,4′-

diaminodiphenylsulfone (DDS, Aradur 9664) were purchased from Huntsman Advanced Materials (Huntsman Corporation, USA). Double autoclaved Milli-Q water (Millipore, Billerica, MA, USA) produced in lab was used in all the experiments.

3.2 PEI assisted exfoliation of WS 2 In a typical experiment, 1 g of WS2 was added to 100 ml of 2% aqueous solution of PEI. The suspension was subjected to bath sonication for 8 h. The suspension was centrifuged at 7000 5 ACS Paragon Plus Environment


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RPM for 15 minutes to allow the sedimentation of the unexfoliated WS2. The supernatant contained the exfoliated WS2-PEI nanosheets which were further centrifuged at 18000 rpm to allow the sedimentation of the WS2-PEI nanosheets which is further washed with water several times and dried at 50 °C for 24 h, Figure 1.

3.3 Preparation Epoxy/WS 2 -PEI nanocomposites WS2-PEI nanosheets were dispersed in ethanol (10 mg/ml) with the aid of sonication for 2 h. WS2-PEI nanosheets suspension was mixed with epoxy which was preheated to 80 °C to reduce its viscosity. The mixture was stirred at high speed to make it homogeneous. Subsequently, the mixture was kept in vacuum oven for 12 h at 90 °C to remove any traces of solvent. The mixture was removed from oven and heated up to 120 °C, following which stoichiometric amount of curing agent was added. The mixture was stirred at high speed to distribute the curing agent uniformly. Post mixing, the mixture was degassed in vacuum oven at 90 °C for 2 h to remove the trapped air. The mixture was finally poured in preheated molds and cured at 150 °C for 2 h, 180 °C 2 h and 200 °C for 2 h. Neat epoxy and epoxy nanocomposites with 0.1, 0.25, 0.5 and 1 wt% of WS2-PEI nanosheets were fabricated using the above processing conditions.

4. Characterization The structure and morphology of WS2-PEI nanosheets was studied using a FEI Sirion XL30 FEG scanning electron microscope (SEM) (FEI, Oregon, USA). The WS2-PEI nanosheets were dispersed in ethanol by bath sonication for 30 minutes. The suspension was drop casted on a freshly cleaned silicon substrate. The samples were coated with Au powder of 10 nm thickness to improve conductivity.


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The morphology of the WS2-PEI nanosheets was studied using transmission electron microscopy (TEM) (JEOL 2000 FX-II TEM, JEOL, Akishima, Tokyo, Japan). WS2-PEI nanosheets were suspended in ethanol and were drop casted on carbon coated Cu grid. The samples were dried overnight and desiccated for 12 h prior to TEM analysis. Average number of layers in exfoliated WS2-PEI nanosheets was found out using atomic force microscopy on a NanoWizard 3 AFM (JPK Instruments AG, Berlin, Germany). The suspension of the WS2-PEI nanosheets in ethanol was drop casted on freshly cleaned silicon substrate and dried. The AFM analysis was conducted in tapping mode. FTIR spectra of bulk WS2 and WS2-PEI nanosheets were recorded on Thermo-Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific, Waltham, Massachusetts, USA) in the 400- 4000 cm-1 region. Confocal microscopy (Zeiss LSM 710, Carl Zeiss Microimaging Inc., Thornwood, USA) was used to study the photoluminescence properties of both bulk and exfoliated WS2.The powdered samples of the bulk WS2 and exfoliated WS2-PEI nanosheets were mounted on a glass slide and were imaged covering the wavelength range 400 to 700 nm. X-ray powder diffraction was performed using X-Pert PRO (PANalytical, Almedo, The Netherlands) equipped with Cu K α tube. Thermo-gravimetric analysis was performed on a NETZSCH STA 409 (NETZSCHGerätebau GmbH, Selb, Germany) under air flow from room temperature to 800 °C at a heating rate of 10 °C min-1. Weight of samples required was 5 to 10 mg. Dynamic mechanical analysis of epoxy nanocomposites was conducted on Gabo eplexor 500N (NETZCSH Gabo Instruments GmbH, Ahlden, Germany) using samples of three-point



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bend geometry with dimensions 5 × 2 × 45 mm3. Temperature was scanned from 30 °C to 330 °C at a rate of 3 °C min-1 with a frequency of 1 Hz. Compression tests were performed according to ASTM D695 using Zwick/Rowell Z100 machine (Zwick Roell, Ulm, Germany). Five replica of each sample with the cylindrical geometry (diameter 6.9 mm, length 10.4 mm) were tested and the average values were reported. The cross-sectional area of each sample was polished using P4000 grade emery paper to ensure absence of any friction between the sample surface and the compression clamps. The flexural properties of the nanocomposites were determined using Zwick/Rowell Z100 (Zwick Roell, Ulm, Germany) machine according to ASTM D790. Five replicas of each composition were tested at a strain rate of 2 mm min-1 and the average values were reported. Samples with dimensions 48 × 3 × 12 mm3 were used. Fracture toughness of the nanocomposites was calculated by performing single edge notch bend tests according to ASTM D5045, using Zwick/Rowell Z100 (Zwick Roell, Ulm, Germany) machine. Samples with dimensions 6 ×12 × 48 mm3 were cast molded. A natural crack was induced in the samples by tapping a razor blade dipped in liquid N2 over the machined notch. The fracture toughness KΙC was calculated using the following formula KΙC =

Pmax

BW1⁄2

f( a⁄W )

(1)

Where Pmax is the maximum load of the load-displacement curve, W is the width of the sample, B is the thickness of the three point bend sample, and a is the length of the pre-crack. f (a/W) is a geometry dependent function given by following equation. f W = 6x1⁄2 a

[1.99-x1-x2.15-3.93x+2.7x2 ] 1+2x (1-x)

3⁄2

(2)

Six replica of each composition were tested and the average values were reported. The samples were loaded till complete fracture occurred. 8 ACS Paragon Plus Environment

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5. Results and Discussion 5.1 Noncovalent functionalization of WS 2 and characterization of WS 2 -PEI nanosheets Branched PEI was used to modify the surface of WS2 nanosheets. PEI is highly reactive and contains large number of amine groups. Amine functionalized polymers have been shown to improve the toughening effect in case of graphene toughened epoxy polymers53-56. PEI helps in better interfacial interaction between epoxy and WS2 nanosheets57 as amine groups can form chemical bonds with the epoxy groups of the matrix. The branched structure of PEI helps in exfoliation and better distribution of WS2 nanosheets in epoxy matrix by preventing agglomeration. The chemical compatibility of PEI with epoxy and intrinsic strength of WS2 combined together fulfills the essential requirement of better interfacial interaction and formation of stronger interface between filler and matrix, Figure 1.



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Figure 1: Schematic illustration of exfoliation of WS2 in the presence of PEI molecules and fabrication of epoxy nanocomposites modified with WS2-PEI.

The morphology of WS2 before and after PEI assisted exfoliation was studied through SEM. Figure 2 (a) and (b) show the SEM micrographs of bulk WS2 and WS2-PEI nanosheets. It can be seen that bulk WS2 has rigid and thick crystals (Figure 2 a) and exfoliated WS2-PEI nanosheets (Figure 2 b) are highly flexible as the crumpling of sheets is evident. The exfoliated WS2-PEI show thinner sheet compared to bulk WS2. The size of the nanosheets appears larger than that of


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bulk WS2. This could be due to overlapping of thin nanosheets that covers a larger area. The morphology of WS2-PEI sheets was further confirmed by TEM analysis. The TEM micrograph in Figure 2 (c) reveals individual transparent and exfoliated nanosheets of WS2-PEI. The average lateral size of the sheets was measured to be 260 ± 45 nm. As shown in Figure 2 (d) and (e) atomic force microscopy analysis revealed the thickness of exfoliated WS2 to be ~3 nm. Monolayer of WS2 has been reported to be 0.7 nm thick58 hence WS2-PEI nanosheets obtained are ~4 layers thick.

Figure 2: SEM micrograph of (a) bulk WS2 platelets, (b) WS2-PEI nanosheets. (c) TEM image of exfoliated WS2-PEI nanosheets. (d) AFM image of the exfoliated WS2-PEI nanosheets. (e) Height profile of the exfoliated WS2-PEI nanosheets.



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FT-IR spectroscopy was used to confirm the successful noncovalent modification of the WS2 nanosheets. Figure 3 (a) shows the FT-IR spectra of bulk WS2 and WS2-PEI nanosheets. Compared to bulk WS2, newly appeared absorption peaks at 3612 and 1647 cm-1 in the spectrum of WS2-PEI nanosheets are attributed to N-H stretching and bending vibration of amine groups of PEI, respectively. The absorption peak at 3057 cm-1 is ascribed to the presence of C-H stretching vibration and peak at 1454 cm-1 is associated with C-H bending vibration of hydrocarbon backbone of PEI molecules. The broad absorption band with the range 1350-1000 cm-1 is attributed to C-N stretching vibration of amine groups of PEI. FT-IR results are indicative of successful noncovalent functionalization of PEI molecules onto WS2-PEI nanosheets. TGA analysis was carried out to further confirm the noncovalent modification of WS2. Figure 3 (b) shows the TGA plots of bulk WS2 and WS2-PEI nanosheets. Bulk WS2 showed high thermal stability as the weight loss is less than 10% at temperature as high as 800 °C. Compared to bulk WS2, WS2-PEI nanosheets showed larger weight loss for the same temperature range. The temperature for 5% weight loss (T-5%) was observed at 384 °C for WS2-PEI which is ~100 °C lower than that observed for bulk WS2 (480 °C). The reduction in T-5% could be attributed to the presence of PEI molecules which decompose at lower temperature due to its organic nature.


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Figure 3: (a) FTIR spectra and (b) TGA curves of bulk WS2 and exfoliated WS2-PEI nanosheets.

The difference in the crystal structure of bulk and exfoliated WS2 nanosheets was studied by XRD analysis. XRD spectra of bulk WS2 and WS2-PEI nanosheets are compared in Figure 4. The intensity of the characteristic peak (002) is significantly reduced for WS2-PEI compared to bulk WS2. This is due to reduction in the crystallite size during exfoliation due to which probability of a set (002) planes satisfying Bragg’s condition reduces. Additionally (004), (103), (006), (110) and (112) peaks observed for bulk WS2 show significant reduction in the intensity for WS2-PEI nanosheets. XRD results also indicate successful PEI assisted exfoliation of WS2.



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Figure 4: XRD spectra of bulk WS2 and exfoliated WS2-PEI nanosheets.

Confocal microscopy was performed to confirm the exfoliation of WS2. The emission photoluminescence images of bulk and exfoliated WS2 are shown in Figure 5. It is evident that bulk WS2 does not show photoluminescence whereas exfoliated WS2-PEI shows excitation dependent photoluminescence. This is due to the changes in the electronic properties of the WS2 as a function of number of layers in the crystal. WS2 shows a change in electronic structure from indirect bandgap for bulk form to direct bandgap for few layered exfoliated form which allows photoluminescence. The quantum size effect and polydispersity of the nanosheets leads to photoluminescence from blue to far red region. The results confirm the exfoliation of bulk WS2 to form few layered WS2-PEI nanosheets.


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Figure 5: Confocal images of bulk WS2 and exfoliated WS2-PEI nanosheets excited from 400 to 700 nm range (Blue to Far red).

5.2 Thermogravimetric analysis The thermal stability of neat epoxy and epoxy nanocomposites was evaluated by thermogravimetric analysis. Figure 6 shows the TGA curves of Epoxy and Epoxy/WS2-PEI nanocomposites, the analyzed data is listed Table 1. The thermal degradation shows similar behavior for all the epoxy nanocomposites as the major weight loss takes places in the temperature range 300-450 °C due to degradation of epoxy network. T-5% shows an increase of 35 °C or more for each composition of WS2-PEI nanosheets modified epoxy nanocomposites. Highest increase in T-5% was observed for epoxy nanocomposite containing 0.1 wt% of WS2-PEI nanosheets (45 °C). In addition, char yield of the all the nanocomposites are higher than that of neat epoxy. Char yield showed gradual enhancement with continuous increase in WS2-PEI nanosheets content. It was observed that nanocomposite with 1 wt% of WS2-PEI nanosheets has maximum char yield (43 wt%) compared to neat epoxy (12 wt%). Thus the results show that WS2-PEI nanosheets significantly enhance the thermal stability of the epoxy nanocomposites. 15 ACS Paragon Plus Environment


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The excellent improvement in thermal properties of the nanocomposites could be attributed to barrier effect of WS2 nanosheets owing to its inherent strength and large aspect ratio. The presence of WS2 sheets restricts the supply of oxygen from the gas phase to bulk phase of the matrix hence delays the thermal degradation process. This is reflected in improved values of T-5% for WS2-PEI modified epoxy nanocomposites. The surface functionalization with PEI helps in improved barrier effect as PEI helps in enhancing the interfacial interaction of WS2 with epoxy matrix due to chemical bond formation between amine groups of PEI and terminal epoxy groups of epoxy matrix. The presence of WS2-PEI also improves effective volume of WS2 in the epoxy matrix. The presence of PEI molecules helps in uniform distribution of WS2 nanosheets by preventing agglomeration owing to their bulky nature.

Figure 6: TGA plots of Epoxy and Epoxy/WS2-PEI nanocomposites.

5.3 Dynamic mechanical analysis Dynamic mechanical analysis (DMA) was conducted for neat epoxy and epoxy nanocomposites modified with WS2-PEI nanosheets to study the thermomechanical behavior.


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Tetrafunctional epoxy are mainly used in the structures employed for high temperature (150 to 200 °C) applications thus the stiffness of the nanocomposites should remain unaltered at high temperatures. Hence any reinforcing agent that improves the mechanical properties without compromising on the thermomechanical properties of the tetrafunctional epoxy nanocomposites is the most suitable candidate for toughening tetrafunctional epoxy polymers. This criterion is fulfilled if the intrinsic thermal stability of the reinforcing agent is significantly high. The thermal stability of WS2 in its bulk as well as exfoliated form is extremely good as it shows no oxidation till 1000 °C. Figure 7 shows the storage modulus and tan δ curves as a function of temperature for neat epoxy and epoxy nanocomposites with WS2-PEI nanosheets. Clearly the storage modulus of all the WS2-PEI nanosheets modified epoxy nanocomposites is much higher than neat epoxy. Incorporation of 0.1 wt% of WS2-PEI nanosheets shows 55 % increase of E′ at 150 °C, 49 % increase of E′ at 200 °C and 91 % increase of E′ at 250 °C. Incorporation of 0.25 wt% of WS2-PEI nanosheets leads to 44 % increase of E′ at 150 °C, 34 % increase of E′ at 200 °C and 63 % increase of E′ at 250 °C. The complete set of data is listed in Table 1. The storage modulus at temperatures below glass transition temperature (Tg) should be high enough to maintain the rigidity of the structural component. The improvement obtained in storage modulus by incorporation of WS2-PEI nanosheets at 250 °C is significantly high (60-90%) which is essential for epoxy nanocomposites used for high temperature applications. The improvement in thermomechanical properties could be attributed to the robust nature of the WS2-PEI nanosheets as the mechanical properties of the nanosheets is much higher than the epoxy matrix. The modified WS2 shows barrier effect by restricting the segmental motion of epoxy chains. Other reasons could be the improved interaction of nanosheet with epoxy matrix and dispersion of the nanosheets in the epoxy matrix owing to the presence of PEI molecules. The presence of



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branched and bulky PEI molecules leads to increase in the effective volume of the WS2 nanosheets in the matrix due to better exfoliation of the bulk WS2 platelets. The chemical bond formation between amine groups of WS2-PEI nanosheets and epoxy matrix leads to stronger interface which contributes to the barrier effect of the WS2-PEI nanosheets. However, the extent of improvement observed in the storage modulus shows a sharp decrease with WS2-PEI nanosheets loading beyond 0.25 wt%. This could be due to reduction in the state of dispersion due to formation of agglomerate particles. The glass transition temperature (Tg) was obtained from the peak values of tan δ versus temperature curves, Figure 7 (b). Tg shows similar values (~281 °C) for neat epoxy and modified epoxy nanocomposites as variation of only ±3 °C was observed. The results show that epoxy nanocomposites modified with WS2-PEI nanosheets have much higher thermomechanical properties compared to neat epoxy.

Table 1: Thermomechanical properties of the Epoxy and Epoxy/WS2-PEI nanocomposites.

Sample

Ta-5% (°C)

Char yield (%)b

Tgc (°C)

Storaged modulus at 150 °C (MPa)



Epoxy

298

12.0

281.0

2482

2435

1403

Epoxy/WS2-PEI-0.1 wt%

343

26.9

282.5

3838

3633

2672


335

41.2

284.3

3580

3265

2286


338

41.3

280.8

3030

3011

1964


336

43.2

282.4

3047

2807

2056

a

T-5%-Temperature for 5% weight loss. At 800 °C c Tg-Glass transition temperature. d DMA results of the Epoxy and Epoxy/WS2-PEI nanocomposites. b


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Figure 7: (a) Storage modulus and (b) tan δ as a function of temperature of Epoxy and Epoxy/WS2-PEI nanocomposites.

5.4 Compressive and flexural properties Figure 8 shows the compressive and flexural properties of neat epoxy and the epoxy nanocomposites modified with WS2-PEI nanosheets. The incorporation of WS2-PEI nanosheets in the range 0.1-1 wt% shows improvement in the compressive and flexural strength of epoxy nanocomposites compared to neat epoxy. The extent of improvement varies with the loading content of WS2-PEI nanosheets. Figure 8 (a) shows the representative stress versus strain curves for all the fabricated epoxy nanocomposites. It is evident that the strain at break shows two fold increase for epoxy nanocomposites modified with low loading percent (below 1 wt%) of WS2PEI nanosheets compared to neat epoxy. Figure 8 (b) compares the compressive strength of neat epoxy and epoxy nanocomposites modified with WS2-PEI nanosheets. Highest increase in compressive strength (43%) was observed for epoxy nanocomposites containing 0.25 wt% of WS2-PEI nanosheets, compared to neat epoxy. The extent of improvement was maintained for nanocomposites with 0.5 wt% of WS2-PEI nanosheets however the improvement reduces to 33% at 1 wt% loading of filler. Similar trend was observed for flexural properties of WS2-PEI 19 ACS Paragon Plus Environment


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modified epoxy nanocomposites. The maximum increase (65%) in the flexural properties was observed for Epoxy/WS2-PEI (0.25 wt%) nanocomposites which was maintained for epoxy with 0.5 wt% of WS2-PEI nanosheets. The significant improvement in the compressive and flexural strength is attributed to (i) intrinsic high mechanical strength of WS2-PEI nanosheets, (ii) strong interaction between WS2-PEI nanosheets and epoxy matrix owing to the presence of PEI molecules containing high density of amine groups, capable of making chemical bonds with epoxy terminal groups and (iii) better dispersion of WS2-PEI nanosheets in the epoxy matrix due to better exfoliation of the nanosheets due to bulky nature of PEI. The WS2-PEI nanosheets and epoxy share a larger interface due to uniform distribution of the nanosheets in the epoxy matrix. The larger and stronger interface helps in better load transfer from epoxy polymeric chains to robust WS2 nanosheets which enhance the reinforcing effect of WS2-PEI. The uniform distribution of the nanosheets in epoxy matrix could be attributed to the branched structure and high molecular weight of PEI which acts as barrier between sheets which help in prevention from agglomeration of WS2 nanosheets. However, the flexural strength shows a prominent dip in the extent of improvement (27%) for nanocomposites containing 1 wt% of the WS2-PEI nanosheets. The decrease in the improvement could be due to poor dispersion as result of formation of agglomerates of the WS2-PEI nanosheets at higher loading levels. The formation of agglomerates leads to interference in epoxy crosslinking reaction during curing and acts as a defect in the structure.


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Figure 8: (a) Typical compression stress versus strain curves and (b) compressive strength of the Epoxy and Epoxy/WS2-PEI nanocomposites as a function of WS2-PEI nanosheets content. (c) Flexural Strength of Epoxy and Epoxy/WS2-PEI nanocomposites as a function of WS2-PEI nanosheets content.

5.5 Fracture properties Mode I fracture toughness tests were conducted to study the fracture properties of the epoxy nanocomposites. Figure 9 (a) shows the representative load versus displacement curves from single edge notch bend tests for neat epoxy and modified epoxy nanocomposites. It can be observed that modified epoxy nanocomposites are able to withstand higher load compared to



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neat epoxy. Figure 9 (b) shows the comparative plot of fracture toughness (KIC) of Epoxy and Epoxy/WS2-PEI nanocomposites. The nanocomposites containing WS2-PEI nanosheets show higher values of KIC compared to neat epoxy. At WS2-PEI nanosheets loading of 0.25 wt% the nanocomposites show the highest increase in KIC (from 0.94 MPa m-1/2 to 1.72 MPa m-1/2) compared to neat epoxy. The excellent improvement in the fracture properties at low loading of WS2-PEI nanosheets could be attributed to good chemical compatibility of the WS2 nanosheets with the epoxy matrix owing to the presence of high density of amine groups of PEI, capable of forming chemical bonds with the matrix. Another contributing reason is better load transfer during mechanical loading of the samples due to the presence of stronger interface between filler and the matrix. The uniform distribution of the nanosheets in the matrix owing to the presence of PEI molecules over the surface of WS2 prevents agglomeration of nanosheets which improves the toughening effect of WS2-PEI nanosheets. Intrinsic higher strength of WS2-PEI nanosheets compared to epoxy matrix makes the final nanocomposites stronger than the neat epoxy. Hence greater extent of interaction between WS2 nanosheets and epoxy due to the presence of PEI polymer chains could be responsible for enhanced fracture properties of the WS2-PEI nanosheets modified epoxy nanocomposites. The extent of improvement shows a decrease for loading of WS2-PEI nanosheets at 0.5 wt% and beyond. Similar trend was observed for compressive and flexural properties. This could be due to onset of aggregation of the WS2-PEI nanosheets at higher loading level which reduces the effective volume of the WS2-PEI nanosheets in the matrix and also reduces the extent of interaction between filler and the matrix.


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Figure 9: Fracture toughness of neat epoxy and Epoxy/WS2-PEI nanocomposites-(a) representative load versus displacement curves and (b) KIC values.

5.6 Fractographic studies Fractured surfaces were investigated under scanning electron microscope to further investigate the interaction between filler and the matrix. As shown in Figure 10 (a) and (b), fractured surface of neat epoxy show typical brittle fracture characteristic with smooth and featureless morphology. The smooth cross-section surfaces of neat epoxy indicate fast crack propagation. In contrast, Epoxy/WS2-PEI nanocomposites show different features with distributed WS2-PEI nanosheets. Uniform distribution of the layered material in the epoxy matrix leads to enhanced effective volume of filler and improved interfacial interaction with the matrix. As can be seen in Figure 10 (c-h), at low weight percent of WS2-PEI nanosheet uniform distribution is achieved owing to the presence of PEI molecules on WS2 nanosheets surfaces which helps in better exfoliation and interaction with the epoxy matrix. This is one of the reasons for high degree of improvement in fracture properties. At loading of WS2-PEI as high as 1 wt%, clear agglomeration of the sheets can be observed in Figure 10 (i) and (j) as many small sheets have agglomerated and resulted in large crumpled sheet. The large sized agglomerates hinder in 23 ACS Paragon Plus Environment


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crosslinks formation during the curing process hence show a lower degree of improvement in the mechanical properties.


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Figure 10: SEM images of fractured surfaces of (a, b) Epoxy, (c, d) Epoxy/WS2-PEI -0.1wt%, (e, f) Epoxy/WS2-PEI-0.25wt%, (g, h) Epoxy/WS2-PEI-0.5wt% and (i, j) Epoxy/WS2-PEI-1.0wt%.

6. Conclusions In this paper, we have demonstrated that WS2 nanosheets can be used to enhance fracture properties of the epoxy nanocomposites. Incorporation of WS2 nanosheets improves mechanical and thermal properties simultaneously. Surface modification of WS2 nanosheets with PEI molecules was carried out in order to enhance the interaction of the WS2 nanosheets with the epoxy matrix. The results showed that 2D form of WS2 has great potential as reinforcing agent for epoxy polymers. The fracture toughness increased from 0.94 MPa m-1/2 to 1.72 MPa m-1/2 for epoxy nanocomposites with WS2-PEI nanosheets loading as low as 0.25 wt%. Compressive and flexural properties also showed significant improvement at the similar loading levels. The storage modulus at 250 °C shows 60-90 % compared to neat epoxy at loading of WS2-PEI nanosheets up to 0.5 wt%. Thermal stabilities of the nanocomposites also showed excellent improvement as T-5% increase by 37 °C for 0.25 wt% loading of WS2-PEI nanosheets. The optimum loading of WS2-PEI nanosheets was observed to be 0.25 wt% as the improvement in the properties shows decrease beyond this loading level.

Notes The authors declare no competing financial interest.


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Acknowledgements The authors are grateful for the financial support from Boeing Company. The authors wish to acknowledge CSIR for providing Megha Sahu with a Senior Research Fellowship (SRF). The authors also acknowledge the Advanced Facility for Microscopy and Microanalysis and Centre for Nano Science and Engineering at Indian Institute of Science for providing access to characterization facilities.

Supporting Information Low magnification SEM micrographs of bulk WS2 and exfoliated WS2-PEI nanosheets, high magnification confocal images of exfoliated WS2-PEI nanosheets, thermomechanical properties (T-10% and storage modulus at 100 °C) of Epoxy and Epoxy/WS2-PEI nanocomposites

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Table of Contents Graphic



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Graphical Abstract. Schematic illustration of exfoliation of WS2 in the presence of PEI Branched PEI was used to modif 406x184mm (96 x 96 DPI)

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Noncovalently Functionalized Tungsten Disulfide Nanosheets for

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