PVB nanocomposites of improved thermo-mechanical and tribological properties

Materials Chemistry and Physics xxx (2016) 1e10

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Inorganic fullerene-like IF-WS2/PVB nanocomposites of improved thermo-mechanical and tribological properties Danica Simi c a, Dusica B. Stojanovi c b, *, Aleksandar Kojovi c b, Mirjana Dimi c a, c b, Radoslav Aleksi cb Ljubica Totovski a, Petar S. Uskokovi a b

Military Technical Institute, Ratka Resanovica 1, 11132 Belgrade, Serbia University of Belgrade, Faculty of Technology and Metallurgy, 11120 Belgrade, Serbia

h i g h l i g h t s
Poly(vinyl butyral)/tungsten disulfide nanocomposites were examined.
Different solvents and deagglomeration methods affect the properties of composites.
Nanoindentation and scratch test, PSD, SEM, DSC and DMTA were analyzed.
Thermo-mechanical and antifriction properties of composite material are improved.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 June 2016 Received in revised form 31 August 2016 Accepted 24 September 2016 Available online xxx

The subject of this research is to explore the possibility of preparation of nanocomposite material of improved thermo-mechanical and tribological properties, using inorganic fullerene-like tungsten disulfide nanostructures (IF-WS2) as reinforcement in poly(vinyl butyral) (PVB). This paper also reports investigation of the effects of using different solvents in preparation of PVB/IF-WS2 nanocomposite on the thermo-mechanical behavior of the resulting material. PVB was dissolved in ethanol, isopropanol, nbutanol and ethyl acetate. IF-WS2 nanoparticles were added to these PVB solutions and dispersed by different deagglomeration techniques. Samples were dried and thin films were obtained. Their microstructure and the quality of IF-WS2 dispersion and deagglomeration in PVB matrix was analyzed by scanning electron microscope (SEM). The reinforcing effect of IF-WS2 is examined by determining hardness, reduced modulus of elasticity and coefficient of friction, by nanoindentation and nanoscratch test, in terms of the different solvents applied in preparation of the samples, mode of stirring and different contents of IF-WS2. The glass transition temperature (Tg) was determined for the prepared samples using differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMA). Storage modulus and mechanical loss factor were observed in a defined temperature range using DMA. © 2016 Elsevier B.V. All rights reserved.

Keywords: Tungsten disulfide Poly(vinyl butyral) Deagglomeration Solid lubricant Scratch test Nanoindentation

1. Introduction Composites with nanofillers are promising materials that combine the advantages of the matrix (usually a polymer) and the fillers. This work represents investigation of composites made of poly(vinyl butyral) - PVB, and inorganic fullerene-like nanoparticles of tungsten disulfide - IF-WS2 of particle sizes in the range of 40e300 nm, quasi-spherical shape, closed-cage layered structure, and chemically inert. PVB resin is a thermoplastic polymer

* Corresponding author. E-mail address: [email protected] (D.B. Stojanovi c).

which has wide application due to its excellent properties [1]: well soluble in alcohols and many other organic solvents, fast drying, fast solvent release and low solvent retention, good film formation, transparent and colorless, good barrier properties, tough polymer with excellent flexibility, broad compatibility with modifying resins and additives, non toxic and low odor, good adhesion to many substrates and strong binding, impact resistance, good tensile strength and elasticity, freezing and aging resistance. Due to these properties, there are many applications of PVB: safety glass, metal primers and coatings, printing inks, temporary binders (ceramics), ballistic protection [1e3]. Transition metal dichalogenides are well known for their solid

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Please cite this article in press as: D. Simic, et al., Inorganic fullerene-like IF-WS2/PVB nanocomposites of improved thermo-mechanical and tribological properties, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.060

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D. Simic et al. / Materials Chemistry and Physics xxx (2016) 1e10

lubricating behavior, even for high-temperature applications, and they can be used as a multifunctional reinforcement for improvement in thermal and mechanical properties of polymers [4e6]. Compared to MoS2, WS2 exhibits higher thermal stability and higher maximum operating temperature [5]. For the first time, Tenne and coworkers researched inorganic fullerene-like materials other than carbon in 1992 [7], and tungsten disulfide was among them. The metal sulfides bond into a sheet which forms nearly spherical, hollow shape. These structures have properties that can be exploited in many applications. One of the most common uses for inorganic fullerenes is, as mentioned, as solid lubricants [8e11]. Each of the layers forming the fullerene is loosely attached to the layers above and below by secondary bonding with covalent bonding in plane. The application of shear stress will cause a surface layer to be sloughed off preventing wear on the lubricated component. The natural spherical shape of the particles causes them to function as nanometric ball bearings rolling with the motion of the components. The excellent mechanical properties of the fullerene prevent them from flattening and breaking down as they roll until a number of surface layers have been removed [7,11,12]. It has also been shown that multilayer tungsten disulfide has outstanding shock resistance properties superior to those of even carbon nanotubes. In contrast to organic (carbon-based) fullerenes, IF-WS2 is easier and much less expensive to produce, as well as chemically stable and much less reactive and less flammable. Organic fullerenes are highly toxic, unlike IF-WS2, as most other inorganic fullerenes [11e13]. Inorganic fullerene IF-WS2 nanoparticles, and their potential usage as fillers in polymer matrix to produce nanocomposites with improved mechanical properties was subject of a small number of researches. Small amount of IFWS2 gave great results in reinforcement of nylon-12 and nylon-6 [14e16]. The combination of PPS/SWCNT-PEI with inorganic fullerene-like tungsten disulfide (IF-WS2) nanoparticles was earlier examined and improvement in the thermal properties of PPS/ SWCNT-PEI nanocomposites was observed with the addition of IF-WS2. As previously observed in PEEK hybrid systems, the dispersion, morphology and thermal properties of PPS/SWCNT nanocomposites could be enhanced by the introduction of small amounts of IF-WS2 [17]. This filler also provided significant improvement of mechanical, tribological and rheological properties in isotactic polypropylene (iPP) [18]. WS2 fillers have also provided significant reduction of the wear rate and friction coefficient of hybrid PTFE/Kevlar fabric composites [19]. Since this kind of nanoparticles often comes aggregated, it is important to emphasize that there is a high spread of the friction results of the lubricants containing IF-WS2 nanoparticles with the size of IF-WS2 aggregates in the lubricant. Their influence on the friction and wear were studied in oil-based lubricants, and it was demonstrated that the repeatability of the antifriction results depends on the homogeneity of the dispersion, of the size of the IFWS2 nanoparticles in the lubricant and the size of the IF-WS2 aggregates [20]. Deagglomeration of nanoparticles by ultrasonic irradiation is very often applied technique, already investigated and prooved to have an impact on improving reinforcing effect of the fillers in composites, leading to enhanced mechanical performance of the composites [21e23]. High intensity ultrasound is used to very effectively disperse powder in a liquid, but it is more complex to achieve dispersion of nanoparticles in high viscosity polymer solutions. We may find very different or even opposite results about the optimum processing conditions of nanoparticle dispersion in polymer solutions, like in epoxy resin [24]. The dispersion quality is usually characterized from transmission electron microscopy or scanning electron microscope image analysis or by measuring the particle size in diluted suspensions. The cluster size in suspensions can be affected by various parameters including particle content,

colloidal stabilization (electrical charges, absorbed surface layers, polymer interaction) [21]. The influence of the main parameters of ultrasonication such as time, power and irradiation modes (continuous, pulsed) on the cluster size of different nanoparticles was investigated in low concentration suspensions in different solvents for polymers such as poly(vinyl alcohol) and polyvinyl butyral [21e26]. Sonication method and duration were optimized through examination of behavior of MoS2, IF-WS2 and BN [13,27]. Nanomechanical and nanotribological properties of polymer composites prepared by these techniques (steering and ultrasonic irradiation) have been studied using the nanoindentation and nanoscratch technique [26,28]. The high-intensity ultrasonic processor was used in examinations of the effects of agglomerated versus deagglomerated multi-walled carbon nanotubes on the thermal and mechanical characteristics of polyethylene oxide, and significant differences were observed [28]. Generally, observing the effect of ultrasonic duration and amplitude it is established that longer ultrasonic irradiation is better for nanofluid preparation, and that higher sonicator amplitude is better for proper dispersion of nanoparticles. Different solvents are often used in this dispersion and deagglomeration methods, and there is also a significant influence of the chemical structure of the used solvent, polymer and nanoparticles. When working with fullerene-like nanoparticles, the aim is to achieve certain exfoliation of the layers. The sonication of the layered compounds in solvents generally gives few-layer nanosheets with lateral dimensions of a few hundred nanometers [29], and taking into account the high surface energy of these layered inorganic materials, the success of the dispersion depends on surface tension and on solubility parameters of the used solvent and polymer (Hansen and Hildebrand solubility parameters) [30,31]. Hansen Solubility Parameters are useful in prediction of the molecular affinities, solubility and solubility-related phenomena. The idea is based on the concept of “like dissolve like” as each molecule is assigned with three parameters following attractions forces: dispersion forces (dD), permanent dipole-permanent dipole forces (dP), and hydrogen bonding (dH) [32]. The goal of this work is to produce PVB/IF-WS2 nanocomposite of enhanced mechanical properties, i.e. to reinforce PVB by adding a small quantity of IF-WS2 nano-particles, and to determinate how different kinds of solvents and ultrasonic deagglomeration techniques affect thermo-mechanical behavior of poly(vinyl butyral)/ tungsten disulfide nanocomposites, that might find further application in more complex composite systems. 2. Materials and experimental 2.1. Materials PVB/IF-WS2 nanocomposites were prepared by the solventcasting technique: fine-grained white powder PVB (Mowital B60H, Kuraray GMBH) was first dissolved (10 wt%) in different solvents: ethanol (Zorka Pharma, Serbia), 2-propanol (LachNer, Chech Republic), ethyl-acetate (CentroHem, Serbia), n-butanol (BetaHem, Serbia). Inorganic fullerene-like tungsten disulfide nanoparticles (IF-WS2, NanoLub™ - ApNano Israel) were added into the PVB solutions in 2 different contents: 1 and 2 wt% of IF-WS2 in PVB. Homogenization and particle deagglomeration were achieved by: 1) stirring (magnetic stirrer), 2) ultrasonic irradiation during 30 min, with pulsations (1 s ON/1 s OFF), at 600 W of power, 3) ultrasonic irradiation during 30 min, without pulsations, at 300 W of power.

Please cite this article in press as: D. Simic, et al., Inorganic fullerene-like IF-WS2/PVB nanocomposites of improved thermo-mechanical and tribological properties, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.060

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Ultrasonic deagglomeration is performed using Sonic Vibra Cell VCX 750, with 19 mm Ti probe, at 20 kHz, in an ice bath to prevent excessive heating. Experimental compositions were mixed at room temperature. After homogenization, mixtures were directly poured into flatbottom dishes and left for the solvents to evaporate. Then the dishes with the samples were placed into a heating oven over the night, and into a vacuum heating oven during one more night, so the prepared samples turned into solid thin films after complete amount of solvents evaporated. More detailed properties of the materials used are given in Tables 1e3 of the Supplementary data. 2.2. Characterization 2.2.1. Particle size analysis The average particle size and particle size distribution were determined by laser particle size analyzer (PSA) Master-sizer 2000 (Micro Precision Hydro 2000 mP sample dispersion unit, Malvern Instruments Ltd.), which covers the particle size range of 0.02e2000 mm. For PSA measurements, the powders were dispersed in ethanol and 2-propanol, and deagglomerated on the ultrasonic probe. 2.2.2. Scanning electron microscopy This characterization technique provides information about the microstructure and the composition of the nanocomposites. The quality of IF-WS2 dispersion and deagglomeration in PVB matrix was analyzed by scanning electron microscope Aztec JEOL JSM6460LV with lanthanum hexaboride filament. Potential of 10e20 kV was applied for most samples. Obtained images are analyzed in software Image-Pro Plus that gives a possibility to determine some important information about the observed samples, in this case average values of particle roundness and mean diameter of nanoparticles in the prepared nanocomposites.

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probe was moved with a constant force of 1 mN and sliding velocity of 0.66 mm/s for 15 s in one direction and then it was returned to the starting position with the same speed and force. Finally the probe was unloaded for 5 s. Indentation testing at nanoscale was used to investigate the nanomechanical properties, including indentation hardness (H) and reduced elastic modulus (Er). The hardness and reduced elastic modulus were calculated from the curves using the Oliver and Pharr method, i.e. prior to experiments, the tip area function was calibrated using this method [33,34]. Nanoscratch tests provide friction coefficient (m), with true nanoscale of the normal (NF) and lateral force (LF). The friction coefficient is defined as the ratio of the LF to the NF. 2.2.5. Dynamic mechanical thermal analysis Dynamic mechanical thermal analysis (DMA, Q800 TA Instruments, USA) for the thin film composite samples was done in order to analyze the influence of IF-WS2 nanoparticles deagglomeration in PVB on the behavior of the fabricated composites. DMA tests were performed in film tension clamp mode at a constant frequency of 1 Hz and amplitude of 20 mm in the temperature range from 25  C to 100  C with a heating rate of 3  C/min in nitrogen atmosphere, in order to determine temperature effects on the storage modulus (E0 ) and mechanical loss factor (tan d). The storage modulus shows the ability of the composites to store elastic energy associated with recoverable elastic deformation. Together with tangent delta, the storage modulus describes the behavior of the composite under stress in a defined temperature range. The storage modulus and tan d were determined for the samples 2 wt% IF-WS2/ PVB/EtOH prepared with different deagglomeration techniques, for the samples of the dimensions 25 mm  6 mm  0.3e0.4 mm. 3. Results and discussion 3.1. PSA results

2.2.3. Differential scanning calorimetry The glass transition temperature (Tg) was determined for the prepared samples using DSC Q20 (TA Instruments), with data acquisition program Universal V4.7A. The measurements were performed under a nitrogen flow of 50 ml min1 in the temperature range from 20  C to 100  C. The samples were first heated from 20 to 100  C at a rate of 10  C min1 and then cooled to 20  C at the same rate. Tg was noted from the second heating. 2.2.4. Nanoindentation and nanoscratch test Nanoindentation is a method of determining materials indentation hardness properties, applied to small volumes. Nanoindenter can operate at high precision with extremely small amounts of material. Since nanoindenter is fundamentally a load controlled system, the displacement of the indenter tip can be measured in picometers. The location of the indentation can also be carefully controlled to test regions of particular interest or ideal distribution. The results from this instrument can be represented on a load displacement curve. With the inclusion of the contact area and tip geometry, the desired properties of modulus and hardness can be determined. Indentation and scratch tests at nanoscale on the IF-WS2/PVB nanocomposite samples were done using the Hysitron TI 950 TriboIndenter, equipped with in situ scanning probe microscopy (SPM), with a Berkovich diamond tip of a geometry as a three-sided pyramidal tip with a total included angle of 142.35 and a halfangle of 65.35 . Indentation was performed in the following procedure: 10 s loading, 15 s hold time, 5 s unloading, with force of 1 mN. For nanoscratch testing discussed in this study, the probe was first indented for 5 s with a ramping force from 0 to 1 mN. The

Particle size distribution obtained using laser particle size analyzer is shown in Fig. 1. The diagrams indicate much better dispersion and deagglomeration achieved for IF-WS2 nanoparticles dispersed in ethanol (Fig. 1a) than in 2-propanol (Fig. 1b). Particles are still agglomerated in both solvents, but the agglomerates have significantly smaller sizes in ethanol. The size of 50% of the particles (d50 value) in ethanol is 296 nm and the same percentage of particles in 2propanol has the size 413 nm. This leads to the conclusion that the dispersion and deagglomeration of IF-WS2 nanoparticles are better in ethanol, so it is more effective to use this solvent in order to achieve the homogeneity of the properties of the composite. 3.2. SEM and TEM analysis In order to get an insight in the onion-like hollow structure of an IF-WS2 nanoparticle, an image made on TEM is given in Fig. 2a, provided by the supplier (NanoMaterials Ltd. - Nanotech Industrial Solutions Inc., Apnano Israel). These nanoparticles represent highly symmetrical spherical structures, composed of 20e100 concentric layers of inorganic compounds. The diameter of the primary particle can range between 30 and 70 nm. These multi-layered particles are extremely thermal- and pressure- resistant. Additionally, their outer layers exfoliate under extreme pressure, to create a continuous lubricating coating layer. According to the supplier, WS2 nano-fullerenes have proven their capacity to reduce wear by up to 30% when added to lubricating fluids, oils and polymers. SEM image of IF-WS2 nanoparticles (Fig. 2b) shows that they are highly agglomerated, so for their application in composite

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Fig. 1. Diagrams of particle size distribution of IF-WS2 in ethanol (a) and 2-propanol (b).

Fig. 2. Images of IF-WS2 nanoparticles: a) TEM and b) SEM.

materials they must be well dispersed and deagglomerated, if homogeneity of material properties is required. SEM images of IFWS2/PVB thin film nanocomposites (Fig. 3) show that also in the composite, the nanoparticles are agglomerated in a different degree depending on method of the sample preparation. Chosen representative SEM images of prepared thin-film nanocomposites 2 wt% IF-WS2/PVB/EtOH, prepared using different deagglomeration techniques, are analyzed in program Image-Pro Plus, and some important parameters were calculated in this software. For sample prepared using magnetic stirrer (Fig. 3a) it is found that nanoparticles are agglomerated in big clusters, having sizes from 0.48 up to 9.8 mm (mean diameter of the agglomerates) and of very irregular shape - roundness up to 9.30, with over 90% of total number of the agglomerates from 0.5 to 1.5 mm. That shows that the method of magnetic stirrer provides low deagglomeration, but good dispersion that is evident from the image. The other method, ultrasonic irradiation at 600 W during 30 min, with pulsations, provided better deagglomeration, but not good homogeneity (Fig. 3b). Here, mean diameters of the particles are from 64 nm to 1.57 mm, and roundness up to 1.97. Lines like wrinkles are visible in images of the samples prepared by this technique, so these samples have most irregular microstructure. Also, there are still some large agglomerates remained inside the wrinkles (Fig. 3 c), of the sizes over 2 mm and of irregular shape (roundness 4.12). Ultrasonic irradiation

without pulsations, at 300 W, gave the best results regarding deagglomeration and dispersion of the nanoparticles (Fig. 3d): sizes of particles/small agglomerates from 58 nm to 1.45 mm, with over 75% of the total number of particles/small agglomerates under 400 nm. SEM analysis of the prepared samples indicates better particle distribution and deagglomeration of IF-WS2 in PVB for lower concentration of nanofiller. 3.3. DSC results The second heating cycle in DSC was observed for the thermal changes of the neat PVB powder and composite samples, with the IF-WS2 nanoparticles as well as without them. The glass transition temperature (Tg) of the samples was determined from these thermograms. DSC results, which provide an insight into the thermal behavior of prepared samples, are given in Table 1 and Fig. 4. The influence of the solvent is visible: PVB dissolved in ethyl acetate has lowest Tg, probably because this solvent most distorts the structure of the polymer. Highest Tg is recorded for PVB dissolved in ethanol. Differences in solution behavior of PVB in various solvents are a consequence of the Hansen solubility parameters that reflect on the DSC results and may have impact on other thermo-mechanical properties after dissolving, investigated and illustrated in the researches of P. Peer et al. [35e37]. Hansen

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Fig. 3. SEM images of thin-film composite samples: a) 2 wt% IF-WS2/2-propanol, magnetic stirrer, b) and c) 2 wt% IF-WS2/2-propanol, ultrasonic irradiation at 600 W, d) 2 wt% IFWS2/EtOH, ultrasonic irradiation at 300 W.

Table 1 Glass transition temperatures obtained by DSC analysis for the examined samples. Influence

Sample

Tg,  C

PVB in different solvents

Dry PVB powder Neat PVB/EtOH Neat PVB/2-propanol Neat PVB/ButOH Neat PVB/EtAc 1 wt% IF-WS2/PVB/2-propanol, ms 1 wt% IF-WS2/PVB/2-propanol us, 600 1 wt% IF-WS2/PVB/2-propanol us, 300 1 wt% IF-WS2/PVB/2-propanol us, 600 2 wt% IF-WS2/PVB/2-propanol us, 600 1 wt% IF-WS2/PVB/2-propanol ms 2 wt% IF-WS2/PVB/2-propanol ms 1 wt% IF-WS2/PVB/EtOH us, 300 W 2 wt% IF-WS2/PVB/EtOH us, 300 W

66.76 60.78 58.64 38.25 23.94 61.63 57.87 64.64 57.87 60.91 61.63 62.37 61.27 67.65

Different deagglom. technique

Different amount of IF-WS2

W W W W

ms e magnetic stirrer, us e ultrasonic irradiation. EtOH e ethanol, ButOH e buthanol, EtAc e ethylacetate.

solubility parameter values for PVB and the chosen solvents are given in Tables 2 and 3, in the Supplementary data. The closer solubility parameters between polymer and solvent represent better miscibility resulting in a larger expansion of the polymer. The glass transition temperature is structure sensitive due to steric effects and a formation's intra- or inter-molecular interactions, and it may indicate change in the structure of a polymer chain and also possible crosslinking. In general, decreasing of Tg indicates a probable scission of the polymer chains [35,38]. DSC results show that adding 1 wt% and 2 wt% of IF-WS2 leads to increase of Tg, that is most emphasized in case of the second ultrasonic irradiation method, at 300 W without pulsations. Adding 1 wt% of nanoparticles resulted in an improvement of Tg up to 10.2% for PVB dissolved in 2-propanol, and 11.3% for PVB with 2 wt% of nanoparticles deagglomerated at 300 W in ethanol. Glass transition temperatures for the prepared samples determined on DSC show increase with increased IF-WS2 concentration. However, for the

ultrasonic irradiation at 600 W the decrease of Tg is recorded for some samples. 3.4. Nanoindentation and nanoscratch test results Taken into consideration that the best results from the particle size analysis regarding deagglomeration and dispersion, further on only samples prepared in this solvent are analyzed and discussed in terms of deagglomeration techniques and content of nanoparticles. Nanoindentation results are given as Er, H and Er/H ratio (Table 2). Regarding the content of IF-WS2, it may be observed that Er and H have shown significant increase with raised particle contents in samples homogenized on magnetic stirrer and by ultrasonic irradiation at 300 W, for each of the loadings of nanoparticles. As expected, for these two methods, higher content of IF-WS2 caused higher increase of Er and H. In case of ultrasonic irradiation at 600 W, for higher content of nanoparticles, lower values of

Please cite this article in press as: D. Simic, et al., Inorganic fullerene-like IF-WS2/PVB nanocomposites of improved thermo-mechanical and tribological properties, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.060

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Fig. 4. DSC thermograms for neat PVB dissolved in 2-propanol and for PVB with 1 wt% IF-WS2 nanoparticles deagglomerated by different methods.

Table 2 Reduced elastic modulus and indentation hardness obtained by nanoindentation in function of deagglomeration techniques and of IF-WS2 particle contents. Sample

Er, GPa

St.dev, GPa

H, GPa

St.dev, GPa

Er/H

Neat PVB/EtOH 1 wt% IF-WS2/PVB/ EtOH, magnetic stirrer EtOH, us, 600 W EtOH, us, 300 W 2 wt% IF-WS2/PVB/ EtOH, magnetic stirrer EtOH, us, 600 W EtOH, us, 300 W

3.84

0.49

0.22

0.04

17.5

3.86 4.28 7.31

1.17 0.32 0.37

0.26 0.21 0.47

0.13 0.02 0.02

14.8 20.4 15.6

4.40 4.52 8.54

0.54 0.59 0.42

0.29 0.19 0.59

0.05 0.05 0.07

15.2 23.8 14.5

indentation hardness are obtained. Reduced elastic modulus has shown a slight increase on samples prepared by magnetic stirrer and ultrasonic deagglomeration at 600 W, but the improvement is the most emphasized for the samples obtained by ultrasonic deagglomeration at 300 W, without pulsations. The best nanoindentation results are obtained for ultrasonic deagglomeration of 2 wt% of IF-WS2 at 300 W. Load vs. displacement (depth) curves obtained as nanoindentation results for PVB dissolved in ethanol, containing 1 wt% and 2 wt% of IF-WS2 (ultrasonic deagglomeration at 300 W), compared to neat PVB dissolved in ethanol, are shown in Fig. 5. Improvement of Er is 90.4% for 1 wt% of the added nanoparticles, and 122.4% for 2 wt% of the nanofiller. For H, improvement is higher: 113.6% for 1 wt% and 168.2% for 2 wt% of nanoparticles added to PVB dissolved in ethanol. The mechanical property ratio Er/H determines the extent of plasticity in the contact region. For metals and ceramics Er/H is typically 100 or greater, while for polymers it is on the order of 10 [39]. Consequently, contact is primarily elastic. The forces of friction are mainly adhesion, deformation and elastic hysteresis. Adhesion responsible in polymers results from the weak bonding forces such as Van der Waals forces and hydrogen bonding, which are also responsible for the cohesion between polymer chains in the bulk of the material. Er/H ratio has the lowest values for the samples prepared by ultrasonic irradiation at 300 W, and highest values at 600 W. There is no significant difference in Er/H values for samples containing 1 wt% and 2 wt % of IF-WS2 nanoparticles: these values are slightly higher for the higher content of the nanofiller in cases of magnetic stirrer and ultrasonic irradiation at 600 W, and the

Fig. 5. Load vs. displacement curves obtained by nanoindentation on a thin film sample PVB/EtOH with different concentrations of IF-WS2 ultrasonicated at 300 W.

opposite for ultrasonic irradiation at 300 W. In situ images of indents in neat PVB dissolved in ethanol and the composite 2 wt% IF-WS2/PVB/EtOH, ultrasonicated at 300 W are given in Fig. 6a and b. Presented image shows that there is no pileup formation around the clear triangular indents and there are no observed cracks in the vicinities of the indents. That indicates that the nanoindentation test is successfully carried out to analyze the nanomechanical properties of PVB and IF-WS2 nanocomposites in this work. It is obvious that adding nanoparticles increases indentation hardness and increases the roughness of the material surface. Rootmean-squared (RMS), Rq roughness is determined from nanoindentation test [40e42]. Rq roughness of neat PVB dissolved in ethanol is 5.203 nm, and for composite 2 wt% IF-WS2/PVB/EtOH the Rq roughness is 37.533 nm. Nanoscratch testing is performed first moving the probe in one direction and then it was returned to the starting position, so there are two values for friction coefficients: m1 in the first scratch and m2 in the opposite direction. Friction coefficients, m ¼ LF/NF (mN/mN), for both directions, are given in Table 3. Friction behavior of the IFWS2/PVB/ethanol nanocomposites is discussed as a function of the content of the nanofiller and the method of particles deagglomeration (Table 3). Decrease in friction coefficient is significant for all the samples, but the most for the samples sonicated at 300 W. For higher contents of the nanoparticles, obtained friction coefficients are lower. In second scratch friction coefficients have lower values, but the samples obtained by ultrasonic irradiation with pulsations at 600 W, gave higher values of m for second scratch. For PVB dissolved in ethanol, prepared using magnetic stirrer, 10.1% decrease of m is achieved for 1 wt% of the nanofiller in the first scratch, and 15.6% decrease for the second scratch. The decrease for the first scratch is 20.2% and 26.6% for the second, for the sample prepared by this method containing 2 wt% of nanofiller. For the ultrasonic irradiation at 600 W, also a decrease of friction coefficient is observed for the first direction of scratching: 27.5% for PVB in ethanol with 1 wt% of IF-WS2, and 30.3% with 2 wt% of IF-WS2. However, an increase of friction coefficient m1 comparing to m2 is observed for ultrasonic irradiation at 600 W. Values of m2 are poorly lower than the starting friction coefficient of neat PVB dissolved in ethanol. This difference in the friction coefficients in two directions

Please cite this article in press as: D. Simic, et al., Inorganic fullerene-like IF-WS2/PVB nanocomposites of improved thermo-mechanical and tribological properties, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.09.060

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Fig. 6. 3D image of the Berkovich nanoindentation imprint: a) neat PVB dissolved in EtOH, b) composite 2 wt% IF-WS2/PVB/EtOH.

Table 3 Friction coefficients obtained by nanoscratch test as a function of deagglomeration techniques and IF-WS2 particle contents. Deagglom. technique

Magnetic stirrer Us 600 W Us 300 W

Sample

Friction coefficient

m1 (mN/mN)

m2 (mN/mN)

Neat PVB/EtOH 1 wt% IF-WS2/PVB/EtOH 2 wt% IF-WS2/PVB/EtOH 1 wt% IF-WS2/PVB/EtOH 2 wt% IF-WS2/PVB/EtOH 1 wt% IF-WS2/PVB/EtOH 2 wt% IF-WS2/PVB/EtOH

0.53e0.56 0.48e0.50 0.42e0.45 0.38e0.41 0.37e0.39 0.35e0.37 0.27e0.31

0.47e0.50 0.45e0.47 0.39e0.41 0.51e0.53 0.50e0.51 0.36e0.37 0.26e0.30

of scratching is probably a consequence of the wrinkles and agglomerates of IF-WS2. For samples with 1 wt% of nanoparticles deagglomerated at 300 W, approximately 34% decrease of friction coefficient is achieved in both scratching directions, and 48% decrease for sample containing 2 wt% of nanoparticles. Approximately equal values of friction coefficients in both directions indicate that nanoparticles are well dispersed and deagglomerated, so they may provide reliable resistance to friction. In Fig. 7 this is shown for the best achieved results e 1 wt% and 2 wt% IF-WS2/PVB/

Fig. 7. Friction vs. time curves obtained by nanoscratch test on a sample PVB/EtOH with different concentration of IF-WS2 ultrasonicated at 300 W: parts of the curves where scratches in first and second direction took place.

EtOH ultrasonicated at 300 W comparing to neat PVB dissolved in ethanol: parts of the friction vs. time curves were extracted that represent the first scratch, between 10th and 25th seconds, and the second scratch in the opposite direction, between 30th and 45th seconds. In situ images of scratch marks in the two materials are given in Fig. 8, neat PVB dissolved in ethanol - a), and composite 2 wt% IFWS2/PVB/EtOH, us 300 W - b). Considering this observed results, the presence of small amounts of IF-WS2 obviously makes this composite a very good antifriction material, suitable for application in coatings or incorporation in other materials where good anti-wear and anti-friction protection is demanded. 3.5. DMA results The results of DMA are presented in Fig. 9, Table 4 and Table 5. At the room temperature, and the temperatures below Tg, the storage modulus is higher for the samples with nanoparticles due to the reinforcement effect of the particles which are more rigid than the matrix. The modulus (E0 ) of thin film samples at 40  C increased with the addition of nanoparticles for all the three applied deagglomeration techniques, and especially for ultrasonic irradiation at 300 W. (Fig. 9a, Table 4). As the temperature increases, E0 decreases for all the composites and this can be attributed to the increase in the molecular mobility of the polymer chains. But for the sample ultrasonicated at 600 W, modulus is the lowest, in accordance with DSC results. At 60  C the highest modulus is observed for the composite prepared at 300 W, and the lowest at 600 W of ultrasonication. These values indicate that deagglomeration and dispersion of nanoparticles were most successfully achieved with ultrasonic irradiation at 300 W, since good homogeneity of the material reflects on the improvement of the mechanical properties. The glass transition temperatures were taken from the tan(d) peaks maximums. The tan(d) peak heights with the corresponding temperatures are given in Table 5. For the samples containing nanoparticles that were deagglomerated at 300 W and on a magnetic stirrer, Tg taken from tan d is shifted towards higher temperatures, and the highest is observed for ultrasonication at 300 W. Composite prepared by ultrasonication at 600 W have tan d peak with the lowest maximum, shifted to a lower temperature and the peak is narrowed, that means the polymer chains have more mobility. The observed DMA results indicate the conclusions in accordance with all the previous methods: the optimum results were obtained for the sample 2 wt% IF-WS2/PVB/EtOH ultrasonicated at 300 W.

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D. Simic et al. / Materials Chemistry and Physics xxx (2016) 1e10

Fig. 8. Nanoscratch test - in situ 3D images of scratches in: a) neat PVB dissolved in EtOH, b) composite 2 wt% IF-WS2/PVB/EtOH.

Table 4 Storage modulus at different temperatures as a function of deagglomeration techniques for neat PVB dissolved in ethanol and for the samples 2 wt% IF-WS2/PVB/ EtOH. Sample

E0 40

Neat PVB/EtOH 2 wt% IF-WS2/PVB/EtOH, magnetic stirrer 2 wt% IF-WS2/PVB/EtOH, us, 600 W 2 wt% IF-WS2/PVB/EtOH, us, 300 W

1299 1381 1577 1787



C,

МPa

E0 60



C,

МPa

548 1113 379 1413

Table 5 Glass transition temperatures at the maximum value of the loss modulus obtained by DMA for neat PVB dissolved in ethanol and for the samples 2 wt% IF-WS2/PVB/ EtOH. Sample

Tg,  C

Tan dmax

Neat PVB/EtOH 2 wt% IF-WS2/PVB/EtOH, magnetic stirrer 2 wt% IF-WS2/PVB/EtOH, us, 600 W 2 wt% IF-WS2/PVB/EtOH, us, 300 W

73.34 74.69 70.50 75.21

1.344 1.376 1.259 1.264

The position and height of the peak are indicative of the structure and properties of a particular composite material. Generally, composites have considerably less damping in the transition region compared to neat polymers because the fillers carry an amount of the load and allow only a part of it to strain the interface. Therefore, energy dissipation will occur in the polymer matrix at the interface and a stronger interface allows less dissipation. 4. Conclusions

Fig. 9. DMA results for neat PVB dissolved in ethanol and for the samples 2 wt% IFWS2/PVB/EtOH: a) Storage modulus vs. temperature dependence, b) Loss modulus vs. temperature dependence.

The possibility is examined to prepare poly(vinyl butyral)/ tungsten disulfide nanocomposite of enhanced thermo-mechanical and tribological properties, i.e. to reinforce PVB by adding a small quantity of IF-WS2 nanoparticles (1 wt% and 2 wt%), and to determinate how different kinds of solvents and ultrasonic deagglomeration techniques affect thermo-mechanical behavior of PVB/IFWS2 nanocomposites. Comparing to methods of magnetic stirrer and ultrasonic irradiation with pulsations, the use of ultrasonication method without pulsations has shown to be the most efficient for IF-WS2 nanoparticles deagglomeration and dispersion in PVB solutions, followed by enhancement of thermal and tribomechanical properties. In all the tests performed, it was found that ultrasonic irradiation at 300 W without pulsations used for nanoparticles deagglomeration gave the best results. Particle size analysis is performed on laser particle size analyzer,

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D. Simic et al. / Materials Chemistry and Physics xxx (2016) 1e10

and it has shown that the dispersion and deagglomeration of IFWS2 nanoparticles are significantly better in ethanol than in 2propanol. SEM analysis shows better dispersion and deagglomeration of IF-WS2 particles at lower concentrations of the filler. Method of magnetic stirrer provides low deagglomeration, but good dispersion; ultrasonic irradiation at 600 W with pulsations gives better deagglomeration, but not good homogeneity; and method of ultrasonication at 300 W without pulsations gave the best results regarding deagglomeration and dispersion of the nanoparticles. Thermal analysis on DSC indicates that nanocomposites show better thermal properties than the neat initial polymer. Adding 1 wt % and 2 wt% of IF-WS2 leads to increase of Tg in cases of magnetic stirrer and ultrasonic irradiation at 300 W, but in the case of ultrasonic irradiation at 600 W, addition of the filler decreases the Tg. PVB dissolved in ethyl acetate has lowest Tg, and the highest Tg is recorded for PVB dissolved in ethanol. Nanomechanical and tribological properties are examined for the samples prepared using ethanol as a solvent, considering DSC and PSA results. Indentation hardness and reduced elastic modulus are increased after adding of nano-IF-WS2. Er and H values are highest for the samples obtained by ultrasonic irradiation without pulsation, at 300 W of power. The largest increase of Er and H was obtained for the samples prepared by this method, containing 2 wt % of IF-WS2: Er increase is 122.4% and H increase is 168.2%, as well as the optimum ratio Er/H. The nanoscratch tests indicate that adding of higher content of IF-WS2 nanoparticles and using the method of ultrasonication at 300 W for their dispersion and deagglomeration can effectively reduce friction coefficient for 48%. DMA results are in accordance with previous methods of examination of composite materials. At the temperatures below Tg, the storage modulus is higher for the samples with nanoparticles due to the reinforcement effect of the particles. At higher temperatures, E0 decreases for all the composites due to the molecular mobility of the polymer chains. The highest values of storage modulus at all the temperatures observed were obtained for the composite material prepared at 300 W of ultrasonication. This sample also has the highest glass transition temperature. The obtained results encourage the use of poly(vinyl butyral)/ tungsten disulfide nanocomposite as a material of improved thermo-mechanical properties, that might find further application in many areas, and especially as antifriction material. Acknowledgments The authors thank to the Ministry of Education, Science and Technological Development of the Republic of Serbia for the financial support of the research through the projects TR 34011 and TR 34034, and to the Military Technical Institute for the material and technical support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.09.060. References [1] http://www.kuraray.eu/en/produkte/product-groups/polyvinyl-butyral/ (Accessed 2 April 2016). [2] http://www.teijinaramid.com/wp-content/uploads/2013/11/Twaron-PVBprepreg-LR.pdf (Accessed 2 April 2016). [3] F. Folgar, B. R. Scott, S. M. Walsh, J. Wolbert, Thermoplastic matrix combat helmet with graphite-epoxy skin, in: 23rd International Symposium on Ballistics, Tarragona, Spain 16e20 April 2007, 883. [4] H.E. Sliney, Solid lubricant materials for high temperatures - a review, Tribol. Int. 15 (1982) 303e315.

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