Polyurethane nanocomposites prepared from solvent-free stable dispersions of functionalized graphene nanosheets in polyols

Polymer 53 (2012) 4931e4939

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Polyurethane nanocomposites prepared from solvent-free stable dispersions of functionalized graphene nanosheets in polyols Anna-Katharina Appel, Ralf Thomann, Rolf Mülhaupt* Freiburg Materials Research Center (FMF) and Institute for Macromolecular Chemistry of the University of Freiburg, Stefan-Meier-Strasse 31, D-79104 Freiburg, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 August 2012 Received in revised form 5 September 2012 Accepted 6 September 2012 Available online 12 September 2012

Novel families of elastomeric polyurethane (PU) nanocomposites are prepared by in-situ polymerization of diisocyanate with functionalized graphene nanosheets dispersions in polyether polyols. The influences of graphene dispersion, interfacial coupling and PU hard segment content on PU nanocomposite morphologies, mechanical and electrical properties are investigated. High shear mechanical dispensing (UT process) and high pressure homogenization (HPH process) enable very effective dispersion of functionalized graphene, prepared by thermally reducing graphite oxide (TRGO), in polyols. The addition of dispersing agents and organophilic TRGO surface modification, reacting TRGO hydroxyl groups with phenyl isocyanate (TRGO-Phi), is not required. The TRGO dispersions in a 6/1 wt.-% blend of polypropylene oxide diol (Mn ¼ 2000 g/mol) with polypropylene oxide triol (Mn ¼ 6000 g/mol) are cured with methylene-diphenyl-4,40 -diisocyanate (MDI) at 60  C. The content of PU hard segments is varied from 23 to 33 wt.-% using 1,4-butanediol (BD) as chain extender. According to the transmission electron microscopic (TEM) analysis of the PU nanocomposite morphologies, the TRGO nanosheets are exclusively allocated in the PU hard phases, thus causing a change of PU morphology. Upon increasing the hard segment content to 33 wt.-%, skeleton-like co-continuous superstructures are formed, paralleled by the simultaneous improvement of Young’s modulus (300%) and the tensile strength (350%) without sacrificing high elongation at break. This behavior is not observed for conventional nanofillers such as nanometer scaled carbon black (CB) and multiwalled carbon nanotubes (CNT), when processed under the same condition. The comparison of TRGO with TRGO-Phi dispersions reveals that covalent coupling of TRGO and copolymerization of hydroxyl-functional TRGO with PU is essential. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Graphene Dispersion Polyurethane nanocomposite

1. Introduction Carbon nanofillers, such as carbon black and carbon nanotubes, are being used extensively for improving thermal, mechanical, electrical properties, UV stability, and abrasion resistance of polymers [1] and [2]. Graphenes represent two dimensional carbon polymers consisting of one-carbon-atom thick and micrometerwide slightly wrinkled single layers of sp2-hybridized carbon atoms arranged in a honeycomb-like lattice [3]. The defect-free single graphene nanosheets, prepared by mechanical peeling of graphite [4], have ultrahigh aspect ratio and exhibit extraordinarily high stiffness with Young’s modulus of 1000 GPa [5], extremely rapid electron transport at room temperature and high electrical conductivity (6000 S cm1) [6] combined with very large specific

* Corresponding author. E-mail addresses: [email protected], freiburg.de (R. Mülhaupt).

[email protected]

0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.09.016

surface area (w2600 m2 g1) [7] and [8]. In PU chemistry hydroxylfunctionalized graphenes are of special interest because they can react with isocyanate to form covalent urethane bonds between the PU matrix and the graphene nanosheets. While carbon nanotubes require special functionalization steps for achieving effective hydroxylation, the functionalization of graphenes can be implemented directly into the graphene synthesis when graphenes are produced by chemical reduction of oxidized graphite. Chemically and thermally reduced graphenes are highly multifunctional, containing hydroxyl, phenol, ketone, carboxyl, lactone, and epoxy groups [9]. Upon heating GO very rapidly up to temperatures well above 400  C, using heating rates as high as 2000  C/min, thermolysis occurs and the decomposition of the functional groups evolves CO and CO2 gases [10,11]. This accounts for expansion and exfoliation of thermally reduced graphite oxide (TRGO) nanosheets, as evidenced by substantially increased specific surface area [10e 12]. During TRGO synthesis the degree of functionalization, as reflected by the oxygen content, increases with decreasing reduction temperature [13e15]. No post treatment functionalization step is

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required. The TRGO oxygen content can be varied between 0.1 and 40 wt.-%. This corresponds to hydroxyl-functionalities varying between 0.01 and 1.5 mol OH/kg. Although the actual structure of graphite oxide is still under debate [9,16,17], it is widely accepted that the hydroxyl groups are located at the edges of graphene planar sheets and defects inside the graphene sheets [9]. It should be noted that TRGO is not completely flat but wrinkled, containing nanometer-sized holes. In comparison to ideal graphenes, the reduced GO nanosheets exhibit somewhat lower specific surface area (600e1500 m2 g1) and lower electrical conductivity (10e 23 Scm1) [10,14]. The presence of functional groups, even in small concentrations, effectively promotes both dispersion of TRGO in various organic media [10,11] as well as polymer melts [18e20]. In a recent advance, it has been demonstrated that functionalized graphenes are readily dispersed in fluids of very different polarity, including water, acetone and even isopropanol, using high pressure homogenization which does not require either organophilic graphene modification or addition of dispersing agents and polymer binders [21]. In the high pressure homogenization the pressurized suspensions of functionalized graphenes pass through the tiny gap inside the homogenizing valve. The resulting very high turbulence and shear forces, combined with compression, acceleration, pressure drop, and impact, account for the simultaneous disintegration of large graphene agglomerates and very effective dispersion of graphenes in the fluid. This high pressure homogenization technology, well-established in food and drug industries, is industrially much more viable with respect to sonication. Among the rapidly expanding families of nanocomposites, graphene/polyurethane (PU) nanocomposites are attracting considerable attention in academia and industry. The dispersion, covalent attachment and orientation of functionalized very large ultrathin carbon 2D molecules offers attractive opportunities for tailoring PU materials exhibiting improved toughness/stiffness balance, high electrical and thermal conductivities, barrier performance against gas and fluid permeation, UV and IR absorption, tear and scratch resistance, corrosion protection as well as enabling response to external mechanical, thermal and electrical stimuli. The attractive key feature is the possibility of copolymerizing graphenes with isocyanate (in-situ polymerization) because graphene hydroxyl groups form urethane bonds with isocyanate groups are incorporated into PU networks. This represents a very versatile synthetic route to novel PU carbon hybrid materials and novel layered systems [22] and [23]. Jeong et al. [24] have reported on graphenereinforced waterborne polyurethane (WPU) prepared by in-situ polymerization. They achieved modulus improvement of WPUs, resulting from graphene reinforcement of the PU matrix, was far superior to that of nanocomposites produced by a conventional physical mixing method. According to Yuan et al. [25], Macosko and coworkers investigated TPU elastomers reinforced with TRGO nanosheets, prepared by means of solvent-mediated mixing, in-situ polymerization and melt compounding [26]. The morphological study revealed that a solvent-based mixing process afforded much more uniform dispersion of graphene nanosheets in the PU matrix when compared with melt compounding. This very effective graphene dispersion proved to be essential for achieving a 10-fold increase of tensile modulus and 90% decrease of nitrogen permeation at only 3 wt.-% graphene content. Electrical conductivity was observed at a very low percolation threshold of 0.5 wt.-% TRGO. Jeong and coworkers employed solvent-mediated dispersions to prepare TPU with uniform dispersion of functionalized graphenes [27]. They observed that the graphene addition hindered the crystallization of the soft segment, which appeared to impair the graphene matrix reinforcement. Another in-situ polymerization process was reported by Hu and coworkers to produce PU/graphene nanocomposites with improved thermal stability, electrical

conductivity, tensile strength (þ239%) and storage modules (þ202%) [22]. Recently, Coleman and coworkers have succeeded to disperse graphene nanosheets, derived directly from graphite without using GO as intermediate, using solutions of PU in DMF and THF [28]. In drop casting processing it was possible to vary the graphene content of PU/graphene nanocomposites between 0 and 90 wt.-% with uniform graphene dispersions up to 55 wt.-% graphene content. However, in the absence of functional groups, graphenes were not covalently bonded to the PU matrix. Many of the above mentioned processes for achieving effective TRGO dispersion are solvent-based. For industrial applications of PU/graphene nanocomposites and improved sustainability it is highly desirable to eliminate the need for solvents and to develop solvent-free insitu polymerization process with easy carbon nanosheets dispersion. Here we report on the solvent-free dispersion of functionalized graphene nanosheets (TRGO) directly in polyols using high speed mechanical dispensing and high pressure homogenizing technology. During the in-situ polymerization these novel TRGO/polyol dispersions are cured with diisocyanate in order to produce PU/ graphene nanocomposites with effective graphene dispersion and covalent PU/graphene coupling. TRGO is compared with conventional carbon nanofillers such as nanometer scaled carbon black (CB) and multiwalled carbon nanotubes (CNT). In order to examine the role of covalent bond formation between TRGO and the PU matrix, all TRGO hydroxyl groups are quantitatively converted with phenyl isocyanate into the corresponding phenylurethane groups (TRGO-Phi). The interaction of TRGO with PU hard segments, prepared by adding various amounts of short chain diol as chain extenders, is examined. 2. Experimental 2.1. Raw materials Graphite powder (Graphit KFL 99.5) was obtained from Kropfmühl AG, Passau, Germany. Multiwalled carbon nanotubes (Baytubes C 150 P) from Bayer AG, Germany, have diameters between 4 and 15 nm, length between 1 and 5 mm and purity over 95%. The conducting carbon black nanofiller (Printex XE 2) with average particle size of 20 nm was supplied by Evonik AG, Germany. The polypropylene oxide diol with number average molar mass of 2000 g/mol (DesmophenÒ L2830) and the corresponding polypropylene oxide triol with number average molar mass of 6000 g/mol (BayflexÒ 5123 X), the catalyst DABCO-25-S and the short chain diol 1,4-butanediol (BD), used as chain extender, modified methylenediphenyl-4,40 -diisocyanate (MDI, Desmodur PF) were purchased from Bayer AG, Germany. The polyols were dried under vacuum at 80  C for 4 h before use and MDI was used as delivered. All the other chemicals were purchased from VWR-Merck and used without further purification. 2.2. Preparation of GO, TRGO and TRGO-Phi TRGO was produced in a two step procedure starting with graphite. At first GO was prepared by oxidation of graphite according to the method of Hummers and Offemann [29]. Graphite (60 g) was stirred in concentrated H2SO4 (1.4 l) at room temperature and NaNO3 (30 g) was added. The mixture was cooled to 0  C and KMnO4 (180 g) was added during 5 h. The reaction mixture was allowed to reach room temperature and stirred for 2 h. The reaction was quenched by pouring the reaction mixture into ice water (2 l) and adding H2O2 (3%, 200 ml). The GO was filtered off, washed with aqueous HCl (3%) and then with water until no AgCl precipitated when aqueous AgNO3 was added to water. The purified chlorine-,

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sulfate- and manganese-free GO was dried in vacuum at 40  C and powdered (60 mm mesh) by a ball mill (Retsch, Haan, Germany) using liquid nitrogen. In the second step, GO was thermally reduced to produce TRGO at 750  C under nitrogen in an automated tube furnace equipped with a rotating quartz tube (Nabertherm, Lilienthal; Germany). TRGO was obtained as a black powder of very low bulk density. For the preparation of TRGO-Phi, TRGO (2.5 g) was treated with an excess ofphenyl isocyanate (15 ml) in toluene (50 ml) under argon for 3 day at a temperature of 50  C. TRGO-Phi was filtered off and washed several times with CHCl3 followed by drying in vacuum at 100  C. 2.3. Preparation of PU/TRGO nanocomposites The preparation of PU/TRGO nanocomposites was carried out by in-situ polymerization in a reactor equipped with a mechanical stirrer. Different PU compounds were prepared by varying the content of hard segments (23 wt.-% for PU23, 29 wt.-% for PU29 and 33 wt.-% for PU33). The hard segment is formed by reacting MDI with 1,4-butanediol. As component for the formation of the PU soft segments a 6/1 blend of polypropylene oxide diol with molar mass of Mn ¼ 2000 g/mol and the polypropylene oxide triol with (Mn ¼ 6000 g/mol) were used. As benchmarks conventional nanofillers such multiwalled carbon nanotubes (CNT) and conductive carbon black (CB) were included in this study. The nanofiller content was varied between 0.25 and 2.0 wt.-%. In a typical procedure for PU33 nanocomposites the 6/1 blend of polypropylene oxide diol (67.8 g) and polypropylene oxide triol (12.0 g) were dried at 80  C under vacuum for 2 h. The dispersion of the fillers in the polyols was achieved by using either a high speed mechanical dispersing (Ultraturrax T 25, IKA) or the high pressure homogenizer (GEA Niro Soavi S.p.A), respectively. Using the Ultraturrax dispersing device, the filler was dispersed in the polyols for 15 min at 10.000 rpm to produce stable TRGO dispersion in the polyols without encountering sedimentation. For effective high pressure homogenization, the carbon nanofiller dispersion was performed in acetone (10 g L1), since a rapid viscosity increase during TRGO dispersions can cause plugging problems in the homogenizer valve and homogenized at 1400 bar to produce a very stable dispersion. Then the polyols were added and acetone was stripped off in vacuum to produce very stable acetone-free dispersions of TRGO, whereas sedimentation was observed for CB and CNT. Prior to cure, the resulting dispersion was degassed in vacuum for 1 h, followed by the addition of the chain extender 1,4Butanediol (6.4 g) and the catalyst DABCO-25-S (0.3 g). In the last step the diisocyanate Desmodur PF (33.1 g) was added. Then the mixture was stirred at room temperature for 1 min, poured into the mold and cured in an oven at 60  C for the duration of 30 min. 2.4. Characterization of GO, TRGO and TRGO-Phi Elemental analysis was performed using a VarioEL elemental analyzer (Elementaranalysensysteme GmbH). Thermogravimetric analysis (TGA) of samples were performed on an STA 409 thermobalance by Netsch at temperatures varied from 50  C to 650  C using a heating rate of 10 Kmin1 under a nitrogen flow (75 ml min1). The surface area was measured using the Brunauer, Emmett and Teller (BET) method [30] in a Porotec Sorptomatic 1990. The Fourier Transform Infrared (FT-IR) spectra were recorded using a Bruker IFS 88 FT-IR spectrometer with KBr pellets. 2.5. Composite characterization The morphology of the PU nanocomposites was examined by transmission electron microscopy (TEM; LEO 912 Omega 120 kV).

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The composites were cryo-microtomed with a diamond knife (Leica Ultracut UCT) at 120  C into 100 nm thick thin sections and placed on 200 mm-mesh Cu grids. Mechanical properties such as tensile strength and elongation at break were performed using a tensile tester (Zwick Z-005). The test specimens were prepared according to DIN 53 504-85.52 ISO R 37 Typ 2. At least five samples were tested toobtain average values according to ISO 527 using a strain rate of 50 mm/min at 25  C. Dynamic mechanical analysis was performed on a TA Dynamic Mechanical Thermal Analyzer Q800. The samples (5 cm  0.2 cm  0.6 cm) were scanned from 70  C to 50  C at a heating rate of 3  C min1. The frequency of dynamic oscillatory loading was 1 Hz. Electrical conductivity was measured on test specimens with DMA geometry (5 cm  0.2 cm  0.6 cm), which were contacted with conductive silver. The resistance of the composites was measured using a Keithly electrometer 617 with a measuring range up to 2  1010 U. The specific resistance was evaluated from the measured resistance using Equation (1), in which A is the cross section area and L the distance between the electrodes:

rsp ¼ rmeasured $A=L

(1)

3. Results and discussion The functionalized graphene/PU nanocomposites were prepared by in-situ polymerization using novel stable TRGO dispersions in polyether polyols as intermediates. The synthetic strategy is displayed in Scheme 1. First graphite was oxidized and then the resulting GO was reduced by heating very rapidly up to 750  C. In the second step, both hydroxyl-functional TRGO and phenylurethane-functionalized TRGO-Phi, were dispersed in the polyether polyols by high speed mechanical dispensing using an UltraturraxÔ dispensing device (UT process) and high pressure homogenization (HPH process). In the third step, the resulting solvent-free stable dispersions of functionalized graphene nanosheets dispersions in polyether polyol were cured with diisocyanate in a mold. During the in-situ polymerization the isocyanate reacts with the TRGO hydroxyl groups, incorporating TRGO nanosheets into the PU network. 3.1. Stable polyol dispersions of functionalized graphenes The functionalized graphene nanosheets (TRGO) were produced following the procedures by Hummers and Offeman [29]. Graphite was intercalated in sulfuric acid and oxidized with potassium permanganate. Then the resulting GO was heated very rapidly up to 750  C in order to thermally reduce GO and to produce TRGO with a carbon content of 86 wt.-% and a specific surface area of 660 m2g1 (cf. Table 1). For applications as nanofiller it is imperative to carefully remove all manganese cations because manganese is redox active and can impair polymer thermooxidative stability. Following synthetic procedures reported by Ruoff et al., all hydroxyl groups of TRGO were converted into phenylurethane groups by reacting TRGO with phenyl isocyanate [31]. The thermogravimetric analysis is displayed in Fig. 1. In contrast to GO, having a low onset of thermal degradation at 100  C with substantial weight loss at 200  C resulting from the thermal GO reduction accompanied gas evolution [32], both TRGO and TRGOPhi are much more stable, because TRGO was thermally reduced by heating at 750  C. In the case of TRGO-Phi the urethane groups degrade at temperatures above 150  C, splitting off phenyl isocyanate with a mass loss of 12%. This can be used to estimate the urethane and hydroxyl group functionalities which are in the order of 1.1 mol/kg. This is in accord with functionalities calculated from the N content in elemental analysis (cf. Table 1), while the hydroxyl

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Scheme 1. Route to PU/TRGO nanocomposites: (a) oxidation of graphite according to Hummers and Offeman [29]; (b) thermal reduction of GO; (c) surface modification with phenyl isocyanate to convert all hydroxyl groups into urethane; (d) polyol dispersion by high shear mechanical dispersing (UT process) or high pressure homogenization (HPH process), (e) addition of the chain extender 1,4-butanediol, (f) cure of the polyol dispersions with diisocyanate.

group number of TRGO is 1.6 mol/kg by means of hydroxyl group titration. The TGA analysis confirms that both TRGO and TRGO-Phi are stable at temperatures encountered during PU processing. FTIR spectra of GO, TRGO and TRGO-Phi are presented in Fig. 2. The spectrum of GO reveals the presence of OeH stretching of absorbed water at 3432 and 1630 cm1, eCH3 stretching at 2965 cm1, eCH2 stretching at 2923 and 2853 cm1, C¼O stretching

at 1728 cm1, CeOH deformation vibration at 1383 cm1 and CeOe C stretching of epoxides at 1050 cm1 [9] and [33]. After thermal reduction at 750  C, most of the characteristic features of GO disappeared, indicating that most of the oxygen containing groups had been removed. The intensity of C¼O stretching at 1728 cm1 decreased, also the signals between 950 and 1383 cm1 became weaker. Most likely, decarboxylation of

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Table 1 Characterization of GO, TRGO and TRGO-Phi. Material

GO TRGO TRGO-Phi a b

Elemental analysis C [%]

H [%]

O [%]

N [%]

OH-number [mol/kg]

BET [m2 g1]

54.19 85.78 84.95

2.59 0.58 1.21

42.51 13.64 12.25

e e 1.31

e 1.60a 1.03b

100 660 e

Hydroxyl group titration. Calculated from N content.

carboxyl groups and thermal decomposition of epoxy groups takes place at elevated temperature. Similar to the spectrum of GO there is a strong and broad absorption at 3432 and 1630 cm1, which resulted from the OeH stretching vibration of absorbed water caused of the high hydrophilicity of the graphene sheets. In the spectrum of TRGO-Phi a new peak appeared at 1553 cm1, which corresponds to the NeH stretching vibration of the urethane. The characteristic peak at 1790 cm1 also increased, which is attributed to the C¼O stretching of urethanes and provided more evidence for successful chemical functionalization. At the same time, the intensity of the peaks associated to the eCH2 stretching vibration at 2923 and 2853 cm1 increased, as expected for the phenyl isocyanate groups on TRGO surface. Functionalization alters surface properties of graphene improving its solubility in organic solvents [13]. TRGO and TRGOPhi dispersions (1 mg/ml) in DI water, THF and chloroform 24 h after ultrasonication are shown in Fig. 3a. TRGO shows no stable dispersions and significant sedimentation occurs even in organic mediums such as THF and chloroform. In contrast to TRGO, TRGO treated with phenyl isocyanate can be well dispersed even in relatively no polar, aprotic mediums such as THF and chloroform which signifies the organophilic surface properties. Both TRGO and TRGO-Phi were very effectively dispersed in polyether polyols with molar masses of 2000 and 6000 g/mol, respectively (cf. Fig. 3b). Interestingly, the organophilic surface modification of TRGO with phenylurethane (TRGO-Phi) was not required for enabling dispersion. In contrast to CNT and CB, the polyol/graphene dispersions are stable and no sedimentation occurs, even after one week. Two different dispersing technologies were applied successfully for producing graphene nanosheets dispersions in polyols: (i) high speed mechanical dispersing using the UltraturraxÔ dispersing device (UT process) and (ii) high pressure homogenization (HPH process). The UT process at

Fig. 1. TGA curves of GO, TRGO and TRGO-Phi under nitrogen atmosphere.

Fig. 2. FTIR spectra of GO, TRGO and TRGO-Phi.

10.000 rpm produces TRGO dispersions directly in the viscous medium of the polyols without requiring solvent addition. In the HPH process, TRGO suspensions in acetone or acetone polyol mixtures, respectively, are pressurized at 1400 bar and pumped through the extremely narrow gap of the high pressure valve. The resulting ultrahigh shear during pressure drop in the homogenizer valve affords highly effective deagglomeration of TRGO and simultaneous formation of stable dispersions. After adding the polyols and stripping off acetone in vacuum very stable solvent-free TRGO dispersions in polyols are obtained. 3.2. PU/graphene nanocomposites The TRGO dispersions in polypropylene oxide, comprising a blend of diol with molar mass of 2000 g/mol and triol having a molar mass of 6000 g/mol (6/1 mixing ratio), were cured with methylene-diphenyl-4,40 -diisocyanate (MDI) for 30 min at 60  C, producing highly flexible PU with PU soft segments. Hard PU segments were incorporating by adding various amounts of the short-chain 1,4-butanediol (BD) as chain extender. As a function of the BD content the hard segment content, calculated from the amount of BD and the corresponding stoichiometric amount of MDI, was increased from 23 wt.-% (PU23) to 29 wt.-% (PU29) and 33 wt.-% (PU33), respectively. The TRGO content was varied between 0 and 2 wt.-%. During the in-situ polymerization the PU network is formed and TRGO is covalently bonded when the hydroxyl groups of TRGO copolymerize with MDI. For comparison also conventional nanofillers, such as nanometer scaled carbon black (CB) and multiwall carbon nanotubes (CNT), were added to the polyol blend and cured with MDI under identical polymerization conditions. Fig. 4 shows TEM images of thin sections prepared from PU and for the corresponding PU/TRGO nanocomposites. When hard segments are incorporated, phase separation occurs. In the absence of TRGO spherical hard phases (dark contrast) are dispersed uniformly in the soft PU matrix (pale contrast). As the hard segment content increases, the average domain size of the hard phases decreases significantly from 0.5 to 2 mm for PU23 to 0.1e1 mm for PU33. The dispersion of TRGO in PU/TRGO is affected by both the type of polyol/TRGO dispersing process and the hard segment content. In all PU/TRGO nanocomposites, displayed in Fig. 3, the TRGO is detected exclusively inside the hard phases which change their shape from spherical to anisotropic shapes resembling that of TRGO. In fact, with increasing hard segment

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Fig. 3. (a) TRGO and TRGO-Phi dispersions in DI water, THF and chloroform (10 g/l) 24 h after ultrasonication for 10 min (b) CNT, CB, TRGO and TRGO-Phi dispersions (10 g/l) in 6/1 polyol blend 1 week after dispersing by UT process.

content, the dispersion of TRGO in the PU matrix is substantially enhanced. For PU33/TRGO (1.5 wt.-%) at high magnification (Fig. 4g and Fig. 4i) the wrinkled structures of fully exfoliated TRGO are clearly visible inside the hard phase. Moreover, the much better

dispersion of individual TRGO particles is achieved at high hard segment content when the polyol/TRGO dispersion is produced with the HPH process (cf. Fig. 4i). This observation confirms that the HPH process is much more effective with respect to TRGO

Fig. 4. TEM micrographs of fracture surfaces: (a) pristine PU23 containing 23 wt.-% hard segments; (b) pristine PU29 containing 29 wt.-% hard segments; (c) pristine PU33 containing 33 wt.-% hard segments; (d) PU23/TRGO (1.5 wt.-%) nanocomposite; (e) PU29/TRGO (1.5 wt.-%) nanocomposite (f) and (g) PU33/TRGO (1.5 wt.-%) nanocomposites, dispersed by UT process, with two different magnifications; (h) and (i) PU33/TRGO (1.0 wt.-%) nanocomposite, dispersed by the HPH process, at two different magnifications.

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Fig. 5. SEM imaging of a PU33/TRGO, containing 1.5 wt.-% TRGO and 33 wt.-% hard segments. PU33/TRGO was prepared by high shear mechanical dispersing in the UT process (a) and by the high pressure homogenizer in the HPH process (b).

dispersion as compared to TRGO dispersions obtained in the UT process. The improved dispersion is also apparent from SEM analysis of fracture surfaces of PU33/TRGO. With the HPH process much more uniform and smooth fracture surfaces are visible in Fig. 5 b, whereas the UT process appears to give incomplete deagglomeration of TRGO with structures of 20 mm dimensions (cf. Fig. 5a). At high hard segment content, the TRGO-containing hard phases appear to assemble, producing a skeleton-like superstructure in the PU matrix. Also the PU/TRGO-Phi nanocomposites exhibited morphologies very similar to those of PU/TRGO with TRGO being exclusively incorporated in the hard segments (cf. Fig. 6a). Obviously, the organophilic surface modification of TRGO with phenylurethanes and nearly full conversion of all hydroxyl groups does not appear to affect the TRGO dispersion. Although the TRGO-Phi does not have free hydroxyl group, the urethane groups can react with MDI to afford allophanate coupling reaction between PU matrix and TRGO-Phi. In sharp contrast to TRGO and TRGO-Phi, both CNT and CB were much more difficult to disperse and failed to afford uniform dispersions (cf.Fig. 6 b and c). The mechanical, thermal and electrical properties of PU and PU nanocomposites, as determined by stress/strain tests, DMA and resistance measurements, are summarized in Table 2. Thermal analysis (DMA) was used to investigate the interaction effect between carbon nanofiller and the soft segment. The glass transition temperatures (Tg) of the soft segment of pristine PU23, PU29 and PU33 are in range between 48 and 30  C. It is obvious that with an increase of the content of hard segments (1,4-Butanediol) the Tg rises from (43  4) C in PU23 to (39  9) C in PU33. The addition of CNT and CB and as well as TRGO has no influence on the

Tg, which demonstrates the interaction between carbon nanofiller and the soft segment is very weak. The influence of TRGO content and content of hard segments on Young’s modulus and tensile strength are graphically displayed in Fig. 7. It is obvious from Fig. 7 that the matrix reinforcement of the PU matrix strongly depends upon the hard segment content. At lower hard segment content of 23 and 29 wt.-%, the incorporation of 2 wt.-% TRGO into PU23 affords only marginally improved stiffness and strength. In sharp contrast, at higher hard segment content of 33 wt.-% and the same TRGO content of 2 wt.-% the Young’s modulus increase by 300% and the tensile strength by 350% with respect to PU. Interestingly, this simultaneous increase of stiffness and strength is achieved without adversely affecting high elongation at break. It is likely that the skeleton-like superstructures, observed only at high content of hard segments, account for stiffness enhancement and very effective energy dissipation. Although more research is needed to examine the micromechanics of PU/TRGO nanocomposites, it appears likely that skeleton-like superstructures afford improved energy dissipation by multiple plastic deformation. The very effective TRGO dispersion in PU is important for achieving improved toughness/stiffness balance. Hence, at 1 wt.-% TRGO content the more uniform TRGO polyol dispersions produced by the HPH process affords higher stiffness and strength with respect to the corresponding nanocomposite using TRGO polyol dispersions prepared by the UT process. Another important requirement for simultaneously improving stiffness and elongation is the covalent attachment and copolymerization of TRGO during in-situ polymerization. TRGO-Phi, in which all hydroxyl groups were scavenged with phenyl isocyanate, affords

Fig. 6. TEM micrographs of fracture surfaces: (a) PU33/TRGO-Phi (1.5 wt.-%) nanocomposite; (b) PU33/CNT (1.5 wt.-%) nanocomposite and (c) PU33/CB (1.5 wt.-%) nanocomposites dispersed by UT process.

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Table 2 Nanocomposites of PU elastomers containing TRGO, TRGO-Phi, CB and CNT. Sample PU23 PU23/2.0% PU23/2.0% PU23/2.0% PU29 PU29/2.0% PU29/2.0% PU29/2.0% PU33 PU33/0.5% PU33/1.0% PU33/1.5% PU33/2.0% PU33/2.0% PU33/2.0% PU33/1.5% PU33/1.0% a b

Young’s modulus [MPa]

TRGO CNT CB TRGO CNT CB TRGO TRGO TRGO TRGO CNT CB TRGO-Phi TRGO (HPH)b

2.9 5.1 5.0 4.5 8.2 15.6 10.8 11.1 11.8 14.3 18.9 27.6 35.1 19.3 15.1 20.0 23.0

                

0.2 0.3 0.3 0.2 0.2 0.3 1.0 1.0 0.3 0.2 0.3 0.6 0.7 0.7 1.8 1.2 0.1

Tensile strength [MPa] 1.6 1.6 1.6 1.7 2.0 2.9 3.5 2.5 3.1 3.8 5.9 6.9 10.6 5.3 2.3 8.9 5.5

                

0.1 0.1 0.1 0.2 0.2 0.2 0.4 0.4 0.1 0.3 0.1 0.2 0.2 0.2 0.1 0.8 0.3

Elongation at break [%] 410 80 300 235 520 209 290 156 650 603 623 510 715 421 112 288 541

                

50 35 40 50 22 20 45 56 22 45 12 10 12 49 11 7 55

Electrical conductivity [Scm1] 1.2 1.4 1.9 1.3 4.8 7.5 7.3 4.8 2.9 2.9 3.6 4.9 4.9 3.2 2.9 3.3 2.9

                

1010 109 109 109 1012 1011 1011 1012 1011 1011 1011 1011 1011 1010 1011 1011 1011

Tga [ C] 43 42 43 44 42 42 41 42 39 39 40 39 40 40 40 41 40

                

4 5 5 6 6 5 7 6 9 9 8 8 8 8 8 8 8

Glass transition temperature (Tg) as measured by DMA. Dispersed by high pressure homogenizer (HPH).

improved stiffness at the expense of elongation at break (cf. sample PU/1.5% TRGO-Phi in Table 2). From Fig. 8 it is apparent that the addition of 2 wt.-% TRGO to PU33 affords far superior mechanical properties with respect to CB and CNT, as reflected by the simultaneous improvement of stiffness, strength and elongation at break. Undoubtedly, the reinforcement

Fig. 8. Mechanical properties of PU33 nanocomposites containing 2.0 wt.-% carbon nanofiller.

of TRGO can be attributed to the good dispersion of the graphene sheets in composites and the strong interaction between TRGO and the PU matrix. The electrical properties of carbon-filled polymers depend on many factors such as filler content, filler percolation, polymer morphology, type of polymer matrix, interfacial coupling, specimen thickness, and even test methods [34] [35], and [14]. Macasko et al. [18] reported percolation threshold values smaller than 1 wt.-% for TPU/graphene nanocomposites, whereas Choi et al. [35] achieved a much lower value with 0.4 wt.-% filler. As is apparent from Table 2, at 2 wt.-% filler content the electrical conductivity was very low around 1012 S cm1 and 109 S cm1 for all carbon nanofillers. This is not surprising, because the high magnification in TEM images in Fig. 4 reveal that isolated TRGO particles are very effectively embedded in a shell of the electrically insulating PU hard segments. As will be published elsewhere in more detail, in coating applications the percolation threshold for PU/TRGO nanocomposites is lowered significantly when the film thickness is reduced. 4. Conclusion

Fig. 7. (a) Young’s modulus and (b) tensile strength of PU/TRGO nanocomposites as a function of TRGO content.

In comparison to conventional high shear mechanical stirring and sonication, both high speed mechanical dispersing (UT process) and especially the high pressure homogenization (HPH

A.-K. Appel et al. / Polymer 53 (2012) 4931e4939

process) are much more effective, enabling the facile dispersion of TRGO in polyether polyols. Stable solvent-free TRGO/polyol dispersions with TRGO content up to 2 wt.-% are readily available. Organophilic graphene modifications, for example by reacting hydroxyl groups with phenyl isocyanate, are not required. Obviously, when TRGO suspensions are pressurized and passed through the extremely narrow gap of the homogenizing valve in the HPH process, ultrahigh shear forces encountered during the pressure drop accounted for the very effective simultaneous disintegration of large TRGO agglomerates and the simultaneous very effective dispersion of TRGO nanosheets in fluids. This HPH process, that is well-established in industrial food and drug production, is readily scaled up and capable of producing graphene/polyol dispersions in industrial scale. The novel TRGO polyol dispersions are easy to handle and can be cured with MDI in conventional PU processing. The addition of functionalized graphene nanosheets changes the PU morphology. According to TEM analysis, the graphenes are incorporated exclusively in the hard phase, thus forming a skeleton-like co-continuous superstructure at high hard segment content of 33 wt.-%. Most likely, the urethane groups form at the TRGO surface hydrogen bridges which cause assembly of the hard segments at the graphene nanosheet surface. At lower hard segment content, the graphene-filled hard phases are not interconnected. The formation of skeleton-like superstructures at high hard segment content affords property synergisms of simultaneously improved stiffness and strength without sacrificing elongation at break. This unique behavior is not paralleled by conventional carbon nanofillers such as conducting carbon black and CNT. When the free TRGO hydroxyl groups are blocked by pretreating TRGO with phenyl isocyanate, the resulting PU/TRGOPhi nanocomposites improve stiffness only at the expense of elongation at break. Obviously, the presence of hydroxyl groups and covalent attachment of graphenes to the hard phases is essential for improving mechanical properties. More research is needed to examine the micromechanics of PU/TRGO as a function of the morphology development and interfacial graphene coupling. The assembly of functionalized graphene nanosheets at PU interfaces offers attractive opportunities for designing new PU carbon hybrid materials with improved property profiles.

Acknowledgment The authors are grateful to Bayer MaterialScience for the financial support of this project and many helpful advices and discussions. We also thank Kropfmühl AG (Dr. Feher) and Evonik for supplying us with graphite and carbon black. The TRGO synthesis was scale-up in the “FUNgraphen” project (project no. 03X0111C),

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sponsored by the German Federal Ministry of Education and Research (BMBF). References [1] Huang J-C. Polymers For Advanced Technologies 2002;21:299. [2] Moniruzzaman M, Winey KI. Macromolecules 2006;39:5194. [3] Boehm H-P, Setton R, Stummp B. Pure And Applied Chemistry 1994;66:1893e 901. [4] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Science 2004;(306):666. [5] Lee C, Wei X, Kysar JW, Hone J. Science 2008;321(5887):385e8. [6] Du X, Skachko I, Barker A, Andrei EY. Nature Nanotechnol 2008;3:491. [7] Wu J, Pisula W, Müllen K. Chemical Reviews 2007;107(3):718e47. [8] Geim AK, Kim P. Scientific American 2008;298:90. [9] Szabó T, Berkesi O, Forgó P, Josepovits K, Sanakis Y, Petridis D, et al. Chemistry of Materials 2006;18(11):2740e9. [10] Schniepp HC, Li J-L, McAllister MJ, Sai H, Herrera-Alonso M, Adamson DH, et al. Journal Of Physical Chemistry B 2006;110(17):8535e9. [11] McAllister MJ, Li J-L, Adamson DH, Schniepp HC, Abdala AA, Liu J, et al. Chemistry of Materials 2007;19(18):4396e404. [12] Dreyer DR, Park S, Bielawski CW, Ruoff RS. Chemical Society Reviews 2010; 39(1):228e40. [13] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Nature 2006;442(7100):282e6. [14] Steurer P, Wissert R, Thomann R, Mülhaupt R. Macromolecular Rapid Communications 2009;30:316e27. [15] Prud`Homme R, Aksay I, and Adamson D. US Patent 2007/0092432A1 2007. [16] Cai W, Piner RD, Stadermann FJ, Park S, Shaibat MA, Ishii Y, et al. Science 2008; 321(5897):1815e7. [17] Lerf A, He HY, Forster M, Klinowski J. Journal of Physical Chemistry B 1998; 102(23):4477e82. [18] Kim H, Abdala AA, Macosko CW. Macromolecules 2010;43(16):6515e30. [19] Verdejo R, Bernal MM, Romasanta LJ, Lopez-Manchado MA. Journal of Materials Chemistry 2010;21(10):3301e10. [20] Ramanathan T, Abdala AA, Stankovich S, Dikin DA, Herrera Alonso M, Piner RD, et al. Nature Nanotechnology 2008;3(6):327e31. [21] Tölle FJ, Fabritius M, Mülhaupt R. Advanced Functional Materials 2012;22(6): 1136e44. [22] Wang X, Hu Y, Song L, Yang H, Xing W, Lu H. Journal of Materials Chemistry 2010;21(12):4222e7. [23] Cai D, Yusoh K, Song M. Nanotechnology 2009:20. [24] Lee Y, Raghu AV, Jeong HM, Kim BK. Macromolecular Chemistry and Physics 2009;210:1247. [25] Ding JN, Y F, X ZC, Liu YB, Yu CT, Yuan NY. Journal of Composite Materials 2011;46(6):4035e41. [26] Kim H, Miura Y, Macosko CW. Chemistry of Materials 2010;22(11):3441e50. [27] Nguyen DA, Lee YR, Raghu AV, Jeong HM, Shin CM, Kim BK. Polymer International 2009;58(4):412e7. [28] Khan U, May P, O’Neill A, and Coleman JN. Carbon;48(14):4035e4041. [29] Hummers WS, Offeman RE. Journal of the American Chemical Society 1958; 80(6):1339. [30] Brunauer S, Emmett PH, Teller E. Journal of the American Chemical Society 1938;60:309. [31] Stankovich S, Piner RD, Nguyen ST, Ruoff RS. Carbon 2006;44(15):3342e7. [32] Xu Z, Gao C. Macromolecules 2010;43(16):6716. [33] Jeong HK, Noh HJ, Kim JY, Jin MH, Park CY, Lee YH. Europhysics Letters 2008;(6):67004. [34] Brosseau C, Queffelec P, Talbot P. Journal of Applied Physics 2001;89:4532. [35] Choi J, Kim D, Ryu K, Lee H-I, Jeong H, Shin C, et al. Macromolecular Research 2011;19(8):809e14.