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Composites Science and Technology 144 (2017) 208e214

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Composites Science and Technology

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The effect of catalyst addition on the structure, electrical andmechanical properties of the cross-linked polyurethane/carbonnanotube composites

Yu.V. Yakovlev a, b, *, Z.O. Gagolkina a, Eu.V. Lobko a, I. Khalakhan b, V.V. Klepko a

a Institute of Macromolecular Chemistry of NAS of Ukraine, 48 Kharkivske chaussee, Kyiv, 02160, Ukraineb Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles University in Prague, V Hole�sovi�ck�ach 2, 180 00, Prague 8, CzechRepublic

a r t i c l e i n f o

Article history:Received 8 November 2016Received in revised form20 March 2017Accepted 21 March 2017Available online 23 March 2017

Keywords:Cross-linked polyurethanesCarbon nanotube nanocompositesElectrical conductivityPercolation thresholdTensile strength

* Corresponding author. Institute of macromoleUkraine, 48 Kharkivske chaussee, Kyiv, 02160, Ukrain

E-mail address: yakovlevyv87@gmail.com (Yu.V. Y

http://dx.doi.org/10.1016/j.compscitech.2017.03.0340266-3538/© 2017 Elsevier Ltd. All rights reserved.

a b s t r a c t

Uniform distribution of filler particles in a polymer matrix is crucial to the improvement of properties ofpolymer composites. In this work, we have shown that control of the polymerization rate by addition ofthe catalyst (Fe(acac)3) hinders filler aggregation and enhances electrical and mechanical properties ofpolyurethane/nanotube composites. Thus, a percolation threshold value of 0.02 wt % obtained for thecomposites with the catalyst was much lower than the value of 0.65 wt % for the composites without thecatalyst. Moreover, the electrical conductivity of the catalytically prepared composites at a nanotubecontent of 3 wt % was two orders of magnitude higher than that of the non-catalytically prepared ones.The tensile strength of both types of composites showed an improvement at lower filler concentrations,however, the increase of filler content led to deterioration of the mechanical properties for the non-catalytically prepared composites. Structure of the composites was investigated by means of opticaland scanning electron microscopy. Additionally, the current-voltage characteristics (J-E) of the com-posites were studied.

© 2017 Elsevier Ltd. All rights reserved.

1. Introduction

Polymer composites based on different kinds of nanoscale fillersare of great scientific interest in the last two decades [1e9]. Theaddition of small amounts of nanofillers allows one to designunique materials, which own both polymer and filler properties.However, the main obstacle to the creation of nanocomposites withhigh performance is the agglomeration tendency of nanofillers. Toachieve a uniform distribution of nanoparticles in polymer matrixboth proper dispersion of the primary aggregates and prevention ofthe particle agglomeration after a mixing process are necessary [1].

In order to address the first problem, application of intensemixing to prepare nanocomposites is important. Among othermixing methods, ultrasonication of nanoparticles in a low viscousmedium allows one to get a proper dispersion of primary aggre-gates [2]. The ultrasonication finds wide use in the preparation of

cular chemistry of NAS ofe.akovlev).

composites by solution mixing [3] or in situ polymerization [4]methods. The second problem arises commonly in the mediawith a low viscosity. Thus, for the polymer composites preparedwith the help of solvent addition, there is a time period betweenthe end of mixing process and complete evaporation of the solvent.This time is long enough for the formation of the large secondaryaggregates, especially when the solvent has a high boiling point. Forthe stabilization of the nanoparticle dispersion different methods,such as chemical modification of filler [5], addition of surfactants[6] and secondary nanoparticles [7], and polymer wrapping [8], canbe used. However, in spite of the possibility to achieve a uniformdistribution of nanoparticles in the polymer matrix, some draw-backs of these methods occur. On the one hand, chemical modifi-cation of the carbon nanotubes (CNT) damages a p conjugation ofthe carbon atoms and, subsequently, impairs the electrical con-ductivity of composites [9,10]. On the other hand, application ofsurfactants and polymer wrapping increases the contact resistancebetween the nanotubes [11], but without degradation of theirproperties.

Another approach that can interrupt filler agglomeration is anacceleration of the polymer network formation during the

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Yu.V. Yakovlev et al. / Composites Science and Technology 144 (2017) 208e214 209

preparation of the composites by in situmethod. One of the ways toaccelerate network formation is a microwave irradiation of a pre-polymer/nanofiller mixture. Inwork [12], the microwave treatmentof polyethylene terephtalate/layered double hydroxide systems hasbeen resulted in the uniform filler distribution. Chang et al. [13]have prepared CNT/epoxy composites by thermal and microwavecuring methods. In this study, the decrease of the curing timeduring microwave treatment helps to achieve a uniform distribu-tion of the nanotubes and to enhance the dielectric properties ofthe nanocomposites.

Furthermore, the addition of the catalyst during in situ poly-merization of polyurethanes can significantly accelerate the ure-thane bonds formation and, therefore, the network formation aswell [14,15]. The main focus of this study is cross-linked poly-urethane/carbon nanotube (CPU/CNT) composites prepared by insitu polymerization with and without the addition of iron acetyla-cetonate (Fe(acac)3) as a catalyst.

2. Experimental

2.1. Materials

For the synthesis of the cross-linked polyurethanes (CPUs) thefollowing reagents were used. Polypropylene glycol with molecularmass 1000 (PPG) (Merck) was dried under pressure of 300 Pa at393 K for 3 h. Toluene diisocyanate (TDI) (2,4-/2,6-isomers¼ 80/20)(Sigma) was distilled under vacuum. 1,1,1-Tris-(hydroxymethyl)-propane (TMP) (98%, Sigma) was dried under vacuum at 313e315 K.Iron (III) acetylacetonate (99%, Sigma) was used as received.Dichloromethane (CH2Cl2) (Sigma) was redistilled.

The multi-walled CNTs («Specmash» Ltd., Ukraine) were madeby chemical vapor deposition (CVD) method at 0.1% of mineraladmixtures. Specific surface area of the nanotubes was 190 m2/g,external diameter was 20 nm, length was 5e10 mm, specific con-ductivity s of pressed nanoparticles (at pressure of 15 TPa) alongthe axis of compression was 10 S/cm.

2.2. Characterization

The reaction kinetic was controlled by the IR-spectroscopy usingFourier transform spectrometer “Tensor-37” (Bruker, Germany).Electrical conductivity of CPU/CNT composites was measured bythe two probe method using an impedancemeter Z-2000 (Elins,Russia) for conductivity above 10�7 S/cm and an alternating currentbridge P5083 (RostokPribor, Ukraine) for lower conductivity. De-pendencies of current density J versus electric field strength E(current-voltage characteristics) of the composites were measuredby UT804 (UNI-T, China) (for current measurements) and UT70С(UNI-T, China) (for voltage measurements) multimeters. TheB5e44A (Priborelectro, Russia) was a source of stabilized voltage.Measurements of the conductivity and JeE characteristics wereperformed on samples with thickness of 0.5 mm and diameter of14 mm.

Measurements of the tensile strength were carry out using 1925RAe10М (Uralpromtek, Russia) under load of 0.5 kN. The stretchingspeed was 40 mm/min. The samples in shape of the spatula wereused. Size of the functional part of the sample was

Fig. 1. Synthesis of prepolyme

150 � 2.5 � 0.5 mm3. The five samples for the one measurementwere used.

Morphology of the composites was investigated by means ofScanning Electron Microscopy (SEM) using a Tescan MIRA III (Tes-can, Czech Republic) microscope operating at 30 keV electron beamenergy and by Optical Microscopy (OM) using a microscope CarlZeiss Primo Star (Carl Zeiss, Germany). Specimens for SEM mea-surements were cut from in situ prepared composite films by knifeand had size of 1 � 5 mm2. The cut surface of the specimens wascovered by thin layer (~2 nm) of Pt. For OM measurements com-posite films were formed between two cover glasses. Thickness ofthe samples for OM measurements was adjusted by 20 mm-thickspacers.

Thermal transitions of the materials were investigated in airatmosphere in the temperature range from 123 to 473 K using DSCQ2000 (TA Instruments, USA). Thermal destruction of the sampleswas investigated in the temperature range from 293 to 973 K in airatmosphere using Derivatograph Q-1500D (MOM, Hungary).

2.3. Synthesis of cross-linked polyurethanes (CPU)

CPUs were synthesized in two stages. At the first stage, pre-polymer (Fig. 1) based on the PPG and TDI was synthesized at 393 Kduring 1.5 h (percentage of isocyanate groups was 5.9%). The PPG/TDI valence ratio was 1/2.

At the second stage, the prepolymer was cross-linked with theTMP (Fig. 2).

The TMPwas dissolved in the prepolymer at 343e348 K in an oilbath during 5 min with constant mixing under argon atmosphere.The prepolymer/TMP valence balance was 3/2.

CNTs were added to the reaction mass (from 0.02 to 3 % wt.) as adispersion in CH2Cl2. Such dispersions of CNTs in CH2Cl2 were ob-tained by using a sonicator UZN-22/44 (UKRROSPRIBOR Ltd,Ukraine) at 22 kHz during 2.5 min. Then the dispersions of CNTswere added to the polymer reaction mass and the sonication wascontinued for 2.5 min. The concentration of CNTs in CH2Cl2 was in arange of 0.015e2.3%. The formation of CPU/CNTs composites wascarried out in Petri dishes at 318 K. The solvent residues wereremoved from the composite films under vacuum to constantweight. For obtaining of the CPU/CNTs/Fe(acac)3 composites thecatalyst was initially solved in CH2Cl2. Then the dispersion of CNTsin CH2Cl2 (as described above) was mixed with this solution con-ditions and added in reaction mixture.

The investigated composites were flexible films. The tempera-ture of destruction of 10% of polymer mass was in the range from530 ± 2 K (for CPU-0 and CPU/CNT) to 558 ± 2 K (for CPU/Fe(acac)3and CPU/CNT/Fe(acac)3). The temperature of the glass transition offlexible oligoether segments in all composites was equal 263 ± 2 K(except CPU/Fe(acac)3 256±2 K).

3. Results and discussion

Fig. 3 represents the kinetics of chemical reaction between theprepolymer and TMP with and without addition of the catalystFe(acac)3. As clearly seen in Fig. 3, the time of NCO-groups con-version for systemswith Fe(acac)3 addition is 5.5 times less than forthe pristine ones. Hence, the catalytic effect of Fe(acac)3 allows to

r based on PPG and TDI.

Fig. 2. Cross-linkage of prepolymer by the TMP with the formation of CPU.

Fig. 3. The degree of NCO-groups conversion of CPU (1) and CPU/Fe(acac)3 (2)matrices.

Yu.V. Yakovlev et al. / Composites Science and Technology 144 (2017) 208e214210

form the cross-linked polyurethane network more quicker. For-mation of the cross-linked polymer network can impede themovement of nanoparticles and, as a result, prevents the aggrega-tion process. Therefore, time of the polymer network cross-linkingis a crucial parameter that controls aggregation of the filler.

Distribution of nanotubes in PU nanocomposites was investi-gated by means of OM (Fig. 4 aed) and SEM (Fig. 4 eef). As can beseen from optical micrographs, composites prepared with thecatalyst addition (Fig. 4 b, d) have a more uniform distribution ofthe nanofiller as compared to the ones prepared without thecatalyst (Fig. 4 a, c). In the SEM images the differences of surfaceroughness can be seen (Fig. 4 e, f). As shown in Fig. 4 e, surface ofCPU/CNT composites is rough. The aggregates of CNTs are pre-sented. On the contrary, CPU/CNT/Fe(acac)3 composites have asmooth surface (Fig. 4 f). There are no visible aggregates of CNTs.Such morphology of the composites can be explained by the uni-formity of filler distribution and by the smaller quantity of aggre-gates in CPU/CNT/Fe(acac)3 composites.

The electrical conductivity data of CPU/CNT and CPU/CNT/Fe(acac)3 composites vs. filler loading are shown in Fig. 5 a. Asevident, both composites show a dramatic increase in the con-ductivity by several orders of magnitude due to the percolationphenomenon.

Formation of the conductive network takes place in the range offiller content 0.5e0.8 wt % for the CPU/CNT composites and below0.1 wt % for the CPU/CNT/Fe(acac)3 ones with total enhancement ofthe conductivity by 3 and 7 orders of magnitude, respectively. Forprecise determination of the percolation threshold, according tothe classical percolation theory, conductivity data of the CPU basedcomposites were analyzed by scaling approach of the percolationtheory [16]:

sðCÞfðC � CcÞt (1)

where t is a critical index, that lays in the range of 1.6e2 for 3Dsystems, Cc is a percolation threshold or critical concentration atwhich formation of the conductive filler network in the insulatingmatrix occurs. As shown in Fig. 5 a, b, Eq. (1) is in a good agreementwith the electrical conductivity data of both composites. The bestfitting to the experimental data resulted in Cc1 ¼ 0.65 wt %, t ¼ 1.54and Cc2 ¼ 0.02 wt %, t ¼ 3.31 for CPU/CNT and CPU/CNT/Fe(acac)3composites, respectively. Percolation transition in PU/CNT com-posites usually takes place in the range from 0.13 to 10 wt % (0.13%[17], 0.39% [18], 3.4% [19], 2.03 vol % (~4%) [20], 4% [21], 5 vol %(~10%) [22]) of the filler content. In our study, an achievement of therelatively low percolation threshold (Cc1 ¼ 0.65 wt %) can beexplained by efficiency of treatment of the primary aggregates bythe sonication method. At the same time, the CPU/CNT/Fe(acac)3composites have much lower percolation threshold (Cc2 ¼ 0.02 wt%). To the best of our knowledge, such ultra low value of thepercolation threshold for polyurethane/CNT composites is reportedfor the first time. The reason of such result can be a good dispersionof the individual CNTs in the catalytically prepared composites.Therefore, more nanotubes are available to contact each other, andthe probability of the formation of the percolation network at lowfiller loading increases. This effect is also consistent with micro-scopy data (Fig. 4).

Fig. 4. OM (aed) and SEM (e, f) images of CPU/CNT composites prepared with (b, d, f) and without (a, c, e) catalyst addition. Composites with 0.02% (a, b), 1% (c, d) and 3% (e, f) ofCNT content were used for OM (aed) and SEM (e, f) methods.

Yu.V. Yakovlev et al. / Composites Science and Technology 144 (2017) 208e214 211

Besides the increase of the interparticle contacts quantity, thenarrowing of the insulating polymer gaps between CNTs due to theaccelerated polymerization of the composites can take place. Instudy [23] has been shown the increase of conductivity of epoxy/silver particles composites by almost two orders of magnitude,when at the gelation stage the degree of cure of epoxywasminimal.In our work we have got the similar result: the conductivity of thecomposites prepared with and without the catalyst additiondiffered by two orders of magnitude at 3 wt % of CNT content, i.e.the conductivity reached values of 1.1$10�4 and 1.1$10�6 S/cm forthe catalytically and non-catalytically prepared composites,respectively.

The critical indexes of CPU/CNT systems deviate from thetheoretically predicted values. The reasons of that can be non sta-tistical distribution of CNTs in polymermatrix, tunneling effects etc.So, for HDPE/carbon black [24], sPS/CNT [25], TPU/CNT [17] and

SCPU/SWNT [19] composites critical indexes were bigger thantheoretical (t ¼ 2) and equal to 3.1, 2.71, 4.6 and 4.7, respectively,due to the tunneling effect. On the contrary, aggregation of the fillerin the epoxy/CNT composites [26] led to decrease of the index to1.2. In order to achieve a deeper understanding of the conductivitymechanisms in the polyurethane/nanotube composites we inves-tigated the current-voltage characteristics (Fig. 6) of the compos-ites. For this purpose composites with two filler loadings above thepercolation threshold were chosen.

All the composites demonstrate non-linear behavior of the J-Echaracteristics, as shown in Fig. 6. Such behavior can not beexplained by the classical percolation theory, which assumes directcontacts between ohmic fillers and, consequently, the linear J-Echaracteristic of the composite. Possible explanation of the non-linear behavior of binary systems can be founded in the frame-work of random resistor network models [27]. In this case, non-

Fig. 5. The electrical conductivity of CPU/CNT and CPU/CNT/Fe(acac)3 composites as a function of filler weight fraction (a) and a log-log plot of sDC vs. C-Cc for the same composites(b).

Yu.V. Yakovlev et al. / Composites Science and Technology 144 (2017) 208e214212

linearity of J-E behavior can be described by the empirical formula:

JðEÞ ¼ sE þ s0Eb (2)

where s and s0 are linear and non-linear conductivity, respectively;J(E) is a current density, E is an electric field, b is an exponent thatusually is in the range of 1e3. The best fitting of the experimentaldata to Eq. (2) was achieved with b ¼ 3 (Fig. 6).

To obtain more information about charge transport in the CPUcomposites, we used an approach that accounts electron tunnelingbetween the CNTs. In some works [28e30] the effect of Zenertunneling in polymers filled with carbonaceous nanofillers wasinvestigated. According to this approach and by taking intoconsideration the linear part of current-voltage characteristic (sE),the J(E) function can be written as follows [30]:

JðEÞ ¼ sE þ AEnexp��B

E

�(3)

where A and B are constants proportional to frequency of thetunneling attempts and the tunneling barrier width, respectively; nis an exponent in range of 1e3. An analysis of the experimental datais shown in Fig. 6. As can be seen from the figure, Eq. (3) is in a goodagreement with the J-E data of CPU based nanocomposites. Thefitting parameters are presented in Fig. 7. As one can see, there is a

Fig. 6. J-E characteristics of CPU/CNT (a) a

tendency towards increasing the tunneling events frequency(parameter A) and decreasing the width of the tunneling barrier(parameter B) with the growth of the CNTconcentration. Moreover,the CPU/CNT/Fe(acac)3 composites had more tunneling events perunit of time and narrower tunneling barrier in comparison to theCPU/CNT ones for both presented concentrations of nanotubes. Inthe study [29] a correlation between the parameters A, B and n ofEq. (3) and state of CNT dispersion has been found. According to thisstudy, composites with uniform distribution of nanotubesdemonstrated the decrease of the B and n parameters and the in-crease of the A parameter comparing to the composites with poorlydistributed filler. In our study, the similar results have shown(Fig. 7). As the CPU/CNT/Fe(acac)3 composites have more uniformdistribution of nanotubes, then the average distance betweennanotubes becomes smaller resulting in the decrease of thetunneling barrier (decrease of B). Additionally, the narrowing of thepolymer gap leads to the increase of amount of the tunnelingelectrons (increase of A).

The addition of the nanotubes to the polymer matrix causes theimprovement of the mechanical properties of composites [31]. Asshown in Fig. 8, addition of small amounts of the nanotubes im-proves the mechanical properties of both CPU/CNT and CPU/CNT/Fe(acac)3 composites due to the reinforcement effect of the filler.The highest value of tensile strength for both systems is 14 MPa atthe 0.75% and 1.5% of CNTs for CPU/CNT and CPU/CNT/Fe(acac)3,

nd CPU/CNT/Fe(acac)3 (b) composites.

Fig. 7. Comparison of the tunneling parameters A and B (Eq. (3)) for CPU basedcomposites.

Yu.V. Yakovlev et al. / Composites Science and Technology 144 (2017) 208e214 213

respectively.However, the addition of successive amounts of nanotubes leads

to decrease the tensile strength values of CPU/CNT composites[32,33]. Moreover, the tensile strength of CPU/CNT/Fe(acac)3 com-posites monotonically increases and reaches the plateau at higherfiller concentrations (>1.5%). Such behavior can be connected withthe dynamics of filler aggregation. After the ultrasonication processgrowth of the aggregates depends either on the time of aggregationor on the filler concentration [34]. When the concentration in-creases, the filler particles tend to form the bigger aggregates. Suchaggregates are weakly linked and act as defects in the composite.Thus, at the bigger concentrations, the reinforcement effect ofnanotubes becomes suppressed by the weakening effect of thelarge aggregates. In the case of the catalytically prepared CPU/CNT/Fe(acac)3 composites, the significant reduction of the polymeriza-tion time leads to the decrease of the aggregate size and, subse-quently, the deterioration of mechanical properties in the fillerconcentration region is less pronounced.

4. Conclusions

Polyurethane based nanocomposites have been prepared in situwith (CPU/CNT/Fe(acac)3) and without (CPU/CNT) addition of the

Fig. 8. Tensile strength vs. filler concentration of CPU/CNT and CPU/CNT/Fe(acac)3composites.

catalyst of polymerization. The addition of catalyst Fe(acac)3 in-creases the rate of PU polymerization by 5.5 times and results in thecomposites with improved properties. Morphological studies showthe uniform distribution of the nanotubes in the PUmatrix for CPU/CNT/Fe(acac)3. The electrical percolation threshold decreases from0.65% for CPU/CNT to 0.02% for CPU/CNT/Fe(acac)3 composites, andthe conductivity reaches the maximum value of 1.1$10�4 S/cm.Investigation of the current-voltage characteristics of the com-posites reveals a non-linear behavior that can be consequence ofelectron transport by tunneling rather than through the directcontacts between CNTs. Moreover, the addition of the catalyst leadsto narrowing of the insulating gap between nanotubes. Addition-ally, the reduced time of the CPU/CNT/Fe(acac)3 composites filmformation leads to the improved mechanical properties at the CNTconcentrations above 1.5%.

Thus, the formation of CPU/CNT composites in situ in the pres-ence of the catalyst of the polymerization opens up new possibil-ities of the control of aggregation of CNTs and the achievement ofthe desirable properties of the composites.

Acknowledgements

The authors thank to the researchers of the Center of CollectiveUse of scientific Equipments (CCUE) of the National academy ofscience of Ukraine “Thermophysics investigation and analysis” forconduction the investigation by using optical microscopy and DSC.

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The effect of catalyst addition on the structure, electrical and mechanical properties of the cross-linked polyurethane/car ...1. Introduction2. Experimental2.1. Materials2.2. Characterization2.3. Synthesis of cross-linked polyurethanes (CPU)

3. Results and discussion4. ConclusionsAcknowledgementsReferences