carbon nanotube composites

Accepted Manuscript The effect of catalyst addition on the structure, electrical and mechanical properties of the cross-linked polyurethane/carbon nanotube composites Yu.V. Yakovlev, Z.O. Gagolkina, Eu.V. Lobko, I. Khalakhan, V.V. Klepko PII:

S0266-3538(16)31671-2

DOI:

10.1016/j.compscitech.2017.03.034

Reference:

CSTE 6719

To appear in:

Composites Science and Technology

Received Date: 8 November 2016 Revised Date:

20 March 2017

Accepted Date: 21 March 2017

Please cite this article as: Yakovlev YV, Gagolkina ZO, Lobko EV, Khalakhan I, Klepko VV, The effect of catalyst addition on the structure, electrical and mechanical properties of the cross-linked polyurethane/carbon nanotube composites, Composites Science and Technology (2017), doi: 10.1016/ j.compscitech.2017.03.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

The effect of catalyst addition on the structure, electrical and mechanical properties of the cross-linked polyurethane/carbon nanotube composites (*)

, Z.O. Gagolkina1, Eu.V. Lobko1, I. Khalakhan2, V.V.

RI PT

Yu.V. Yakovlev1,2 Klepko1 1

Institute of macromolecular chemistry of NAS of Ukraine, 48 Kharkivske

2

SC

chaussee, Kyiv 02160, Ukraine

Faculty of Mathematics and Physics, Department of Surface and Plasma

M AN U

Science, Charles University in Prague,V Holešovičkách 2, 180 00 Prague 8, Czech Republics Abstract

Uniform distribution of filler particles in a polymer matrix is crucial to the

TE D

improvement of properties of polymer composites. In this work, we have shown that control of the polymerization rate by addition of the catalyst (Fe(acac)3) hinders filler aggregation

and

enhances

electrical

and

mechanical

properties

of

EP

polyurethane/nanotube composites. Thus, a percolation threshold value of 0.02 wt. % obtained for the composites with the catalyst was much lower than the value of

AC C

0.65 wt. % for the composites without the catalyst. Moreover, the electrical conductivity of the catalytically prepared composites at a nanotube content 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 optical and scanning electron microscopy. Additionally, the current-voltage characteristics (J-E) of the composites were studied.

1

ACCEPTED MANUSCRIPT Keywords: cross-linked polyurethanes; carbon nanotube nanocomposites; electrical conductivity; percolation threshold; tensile strength.

RI PT

1. Introduction Polymer composites based on different kinds of nanoscale fillers are of great scientific interest in the last two decades [1-9]. The addition of small amounts of

SC

nanofillers allows one to design unique materials, which own both polymer and filler properties. However, the main obstacle to the creation of nanocomposites with high

M AN U

performance is the agglomeration tendency of nanofillers. To achieve a uniform distribution of nanoparticles in polymer matrix both proper dispersion of the primary aggregates and prevention of the particle agglomeration after a mixing process are necessary [1].

In order to address the first problem, application of intense mixing to prepare

TE D

nanocomposites is important. Among other mixing methods, ultrasonication of nanoparticles in a low viscous medium allows one to get a proper dispersion of primary

EP

aggregates [2]. The ultrasonication finds wide use in the preparation of composites by solution mixing [3] or in situ polymerization [4] methods. The second problem arises

AC C

commonly in the media with a low viscosity. Thus, for the polymer composites prepared with the help of solvent addition, there is a time period between the end of mixing process and complete evaporation of the solvent. This time is long enough for the formation of the large secondary aggregates, especially when the solvent has a high boiling point. For the 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], can be used. However, in spite of the possibility to achieve a uniform distribution of nanoparticles in the polymer matrix,

2

ACCEPTED MANUSCRIPT

some drawbacks of these methods occur. On the one hand, chemical modification of the carbon nanotubes (CNT) damages a π conjugation of the carbon atoms and, subsequently, impairs the electrical conductivity of composites [9, 10]. On the other hand, application of surfactants and polymer wrapping increases the contact resistance

RI PT

between the nanotubes [11], but without degradation of their properties. Another approach that can interrupt filler agglomeration is an acceleration of the polymer network formation during the preparation of the composites by in situ method.

SC

One of the ways to accelerate network formation is a microwave irradiation of a prepolymer/nanofiller mixture. In work [12], the microwave treatment of polyethylene

M AN U

terephtalate/layered double hydroxide systems has been resulted in the uniform filler distribution. Chang et. al. [13] have prepared CNT/epoxy composites by thermal and microwave curing methods. In this study, the decrease of the curing time during microwave treatment helps to achieve a uniform distribution of the nanotubes and to enhance the dielectric properties of the nanocomposites.

TE D

Furthermore, the addition of the catalyst during in situ polymerization of polyurethanes can significantly accelerate the urethane bonds formation and, therefore,

EP

the network formation as well [14, 15]. The main focus of this study is cross-linked polyurethane/carbon

nanotube

(CPU/CNT)

composites

prepared

by

in

situ

AC C

polymerization with and without the addition of iron acetylacetonate (Fe(acac)3) as a catalyst.

2. Experimental 2.1. Materials

For the synthesis of the cross-linked polyurethanes (CPUs) the following reagents were used. Polypropylene glycol with molecular mass 1000 (PPG) (Merck) was dried under pressure of 300 Pa at 393 K for 3 h. Toluene diisocyanate (TDI) (2,4/2,6-isomers = 80/20) (Sigma) was distilled under vacuum. 1,1,1-Tris-(hydroxymethyl)-

3

ACCEPTED MANUSCRIPT

propane (TMP) (98 %, Sigma) was dried under vacuum at 313 – 315 K. Iron (III) acetylacetonate (99 %, Sigma) was used as received. Dichloromethane (CH2Cl2) (Sigma) was redistilled. The multi-walled CNTs («Specmash» Ltd., Ukraine) were made by chemical

RI PT

vapor deposition (CVD) method at 0.1 % of mineral admixtures. Specific surface area of the nanotubes was 190 m2/g, external diameter was 20 nm, length was 5 – 10 µm, specific conductivity σ of pressed nanoparticles (at pressure of 15 TPa) along the axis of

SC

compression was 10 S/cm. 2.2. Characterization

M AN U

The reaction kinetic was controlled by the IR-spectroscopy using Fourier transform spectrometer “Tensor-37” (Bruker, Germany). Electrical conductivity of CPU/CNT composites was measured by the two probe method using an impedancemeter Z-2000 (Elins, Russia) for conductivity above 10-7 S/cm and an alternating current bridge P5083 (RostokPribor, Ukraine) for lower conductivity.

TE D

Dependencies of current density J versus electric field strength E (current-voltage characteristics) of the composites were measured by UT804 (UNI-T, China) (for current

EP

measurements) and UT70С (UNI-T, China) (for voltage measurements) multimeters. The B5–44A (Priborelectro, Russia) was a source of stabilized voltage. Measurements

AC C

of the conductivity and J–E characteristics were performed on samples with thickness of 0.5 mm and diameter of 14 mm. Measurements of the tensile strength were carry out using 1925 RA–10 М

(Uralpromtek, Russia) under load of 0.5 kN. The stretching speed was 40 mm/min. The samples in shape of the spatula were used. Size of the functional part of the sample was 150×2.5×0.5 mm3. The five samples for the one measurement were used. Morphology of the composites was investigated by means of Scanning Electron Microscopy (SEM) using a Tescan MIRA III (Tescan, Czech Republic) microscope

4

ACCEPTED MANUSCRIPT

operating at 30 keV electron beam energy and by Optical Microscopy (OM) using a microscope Carl Zeiss Primo Star (Carl Zeiss, Germany). Specimens for SEM measurements were cut from in situ prepared composite films by knife and had size of 1x5 mm2. The cut surface of the specimens was covered by thin layer (~2 nm) of Pt. For

RI PT

OM measurements composite films were formed between two cover glasses. Thickness of the samples for OM measurements was adjusted by 20 µm-thick spacers.

Thermal transitions of the materials were investigated in air atmosphere in the

SC

temperature range from 123 to 473 K using DSC Q2000 (TA Instruments, USA). Thermal destruction of the samples was investigated in the temperature range from 293

M AN U

to 973 K in air atmosphere using Derivatograph Q-1500D (MOM, Hungary). 2.2. Synthesis of cross-linked polyurethanes (CPU)

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

TE D

At the second stage, the prepolymer was cross-linked with the TMP (Fig. 2). The TMP was dissolved in the prepolymer at 343 – 348 K in an oil bath during

EP

5 min with constant mixing under argon atmosphere. The prepolymer/TMP valence balance was 3/2.

AC C

CNTs were added to the reaction mass (from 0.02 to 3 % wt.) as a dispersion in CH2Cl2. Such dispersions of CNTs in CH2Cl2 were obtained by using a sonicator UZN22/44 (UKRROSPRIBOR Ltd, Ukraine) at 22 kHz during 2.5 min. Then the dispersions of CNTs were added to the polymer reaction mass and the sonication was continued for 2.5 min. The concentration of CNTs in CH2Cl2 was in a range of 0.015 – 2.3 %. The formation of CPU/CNTs composites was carried out in Petri dishes at 318 K. The solvent residues were removed from the composite films under vacuum to constant weight. For obtaining of the CPU/CNTs/Fe(acac)3 composites the catalyst was initially solved in

5

ACCEPTED MANUSCRIPT

CH2Cl2. Then the dispersion of CNTs in CH2Cl2 (as described above) was mixed with this solution conditions and added in reaction mixture. The investigated composites were flexible films. The temperature of destruction of 10 percent of polymer mass was in the range from 530±2 K (for CPU-0 and

RI PT

CPU/CNT) to 558±2 K (for CPU/Fe(acac)3 and CPU/CNT/Fe(acac)3). The temperature of the glass transition of flexible oligoether segments in all composites was equal 263±2 K (except CPU/Fe(acac)3 256±2K).

SC

3. Results and discussion

Fig. 3 represents the kinetics of chemical reaction between the prepolymer and

M AN U

TMP with and without addition of the catalyst Fe(acac)3. As clearly seen in the Fig. 3, the time of NCO-groups conversion for systems with Fe(acac)3 addition is 5.5 times less than for the pristine ones. Hence, the catalytic effect of Fe(acac)3 allows to form the cross-linked polyurethane network more quicker. Formation of the cross-linked polymer network can impede the movement of nanoparticles and, as a result, prevents the

TE D

aggregation process. Therefore, time of the polymer network cross-linking is a crucial parameter that controls aggregation of the filler.

EP

Distribution of nanotubes in PU nanocomposites was investigated by means of OM (Fig. 4 a-d) and SEM (Fig. 4 e-f). As can be seen from optical micrographs,

AC C

composites prepared with the catalyst addition (Fig. 4 b, d) have a more uniform distribution of the nanofiller as compared to the ones prepared without the catalyst (Fig. 4 a, c). In the SEM images the differences of surface roughness can be seen (Fig. 4 e, f). As shown in Fig. 4 e, surface of CPU/CNT composites is rough. The aggregates of CNTs are presented. On the contrary, CPU/CNT/Fe(acac)3 composites have a smooth surface (Fig. 4 f). There are no visible aggregates of CNTs. Such morphology of the composites can be explained by the uniformity of filler distribution and by the smaller quantity of aggregates in CPU/CNT/Fe(acac)3 composites.

6

ACCEPTED MANUSCRIPT

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

RI PT

Formation of the conductive network takes place in the range of filler content 0.5 – 0.8 wt. % for the CPU/CNT composites and below 0.1 wt. % for the CPU/CNT/Fe(acac)3 ones with total enhancement of the conductivity by 3 and 7 orders

SC

of magnitude, respectively. For precise determination of the percolation threshold, according to the classical percolation theory, conductivity data of the CPU based

M AN U

composites were analyzed by scaling approach of the percolation theory [16]: ߪሺ‫ܥ‬ሻ ∝ ሺ‫ ܥ‬− ‫ܥ‬௖ ሻ௧

1

where t is a critical index, that lays in the range of 1.6 – 2 for 3D systems, Cc is a percolation threshold or critical concentration at which formation of the conductive

TE D

filler network in the insulating matrix occurs. As shown in Fig. 5 a, b, Eq. (1) is in a good agreement with the electrical conductivity data of both composites. The best fitting to the experimental data resulted in Cc1=0.65 wt. %, t=1.54 and Cc2=0.02 wt. %,

EP

t=3.31 for CPU/CNT and CPU/CNT/Fe(acac)3 composites, respectively. Percolation transition in PU/CNT composites usually takes place in the range from 0.13 to 10 wt. % [17],

0.39 % [18],

AC C

(0.13 %

3.4 %

[19],

2.03 vol. % (~4 %) [20],

4%

[21],

5 vol. % (~10 %) [22]) of the filler content. In our study, an achievement of the relatively low percolation threshold (Cc1=0.65 wt. %) can be explained by efficiency of treatment of the primary aggregates by the sonication method. At the same time, the CPU/CNT/Fe(acac)3

composites

have

much

lower

percolation

threshold

(Cc2=0.02 wt. %). To the best of our knowledge, such ultra low value of the percolation threshold for polyurethane/CNT composites is reported for the first time. The reason of such result can be a good dispersion of the individual CNTs in the catalytically prepared

7

ACCEPTED MANUSCRIPT

composites. Therefore, more nanotubes are available to contact each other, and the probability of the formation of the percolation network at low filler loading increases. This effect is also consistent with microscopy data (Fig. 4). Besides the increase of the interparticle contacts quantity, the narrowing of the

RI PT

insulating polymer gaps between CNTs due to the accelerated polymerization of the composites can take place. In study [23] has been shown the increase of conductivity of epoxy/silver particles composites by almost two orders of magnitude, when at the

SC

gelation stage the degree of cure of epoxy was minimal. In our work we have got the similar result: the conductivity of the composites prepared with and without the catalyst

M AN U

addition differed 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 for the catalytically and noncatalytically prepared composites, respectively.

The critical indexes of CPU/CNT systems deviate from the theoretically predicted values. The reasons of that can be non statistical distribution of CNTs in

TE D

polymer matrix, tunneling effects etc. So, for HDPE/carbon black [24], sPS/CNT [25], TPU/CNT [17] and SCPU/SWNT [19] composites critical indexes were bigger than

EP

theoretical (t = 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 filler in the epoxy/CNT composites [26] led

AC C

to decrease of the index to 1.2. In order to achieve a deeper understanding of the conductivity mechanisms in the polyurethane/nanotube composites we investigated the current-voltage characteristics (Fig. 6) of the composites. For this purpose composites with two filler loadings above the percolation threshold were chosen. All the composites demonstrate non-linear behavior of the J-E characteristics, as shown in Fig. 6. Such behavior can not be explained by the classical percolation theory, which assumes direct contacts between ohmic fillers and, consequently, the linear J-E characteristic of the composite. Possible explanation of the non-linear behavior of

8

ACCEPTED MANUSCRIPT

binary systems can be founded in the framework of random resistor network models [27]. In this case, non-linearity of J-E behavior can be described by the empirical formula: ‫ܬ‬ሺ‫ܧ‬ሻ = ߪ‫ ܧ‬+ ߪ′‫ ܧ‬௕

2

RI PT

where σ and σ’ are linear and non-linear conductivity, respectively; J(E) is a current density, E is an electric field, b is an exponent that usually is in the range of 1 – 3. The best fitting of the experimental data to Eq.(2) was achieved with b = 3 (Fig. 6).

SC

To obtain more information about charge transport in the CPU composites, we used an approach that accounts electron tunneling between the CNTs. In some works

M AN U

[28–30] the effect of Zener tunneling in polymers filled with carbonaceous nanofillers was investigated. According to this approach and by taking into consideration the linear part of current-voltage characteristic (σE), the J(E) function can be written as follows [30]:

TE D

‫ܬ‬ሺ‫ܧ‬ሻ = ߪ‫ ܧ‬+ ‫ ܧܣ‬௡ ݁‫ ݌ݔ‬൬

−‫ܤ‬ ൰ ‫ܧ‬

3

where A and B are constants proportional to frequency of the tunneling attempts and the

EP

tunneling barrier width, respectively; n is an exponent in range of 1 – 3. An analysis of the experimental data is shown in Fig. 6. As can be seen from the figure, Eq. (3) is in a

AC C

good agreement with the J-E data of CPU based nanocomposites. The fitting parameters are presented in Fig. 7. As one can see, there is 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 CNT concentration. Moreover, the CPU/CNT/Fe(acac)3 composites had more tunneling events per unit of time and narrower tunneling barrier in comparison to the CPU/CNT ones for both presented concentrations of nanotubes. In the study [29] a correlation between the parameters A, B and n of Eq.(3) and state of CNT dispersion has been found. According to this study,

9

ACCEPTED MANUSCRIPT

composites with uniform distribution of nanotubes demonstrated the decrease of the B and n parameters and the increase of the A parameter comparing to the composites with poorly distributed filler. In our study, the similar results have shown (Fig. 7). As the CPU/CNT/Fe(acac)3 composites have more uniform distribution of nanotubes, then the

RI PT

average distance between nanotubes becomes smaller resulting in the decrease of the tunneling barrier (decrease of B). Additionally, the narrowing of the polymer gap leads to the increase of amount of the tunneling electrons (increase of A).

SC

The addition of the nanotubes to the polymer matrix causes the improvement of the mechanical properties of composites [31]. As shown in Fig. 8, addition of small

M AN U

amounts of the nanotubes improves 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 at the 0.75 % and 1.5 % of CNTs for CPU/CNT and CPU/CNT/Fe(acac)3, respectively.

However, the addition of successive amounts of nanotubes leads to decrease the

TE D

tensile strength values of CPU/CNT composites [32, 33]. Moreover, the tensile strength of CPU/CNT/Fe(acac)3 composites monotonically increases and reaches the plateau at

EP

higher filler concentrations (>1.5 %). Such behavior can be connected with the dynamics of filler aggregation. After the ultrasonication process growth of the

AC C

aggregates depends either on the time of aggregation or on the filler concentration [34]. When the concentration increases, the filler particles tend to form the bigger aggregates. Such aggregates are weakly linked and act as defects in the composite. Thus, at the bigger concentrations, the reinforcement effect of nanotubes becomes suppressed by the weakening effect of the large aggregates. In the case of the catalytically prepared CPU/CNT/Fe(acac)3 composites, the significant reduction of the polymerization time leads to the decrease of the aggregate size and, subsequently, the deterioration of mechanical properties in the filler concentration region is less pronounced.

10

ACCEPTED MANUSCRIPT

4. Conclusions

Polyurethane based nanocomposites have been prepared in situ

with

(CPU/CNT/Fe(acac)3) and without (CPU/CNT) addition of the catalyst of polymerization. The addition of catalyst Fe(acac)3 increases the rate of PU

RI PT

polymerization by 5.5 times and results in the composites with improved properties. Morphological studies show the uniform distribution of the nanotubes in the PU matrix for CPU/CNT/Fe(acac)3. The electrical percolation threshold decreases from 0.65 % for CPU/CNT to 0.02 % for CPU/CNT/Fe(acac)3 composites, and the conductivity reaches

SC

the maximum value of 1.1·10-4 S/cm. Investigation of the current-voltage characteristics

M AN U

of the composites reveals a non-linear behavior that can be consequence of electron transport by tunneling rather than through the direct contacts between CNTs. Moreover, the addition of the catalyst leads to narrowing of the insulating gap between nanotubes. Additionally, the reduced time of the CPU/CNT/Fe(acac)3 composites film formation leads to the improved mechanical properties at the CNT concentrations above 1.5%.

TE D

Thus, the formation of CPU/CNT composites in situ in the presence of the catalyst of the polymerization opens up new possibilities of the control of aggregation

EP

of CNTs and the achievement of the desirable properties of the composites.

AC C

Acknowledgements

The authors thank to the researchers of the Center of Collective Use of scientific

Equipments (CCUE) of the National academy of science of Ukraine “Thermophysics investigation and analysis” for conduction the investigation by using optical microscopy and DSC. References

11

[1]

ACCEPTED MANUSCRIPT

D. Carponcin, E. Dantras, G. Aridon, F. Levallois, L. Cadiergues, C. Lacabanne,

Evolution of dispersion of carbon nanotubes in Polyamide 11 matrix composites as determined by DC conductivity, Compos. Sci. Technol. 72 (2012) 515–520. [2] Y.Y. Huang, E.M. Terentjev, Dispersion of Carbon Nanotubes: Mixing, Sonication,

RI PT

Stabilization, and Composite Properties, Polymers. 4 (2012) 275–295. [3] K. Ke, Y. Wang, X.-Q. Liu, J. Cao, Y. Luo, W. Yang, B.-H. Xie, M.-B. Yang, A comparison of melt and solution mixing on the dispersion of carbon nanotubes in a

SC

poly(vinylidene fluoride) matrix, Compos. Part B Eng. 43 (2012) 1425–1432.

[4] M. Lahelin, A. Vesterinen, A. Nykänen, J. Ruokolainen, J. Seppälä, In situ

M AN U

polymerization of methyl methacrylate/multi-walled carbon nanotube composites using cationic stearyl methacrylate copolymers as dispersants, Eur. Polym. J. 47 (2011) 873–881.

[5] N.G. Sahoo, S. Rana, J.W. Cho, L. Li, S.H. Chan, Polymer nanocomposites based on functionalized carbon nanotubes, Prog. Polym. Sci. 35 (2010) 837–867.

TE D

[6] Y. Geng, M.Y. Liu, J. Li, X.M. Shi, J.K. Kim, Effects of surfactant treatment on mechanical and electrical properties of CNT/epoxy nanocomposites, Compos. Part

EP

Appl. Sci. Manuf. 39 (2008) 1876–1883. [7] J. Sumfleth, L.A.S. de Almeida Prado, M. Sriyai, K. Schulte, Titania-doped multi-

AC C

walled carbon nanotubes epoxy composites: Enhanced dispersion and synergistic effects in multiphase nanocomposites, Polymer. 49 (2008) 5105–5112.

[8] Y. Huang, Y. Zheng, W. Song, Y. Ma, J. Wu, L. Fan, Poly(vinyl pyrrolidone) wrapped multi-walled carbon nanotube/poly(vinyl alcohol) composite hydrogels, Compos. Part Appl. Sci. Manuf. 42 (2011) 1398–1405. [9] P.C. Ma, J.-K. Kim, B.Z. Tang, Effects of silane functionalization on the properties of carbon nanotube/epoxy nanocomposites, Compos. Sci. Technol. 67 (2007) 2965– 2972.

12

[10]

ACCEPTED MANUSCRIPT

F. Buffa, G.A. Abraham, B.P. Grady, D. Resasco, Effect of nanotube

functionalization on the properties of single-walled carbon nanotube/polyurethane composites, J. Polym. Sci. Part B Polym. Phys. 45 (2007) 490–501. [11] J. Vilčáková, R. Moučka, P. Svoboda, M. Ilčíková, N. Kazantseva, M. Hřibová, M.

RI PT

Mičušík, M. Omastová, Effect of Surfactants and Manufacturing Methods on the Electrical and Thermal Conductivity of Carbon Nanotube/Silicone Composites, Molecules. 17 (2012).

S. Martínez-Gallegos, M. Herrero, V. Rives, In situ microwave-assisted

SC

[12]

polymerization of polyethylene terephtalate in layered double hydroxides, J. Appl.

[13]

M AN U

Polym. Sci. 109 (2008) 1388–1394.

J. Chang, G. Liang, A. Gu, S. Cai, L. Yuan, The production of carbon

nanotube/epoxy composites with a very high dielectric constant and low dielectric loss by microwave curing, Carbon. 50 (2012) 689–698. [14]

Saunders, J.H. & Frish, K.C.,. Polyurethanes. Chemistry and Technology Part 1

[15]

TE D

Chemistry, Interscience Publisher John Wiley&Sons, New York-London, 1962. Ana L. Silva and Joao C. Bordado, Recent Developments in Polyurethane

[16]

EP

Catalysis: Catalytic Mechanisms Review, Сatalysis reviews, 46 (2004) 31–51. D. Stauffer, A. Aharony, Introduction to percolation theory, 2nd ed., Taylor &

AC C

Francis, London, 2003. [17]

R. Zhang, A. Dowden, H. Deng, M. Baxendale, T. Peijs, Conductive network

formation in the melt of carbon nanotube/thermoplastic polyurethane composite, Compos. Sci. Technol. 69 (2009) 1499–1504.

[18]

J. Chen, Z. Zhang, W. Huang, J. Li, J. Yang, Y. Wang, Z. Zhou, J. Zhang,

Carbon nanotube network structure induced strain sensitivity and shape memory behavior changes of thermoplastic polyurethane, Mater. Des. 69 (2015) 105–113.

13

[19]

ACCEPTED MANUSCRIPT

Z. Liu, G. Bai, Y. Huang, F. Li, Y. Ma, T. Guo, X. He, X. Lin, H. Gao, Y. Chen,

Microwave Absorption of Single-Walled Carbon Nanotubes/Soluble Cross-Linked Polyurethane Composites, J. Phys. Chem. C. 111 (2007) 13696–13700. [20]

H.

He,

X.-B.

Xu,

D.-F.

Zhang,

An

aligned

macro-porous

carbon

vapors, Sens. Actuators B Chem. 176 (2013) 940–944. [21]

RI PT

nanotube/waterborne polyurethane sensor for the detection of flowing organic

B. Fernández-d’Arlas, U. Khan, L. Rueda, J.N. Coleman, I. Mondragon, M.A.

SC

Corcuera, A. Eceiza, Influence of hard segment content and nature on polyurethane/multiwalled carbon nanotube composites, Compos. Sci. Technol. 71

[22]

M AN U

(2011) 1030–1038.

H. Koerner, W. Liu, M. Alexander, P. Mirau, H. Dowty, R.A. Vaia,

Deformation–morphology correlations in electrically conductive carbon nanotube— thermoplastic polyurethane nanocomposites, Polymer. 46 (2005) 4405–4420. [23]

X. Zhang, H. Sun, C. Yang, K. Zhang, M.M.F. Yuen, S. Yang, Highly

TE D

conductive polymer composites from room-temperature ionic liquid cured epoxy resin: effect of interphase layer on percolation conductance, RSC Adv. 3 (2013)

[24]

EP

1916–1921.

S.H. Foulger, Electrical properties of composites in the vicinity of the

AC C

percolation threshold, J. Appl. Polym. Sci. 72 (1999) 1573–1582. [25]

C.-L. Huang, C. Wang, Rheological and conductive percolation laws for

syndiotactic polystyrene composites filled with carbon nanocapsules and carbon nanotubes, Carbon. 49 (2011) 2334–2344.

[26]

J.K.W. Sandler, J.E. Kirk, I.A. Kinloch, M.S.P. Shaffer, A.H. Windle, Ultra-low

electrical percolation threshold in carbon-nanotube-epoxy composites, Polymer. 44 (2003) 5893–5899.

14

[27]

ACCEPTED MANUSCRIPT

Y. Gefen, W.-H. Shih, R.B. Laibowitz, J.M. Viggiano, Nonlinear behavior near

the percolation metal-insulator transition, Phys. Rev. Lett. 57 (1986) 3097. [28]

L. He, S.-C. Tjong, Universality of Zener tunneling in carbon/polymer

composites, Synth. Met. 161 (2012) 2647–2650. L. He, S.-C. Tjong, Carbon nanotube/epoxy resin composite: Correlation

RI PT

[29]

between state of nanotube dispersion and Zener tunneling parameters, Synth. Met. 162 (2012) 2277–2281.

L.X. He, Zener tunneling in conductive graphite/epoxy composites: Dielectric

SC

[30]

breakdown aspects, Express Polym. Lett. 7 (2013) 375–382.

H.-C. Kuan, C.-C.M. Ma, W.-P. Chang, S.-M. Yuen, H.-H. Wu, T.-M. Lee,

M AN U

[31]

Synthesis, thermal, mechanical and rheological properties of multiwall carbon nanotube/waterborne polyurethane nanocomposite, Compos. Sci. Technol. 65 (2005) 1703–1710. [32]

W. Chen, X. Tao, Y. Liu, Carbon nanotube-reinforced polyurethane composite

[33]

TE D

fibers, Compos. Sci. Technol. 66 (2006) 3029–3034. M. Wong, M. Paramsothy, X.J. Xu, Y. Ren, S. Li, K. Liao, Physical interactions

[34]

EP

at carbon nanotube-polymer interface, Polymer. 44 (2003) 7757–7764. L.N. Lisetski, N.I. Lebovka, S.V. Naydenov, M.S. Soskin, Dispersions of multi-

AC C

walled carbon nanotubes in liquid crystals: A physical picture of aggregation, J. Mol. Liq. 164 (2011) 143–147.

15

List of the figures:

ACCEPTED MANUSCRIPT

Figure 1. Synthesis of prepolymer based on PPG and TDI. Figure 2. Cross-linkage of prepolymer by the TMP with the formation of CPU. Figure 3.

The

degree

of

NCO-groups

conversion

of

CPU (1)

and

RI PT

CPU/Fe(acac)3 (2) matrices. Figure 4. OM (a-d) 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 %

SC

(c, d) and 3 % (e, f) of CNT content were used for OM (a-d) and SEM (e, f) methods.

Figure 5. The electrical conductivity of CPU/CNT and CPU/CNT/Fe(acac)3

for the same composites (b).

M AN U

composites as a function of filler weight fraction (a) and a log-log plot of σDC vs. C-Cc

Figure 6. J-E characteristics of CPU/CNT (a) and CPU/CNT/Fe(acac)3 (b) composites.

based composites.

TE D

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

Figure 8. Tensile strength vs. filler concentration of CPU/CNT and

AC C

EP

CPU/CNT/Fe(acac)3 composites.

16

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT