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Aug 6, 2014 - Masterbatches Produced by In Situ. Polymerization and by Melt Extrusion. Rafael S. Araujo, Renato J. B. Oliveira, Maria de F atima V. Marques*.

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Preparation of Nanocomposites of Polypropylene with Carbon Nanotubes via Masterbatches Produced by In Situ Polymerization and by Melt Extrusion tima V. Marques* Rafael S. Araujo, Renato J. B. Oliveira, Maria de Fa

In the present study, a new polypropylene (PP)/multi-walled carbon nanotube (MWCNT) masterbatch is synthesized by in situ polymerization and compared with a masterbatch obtained by melt mixing. Both masterbatches are used for the realization of PP/MWCNT nanocomposites by mixing with a commercial PP in a laboratory extruder. It is shown that the use of masterbatch synthesized in situ allows providing additional enhanced thermal stability and mechanical properties. Samples are characterized according to their thermo dynamicmechanical properties, thermal stability, and degree of crystallinity, as well as by scanning electron microscopy. DMA analysis shows that there is a sharp increase in both storage and loss moduli of materials even with very low CNT content, in comparison with the neat PP. By means of thermogravimetric analysis, it is found that the thermal stability of nanocomposites is also increased. Furthermore, the degree of crystallinity of the materials containing CNTs is increased to higher value than that of neat PP, suggesting that the carbon nanotubes act as nucleating agent. A sharper increase of Xc is observed in the composites with low CNT content prepared in the method using meltmasterbatch, suggesting that there are more agglomerations of CNTs in this material.

1. Introduction Polypropylene (PP) is one of the most widely used thermoplastics, with applications in several segments, such as in the automotive industry, electrical and electronics, construction, packaging etc..[1–8] In their formulation, materiProf. M. de F. V. Marques Instituto de Macromoleculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, IMA-UFRJ E-mail: [email protected] R. S. Araujo, R. J. B. Oliveira Avenida Horacio Macedo, 2030. Centro de Tecnologia. Bloco J. Rio de Janeiro, RJ, Brazil Macromol. React. Eng. 2014, 8, 747–754 ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

als based on PP usually contain inorganic reinforcing fillers in order to improve their properties.[7–12] Carbon nanotubes are very resistant synthetic nanofillers and provide excellent mechanical properties. Polyolefin nanocomposites containing single wall (SWCNT) and multi-walled (MWCNT) carbon nanotubes have been extensively researched to increase the electrical, thermal, and mechanical properties of materials.[10–20] High performance is achieved with very low percentages of CNT, which has a practical importance since it is possible to obtain lightweight final materials.[12] Nitric acid or a mixture of nitric and sulfuric acids can be used to purify CNT while modifying their surfaces by introducing functional groups

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DOI: 10.1002/mren.201400012

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such as carboxyl and hydroxyl.[8,10,17,21] The optimization of thermal and mechanical properties in composites reinforced with CNT is obtained when the dispersion and the interfacial interaction between the filler and the polymer is efficient. Some studies in the literature have shown that the in situ polymerization technique tends to achieve a more efficient dispersion of nanofillers. This allows obtaining nanocomposites with higher levels of nanofiller, leading consequently to materials with improved mechanical and thermal properties.[12,21–25] Despite recent advances in metallocene systems, the Ziegler-Natta catalysts still dominate the industrial production of PP, with many studies regarding the optimization of the performance of the families of MgCl2/TiCl4 compounds.[26] One of the reasons for the continued development of these systems that have more than 50 years, lies in its low cost and yet with similar levels of efficiency compared with metallocene systems, particularly in obtaining polypropylene homopolymer. The challenge in the in situ polymerization using supported Ziegler-Natta catalysts containing CNT is the complexity of the process of obtaining these catalysts, with its various stages of preparation, to the heterogeneous polymerization.[12,26] In situ polymerization is supposed to be an efficient method to perform uniform dispersion of nanoparticles in the polymer matrix, although few studies in the literature reported the preparation of PP nanocomposites with carbon nanofiller by this technique.[23] In the work of Funck and Kaminsky,[24] nanocomposite obtained by in situ polymerization was the method used to produce PP/MWCNT from a metallocene catalyst based on zirconium. By means of SEM analysis, the authors found that there was a good dispersion of the nanofiller in the polymer matrix. The work of Huang et al.[26] is the first example related to the synthesis of nanocomposites of polypropylene/graphene (PP/GO) via in situ polymerization using Ziegler-Natta catalysis, where oxidized graphite was incorporated in the Mg/Ti particle, resulting in a supported catalyst. After polymerization, it was found that the nanosheets were exfoliated and well dispersed in the PP matrix, even at levels of 4.9 wt.-% reaching high electrical conductivity. Thus, in the present work we prepared a Ziegler-Natta catalyst based on titanium species supported on magnesium chloride by the emulsion technique introducing CNT in the catalyst particle. A new method of preparation of nanocomposites is discussed, including the in situ polymerization to obtain a masterbatch of PP/CNT and blending this material in the molten state with a commercial PP matrix. This method was compared with the traditional one where a masterbatch is prepared by direct mixing the matrix with CNT, to be similarly mixed in the extruder to the appropriate concentration in the final nanocomposite.

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2. Experimental Section 2.1. Materials Multi-walled carbon nanotubes (MWCNT)–FeCoMgO 93% carbon–were donated by the Laboratory of Nanomaterials, Department of Physics–ICEX–UFMG. Sulfuric and nitric acids (Vetec Quimica, Brazil) were used to treat CNT. The PP matrix used was commercial HP550K (PP homopolymer commercial, Braskem, Brazil, MFI ¼ 3.5 g/10 min). Anhydrous MgCl2 (Toho Titanium Co. Ltd., Japan), anhydrous ethanol (Vetec Quimica, Brazil) were used to produce the alcoholic adduct as support precursor and isoparaffin (Ipiranga, Brazil) was used to precipitate adduct. TiCl4 (Aldrich, USA) was used for the dealcoholization of adduct, producing the catalytic support. The internal electron donor (DI, Elekeiroz S.A, Brazil) n-butyl phthalate was introduced and TiCl4 was added to obtain the Ziegler-Natta catalyst. The cocatalyst triethylaluminium (TEA, Akzo Nobel, EUA) and external donor (diphenyl-dimethoxysilane, ToKyo Kasei LTD., Japan) were introduced in the polymerization reactor. Propylene monomer used in polymerization conducted in hexane (Vetec, Brazil) was supplied by White Martins.

2.2. Methods 2.2.1. Treatment of Multi-Walled Carbon Nanotubes (MWCNT) Initially, MWCNT were treated in a mixture of concentrated sulfuric and nitric acids, aiming to remove impurities and to introduce hydroxyl groups on the CNT surface. These treated CNT was named CNTRA-04.

2.2.2. Preparation of Ziegler-Natta Catalyst Supported on MgCl2 Containing MWCNT The synthesis of PP/CNT masterbatch prepared by in situ polymerization was carried out using a Ziegler-Natta catalyst containing MWCNT obtained by emulsion technique.[26] The catalyst was prepared in the following way: MWCNT was added to the reactor containing mineral oil, having undergone sonication for 1 h. After this period, anhydrous ethanol was added under stirring and after 5 min anhydrous magnesium chloride was introduced. The ratio between the MgCl2 and EtOH was 1:3. The system was heated to the melt of the adduct and the melt was transferred under inert atmosphere and stirring to a flask containing isoparaffin at –40 8C, where it remained for 2 h under stirring. Adduct was desalcoolated by a chemical process during 4 h at 60 8C with the used of TiCl4. The amount of TiCl4 was determined from the molar ratio Ti to ethanol 1/2:1. The desalcoolated support was then washed to eliminate non active titan alkoxides. To control isotactic index of PP, the catalyst was prepared with the addition of n-butyl phthalate as internal donor (ID), and TiCl4 was introduced for fixing on the surface of the support. DI was added to the system at 60 8C and its volume was determined from the molar ratio of 1:8 between DI/MgCl2. Then, the temperature was raised slowly to 100 8C where it was maintained for 2 h. After

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this time, 10 ml of TiCl4 was added. The temperature was raised slowly to 110 8C and held for 2 h. Soon after, the supernatant was removed and the solid catalyst was washed with dry hexane at 60 8C until no displacement of HCl vapors was observed. The catalyst obtained suffered drying under nitrogen flow until constant mass.

2.2.3. Preparation of Masterbatch by In Situ Polymerization 0.100 g of catalyst was used in the polymerization for the synthesis of the masterbatch. The polymerization was carried out in a reactor € chi with mechanical stirrer and jacketed glass beaker of 1 L Bu connected to a thermostatic bath Haake model N6. A volume of 80 ml of hexane was introduced; the cocatalyst triethylaluminum (TEA) and the external donor (diphenyl dimethoxy silane) were also transferred to the reactor, followed by stirring. Afterwards, the catalyst was added and finally propylene monomer was fed to the pressure of 4 bar. The water bath was turned on and polymerization temperature was maintained at 70 8C. At the end of the reaction, the catalyst was deactivated with an alcoholic solution. The masterbatch was vacuum filtered and suffered drying oven until constant weight.

2.2.4. Obtaining Masterbatch by Mixing in the Melt Masterbatch of PP/CNT was prepared with 5 wt.-% of treated MWCNT (CNTRA-04). The procedure was carried in twin-screw mini-extruder Haake Minilab under screw speed of 120 rpm for 10 min at 180 8C.

2.2.5. Obtaining the Nanocomposites in Mini-Extruder Both masterbatches were used at a later stage for dilutions in the same PP matrix to produce the final nanocomposites. Nanocomposites were prepared by blending both masterbatches containing the MWCNT with the commercial PP matrix under screw speed 120 rpm for 10 min at 180 8C. Nanocomposites were prepared with final concentration of CNT in the range of 0.01– 1.0 wt.-% (from masterbatch by melt blending) and 0.01–0.3% wt. (from in situ polymerization).

2.2.6. Techniques of Characterization Dynamic-mechanical tests (DMA) were performed under the following conditions: Frequency 1 Hz, Temperature range: –20 to 120 8C isotherm: 10 min at –20 8C; single Cantilever was used and the device DMA was Q800 TA Instruments. Thermal degradation of the polymeric materials was evaluated by weight loss, and such analysis was conducted on a device TGA Q500 from TA Instruments employing a heating rate of 10 8C  min1 ranging from 30 to 700 8C under an atmosphere of N2. To determine the melting temperature (Tm), the degree of crystallinity (Xc), and crystallization temperature (Tc), differential scanning calorimeter (DSC) TA Instruments Q1000 model was used. The conditions for analysis were as follows: heating from 30 to 200 8C with a rate of 10 8C  min1, rapid cooling, heating from 30 to 200 8C ay 10 8C  min1 and cooling at 10 8C  min1 to 30 8C. The degree of crystallinity of each sample was determined by the value of enthalpy of melting of the sample (DHm)

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and the enthalpy of melting of 100% polypropylene crystal (190 J  g1). The morphology of MWCNT/PP composites was observed using a scanning electron microscopy (SEM, Model JEOL JSM-840A). XRD measurements were performed in a powder sample form in X-ray diffractometer Rigaku model Miniflex, using a potential difference at 30 kV and tube electric current 15 mA. The scan was performed in the range of 10-  358, with the speed of the goniometer 0.058  min1. The radiation used was CuKa with ˚. l ¼ 1.5418 A

3. Results and Discussion After preparation of both masterbatches, one by in situ polymerization using the prepared Ziegler-Natta catalyst containing MWCNT and the other by direct melt mixing of PP with MWCNT, they were blended with commercial polypropylene to achieve concentrations of 0.01–0.3 or 1.0wt.% in final nanocomposites. The results of dynamicmechanical analysis (DMA) of the final nanocomposites are shown in Table 1. Figure 1 reports the curves of storage modulus (E0 ) and mechanical loss factor (tan d) versus temperature. It is evident a high increase in the storage modulus throughout the temperature range studied for the nanocomposites obtained from the PP/MWCNT masterbatch synthesized by in situ polymerization (Figure 1a) when compared with the neat PP (PP550-P). Moreover, the increase of content from 0.1 to 0.3wt.-% resulted in even higher value of E0 . The same trend was not observed for the materials obtained from masterbatch by melt blending (Figure 1c). The behavior concerning the influence of the nanofiller content on the storage modulus are shown in Figure 2, where it is observed that for the materials prepared by melt blending method, the addition of a higher amount of CNT tends to lower the values of E0 . This is an indication of lower ability for dispersion of the nanofillers in the matrix using this method. Furthermore, for nanocomposites prepared from masterbatch by in situ polymerization the moduli are significantly higher even in small quantities. This is because in this method, the polymer is growing directly on the surface of CNT leading probably to disentanglement of the bundles and providing better dispersion of filler in the polymer matrix. Samples prepared with masterbatch by melt blending containing 0.5 and 1.0% CNT have lower moduli than the other materials, showing the difficulty to disperse carbon nanotubes in polypropylene. Additionally, the loss moduli (E00 ) of the nanocomposites were also sharply increased, resulting in great augment especially for the nanocomposites obtained from in situ polymerization masterbatch. It is well known that the tan d curve of PP exhibits three relaxations localized near –80 8C (g), 10 8C (b), and 100 8C (a). In the present work, the study focused on the b-relaxation of

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Table 1. Results of dynamic-mechanical analysis (DMA) of the final nanocomposites.

Material

Neat PP In situ-Blending

In situ-Masterbatch Melt-Blending

Melt-Masterbatch

Sample

PP550-P

CNT (% wt.)

E0 (MPa)

E00 (MPa)

Tg (8C)

Tan dMax

0

1919

52

10.0

0.079

PPNT1-001

0.01

2550

178

14.0

0.078

PPNT1-01

0.1

2522

167

11.1

0.083

PPNT1-03

0.3

2806

184

14.5

0.079

PP1/CNT1

2.2

1880

120

5.5

0.139

DPPCNT1-001

0.01

2202

174

14.6

0.093

DPPCNT1-01

0.1

1645

113

12.4

0.083

DPPCNT1-03

0.3

1572

115

10.4

0.093

DPPCNT1-05

0.5

960

56

16.2

0.064

DPPCNT1-1

1.0

1233

86

13.7

0.080

PP/CNTR04

5.0

2659

205

13.7

0.077

Where: PP550-P ¼ neat PP after processing, PP/CNTRA04 ¼ Masterbatch prepared by melt blending with 5 wt.-% of MWCNT (CNTRA04). E0 and E00 at 25 8C; Tg measured by Tan d curve.

iPP since it corresponds to the glass rubber transition of the amorphous fraction and the temperature of the peak maximum is designed as the glass transition temperature (Tg). For all the obtained nanocomposites, the glass transition temperature was also enhanced in relation to neat PP, demonstrating that the mobility of the polymer chain is reduced in the presence of MWCNT. This behavior can be attributed to the reinforcing effect of CNT, hindering the movements of the chains, since the fillers act as restriction sites for the PP segments. The effect of nanotube on the damping behavior is shown by the plot of tan d versus temperature in Figure 1b and d. The height and width of the peaks provide additional Figure 1. Storage modulus and tan delta of PP/CNT nanocomposites obtained from masterbatches of in situ information about the relaxation behavior of these polymerization (PPNT1) (a, b) and melt blending (DPPNT1) (c, d). samples. The height of the peak (tan dMax) for samples of PP-550P ¼ Commercial PP after processing. PP/CNT nanocomposites tended to increase in comparison with the neat PP when low CNT was introduced. This implies that these samples exhibit less elastic behavior. The significant increase in width of the peak for the nanocomposite samples suggests a broader distribution of relaxation times, presumably due to more nanotube–polymer interactions, and hence restricted mobility. Generally, the incorporation of nanofiller in the polymer matrix increases the thermal stability, as the filler acts as an insulator or even as a barrier to the mass transport of volatiles during degradation. 0 Figure 2. (a) Storage (E ) moduli in function of CNT amount (wt.-%) for processed PP/CNT nanocomposites from masterbatches of in situ polymerization (PPNT1) and melt However, this behavior requires superior filler dispersion in the polymer matrix. In blending (DPPNT1). Neat PP ¼ Pure PP after processing.

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Figure 3. TGA curves of PP and PP reinforced with carbon nanotubes.

Figure 3, the curves of mass loss with the increase of temperature for the polymeric materials are shown. One can observe an increase in thermal stability of all prepared nanocomposites in relation to the neat PP, shifting the degradation temperatures to higher values, showing that carbon nanotube even at low contents in PP becomes an excellent insulator and a mass transport barrier. Moreover, the profile of the DPPCNT1 (both with 0.1 and 0.3% CNT) are very much alike that of neat PP. On the other hand, for the PPNT nanocomposites prepared with the masterbatch obtained by in situ polymerization, the shape of the curve has changed. It shows that, achieving higher degradation temperature, a sharper decrease in mass occurs with increasing temperature. The volatile evolution takes place in a single step leaving a low quantity of residue. It is generally assumed that the residue correspond to the amount of CNT, although for the samples prepared in this

work, the low content of CNT used makes this measure not precise. The sharp mass loss is expected to be due to some interaction of PP chains with carbon nanotubes in a more uniformly dispersed PPNT nanocomposite with 0.1 wt.-%. Table 2 shows the results obtained by thermogravimetric analysis, where the values of initial degradation temperature (Tonset) and temperature of maximum degradation rate (Tmax) are presented, as well as the content of residual materials determined with the mass loss. Both Tonset and Tmax of the nanocomposites were shifted to higher temperatures when compared to the neat PP. It is also noted that the thermal stability was higher for PP containing 0.1% CNT, which shows that at this amount higher dispersion of nanotubes was obtained and possibly the increase of its content in PP 0.3% promotes some agglomeration of CNT. The difference in Tmax and Tonset is much lower in the composites containing 0.1 wt.-% of CNT. The thermal properties of the materials were also evaluated by differential scanning calorimetry, and the crystallization temperature (Tc), melting temperature (Tm), and crystallinity degree of the materials were determined. The results are listed in Table 3. The crystallization temperature of the nanocomposites has shifted to higher values, especially in the composites prepared with the masterbatch of in situ polymerization, while the melting temperature of all polymers was roughly equal. Crystallization of polymer in the PP/ MWCNT nanocomposites begins at a temperature higher than that in pure PP. It is assumed that an increase in Tc is associated with an increased number of heterogeneous nuclei for crystallization. These results reveal that the MWCNT interact with the polymer chain and acts as effective nucleating agent for PP even at low percentages.

Table 2. Temperatures degradation and residue content of the samples analyzed by TGA.

Material

Neat PP In situ-Blending

In situ-Masterbatch Melt-Blending

Melt-Masterbatch

Sample

Tonset (8C)

Tmax (8C)

Tmax–Tonset (8C)

Residue (%)

PP550-P

361

408

47

0.80

PPNT1-001

356

397

41

0.34

PPNT1-01

445

465

20

0.60

PPNT1-03

382

433

51

0.43

PP/CNT1

444

462

18

2.38

DPPCNT1-001

348

389

41

0.34

DPPCNT1-01

434

455

21

0.27

DPPCNT1-03

375

415

40

0.26

DPPCNT1-05

431

452

21

0.96

DPPCNT1-1

442

462

20

1.21

PP/CNTR04

425

455

30

4.5

Where: PP550-P ¼ neat PP after processing, PP/CNTRA04 ¼ Masterbatch prepared by melt blending with 5 wt.-% of CNTRA04.

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Macromol. React. Eng. 2014, 8, 747–754 ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Table 3. Properties obtained by differential scanning calorimetry.

Material

Neat PP In situ-Blending

In situ-Masterbatch Melt-Blending

Melt-Masterbatch

Sample

Tc (8C)

Tm (8C)

Xc (%)

PP550-P

120

163

40.1

PPNT1-001

126

164

37.5

PPNT1-01

127

164

47.5

PPNT1-03

126

164

50.1

PPNT1

122

158

33.0

DPPCNT1-001

120

164

47.6

DPPCNT1-01

121

163

60.5

DPPCNT1-03

123

163

58.4

DPPCNT1-05

123

163

32.4

DPPCNT1-1

125

164

34.8

PP/CNTR04

130

165

41.8

Where: PP550-P ¼ neat PP after processing.

Interestingly, the shift of Tc was higher in the nanocomposites prepared using the in situ-masterbatch, which means that it produces a higher heterogeneous nucleation effect for PP. The degree of crystallinity (Xc) of the nanocomposites has also increased, indicating once again an action of MWCNT as nucleating agent on the PP matrix. Enhance was superior for the composites prepared with the melt blending masterbatch at low concentration of CNT. The addition of external nucleating sites, such as carbon nanotubes, leads to an increase of crystallization rate and consequently to an increase of Xc. In addition, the addition of higher amounts of CNT in this set of materials has decreased Xc as previously reported for nanocomposites with contents higher than 0.5%. The relatively high cooling rate of 10 8C  min1 gives limited time to the polymer to be crystallized, and, as a result, Xc has decreased. Figure 4 show the DSC profiles for melting (second heat) and crystallizing (cooling) where the peaks observed for the set of materials obtained from in situ polymerization (PPNT01 and PPNT03) are narrower, which are directly related to higher perfection of the crystals with respect to neat polypropylene PP550. Any signal of trans-crystallinity is observed. The morphology related to the dispersion of CNT in the polymeric matrix is shown in the Figure 5. It can be seen that in the nanocomposites obtained with the masterbatch prepared by melt blending (a,b) and then dispersed in the commercial PP, the carbon nanotubes are clearly visible and for the concentration of 0.1 wt.-%, it is possible to see aggregates in which we observe a low amount of selforganized MWCNT in bundles. On the contrary, in the

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Figure 4. DSC profiles of polymers: (a) heating and (b) cooling.

nanocomposites prepared using the masterbatch synthesized by in situ polymerization (c, d) it was only possible to observe some dispersed nanotubes using secondary electron imaging as mode of operation, BF (bright-field) detectors for SEM, and very low amounts of nanotubes are apparent on the surface of the cryofractured sample. Moreover, the samples with 0.1 and 0.3% CNT did not show the presence of any agglomerate. Moreover, nanocomposites of PP/CNT prepared by in situ-masterbatch show relatively smooth fractured surface while rough fractured surface with obvious cracks and fibrils are observed for the PP nanocomposites from meltmasterbatch. Finally, X-ray diffraction technique was also used to investigate the change in the crystal structure of the PP as a result of the introduction of CNT. Figure 6 shows the XRD patterns of treated CNT (CNTRA-04), the neat PP (PP550), as well as the nanocomposites of PP/CNT, with different loadings. Five strong peaks are shown in the XRD profile of neat PP and, according to the literature,[27,28] the profile corresponds to the most usual alpha-form of PP crystals. The addition of 0.1 and 0.3 wt.-% of CNT shifted these peaks to smaller angles, which clearly indicates that small amounts of CNT fillers affect the morphology of the PP crystals in the PP/CNT nanocomposites. Also important is

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Figure 5. SEM micrographs of the nanocomposites. (a) PPNT01, (b) PPNT03, (c) DPPNT01 e (d) DPPNT03.

that the peaks are broader and their relative intensity was modified. This means that the crystalline structure of PP in the nanocomposites have more imperfect crystals with orientation. However, it exhibits complete absence of the bcrystal form, which would appear as two strong peaks at 2u of 16.2 and 21.28.[5]

4. Conclusion

Figure 6. XRD patterns of materials include CNT filler, pure PP and nanocomposites.

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In view of what has been addressed, it can be concluded that the method employing the masterbatch synthesized by in situ polymerization for the preparation of nanocomposites of PP with carbon nanotubes was more successful than the traditional method by melt blending. It was observed by DMA that the storage and loss moduli of PP/CNT nanocomposites thus obtained resulted in much higher values, even with very low content of CNT in the final material (0.1

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and 0.3 wt.-%). Furthermore, it was found that the thermal stability of nanocomposites measured by TGA also sharply increased. DSC analyses showed that the MWCNT acted as a nucleating agent to the PP matrix, since both the crystallization temperature and the crystallinity degree of the nanocomposites were higher than that of the neat PP. The increase in Tc and Xc was specially higher for the nanocomposites obtained from masterbatch prepared by melt blending, despite they resulted in lower stiffness. This means that CNT led to an increase in the rate of polymer crystallization with no substantial changes in the crystalline structure, as confirmed by X-ray diffraction. XRD analysis also showed that PP remains crystallizing exclusively in primarily monoclinic (a-form) in the nanocomposite, and therefore the presence of MWCNT did not influence the main crystal structure of PP matrix. These results confirmed that the masterbatch synthesize in the laboratory produced a very positive effect on the commercial polymer, providing a sharp increase of its properties.

Acknowledgements: This work was financially supported by CNPq and CAPES (Brazil), and the European project FP7-PEOPLEIRSES-2011-295262-VAIKUTUS.

Received: April 2, 2014; Revised: May 23, 2014; Published online: August 6, 2014; DOI: 10.1002/mren.201400012 Keywords: carbon nanotubes; masterbatch; nanocomposite; polypropylene

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