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Thermoplastic elastomers with multi-walled carbon nanotubes: Influence of dispersion methods on morphology
Composites Science and Technology 70 (2010) 1006–1010
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Thermoplastic elastomers with multi-walled carbon nanotubes: Influence of dispersion methods on morphology Georg Broza * Institute of Polymer Composites, Hamburg University of Technology, Denickestrasse 15, D-21073 Hamburg, Germany
a r t i c l e
i n f o
Article history: Received 24 November 2009 Received in revised form 15 February 2010 Accepted 21 February 2010 Available online 1 March 2010 Keywords: A. Carbon nanotubes A. Nanocomposites B. Interfaces D. Scanning/transmission electron microscopy (STEM) D. X-ray diffraction (XRD)
a b s t r a c t PBT-block-PTMO thermoplastic elastomers derived from dimethyl terephthalate (DMT), 1,4-butanediol(BD) and poly(tetramethylene glycol) (PTMG) were synthesized in the presence of oxidized multiwalled carbon nanotubes (MWCNTs) by a two-stage process involving transesterification and in situ polycondensation. Two procedures were applied to nanotubes in the polymer matrices were used. In procedure 1, nanotubes were dispersed in DMT + BD before the transesterification, while in procedure 2 nanotubes were dispersed in PTMG after transesterification. The mole ratio of the starting components was selected to produce copolymers with a constant hard to soft segment weight ratio of 45:55. Characterization of the new nanocomposites was performed by transmission electron microscopy (TEM), scanning electron microscopy (SEM), small- and wide-angle X-ray scattering (SAXS/WAXS). A better nanotube dispersion can be achieved when oxidized MWCNTs are added to the DMT + BD monomers before transesterification (procedure 1). Oxidized MWCNTs exhibit strong interfacial adhesion to the polymer matrix for both procedures. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Nano-structured modification of polymers has opened new perspectives for multifunctional materials [1]. Combination of carbon nanotubes (CNTs) with polymer matrices can be considered as a step forward in polymer and composites technology. This has been shown for thermoset polymers [2–4] and recently also for several thermoplastics [5–12]. Chemical functionalization and solubilization of carbon nanotubes has emerged as an effective means for homogeneous dispersion of CNTs in solution. Functionalization is a surface treatment which creates functional groups at surfaces of filler nanoparticles. The filler functionalization brings about favourable interactions between the functional groups and the polymer matrix [13–15]. This is particularly important for fillers with a strong tendency towards agglomeration – as for instance carbon nanotubes. Enhanced filler–matrix interactions facilitate the dispersion of the filler in the polymer matrix and strengthen the interfacial bonding, leading to better reinforcement effects. In this respect, a main challenge consists on the improvement of the dispersion of CNTs in a polymer matrix during processing. There are several techniques to improve the dispersion of CNTs in a polymer matrix, such as melt blending [16–19], in situ polymerisation or polycondensation on the presence of nanotubes [10,11,20], and solution mixing [21,22]. Direct mixing and sonica* Tel.: +49 40428783 496; fax: +49 40428782 002. E-mail address: [email protected] 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.02.021
tion have been the most common techniques to disperse CNTs within thermoset polymer resins [23–25], and thermoplastic polymeric matrices [26–29]. However, these methods were found to be ineffective to completely eliminate agglomeration of CNTs. In situ polycondensation of monomers in the presence of nanofillers is a promising approach for a more homogeneous distribution, due to a closer contact of polymer and filler during synthesis [10]. Dispersion of CNTs in 1,4-butanediol before transesterification has been utilized previously for the synthesis of PBT/PTMO- and PBT-containing nanocomposites. Methods of nanocomposite preparation have been described before [11,30]. In this work multi-wall carbon nanotubes (MWCNTs) were used to prepare composites with poly(ether-b-ester)s. These copolymers are based on semicrystalline poly(butylene terephthalate) (PBT) blocks and amorphous poly(tetramethylene oxide) (PTMO) blocks. MWCNTs can be oxidized in order to enhance solubility of the nanotubes in polar solvents [31–34]. Introduction of oxygen-containing surface groups on the nanotube surface, e.g. carboxylate groups, leads to an electrostatic stabilisation in polar solvents due to development of strong hydrogen bonds with the solvent [35,36]. Since PBT/PTMO copolymer used in the present work is synthesized based on dimethyl terephthalate (DMT), butanediol (BD) and poly(tetramethylene glycol) (PTMG), then chemical properties of the functionalized carbon nanotubes should be tailored considering the chemical reactions involved during the synthesis. It will be shown that introduction of carbonyl groups on surfaces of
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carbon nanotubes is a plausible method to optimize dispersion in the PBT/PTMO with MWCNT nanocomposite and to improve filler adhesion to the polymer matrix. The aim of this work is to advance the general understanding of interactions between CNTs and a polymer matrix by investigating nanocomposites prepared by using two different methods to incorporate introduction of oxidized MWCNTs into the polymer matrix. In a first method, MWCNTs were added prior to the transesterification reaction and in the second method they were added after the transesterification. It will be shown that the different methods lead to different composite morphology. 2. Experimental 2.1. Material characterization Commercially available reagents for poly(ether–ester) block copolymers synthesis were obtained as follows: dimethyl terephthalate (DMT) from (DuPont, NC, USA); 1,4-butanediol (BD) from (BASF, Germany); poly(tetramethylene glycol) (PTMG; Mn = 970 g/ mol) also from DuPont. MWCNTs were supplied by Nanocyl S.A. (Namur, Belgium) – produced by catalytic chemical vapour deposition method (CCVD). Typically, their outer diameter range is 3–15 nm, length up to 50 lm. 2.2. Electron microscopy (TEM and SEM) Transmission electron microscopy (TEM) observations were conducted to determine the dispersion state of MWCNTs either within DMT + BD or within PTMG in the polymer matrix. TEM images were taken using a Philips EM 400 machine at 100 kV. A LEO 1530 field emission scanning electron microscope (SEM) with different accelerating voltages (1.5–3 kV) was employed to observe the morphology of the tensile fractured nanocomposites.
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3.1.1. Dispersion procedure 1 Before the first step of transesterification, 50 wt.% of total amount of dimethyl terephthalate (DMT) was melted in butanediol (BD) at 140 °C. MWCNTs (0.3 wt.%) was added to this mixture. Previously MWCNTs were ultrahigh speed stirred for 5 min. at 20,000 rpm (step 1) and then sonicated for 5 min at 75 °C (step 2). To optimize the dispersion, both steps were repeated six times. The molten dispersion was casted onto a plate, and after cooling down to room temperature the material has solidified. TEM and SEM characterization provides a clear evidence of nanotube surface coverage. In Fig. 1a (TEM image) it is shown a MWCNT covered by monomers. The diameter of the covered MWCNT is in the range from 33 to 42 nm. The core formed by the nanotube with a diameter of about 8 nm can also be seen in Fig. 1a. At lower magnification the SEM image in Fig. 1b shows the covered nanotubes with diameters in the range from 41 to 74 nm. These results show that oxidized MWCNTs are very suitable for the coating process. PBT/ PTMO with MWCNT nanocomposites were synthesised in a twostage process [11]: transesterification followed by polycondensation in the melt, with PTMO content of 55 wt.%. In a steel reactor (Autoclave Eng. Inc., USA) we put (i) 50 wt.% of dimethyl terephthalate (DMT) and catalyst mixed, additionally the crystallized MWCNT/DMT + BD powder (with 50 wt.% of total DMT). 3.1.2. Dispersion procedure 2 After the transesterification process, the MWCNTs were dispersed in PTMG to produce PBT/PTMO + CNTs nanocomposites. PTMG has higher molecular weight than glycol, hence it is more viscous and dispersion is more difficult, even though the mixture was heated all the time. MWCNTs (0.3 wt.%) were mixed with molten PTMG and stirred for 10 min and sonicated for 10 min at room temperature and introduced into the reactor after the transestrification process. To optimize the dispersion, both steps were repeated ten times. In this case, PBT/PTMO + MWCNT composites were synthesised by the described two-stage process but now 100 wt.% of DMT + BD and MWCNT + PTMG mixture were slowly
2.3. Small and wide-angle X-ray scattering (SAXS/WAXS) Small and wide-angle X-ray scattering (SAXS/WAXS) experiments were performed in transmission at the beamline A2 of HASYLAB/DESY, Hamburg, with 8 keV X-rays and two linear detectors. The A2 beamline is particularly useful for this purpose since it allows conducting SAXS and WAXS experiments simultaneously. The raw data were background and detector response corrected. The SAXS scattering-vectors were calibrated using a rat-tail tendon protein. The WAXS data were calibrated using reflections of poly(butylene terephthalate) (PBT) film. 3. Results and discussion 3.1. Preparation of the nanocomposites MWCNTs, as received, were added to a mixture containing concentrated sulphuric and nitric acids, 3:1 by volume. This procedure is known to introduce carboxylate groups onto the outermost shell of the MWCNTs. In addition, this acid mixture intercalates and exfoliates graphite sheets and, therefore, helps to separate carbon nanotubes from each other. The suspension was sonicated in an ultrasonic bath for 30 min and then heated up to 90 °C for 3 h. After centrifugation and washing with hot distillated water, the suspension was stirred in sodium hydrogen carbonate solution overnight and washed again according to a procedure of Funk and Kaminsky [37]. For the synthesis of PBT/PTMO + MWCNTs, two different ways of introducing oxidized MWCNTs in polymer matrices were used:
Fig. 1. (a) TEM image and (b) SEM image showing the dispersion of oxidized MWCNTs due to coating nanotubes surfaces by DMT + BD.
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injected. Nanocomposites containing 0.3 wt.% of MWCNT were extruded from the reactor by compressed nitrogen and cooled down to room temperature. 3.2. Structure High intensity of synchrotron radiation enables investigation of SAXS patterns of materials which have weak scattering properties – such as polymers. In the case of polymer nanocomposites containing carbon nanotubes, CNTs lead to a strong radiation absorption by the sample; this makes the data acquisition by conventional SAXS equipment (i.e., those with a conventional X-ray source) very difficult. Figs. 2 and 3 show the results of SAXS and WAXS measurements, respectively. The SAXS curve of the PBT/PTMO nanocomposites with dispersion procedure 1 (MWCNTs in the DMT + BD) has a correlation peak at low angles – associated with the crystalline layers separated by the amorphous polymer chains. The position of the maximum of this peak corresponds to a long-period (distances between the crystalline PBT lamellae) of L = 14.2 nm. For the nanocomposite with CNTs dispersed by procedure 2 (MWCNT dispersed in PTMG) a similar peak but of higher intensity is observed. The long-period value in this case was slightly smaller (L = 12.7 nm). The scattering power, Q, of the nanocomposites prepared by procedure 1 (MWCNT in DMT + BD) seems to be lower than that for nanocomposites prepared by procedure 2 (MWCNT in PTMG). This decrease might be explained, in a first approach, in terms of an increased amount of the crystalline phase for samples prepared by procedure 2. Considering that the amount of CNTs in the system is expected to be much lower than the crystalline fraction of PBT in the nanocomposite, the twocomponent model, in which dense PBT lamellae (phase 1) are dispersed in an amorphous PBT phase (phase 2) is applicable. For such systems [38]:
Q¼
Z
IðqÞq2 dq ¼ 2p2 ðq2 q1 Þ2 V U1 ð1 U1 Þ
Fig. 3. WAXS patterns of PBT/PTMO nanocomposites containing MWCNTs with different types of dispersion.
According to Eq. (1), Q also depends on the density contrast between the two phases. In this case, the higher SAXS intensity exhibited by samples prepared by procedure 2 can be attributed to a higher density difference between amorphous and crystalline phases due to the presence of MWCNT. In procedure 2, MWCNTs are mixed with PTMG. In this case, SAXS measurements would indicate that MWCNT tend to be allocated preferentially in the amorphous PTMO domains. While for samples prepared by procedure 1, a better dispersion of MWCNT in both PBT and PTMO, lead-
ð1Þ
where V is the irradiated volume, q1, q2, are the electron density of crystalline and amorphous PBT phases respectively and U1 is the crystalline volume fraction. However, WAXS patterns for these samples clearly demonstrate that the peaks associated with the crystalline regions of PBT segments in the PBT/PTMO + CNTs nanocomposites are similar for all samples regardless of dispersion procedure. This is expected considering the good nucleation capability of CNTs [39–41]. The sharp peaks reflect a good crystal quality, which can be understood considering the capability of PBT to crystallise in the presence of carbon nanotubes.
Fig. 2. SAXS of PBT/PTMO nanocomposites containing MWCNTs with different types of dispersion.
Fig. 4. (a and b) SEM micrographs of fracture surfaces of granulated PBT/PTMO nanocomposites with 0.3 wt.% MWCNTs dispersed in DMT + BD (procedure 1).
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ing to a lower density contrast, can explain a lower SAXS intensity in comparison with that of samples prepared by procedure 2. 3.3. Morphology SEM investigation show that the different dispersion procedures led to different morphologies of the nanocomposites. Figs. 4a,b and 5a,b show SEM images of the fracture surfaces for samples prepared by procedure 1 (MWCNTs dispersed in DMT + BD) and by procedure 2 (MWCNTs dispersed in PTMG) respectively. It is evident that procedure 1 produces less agglomeration in comparison to procedure 2. A poorer distribution of nanotubes for procedure 2 (MWCNTs in PTMG) can be expected since PTMG has higher molecular weight and is more viscous than butanediol. These agglomerates can act as stress concentrators and initiators of mechanical failure and fracture of the specimens. According to molecular dynamics computer simulations of two phase systems, cracks start to propagate at interfaces [42–44]. The importance of interfaces for properties of multiphase polymeric materials has been discussed by Kopczynska and Ehrenstein [45]. Fracture surfaces of the nanocomposites were also examined at high magnification with field emission scanning electron microscopy (FESEM). Various electron microscopy techniques and their applications to nanocomposites have been reviewed by Adhikari and Michler [46]. In our case the difference observed for both methods of dispersion can be attributed to the higher reactivity of CNTs and therefore better CNT functionalization with the polymeric matrix – what in turn explains stronger nanotubes surface covering with DMT + BD when following procedure 1.
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Fig. 6 shows the fracture surface of a nanocomposite with dispersion of CNTs in DMT + BD (procedure 1). In this one sees image a nanotube, efficiently coated by the polymer matrix. The polymer matrix coating layer varies from 20 nm to 70 nm in thickness along the length of the nanotube. This image suggests that a good adhesion exists between the filler and the matrix, because of the absence of pull-out effect. However, one can detect a reduction of the nanotube coating (20 nm) probably due to forces causing the fracture of the sample that are capable to break the polymer matrix but not to separate the nanotube from the matrix. The oxidation process of the nanotubes leads to the formation of carboxylic groups on the nanotube surface. The matrix can directly interact with the nanotubes, providing good adhesion of CNTs to the PBT/PTMO copolymer matrix. The improved polymer-nanotube bonding, observed by SEM, obtained by procedure 1 is in a good agreement with the TEM observations of Fig. 1a showing a layer of crystallized powder of DMT + BD on MWCNTs surface. Fig. 7 shows the fracture surface of a nanocomposite with dispersion of CNTs in PTMG (procedure 2). Also an excellent wetting of the carbon nanotube by the matrix is observed as derived by the thickness the coated nanotube (100 nm). In comparison with the dispersion procedure 1, the coating layer of the nanotubes is about 30 nm thicker. Due to the void growth and the good CNT–matrix bonding, the outer layer of the tube remains bonded to the bridge, and the inner tubes are not subjected to a pull-out effect.
Fig. 6. SEM micrographs with high magnification of block copolymers with carbon nanotubes dispersed in DMT + BD (procedure 1) showing the total coverage of the tubes by the PBT/PTMO block copolymer.
Fig. 5. (a and b) SEM micrographs of fracture surfaces of granulated PBT/PTMO nanocomposites with 0.3 wt.% MWCNTs dispersed in PTMG (procedure 2).
Fig. 7. SEM micrographs with high magnification of PBT/PTMO block copolymer nanocomposite with nanotubes dispersed in PTMG (procedure 2).
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4. Concluding remarks In this work it was shown that thermoplastic elastomers based PBT-block-PTMO can be synthesized in the presence of oxidized multi-walled carbon nanotubes (MWCNTs) with both, a homogenous distribution of the nanoadditives and a good wetting of the nanotubes by polymer. The level of nanotube aggregation within the polymer matrix depends on the procedure followed to synthesize the PBT/PTMO with MWCNT nanocomposite. A better dispersion can be achieved when oxidized MWCNTs are added to the DMT + BD monomers before transesterification (procedure 1). In this case an effective coating of the nanotube by the crystallized powder of DMT + BD is obtained. By mixing MWCNT with PTMG after transesterification (procedure 2) a higher amount of nanotube aggregation is observed. Due to the oxidized nature of the nanotube surface, for both procedures an efficient coating of the nanotube surface by the polymer matrix is obtained. Acknowledgements The author gratefully acknowledge the support by the Deutsche Forschungsgemeinschaft (DFG), Bonn, contract SCHU 926-14-1 and by the Polish Ministry of Science and High Education, Warsaw, contract 2/DFG/2007/02. Furthermore, thanks are due to Dr. L.A.S. de A. Prado at the Hamburg University of Technology for performing the simultaneous SAXS/WAXS measurements at A2 beamline (HASYLAB at Deutsches Elektronsynchrotron DESY, Hamburg). References [1] Thostensen ET, Ren Z, Chou TW. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61(13):1899–912. [2] Gojny FH, Wichmann MHG, Köpke U, Fiedler B, Schulte K. Carbon nanotubereinforced epoxy-composites: enhanced stiffness and fracture toughness at low nanotube content. Compos Sci Technol 2004;64(15):2363–71. [3] Li QQ, Zaiser M, Koutsos V. Carbon nanotube/epoxy resin composites using a block copolymer as a dispersing agent. Phys Status Solidi A 2004;201(13):89–91. [4] Song YS, Yuon JR. Influence of the dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites. Carbon 2005;43(7):1378–85. [5] Tang W, Santare MH, Advani SG. Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite films. Carbon 2003;41(14):2779–85. [6] Manchado LMA, Valentini L, Biagiotti J, Kenny JM. Thermal and mechanical properties of single-walled carbon nanotubes–polypropylene composites prepared by melt processing. Carbon 2005;43(7):1499–505. [7] Pötschke P, Bhattacharyya AR. Carbon nanotube filled polycarbonate composites produced by melt mixing and their use in blends. Polym Preprint 2003;44(1):760–1. [8] Giraldo LF, Brostow W, Devaux E, Lopez BL, Perez LD. Scratch and wear resistance of polyamide 6 reinforced with multiwall carbon nanotubes. J Nanosci Nanotechnol 2008;8(6):3176–83. [9] Giraldo LF, Lopez BL, Brostow W. Effects of the type of carbon nanotubes on tribological properties of polyamide 6. Polym Eng Sci 2009;49(5):896–902. [10] Kwiatkowska M, Broza G, Schulte K, Roslaniec Z. The in-situ synthesis of poly(butylene terephthalate)/carbon nanotubes composites. Rev Adv Mater Sci 2006;12(2):154–9. [11] Roslaniec Z, Broza G, Schulte K. Nanocomposites based on multiblock polyester elastomers (PEE) and carbon nanotubes. Compos Interf 2003;10(1):95–102. [12] Broza G, Piszczek K, Schulte K, Sterzynski T. Nanocomposites of poly(vinyl chloride) with carbon nanotubes (CNT). Compos Sci Technol 2007;67(5):890–4. [13] Gojny FH, Schulte K. Functionalisation effect on the thermo-mechanical behaviour of multi-wall carbon nanotube/epoxy composites. Compos Sci Technol 2004;64(15):2303–8. [14] Qin S, Qin D, Ford WT, Resasco DE, Herrera JE. Polymer brushes on singlewalled carbon nanotubes by atom transfer radical polymerization of n-butyl methacrylate. J Am Chem Soc 2004;126(1):170–6. [15] Hao K, Chao G, Deyue Y. Controlled functionalization of multiwalled carbon nanotubes by in situ atom transfer radical polymerization. J Am Chem Soc 2004;126(2):412–3. [16] Haggenmüller R, Gommans HH, Rinzler AG, Fischer JE, Winey KI. Aligned single-wall carbon nanotubes in composites by melt processing methods. Chem Phys Lett 2000;330(3–4):219–25.
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Thermoplastic elastomers with multi-walled carbon nanotubes: Influence of dispersion methods on morphology Georg Broza * Institute of Polymer Composites, Hamburg University of Technology, Denickestrasse 15, D-21073 Hamburg, Germany
a r t i c l e
i n f o
Article history: Received 24 November 2009 Received in revised form 15 February 2010 Accepted 21 February 2010 Available online 1 March 2010 Keywords: A. Carbon nanotubes A. Nanocomposites B. Interfaces D. Scanning/transmission electron microscopy (STEM) D. X-ray diffraction (XRD)
a b s t r a c t PBT-block-PTMO thermoplastic elastomers derived from dimethyl terephthalate (DMT), 1,4-butanediol(BD) and poly(tetramethylene glycol) (PTMG) were synthesized in the presence of oxidized multiwalled carbon nanotubes (MWCNTs) by a two-stage process involving transesterification and in situ polycondensation. Two procedures were applied to nanotubes in the polymer matrices were used. In procedure 1, nanotubes were dispersed in DMT + BD before the transesterification, while in procedure 2 nanotubes were dispersed in PTMG after transesterification. The mole ratio of the starting components was selected to produce copolymers with a constant hard to soft segment weight ratio of 45:55. Characterization of the new nanocomposites was performed by transmission electron microscopy (TEM), scanning electron microscopy (SEM), small- and wide-angle X-ray scattering (SAXS/WAXS). A better nanotube dispersion can be achieved when oxidized MWCNTs are added to the DMT + BD monomers before transesterification (procedure 1). Oxidized MWCNTs exhibit strong interfacial adhesion to the polymer matrix for both procedures. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Nano-structured modification of polymers has opened new perspectives for multifunctional materials [1]. Combination of carbon nanotubes (CNTs) with polymer matrices can be considered as a step forward in polymer and composites technology. This has been shown for thermoset polymers [2–4] and recently also for several thermoplastics [5–12]. Chemical functionalization and solubilization of carbon nanotubes has emerged as an effective means for homogeneous dispersion of CNTs in solution. Functionalization is a surface treatment which creates functional groups at surfaces of filler nanoparticles. The filler functionalization brings about favourable interactions between the functional groups and the polymer matrix [13–15]. This is particularly important for fillers with a strong tendency towards agglomeration – as for instance carbon nanotubes. Enhanced filler–matrix interactions facilitate the dispersion of the filler in the polymer matrix and strengthen the interfacial bonding, leading to better reinforcement effects. In this respect, a main challenge consists on the improvement of the dispersion of CNTs in a polymer matrix during processing. There are several techniques to improve the dispersion of CNTs in a polymer matrix, such as melt blending [16–19], in situ polymerisation or polycondensation on the presence of nanotubes [10,11,20], and solution mixing [21,22]. Direct mixing and sonica* Tel.: +49 40428783 496; fax: +49 40428782 002. E-mail address: [email protected] 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.02.021
tion have been the most common techniques to disperse CNTs within thermoset polymer resins [23–25], and thermoplastic polymeric matrices [26–29]. However, these methods were found to be ineffective to completely eliminate agglomeration of CNTs. In situ polycondensation of monomers in the presence of nanofillers is a promising approach for a more homogeneous distribution, due to a closer contact of polymer and filler during synthesis [10]. Dispersion of CNTs in 1,4-butanediol before transesterification has been utilized previously for the synthesis of PBT/PTMO- and PBT-containing nanocomposites. Methods of nanocomposite preparation have been described before [11,30]. In this work multi-wall carbon nanotubes (MWCNTs) were used to prepare composites with poly(ether-b-ester)s. These copolymers are based on semicrystalline poly(butylene terephthalate) (PBT) blocks and amorphous poly(tetramethylene oxide) (PTMO) blocks. MWCNTs can be oxidized in order to enhance solubility of the nanotubes in polar solvents [31–34]. Introduction of oxygen-containing surface groups on the nanotube surface, e.g. carboxylate groups, leads to an electrostatic stabilisation in polar solvents due to development of strong hydrogen bonds with the solvent [35,36]. Since PBT/PTMO copolymer used in the present work is synthesized based on dimethyl terephthalate (DMT), butanediol (BD) and poly(tetramethylene glycol) (PTMG), then chemical properties of the functionalized carbon nanotubes should be tailored considering the chemical reactions involved during the synthesis. It will be shown that introduction of carbonyl groups on surfaces of
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carbon nanotubes is a plausible method to optimize dispersion in the PBT/PTMO with MWCNT nanocomposite and to improve filler adhesion to the polymer matrix. The aim of this work is to advance the general understanding of interactions between CNTs and a polymer matrix by investigating nanocomposites prepared by using two different methods to incorporate introduction of oxidized MWCNTs into the polymer matrix. In a first method, MWCNTs were added prior to the transesterification reaction and in the second method they were added after the transesterification. It will be shown that the different methods lead to different composite morphology. 2. Experimental 2.1. Material characterization Commercially available reagents for poly(ether–ester) block copolymers synthesis were obtained as follows: dimethyl terephthalate (DMT) from (DuPont, NC, USA); 1,4-butanediol (BD) from (BASF, Germany); poly(tetramethylene glycol) (PTMG; Mn = 970 g/ mol) also from DuPont. MWCNTs were supplied by Nanocyl S.A. (Namur, Belgium) – produced by catalytic chemical vapour deposition method (CCVD). Typically, their outer diameter range is 3–15 nm, length up to 50 lm. 2.2. Electron microscopy (TEM and SEM) Transmission electron microscopy (TEM) observations were conducted to determine the dispersion state of MWCNTs either within DMT + BD or within PTMG in the polymer matrix. TEM images were taken using a Philips EM 400 machine at 100 kV. A LEO 1530 field emission scanning electron microscope (SEM) with different accelerating voltages (1.5–3 kV) was employed to observe the morphology of the tensile fractured nanocomposites.
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3.1.1. Dispersion procedure 1 Before the first step of transesterification, 50 wt.% of total amount of dimethyl terephthalate (DMT) was melted in butanediol (BD) at 140 °C. MWCNTs (0.3 wt.%) was added to this mixture. Previously MWCNTs were ultrahigh speed stirred for 5 min. at 20,000 rpm (step 1) and then sonicated for 5 min at 75 °C (step 2). To optimize the dispersion, both steps were repeated six times. The molten dispersion was casted onto a plate, and after cooling down to room temperature the material has solidified. TEM and SEM characterization provides a clear evidence of nanotube surface coverage. In Fig. 1a (TEM image) it is shown a MWCNT covered by monomers. The diameter of the covered MWCNT is in the range from 33 to 42 nm. The core formed by the nanotube with a diameter of about 8 nm can also be seen in Fig. 1a. At lower magnification the SEM image in Fig. 1b shows the covered nanotubes with diameters in the range from 41 to 74 nm. These results show that oxidized MWCNTs are very suitable for the coating process. PBT/ PTMO with MWCNT nanocomposites were synthesised in a twostage process [11]: transesterification followed by polycondensation in the melt, with PTMO content of 55 wt.%. In a steel reactor (Autoclave Eng. Inc., USA) we put (i) 50 wt.% of dimethyl terephthalate (DMT) and catalyst mixed, additionally the crystallized MWCNT/DMT + BD powder (with 50 wt.% of total DMT). 3.1.2. Dispersion procedure 2 After the transesterification process, the MWCNTs were dispersed in PTMG to produce PBT/PTMO + CNTs nanocomposites. PTMG has higher molecular weight than glycol, hence it is more viscous and dispersion is more difficult, even though the mixture was heated all the time. MWCNTs (0.3 wt.%) were mixed with molten PTMG and stirred for 10 min and sonicated for 10 min at room temperature and introduced into the reactor after the transestrification process. To optimize the dispersion, both steps were repeated ten times. In this case, PBT/PTMO + MWCNT composites were synthesised by the described two-stage process but now 100 wt.% of DMT + BD and MWCNT + PTMG mixture were slowly
2.3. Small and wide-angle X-ray scattering (SAXS/WAXS) Small and wide-angle X-ray scattering (SAXS/WAXS) experiments were performed in transmission at the beamline A2 of HASYLAB/DESY, Hamburg, with 8 keV X-rays and two linear detectors. The A2 beamline is particularly useful for this purpose since it allows conducting SAXS and WAXS experiments simultaneously. The raw data were background and detector response corrected. The SAXS scattering-vectors were calibrated using a rat-tail tendon protein. The WAXS data were calibrated using reflections of poly(butylene terephthalate) (PBT) film. 3. Results and discussion 3.1. Preparation of the nanocomposites MWCNTs, as received, were added to a mixture containing concentrated sulphuric and nitric acids, 3:1 by volume. This procedure is known to introduce carboxylate groups onto the outermost shell of the MWCNTs. In addition, this acid mixture intercalates and exfoliates graphite sheets and, therefore, helps to separate carbon nanotubes from each other. The suspension was sonicated in an ultrasonic bath for 30 min and then heated up to 90 °C for 3 h. After centrifugation and washing with hot distillated water, the suspension was stirred in sodium hydrogen carbonate solution overnight and washed again according to a procedure of Funk and Kaminsky [37]. For the synthesis of PBT/PTMO + MWCNTs, two different ways of introducing oxidized MWCNTs in polymer matrices were used:
Fig. 1. (a) TEM image and (b) SEM image showing the dispersion of oxidized MWCNTs due to coating nanotubes surfaces by DMT + BD.
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injected. Nanocomposites containing 0.3 wt.% of MWCNT were extruded from the reactor by compressed nitrogen and cooled down to room temperature. 3.2. Structure High intensity of synchrotron radiation enables investigation of SAXS patterns of materials which have weak scattering properties – such as polymers. In the case of polymer nanocomposites containing carbon nanotubes, CNTs lead to a strong radiation absorption by the sample; this makes the data acquisition by conventional SAXS equipment (i.e., those with a conventional X-ray source) very difficult. Figs. 2 and 3 show the results of SAXS and WAXS measurements, respectively. The SAXS curve of the PBT/PTMO nanocomposites with dispersion procedure 1 (MWCNTs in the DMT + BD) has a correlation peak at low angles – associated with the crystalline layers separated by the amorphous polymer chains. The position of the maximum of this peak corresponds to a long-period (distances between the crystalline PBT lamellae) of L = 14.2 nm. For the nanocomposite with CNTs dispersed by procedure 2 (MWCNT dispersed in PTMG) a similar peak but of higher intensity is observed. The long-period value in this case was slightly smaller (L = 12.7 nm). The scattering power, Q, of the nanocomposites prepared by procedure 1 (MWCNT in DMT + BD) seems to be lower than that for nanocomposites prepared by procedure 2 (MWCNT in PTMG). This decrease might be explained, in a first approach, in terms of an increased amount of the crystalline phase for samples prepared by procedure 2. Considering that the amount of CNTs in the system is expected to be much lower than the crystalline fraction of PBT in the nanocomposite, the twocomponent model, in which dense PBT lamellae (phase 1) are dispersed in an amorphous PBT phase (phase 2) is applicable. For such systems [38]:
Q¼
Z
IðqÞq2 dq ¼ 2p2 ðq2 q1 Þ2 V U1 ð1 U1 Þ
Fig. 3. WAXS patterns of PBT/PTMO nanocomposites containing MWCNTs with different types of dispersion.
According to Eq. (1), Q also depends on the density contrast between the two phases. In this case, the higher SAXS intensity exhibited by samples prepared by procedure 2 can be attributed to a higher density difference between amorphous and crystalline phases due to the presence of MWCNT. In procedure 2, MWCNTs are mixed with PTMG. In this case, SAXS measurements would indicate that MWCNT tend to be allocated preferentially in the amorphous PTMO domains. While for samples prepared by procedure 1, a better dispersion of MWCNT in both PBT and PTMO, lead-
ð1Þ
where V is the irradiated volume, q1, q2, are the electron density of crystalline and amorphous PBT phases respectively and U1 is the crystalline volume fraction. However, WAXS patterns for these samples clearly demonstrate that the peaks associated with the crystalline regions of PBT segments in the PBT/PTMO + CNTs nanocomposites are similar for all samples regardless of dispersion procedure. This is expected considering the good nucleation capability of CNTs [39–41]. The sharp peaks reflect a good crystal quality, which can be understood considering the capability of PBT to crystallise in the presence of carbon nanotubes.
Fig. 2. SAXS of PBT/PTMO nanocomposites containing MWCNTs with different types of dispersion.
Fig. 4. (a and b) SEM micrographs of fracture surfaces of granulated PBT/PTMO nanocomposites with 0.3 wt.% MWCNTs dispersed in DMT + BD (procedure 1).
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ing to a lower density contrast, can explain a lower SAXS intensity in comparison with that of samples prepared by procedure 2. 3.3. Morphology SEM investigation show that the different dispersion procedures led to different morphologies of the nanocomposites. Figs. 4a,b and 5a,b show SEM images of the fracture surfaces for samples prepared by procedure 1 (MWCNTs dispersed in DMT + BD) and by procedure 2 (MWCNTs dispersed in PTMG) respectively. It is evident that procedure 1 produces less agglomeration in comparison to procedure 2. A poorer distribution of nanotubes for procedure 2 (MWCNTs in PTMG) can be expected since PTMG has higher molecular weight and is more viscous than butanediol. These agglomerates can act as stress concentrators and initiators of mechanical failure and fracture of the specimens. According to molecular dynamics computer simulations of two phase systems, cracks start to propagate at interfaces [42–44]. The importance of interfaces for properties of multiphase polymeric materials has been discussed by Kopczynska and Ehrenstein [45]. Fracture surfaces of the nanocomposites were also examined at high magnification with field emission scanning electron microscopy (FESEM). Various electron microscopy techniques and their applications to nanocomposites have been reviewed by Adhikari and Michler [46]. In our case the difference observed for both methods of dispersion can be attributed to the higher reactivity of CNTs and therefore better CNT functionalization with the polymeric matrix – what in turn explains stronger nanotubes surface covering with DMT + BD when following procedure 1.
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Fig. 6 shows the fracture surface of a nanocomposite with dispersion of CNTs in DMT + BD (procedure 1). In this one sees image a nanotube, efficiently coated by the polymer matrix. The polymer matrix coating layer varies from 20 nm to 70 nm in thickness along the length of the nanotube. This image suggests that a good adhesion exists between the filler and the matrix, because of the absence of pull-out effect. However, one can detect a reduction of the nanotube coating (20 nm) probably due to forces causing the fracture of the sample that are capable to break the polymer matrix but not to separate the nanotube from the matrix. The oxidation process of the nanotubes leads to the formation of carboxylic groups on the nanotube surface. The matrix can directly interact with the nanotubes, providing good adhesion of CNTs to the PBT/PTMO copolymer matrix. The improved polymer-nanotube bonding, observed by SEM, obtained by procedure 1 is in a good agreement with the TEM observations of Fig. 1a showing a layer of crystallized powder of DMT + BD on MWCNTs surface. Fig. 7 shows the fracture surface of a nanocomposite with dispersion of CNTs in PTMG (procedure 2). Also an excellent wetting of the carbon nanotube by the matrix is observed as derived by the thickness the coated nanotube (100 nm). In comparison with the dispersion procedure 1, the coating layer of the nanotubes is about 30 nm thicker. Due to the void growth and the good CNT–matrix bonding, the outer layer of the tube remains bonded to the bridge, and the inner tubes are not subjected to a pull-out effect.
Fig. 6. SEM micrographs with high magnification of block copolymers with carbon nanotubes dispersed in DMT + BD (procedure 1) showing the total coverage of the tubes by the PBT/PTMO block copolymer.
Fig. 5. (a and b) SEM micrographs of fracture surfaces of granulated PBT/PTMO nanocomposites with 0.3 wt.% MWCNTs dispersed in PTMG (procedure 2).
Fig. 7. SEM micrographs with high magnification of PBT/PTMO block copolymer nanocomposite with nanotubes dispersed in PTMG (procedure 2).
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4. Concluding remarks In this work it was shown that thermoplastic elastomers based PBT-block-PTMO can be synthesized in the presence of oxidized multi-walled carbon nanotubes (MWCNTs) with both, a homogenous distribution of the nanoadditives and a good wetting of the nanotubes by polymer. The level of nanotube aggregation within the polymer matrix depends on the procedure followed to synthesize the PBT/PTMO with MWCNT nanocomposite. A better dispersion can be achieved when oxidized MWCNTs are added to the DMT + BD monomers before transesterification (procedure 1). In this case an effective coating of the nanotube by the crystallized powder of DMT + BD is obtained. By mixing MWCNT with PTMG after transesterification (procedure 2) a higher amount of nanotube aggregation is observed. Due to the oxidized nature of the nanotube surface, for both procedures an efficient coating of the nanotube surface by the polymer matrix is obtained. Acknowledgements The author gratefully acknowledge the support by the Deutsche Forschungsgemeinschaft (DFG), Bonn, contract SCHU 926-14-1 and by the Polish Ministry of Science and High Education, Warsaw, contract 2/DFG/2007/02. Furthermore, thanks are due to Dr. L.A.S. de A. Prado at the Hamburg University of Technology for performing the simultaneous SAXS/WAXS measurements at A2 beamline (HASYLAB at Deutsches Elektronsynchrotron DESY, Hamburg). References [1] Thostensen ET, Ren Z, Chou TW. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61(13):1899–912. [2] Gojny FH, Wichmann MHG, Köpke U, Fiedler B, Schulte K. Carbon nanotubereinforced epoxy-composites: enhanced stiffness and fracture toughness at low nanotube content. Compos Sci Technol 2004;64(15):2363–71. [3] Li QQ, Zaiser M, Koutsos V. Carbon nanotube/epoxy resin composites using a block copolymer as a dispersing agent. Phys Status Solidi A 2004;201(13):89–91. [4] Song YS, Yuon JR. Influence of the dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites. Carbon 2005;43(7):1378–85. [5] Tang W, Santare MH, Advani SG. Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite films. Carbon 2003;41(14):2779–85. [6] Manchado LMA, Valentini L, Biagiotti J, Kenny JM. Thermal and mechanical properties of single-walled carbon nanotubes–polypropylene composites prepared by melt processing. Carbon 2005;43(7):1499–505. [7] Pötschke P, Bhattacharyya AR. Carbon nanotube filled polycarbonate composites produced by melt mixing and their use in blends. Polym Preprint 2003;44(1):760–1. [8] Giraldo LF, Brostow W, Devaux E, Lopez BL, Perez LD. Scratch and wear resistance of polyamide 6 reinforced with multiwall carbon nanotubes. J Nanosci Nanotechnol 2008;8(6):3176–83. [9] Giraldo LF, Lopez BL, Brostow W. Effects of the type of carbon nanotubes on tribological properties of polyamide 6. Polym Eng Sci 2009;49(5):896–902. [10] Kwiatkowska M, Broza G, Schulte K, Roslaniec Z. The in-situ synthesis of poly(butylene terephthalate)/carbon nanotubes composites. Rev Adv Mater Sci 2006;12(2):154–9. [11] Roslaniec Z, Broza G, Schulte K. Nanocomposites based on multiblock polyester elastomers (PEE) and carbon nanotubes. Compos Interf 2003;10(1):95–102. [12] Broza G, Piszczek K, Schulte K, Sterzynski T. Nanocomposites of poly(vinyl chloride) with carbon nanotubes (CNT). Compos Sci Technol 2007;67(5):890–4. [13] Gojny FH, Schulte K. Functionalisation effect on the thermo-mechanical behaviour of multi-wall carbon nanotube/epoxy composites. Compos Sci Technol 2004;64(15):2303–8. [14] Qin S, Qin D, Ford WT, Resasco DE, Herrera JE. Polymer brushes on singlewalled carbon nanotubes by atom transfer radical polymerization of n-butyl methacrylate. J Am Chem Soc 2004;126(1):170–6. [15] Hao K, Chao G, Deyue Y. Controlled functionalization of multiwalled carbon nanotubes by in situ atom transfer radical polymerization. J Am Chem Soc 2004;126(2):412–3. [16] Haggenmüller R, Gommans HH, Rinzler AG, Fischer JE, Winey KI. Aligned single-wall carbon nanotubes in composites by melt processing methods. Chem Phys Lett 2000;330(3–4):219–25.
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