An exfoliation of organoclay in thermotropic liquid crystalline polyester nanocomposites

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Polymer 43 (2002) 2969±2974 www.elsevier.com/locate/polymer An exfoliation of organoclay in thermotropic liquid crystalline polyester nanocomposites Jin-Hae Chang a,*, Bo-Soo Seo a, Do-Hoon Hwang b a Department of Polymer Science and Engineering, Kumoh National University of Technology, Kumi 730-701, South Korea b Department of Applied Chemistry, Kumoh National University of Technology, Kumi 730-701, South Korea Dedicated to Prof Jung-Il Jin of Korea University, Seoul, Korea, on the occasion of his 60th birthday Received October 2001; received in revised form January 2002; accepted February 2002 Abstract A thermotropic liquid crystalline polyester (TLCP) with an alkoxy side-group was synthesized from 2-ethoxyhydroquinone and 2bromoterephthalic acid Nanocomposites of TLCP with Cloisite 25A (C25A) as an organoclay were prepared by the melting intercalation method above the melt transition temperature (Tm) of the TLCP Liquid crystallinity, morphology, and thermo-mechanical behaviors were examined with increasing organoclay content from to 6% Liquid crystallinity of the C25A/TLCP hybrids was observed when organoclay content was up to 6% Regardless of the clay content in the hybrids, the C25A in TLCP was highly dispersed in a nanometer scale The hybrids (0±6% C25A/TLCP) were processed for ®ber spinning to examine their tensile properties Ultimate strength and initial modulus of the TLCP hybrids increased with increasing clay content and the maximum values of the mechanical properties were obtained from the hybrid containing 6% of the organoclay Thermal, morphological and mechanical properties of the nanocomposites were examined by differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), polarized optical microscope, electron microscopes (SEM and TEM), and capillary rheometer q 2002 Elsevier Science Ltd All rights reserved Keywords: Thermotropic liquid crystalline polyester; Organoclay; Nanocomposite Introduction Thermotropic liquid crystalline polymers have already been established as high performance commercial engineering polymers This is due to their speci®c chemical structures, high strengths, high moduli, low viscosities, and other good mechanical properties [1±4] The structureproperty relationships of thermotropic liquid crystalline polyesters (TLCPs) have been the subject of much research [4±7] In spite of their inferior physical strength when compared with lyotropic liquid crystalline polyamides, TLCPs are attracting a great deal of interest based on their melt processability [8,9] Although wholly aromatic TLCPs exhibit very attractive mechanical properties, they generally have high melting points, thus making them dif®cult to process [10,11] Inclusion of ¯exible alkyl groups in otherwise wholly aromatic polyesters not only lowers the melting point, but also improves solubility and increases mixing entropy Thus, despite the predictable reduction in mechanical properties, * Corresponding author Tel.: 182-54-467-4292; fax: 182-42-483-6155 E-mail address: changjinhae@hanmail.net (J.-H Chang) these polyesters possess considerable advantages in some applications and show improved interfacial adhesion between the two phases [5,12,13] Nanocomposites possess unique properties, such as stiffness, strength and gas permeability, for their dispersion structure [14±18] The methods used for creating nanocomposites include in situ polymerization, solution intercalation, and melting intercalation [19,20] Of them, melting intercalation can be used with the most polymers, especially thermoplastic materials, but it needs a polymer that has good process properties in the melting state In recent years much attention has been paid to layered clay/polymer nanocomposites, since these represent advanced plastic materials prepared via the melting intercalation method In our previous paper [21], large improvements were achieved in the thermal stabilities of TLCP nanocomposites by using organo-montmorillonite This enhancement of the thermal stabilities explains reasonably well the dispersed structure of clay in the nanocomposites caused by the formation of the large aspect ratio of the clay particles For this paper, we synthesized TLCP with an alkoxy side group base on a nematic liquid crystalline phase We also examined the correlation between the thermo-mechanical 0032-3861/02/$ - see front matter q 2002 Elsevier Science Ltd All rights reserved PII: S 0032-386 1(02)00125-8 2970 J.-H Chang et al / Polymer 43 (2002) 2969±2974 earlier described by us [22], as well as by Lenz et al [23] The ethoxy side group and Br on the TLCP not only lowers the melting point, but also improves some applications, as mentioned in the previous section The polymer formed was thoroughly washed with methanol, with dilute HCl, and then with water prior to drying at 60 8C in a vacuum oven Inherent viscosity of the TLCP was 0.64 dL/g which was measured at 30 8C at a concentration of 0.2 g/dL solutions in a phenol/1,1,2,2-tetrachloroethane ˆ 50/50 (v/v) mixture Fig shows the thread nematic textures for pure TLCP at both 197 and 210 8C 2.3 Preparation of C25A /TLCP nanocomposites Fig Optical micrographs of TLCP taken at (a) 197 8C and (b) 210 8C ( £ 250) properties and the clay content in TLCP nanocomposites with variances in the dispersed morphology of the clay particles The general goal of this work was to use a minimum amount of clay in the hybrids and still obtain thermo-mechanical properties signi®cantly superior to those of matrix polymer Experimental 2.1 Monomer synthesis All reagents were purchased from Aldrich Chemical Co Commercially available solvents were puri®ed by distillation The compound 2-ethoxyhydroquinone was synthesized via a multi-step route, and 2-bromoterephthalic acid was purchased from Aldrich Chemicals 2.2 Polymer preparation The TLCP was prepared by direct polycondensation of equivalent weights of the appropriate 2-ethoxyhydroquinone and 2-bromoterephthalic acid in the presence of thionyl chloride and pyridine The detailed procedure was Cloisite 25A (organically modi®ed MMT; C25A) was obtained from Southern Clay Product, Co Since the synthetic procedures for C25A/TLCP nanocomposites with different weight percent (wt%) organoclay are very similar, only a representative example for the preparation of the C25A/TLCP (2 wt%) is given 50 g of TLCP and g of C25A were dry-mixed and melt-blended at 190 8C, within the nematic region of the polymer, for 30 using a mechanical mixer For simplicity, the hybrids will be referred to as 0% C25A/TLCP, 2% C25A/TLCP, 4% C25A/TLCP, and so on, in which C25A and TLCP represent the organoclay and polymer components used to prepare the hybrids, respectively, and the number denotes the organoclay weight percent in the hybrid 2.4 Extrusion The TLCP hybrids were processed for ®ber spinning to examine their tensile properties The dried blends were pressed at 160 8C, 2500 kg/cm for a few minutes on a hot press The ®lm-type blends were dried in a vacuum oven for 24 h prior to being extruded through the die of a capillary rheometer From the capillary rheometer, the hot extrudates were immediately drawn at constant take-up speed to form extended extrudates having the same diameters The cylinder temperature of the extruder was 190 8C and the mean residence time in the capillary rheometer was about 2±3 To identify chemical reactions such as transesteri®cation and thermal degradation at the processing temperature, annealing was conducted for 4% C25A/TLCP hybrid at 190 8C DSC thermograms of the heat-treated hybrids are shown in Fig When heat treatment time increased from 10 to 60 at 190 8C, there were no signi®cant changes in the DSC scans Chemical changes thus not take place to any appreciable extent at the extrusion processing temperature 190 8C It was also con®rmed by 1H- and 13C-NMR spectroscopy that no detectable transesteri®cation reaction occurred in TLCP under the processing condition J.-H Chang et al / Polymer 43 (2002) 2969±2974 Fig DSC thermograms of 4% C25A in TLCP hybrid annealed at 190 8C for different times 2.5 Characterization The thermal and the thermogravimetric analyses of hybrids were carried out under N2 atmosphere on Du Pont 910 equipment The samples were heated and cooled at a rate of 20 8C /min Wide-angle X-ray diffraction (XRD) measurements were performed at room temperature on a Rigaku (D/Max-IIIB) X-ray Diffractometer, using Ni®ltered Co-Ka radiation The scanning rate was 28/min over a range of 2u ˆ 2±308 Tensile properties of the extrudate were determined using an Instron Mechanical Tester (Model 5564) at a crosshead speed of mm/min The specimens were prepared by cutting strips by 70 mm long An average of at least eight individual determinations was obtained The experimental uncertainties in tensile strength and modulus were ^1 MPa and ^0.05 GPa, respectively A polarizing microscope (Leitz, Ortholux) equipped with a Mettler FP-5 hot stage was used to examine the liquid crystalline behavior The morphology of the fractured surfaces of the extrusion samples was investigated using a Hitachi S-2400 scanning electron microscope (SEM) The fractured surfaces were sputter-coated with gold for enhanced conductivity using an SPI Sputter Coater TEM photographs of ultrathin section polymer/organoclay hybrid samples were taken on an EM 912 OMEGA (CARL ZEISS) transmission electron microscope using an acceleration voltage of 120 kV Results and discussion 3.1 Dispersibility of organoclay in TLCP The XRD patterns of C25A, pure TLCP, and their TLCP 2971 Fig XRDs of C25A and C25A/TLCP hybrids hybrids with 2±6% C25A were represented in the region from 2u ˆ 2±158 in Fig The interlayer spacing was Ê ) for C25A A peak observed in 2u ˆ 5.648 (d ˆ 18.14 A Ê ) for pure TLCP was observed in 2u ˆ 4.698 (d ˆ 21.98 A When the amount of organoclay increased from to 6%, C25A/TLCP hybrids showed a same peak at the same position (2u ˆ 4.698) In the TLCP hybrids with 2±6% C25A, no obvious clay peaks appeared in their X-ray diffraction curves This indicated that these clay layers were exfoliated and dispersed homogeneously in the TLCP matrix This was also direct evidence that the C25A/TLCP hybrids formed nanocomposites Unfortunately, XRD is unable to detect regular stacking Ê One may note that the commonly used exceeding 88 A de®nition of an exfoliated nanocompoisite is based on layer spacing larger than this value In reality, it was the electron microscopic analyses that evidenced the formation of nanocomposites Fractured surfaces of the ®lms were viewed under SEM A comparative analysis of the SEM photograph for TLCP hybrids with different clay content exhibiting the ®brous and platelet orientation distribution morphology including overall projection, as shown in Fig More direct evidence of the formation of a true nanocomposite is provided by TEM of an ultramicrotomed section TEM micrographs of TLCP with different C25A content from to 6% are shown in Fig 5(a)±(c), respectively The dark lines are the intersections of the clay layer of 1nm-thickness and the spaces between the dark lines are interlayer spaces This TEM photograph proves that most clay layers of organoclay were exfoliated and dispersed homogeneously into the TLCP matrix This is consistent with the observation of XRD studies shown in Fig In conclusion, we were able to successfully synthesize TLCP nanocomposites using C25A via a melting intercalation 2972 J.-H Chang et al / Polymer 43 (2002) 2969±2974 are slightly increased to 150 8C (see Table 1) This increase in the thermal behavior of the hybrids may result from the heat insulation effect of the clay layer structure, as well as the strong interaction between the organoclay and TLCP molecular chains The isotropic transition temperatures (Ti) of pure TLCP was virtually unchanged regardless of organoclay loading, compared with the TLCP hybrids Fig shows the thread nematic textures for and 6% C25A/TLCP hybrids, respectively Regardless of the clay content in the hybrids, liquid crystallinity of the C25A/ TLCP hybrids was observed when organoclay content was up to 6% In addition to having a higher melting point, thermal degradation properties of TLCP hybrids also show improvement Fig SEM photomicrographs of (a) 0% (pure TLCP), (b) 2%, (c) 4%, and (d) 6% C25A in TLCP hybrids method Considering the preceding results, the existing state of clay particles could be determined to affect the thermal behaviors and the tensile mechanical properties for each organoclay/polymer hybrid 3.2 Thermal behaviors The thermal properties of TLCP hybrids with different contents of C25A are listed in Table The glass transition temperatures (Tg) of TLCP hybrids linearly increased from 92 to 98 8C with clay loading from to wt% and leveled off at the content range of more than wt% of organoclay The increase in the Tg of these hybrids could be the result of two factors First, the effect of small amounts of dispersed clay layers on the free volume of TLCP is signi®cant, and does in¯uence the glass transition temperature of TLCP hybrids The second factor is ascribed to the con®nement of the intercalated polymer chains within the clay galleries, which prevents segmental motions of the polymer chains DSC traces of the pure TLCP and the hybrids are shown in Fig The endothermic peak of the pure TLCP appears at 143 8C and corresponds with the melt transition temperature (Tm) Maximum transition peaks of the TLCP hybrids containing different clay contents in the DSC thermograms Table Thermal behavior values of C25A/TLCP hybrids clay (wt%) Tg (8C) Tm (8C) Ti (8C) TD i a (8C) wtR600 b (%) (pure TLCP) 92 95 98 98 143 150 150 149 225 226 225 225 330 352 352 353 37 42 44 47 a b Initial weight reduction onset temperature Weight percent of residue at 600 8C Fig TEM photomicrographs of (a) 2%, (b) 4%, and (c) 6% C25A in TLCP hybrids J.-H Chang et al / Polymer 43 (2002) 2969±2974 2973 Fig TGA thermograms of C25A and C25A/TLCP hybrids Fig DSC thermograms of C25A and C25A/TLCP hybrids A comparative thermal gravimetric analysis (TGA) of pure TLCP and three nanocomposites with 2±6% C25A is shown in Table and Fig TGA curves not show weight loss below 100 8C, as shown in Fig 8, indicating no water remained in the samples The weight loss due to the decomposition of TLCP and its hybrids was nearly the same until a temperature of about 300 8C After this point, the initial thermal degradation temperature (TDi ) was in¯uenced by organoclay loading in hybrids Table summarizes TDi of the C25A/TLCP hybrids (at 2% weight loss) increased with the amount of organoclay TDi was observed at 352±353 8C depending on the composition of the clay from to wt% in the TLCP hybrids, with a maximum increase of 23 8C in the case of the 6% C25A/TLCP as compared with that of the pure TLCP Weight of the residue at 600 8C increased with clay loading from to 6%, ranging from 37 to 47% This enhancement of the char formation is ascribed to the high heat resistance exerted by the clay itself Considering the above results, it is consistently believable that the introduction of inorganic components into organic polymers can improve their thermal stability on the basis of the fact that clays have good thermal stability [24,25] 3.3 Tensile properties The pure TLCP and the TLCP hybrids were extruded through a capillary die with draw ratio (DR) ˆ to examine the tensile strength and modulus of the extrudates The DR was calculated from the ratio of the diameter of the drawn extrudate to that of the extruder die The tensile mechanical properties of pure TLCP and its hybrid ®bers are listed in Table The tensile strength and initial modulus of C25A/TLCP hybrids increased with corresponding increases in the amount of organoclay The Table Tensile properties of C25A/TLCP hybrid ®bers Fig Optical micrographs of (a) 2% and (b) 6% C25A in TLCP hybrids taken at 200 8C ( £ 250) Clay (wt%) Ult Str (MPa) Ini Modu (GPa) E.B a (%) 11.03 15.10 16.15 17.28 2.91 4.03 4.38 5.76 1 a Elongation percent at break 2974 J.-H Chang et al / Polymer 43 (2002) 2969±2974 ultimate tensile strength of TLCP hybrid ®bers increased as the organoclay contents increased When the C25A was increased from to 6% in hybrids, the strength linearly improved from 11.03 to 17.28 MPa The ultimate strength of 6% C25A/TLCP was 1.6 times higher than that of pure TLCP The same kind of behavior was observed for the initial moduli For example, the initial tensile modulus of 2% C25A was 4.03 GPa, which was about 140% higher than the modulus of pure TLCP When the organoclay in TLCP reaches 6%, the modulus increases about 2.0 fold (5.76 GPa) over that of the pure TLCP This large increase in tensile property of hybrids owing to the presence of organoclay can be explained as follows: the amount of the increase of tensile property by clay layers depends on the interactions between rigid, rod-shaped TLCP molecules and layered organoclays, as well as on the rigid nature of the clay layers Moreover, the clay was much more rigid than the TLCP molecules, and did not deform or relax as the TLCP molecules did This improvement was possible because organoclay layers could be highly dispersed and exfoliated in the TLCP matrix This is consistent with the general observation that the introduction of organoclay into a matrix polymer increases its strength and modulus [26,27] The percent elongation at break of all samples, however, decreases from to 1% and then remains constant with clay addition Conclusions An aromatic thermotropic LCP with ethoxy side group was synthesized and its optical texture was nematic The addition of 2±6% C25A to a TLCP maintains liquid crystallinity C25A was exfoliated and dispersed homogeneously in the matrix polymer This was direct evidence that the C25A/TLCP hybrids formed nanocomposites This was also cross-checked using XRD and TEM In general, thermal behaviors (Tg, Tm, and TDi ) of the hybrids were enhanced with increasing clay content from to wt% On the other hand, the isotropic transition temperatures (Ti) of the hybrids were unchanged regardless of organoclay loading Hybrids of different C25A contents were extruded with DR ˆ from a capillary rheometer to investigate the mechanical properties of the hybrids The ultimate strength and initial modulus of the hybrids increased with increasing C25A content When the amount of organoclay in TLCP reached wt%, a 1.6-fold increase in the ultimate strength and a 2.0-fold increase in the initial modulus were obtained, as compared with the strength and modulus of the pure polymer matrix In this system, it was found that small additions of organoclay were enough to improve the properties of the matrix polymer, TLCP Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2000-041-E00358) References [1] Kiss G Polym Engng Sci 1987;27:410 [2] Blizard KG, Baird DG Polym Engng Sci 1987;27:653 [3] Dutta D, Fruitwala H, Kohli A, Weiss RA Polym Engng Sci 1990;30:1005 [4] Joseph EG, Wilkes GL, Baird DG Polymeric liquid crystals New York: Plenum Press, 1985 [5] Heitz T, Rohrbach P, Hocker H Macromol Chem 1989;190:3295 [6] Jackson Jr WJ, Kuhfuss HF J Polym Sci A Polym Chem 1976;14:2043 [7] Sukhadia AM, Done D, Baird DG Polym Engng Sci 1980;30:519 [8] Kenig S Polym Engng Sci 1987;27:887 [9] Lenz W Faraday Disc Chem Soc 1985;79:21 [10] Isayev AI, Modic M Polym Compos 1987;8:158 [11] Chung T Plast Engng 1987;October:39 [12] Chang J-H, Farris RJ Polym J 1995;27:780 [13] Chang J-H, Jo B-W J Appl Polym Sci 1996;60:939 [14] Giannelis EP Adv Mater 1996;8:29 [15] Lagaly G Appl Clay Sci 1999;15:1 [16] Usuki A, Koiwai A, Kojima Y, Kawasumi M, Okada A, Kurauchi T, Kamigaito O J Appl Polym Sci 1995;55:119 [17] Yano K, Usuki A, Okada A, Kurauchi T, Kamigaito O J Polym Sci A Polym Chem 1993;31:2493 [18] LeBaron PC, Wang Z, Pinnavaia TJ Appl Clay Sci 1999;15:11 [19] Yang F, Ou Y, Yu Z J Appl Polym Sci 1998;69:355 [20] Kato M, Usuki A Polymer-clay nanocomposites, Wiley Series in Polymer Science New York: John Wiley & Sons, 2000 Chapter [21] Chang J-H, Park D-K Polymer (Korea) 2000;24:399 [22] Chang J-H, Jo B-W, Jin J-I Korea Polym J 1994;2:140 [23] Lenz RW, Frukawa A, Bhowmik P, Go RO, Majusz J Polymer 1991;32:1703 [24] Petrovic XS, Javni I, Waddong A, Banhegyi G J Appl Polym Sci 2000;76:133 [25] Frischer HR, Gielgens LH, Koster TPM Acta Polym 1999;50:122 [26] Wang Z, Pinnavaia TJ Chem Mater 1998;10:3769 [27] Li J-X, Wu J, Chan C-M Polymer 2000;41:6935 EUROPEAN POLYMER JOURNAL European Polymer Journal 43 (2007) 374–379 www.elsevier.com/locate/europolj Macromolecular Nanotechnology – Short communication Critical aspects related to processing of carbon nanotube/unsaturated thermoset polyester nanocomposites A Tug˘rul Seyhan a, Florian H Gojny b, Metin Tanog˘lu a,* , Karl Schulte b _ _ Izmir Institute of Technology (IZTECH), Mechanical Engineering Department, 35437 Izmir, Turkey Polymer Composites, Technische Universita¨t Hamburg-Harburg (TUHH), Denickestrasse 15, 21073 Hamburg, Germany a MACROMOLECULAR NANOTECHNOLOGY b Received 17 June 2006; received in revised form 16 August 2006; accepted 14 November 2006 Abstract Carbon nanotubes (CNTs) have outstanding mechanical, thermal and electrical properties As a result, particular interest has been recently given in exploiting these properties by incorporating carbon nanotubes into some form of matrix Although unsaturated polyesters with styrene have widespread use in the industrial applications, surprisingly there is no study in the literature about CNT/thermoset polyester nanocomposite systems In the present paper, we underline some important issues and limitations during the processing of unsaturated polyester resins with different types of carbon nanotubes In that manner, 3-roll mill and sonication techniques were comparatively evaluated to process nanocomposites made of CNTs with and without amine (NH2) functional groups and polyesters It was found that styrene evaporation from the polyester resin system was a critical issue for nanocomposite processing Rheological behaviour of the suspensions containing CNTs and tensile strengths of their resulting nanocomposites were characterized CNT/polyester suspensions exhibited a shear thinning behaviour, while polyester resin blends act as a Newtonian fluid It was also found that nanotubes with amine functional groups have better tensile strength, as compared to those with untreated CNTs Transmission electron microscopy (TEM) was also employed to reveal the degree of dispersion of CNTs in the matrix Ó 2006 Elsevier Ltd All rights reserved Keywords: Carbon nanotubes; Thermosetting resin; Mechanical properties; Viscosity Introduction Scientific and industrial efforts have been recently focused on nanotechnology and nanomaterials Nanomaterials are exhibiting some superior properties, as compared to their micro or macro size counterparts Carbon nanotubes (CNTs) are composed * Corresponding author Tel.: +90 232 750 7806; fax: +90 232 750 7890 E-mail address: metintanoglu@iyte.edu.tr (M Tanog˘lu) of thin tubes with diameters of only a few nanometers, but a length of few microns They exhibit higher aspect ratio, extraordinary mechanical, thermal and electrical properties, which make them prime candidates as reinforcing constituents in various polymers for the production of nanocomposites Although there is a number of work published [1–5] on CNT reinforced polymer composites, realization of the expected enhancement in the properties of the composites, such as mechanical properties has not entirely been established so far This is because of 0014-3057/$ - see front matter Ó 2006 Elsevier Ltd All rights reserved doi:10.1016/j.eurpolymj.2006.11.018 A.T Seyhan et al / European Polymer Journal 43 (2007) 374–379 no reported work in the literature on the processing and properties of CNT/polyester systems Thus, CNTs have a great potential to improve the properties of a low cost resin like polyester at very low filler content and to induce new characteristics such as electrical conductivity In this paper, we address some critical aspects on the processing of CNT/ polyester nanocomposites prepared with the use of 3-roll-milling and also sonication techniques Transmission electron microscopy (TEM) was employed to reveal the degree of dispersion of carbon nanotubes with and without functional groups in the involved resin Some rheological and mechanical properties of the composites are also discussed Experimental details An isophtalic commercial unsaturated polyester resin Cam Elyaf 266 with 35 wt.% of styrene was obtained from CAM ELYAF Inc, Turkey Also, special polyester resin blends, composed of an allylic based polyester resin Poliya 240 with negligible amount of styrene and Poliya 420 without any sty_ rene were obtained from POLIYA POLYESTER Corp., Turkey Double-wall carbon nanotubes (DWCNT) and multi-walled carbon nanotubes (MWCNT) with and without amine functional group (NH2) produced by chemical vapor deposition (CVD) were obtained from Nanocyl (Namur/Belgium) and used as additives in the involved resin systems DWCNTs and MWCNTs have average diameters of 2.8 and 15 nm, respectively, with a length of approximately 50 lm Cobalt naphthanate (CoNAP) and methyl ethyl ketone peroxide (MEKP) were used as an accelerator and initiator, respectively, to polymerize the resin suspensions that contain various amounts of CNTs To prepare CNT/polyester nanocomposites, the first approach was the utilization of the 3-roll-milling process, successfully employed to process epoxy resins, employing to a commercial unsaturated polyester resin In this manner, the samples were prepared under excessive shear forces for the dispersion of 0.1, 0.3, and 0.5 wt.% of carbon nanotubes in Cam Elyaf 266 resin, setting the dwell time of CNTs/polyester suspension on the rolls for about The resin suspensions were polymerized with the addition of 0.3 wt.% of CoNAP and wt.% of MEKP into the system During the application of this technique, we have experienced some difficulties The major concern was the styrene evaporation from the polyester resin during the processes, which MACROMOLECULAR NANOTECHNOLOGY the fact that nanotubes have strong tendency to exist in agglomerated form via their huge surface area, which leads to non-homogeneous dispersion and random distribution of the nanotubes inside the resin Therefore, homogeneous dispersion of CNTs in the polymer matrix is one of the key factors to enhance mechanical properties of the composites [1–9] The common dispersion techniques for processing CNT/polymer composites have been direct mixing and sonication [1–15] In addition, Gojny et al [6] showed that the utilization of 3roll-milling, which applies intensive shear forces on the processed compounds, is an appropriate technique to exfoliate and disperse carbon nanotubes in an epoxy resin They also concluded that 3-roll-milling technique provided a better dispersion of CNTs in the epoxy resin resulting in higher mechanical properties, as compared to those prepared by sonication Furthermore, besides the physical approaches for the CNT dispersion, some other attempts including the use of surfactants and chemical functionalization of the CNT-surfaces have been made in order to alter the degree of dispersion and to tailor the interface between the matrix and carbon nanotubes In the near future, the further development in chemical functionalization of nanotubes may be the key challenge for advanced nanocomposites with the desired properties Consequently, it is obvious that a better understanding of the relationship between processing, interfacial optimization, surface chemistry and composite properties is necessary for the potential future applications of CNTs in polymer matrices Unsaturated polyesters (UP) with good cost/ property relation have been the most commonly employed matrix materials for glass fiber reinforced polymer composite parts UP based materials have been utilized in many applications including automotive, construction, transportation, storage tanks and piping industry Unsaturated polyesters become insoluble and infusible by crosslinking with a monomer, which is usually styrene The miscibility of the resin and the styrene depends on the resin composition Commercial polyester resins contain about 30–40% by mass of styrene Polyester resins are versatile, quick curing, and have a long shelf life at room temperature The disadvantages of these thermoset resins are self-polymerization at higher temperatures and significantly higher cure shrinkage, as compared to epoxy Despite the fact that polyester resins have been commonly employed in many industrial applications, to our knowledge, there is 375 MACROMOLECULAR NANOTECHNOLOGY 376 A.T Seyhan et al / European Polymer Journal 43 (2007) 374–379 caused a dramatic increase of the viscosity Styrene evaporation was accelerated due to heat occurred on the rolling mills due to higher shear effect The polyester resin with high viscosity stacked on the rolls and it caused some difficulties for the collection of the resin, due to the uncontrolled styrene evaporation, and thus the final styrene compositions within the resin blends were unknown Alternatively, the sonication method was employed with the same CNT/resin systems Some problems with the sonication method similar to 3-roll-milling process were observed Even though the sonication bath was cooled by water, the local heating due to energy created within the resin system, caused styrene evaporation from the polymer suspension, leading to a more viscous resin In addition, it was observed that nanotubes were agglomerated in the volumes closer to the tip of the sonicator Van der Waals attractive forces between the CNT-surfaces are known to be sensitive to heat, so increasing agglomeration occurred [2,6] To overcome the difficulties associated with styrene evaporation, we switched to a resin system, containing negligible amount of styrene (Poliya 240) and non styrene (Poliya 420) With the corresponding novel polyester resin systems nanocomposites were prepared by setting the appropriate gelation time and viscosity for the 3-roll-milling processing The styrene was added to the system after 3-roll-milling After some experimental trials, polyester resin blends were formulated based on 45 wt.% of Poliya 420, 30 wt.% of Poliya 240 and 25 wt.% of styrene with the presence of 0.2 wt.% of CoNAP and 1.5 wt.% of MEKP The polymer mixture to be used during 3-roll-milling process was prepared by hand-mixing of two types of the polyester resin at the given ratio for 10 Nanocomposite samples were prepared by the dispersion of the 0.1, 0.3, and 0.5 wt.% of the carbon nanotubes within the polyester resin blend After collecting the CNT containing polyester suspension by spatula from the 3-roll-milling, 25 wt.% of styrene was added to the involved resin system The whole system was then subjected to the intensive mixing for half an hour using magnetic stirrer and finally poured down into an aluminum mold and cured at room temperature followed by post curing in an oven at 110 °C for h Although Poliya 240 with a lower amount of styrene was introduced to 3-roll-milling, comparable viscosity increase of the resin, and problems due to the stacking of the high viscosity resin on the rolls were observed However, note that in this second approach, we diminished difficulties with styrene evaporation and unknown styrene content in the final product by using low styrene containing resin For that reason, in our further experimental investigations, we focused just on investigating the properties of the nanomaterials prepared by the second approach The dispersion of the CNTs within the composites was characterized by transmission electron microscopy (TEM) using a Philips EM 400 at 120 kV acceleration voltages The ultra thin TEM samples with a thickness of 50 nm were prepared by ultramicrotome cutting at room temperature TA Instruments RDA III with parallel plate rheometer geometry (500 lm gap, and 50 mm plate diameter) was used to analyze the rheological behaviour of the polyester suspensions with different carbon nanotubes loadings Tests were performed in steady state modes at room temperature in order to avoid styrene evaporation during the measurements For that reason, liquid samples were taken from the collected resin suspension from the 3-roll-mill Steady shear rates (SSS) were used to investigate the flow properties of the polyester suspensions by considering the viscosity as a function of increasing shear rates Mechanical tensile properties of the composites were determined according to DIN EN ISO 527.1 Dog bone specimens were prepared by countersinking using a Mutronic Dear Drive 2000 The tensile samples were tested using a Zwick Z010 Universal tensile testing machine at a cross head speed of mm/min The elongation of the specimens during the test was also measured Results and discussion The 3-roll-milling process via intensive shear forces seems to be more convenient technique than traditional ones such as sonication and direct mixing for the dispersion of carbon nanotubes within a liquid polymer resin Fig shows the TEM micrographs of MWCNTs and DWCNTs with and without functional groups in the polyester resin blend for 0.3 wt.% of loading MWCNTs with functional groups exhibited better local dispersion in the polyester matrix, as compared to DWCNTs with and without treatment In general, DWCNTs were observed to be more agglomerated form caused by their pronounced higher surface area In the literature, rheological behaviour of the polymer suspension was associated with the prediction state of 377 Fig TEM micrographs of nanocomposites prepared from POLIYATM polyester at 0.3 wt.% loading of (a) MWCNT, (b) MWCNT– NH2, (c) DWCNT (d) DWCNT–NH2 10 Neat polyester resin 0.1 wt.% MWCNT-NH2 0.3 wt.% MWCNT-NH2 0.5 wt.% MWCNT-NH2 10 10 -1 10 Neat polyester resin 0.1wt.% MWCNT 0.3wt.% MWCNT 0.5wt.% MWCNT 10 Viscosity [Pa.s] 10 Viscosity [Pa.s] CNTs dispersion within the corresponding resin [11] Figs and give the viscosity as a function of shear rate for the Poliya polyester based suspensions containing MWCNTs, MWCNT–NH2, respectively at different loading rates As seen in the figures, shear thinning behavior was observed for the samples containing either MWCNT or -2 10 -1 10 10 10 -1 10 10 Shear rate [s ] 10 Fig Viscosity of the polyester suspension with MWCNT–NH2 as a function of shear rate -1 10 -2 10 -1 10 10 10 10 10 -1 Shear rate [s ] Fig Viscosity of the polyester suspension with MWCNTs as a function of shear rate MWCNT–NH2, such that viscosity is reducing with the increase of shear rates The viscosity of polyester suspensions with MWCNT decreases sharply at 0.1 wt%, but MWCNT–NH2 has not the same behavior This might be due to the fact that nanotubes with amine functional groups reveal better compatibility or chemical interaction with the polyester chains within the system Carbon nanotubes MACROMOLECULAR NANOTECHNOLOGY A.T Seyhan et al / European Polymer Journal 43 (2007) 374–379 378 A.T Seyhan et al / European Polymer Journal 43 (2007) 374–379 Ultimate tensile strength [MPa] 28 MWCNT MWCNT-NH 26 24 N E A T 22 20 R E S I N 0.1 0.3 0.5 CNT filler ratio [wt.%] Fig Ultimate tensile strength (UTS) of the CNT/polyester nanocomposites with MWCNT and MWCNT–NH2 as a function of CNT filler ratio 30 Ultimate tensile strength [MPa] MACROMOLECULAR NANOTECHNOLOGY have a high aspect ratio, which alters significantly the flow characteristics of involved polymer suspension In order to investigate the nanofiller effect with and without chemical functional groups on the mechanical properties of the composites, tensile tests were conducted The tensile properties of Poliya polyester blend were much lower, as compared to a common commercial polyester resin in the market Note that both of the components of Poliya were specially snythesized and their individual mechanical properties were lower than those of a commercial polyester resin The Figs and show DWCNT DWCNT-NH 28 26 24 N E A T 22 R E S I N 20 0.1 0.3 CNT filler ratio [wt.%] 0.5 Fig Ultimate tensile strength (UTS) of the CNT/polyester nanocomposites with DWCNT and DWCNT–NH2 as a function of filler ratio the tensile strength of the resulting nanomaterials As it can be seen in the figures, there are some differences between the MWCNT reinforced nanocomposites with and without amine functional groups, as compared to neat polyester resin Moreover, at each loading rate, composite specimens containing MWCNTs with amine functional group have higher values than those with MWCNTs without any functional group For instance, the nanocomposites with 0.5 wt.% of MWCNT–NH2 have about 6% and 15% higher strength than those with the same loading rate of MWCNTs and the neat resin, respectively The same findings were also valid for the composites with DWCNTs and DWCNT– NH2 The nanocomposites with DWCNT–NH2 at 0.5 wt.% loading ratio have about 17% and 5% of higher strength values than the neat resin and the ones with DWCNTs Note that nanocomposites with DWCNTs with either functional group or not have higher strengths than those with MWCNTs or MWCNT–NH2 at each loading rate This can be explained by the higher surface area of the double wall carbon nanotubes, which may result in a better load transfer efficiency at the interface region as well as amine functional groups over CNTs which is supposed to promote the dispersion and pronounced covalent bonding to some extent Conclusion In this paper, we have focused principally on investigating three common key issues (i) availability of blending of polyester resin with very low amount of carbon nanotubes, highlighting some critical aspects and some limitations for the process, (ii) dispersion state of carbon nanotubes within the corresponding resin, (iii) interfacial adhesion/interactions of carbon nanotubes with the polyester resin system We have concluded that the styrene evaporation and self-polymerization via too much heat occurred are the two major issues to be considered, when a thermoset polyester resin is blended with carbon nanotubes by employing 3-roll-milling and sonication techniques We also revealed that 3-roll-milling method is more adequate technique for the dispersion of CNTs within a thermoset polyester resin blend, as compared to methods such as sonication and direct mixing Furthermore, the fact that CNTs with amine functional groups exhibited relatively enhanced dispersion state within the polyester resin blend, resulting in better tensile mechanical properties is evidence for that appropriate chemical functionalization of carbon nanotubes would be the key for the potential future applications In the further studies, we are going to concentrate on developing CNT/polyester master batches by means of different types of functional groups and without styrene in order to obtain desired microstructure and mechanical properties of the nanocomposites Acknowledgement Authors acknowledge the financial support from _ TUBITAK-JULICH Project References [1] Rosen R, Zheng B, Liu J Adv Mater 2002;19:14 [2] Thostenson ET, Li C, Chou TW Compos Sci Technol 2005;65:491 379 [3] Gojny FH, Wichmann MHG, Fiedler B, Schulte K Compos Sci Technol 2005;65:2300 [4] Gojny FH, Nastalczyk J, Roszlanic Z, Schulte K Chem Phys Lett 2003;370:820 [5] Gojny FH, Schulte K Compos Sci Technol 2004;64:2303 [6] Gojny FH, Wichmann MHG, Ko¨pke U, Fiedler B, Schulte K Compos Sci Technol 2005;64:2363 [7] Yasmin A, Abot JL, Daniel IM Scripta Mater 2003;49:81 [8] Ai A, Bai S, Cheng HM, Bai JB Compos Sci Technol 2002;62:1993 [9] Andrews R, Jacques D, Minot M, Rantell T Macromol Mater Eng 2002;287:395 [10] Weg UD, Benoit JM, Lebedkin S Curr Appl Phys 2002;21: 497 [11] Po¨tschke P, Fornes TD, Paul DR Polymer 2004;45:8863 [12] Aizawa M, Shaffer Milo SP Chem Phys Lett 2003;368:121 [13] Kymakis E, Alexandou I, Amaratunga GAJ Synthetic Met 2002;127:59 [14] Sandler JKW, Kirk JE Polymer 2003;44:5893 [15] Lau Kin-Tak, Hui David Carbon 2002;40:1597 MACROMOLECULAR NANOTECHNOLOGY A.T Seyhan et al / European Polymer Journal 43 (2007) 374–379 [...]... focused principally on investigating three common key issues (i) availability of blending of polyester resin with very low amount of carbon nanotubes, highlighting some critical aspects and some limitations for the process, (ii) dispersion state of carbon nanotubes within the corresponding resin, (iii) interfacial adhesion/interactions of carbon nanotubes with the polyester resin system We have concluded... can be seen in the figures, there are some differences between the MWCNT reinforced nanocomposites with and without amine functional groups, as compared to neat polyester resin Moreover, at each loading rate, composite specimens containing MWCNTs with amine functional group have higher values than those with MWCNTs without any functional group For instance, the nanocomposites with 0.5 wt.% of MWCNT–NH2... of MWCNT–NH2 have about 6% and 15% higher strength than those with the same loading rate of MWCNTs and the neat resin, respectively The same findings were also valid for the composites with DWCNTs and DWCNT– NH2 The nanocomposites with DWCNT–NH2 at 0.5 wt.% loading ratio have about 17% and 5% of higher strength values than the neat resin and the ones with DWCNTs Note that nanocomposites with DWCNTs with... snythesized and their individual mechanical properties were lower than those of a commercial polyester resin The Figs 4 and 5 show DWCNT DWCNT-NH 28 2 26 24 N E A T 22 R E S I N 20 0 0.1 0.3 CNT filler ratio [wt.%] 0.5 Fig 5 Ultimate tensile strength (UTS) of the CNT /polyester nanocomposites with DWCNT and DWCNT–NH2 as a function of filler ratio the tensile strength of the resulting nanomaterials As it can be... significantly the flow characteristics of involved polymer suspension In order to investigate the nanofiller effect with and without chemical functional groups on the mechanical properties of the composites, tensile tests were conducted The tensile properties of Poliya polyester blend were much lower, as compared to a common commercial polyester resin in the market Note that both of the components of Poliya... than those with MWCNTs or MWCNT–NH2 at each loading rate This can be explained by the higher surface area of the double wall carbon nanotubes, which may result in a better load transfer efficiency at the interface region as well as amine functional groups over CNTs which is supposed to promote the dispersion and pronounced covalent bonding to some extent 4 Conclusion In this paper, we have focused principally... direct mixing Furthermore, the fact that CNTs with amine functional groups exhibited relatively enhanced dispersion state within the polyester resin blend, resulting in better tensile mechanical properties is evidence for that appropriate chemical functionalization of carbon nanotubes would be the key for the potential future applications In the further studies, we are going to concentrate on developing... evaporation and self-polymerization via too much heat occurred are the two major issues to be considered, when a thermoset polyester resin is blended with carbon nanotubes by employing 3-roll-milling and sonication techniques We also revealed that 3-roll-milling method is more adequate technique for the dispersion of CNTs within a thermoset polyester resin blend, as compared to methods such as sonication and... future applications In the further studies, we are going to concentrate on developing CNT /polyester master batches by means of different types of functional groups and without styrene in order to obtain desired microstructure and mechanical properties of the nanocomposites Acknowledgement Authors acknowledge the financial support from _ TUBITAK-JULICH 5 Project References [1] Rosen R, Zheng B, Liu J Adv...378 A.T Seyhan et al / European Polymer Journal 43 (2007) 374–379 Ultimate tensile strength [MPa] 28 MWCNT MWCNT-NH 2 26 24 N E A T 22 20 R E S I N 0 0.1 0.3 0.5 CNT filler ratio [wt.%] Fig 4 Ultimate tensile strength (UTS) of the CNT /polyester nanocomposites with MWCNT and MWCNT–NH2 as a function of CNT filler ratio 30 Ultimate tensile strength [MPa] MACROMOLECULAR NANOTECHNOLOGY have a high
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