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Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites
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JTICE-918; No. of Pages 7 Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
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Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites Min-Hua Chung a,b, Li-Ming Chen a, Wei-Hsiang Wang a, Yishao Lai b, Ping-Feng Yang b, Hong-Ping Lin a,* a b
Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan Material Lab, Advanced Semiconductor Engineering. Inc., Nantze Export Processing Zone, 811 Nantze, Kaohsiung, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 January 2014 Received in revised form 2 May 2014 Accepted 11 May 2014 Available online xxx
Mesoporous silica coated multi-wall carbon nanotubes (denoted as CNTs@MS) were prepared using sodium silicate as the silica source and gelatin as the surface-activation agent. The effects of CNTs@MS on the mechanical and thermal properties of epoxy composite are investigated in this study. The electron microscopy images and Fourier transform infrared spectra demonstrate integral coating of the mesoporous silica on the CNTs. Because of the polar silica shell, the CNTs@MS exhibited uniform dispersion in epoxy-based nanocomposite. The thermal and mechanical properties of nanocomposites were characterized using dynamic mechanical analysis (DMA), thermo-mechanical analysis (TMA) and thermal conductivity measurement. These results show that the storage modulus and thermal conductivity increased along with the amount of CNTs@MS (0.25, 0.5, 1.0 and 2.0 wt%). The coefficient of thermal expansion decreased gradually, because the dipole–dipole interactions between the silica and epoxy polymer and confinement space of the mesoporous structure reduced the thermal mobility of the epoxy polymer inside the mesopore space. ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: CNTs Mesoporous silica Epoxy nanocomposites Properties
1. Introduction There is now an increasing demand for high-quality, advanced nano-structured composites with superior characteristics [1]. In particular, the efficient thermal management, mechanical protection and electrical insulating properties of these novel composites are needed for electronic packaging applications. Due to their, unique structural and transport properties (i.e., high strength, high modulus, high thermal and electrical conductivity, and low density), carbon nanotubes (CNTs) have attracted a great deal of research attention in this area [2]. Among the various polymer composites, epoxy-based systems are very important materials for electronic packaging applications, although their low thermal conductivity and weak mechanical properties remain major weaknesses. While CNT-reinforced epoxy composites have been developed, several processing challenges remain to be overcome in order to ensure the best performance is obtained [3,4]. CNTs that are subject to significant van-der Waals interactions typically entangle into bundles, and have a low dispersity in solvent. Similarly, when CNTs are mixed with a
* Corresponding author. Tel.: +886 6 2757575x65342. E-mail address: [email protected] (H.-P. Lin).
polymer, they tend to exist as entangled agglomerates that hinder a homogeneous dispersion. Although much research has been carried out on surface functionalization of CNTs to improve dispersion and increase interfacial adhesion with polymer resin [5–9], there are disadvantages to these approaches, such as the need to use highly corrosive inorganic acids, the formation of numerous defects on the CNTs walls, decreases in the aspect ratio of CNTs, and low yields of the functionalized CNTs. Moreover, destructive surface modification of CNTs would degrade the mechanical and electric- and thermal-transport properties [10–12]. It is thus necessary to develop new methods to stably disperse CNTs into various solvents or polymer matrices without destroying their original structures. Coating a high-compatibility layer on CNTs is a convenient way to improve their dispersity. Silica has been widely applied as an oxide shell to achieve this, primarily due to its biocompatiblility, excellent chemical stability and the fact that it can be easily functionalized for conjugation purposes [13,14]. In addition, silica has long been widely used as filler in electronic packaging material, due to its good mechanical properties and low coefficient of thermal expansion. Coating CNTs with a silica shell can not only improve the dispersion that occurs between CNTs and polymer matrix, but also retain the whole structure of the CNTs [15–20]. Although some synthesis methods for silica-coating or -decoration
http://dx.doi.org/10.1016/j.jtice.2014.05.009 1876-1070/ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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onto CNTs have been reported [3,17,18], a chemical oxidation process is still needed to functionalize the surface of the CNTs with carboxylic acid and hydroxyl groups before silane-grafting and silica coating. CNT@SiO2 had be already prepared using these complicated synthesis methods. The addition of CNT@SiO2, has been shown to improve, the mechanical strength, thermal expansion properties, and thermal conductivities of epoxy composites. These improvements were ascribed to the dualfunctions of the silica shell with regard to tuning modulus matching and improving the interaction between the epoxy and CNTs@SiO2 [3,15]. However, the oxidation process can lead to unwanted destruction of the unique properties of CNTs. Some methods have thus been developed to prepare mesoporous silica-coated CNTs composites by using anionic surfactant (e.g. sodium dodecyl sulfate, SDS) [17] or cationic surfactant (e.g. cetyltrimethyl ammonium bromide, CTAB) [18]. However, these synthetic compositions require a toxic surfactant, organic silanes and silica source (e.g. tetraethyl orthosilicate, TEOS) and an organic solvent. In this paper, we present a simple synthesis method to prepare CNT@mesoporous silica by using a natural polymer gelatin with high bio-compatibility and sodium silicate as the silica source, applying a sol–gel reaction without using any highly corrosive inorganic acid, organic solvent or silane. Distinct from the surfactant, the non-polar amino acid chains of the gelatin could wrap and immobilize on the side wall of CNTs through a hydrophobic–hydrophobic interaction [21]. In addition, based on silica chemistry, the amide group (–CO–NH2) in the gelatin can have a high affinity to strongly interact with silanol groups (Si–OH) on the silicate species via hydrogen-bonding interaction [22]. The resulting CNTs@MS with an integral mesoporous silica coating is thus almost discrete, without forming a serious aggregation. With regard to the compatibility and thermal expansion coefficient of a polymer, mesoporous silica has superior properties to that of a silica layer without porosity, such as a larger surface area and greater pore volume. Since the size of mesopores is greater than the dimensions of polymer molecules, the penetration of the epoxy polymer into the mesopore space is easily carried out during the fabrication of CNTs@MS/epoxy polymer composites. In the presence of a mesoporous silica layer, the dispersion and compatibility of CNTs@MS in an epoxy-based matrix are both better than seen with CNTs. The thermal mobility of epoxy polymers is partially confined inside the mesopore space. In other
words, the thermal expansion of the composites during thermal treatment is mainly caused by the polymer located outside the mesopores and it provides a path for reduction of thermal expansion [23,24]. Therefore the CNTs@MS-epoxy composite demonstrates higher mechanical strength, and greater thermal conductivity than that of the CNTs-epoxy composite. 2. Experiments 2.1. Materials MW-CNTs with an average diameter of 20–30 nm and length of 5–15 mm were obtained from Applied Technologies, Inc., China. Diglycidyl Ether of Bisphenol-A epoxy resin, EEW 188–192 (DGEBA, BE188), was supplied by ChangChun Group, Taiwan. The curing agent, 4,4-diaminodiphenyl sulfone, HEW 62 (DDS), was obtained from Acroˆs. In order to synthesize the mesoporous silica on CNTs, sodium silicate (27% in water) and sulfuric acid (95–97%) were purchased from Aldrich. Gelatin was purchased from Acroˆs. All chemicals were used directly, without further purification. 2.2. Coating SiO2 on MWCNTS To synthesize the CNTs@MS composites, a gelatin solution (0.75 g gelatin dissolved in 11.25 mL H2O) was mixed with a CNTs dispersed solution (1.0 g of CNTs dispersed in 100 mL H2O), and stirred for 12.0 h. An acidified sodium silicate solution with a pH of around 4.0 was then added to this. The acidified sodium silicate solution was prepared by quickly acidifying a sodium silicate solution (5.0 g sodium silicate dissolved in 93.75 mL H2O) with an appropriate amount of 2.0 M sulfuric acid solution. The CNTs–gelatin–silicate solution was further stirred for 2.0 h, and then sealed in a polypropylene bottle for hydrothermal treatment for 24 h. The gelatin was finally removed by calcination at 450 8C. Scheme 1 shows the synthetic process. 2.3. Preparation of CNTs@MS/epoxy composite The CNTs@MS was dispersed in acetone by ultrasonication (100 W, in a water bath) for 1.0 h before adding the epoxy monomer. The dispersion was mixed with DGEBA, and the weight ratio of the CNTs@MS was set at 0.25, 0.50, 1.0 and 2.00 wt.%. To
Scheme 1. The synthesis processes of the CNTs@mesoporous silica.
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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Scheme 2. The processing steps used to fabricate the CNTs@MS–epoxy composite composites.
form a homogeneous slurry, the CNTs@MS–DGEBA mixture was subjected to under ultrasonication (100 W, in a water bath) for another 1.0 h, and stirred at 90 8C for 12.0 h. This treatment can reduce the aggregation of CNTs@MS and remove the remaining solvent to get rid of air bubbles. The curing agent DDS was added into the slurry at an epoxy-to-curing agent weight ratio of 3:1. The mixture was then stirred for 1.0 h at 110 8C in order to get optimum solubility of the DDS curing agent. The resulting glue-like gel was then molded into a flat plate and pre-cured at 120 8C for 2.0 h, followed by post-curing at 200 8C for 4.0 h to form CNTs@SiO2–epoxy composites. Scheme 2 shows the processing steps used to fabricate the CNTs@MS–epoxy composite composites. 2.4. Characterization The morphology of CNTs@MS and the dispersion states of CNTs@MS in the epoxy matrix were examined by field emission scanning electron microscopy (FE-SEM, JEOL JSM7000F, USA) and transmission electron microscopy (TEM, Hitachi H-7100, Japan). The Fourier transform infrared spectrum (FT-IR) of CNTs@SiO2 was recorded on a Perkin-Elmer GX50003. Specimens were pressed into pellets with potassium bromide (KBr) and scanned from 4000 to 500 cm 1 at a resolution 4.0 cm 1. N2 adsorption–desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2020 apparatus. Before analyzing, the sample was outgassed at 120 8C for about 6 h in 10 3 Torr. TGA were conducted with a TA Q-50 thermogravimetric system. In a typical experiment, ca. 10 mg of the sample was heated to 800 8C at 30 8C/min under air. The storage modulus and Tand were determined on a dynamic mechanical analyzer (DMA Q800, TA Instruments). The samples with dimensions of 20 mm 12 mm 2 mm were tested in a three point bending mode from room temperature to 270 8C at a heating rate of 3 8C/min and an oscillation frequency of 1 Hz. The thermal-expansion coefficient was obtained with a thermo mechanical analyzer (TMA Q400, TA Instruments). The range of temperature was from room temperature to 220 8C at 20 8C/min by first run to release the internal stress, followed by a second scan from 0 to 270 8C at 5 8C/min. The thermal conductivities of the composite were measured using a hot disk thermal conductivity analyzer (TPS2500, Sweden) at room temperature.
3. Results and discussion 3.1. Characterizations on the CNTs@MS TEM images of the CNTs@MS are shown in Fig. 1a, and these were obtained to assess the integrity of the mesoporous silica coating on the CNTs. It can be clearly seen that the mesoporous silica homogeneously coated the CNTs. Because of the wellcontrolled reaction conditions (pH, CNTs/gelatin and silica/gelatin weight ratios, and water content), there were few mesoporous silica particles outside the CNTs@MS. To further confirm the high integrity of the mesoporous silica replica, the CNTs@MS was calcined at 800 8C to remove the CNTs core, and the remaining silica replica was in the form of hollow tubules with a diameter close to that of the CNTs. The thickness of the mesoporous silica layer was around 10.0 nm. In the presence of the mesoporous silica shell, the CNTs@MS sample has an obvious capillary condensation at P/P0 = 0.5–0.7 in the N2 adsorption–desorption isotherm. The pore size calculated by the BJH method is around 6.0 nm. The mesoporous silica shell makes the BET surface area (about 560 m2 g 1) of the CNTs@MS higher than that of CNTs (180 m2 g 1). When comparing the TGA curves, the silica content in the CNTs@MS is around 65 wt.%. The mesoporous silica shell makes the CNTS@MS more thermally stable than the CNTs (Fig. 1d). There is also an additional intense vibration band of the Si–O–Si group, which confirms the presence of a mesoporous silica coating on the CNTs (Fig. S1). 3.2. Characterizations on the dispersion Representative SEM images of the fractured composites are shown in Fig. 2, and these are used to better understand the dispersity of the CNTs@MS and CNTs in the epoxy matrices. The as-received CNTs presented mainly in the agglomerated form, because the hydrophobic CNTs tend to self-aggregate and separate out from the polar epoxy matrix (Fig. 2a, b). There are many CNTs on the fractured surface, which indicates that the interaction between CNTs and epoxy is weak. Distinct from the CNTs, the CNTs@MS fillers (65 wt.% mesoporous silica content) with the polar mesoporous silica shell can have a high dispersity in the epoxy matrix. Therefore, large CNTs@MS
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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Fig. 1. (a) TEM image of the CNTs@MS. (b) Mesoporous silica replicas of the CNTs. The arrow indicates the hollow interior of the mesoporous silica nanotubes. (c) N2 adsorption–desorption isotherms, (d) TGA curves of the CNTs (curve I) and CNTs@MS after gelatin removal (curve II).
aggregates were rarely observed on the fractured surface, which means the CNTs@MS is strongly bound with the epoxy matrix (Fig. 2c, d). In addition, the well-dispersed state of the CNTs@MS in the epoxy composites was further confirmed by the microtome TEM image (Fig. 3). One can clearly see the well-dispersion of the CNTs@MS on the submicron-scale in the epoxy resin matrix. In addition, dispersity states of as-received CNTs–epoxy and CNTs@MS–epoxy composites can also be simply observed by illumination on the molded specimen (with thickness of 2.0 mm) by a LED light behind the specimen (Fig. S2). 3.2.1. Thermo-mechanical properties The uniform silica shell will not only promote the dispersion of CNTs in the epoxy matrix but also serve as an intermediate layer to alleviate the modulus mismatch between the stiff CNTs and the
soft epoxy matrix. This result implies that the interfacial interaction would be benefit to give enhanced thermal and mechanical properties in the composites [3,25]. The glass transition temperature (Tg) of different nanocomposite as the peak values of tan delta obtained by DMA analysis in the range of 200–240 8C. The CNTs@MS–epoxy composite has a higher Tg (231.8 8C) than that of the neat epoxy (221.0 8C). The increase in the Tg can be attributed to the well-dispersion of the CNTs@MS producing a network which restricted the motion of epoxy segmental chain in the mesopores of the silica layer. In addition, the CNTs@MS–epoxy composite also has higher storage modulus than those of the CNTs–epoxy one at the same weight content of 1.0 wt.% (Fig. 4). Although the CNTs have high mechanical strength, a poor-dispersion and incompatibility in the epoxy matrix lowered the improvement on the mechanical strength. The hydrophilic
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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Fig. 2. FESEM images of (a), (b) CNTs and (c),(d) CNTs@MS in epoxy matrix.
silica shell contributes to improve both the compatibility of the filler with epoxy network and served as a medium to improve interfacial adhesion for well dispersion and restrict the motion of epoxy matrix.
Fig. 3. Microtome TEM images of CNTs@MS in epoxy matrix.
The dependency of the storage modulus and tan delta of the neat epoxy on the content of the added CNTs@MS in the epoxy matrixes were also examined by DMA analysis (Fig. 5). As increasing the content of the CNTs@MS, the improvement on the storage modulus became larger. Due to the presence of the CNTs@MS with high mechanical strength and a well dispersion, the CNTs@MS facilitates the capability of energy storage of epoxy and led to the enhancement of storage modulus. This linear relationship between the storage modulus and the content of the added CNTs@MS indicates the CNTs@MS is almost homogeneously dispersed in the epoxy even at high CNTs@MS content [3,5,6]. Moreover, there was a stronger effect of CNTs@MS in the rubbery region at elevated temperature where the improvement in elastic properties of the composite was obviously observed. This behavior can be explained in terms of the mesoporous silica shell improved modulus mismatch and reduced the mobility of the epoxy matrix, so as to increasing the thermal stability at higher temperature. Fig. 6 shows the coefficient of thermal expansion (CTE, i.e. the slopes of the curves) of the CNTs@MS–epoxy composites at different CNTs@MS contents. The CTE decrease from 78.0 to 62.1 ppm/8C with 2 wt.% CNTs@MS content in the temperature region lower than glass transition temperature (Tg) and decreases from 158.6 to 124.0 ppm/8C in the temperature region over Tg, respectively. These values were clearly seen that the CTE values get smaller as the increase of the content and reduced by approximately 21% in comparison with neat epoxy matrix. There are two major factors that contribute to the reduce in CTE property: (i) the interfacial interactions between the epoxy matrix and CNTs@MS surface; and (ii) the physically confinement of the
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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Fig. 6. (a), (b) The coefficient of thermal expansion of neat epoxy and its composites at different CNTs@MS contents.
Fig. 4. (a), (b) Storage modulus and tan delta of neat epoxy and its composites filled with CNTs and CNTs@MS, respectively.
epoxy matrix surrounded by robust mesoporous silica shell, that is, the epoxy polymer can easily penetrated into the silica mesopores by as a result of capillary force [23,24]. Therefore, with the increase of the amount of the physically confined epoxy matrix, the CTE values of nanocomposites were gradually decreased. This result would also be proven in Fig. 6b where the addition of CNTs@MS has lower CTE than CNTs performed.
CNTs@MS contents, as shown in Fig. 7. For CNTs incorporated in the epoxy matrix, the thermal conductivity increased by 57.92% (from 0.2417 to 0.3817 W/mK) at 2 wt.% loading. However, the CNTs@MS caused a greater improvement in the thermal conductivity of the epoxy matrix by 95.24% (from 0.2417 to 0.4719 W/mK) at 2 wt% loading. Previous studies showed that decreasing the modulus mismatch between the polymer matrix and solid filler would decrease the thermal interfacial resistance, thereby improving the thermal conductivity of the polymer composites [3,25]. In addition, the high aspect ratio and non-destructive sidewalls of CNTs also the lead to high thermal conductivity [26,27]. Even although the silica-coated CNTs do have higher
3.2.2. Thermal conductivity properties The thermal conductivities of the CNTs–epoxy and CNTs@MS– epoxy composites both increased along with the CNTs and
Fig. 5. Storage modulus and Tg of epoxy with different content of CNTs@MS.
Fig. 7. Thermal conductivities of neat epoxy and its composites filled with CNTs and CNTs@MS–epoxy composites with different filler contents.
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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thermal conductivity values than the raw CNTs incorporated in the epoxy matrix, the improvement is still less than that predicted by Nan’s model [28]. In general, thermal conductivity can be improved by two major factors. (i) de-bundling of CNTs and a uniform dispersion in the matrix; and (ii) strong adhesion to the matrix through effective interfacial bonding. In this study, the uniform silica shell on the CNTs not only serves as a medium to promote dispersion and improve the modulus mismatch of CNTs in the epoxy matrix but also maintain the individual CNTs with a high aspect ratio and nondestructive walls. However, the silica shell still does not have interfacial bonding as strong as the chemical bonding between CNTs and epoxy matrix, and this limits the increase of thermal conductivity [27,29,30]. 4. Conclusion This study fabricated mesoporous silica shells onto the surface of CNTs, and then incorporated them into an epoxy matrix. The mesoporous structure of the silica layer makes modulus matching possible, providing compatibility and dispersibility between the epoxy and CNTs. Moreover, it also enables physical confinement of the epoxy matrix into robust mesopores. In sum, the mesoporous silica coated CNTs had better mechanical properties (storage modulus, glass transition temperature, coefficient of thermal expansion, etc.) and thermal conductivity when incorporated into an epoxy matrix than the as-received CNTs system. Acknowledgements The authors would like to acknowledge the support of the Material Lab, at Advanced Semiconductor Engineering Incorporated in performing all measurements. This research received funding from Ministry of Science and Technology (NSC100-2113-M-006006-MY3) and the Headquarters of University Advancement at the National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan.
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Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
JTICE-918; No. of Pages 7 Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
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Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites Min-Hua Chung a,b, Li-Ming Chen a, Wei-Hsiang Wang a, Yishao Lai b, Ping-Feng Yang b, Hong-Ping Lin a,* a b
Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan Material Lab, Advanced Semiconductor Engineering. Inc., Nantze Export Processing Zone, 811 Nantze, Kaohsiung, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 January 2014 Received in revised form 2 May 2014 Accepted 11 May 2014 Available online xxx
Mesoporous silica coated multi-wall carbon nanotubes (denoted as CNTs@MS) were prepared using sodium silicate as the silica source and gelatin as the surface-activation agent. The effects of CNTs@MS on the mechanical and thermal properties of epoxy composite are investigated in this study. The electron microscopy images and Fourier transform infrared spectra demonstrate integral coating of the mesoporous silica on the CNTs. Because of the polar silica shell, the CNTs@MS exhibited uniform dispersion in epoxy-based nanocomposite. The thermal and mechanical properties of nanocomposites were characterized using dynamic mechanical analysis (DMA), thermo-mechanical analysis (TMA) and thermal conductivity measurement. These results show that the storage modulus and thermal conductivity increased along with the amount of CNTs@MS (0.25, 0.5, 1.0 and 2.0 wt%). The coefficient of thermal expansion decreased gradually, because the dipole–dipole interactions between the silica and epoxy polymer and confinement space of the mesoporous structure reduced the thermal mobility of the epoxy polymer inside the mesopore space. ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: CNTs Mesoporous silica Epoxy nanocomposites Properties
1. Introduction There is now an increasing demand for high-quality, advanced nano-structured composites with superior characteristics [1]. In particular, the efficient thermal management, mechanical protection and electrical insulating properties of these novel composites are needed for electronic packaging applications. Due to their, unique structural and transport properties (i.e., high strength, high modulus, high thermal and electrical conductivity, and low density), carbon nanotubes (CNTs) have attracted a great deal of research attention in this area [2]. Among the various polymer composites, epoxy-based systems are very important materials for electronic packaging applications, although their low thermal conductivity and weak mechanical properties remain major weaknesses. While CNT-reinforced epoxy composites have been developed, several processing challenges remain to be overcome in order to ensure the best performance is obtained [3,4]. CNTs that are subject to significant van-der Waals interactions typically entangle into bundles, and have a low dispersity in solvent. Similarly, when CNTs are mixed with a
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polymer, they tend to exist as entangled agglomerates that hinder a homogeneous dispersion. Although much research has been carried out on surface functionalization of CNTs to improve dispersion and increase interfacial adhesion with polymer resin [5–9], there are disadvantages to these approaches, such as the need to use highly corrosive inorganic acids, the formation of numerous defects on the CNTs walls, decreases in the aspect ratio of CNTs, and low yields of the functionalized CNTs. Moreover, destructive surface modification of CNTs would degrade the mechanical and electric- and thermal-transport properties [10–12]. It is thus necessary to develop new methods to stably disperse CNTs into various solvents or polymer matrices without destroying their original structures. Coating a high-compatibility layer on CNTs is a convenient way to improve their dispersity. Silica has been widely applied as an oxide shell to achieve this, primarily due to its biocompatiblility, excellent chemical stability and the fact that it can be easily functionalized for conjugation purposes [13,14]. In addition, silica has long been widely used as filler in electronic packaging material, due to its good mechanical properties and low coefficient of thermal expansion. Coating CNTs with a silica shell can not only improve the dispersion that occurs between CNTs and polymer matrix, but also retain the whole structure of the CNTs [15–20]. Although some synthesis methods for silica-coating or -decoration
http://dx.doi.org/10.1016/j.jtice.2014.05.009 1876-1070/ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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onto CNTs have been reported [3,17,18], a chemical oxidation process is still needed to functionalize the surface of the CNTs with carboxylic acid and hydroxyl groups before silane-grafting and silica coating. CNT@SiO2 had be already prepared using these complicated synthesis methods. The addition of CNT@SiO2, has been shown to improve, the mechanical strength, thermal expansion properties, and thermal conductivities of epoxy composites. These improvements were ascribed to the dualfunctions of the silica shell with regard to tuning modulus matching and improving the interaction between the epoxy and CNTs@SiO2 [3,15]. However, the oxidation process can lead to unwanted destruction of the unique properties of CNTs. Some methods have thus been developed to prepare mesoporous silica-coated CNTs composites by using anionic surfactant (e.g. sodium dodecyl sulfate, SDS) [17] or cationic surfactant (e.g. cetyltrimethyl ammonium bromide, CTAB) [18]. However, these synthetic compositions require a toxic surfactant, organic silanes and silica source (e.g. tetraethyl orthosilicate, TEOS) and an organic solvent. In this paper, we present a simple synthesis method to prepare CNT@mesoporous silica by using a natural polymer gelatin with high bio-compatibility and sodium silicate as the silica source, applying a sol–gel reaction without using any highly corrosive inorganic acid, organic solvent or silane. Distinct from the surfactant, the non-polar amino acid chains of the gelatin could wrap and immobilize on the side wall of CNTs through a hydrophobic–hydrophobic interaction [21]. In addition, based on silica chemistry, the amide group (–CO–NH2) in the gelatin can have a high affinity to strongly interact with silanol groups (Si–OH) on the silicate species via hydrogen-bonding interaction [22]. The resulting CNTs@MS with an integral mesoporous silica coating is thus almost discrete, without forming a serious aggregation. With regard to the compatibility and thermal expansion coefficient of a polymer, mesoporous silica has superior properties to that of a silica layer without porosity, such as a larger surface area and greater pore volume. Since the size of mesopores is greater than the dimensions of polymer molecules, the penetration of the epoxy polymer into the mesopore space is easily carried out during the fabrication of CNTs@MS/epoxy polymer composites. In the presence of a mesoporous silica layer, the dispersion and compatibility of CNTs@MS in an epoxy-based matrix are both better than seen with CNTs. The thermal mobility of epoxy polymers is partially confined inside the mesopore space. In other
words, the thermal expansion of the composites during thermal treatment is mainly caused by the polymer located outside the mesopores and it provides a path for reduction of thermal expansion [23,24]. Therefore the CNTs@MS-epoxy composite demonstrates higher mechanical strength, and greater thermal conductivity than that of the CNTs-epoxy composite. 2. Experiments 2.1. Materials MW-CNTs with an average diameter of 20–30 nm and length of 5–15 mm were obtained from Applied Technologies, Inc., China. Diglycidyl Ether of Bisphenol-A epoxy resin, EEW 188–192 (DGEBA, BE188), was supplied by ChangChun Group, Taiwan. The curing agent, 4,4-diaminodiphenyl sulfone, HEW 62 (DDS), was obtained from Acroˆs. In order to synthesize the mesoporous silica on CNTs, sodium silicate (27% in water) and sulfuric acid (95–97%) were purchased from Aldrich. Gelatin was purchased from Acroˆs. All chemicals were used directly, without further purification. 2.2. Coating SiO2 on MWCNTS To synthesize the CNTs@MS composites, a gelatin solution (0.75 g gelatin dissolved in 11.25 mL H2O) was mixed with a CNTs dispersed solution (1.0 g of CNTs dispersed in 100 mL H2O), and stirred for 12.0 h. An acidified sodium silicate solution with a pH of around 4.0 was then added to this. The acidified sodium silicate solution was prepared by quickly acidifying a sodium silicate solution (5.0 g sodium silicate dissolved in 93.75 mL H2O) with an appropriate amount of 2.0 M sulfuric acid solution. The CNTs–gelatin–silicate solution was further stirred for 2.0 h, and then sealed in a polypropylene bottle for hydrothermal treatment for 24 h. The gelatin was finally removed by calcination at 450 8C. Scheme 1 shows the synthetic process. 2.3. Preparation of CNTs@MS/epoxy composite The CNTs@MS was dispersed in acetone by ultrasonication (100 W, in a water bath) for 1.0 h before adding the epoxy monomer. The dispersion was mixed with DGEBA, and the weight ratio of the CNTs@MS was set at 0.25, 0.50, 1.0 and 2.00 wt.%. To
Scheme 1. The synthesis processes of the CNTs@mesoporous silica.
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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Scheme 2. The processing steps used to fabricate the CNTs@MS–epoxy composite composites.
form a homogeneous slurry, the CNTs@MS–DGEBA mixture was subjected to under ultrasonication (100 W, in a water bath) for another 1.0 h, and stirred at 90 8C for 12.0 h. This treatment can reduce the aggregation of CNTs@MS and remove the remaining solvent to get rid of air bubbles. The curing agent DDS was added into the slurry at an epoxy-to-curing agent weight ratio of 3:1. The mixture was then stirred for 1.0 h at 110 8C in order to get optimum solubility of the DDS curing agent. The resulting glue-like gel was then molded into a flat plate and pre-cured at 120 8C for 2.0 h, followed by post-curing at 200 8C for 4.0 h to form CNTs@SiO2–epoxy composites. Scheme 2 shows the processing steps used to fabricate the CNTs@MS–epoxy composite composites. 2.4. Characterization The morphology of CNTs@MS and the dispersion states of CNTs@MS in the epoxy matrix were examined by field emission scanning electron microscopy (FE-SEM, JEOL JSM7000F, USA) and transmission electron microscopy (TEM, Hitachi H-7100, Japan). The Fourier transform infrared spectrum (FT-IR) of CNTs@SiO2 was recorded on a Perkin-Elmer GX50003. Specimens were pressed into pellets with potassium bromide (KBr) and scanned from 4000 to 500 cm 1 at a resolution 4.0 cm 1. N2 adsorption–desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2020 apparatus. Before analyzing, the sample was outgassed at 120 8C for about 6 h in 10 3 Torr. TGA were conducted with a TA Q-50 thermogravimetric system. In a typical experiment, ca. 10 mg of the sample was heated to 800 8C at 30 8C/min under air. The storage modulus and Tand were determined on a dynamic mechanical analyzer (DMA Q800, TA Instruments). The samples with dimensions of 20 mm 12 mm 2 mm were tested in a three point bending mode from room temperature to 270 8C at a heating rate of 3 8C/min and an oscillation frequency of 1 Hz. The thermal-expansion coefficient was obtained with a thermo mechanical analyzer (TMA Q400, TA Instruments). The range of temperature was from room temperature to 220 8C at 20 8C/min by first run to release the internal stress, followed by a second scan from 0 to 270 8C at 5 8C/min. The thermal conductivities of the composite were measured using a hot disk thermal conductivity analyzer (TPS2500, Sweden) at room temperature.
3. Results and discussion 3.1. Characterizations on the CNTs@MS TEM images of the CNTs@MS are shown in Fig. 1a, and these were obtained to assess the integrity of the mesoporous silica coating on the CNTs. It can be clearly seen that the mesoporous silica homogeneously coated the CNTs. Because of the wellcontrolled reaction conditions (pH, CNTs/gelatin and silica/gelatin weight ratios, and water content), there were few mesoporous silica particles outside the CNTs@MS. To further confirm the high integrity of the mesoporous silica replica, the CNTs@MS was calcined at 800 8C to remove the CNTs core, and the remaining silica replica was in the form of hollow tubules with a diameter close to that of the CNTs. The thickness of the mesoporous silica layer was around 10.0 nm. In the presence of the mesoporous silica shell, the CNTs@MS sample has an obvious capillary condensation at P/P0 = 0.5–0.7 in the N2 adsorption–desorption isotherm. The pore size calculated by the BJH method is around 6.0 nm. The mesoporous silica shell makes the BET surface area (about 560 m2 g 1) of the CNTs@MS higher than that of CNTs (180 m2 g 1). When comparing the TGA curves, the silica content in the CNTs@MS is around 65 wt.%. The mesoporous silica shell makes the CNTS@MS more thermally stable than the CNTs (Fig. 1d). There is also an additional intense vibration band of the Si–O–Si group, which confirms the presence of a mesoporous silica coating on the CNTs (Fig. S1). 3.2. Characterizations on the dispersion Representative SEM images of the fractured composites are shown in Fig. 2, and these are used to better understand the dispersity of the CNTs@MS and CNTs in the epoxy matrices. The as-received CNTs presented mainly in the agglomerated form, because the hydrophobic CNTs tend to self-aggregate and separate out from the polar epoxy matrix (Fig. 2a, b). There are many CNTs on the fractured surface, which indicates that the interaction between CNTs and epoxy is weak. Distinct from the CNTs, the CNTs@MS fillers (65 wt.% mesoporous silica content) with the polar mesoporous silica shell can have a high dispersity in the epoxy matrix. Therefore, large CNTs@MS
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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Fig. 1. (a) TEM image of the CNTs@MS. (b) Mesoporous silica replicas of the CNTs. The arrow indicates the hollow interior of the mesoporous silica nanotubes. (c) N2 adsorption–desorption isotherms, (d) TGA curves of the CNTs (curve I) and CNTs@MS after gelatin removal (curve II).
aggregates were rarely observed on the fractured surface, which means the CNTs@MS is strongly bound with the epoxy matrix (Fig. 2c, d). In addition, the well-dispersed state of the CNTs@MS in the epoxy composites was further confirmed by the microtome TEM image (Fig. 3). One can clearly see the well-dispersion of the CNTs@MS on the submicron-scale in the epoxy resin matrix. In addition, dispersity states of as-received CNTs–epoxy and CNTs@MS–epoxy composites can also be simply observed by illumination on the molded specimen (with thickness of 2.0 mm) by a LED light behind the specimen (Fig. S2). 3.2.1. Thermo-mechanical properties The uniform silica shell will not only promote the dispersion of CNTs in the epoxy matrix but also serve as an intermediate layer to alleviate the modulus mismatch between the stiff CNTs and the
soft epoxy matrix. This result implies that the interfacial interaction would be benefit to give enhanced thermal and mechanical properties in the composites [3,25]. The glass transition temperature (Tg) of different nanocomposite as the peak values of tan delta obtained by DMA analysis in the range of 200–240 8C. The CNTs@MS–epoxy composite has a higher Tg (231.8 8C) than that of the neat epoxy (221.0 8C). The increase in the Tg can be attributed to the well-dispersion of the CNTs@MS producing a network which restricted the motion of epoxy segmental chain in the mesopores of the silica layer. In addition, the CNTs@MS–epoxy composite also has higher storage modulus than those of the CNTs–epoxy one at the same weight content of 1.0 wt.% (Fig. 4). Although the CNTs have high mechanical strength, a poor-dispersion and incompatibility in the epoxy matrix lowered the improvement on the mechanical strength. The hydrophilic
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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Fig. 2. FESEM images of (a), (b) CNTs and (c),(d) CNTs@MS in epoxy matrix.
silica shell contributes to improve both the compatibility of the filler with epoxy network and served as a medium to improve interfacial adhesion for well dispersion and restrict the motion of epoxy matrix.
Fig. 3. Microtome TEM images of CNTs@MS in epoxy matrix.
The dependency of the storage modulus and tan delta of the neat epoxy on the content of the added CNTs@MS in the epoxy matrixes were also examined by DMA analysis (Fig. 5). As increasing the content of the CNTs@MS, the improvement on the storage modulus became larger. Due to the presence of the CNTs@MS with high mechanical strength and a well dispersion, the CNTs@MS facilitates the capability of energy storage of epoxy and led to the enhancement of storage modulus. This linear relationship between the storage modulus and the content of the added CNTs@MS indicates the CNTs@MS is almost homogeneously dispersed in the epoxy even at high CNTs@MS content [3,5,6]. Moreover, there was a stronger effect of CNTs@MS in the rubbery region at elevated temperature where the improvement in elastic properties of the composite was obviously observed. This behavior can be explained in terms of the mesoporous silica shell improved modulus mismatch and reduced the mobility of the epoxy matrix, so as to increasing the thermal stability at higher temperature. Fig. 6 shows the coefficient of thermal expansion (CTE, i.e. the slopes of the curves) of the CNTs@MS–epoxy composites at different CNTs@MS contents. The CTE decrease from 78.0 to 62.1 ppm/8C with 2 wt.% CNTs@MS content in the temperature region lower than glass transition temperature (Tg) and decreases from 158.6 to 124.0 ppm/8C in the temperature region over Tg, respectively. These values were clearly seen that the CTE values get smaller as the increase of the content and reduced by approximately 21% in comparison with neat epoxy matrix. There are two major factors that contribute to the reduce in CTE property: (i) the interfacial interactions between the epoxy matrix and CNTs@MS surface; and (ii) the physically confinement of the
Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009
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Fig. 6. (a), (b) The coefficient of thermal expansion of neat epoxy and its composites at different CNTs@MS contents.
Fig. 4. (a), (b) Storage modulus and tan delta of neat epoxy and its composites filled with CNTs and CNTs@MS, respectively.
epoxy matrix surrounded by robust mesoporous silica shell, that is, the epoxy polymer can easily penetrated into the silica mesopores by as a result of capillary force [23,24]. Therefore, with the increase of the amount of the physically confined epoxy matrix, the CTE values of nanocomposites were gradually decreased. This result would also be proven in Fig. 6b where the addition of CNTs@MS has lower CTE than CNTs performed.
CNTs@MS contents, as shown in Fig. 7. For CNTs incorporated in the epoxy matrix, the thermal conductivity increased by 57.92% (from 0.2417 to 0.3817 W/mK) at 2 wt.% loading. However, the CNTs@MS caused a greater improvement in the thermal conductivity of the epoxy matrix by 95.24% (from 0.2417 to 0.4719 W/mK) at 2 wt% loading. Previous studies showed that decreasing the modulus mismatch between the polymer matrix and solid filler would decrease the thermal interfacial resistance, thereby improving the thermal conductivity of the polymer composites [3,25]. In addition, the high aspect ratio and non-destructive sidewalls of CNTs also the lead to high thermal conductivity [26,27]. Even although the silica-coated CNTs do have higher
3.2.2. Thermal conductivity properties The thermal conductivities of the CNTs–epoxy and CNTs@MS– epoxy composites both increased along with the CNTs and
Fig. 5. Storage modulus and Tg of epoxy with different content of CNTs@MS.
Fig. 7. Thermal conductivities of neat epoxy and its composites filled with CNTs and CNTs@MS–epoxy composites with different filler contents.
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thermal conductivity values than the raw CNTs incorporated in the epoxy matrix, the improvement is still less than that predicted by Nan’s model [28]. In general, thermal conductivity can be improved by two major factors. (i) de-bundling of CNTs and a uniform dispersion in the matrix; and (ii) strong adhesion to the matrix through effective interfacial bonding. In this study, the uniform silica shell on the CNTs not only serves as a medium to promote dispersion and improve the modulus mismatch of CNTs in the epoxy matrix but also maintain the individual CNTs with a high aspect ratio and nondestructive walls. However, the silica shell still does not have interfacial bonding as strong as the chemical bonding between CNTs and epoxy matrix, and this limits the increase of thermal conductivity [27,29,30]. 4. Conclusion This study fabricated mesoporous silica shells onto the surface of CNTs, and then incorporated them into an epoxy matrix. The mesoporous structure of the silica layer makes modulus matching possible, providing compatibility and dispersibility between the epoxy and CNTs. Moreover, it also enables physical confinement of the epoxy matrix into robust mesopores. In sum, the mesoporous silica coated CNTs had better mechanical properties (storage modulus, glass transition temperature, coefficient of thermal expansion, etc.) and thermal conductivity when incorporated into an epoxy matrix than the as-received CNTs system. Acknowledgements The authors would like to acknowledge the support of the Material Lab, at Advanced Semiconductor Engineering Incorporated in performing all measurements. This research received funding from Ministry of Science and Technology (NSC100-2113-M-006006-MY3) and the Headquarters of University Advancement at the National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan.
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Please cite this article in press as: Chung M-H, et al. Effects of mesoporous silica coated multi-wall carbon nanotubes on the mechanical and thermal properties of epoxy nanocomposites. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.05.009