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A method of visualization of inorganic nanoparticles dispersion in nanocomposites
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Composites: Part B 39 (2008) 196–201 www.elsevier.com/locate/compositesb
A method of visualization of inorganic nanoparticles dispersion in nanocomposites S. Lingaiah
a,*
, R. Sadler a, C. Ibeh b, K. Shivakumar
a
a b
Center for Composite Materials Research, North Carolina A&T State University, Greensboro, NC 27411, USA Center for Nanocomposites and Multifunctional Materials, Pittsburg State University, Pittsburg, KS 66762, USA Available online 12 March 2007
Abstract Air plasma etching followed by scanning electron microscope (SEM) imaging technique for visualizing the inorganic nanofillers in polymeric composites is a promising technique for quick screening of exfoliation and dispersion of nanofillers in nanocomposites in manufacturing and materials development environments. The method was applied to three different nanocomposites, namely, montmorillonite (MMT) clay vinyl ester, carbon nanotube (CNT) vinyl ester and MMT epoxy nanocomposites samples produced using high shear mixing. Results were compared with transmission electron microscope data. The etch rate for 2%MMT/vinyl ester and 1%MMT/epoxy nanocomposites varied from 1 to 2 nm/min. 2007 Published by Elsevier Ltd. Keywords: A. Nanocomposite; A. Montmorillonite; E. Plasma etching; D. SEM; TEM
1. Introduction The utility of introducing inorganic nanomaterials as additives into polymer systems has resulted in nanofilled polymer composite materials exhibiting multi-functional performance characteristics beyond what traditional polymer composites possess. Multi-functional features attributable to polymer nanocomposites consist of improved share modulus, flame resistance, moisture and chemical resistance, and decreased permeability. Through, controlled alteration of the additive at the nanoscale level, one can maximize properties of selected polymers systems to meet or exceed the requirements of current military, and/or civilian systems [1–18]. Montmorillonite (MMT) clay is one of the reinforcing agents commonly used in polymers systems because of its availability and lower cost. In order to obtain superior performance in nanocomposites, the MMT clay must be exfoliated and dispersed well. This is
*
Corresponding author. E-mail address: [email protected] (S. Lingaiah).
1359-8368/$ - see front matter 2007 Published by Elsevier Ltd. doi:10.1016/j.compositesb.2007.02.027
being achieved by a number of mixing techniques, all involving high shear. Transmission electron microscope (TEM) is the primary technique of determining the extent of exfoliation and dispersion in nanocomposites [10–18]. This technique gives a local view of the nanocomposite to a high degree of resolution on a 2-D plane. X-ray diffraction (XRD), small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) [1,10–18] are the commonly used instruments for measuring the degree of exfoliation and dispersion of MMT clay in a polymer matrix. Each of these methods has their merits and demerits. These are the research laboratory equipments and are not suitable in a manufacturing environment, where a quick pictorial (3-D) assessment is needed. A novel technique based on a low temperature plasma etching followed by SEM was developed to assess the degree of exfoliation and dispersion of inorganic nanoplatelets in a polymer composite [25–27]. The purpose of this paper is not to evaluate degree of exfoliation and dispersion of MMT and carbon nanotubes but to offer a tool for visualization of nanosize inorganic particles in resin composite.
S. Lingaiah et al. / Composites: Part B 39 (2008) 196–201
2. Plasma etching 2.1. Methodology Plasma etching is a chemical process where an RF discharge on parent gases generates reactive species such as atoms, free radicals and ions. The discharge occurs in a vacuum chamber containing the substrates to be etched. The reactive species react at the substrate surface and form volatile compounds. These compounds are removed from the surface by the vacuum pump. Main advantages of plasma etching are uniform etching of exposed surfaces and the results are controllable and repeatable [19–24]. The Fig. 1 shows a schematic view of the plasma etching setup.
2.2. Sample preparation The 2%MMT/vinyl ester samples were prepared by mixing 3 g (2% by weight) of Cloisite 10A in 150 g of Derakane 510A-40 epoxy vinyl ester resin (VE). The MMT was dispersed in the VE using a high shear IKA mixer with a 19 mm mixing shaft set at about 6000 rpm. The mixing was accomplished by three 10 min mixing cycles with a cooling period between each cycle. The MMT/VE mixture was then catalyzed using 0.3% cobalt naphthenate (6%) promoter, 0.75% cumene hydroperoxide catalyst and 0.075% 2,4-pentanedione retarder. Each ingredient was weighed and hand-stirred into the MMT/VE mixture before the next ingredient was added. The catalyzed mixture was degassed in a vacuum chamber to remove the entrapped air and casted into a thin sheet in a 50 mm aluminum pan. The material was cured at room temperature for about 72 h and post cured for 8 h at 82 C. The 3 roll milling is a high shear rolling technique provides a good dispersion of nanoparticles in resin matrix [11]. The MMT samples were prepared by mixing 1% of MMT with Epoxy EPON 9504 and curing agent 9554 in the ratio 100:26. The MMT was dispersed in Epoxy using 3 roll milling. The gap between Nip-rolls and Take-off rolls is 0.0015 in. The number of passes through the mill was doubled starting at 3 and ending at 292 passes. Two hours Gas Inlet
RF Electrodes Etching Chamber
Vacuum Pump
Aluminum Tray
RF Generator
Samples
Fig. 1. Schematic of Etching system.
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post sonication was performed for 110 passes sample. The catalyzed mixture was degassed in a vacuum chamber to remove the entrapped air and casted into a thin sheet in a 50 mm aluminum pan. The material was post cured at temperature 95 C for 1 h. The CNT/VE samples were prepared by mixing 5.5 g of oxidized and unoxidized CNT in 275 g of vinyl ester resin (2% by weight). Additional 100 g of styrene was added to the mixture for better dispersion and sonicated for 1 h duration. The catalyzed mixture was degassed in a vacuum chamber to remove the entrapped air. Then the cast was cured at room temperature for 24 h and post cured at 180 F for about 8 h. 2.3. Etching The nanocomposite samples [5 · 5 mm] were exposed to ionized dry air in a low temperature plasma chamber. The etching chamber consists of a quartz tube about 250 mm in diameter and 450 mm deep and it is surrounded by RF electrodes that ionize the air (or gas). Samples were placed on a perforated aluminum tray in the middle of the etching chamber (see Fig. 1). The RF power is turned on to warmup the electronic system. The chamber was evacuated to about 0.001 Torr. Dry air was allowed slowly into the chamber. After the gas pressure was stabilized to 0.3 Torr, the RF power was increased to 250 W. Care was exercised to prevent the plasma from entering the vacuum plumbing. The Oxygen in the air plasma oxidizes the polymer and exposes the MMT silicates. Amount of etch depends on the duration of exposure, RF power, and the gas pressure. Several samples were etched for different durations at 250 W power and 0.3 Torr chamber pressure. 2.4. TEM sample preparation The material was sliced into ultra-thin films (around 70 nm) at room temperature using a Reichert–Jung Ultracut E microtome with a diamond knife. The ultra-thin films was floated onto water and mounted on carbon coated copper grids. Bright field TEM images of the sample were obtained using a JEOL’s JEM 1200ex TEM at 80 KV. The contrast between the clay layers and the polymer phase is sufficient for imaging the sample. 3. Results and discussion 3.1. Etch rate The etch rate of epoxy, 1%MMT/epoxy, VE and 2%MMT/VE composites were determined by weight loss technique [19]. Fig. 2 shows etch rate in nm/min versus etch time duration in minutes. Etch rate of 1%MMT/epoxy (EPON 9504) nanocomposite was slightly greater than epoxy resin for all etch durations, whereas the etch rate of 2%MMT/VE nanocomposite was slightly less than VE
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Etch Rate (nm/min)
2.5
2.0
1.5
1.0
0.5
0.0 0
10
20
30
40
50
60
70
80
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Fig. 2. Etch rate of 1%MMT/epoxy, epoxy, 2%MMT/VE & VE nanocomposites.
at higher etching rate. The etch rates of these nanocomposites varies from 1 to 2 nm for all etch durations. 3.2. SEM results Plasma etch samples were coated with gold using sputtering system to improve the surface conductivity and the SEM images were taken at different magnifications. Fig. 3 shows the SEM of 2%MMT/VE composite for 32, 50 and 90 min etch durations. As the etch duration was increased, more exposed platelets are seen. The images also reveal that the samples were not fully dispersed. Fig. 4 shows etched 2%MMT/VE nanocomposites for 50 min at 1 K and 20 K magnification, respectively. Fig. 4a illustrates a uniform dispersion at a microscopic scale and shows that the material is microscopically homogeneous. Fig. 4b illustrates the exfoliation of nanoclay into
platelets of varying degree. The details of folding of platelets can be seen. Figs. 3 and 4, illustrates the potential of this experimental method to evaluate dispersion of MMT or any other inorganic fillers at microscopic to nearly nano (submicron) levels. Further, the method allows the experimentation of many sites of the sample at one setting of the SEM. Fig. 5a shows the SEM of plasma etched (15 min) unoxidized CNT/VE composite with addition of styrene. The mixture was sonicated for 2 h. The dispersion of CNT in resin matrix is not uniform. The picture looks like ‘‘birdnest’’ appearance of clumps of nano CNT’s due to lack of blending. Fig. 5b shows the SEM image of oxidized CNT/VE nanocomposite. Here the dispersion is good compared to unoxidized CNT. Hence, oxidized CNT provides better dispersion in resin matrix compared to unoxidized CNT. 3.3. TEM results The TEM images of the 1%MMT/epoxy and 2%MMT/ VE nanocomposites are shown in Fig. 6. Presence of Al and Mg elements in silicates make the clay platelets to appear dark compared to the matrix. Fig. 6a and b shows TEM image of 2%MMT/VE nanocomposites. These pictures show some amount exfolication but poor dispersion of MMT clay. Fig. 6c and d shows the TEM images of 1%MMT/epoxy nanocomposites prepared by 3 roll mill mixing (292 passes) and shows small aggregates containing some clay layers evenly distributed in epoxy matrix. Fig. 6e and f shows TEM images of 1%MMT/epoxy nanocomposites prepared by 3 roll milling (110 passes) and sonicated for 2 h after milling. These pictures i.e. Fig. 6e and f shows very small aggregates containing 3–7 silicate layers. The size of the aggregates was smaller than the sample without sonication, which indicates that the sonication introduces
Fig. 3. SEM images of 2%MMT/VE samples at different etch time (· 5 K).
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Fig. 4. SEM images of 2%MMT/VE at different magnifications.
Fig. 5. SEM images of unoxidized & oxidized CNT/VE nanocomposites.
the force to help the clay dispersion. The d-spacing of all these nanocomposites was around 2.5 nm. As explained previously, TEM provides a 2-D image of the dispersed clay platelets that are within a 70 nm slice. Platelets outside this thickness require additional slices to visualize the 3-D dispersion. The platelets that are laying parallel to the cut could be missed.
3.4. Process problems Fig. 7 shows the improper curing of nanocomposite which has resulted in accelerated etching of resin for MMT/Epoxy nanocomposites, plasma etched for 10 min. The accelerated etching of resin could be due to uncured resin, trapped air bubbles or it could be due to coupling
Fig. 6. TEM image of 1%MMT/epoxy & 2%MMT/VE nanocomposites.
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Fig. 7. SEM images of MMT/Epoxy and MMT/VE nanocomposites.
of the RF field to iron content. Fig. 7a and b shows the SEM pictures of MMT/Epoxy nanocomposite processed through 3 roll mill (200 passes) and the SEM analysis was carried out at University of North Carolina at Greensboro and Indian Institute of Science at Bangalore, India, respectively. Fig. 7c shows the SEM image for MMT/VE nanocomposites with addition of styrene during processing. Addition of styrene is more susceptible for accelerating etching of matrix in nanocomposites because of improper curing. 4. Conclusions The paper presents a plasma etching and scanning electron microscope (SEM) imaging technique of visualizing the inorganic nanofillers in polymeric composites. This technique is promising for quick screening of exfoliation and dispersion of nanofillers in nanocomposites in a manufacturing and/or materials development environments. The method was applied to study MMT clay in vinyl ester, MMT/epoxy and carbon nanotube (CNT) in vinyl ester nanocomposites samples made by high shear mixing using IKA and 3 roll milling. Results were compared with transmission electron microscope (TEM) data. The sonicated sample of 110 passes made by 3 roll mills showed better exfoliation with very small aggregates containing 3–7 silicate layers compared to samples without sonication. The d-spacing of all these nanocomposites was around 2.5 nm. The plasma etch followed by SEM provides microscopic to submicroscopic 3-D details of the inorganic fillers in a polymer composite. The etch rate for vinyl ester, 2%MMT/vinyl ester, epoxy and 1%MMT/epoxy nanocomposites varied between 1 and 2 nm/min, respectively. The improper curing of nanocomposite can result in accelerated etching and/or melting of resin in nanocomposites. Acknowledgements The authors wish to thank the Office of Naval Research (Grant Nos. N00014-01-1033 and N000140510532) and Dr Yapa Rajapakse, Program Manager for ship structure. They also acknowledge the support of United Negro College Fund Special Programs Corp. (Grant No. W911NF04-1-0045) and Prof. Bruce Kirchoff, UNCG, for providing SEM facility. They also thank Prof. Miriam Rafailovich,
Prof. Jonathan Sokolov and Mr. Mayu Si, State University of New York at Stony Brook, Stony Brook, New York for TEM analysis. Finally, they thank, Mr. Gowtham, Mr. John Steelman and Mr. Patrick Stuart, students at Center for Composite Materials Research for their help in preparing the samples. References [1] Chenggang C, David C. Preparation, characterization and nanostructural evaluation of epoxy nanocomposites. J Appl Polym Sci 2003;90:2276–87. [2] Tamaki R, Choi J, Laine RM. Polyimide nanocomposite from octe silsequioxane. Chem Mater 2003;15:739–97. [3] Pyun J, Matyjaszewski K, Kowalewski T, Savin D, Patternson G, Kickelbick G, et al. Synthesis of well-defined block copolymers tehered to polysilsesquioxane nanoparticle and their nanoscale morphology on surfaces. J Am Chem Soc 2001;123:9445–6. [4] Li GZ, Wang L, Toghiani H, Daulton TL, Koyama K, Pittman CU, et al. Viscoelastic and mechanical properties of epoxy/multifunctional polyhedral oligomeric silsesquioxane nanocomposites and epoxy/ladder like polyphenylsilsesquioxane blends. Macromolecules 2001;34:8686–93. [5] He JA, Valluzzi R, Yang K, Dolukhanyan T, Sung C, Kumar J, et al. Electrostatic multilayer deposition of a gold-dendrimer nanocomposite. Chem Mater 1999;11:3268–74. [6] Percy MJ, Barthet C, Lobb JC, Khan MA, Lascelles SF, Vamvakaki M, et al. Synthesis and characterization of vinyl polymer-silica colloidal nanocomposite. Langmuir 2000;16:6913–20. [7] Patton RD, Pittman Jr CU, Wang L, Hill JR. Vapor grown carbon fiber composites with epoxy and poly (phenylene sulfide) matrices. Composites Part A 1999;31:1081–91. [8] Lan T, Pinnavaia TJ. Clay-reinforced epoxy nanocomposites. Chem Mater 1994;6:2216–9. [9] Koo JH, Pilato LA, Winzek P, Shivakumar KN, Pitman Jr CU, Luo ZP. Thermo-oxidative studies of nanomodified carbon/carbon composites. In: SAMPE Conference, vol. 1. California: Long Beach Convention Center; 2004, pp. 1–15. [10] Qi W, Chunfang S, Weiwei L. Study of the exfoliation process of epoxy-clay nanocomposites by different curing agents. J Appl Polym Sci 2003;90:511–7. [11] Hai-jun L, Guo-Zheng L, Xiao-yan M, Bao-yan Z, Xiang-bao C. Epoxy/clay nanocomposites: further exfoliation of newly modified clay induced by shearing force of ball milling. Polym Int 2004;53:1545–53. [12] Yoonessi M, Toghiani H, William LK, Pittman Jr CU. Preparation characterization and properties of exfoliated/delaminated organically modified clay/dicyclopentadiene resin nanocomposites. Macromolecules 2004;37:2511–8. [13] Hiroaki M, Michael J, Lawrence TD. Amine-cured epoxy/clay nanocomposotes I. Processing and chemical characterization. J Polym Sci: Part B: Polym Phys 2004;42:4384–90.
S. Lingaiah et al. / Composites: Part B 39 (2008) 196–201 [14] Du XS, Xiao M, Meng YZ. Synthesis and characterization of polyaniline/graphite conducting nanocomposites. J Polym Phys: Part B: Polym Phys 2004;42:1972–8. [15] Delozier DM, Orwoll RA, Cahoon JF, Ladislaw JS, Smith Jr JG, Connell JW. Polymide nanocomposites prepared from high-temperature, reduced charge organoclays. Polymer 2003;44:2231–41. [16] Jyi JL, Isaac MD. Characterization and modeling of mechanical behavior of polymer/clay nanocomposites. Compos Sci Technol 2003;63:1607–16. [17] Xu LR, Vikram B, Wei HZ, Li J, Lukehart CM. Mechanical property characterization of a polymer nanocomposite reinforced by graphitic nanofibers with reactive linkers. J Compos Mater 2004; 38:1563–82. [18] Gilman JW, Morgan AB. Characterization of polymer-layered silicon (clay) nanocomposite by transmission electron microscope and X-ray diffraction: A comparative study. J Appl Polym Sci 2003; 87:1329–38. [19] Lin S, Chang JP. Plasma etching of high dielectric constant materials on silicon in halogen chemistries. J Vac Sci Technol A 2004;22:88–95. [20] Kim DP, Kim C. Etching characteristics of SrBi2Ta2O9 thin films in a Cl2/CF4/Ar plasma. J Korean Phys Soc 2002;41:439–44.
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[21] Christopher DH, Gray SM. Advantages of plasma etch modeling using neural networks over statistical techniques. IEEE T Semiconduct M 1993;6:103–11. [22] Marnor M, Auret FD, Goodman SA, Malherbe JB. Argon plasma sputter etching induced defect levels in strained, epitaxial p-type Si– Ge alloys. Thin Solid Films 1999;343–344:416–9. [23] Kim MC, Yang SH, Boo JH, Han JG. Surface treatment of metals using an atmospheric pressure plasma jet and their surface characteristics. Surf Coat Tech 2003;174–175:839–44. [24] Chen Y, Hong L, Yu R, Wang H, Zhu L. Study of Bombyx mori silk streated by oxygen plasma. J Zhejiang Univ Sci 2004;5:918–22. [25] Shivakumar KN, Sadler R, Shivalingappa L, Sharp M. Plasma etching to enhance visibility of nanoparticles in nanocomposites. Comput Mater Cont 2004;1(4):309–18. [26] Shivakumar KN, Shivalingappa L, Sadler R, Miriam Rafailovich, Mayu Si, Jonathan Sokolov. Plasma etching to enhance visibility of nanoparticle in nanocomposites. In: 46th AIAA/ASME/ASCE/ AHS/IASC Structures, Structural Dynamics and Material Conference, 18–21 April, Austin; 2005. [27] Shivalingappa L, Shivakumar KN, Sadler R, Sharp M. A method of visualization of dispersion of nanoplatelets in nanocomposites. Composite Science and Technology, in press.
Composites: Part B 39 (2008) 196–201 www.elsevier.com/locate/compositesb
A method of visualization of inorganic nanoparticles dispersion in nanocomposites S. Lingaiah
a,*
, R. Sadler a, C. Ibeh b, K. Shivakumar
a
a b
Center for Composite Materials Research, North Carolina A&T State University, Greensboro, NC 27411, USA Center for Nanocomposites and Multifunctional Materials, Pittsburg State University, Pittsburg, KS 66762, USA Available online 12 March 2007
Abstract Air plasma etching followed by scanning electron microscope (SEM) imaging technique for visualizing the inorganic nanofillers in polymeric composites is a promising technique for quick screening of exfoliation and dispersion of nanofillers in nanocomposites in manufacturing and materials development environments. The method was applied to three different nanocomposites, namely, montmorillonite (MMT) clay vinyl ester, carbon nanotube (CNT) vinyl ester and MMT epoxy nanocomposites samples produced using high shear mixing. Results were compared with transmission electron microscope data. The etch rate for 2%MMT/vinyl ester and 1%MMT/epoxy nanocomposites varied from 1 to 2 nm/min. 2007 Published by Elsevier Ltd. Keywords: A. Nanocomposite; A. Montmorillonite; E. Plasma etching; D. SEM; TEM
1. Introduction The utility of introducing inorganic nanomaterials as additives into polymer systems has resulted in nanofilled polymer composite materials exhibiting multi-functional performance characteristics beyond what traditional polymer composites possess. Multi-functional features attributable to polymer nanocomposites consist of improved share modulus, flame resistance, moisture and chemical resistance, and decreased permeability. Through, controlled alteration of the additive at the nanoscale level, one can maximize properties of selected polymers systems to meet or exceed the requirements of current military, and/or civilian systems [1–18]. Montmorillonite (MMT) clay is one of the reinforcing agents commonly used in polymers systems because of its availability and lower cost. In order to obtain superior performance in nanocomposites, the MMT clay must be exfoliated and dispersed well. This is
*
Corresponding author. E-mail address: [email protected] (S. Lingaiah).
1359-8368/$ - see front matter 2007 Published by Elsevier Ltd. doi:10.1016/j.compositesb.2007.02.027
being achieved by a number of mixing techniques, all involving high shear. Transmission electron microscope (TEM) is the primary technique of determining the extent of exfoliation and dispersion in nanocomposites [10–18]. This technique gives a local view of the nanocomposite to a high degree of resolution on a 2-D plane. X-ray diffraction (XRD), small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) [1,10–18] are the commonly used instruments for measuring the degree of exfoliation and dispersion of MMT clay in a polymer matrix. Each of these methods has their merits and demerits. These are the research laboratory equipments and are not suitable in a manufacturing environment, where a quick pictorial (3-D) assessment is needed. A novel technique based on a low temperature plasma etching followed by SEM was developed to assess the degree of exfoliation and dispersion of inorganic nanoplatelets in a polymer composite [25–27]. The purpose of this paper is not to evaluate degree of exfoliation and dispersion of MMT and carbon nanotubes but to offer a tool for visualization of nanosize inorganic particles in resin composite.
S. Lingaiah et al. / Composites: Part B 39 (2008) 196–201
2. Plasma etching 2.1. Methodology Plasma etching is a chemical process where an RF discharge on parent gases generates reactive species such as atoms, free radicals and ions. The discharge occurs in a vacuum chamber containing the substrates to be etched. The reactive species react at the substrate surface and form volatile compounds. These compounds are removed from the surface by the vacuum pump. Main advantages of plasma etching are uniform etching of exposed surfaces and the results are controllable and repeatable [19–24]. The Fig. 1 shows a schematic view of the plasma etching setup.
2.2. Sample preparation The 2%MMT/vinyl ester samples were prepared by mixing 3 g (2% by weight) of Cloisite 10A in 150 g of Derakane 510A-40 epoxy vinyl ester resin (VE). The MMT was dispersed in the VE using a high shear IKA mixer with a 19 mm mixing shaft set at about 6000 rpm. The mixing was accomplished by three 10 min mixing cycles with a cooling period between each cycle. The MMT/VE mixture was then catalyzed using 0.3% cobalt naphthenate (6%) promoter, 0.75% cumene hydroperoxide catalyst and 0.075% 2,4-pentanedione retarder. Each ingredient was weighed and hand-stirred into the MMT/VE mixture before the next ingredient was added. The catalyzed mixture was degassed in a vacuum chamber to remove the entrapped air and casted into a thin sheet in a 50 mm aluminum pan. The material was cured at room temperature for about 72 h and post cured for 8 h at 82 C. The 3 roll milling is a high shear rolling technique provides a good dispersion of nanoparticles in resin matrix [11]. The MMT samples were prepared by mixing 1% of MMT with Epoxy EPON 9504 and curing agent 9554 in the ratio 100:26. The MMT was dispersed in Epoxy using 3 roll milling. The gap between Nip-rolls and Take-off rolls is 0.0015 in. The number of passes through the mill was doubled starting at 3 and ending at 292 passes. Two hours Gas Inlet
RF Electrodes Etching Chamber
Vacuum Pump
Aluminum Tray
RF Generator
Samples
Fig. 1. Schematic of Etching system.
197
post sonication was performed for 110 passes sample. The catalyzed mixture was degassed in a vacuum chamber to remove the entrapped air and casted into a thin sheet in a 50 mm aluminum pan. The material was post cured at temperature 95 C for 1 h. The CNT/VE samples were prepared by mixing 5.5 g of oxidized and unoxidized CNT in 275 g of vinyl ester resin (2% by weight). Additional 100 g of styrene was added to the mixture for better dispersion and sonicated for 1 h duration. The catalyzed mixture was degassed in a vacuum chamber to remove the entrapped air. Then the cast was cured at room temperature for 24 h and post cured at 180 F for about 8 h. 2.3. Etching The nanocomposite samples [5 · 5 mm] were exposed to ionized dry air in a low temperature plasma chamber. The etching chamber consists of a quartz tube about 250 mm in diameter and 450 mm deep and it is surrounded by RF electrodes that ionize the air (or gas). Samples were placed on a perforated aluminum tray in the middle of the etching chamber (see Fig. 1). The RF power is turned on to warmup the electronic system. The chamber was evacuated to about 0.001 Torr. Dry air was allowed slowly into the chamber. After the gas pressure was stabilized to 0.3 Torr, the RF power was increased to 250 W. Care was exercised to prevent the plasma from entering the vacuum plumbing. The Oxygen in the air plasma oxidizes the polymer and exposes the MMT silicates. Amount of etch depends on the duration of exposure, RF power, and the gas pressure. Several samples were etched for different durations at 250 W power and 0.3 Torr chamber pressure. 2.4. TEM sample preparation The material was sliced into ultra-thin films (around 70 nm) at room temperature using a Reichert–Jung Ultracut E microtome with a diamond knife. The ultra-thin films was floated onto water and mounted on carbon coated copper grids. Bright field TEM images of the sample were obtained using a JEOL’s JEM 1200ex TEM at 80 KV. The contrast between the clay layers and the polymer phase is sufficient for imaging the sample. 3. Results and discussion 3.1. Etch rate The etch rate of epoxy, 1%MMT/epoxy, VE and 2%MMT/VE composites were determined by weight loss technique [19]. Fig. 2 shows etch rate in nm/min versus etch time duration in minutes. Etch rate of 1%MMT/epoxy (EPON 9504) nanocomposite was slightly greater than epoxy resin for all etch durations, whereas the etch rate of 2%MMT/VE nanocomposite was slightly less than VE
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Etch Rate (nm/min)
2.5
2.0
1.5
1.0
0.5
0.0 0
10
20
30
40
50
60
70
80
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Fig. 2. Etch rate of 1%MMT/epoxy, epoxy, 2%MMT/VE & VE nanocomposites.
at higher etching rate. The etch rates of these nanocomposites varies from 1 to 2 nm for all etch durations. 3.2. SEM results Plasma etch samples were coated with gold using sputtering system to improve the surface conductivity and the SEM images were taken at different magnifications. Fig. 3 shows the SEM of 2%MMT/VE composite for 32, 50 and 90 min etch durations. As the etch duration was increased, more exposed platelets are seen. The images also reveal that the samples were not fully dispersed. Fig. 4 shows etched 2%MMT/VE nanocomposites for 50 min at 1 K and 20 K magnification, respectively. Fig. 4a illustrates a uniform dispersion at a microscopic scale and shows that the material is microscopically homogeneous. Fig. 4b illustrates the exfoliation of nanoclay into
platelets of varying degree. The details of folding of platelets can be seen. Figs. 3 and 4, illustrates the potential of this experimental method to evaluate dispersion of MMT or any other inorganic fillers at microscopic to nearly nano (submicron) levels. Further, the method allows the experimentation of many sites of the sample at one setting of the SEM. Fig. 5a shows the SEM of plasma etched (15 min) unoxidized CNT/VE composite with addition of styrene. The mixture was sonicated for 2 h. The dispersion of CNT in resin matrix is not uniform. The picture looks like ‘‘birdnest’’ appearance of clumps of nano CNT’s due to lack of blending. Fig. 5b shows the SEM image of oxidized CNT/VE nanocomposite. Here the dispersion is good compared to unoxidized CNT. Hence, oxidized CNT provides better dispersion in resin matrix compared to unoxidized CNT. 3.3. TEM results The TEM images of the 1%MMT/epoxy and 2%MMT/ VE nanocomposites are shown in Fig. 6. Presence of Al and Mg elements in silicates make the clay platelets to appear dark compared to the matrix. Fig. 6a and b shows TEM image of 2%MMT/VE nanocomposites. These pictures show some amount exfolication but poor dispersion of MMT clay. Fig. 6c and d shows the TEM images of 1%MMT/epoxy nanocomposites prepared by 3 roll mill mixing (292 passes) and shows small aggregates containing some clay layers evenly distributed in epoxy matrix. Fig. 6e and f shows TEM images of 1%MMT/epoxy nanocomposites prepared by 3 roll milling (110 passes) and sonicated for 2 h after milling. These pictures i.e. Fig. 6e and f shows very small aggregates containing 3–7 silicate layers. The size of the aggregates was smaller than the sample without sonication, which indicates that the sonication introduces
Fig. 3. SEM images of 2%MMT/VE samples at different etch time (· 5 K).
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Fig. 4. SEM images of 2%MMT/VE at different magnifications.
Fig. 5. SEM images of unoxidized & oxidized CNT/VE nanocomposites.
the force to help the clay dispersion. The d-spacing of all these nanocomposites was around 2.5 nm. As explained previously, TEM provides a 2-D image of the dispersed clay platelets that are within a 70 nm slice. Platelets outside this thickness require additional slices to visualize the 3-D dispersion. The platelets that are laying parallel to the cut could be missed.
3.4. Process problems Fig. 7 shows the improper curing of nanocomposite which has resulted in accelerated etching of resin for MMT/Epoxy nanocomposites, plasma etched for 10 min. The accelerated etching of resin could be due to uncured resin, trapped air bubbles or it could be due to coupling
Fig. 6. TEM image of 1%MMT/epoxy & 2%MMT/VE nanocomposites.
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Fig. 7. SEM images of MMT/Epoxy and MMT/VE nanocomposites.
of the RF field to iron content. Fig. 7a and b shows the SEM pictures of MMT/Epoxy nanocomposite processed through 3 roll mill (200 passes) and the SEM analysis was carried out at University of North Carolina at Greensboro and Indian Institute of Science at Bangalore, India, respectively. Fig. 7c shows the SEM image for MMT/VE nanocomposites with addition of styrene during processing. Addition of styrene is more susceptible for accelerating etching of matrix in nanocomposites because of improper curing. 4. Conclusions The paper presents a plasma etching and scanning electron microscope (SEM) imaging technique of visualizing the inorganic nanofillers in polymeric composites. This technique is promising for quick screening of exfoliation and dispersion of nanofillers in nanocomposites in a manufacturing and/or materials development environments. The method was applied to study MMT clay in vinyl ester, MMT/epoxy and carbon nanotube (CNT) in vinyl ester nanocomposites samples made by high shear mixing using IKA and 3 roll milling. Results were compared with transmission electron microscope (TEM) data. The sonicated sample of 110 passes made by 3 roll mills showed better exfoliation with very small aggregates containing 3–7 silicate layers compared to samples without sonication. The d-spacing of all these nanocomposites was around 2.5 nm. The plasma etch followed by SEM provides microscopic to submicroscopic 3-D details of the inorganic fillers in a polymer composite. The etch rate for vinyl ester, 2%MMT/vinyl ester, epoxy and 1%MMT/epoxy nanocomposites varied between 1 and 2 nm/min, respectively. The improper curing of nanocomposite can result in accelerated etching and/or melting of resin in nanocomposites. Acknowledgements The authors wish to thank the Office of Naval Research (Grant Nos. N00014-01-1033 and N000140510532) and Dr Yapa Rajapakse, Program Manager for ship structure. They also acknowledge the support of United Negro College Fund Special Programs Corp. (Grant No. W911NF04-1-0045) and Prof. Bruce Kirchoff, UNCG, for providing SEM facility. They also thank Prof. Miriam Rafailovich,
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