- Email: [email protected]
montmorillonite nanocomposites by positron
ARTICLE IN PRESS
Radiation Physics and Chemistry 76 (2007) 146–149 www.elsevier.com/locate/radphyschem
Study on the microstructure and mechanical properties for epoxy resin/montmorillonite nanocomposites by positron B. Wanga,, N. Qia, W. Gonga, X.W. Lia, Y.P. Zhenb a Department of Physics, Wuhan University, Wuhan 430072, China Department of Chemical Engineering, Northwester Polytechnical University, Xian 710072, China
b
Abstract Positron annihilation lifetimes have been measured for epoxy resin/organic montmorillonite (OMMT) nanocomposites. Effects of different dispersion states of nano-layered OMMT on the positron annihilation parameters and the mechanical properties were studied. We found that the ortho-positronium (o-Ps) intensity decreased with increasing OMMT content, which indicated that the interaction between the host and nanofillers restrained the segmental motion, resulting in a decrease of the free volume. On the other hand, it is very interesting to observe a good correlation between the interfacial interaction and mechanical properties, suggesting that the dispersion states of OMMT and interfacial property between clay layers and matrix played an important role in determining the mechanical properties. r 2006 Elsevier Ltd. All rights reserved. Keywords: Epoxy; Nanocomposite; Positron; Mechanical property
1. Introduction Epoxy resins have been widely used in adhesives, construction materials, composites, laminates, surface coating, automotive, electronics, aircraft and spacecraft industries owing to their high strength and stiffness, low viscosity, volatily and shrinkage during cure, low creep and good adhesion to many substrates (Barcia et al., 2003). However, the major drawback of these resins is their brittleness. Many efforts have been made to enhance the toughness of epoxy resins by the addition of a second component such as a rubber, a thermoplastic modifier, or even a hard inorganic glass. In recent years, one of the most effective methods is to prepare nanocomposite materials by introducing nano-scale
Corresponding author. Tel.: +86 27 7812 661.
E-mail address: [email protected] (B. Wang).
layered silicate into the matrix. Polymer/layered nanocomposite have attracted a great deal of attention, because these nanocomposites exhibit marked improved physical and chemical properties when compared with pure polymers or conventional microcomposites. Though significant progress has been made in developing nanocomposites with different polymer matrices, a general understanding has yet to emerge. For example, what allows nanocomposites to be both stiffer and tougher than conventional composites without sacrificing other properties? Why do they display better thermal stability versus unfilled polymers? A major challenge to further development of nanocomposites is the lack of even simple structure-property model (Schmidt et al., 2002). In this work, the microstructure and mechanical property of epoxy resin/organic montmorillonite (OMMT) nanocomposites and the effect of interfacial property on the mechanical properties have been studied by positron.
0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2006.03.021
ARTICLE IN PRESS B. Wang et al. / Radiation Physics and Chemistry 76 (2007) 146–149
2. Experiments Epoxy resin (diglycidyl ether of bisphenol A, DGEBA) CYD-128, OMMT and methyl four phathalic anhydride (MeTPHTA), 2-ethy-4methyl imidazole as a curing agent and accelerator were used to make the epoxy resin nanocomposites using the melting blend technology. The dispersibility of the OMMT layer in samples was evaluated by X-ray diffraction (XRD) measurement using a Rigaku RAD-B diffractometer with Cu Ka radiation generated at 40 keV and 30 mA and scanning rate of 1.51/min. The impact and flexural strength measurements were carried out using an impact tester (XCJ-400) according to the ASTM-D256 and universal test machine (ZMGI250, Industrirerk Co. Ltd., Germany). Positron annihilation lifetime spectra were measured using a fast-fast coincidence spectrometer. The time resolution of the system was found to be a sum of two Gaussian with (fwhm)1 ¼ 280 ps (92%), and (fwhm)1 ¼ 260 ps (8%). A 25 mCi 22Na positron source was sandwiched between two pieces of identical samples. Each spectrum contained approximately 106 counts.
3. Results and discussion 3.1. Microstructural and interfacial properties characterization Fig. 1 shows the XRD patterns for the samples with different OMMT content (wt%). From this figure, the
147
marked diffraction peaks of OMMT were not observed for the samples with a OMMT content of 1 and 2 wt% which indicates the formation of more exfoliated structure in these samples. On the other hand, a weak diffraction peak was observed for the other samples with OMMT content of 3–6 wt%, which reveals that the layered OMMT was not entirely exfoliated, that is, a mixture of partially intercalated and partially exfoliated structures is revealed. Effect of this structural feature on the free volume and the mechanical properties is very important (see below). All positron annihilation lifetime spectra measured were resolved into three components using PATFIT after source and background are subtracted. As it is well known, the longest-lived component is assigned to the ortho-positronium (o-Ps) pick-off annihilation in the free volume. The main attention in this paper is paid to the variations of o-Ps annihilation parameters and the intensity of the second component because o-Ps component is significantly sensitive to the change in free volume and the second component intensity could give much information about the interface (Wang et al., 1994). The variations of o-Ps lifetime t3 and its intensity I3 are shown in Figs. 2–3, respectively. From Figs. 2–3, we found that t3 decreases with increasing OMMT content up to 2 wt%, then it remains nearly constant at the same value that the one measured for the unfilled sample, while I3 diminishes with increase of OMMT content. The changes in o-Ps annihilation parameters could be attributed to the interaction between matrix and filled OMMT, because the interaction can restrict the segmental chain motion, leading to the decrease in
peak OMMT 6% Intensity (a.u.)
5% 4% 3%
2% 1% 2
4
6 2θ (degree)
8
Fig. 1. XRD patterns of OMMT and epoxy nanocomposites.
10
ARTICLE IN PRESS B. Wang et al. / Radiation Physics and Chemistry 76 (2007) 146–149
148 1.9
60
Intermediate-lived intensity I2 (%)
o-Ps lifetime τ3 (ns)
58
1.8
1.7
1.6 0
1
2
3
4
5
56 54 52 50 48 46
6
OMMT content (wt/%)
44
Fig. 2. Variation of o-Ps lifetime versus OMMT content (%wt%).
42 -1
0
1
2
3
4
5
6
OMMT content (wt/%) Fig. 4. Variation of I2 versus OMMT content (wt%).
23
o-Ps lifetime I3 (%)
fraction f as follows: F M IC 3 ¼ f I 3 þ ð1 fÞI 3 ,
22
(1)
where the superscripts C, F, M refer to the composite, filler and matrix, respectively, this phenomenon has also be observed in Winberg’s work (Winberg et al., 2004). On the other hand, the variation of intermediate lived component intensity I2 with OMMT content wt% as shown in Fig. 4 can not follow the linearly additive (simple mixing rule) according to the weight fraction f:
21
20
F M IC 2 ¼ I 2 f þ I 2 ð1 fÞ.
19 -1
0
1
2 3 4 OMMT content (wt/%)
5
6
Fig. 3. Variation of o-Ps intensity versus OMMT content (wt%).
(2)
As mentioned above, we conclude that the interaction between OMMT and epoxy matrix exists (Wang et al., 1994). 3.2. Effect of the filler content on the mechanical properties
the free volume concentration. When the OMMT content is 42 wt%, agglomeration of OMMT can occur, exfoliation and intercalation structures coexist, which gives rise to the decrease of the interfacial interaction. So the effect of OMMT on the size of the free volume can be neglected. However, the continued decrease of the o-Ps intensity and a drastic drop of I3 at OMMT content of 2 wt% were observed when the OMMT content is 43 wt% in Fig. 3 (in order to confirm this observation, experiments were repeated for three times), this variation of I3 does not follow the linearly additive relationship according to the weight
The plots of the flexural and impact strengths as a function of the OMMT content are shown in Fig. 5. From this figure, a significant increase in the flexural and the impact strengths when compared to that of unfilled epoxy is observed. Further, we can see that they increase up to an OMMT content of 2 wt%, followed by a decrease with increasing the OMMT content, which is similar to the variation of I2 in Fig. 4. This observation suggests that the interfacial interaction plays an important role in determining flexural and impact strengths (Wang et al., 1994; Ray and Okamoto,
ARTICLE IN PRESS B. Wang et al. / Radiation Physics and Chemistry 76 (2007) 146–149
149
16 120
12
100
10 80 8
6
Flexural Strength (MPa)
Impact Strength (KJ/m2)
14
60 Impact Strength
4
Flexural Strength 40 0
2 4 OMMT Content (wt%)
6
Fig. 5. Variations of mechanical properties versus OMMT content (wt%).
2003). When the OMMT content is p2 wt%, nano-scale layered OMMT are well dispersed in the epoxy matrix, which brings about the formation of the exfoliated structure. Exfoliated structure is more desirable in enhancing the mechanical properties due to a stronger interfacial interaction (Jordan et al., 2005). For OMMT content 42 wt%, the exfoliation and intercalation structures coexist, the interfacial interaction between OMMT and matrix is reduced, resulting in poorer mechanical properties.
4. Conclusions Positron annihilation lifetime measurements were performed. Effects of OMMT content on the positron annihilation parameters and mechanical properties have been discussed. The analysis of positron lifetime results reveals that the dispersion states of nano-scale OMMT layers in epoxy resin/OMMT nanocomposite play an important role in determining the interfacial property and the interaction between the OMMT and epoxy matrix. Exfoliated structure enhances the flexural and impact strengths of nanocomposites due to the strong interfacial interaction between OMMT and epoxy matrix.
Acknowledgements This work is supported by the National Nature Science Foundation and Key Laboratory of Nuclear Solid State Physics of Hubei Province.
References Barcia, F.L., Amaral, T.P., Soares, B.G., 2003. Synthesis and properties of epoxy resin modified with epoxy-terminated liquid polybutadiene. Polymer 44, 5811–5819. Jordan, J., Jacob, K.I., Tannenbaum, R., et al., 2005. Experimental trends in polymer nanocompopsites—a review. Mater. Sci Eng. A 393, 1–11. Ray, S.S., Okamoto, M., 2003. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog. polym. Sci. 28, 1539–1641. Schmidt, D., Shah, D., Giannelis, E.P., 2002. New advances in polymer/layered silicate nanocomposites. Curr. Opin. Solid Mater. Sci. 6, 205–212. Wang, S.J., Wang, C.L., Zhu, X.G., Qi, Z.N., 1994. Structural characteristics of HDPE/CaCO3 polymer composites probed by positron annihilation. Phys. Stat. Sol. A 142, 275–280. Winberg, P., Eldrup, M., Maurer, F.H.J., 2004. Nanocopic properties of silica filled polydimethysiloxane by means of positron annihilation lifetime spectroscopy. Polymer 45, 8253–8282.
Radiation Physics and Chemistry 76 (2007) 146–149 www.elsevier.com/locate/radphyschem
Study on the microstructure and mechanical properties for epoxy resin/montmorillonite nanocomposites by positron B. Wanga,, N. Qia, W. Gonga, X.W. Lia, Y.P. Zhenb a Department of Physics, Wuhan University, Wuhan 430072, China Department of Chemical Engineering, Northwester Polytechnical University, Xian 710072, China
b
Abstract Positron annihilation lifetimes have been measured for epoxy resin/organic montmorillonite (OMMT) nanocomposites. Effects of different dispersion states of nano-layered OMMT on the positron annihilation parameters and the mechanical properties were studied. We found that the ortho-positronium (o-Ps) intensity decreased with increasing OMMT content, which indicated that the interaction between the host and nanofillers restrained the segmental motion, resulting in a decrease of the free volume. On the other hand, it is very interesting to observe a good correlation between the interfacial interaction and mechanical properties, suggesting that the dispersion states of OMMT and interfacial property between clay layers and matrix played an important role in determining the mechanical properties. r 2006 Elsevier Ltd. All rights reserved. Keywords: Epoxy; Nanocomposite; Positron; Mechanical property
1. Introduction Epoxy resins have been widely used in adhesives, construction materials, composites, laminates, surface coating, automotive, electronics, aircraft and spacecraft industries owing to their high strength and stiffness, low viscosity, volatily and shrinkage during cure, low creep and good adhesion to many substrates (Barcia et al., 2003). However, the major drawback of these resins is their brittleness. Many efforts have been made to enhance the toughness of epoxy resins by the addition of a second component such as a rubber, a thermoplastic modifier, or even a hard inorganic glass. In recent years, one of the most effective methods is to prepare nanocomposite materials by introducing nano-scale
Corresponding author. Tel.: +86 27 7812 661.
E-mail address: [email protected] (B. Wang).
layered silicate into the matrix. Polymer/layered nanocomposite have attracted a great deal of attention, because these nanocomposites exhibit marked improved physical and chemical properties when compared with pure polymers or conventional microcomposites. Though significant progress has been made in developing nanocomposites with different polymer matrices, a general understanding has yet to emerge. For example, what allows nanocomposites to be both stiffer and tougher than conventional composites without sacrificing other properties? Why do they display better thermal stability versus unfilled polymers? A major challenge to further development of nanocomposites is the lack of even simple structure-property model (Schmidt et al., 2002). In this work, the microstructure and mechanical property of epoxy resin/organic montmorillonite (OMMT) nanocomposites and the effect of interfacial property on the mechanical properties have been studied by positron.
0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2006.03.021
ARTICLE IN PRESS B. Wang et al. / Radiation Physics and Chemistry 76 (2007) 146–149
2. Experiments Epoxy resin (diglycidyl ether of bisphenol A, DGEBA) CYD-128, OMMT and methyl four phathalic anhydride (MeTPHTA), 2-ethy-4methyl imidazole as a curing agent and accelerator were used to make the epoxy resin nanocomposites using the melting blend technology. The dispersibility of the OMMT layer in samples was evaluated by X-ray diffraction (XRD) measurement using a Rigaku RAD-B diffractometer with Cu Ka radiation generated at 40 keV and 30 mA and scanning rate of 1.51/min. The impact and flexural strength measurements were carried out using an impact tester (XCJ-400) according to the ASTM-D256 and universal test machine (ZMGI250, Industrirerk Co. Ltd., Germany). Positron annihilation lifetime spectra were measured using a fast-fast coincidence spectrometer. The time resolution of the system was found to be a sum of two Gaussian with (fwhm)1 ¼ 280 ps (92%), and (fwhm)1 ¼ 260 ps (8%). A 25 mCi 22Na positron source was sandwiched between two pieces of identical samples. Each spectrum contained approximately 106 counts.
3. Results and discussion 3.1. Microstructural and interfacial properties characterization Fig. 1 shows the XRD patterns for the samples with different OMMT content (wt%). From this figure, the
147
marked diffraction peaks of OMMT were not observed for the samples with a OMMT content of 1 and 2 wt% which indicates the formation of more exfoliated structure in these samples. On the other hand, a weak diffraction peak was observed for the other samples with OMMT content of 3–6 wt%, which reveals that the layered OMMT was not entirely exfoliated, that is, a mixture of partially intercalated and partially exfoliated structures is revealed. Effect of this structural feature on the free volume and the mechanical properties is very important (see below). All positron annihilation lifetime spectra measured were resolved into three components using PATFIT after source and background are subtracted. As it is well known, the longest-lived component is assigned to the ortho-positronium (o-Ps) pick-off annihilation in the free volume. The main attention in this paper is paid to the variations of o-Ps annihilation parameters and the intensity of the second component because o-Ps component is significantly sensitive to the change in free volume and the second component intensity could give much information about the interface (Wang et al., 1994). The variations of o-Ps lifetime t3 and its intensity I3 are shown in Figs. 2–3, respectively. From Figs. 2–3, we found that t3 decreases with increasing OMMT content up to 2 wt%, then it remains nearly constant at the same value that the one measured for the unfilled sample, while I3 diminishes with increase of OMMT content. The changes in o-Ps annihilation parameters could be attributed to the interaction between matrix and filled OMMT, because the interaction can restrict the segmental chain motion, leading to the decrease in
peak OMMT 6% Intensity (a.u.)
5% 4% 3%
2% 1% 2
4
6 2θ (degree)
8
Fig. 1. XRD patterns of OMMT and epoxy nanocomposites.
10
ARTICLE IN PRESS B. Wang et al. / Radiation Physics and Chemistry 76 (2007) 146–149
148 1.9
60
Intermediate-lived intensity I2 (%)
o-Ps lifetime τ3 (ns)
58
1.8
1.7
1.6 0
1
2
3
4
5
56 54 52 50 48 46
6
OMMT content (wt/%)
44
Fig. 2. Variation of o-Ps lifetime versus OMMT content (%wt%).
42 -1
0
1
2
3
4
5
6
OMMT content (wt/%) Fig. 4. Variation of I2 versus OMMT content (wt%).
23
o-Ps lifetime I3 (%)
fraction f as follows: F M IC 3 ¼ f I 3 þ ð1 fÞI 3 ,
22
(1)
where the superscripts C, F, M refer to the composite, filler and matrix, respectively, this phenomenon has also be observed in Winberg’s work (Winberg et al., 2004). On the other hand, the variation of intermediate lived component intensity I2 with OMMT content wt% as shown in Fig. 4 can not follow the linearly additive (simple mixing rule) according to the weight fraction f:
21
20
F M IC 2 ¼ I 2 f þ I 2 ð1 fÞ.
19 -1
0
1
2 3 4 OMMT content (wt/%)
5
6
Fig. 3. Variation of o-Ps intensity versus OMMT content (wt%).
(2)
As mentioned above, we conclude that the interaction between OMMT and epoxy matrix exists (Wang et al., 1994). 3.2. Effect of the filler content on the mechanical properties
the free volume concentration. When the OMMT content is 42 wt%, agglomeration of OMMT can occur, exfoliation and intercalation structures coexist, which gives rise to the decrease of the interfacial interaction. So the effect of OMMT on the size of the free volume can be neglected. However, the continued decrease of the o-Ps intensity and a drastic drop of I3 at OMMT content of 2 wt% were observed when the OMMT content is 43 wt% in Fig. 3 (in order to confirm this observation, experiments were repeated for three times), this variation of I3 does not follow the linearly additive relationship according to the weight
The plots of the flexural and impact strengths as a function of the OMMT content are shown in Fig. 5. From this figure, a significant increase in the flexural and the impact strengths when compared to that of unfilled epoxy is observed. Further, we can see that they increase up to an OMMT content of 2 wt%, followed by a decrease with increasing the OMMT content, which is similar to the variation of I2 in Fig. 4. This observation suggests that the interfacial interaction plays an important role in determining flexural and impact strengths (Wang et al., 1994; Ray and Okamoto,
ARTICLE IN PRESS B. Wang et al. / Radiation Physics and Chemistry 76 (2007) 146–149
149
16 120
12
100
10 80 8
6
Flexural Strength (MPa)
Impact Strength (KJ/m2)
14
60 Impact Strength
4
Flexural Strength 40 0
2 4 OMMT Content (wt%)
6
Fig. 5. Variations of mechanical properties versus OMMT content (wt%).
2003). When the OMMT content is p2 wt%, nano-scale layered OMMT are well dispersed in the epoxy matrix, which brings about the formation of the exfoliated structure. Exfoliated structure is more desirable in enhancing the mechanical properties due to a stronger interfacial interaction (Jordan et al., 2005). For OMMT content 42 wt%, the exfoliation and intercalation structures coexist, the interfacial interaction between OMMT and matrix is reduced, resulting in poorer mechanical properties.
4. Conclusions Positron annihilation lifetime measurements were performed. Effects of OMMT content on the positron annihilation parameters and mechanical properties have been discussed. The analysis of positron lifetime results reveals that the dispersion states of nano-scale OMMT layers in epoxy resin/OMMT nanocomposite play an important role in determining the interfacial property and the interaction between the OMMT and epoxy matrix. Exfoliated structure enhances the flexural and impact strengths of nanocomposites due to the strong interfacial interaction between OMMT and epoxy matrix.
Acknowledgements This work is supported by the National Nature Science Foundation and Key Laboratory of Nuclear Solid State Physics of Hubei Province.
References Barcia, F.L., Amaral, T.P., Soares, B.G., 2003. Synthesis and properties of epoxy resin modified with epoxy-terminated liquid polybutadiene. Polymer 44, 5811–5819. Jordan, J., Jacob, K.I., Tannenbaum, R., et al., 2005. Experimental trends in polymer nanocompopsites—a review. Mater. Sci Eng. A 393, 1–11. Ray, S.S., Okamoto, M., 2003. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog. polym. Sci. 28, 1539–1641. Schmidt, D., Shah, D., Giannelis, E.P., 2002. New advances in polymer/layered silicate nanocomposites. Curr. Opin. Solid Mater. Sci. 6, 205–212. Wang, S.J., Wang, C.L., Zhu, X.G., Qi, Z.N., 1994. Structural characteristics of HDPE/CaCO3 polymer composites probed by positron annihilation. Phys. Stat. Sol. A 142, 275–280. Winberg, P., Eldrup, M., Maurer, F.H.J., 2004. Nanocopic properties of silica filled polydimethysiloxane by means of positron annihilation lifetime spectroscopy. Polymer 45, 8253–8282.