Cryogenic mechanical behaviors of carbon nanotube reinforced composites based on modified epoxy by poly(ethersulfone)

Composites: Part B 43 (2012) 22–26

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Cryogenic mechanical behaviors of carbon nanotube reinforced composites based on modified epoxy by poly(ethersulfone) Jiao-Ping Yang a, Zhen-Kun Chen a, Qing-Ping Feng a,⇑, Yin-Hu Deng a, Yu Liu a, Qing-Qing Ni b, Shao-Yun Fu a,⇑ a b

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China Department of Functional Machinery & Mechanics, Shinshu University, 3-15-1 Tokida, Ueda, Japan

a r t i c l e

i n f o

Article history: Available online 28 April 2011 Keywords: A. Epoxy A. Poly(ethersulfone) A. Carbon nanotubes B. Cryogenic mechanical properties

a b s t r a c t Cryogenic mechanical properties are important parameters for epoxy resins used in cryogenic engineering areas. In this study, multi-walled carbon nanotubes (MWCNTs) were employed to reinforce diglycidyl ether of bisphenol F (DGBEF)/diethyl toluene diamine (DETD) epoxy system modified by poly(ethersulfone) (PES) for enhancing the cryogenic mechanical properties. The epoxy system was properly modified by PES in our previous work and the optimized formulation of the epoxy system was reinforced by MWCNTs in the present work. The results show that the tensile strength and Young’s modulus at 77 K were enhanced by 57.9% and 10.1%, respectively. The reported decrease in the previous work of the Young’s modulus of the modified epoxy system due to the introduction of flexible PES is offset by the increase of the modulus due to the introduction of MWCNTs. Meanwhile, the fracture toughness (KIC) at 77 K was improved by about 13.5% compared to that of the PES modified epoxy matrix when the 0.5 wt.% MWCNT content was introduced. These interesting results imply that the simultaneous usage of PES and MWCNTs in a brittle epoxy resin is a promising approach for efficiently modifying and reinforcing epoxy resins for cryogenic engineering applications. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Epoxy resins have applications in cryogenic engineering areas in the temperature range of liquid nitrogen 77 K-liquid helium 4.2 K due to their low cost, easy processability, good thermal, mechanical and insulating properties [1–5]. In order to develop high performance epoxy resins for cryogenic engineering applications, it is necessary to modify brittle epoxy resins [1,4]. Incorporation of soft modifiers such as flexible polyurethane [6], hyperbranched polyester (H30) [7], flexible diamines (D-230 and D-400) [8], nano-sized rubber particles [9] and thermoplastic poly(ethersulfone) (PES) [10] is an effective way to improve their cryogenic mechanical properties including strength, elongation and fracture resistance at low temperatures. However, the modulus of the epoxy resins was inevitably decreased by these soft polymer modifiers. Carbon nanotubes (CNTs) are long cylinders of covalently bonded carbon atoms and have a diameter from a few angstroms to several tens of nanometers across. CNTs have exceptional mechanical properties [11–15] and thus extensive research work has been carried out on carbon nanotube reinforced polymer ⇑ Corresponding authors. Address: Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Tel./fax: +86 10 82543752. E-mail addresses: [email protected] (Q.-P. Feng), [email protected] (S.-Y. Fu). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.04.025

composites [11–17]. The previous research work was mainly focused on room temperature mechanical properties. However, little work has been reported on cryogenic mechanical properties of CNT/polymer composites. In our previous work [18], the addition of MWCNTs to epoxy resins at a proper content led to effective enhancements in the cryogenic tensile strength and Young’s modulus. But the enhancing degree in the cryogenic fracture resistance by introduction of MWCNTs was not satisfactory as that by introduction of soft modifiers such as PES [10]. The major objective of the present work is to combine the advantages of PES and carbon nanotubes in improving the cryogenic mechanical properties of epoxy resins. MWCNTs were employed to reinforce the modified epoxy resins by PES to enhance the overall cryogenic mechanical properties. The tensile strength, modulus and fracture toughness at 77 K are examined for MWCNT reinforced composites. It is observed that the strength, modulus and fracture toughness are all enhanced by the simultaneous usage of PES and MWCNTs. 2. Experimental 2.1. Materials The epoxy resin used in this work was diglycidyl ether of bisphenol-F (DGEBF, D.E.R.354, Dow Chemical Co., USA) with the

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J.-P. Yang et al. / Composites: Part B 43 (2012) 22–26

epoxide weight equivalence in the range 167–174. The curing agent was diethyl toluene diamine (DETD, ETHACURE-100, Albemarle Co., USA), which is a mixture of 2,4- and 2,6-isomers. The thermoplastic polymer poly(ethersulfone) (PES) was kindly provided by Jilin University. The number average molecular weight (Mn) of the PES is 30,000, which can be melted at 130 °C. Multiwalled carbon nanotubes (MWCNTs) produced by Showa Denko K.K. (Japan) were used for preparation of epoxy composites and the MWCNTs have a length of 10–20 lm and a diameter of 50– 116 nm [18,19]. 2.2. Preparation of samples The blends of MWCNTs and DGEBF resin were prepared with a three-roll mill (see Fig. 1a). And the schematic diagram showing the general configuration of the three roll mill for blending epoxy and MWCNTs is exhibited in Fig. 1b. PES was first melted at 130 °C, and then the melted PES with a proper content (15.0 phr) related to the epoxy resin and the stoichiometric amount of curing agent were added to the DGEBF resin/MWCNT blends. The optimal PES content relative to the DGEBF epoxy resin is 15.0 phr at which the modified epoxy system showed the optimal overall mechanical performance [10]. The as-obtained mixture was degassed under vacuum for 30 min and cast in a steel mold, then cured at 80 °C for 8 h and post-cured at 130 °C for 12 h. After curing, the blends were allowed to be naturally cooled to room temperature. Formulations of epoxy composites are listed in Table 1. 2.3. Characterization 2.3.1. Mechanical testing The tensile samples were prepared according to the recommendation of ASTM D638–96. The tensile properties of the cured specimens at 77 K were measured by a WD-10A Mechanical Tester using a 10 kN load cell with a crosshead speed of 2 mm/min. The cryogenic temperature condition was achieved by dipping the clamps and the samples in a liquid nitrogen filled cryostat designed in our laboratory. The entire testing was conducted while the specimen and the loading fixture were submerged in liquid nitrogen [20]. The fracture toughness (KIC) test was carried out using threepoint-bend specimens with dimensions of 90  20  5 mm3 according to the recommendation of GB 4161-84. A pre-crack was made in the specimen by lightly tapping a sharp fresh razor blade into the bottom of the slot with 9 mm depth. The slot was sawed by a HC-400 digital manual dicing cutter. This method can yield a very natural crack over several millimeters. The threepoint-bend specimens was accomplished on a WD-10A Mechanical Tester at a crosshead speed of 2 mm/min. Similar to the cryogenic tensile testing [18,20,21], the specimens and the loading fixture were submerged in liquid nitrogen. The actual crack length was

Table 1 Formulations of MWCNT/PES/epoxy composites. DGEBF (g)

DETD (g)

PES (g)

MWCNTs (g)

100.00 99.80 99.50 99.0 97.50

25.80 25.74 25.70 25.50 25.20

15.00 14.97 14.90 14.80 14.60

0.0 0.2 0.5 1.0 2.5

measured after the fracture testing by an optical microscope (JQC-15J) with the accuracy of 0.01 mm. At least five specimens were tested for each composition. The calculation of the criticalstress-intensity factor (KIC) is referred to the following relationship.

K IC ¼

SP Q BW 3=2

f ðxÞ ¼

a¼

f ðxÞ

ð1Þ

3a1=2 f1:99  að1  aÞð2:15  3:93a þ 2:7a2 Þg 2ð1 þ 2aÞð1  aÞ3=2

a W

ð2Þ

ð3Þ

where S is the tested span, PQ the peak load, f the shape factor, B the specimen thickness (5 mm), W the specimen width (20 mm) and a the crack length. 2.3.2. Scanning electron microscopy (SEM) The fracture surfaces of the broken specimens were observed by scanning electron microcopy (SEM) using a HITACHI S-4300 microscope. Prior to examination, the fracture surfaces of the specimens were cleaned with alcohol and spray coated with a thin layer of evaporated gold to improve conductivity. 3. Results and discussions 3.1. Dispersion of MWCNTs The pristine MWCNTs used in this paper are the same as in our previous work [18,19]. In order to homogeneously disperse MWCNTs in the epoxy matrix, a huge energy input has to be ensured to overcome the nanotube-nanotube adhesion by van der Waals-forces [22]. On the three-roll mill used in this work shown in Fig. 1a and b, the feed roll (1) and apron roll (3) rotate in the same direction while the roll (2) between the feed and apron roll, which called center roll, rotates in the opposite direction. By setting the higher angular velocity of the center roll (2) than that of the feed roll (1) (x3 = 3x2 = 9x1), high shear rates can be gained. The gap size between the rolls was kept constant at 5 lm. As the CNT/epoxy resin suspension is fed into the narrow gap between feed and center rolls, the liquid stuff flows down and covers the adjacent rolls through its surface tension under intensive shear

Fig. 1. (a) The three-roll mill and (b) schematic diagram showing the general configuration of a three roll mill.

J.-P. Yang et al. / Composites: Part B 43 (2012) 22–26

forces. Consequently, the MWCNTs can be evenly dispersed in the epoxy resin matrix as stated in the literature [23]. In this work, MWCNT/PES/epoxy composite samples were dipped in liquid nitrogen for about 10 min that is sufficient for the full thickness of the sample to thermally equilibrate at 77 K and then were quickly broken into two parts. The dispersion of MWCNTs in the epoxy samples was observed from the breached cross-sections of the samples using SEM and shown in Fig. 2. From Fig. 2a and b, it is clear that using the three-roll mill with highshear speed provides a respectable MWCNT dispersion throughout the epoxy matrix with the MWCNT content of 0.2 and 0.5 wt.%. And it is observed that MWCNTs were pulled out from the epoxy matrix. When the MWCNT content is increased to 1.0 wt.% and 2.5 wt.%, CNT aggregates can be clearly observed from Fig. 2c-d. Here the dispersion degree of MWCNTs is lower than that in our previous work [24] due to the increase in viscosity by introduction of PES. The cryogenic mechanical properties of MWCNT/PES/epoxy composites will be discussed below. 3.2. Tensile properties The stress–strain curves of epoxy and MWCNT/PES/epoxy composites at 77 K are shown in Fig. 3. It displays that both neat epoxy resin and MWCNT/epoxy composites show linear relationship between stress and strain, and thus exhibit brittle behaviors at liquid nitrogen temperature. Addition of MWCNTs at proper contents has led to improvements in cryogenic tensile properties. Average and standard deviation values of tensile strength, Young’s modulus and ultimate failure strain at 77 K obtained from the tensile stress–strain curves via the commercial REGER testing Software are listed in Table 2. A significant enhancement in the cryogenic tensile strength has been observed by the addition of MWCNTs at appropriate contents. It reaches 134.19 MPa at the MWCNT content of 0.5 wt.%, corresponding to an improvement of 57.9% when compared with that of the PES modified epoxy resin, followed by a decrease when the MWCNT content was further increased to 1.0 and 2.5 wt.%. In Table 2, the Young’ modulus at 77 K of the MWCNT/PES/ epoxy composites are found to increase with the increase of MWCNT content. This can be owing to the introduction of the

200

Epoxy (15 phr PES) 0.2 wt% CNTs 0.5 wt% CNTs 1.0 wt% CNTs 2.5 wt% CNTs

160

Stress /MPa

24

120

80

40

0 0

1

2

3

4

Strain /% Fig. 3. Typical stress–strain cures of MWCNT/PES/epoxy composites at 77 K.

MWCNTs with a high Young’s modulus. From our previous work [10], the Young’s modulus of the DGBEF/DETD system used in this work is 4.62 GPa. The Young’s modulus of this epoxy system modified by PES is 4.36 GPa. The Young’s modulus of MWCNT/PES/ epoxy composites with the 0.2–2.5 wt.% MWCNTs is 4.73– 5.02 GPa. According to these results, it is clear that the decrease in the Young’s modulus of pure DGBEF/DETD system caused by the introduction of flexible PES can be offset by the addition of MWCNTs. It can also be clearly seen from Table 2 that the failure strain of epoxy resins at 77 K increases initially up to the maximum at the MWCNT content of 0.5 wt.% and then decreases with further increasing the MWCNT content. When the MWCNT content is high, the aggregated MWCNTs observed as shown in Fig. 2c and d would give rise to weak MWCNT/epoxy interactions and act as stress concentration sites, leading to the reduction of the failure stain. Fig. 4 showed the MWCNT pullout from the tensile fracture surfaces of MWCNT/PES/epoxy composites at 77 K with the MWCNT content of 0.2 and 0.5 wt.%. And the diameters of the pulled-out CNTs were measured using the software SemAfore 4.0 as shown in Fig. 4. It can be seen from the Fig. 4 that the MWCNT diameters are in the range of 100–200 nm with an average value of 150 nm,

(a)

(b)

(c)

(d)

Aggregates Aggregates

Fig. 2. Dispersion of MWCNTs in the epoxy composites with the MWCNT content of (a) 0.2 wt.%,(b) 0.5 wt.%,(c) 1.0 wt.% and (d) 2.5 wt.%.

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J.-P. Yang et al. / Composites: Part B 43 (2012) 22–26 Table 2 The tensile properties of MWCNT/PES/epoxy composites.

2.1

Young’s modulus (GPa)

Failure strain (%)

0 0.2 0.5 1.0 2.5

84.9 ± 53.69 120.48 ± 6.23 134.19 ± 9.07 117.44 ± 7.16 78.66 ± 8.87

4.36 ± 0.13 4.73 ± 0.06 4.80 ± 0.05 4.92 ± 0.08 5.02 ± 0.06

1.95 ± 0.08 2.54 ± 0.06 2.80 ± 0.03 2.39 ± 0.05 1.57 ± 0.07

2.0 1/2

Tensile strength (MPa)

K IC /MPa.m

MWCNTs content (wt.%)

1.9

1.8

which is much higher than that of the pristine MWCNTs (50– 116 nm with an average value of 83 nm) [18]. This indicates that the surfaces of pulled out MWCNTs are quite rough and have been attached with the PES modified epoxy matrix. Namely, there is a good interfacial adhesion between MWCNTs and PES modified epoxy matrix at 77 K. When the temperature was cooled down to cryogenic one, both epoxy matrix and MWCNTs will contract. Since the coefficient of thermal expansion (CTE) for the epoxy matrix (about 5.1  105 K1) [25] is much higher than that of MWCNTs (lower than 0.73–1.49  105 K1) [17] in the temperature range of liquid nitrogen temperature and RT, the contraction of epoxy matrix due to the difference in temperature is bigger than that of MWCNTs. Thus, a strong interfacial adhesion between MWCNTs and epoxy matrix within the MWCNT/PES/epoxy composites can be gained at cryogenic liquid nitrogen temperature. It is well known that the tensile strength of MWCNTs is much higher than that of the epoxy resins [11]. The good dispersion of MWCNTs as shown in Fig. 2a and b and the good interfacial adhe-

0.2 wt% MWCNTs

0.5 wt% MWCNTs

1.7

0.0

0.5

1.0

1.5

2.0

2.5

MWCNT content /wt% Fig. 5. Fracture toughness KIC at 77 K of MWCNT/PES/epoxy composites.

sion both are in favor of achieving high strength composites. Thus the tensile strength at 77 K was increased for the MWCNT/PES/ epoxy composites with the 0.2 wt.% and 0.5 wt.% MWCNT content. The decrease in the tensile strength at 77 K for the MWCNT/PES/ epoxy composites with a relatively high content of 1.0 wt.% and 2.5 wt.% MWCNT content was also observed, possibly resulting from the agglomeration of MWCNTs in the epoxy matrix at the relatively high CNT contents as shown in Fig. 2c and d. These agglomerations might act as defects decreasing the cryogenic tensile strength. 3.3. Fracture toughness The critical-stress-intensity factor KIC at 77 K of MWCNT/PES/ epoxy composites is plotted as a function of the MWCNT content in Fig. 5. KIC was evaluated in terms of Eq. (1) based on the peak load. Comparison of the results at peak load does not give an exact representation since the crack length is different at this point but, it does give a good indication. The graph includes the average values and the standard deviations corresponding to five tests for the epoxy resin and the MWCNT/PES/epoxy composites with different MWCNT contents. The fracture toughness increased with increasing the MWCNT content. The maximum KIC reached 2.02 MPa.m1/2 with an improvement of 13.5% at 0.5 wt.% MWCNT content from 1.78 MPa m1/2 of the PES modified epoxy matrix. Although the KIC at 77 K of MWCNT/PES/ epoxy composites was decreased by the further increase of MWCNT content to 1.0 wt.% and 2.5 wt.%, it was still higher than that of the epoxy matrix. When MWCNTs are pulled out from the epoxy matrix, the pullout energy would make a significant contribution to the fracture toughness [26,27]. Hence the KIC at 77 K of MWCNT/PES/epoxy composite increased with the increase of CNT content at low CNT contents. However, aggregates of MWCNTs would give rise to weak MWCNT–epoxy interactions at high CNT contents and high stress concentrations similar to clay–epoxy composites [20,28]. Therefore, the slight decrease of KIC at 77 K would be caused by MWCNT aggregation for the MWCNT/PES/epoxy composites with 1.0 and 2.5 wt.% MWCNT content, which can be observed from Fig. 2c and d. From the failure strain results of MWCNT/PES/epoxy composites shown above, the brittle epoxy matrix has been effectively toughened by the introduction of MWCNTs, which is consistent with the results for the fracture toughness (KIC). 4. Conclusions

Fig. 4. SEM images showing MWCNT pullout from the tensile fracture surfaces of MWCNT/PES/epoxy composites at 77 K.

MWCNTs have been employed to enhance the cryogenic mechanical properties of DGBEF/DETD epoxy system modified by PES. The results showed that both the tensile strength and fracture

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toughness (KIC) at 77 K of MWCNT/PES/epoxy composites have been simultaneously enhanced with a maximum value at the MWCNT content of 0.5 wt.%, corresponding to an improvement of 13.5% and 57.9% respectively, compared to that of the PES modified epoxy matrix. At the same MWCNT, the tensile modulus at 77 K was also improved by 10.1%. The decrease in the Young’s modulus of the pure DGBEF/DETD epoxy system caused by the introduction of flexible PES can be offset by the increase due to the addition of MWCNTs. These interesting results indicate that the CNTs are promising fillers for enhancing the cryogenic mechanical properties of epoxy resins for cryogenic engineering applications. Acknowledgements We gratefully acknowledge that this work is funded by National Natural Science Foundation of China (Nos. 10972216, 51073169 and 11002141), the National Basic Research Program of China (No. 2010CB934500), and the State Key Laboratory of Explosion Science and Technology (No. KFJJ10-3M). References [1] Ueki T, Nishijima S, Izumi Y. Designing of epoxy resin systems for cryogenic use. Cryogenics 2005;45(2):141–8. [2] Yokozeki T, Ogasawara T, Aoki T, Ishikawa T. Experimental evaluation of gas permeability through damaged composite laminates for cryogenic tank. Compos Sci Technol 2009;69:1334–40. [3] Nishijima S, Honda Y, Okada T. Application of the positron annihilation method for evaluation of organic materials for cryogenic use. Cryogenics 1995;35(11):779–81. [4] Nishijma S, Honda Y, Tagawa S, Okada T. Study of epoxy resin for cryogenic use by positron annihilation method. J Radioanal Nucl Chem 1996;211(1):93–101. [5] Nobelen M, Hayes BS, Seferis JC. Cryogenic microcracking of rubber toughened composites. Polym Compos 2003;24(6):723–30. [6] Fu SY, Pan QY, Huang CJ, Yang G, Liu XH, Ye L, et al. A preliminary study on cryogenic mechanical properties of epoxy blends matrices and SiO2/epoxy nanocomposites. Key Eng Mater 2006;312:211–6. [7] Yang JP, Chen ZK, Yang G, Fu SY, Ye L. Simultaneous improvements in the cryogenic tensile strength, ductility and impact strength of epoxy resins by a hyperbranched polymer. Polymer 2008;49(13–14):3168–75. [8] Yang G, Fu SY, Yang JP. Preparation and mechanical properties of modified epoxy resins with flexible diamines. Polymer 2007;48(1):302–10. [9] Chen ZK. Cryogenic mechanical properties of nanofiller modified DGEBF epoxy resins. PhD thesis. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, May 2009.

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