Materials Letters 57 (2003) 2940 – 2944 www.elsevier.com/locate/matlet
Effects of nanoparticles SiO2 on the performance of nanocomposites Yaping Zheng a, Ying Zheng b,*, Rongchang Ning a b
a Department of Chemical Engineering, Northwestern Polytechnical University, Xian 710072, China Department of Chemical Engineering, University of New Brunswick, 15 Dineen Drive, P.O. 4400, Fredericton, N.B., Canada E3B 5A3
Received 28 September 2002; accepted 28 October 2002
Abstract New developments in the synthesis of nanoparticles SiO2 have enabled the processing of exciting new nanoparticle/epoxy composites. Ultrasonic and mechanical methods were used to disperse the nanoparticles in epoxy resin. The nanocomposites were characterized by tensile and impact testing as well as TEM studies. Additionally, the effects of nanometer-sized SiO2 particles on free volume of nanocomposites were studied using positron annihilation lifetime spectroscopy. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanocomposites; Epoxy resin; Positron annihilation
1. Introduction Epoxy resins are used in a variety of applications since their properties, such as thermal stability, mechanical response, low density and electrical resistance, can be varied considerably. The important factors influencing their performance are the molecular architecture, curing conditions and the ratio of the epoxy resin and the curing agent(s). The use of an additional phase (e.g. inorganic fillers) to strengthen the properties of epoxy resins has been a common practice. Developments in the synthesis of nanometer-sized particles have made it possible to process nanocomposites. Nanoparticles can fill up the weak microregions of resins to boost the interaction forces at the polymer –
* Corresponding author. Fax: +1-506-453-3591. E-mail address:
[email protected] (Y. Zheng).
filler interfaces. A dramatic increase in the interfacial area between fillers and polymer can significantly improve the properties of the polymer [1]. The reinforcement efficiency is reported to show strong dependence on dispersion of nanoparticles. Well-dispersed nanoparticles can effectively enhance the comprehensive properties of nanocomposites, which are unique and different from any other current composites with typical filler amounts of less than 5 wt.% [2,3]. Both macroscopic and microscopic measurements are normally performed for a newly synthesized composite to achieve an overall understanding. The positron annihilation lifetime spectroscopy (PALS) technique has been widely used in the study of polymeric systems at the molecular level in the recent years. This technique utilizes the interactions between the positrons and the electrons from the host material. Although there has been an observed increase of the number of papers concerning PLAS applications in the
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01401-5
Y. Zheng et al. / Materials Letters 57 (2003) 2940–2944
study of microstructure of polymers and polymeric blends [8,9], little attention has still been addressed to the effect of nanoparticles on nanocomposites [4]. In this paper, an epoxy resin (CYD-128) is processed with various amounts of nanometer-sized SiO2 particles. The SiO2 nanoparticles have shown a positive effect on the mechanical and thermal properties of the nanocomposites. The positron annihilation lifetime spectroscopy technique was applied to probe the microstructure of the nanocomposites.
2. Experiment 2.1. Materials The CYD-128 epoxy resin and curing agents were used for this investigation. The CYD-128 obtained from YueYang Chemical is a diglycidyl ether of bisphenol A resin with an average molecular weight of 385 g/mol. Aromatic hardener (JHB-590) with an acid value of 660 –685 mgKOH/g was produced by DalianJinshi Chemical Industry. Nanoparticles SiO2 of mean diameter 15 nm, purchased from Mingri Nanomaterial, have a density of 0.22 g/cm3 and a surface area of 160 F 20 cm2/g. Hydroxide group content on the surface is 36%. The ultraviolet radiation reflectivity is greater than 83%. SiO2 content is more than 99.9%. 2.2. Nanocomposite preparation The nanometer-sized SiO2 particles pretreated by a coupling agent were added to the epoxy resin at 120 jC to lower the viscosity of the resin. The nanoparticles and resin were dispersed in three different ways as described in Section 3. An appropriate amount of hardener was then added to the epoxy resin as its temperature is lowered to 70 –80 jC. After degasification, the mixture was quickly poured into a preheated steel mold coated with the mold release agent. The mold was kept at 130 jC for 5 h and then at 150 jC for 5 h to form the specimens.
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tester (Instron 1196) at a cross-head speed of 4 mm/ min following ASTM-D638. Ten specimens of each sample were tested and the mean values and standard deviations were calculated. The morphology of composites was examined by AMRAY 1000 model scanning electron microscopy (SEM). A compressive molded specimen was deposited on double-sided scotch and examined at the fracture surface. The specimen was coated with gold to improve SEM imaging. Transmission electron microscope (TEM) examination was made for ultra thin films on a Joel H-600 instrument at an accelerating voltage of 100 kV. These films were obtained from compressive molded plaques after embedding epoxy resin matrix. 2.4. PALS measurement Positron annihilation lifetime spectroscopy (PALS) measurements were carried out at 298 K using a fast – fast coincidence system, with time resolution of 2.9 10 1 s, given by 60Co prompt curve. The 22 Na positron source with activity of approximately 1.6 10 6 Bq was sandwiched between two foils of Mylar with about 10% of source correction. Millions of counts were collected for each spectrum in about 60 min. The samples for positron lifetime have a size of 2 10 10 mm. According to the free-volume model, the lifetime of the o-Ps localized inside a rigid and spherical potential well of radium R0, and free volume of radius R, below which no electron is found, is given by the following expression [4]: k3 ¼ ð1=s3 Þ ¼ 2½1 R=R0 þ 1=2psinð2pR=R0 Þ ð1Þ ˚ is the width of the electron where (R0 R) = 1.656 A layer at the internal surface of the potential well where the Ps annihilates at a constant rate (k3) of 2 ns 1. The total fractional free volume may be estimated by the simple equation: h ¼ CVI3
ð2Þ
2.3. Characterization of the nanocomposites The impact tests were performed using an impact tester (XCJ-400) according to the ASTM-D256. The tensile experiments were carried out with a tensile
˚ 3, I3 in % and C is a constant where V ¼ ð4pR3 Þ=3 A empirically determined from comparison with pressure –volume –temperature data, and is often found to be approximately 1.8 10 3 in polymers.
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3. Results and discussion 3.1. Dispersion of nanoparticles The dispersion of nanometer-sized particles in the polymer matrix is reported to have a significant impact on the mechanical properties of nanocomposites [5]. As the nanoparticles have a strong tendency to agglomerate, homogeneous dispersion of the nanoparticles in the polymer has been considered as a difficult process. A good dispersion may be achieved by surface modification of the nanoparticles under an appropriate processing condition [6]. In this work, three different approaches have been utilized to disperse nanoparticles in the epoxy resin. 1. The epoxy resin CYD-128 was mixed with unpretreated SiO2 nanoparticles using ultrasonic energy under the temperature of 120 jC. 2. The mixture of epoxy and nanoparticles was treated with ultrasonic waves for twenty minutes at the same temperature. The only difference was that SiO2 particle was pretreated with a couple agent. 3. The pretreated SiO2 nanoparticles were dispersed in the epoxy resin by ultrasonic waves for 20 min followed by a high-speed homogenizer with a rotational speed of 24,000 rpm [7]. The properties of the nanocomposites prepared by the three approaches are listed in Table 1 to compare with those of the pure epoxy resin. The experimental observation with eyes indicates that there are agglomerations on the surface of the specimen cast with the first approach while no agglomerations appear on the surface of the other two speci-
Table 1 Effects of nanoparticle dispersion on the properties of nanocomposites Epoxy resin/TiO2 (g/g)
Treatment methods
Tensile strength (MPa)
Tensile modulus (GPa)
Impact strength (kJ m 2)
100/0 100/3 100/3 100/3
– 1 2 3
35.33 38.33 45.88 75.68
3.17 3.21 3.43 3.57
10.2 11.2 12.68 15.94
Fig. 1. TEM photograph of SiO2/epoxy nanocomposites (12,000).
mens, suggesting the coupling agent is quite useful in dispersing nanoparticles in the epoxy. With the assistance of a high-speed homogenizer, a relative uniform distribution of nanoparticles was achieved, evidenced by the TEM study (Fig. 1). From the macroscopic level, the mechanical testing results demonstrate the properties of the nanocomposite with a uniform distribution of nanoparticles are greatly improved. The enhancements in tensile strength, tensile modulus and impact strength has reached up to 114%, 12.6% and 56%, respectively, in comparison with the pure epoxy resin. In contrast, the nanocomposite having a poor distribution of nanoparticles (prepared by the first approach) exhibits little improvement in its mechanical properties. This result confirms the reported observations [5]. 3.2. Free volume of nanocomposites Each spectrum was resolved into three components using the computer program PATFIT (finite-term lifetime analysis). After the background and source correction were subtracted, the variance of fit was less than 1.2. The average radius R of free-volume holes can be evaluated from s3 and I3 is directly proportional to the concentration of free-volume holes. The meas-
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Table 2 Effect of nanometer SiO2 on parameters of free volume CYD-128/SiO2 (g/g)
s3 (ns)
I3 (%)
˚ 3) V (A
h (%)
Impact strength (kJ m 2)
100/0 100/1 100/2 100/3 100/4 100/5
1.6895 F 0.0141 1.6875 F 0.0115 1.7074 F 0.0118 1.7093 F 0.0112 1.6754 F 0.0108 1.7382 F 0.0116
20.1220 F 0.2496 21.7028 F 0.2254 23.0594 F 0.2498 19.0892 F 0.2645 23.4502 F 0.2306 22.5612 F 0.2297
69.13645 68.96629 70.66472 70.82748 67.93922 73.31612
2.504095 2.694171 2.933075 2.433672 2.867739 2.977379
10.2 12.14 13.01 15.94 12.68 10.8
ured s3 and I3 for the nanocomposites are shown in Table 2. The total fractional free volume, h, and the free-volume size, V, computed using Eqs. (1) and (2) are plotted against the amount of SiO2 in Fig. 2. The changes of s3 in Table 2 correspond to the variations of the free-volume size (V) from 69 to 74 ˚ 3. The intensities (I3) of all specimens were generA ally constant with adding the SiO2 nanoparticles, indicating the number of o-Ps trapping sites keeps constant. The total fractional free volume, the products of V and I 3, has little variations. On one hand, the nanoparticles tend to occupy small ‘‘holes’’ in the
epoxy resin and to act as a bridge to make more molecular interconnected, resulting in a reduction in the total free volume as well as an increase in the cross-linking density [8]. Consequently, nanocomposites show a distinct improvement in their mechanical properties such as impact strength (Table 2). It is due to the presence of nanoparticles in free volume that limits the chain segmental motions and reduces the flexibility of the matrix reins. This, on the other hand, regulates the packing density thus the free volume size is likely to expend. Furthermore, there is a larger number of nonpairing atoms covering the surfaces of nanoparticles. This intense electronic density of the nanoparticles may trap a very thin layer of air during the preparation process, which air membrane is no doubt to multiply the free volume sizes [9]. A comprehensive explanation with respect to this contradiction between the theoretical expectations and experimental phenomena is beyond this study. Apparently, more efforts are needed to fully understand the ambiguity. 3.3. Effects of nanoparticles on the impact strength of nanocomposites
Fig. 2. Fracture morphology of the composite impact specimens.
As shown in Table 2, the impact strength of nanocomposites is enhanced as more SiO2 nanoparticles are added to the system until 3 wt.% and then the impact strength drops with further addition of nanoparticles. Inhomogeneous dispersion of nanoparticles may be responsible for it. The presence of excessive nanoparticles makes uniform dispersion impossible even with the high-speed homogenizer. Nanoparticles function in two ways: (1) serving as a binding agent to modify the morphological structure of the CYD-128; (2) acting as stress concentrators to promote cavitation at the particle – polymer boundaries. On the basis of
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through these new surfaces so as to increase the impact strength of the nanocomposite.
4. Conclusions The studies on the morphology and mechanical properties show that the introduction of SiO2 nanoparticles in the CYD-128 matrix polymer has dramatic effects on nanocomposites. Uniform dispersion of nanoparticles is critical to the morphological structure of nanocomposites, which in turn affects the impact strength of the SiO2/CYD-128. Much more interfacial surfaces can be generated between polymer and nanoparticles, which assists in absorbing the stress. The free volume parameters (s3 and I3) of nanocomposites are found to change with the addition of nanoparticles.
References Fig. 3. Fracture morphology of the SiO2 nanocomposite impact specimens.
this proposed morphological structure, numerous cavitation sites will be created at the interface between the SiO2 particles and the amorphous layers. When the matrix is subject to impact, the cavities formed will release the plastic constraint in the matrix and trigger large-scale plastic deformation, significantly improving the fracture toughness of the matrix [10]. However, stress concentration sources that lead to the reduction of impact strength are formed when significant aggregates occur in the nanocomposites. Fig. 2 is the SEM photograph of pure epoxy resin, showing the small crazes and the clear river lines with smooth surface on the fracture surface. On the other hand, the SEM of the nanocomposite with 3 wt.% of SiO 2 (Fig. 3) demonstrates river lines crowded together many Compared with Fig. 2, it is apparent that massive new surfaces were created under impact loading. Much more impact energy can be dissipated
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