Mechanical Properties of Nanocomposites from Ball Milling Grafted Nano-Silica/Polypropylene Block Copolymer

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1 Mechanical Properties of Nanocomposites from Ball Milling Grafted Nano-Silica/Polypropylene Block Copolymer Mechanical Properties of Nanocomposites from Ball Milling Grafted Nano-Silica/Polypropylene Block Copolymer Wen Hong Ruan 1,, Ming Qiu Zhang *, Min Zhi Rong and Klaus Friedrich 3 1 Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Zhongshan University, Guangzhou 51075, P.R. China Materials Science Institute, Zhongshan University, Guangzhou 51075, P.R. China 3 Institute for Composite Materials (IVW), University of Kaiserslautern, D Kaiserslautern, Germany Received: 4 August 003 Accepted: 5 November 003 SUMMARY Nanocomposites consisting of ethylene-propylene block copolymer filled with nanosilica (pre-treated by ball milling aided graft polymerisation) were prepared by a conventional compounding technique. The mechanical performance of the nanocomposites and the morphological changes induced by the addition of the nanoparticles were investigated. It was confirmed that the copolymer chains were chemically bonded to the silica particles during mechanochemical grafting in the ball mill. Morphology observations revealed that strong interfacial interaction between the grafting polymer (i.e., poly(butyl acrylate)) and the matrix (i.e., ethylene-propylene block copolymer) is critical for bringing the reinforcing effect of the nanoparticles into play. Owing to the enhanced interfacial interaction, the grafted nanoparticles exhibited a nucleating effect and improved the crystallinity of the polymer matrix. In addition, the particles also had a toughening effect on the amorphous polypropylene phase because of entanglements between the grafting polymer and the matrix. Insufficient interaction between the nanoparticles and ethylene-propylene rubber phase of the copolymer matrix actually introduces restraints. As a result, the tensile strength and modulus of the nanocomposites can be significantly increased by using low loadings of the treated nanoparticles. The decrease in the notched Charpy impact strength was insignificant in comparison to that of conventional micron-scale inorganic particles filled composites. The technical route proposed is therefore feasible for fabricating polymer composites with inorganic nanoparticles. INTRODUCTION Polymer based nanocomposites, in which inorganic nanoparticles are dispersed in organic polymer matrices, have attracted materials scientists attention owing to their unique properties, resulting from their nano-scale microstructure. Because of their large surface area, nanoparticles interact with the polymer matrix and carry a load effectively. The mechanical properties of these composites can be significantly improved at low filler content if the ultrafine phase dimensions of the nanoparticles are maintained 1,. *Correspondence author: Prof. Ming Qiu Zhang, Materials Science Institute, Zhongshan University, Guangzhou 51075, P.R. China. Tel.: Fax: ceszmq@zsu.edu.cn To make successful nanocomposites, it is very important to be able to disperse the nanoparticles throughout the matrix. However, a homogeneous dispersion of nanoparticles is very difficult to achieve, because they have a high surface energy, and tend to agglomerate. Moreover hydrophilic nanoparticles are incompatible with hydrophobic polymers. Consequently, a number of loose clusters of particles appear in the composite materials and lead to properties even worse than those of conventional inorganic/organic systems. To break up the agglomerates, researchers have proposed two approaches: - (a) in-situ polymerisation of monomers in the presence of nanoparticles 3 and (b) intercalation polymerisation techniques 4. Although nanoparticles can be dispersed uniformly, these methods involve complex processing procedures, special conditions and high cost and are limited to a laboratory scale. In Polymers & Polymer Composites, Vol. 1, No. 4,

2 Wen Hong Ruan, Ming Qiu Zhang, Min Zhi Rong and Klaus Friedrich addition, most nanoparticles available on the market are non-layered inorganic particulates. By examining the current technical position and the feasibility of all the available processing methods, we inferred that melt blending nanoparticles with a polymer is the best compounding technique for the mass production of nanocomposites at low cost. The problem of melt blending is that nanoparticle agglomerates are hard to cleave with the limited shear forces offered by a conventional mixer in polymer melts. This is true even when a coupling agent is used 5. (Since the coupling agent can only react with the exterior particles, the agglomerates will maintain their friable structure in the composites, and provide neither reinforcing nor toughening effects 6.) Many researchers consider that it might be impossible to achieve a nano-scale dispersion of the particles during melt mixing, owing to the high viscosity of the polymer melt, as well as the poor interaction between the hydrophilic fillers and the hydrophobic matrix 7-9. Modification of nanoparticle agglomerates to utilise non-layered inorganic nanoparticles has become the focus of a new research initiative in our group. Irradiation grafting polymerisation was used to modify the nanoparticles and then the grafted particles were mechanically mixed with the polymer as usual 10. The results showed that the strength and toughness of the composites could be improved by the addition of quite a small amount of grafted nanoparticles (typically less than 3% by volume), This finding is in striking contrast to experience with micrometer-sized particulate composites, in which 0% by volume of the fillers are usually needed for performance enhancement 15. A double percolation mechanism of stress volumes, characterised by the appearance of connected shear yielded networks throughout the composite, was proposed to explain the special effects generated by small amounts of grafted nanoparticles 14. Under these circumstances, a homogeneous distribution of the nanoparticles is no longer critical, because all the nanoparticles are connected by the molecules of the grafting polymers, and stress can be easily transferred to the particles through the entangled grafting polymer chains and matrix chains. In the work presented here, to develop a more satisfactory industrial route for the grafting polymerisation treatment of inorganic nanoparticles, nano-silica (SiO ) was first treated by ball milling in the presence of butyl acrylate monomer. It was expected that mechanochemically initiated in-situ grafting polymerisation of the monomer 16 onto the nanoparticles would take place. Then the modified nanoparticles were melt compounded with ethylene propylene block copolymer, synthesised by introducing a small amount of polyethylene segments onto polypropylene backbones via block copolymerisation. It was chosen as the matrix because of its practical and theoretical importance, as well as increasing demands for special applications of the polymer. The mechanical performance of the nanocomposites, and the possible morphological changes induced by the addition of the nanoparticles, are discussed in detail to explain the role of the treated nanoparticles and the feasibility of the method suggested above. EXPERIMENTAL Ethylene-propylene block copolymer EPS30R was supplied by the Qilu Petrochemical Industrial Co., China. It has a melt flow index of 1.9 g/10 min (ASTM D569-90). Precipitated silica with an average primary particle size of 10 nm and a specific surface area of 640 m /g was produced by Zhejiang Zhoushan Mingri Nanomaterials Ltd., China. Commercial butyl acrylate was used as the grafting monomer without further purification. Commercial isobutyronitrile (AIBN) was used as initiator. A typical mechanochemically initiated in-situ grafting polymerisation onto nano-sio proceeded as follows. The nanoparticles were preheated at 140 o C under vacuum for 5 h to eliminate any adsorbed water on the surface of the particles. Then, a mixture of monomer/nanoparticles (/1 by weight) and a certain amount of initiator was ground by a planetary ball miller at 500 rpm for 4 h under atmospheric pressure at room temperature. The concentrations of the monomer and the initiator, the manner of incorporation of the components, and the milling conditions, were changed in order to study their influence on the reaction processes. Certain amounts of milling products were extracted by acetone in a Soxhlet apparatus for 48h to isolate the homopolymerised butyl acrylate, which was generated simultaneously with the grafting poly(butyl acrylate) (PBA). The residual material was then dried under vacuum at 80 o C for 1 h. The increase in weight of nano-sio caused by the presence of the 58 Polymers & Polymer Composites, Vol. 1, No. 4, 004

3 Mechanical Properties of Nanocomposites from Ball Milling Grafted Nano-Silica/Polypropylene Block Copolymer grafting polymer was determined by a Netzsch TG 09 thermogravimetric analyser (TGA) under a nitrogen atmosphere. The chemical structures of the nanoparticles before and after the modification were characterised using a Bruker Equinox 55 Fourier transform infrared spectroscope (FT-IR). The nanoparticles were compounded directly with the EPS30R matrix in the mixer of a Haake Rheocord 300p torque rheometer at 180 o C for 10 min. The rotation speed of the mixing rotors was set to 60 rpm. Then the compounds were broken up and injection moulded into standard bars for mechanical tests with a Y-350 vertical injection moulding machine at 00 o C. Tensile testing was conducted on ASTM D Type V specimens by means of a Hounsfield H10K- S universal testing machine at a crosshead speed of 50 mm/min. The notched Charpy impact strength was determined according to ISO 179-, using an API advanced pendulum impact device at a rate of 3.8 m/s. The dimensions of the specimens were mm 3 and they were notched with an ASN automatic sample notcher. The initial crack length was mm and the span was 40 mm 17. The fractured surfaces were studied by a JEOL-5400 scanning electron microscope (SEM). The nonisothermal melting and crystallisation behaviour of the materials was examined by a TA MDSC910 differential scanning calorimeter (DSC). The heating and cooling rates were both 10 o C/min. Dynamic mechanical analysis (DMA) was carried out on a TA DMA 980 analyzer with a dual cantilever clamp under a nitrogen atmosphere. The test frequency was 1Hz and the test temperature was raised from -100 o C to +100 o C at 3 o C/min. RESULTS AND DISCUSSION 1. Effect of Mechanochemical Grafting Polymerisation onto Nano-SiO Particles Since the present programme was designed to study the effect of modified nano-silica on the mechanical behaviour of PP composites, any variation in the chemical structure of the particles under ball milling pre-treatment needed to be known at the very beginning. The FT-IR spectra of untreated and treated nano-silica (from which the homopolymer had already been extracted, see EXPERIMENTAL section) are shown in Figure 1. In comparison with the spectrum of SiO asreceived, the absorption peak at 1717 cm -1 in the spectrum of the treated sample indicates the existence of a carbonyl group, proving that polybutyl acrylate (PBA) has been chemically bonded to the surface of the nano-silica during the grinding process. The weighing data and the thermogravimetric analysis of the samples also suggest that using in-situ grafting polymerisation in a mechanochemical environment works. Changing the concentrations of the monomer and the initiator and the milling conditions (such as milling time and speed), enabled a series of modified nano-silicas with different monomer conversions and percentage grafting to be obtained. The PBA grafted nano-silica (denoted by SiO -g-pba) with a conversion of 0.85% and a percentage grafting of 5.96% was chosen to prepare the composites studied thereafter. Figure 1. FT-IR spectra of SiO as-received and grafted SiO Polymers & Polymer Composites, Vol. 1, No. 4,

4 Wen Hong Ruan, Ming Qiu Zhang, Min Zhi Rong and Klaus Friedrich. Tensile Properties of the Nanocomposites Figure shows the results of tensile testing of the nanocomposites as a function of silica content. The introduction of untreated and treated nano-silica improved the tensile strength of the composites over almost the whole range of filler loading (Figure (a)) The decrease in tensile strength of SiO /EPS30R above 0.84 vol% can be explained by a change in the dispersion status of the fillers, leading to weakened interfacial interactions. It is believed that a higher filler loading is detrimental to its dispersion in the polymer matrix. Compared to the untreated samples, the tensile strength of SiO -g-pba/eps30r was improved much more positively. Such a strong dependence of tensile strength on filler loading is hard to find in micron particulates composites. For example, the tensile strength of the same polymer decreases remarkably with a rise in the amount of microsized calcium carbonate 18. In addition, the current grafted nanoparticles seem to have a higher reinforcing capacity than those treated by irradiation-graft polymerisation. In the case of poly(methyl methacrylate) grafted nano-silica/isotactic polypropylene, in which the grafting polymers were attached to the particles surfaces by γ-ray irradiation, the highest rate of tensile strength increase was about 9%. It was much lower than 35% for SiO -g-pba/ EPS30R, as illustrated in Figure (a). During ball milling, the grafting monomers can penetrate into the agglomerated nanoparticles easily, owing to their low molecular weight, and they can react with the activated sites of the nanoparticles inside, as well as outside, the agglomerates. The main roles of milling are, apart from providing mechanical stress for the mechanochemical reaction, homogenisation of the reaction mixture and disintegration of the severely agglomerated nanoparticles. As a result, the following effects can be obtained: (i) (ii) (iii) Agglomerates of the pretreated nanoparticles become smaller and much stronger because they turn into a nano-composite microstructure, consisting of the primary particles and the grafted, homopolymerised secondary polymer An increase in the hydrophobic nature of the nanoparticles due to the grafting polymers is beneficial for filler/matrix compatibility; The filler/matrix interaction is enhanced by inter-diffusion and entanglement between the grafting polymer and the polymer matrix. It is well known that the interfacial adhesion and particle dispersion markedly influence the mechanical behaviour of particulate filled polymer composites Because of the above-mentioned effects, it can be inferred that nanoparticles modified by ball milling would be very effective in improving the strength of the thermoplastic matrix. Since the polymer matrix used here was an ethylenepropylene block copolymer obtained by sequential polymerisation in the reactor, it consisted of polypropylene (PP) homopolymer, amorphous ethylene propylene rubber (EPR), crystallisable propylene ethylene copolymer and polyethylene (PE) homopolymer as well. To facilitate our understanding of the reinforcing mechanism of the nanoparticles, the influence of the particles on the crystalline characteristics of the matrix needed to be investigated. Table 1 reports DSC studies of non-isothermal crystallisation and melting Table 1. Non-isothermal crystallisation and melting data of EPS30R and its nanocomposites Samples T ma ( C) T cb ( C) T c d ( C) X c (%) EPS30R e SiO / EPS30R f SiO -g-pba/eps30r a T m denotes the peak melting temperature b T c denotes the peaking crystallisation temperature c T = T - T, denoting the supercooled temperature m c d X c denotes the matrix crystallinity econtent of SiO : 1.30 vol% fcontent of SiO : 1.36 vol% 60 Polymers & Polymer Composites, Vol. 1, No. 4, 004

5 Mechanical Properties of Nanocomposites from Ball Milling Grafted Nano-Silica/Polypropylene Block Copolymer Figure. Tensile properties of PP block copolymer based nanocomposites as a function of nano-sio volume fraction: (a) tensile strength, (b) Young s modulus, and (c) elongation-to-break Polymers & Polymer Composites, Vol. 1, No. 4,

6 Wen Hong Ruan, Ming Qiu Zhang, Min Zhi Rong and Klaus Friedrich behaviour of the matrix and its nanocomposites. The experimental data demonstrate that both untreated and PBA-grafted SiO particles exerted a nucleating effect on the crystallisation of the matrix polymer. The supercooling temperature, T, of the polymer decreased with the addition of untreated SiO and SiO -g-pba, indicating that the crystallisation became easier in the nanocomposites owing to the nucleation effect of the fillers. On the other hand, the results show that the melting point of EPS30R was not affected by the fillers, meaning that no transition of the crystal form occurred. However, the crystallinity increased significantly when the SiO -g-pba particles were incorporated. Some studies have shown that filler particulates have an important influence on the crystallisation of polymer matrices 3-5, both because the fillers act as nucleation agents, and also because the filler/matrix interfacial stress can induce changes in the microstructure (lamellar ordering). Since uniformly dispersed fillers produce a higher interfacial stress accumulation and stronger interfacial bonding ensures better interfacial stress transfer, both factors facilitate interfacial stress-induced crystallisation. As mentioned above, the composites based on nanosilica grafted by ball milling has enhanced interfacial adhesion and more uniform particle dispersion than was the case with the untreated particles. Therefore, SiO -g-pba filled composites have higher crystallinity and hence a more pronounced improvement in their tensile strength. In general, the tensile strength of a particulate composite is reduced with filler content, following a power law in the case of poor filler/matrix bonding 6, 7. The results given in Figure (a) demonstrate that the reinforcing effect of nanoparticles on the polymer can be realised so long as the particles are grafted and properly dispersed in the matrix polymer. Figure (b) illustrates the Young s modulus of the composites as a function of silica content. Again, both the untreated and the treated nanoparticles can impart the high stiffness of the fillers (the modulus of silica =70 GPa) to the matrix polymer. The modulus of the composites filled with untreated silica increased almost linearly with the addition of the filler, while SiO g- PBA weakens the stiffening effect of nano-sio. Since the tensile modulus is determined within a small strain range, relatively soft layers at the particles/ matrix interface formed by the grafting of PBA and the homopolymerised PBA introduced onto the nanosilica tend to hinder a complete stress transfer, and decrease the stiffening efficiency of the fillers 8. Unlike the tensile strength and modulus, the incorporation of untreated and treated SiO reduced the elongation-to-break of the composites (Figure (c)). An improvement in the crystallinity of the matrix was detected by DSC and might be one of the reasons for the reduction in elongation-to-break. On the other hand, since amorphous ethylene-propylene rubber (EPR) is one of the main compositions of the matrix polymer, its content and properties would be the main influence on the toughness of the polymer. The declining trends exhibited in Figure (c) imply that the fillers caused a reduction in matrix deformation, due to the introduction of mechanical restraints to the rubber phases. Owing to the fact that the grafting of PBA onto the nanoparticle surfaces makes an effective contribution to interfacial viscoelastic deformation and matrix yielding, the elongation-to-break of SiO g-pba/eps30r composites was still higher than that of SiO /EPS30R composites. 3. Impact Resistance of the Nanocomposites Figure 3 illustrates the notched Charpy impact strength of the EPS30R based composites as a function of nano-silica content. Although both untreated and PBA grafted nano-sio particles have a negative effect, the trends in the impact resistance are quite different for the two composites. On the whole, the impact strength of the composites filled with untreated SiO decreased more steeplywith increasing filler content, which is similar to the behaviour observed in micro-particulate filled polymeric composites with poor interfacial bonding 9, 30. With respect to SiO - g-pba/eps30r composites, the impact strength showed a reduction as the nanoparticles were added and then increased slightly with a further rise in the particulate fraction. The decrease in the impact toughness of SiO -g-pba/eps30r composites was about 6% compared with the unfilled matrix. Recalling that the impact strength was decreased by 46% in micro-sized calcium carbonate filled EPS30R 18, 46% in talc filled propylene-ethylene block copolymer 31, and 58% in glass spheres filled propylene-ethylene block copolymer 31, respectively, the conclusion can be drawn that the loss of impact strength in grafted nanoparticle-filled EPR30S was insignificant in comparison to conventional micronscale inorganic particle filled composites. Many explanations have been proposed for brittle-totough transitions in toughened PP Cavitation of the rubber particles, absorption of impact energy by matrix crazes, and overlapped stress fields that inhibit crack propagation are the main mechanisms of plastic deformation. Thus, the content, size and spatial packing of the amorphous EPR phase and its 6 Polymers & Polymer Composites, Vol. 1, No. 4, 004

7 Mechanical Properties of Nanocomposites from Ball Milling Grafted Nano-Silica/Polypropylene Block Copolymer Figure 3. Notched Charpy impact strength of PP block copolymer based nanocomposites as a function of nano-sio volume fraction compatibility with PP homopolymers in the matrix polymer EPS30R used in the present work are important parameters for controlling the fracture toughness of the composites. The addition of the nano-fillers indeed introduces mechanical restraints and reduces deformationability of the rubber phases, reducing the impact strength of the composites as a result. Nevertheless, compared to the untreated case, the mechanical load seems to be more effectively transferred from the matrix to the modified particles through the interfacial bonding effect of the grafting polymer PBA, which can result in interdiffusion and entanglements between the molecules of the grafting polymers and those of the matrix. The inherent flexibility of the PBA chains also makes a contribution to the resistance to crack propagation. Thus it can be concluded that the slight increase in the notched Charpy impact strength of SiO -g-pba/eps30r composites over 0.4 vol% of SiO might be attributed to the increased amount of grafting PBA. Figure 4. SEM micrographs of tensile fractured surface of (a) neat EPS30R, (b) SiO as-received/eps30r (content of SiO =1.30 vol%), and (c) SiO -g-pba/eps30r (content of SiO =1.36 vol%) 4. Fractography Fractography helps us to understand the fracture processes occurring during mechanical testing. Figure4 shows SEM micrographs of the tensile fractured surfaces of unfilled EPS30R and its composites. The neat EPS30R had an uneven appearance, full of relatively smooth bumps, (Figure 4(a)), indicating that although ductile failure occurred, the resistance of the polymer to crack propagation was still weak. In Figure 4(b) the elongated bumps could be observed on the fracture surface of the composites containing untreated SiO, accompanied by a few fibrils. These might have been responsible for the improvement in tensile strength Polymers & Polymer Composites, Vol. 1, No. 4,

8 Wen Hong Ruan, Ming Qiu Zhang, Min Zhi Rong and Klaus Friedrich of the composites. However, many particle agglomerates ( 1 µm in size) were faintly observed in the matrix. In contrast, the fracture surface of the nanocomposite incorporating PBA-grafted nanoparticles was full of extensive matrix fibrils (Figure 4(c)), showing clear evidence of plastic stretching of the matrix ligaments. During the tensile test, the modified nanoparticles would have acted as stress concentrators and the yielding process of the matrix propagated through the ligaments between the dispersed particles, when interfacial interaction was strong enough. In accordance with the model describing double percolation of yielded zones 14, the appearance of extensive fibrils would result from the superposition of stress volumes around the nanoparticles. Therefore, it can be proved that the grafting polymers on the nanoparticles enhanced filler dispersion and interfacial interaction, dissipating more energy through matrix stretching, which might account for the measured reinforcing effects in SiO - g-pba/eps30r composites. Figure 5. SEM micrographs of notched Charpy impact fractured surface of (a) neat EPS30R, (b) SiO as-received/ EPS30R (content of SiO =1.30 vol%), and (c)~(e) SiO -g- PBA/EPS30R (content of SiO =1.36 vol%) Figure 5 illustrates some fracture surface micrographs of those specimens tested by notched Charpy impact measurements. Neat EPS30R (Figure 5 (a)) shows a ductile fracture surface, similar to the other ethylenepropylene block copolymer. Addition of untreated nanoparticles made no evident change to the fracture surface of the composites (Figure 5(b)) except for the appearance of the particle agglomerates under high magnification. In contrast, the micrographs of SiO - g-pba/eps30r composites showed distinct features (Figure 5(c)-(e)). A number of concentric matrix circles around particle-like object could be found (Figure 5(c)). From the magnified photomicrograph in Figure 5(d) one can clearly see that the particle-like objects introduce cavities, followed by a circular movement of the matrix. High magnification picture taken from the particle-like object (Figure 5(e)) demonstrates that it indeed was a cluster of microfibrils, which might be composed of stretched matrix and modified nanoparticle agglomerates. Owing to the interaction between the molecules of the grafting polymers and the matrix, the microfibrils were intertwined to form a network. These networks actually introduced restraints to the rubber phases of the matrix polymer, and acted as stress concentrators, inducing multiple crazing and plastic deformation of the matrix. Once debonding of the modified nanoparticle agglomerates occurred, the shear stress was locally relieved and the deformation circles formed due to a gradual contraction of the matrix. The presence of concentric circles around nanoparticle agglomerations must have consumed a considerable amount of energy during the yielding process. It is noteworthy that these 64 Polymers & Polymer Composites, Vol. 1, No. 4, 004

9 Mechanical Properties of Nanocomposites from Ball Milling Grafted Nano-Silica/Polypropylene Block Copolymer circles overlapped and pervaded the entire matrix in the deformation zone, which is actually an indication of the percolation behaviour induced by the modified nanoparticles. In consideration of the results of impact testing (Figure 3), it might be deduced that the matrix ductility had been suppressed by the constrained rubber phase. In other words, the toughening effect of modified nanoparticles was somewhat shielded by a reduction in matrix deformability caused by restraints to the rubber phase. 5. Dynamic Mechanical Analysis of the Nanocomposites Dynamic mechanical analysis (DMA) is able to reflect the interaction between the fillers and the polymer matrix by measuring the viscoelastic response of the composites. Figure 6 shows DMA curves of EPS30R and its composites, and the characteristic parameters are listed in Table. Separately detected tan δ peaks of amorphous PP and EPR glass transitions indicate that phase separation took place during the copolymerisation of EPS30R. Incorporating untreated and modified nano-silica into the matrix did not affect the glass transition temperature of the EPR phase, but it decreased the area under the peak of the loss modulus of the EPR phase (S 1 ), a parameter related to the degree of interaction between the phases. The reduction in S 1 means that the interaction between the nanoparticles and the rubber phase of the matrix polymer was not strong enough, and the nanoparticles in fact introduced restraints to the rubber phase. In contrast, Figure 6. Loss tangent, tan δ, versus temperature of PP block copolymer and its nanocomposites. Content of SiO in SiO as-received/eps30r=1.30 vol%, and content of SiO in SiO -g-pba/eps30r = 1.36 vol%. Table. Characteristic DMA parameters of EPS30R and its nanocomposites Samples T a g1 ( C) T b g ( C) E 1 c ( GPa) d S 1 ( MPa min) e S (MPa min) EPS30R f SiO / EPS30R g SiO -g-pba/eps30r a T g1 denotes the peak glass transition temperature of amorphous b T g denotes the peak glass transition temperature of PP phase c E 1 denotes the storage modulus corresponding to the onset of glass d S 1 denotes the area under the peak of loss modulus of EPR phase e S denotes the area under the peak of loss modulus of PP phase f Content of SiO : 1.30 vol% g Content of SiO : 1.36 vol% EPR phase transition of EPR phase Polymers & Polymer Composites, Vol. 1, No. 4,

10 Wen Hong Ruan, Ming Qiu Zhang, Min Zhi Rong and Klaus Friedrich a shift of the T g of the PP phase to lower temperature was observed for the nanocomposites, especially for SiO -g-pba/eps30r. Meanwhile the area under the peak of the loss modulus of the PP phase (S ) was slightly increased, indicating that actually the nanoparticles have a toughening effect on the amorphous PP phase due to the enhanced interaction by the entanglement between the molecules of grafting polymer and those of the amorphous PP phase. On the other hand, the fillers increased the storage moduli, E 1, of the composites, and E 1 of SiO /EPS30R was higher than that of the SiO -g-pba/eps30r, due to the relatively low stiffness at the particle/matrix interface in the latter composites. CONCLUSIONS This work shows that the tensile performance of ethylene-propylene block copolymer can be improved by a low loading of nano-silica, pre-treated by ball milling-initiated graft polymerisation and incorporated into the matrix polymer using a conventional compounding technique. It is confirmed that the PBA chains are chemically bonded to the surfaces of nano-silica through ball milling. Hence an increase in the hydrophobicity of the nanoparticles due to the presence of the grafting polymers improves filler/matrix compatibility, and the entanglement of the grafting polymer and the matrix substantially enhances the filler/matrix interaction. As a result, the reinforcing effects of the nanoparticles on polymeric materials could be brought into play. The tensile strength and Young s modulus of the EPS30R PP block copolymer are significantly increased, while the decrease in the notched Charpy impact strength is insignificant in comparison to conventional micron-scale inorganic particle composites. Morphological observations of the composites prove the effectiveness of the grafted nanoparticles. Characteristic parameters of the composites measured by DSC and DMA reveal the interaction between nanoparticles and polymer matrix. The grafted nanoparticles exhibit a nucleating effect and improve the crystallinity of the matrix polymer due to enhanced interfacial bonding. These particles also have a toughening effect on the amorphous PP phase of the matrix due to its enhanced interaction by the entanglement between the grafting polymer and the amorphous PP phase. Insufficient interaction between the nanoparticles and the rubber phase of the matrix actually introduces restraints to the rubber phase, and might be responsible for the reduction in the toughness of the matrix. The technical route proposed in the current work is proved to be feasible for fabricating polymer composites with inorganic nanoparticles. The interfacial characteristics between the grafted nanoparticles and the polymer matrix can be tailored by changing the species of the grafting monomers and the ball milling conditions. Further studies should be made to identify the mechanochemical mechanism of the ball milling process and to improve the interaction between the nanoparticles and the rubber phase of the block copolymer, to achieve greater toughening effects. ACKNOWLEDGEMENTS The authors are grateful to the support of the Deutsche Forschungsgemeinschaft (DFG FR675/40-4) for the cooperation between the German and Chinese institutes on the topic of nanocomposites. Further thanks are due to the Team Project of the Natural Science Foundation of Guangdong, China (Grant: ). REFERENCES 1. R. Dagni, C&EN 7 (1999) M.Z. Rong, M.Q. Zhang, Y.X. Zheng, H.M. Zeng, R. Walter and K. Friedrich, Polymer 4 (001) B.M. Novak, Adv. Mater. 5 (1993) E.P. Giannelis, Adv. Mater. 8 (1996) W.P. Xu, R. Huang, B.H. Cai and W.Y. Fan, Chin. Plast. 1(6) (1998) [in Chinese]. 6. M. Sumita, Y. Tsukumo, K. Miyasaka and K. Ishikawak. J. Mater. Sci. 18 (1983) S.C. Jana and S. Jain, Polymer 4 (001) J.X. Li, M. Silverstein, A. Hiltner and E. Baer, J. Appl. Polym. Sci. 5 (1994) Y. Wang and J.S. Huang, J. Appl. Polym. Sci. 60 (1996) M.Z. Rong, M.Q. Zhang, Y.X. Zheng and H.M. Zeng, Chinese Patent (Application No.: CN ), M.Z. Rong, M.Q. Zhang, Y.X. Zheng, H.M. Zeng, 66 Polymers & Polymer Composites, Vol. 1, No. 4, 004

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12 Wen Hong Ruan, Ming Qiu Zhang, Min Zhi Rong and Klaus Friedrich 68 Polymers & Polymer Composites, Vol. 1, No. 4, 004