J. Cent. South Univ. Technol. (2008) 15(s1): 076-079

DOI: 10.1007/s11771-008-318-y

Rheological behavior of novel polyamide 6/silica nanocomposites containing epoxy resins

ZHAO Cai-xian(赵才贤)^{1, 2}, ZHANG Ping(张 平)¹ , CHEN Guang-bing(陈广兵)², WANG Xia-yu(王霞瑜)²

(1. Institute of Fundamental Mechanics and Material Engineering, Xiangtan University, Xiangtan 411105, China;

2. Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education,

Xiangtan University, Xiangtan 411105, China)

Abstract: A novel polyamide 6/silica nanocomposite containing epoxy resins (EPA6N) was prepared via in situ polymerization using tetraethoxysilane (TEOS) as the precursor of silica. The dynamic rheological properties of pure PA6 and EPA6N at temperatures of 225 and 235 ℃ were investigated. The results of transmission electron microscopy (TEM) and atomic force microscopy (AFM) indicate that the silica particles are well dispersed in the polyamide 6 matrix on about 30 nm in diameter, which demonstrates that this method can effectively avoid agglomeration of the inorganic particles. The rheological results suggest that pure PA6 shows Newtonian behavior. However, the novel EPA6N exhibits a solid-like rheological behavior, which is due to the small size, large surface of silica particles and the stronger polyamide 6-silica chemical bond formed through the reactions of epoxy resins with end groups of PA6 molecular chains. The EPA6N also exhibits higher melt viscosity, storage modulus and loss modulus than those of pure PA6.

Key words: polyamide 6; nanocomposites; epoxy resins; rheological behavior; viscosity

1 Introduction

In recent years, polymer-based organic-inorganic hybrid materials, considered innovative advanced materials, have gained increasing attention in the field of material science^[1-3]. Inorganic materials, such as silica, exhibit excellent thermal stability and hardness. The incorporation of well-dispersed nano-silica particles into polymer matrices has been shown to be an extremely effective way to improve the thermal and mechanical properties of polymers. In general, the methods used for preparation of nanocomposites include sol-gel processing, intercalation, blending and in situ polymerization^[4-10]. In situ polymerization is a method in which fillers or reinforcements are dispersed in an appropriate monomer first and then the mixture is polymerized using a technique similar to bulk polymerization. In previous work^[11-12], novel polyamide 6/silica nanocomposites (EPA6N) via in situ polymerization were prepared by using tetraethoxysilane (TEOS) as the precursor of silica, which was modified by diglycidyl ether of bisphenol A (DGEBA) type epoxy resin. The novel PA6N showed dramatic increases in mechanical properties, especially in impact strength, which was improved 3.6 times than that of pure PA6.

Melt-state rheological properties are important to gauge the processability of the dispersion state of the filler and to a lesser extent the interactions between the polymer and the filler. According to Ref.[13] viscoelastic measurements in the melt (rheometry) are highly sensitive to the nanoscale structure of the hybrids and appear to be a powerful method to probe the developed structure of such materials. Therefore, detailed rheological studies are important to allow evaluation of the dispersion techniques of polymer nanocomposites.

So far, there are some papers on studying the melt rheological behavior of PA6 nanocomposites^[14-17], but no publication has been found concerning the rheological behavior of the EPA6N. The purpose of this paper is to examine the rheological properties of EPA6N prepared via in situ polymerization. The influence of silica particles on complex viscosity, storage modulus and loss modulus were investigated. The materials were also characterized by SEM and AFM.

2 Experimental

2.1 Materials

ε-caprolactam and DGEBA type epoxy resin with relative molecular mass (W_ep) of 196, were both purchased from Yueyang Chemical Plant, Hunan, China. Tetraethoxysilane(TEOS) and tetrahydrofuran(THF), analytical reagent grade, were obtained from Xilong Chemical Fractory, Guangdong, China, and used without further purification. Tetraisopropyl titanate(TPT) was ordered from Dupont Co. Ltd. and used as received.

2.2 Preparation of novel EPA6N

The novel EPA6N (silica content is 3%) and pure PA6 samples were prepared according to Refs.[11-12].

2.3 Morphological characterization

Thin plates were cut by ultramicrotomy from the systems and observed under a TECNAI G² 20 transmission electron microscope (TEM) at a high voltage of 120 kV.

In addition to TEM analysis, atomic force microscopy (AFM) image was obtained with a Veeco- digital instruments multimode SPM (scanning probe microscopy) with nanoscope Ⅲa with tapping mode (model RTESP) probes for morphological investigation. The scan rate was 1.000 Hz and numbers of samples were 512.

2.4 Rheological measurements

Prior to rheological measurements, samples were dried at 80 ℃ for a minimum of 24 h under vacuum. Dynamic rheological measurements in the melt were performed using a RMS 605 rheometrics mechanical spectrometer at 225 and 235 ℃ under nitrogen atmosphere, respectively. The rheological properties of the pure PA6 and EPA6N were measured in oscillatory shear mode using 50 mm-diameter parallel plates at a gap of 1.8 mm. The dynamic moduli were recorded as functions of angular grequency in the range from 0.1 to 100 rad/s at a strain magnitude of 1%.

3 Results and discussion

3.1 Morphologies of SiO₂

Microscopic control was conducted, in order to get insight into the silica dispersion state within the polyamide 6 matrix. Fig.1 illustrates the extreme morphologies of the sample EPA6N. The surface topography structure of EPA6N was characterized by AFM (Fig.2). It is evident that the silica particles are well dispersed in the polyamide 6 matrix on about 30 nm in diameter.

3.2 Dynamic rheological properties

For polymer composite systems, the size, shape and concentration of the filler can have a significant effect on the rheological properties in the melt state. Figs.3 and 4 show the complex viscosity behavior of pure PA6 and EPA6N at temperatures of 225 and 235 ℃, respectively.

Fig.1 TEM photograph of EPA6N

Fig.2 AMF phase image of EPA6N

Fig.3 Complex viscosity from oscillatory rheometry at 225 ℃

It can be seen from Figs.3 and 4 that the EPA6N exhibits shear thinning behavior, whilst the pure PA6 exhibits Newtonian behavior at both temperatures. Furthermore, the absolute value of the melt viscosity of EPA6N is significantly higher than that of pure PA6, and the EPA6N exhibits decreasing dynamic viscosity with increasing frequency. This is due to the small size of silica particles (about 30 nm, Figs.1 and 2) and the stronger polymer-silica chemical bond formed through the reactions of epoxy resins with end groups of polyamide 6 chains^[12], which prevent the movement of the macromolecular chains, accordingly leading to an increased viscosity under shearing stress.

Fig.4 Complex viscosity from oscillatory rheometry at 235 ℃

Figs.5 and 6 present the storage modulus as a function of frequency for pure PA6 and EPA6N at temperatures of 225 and 235 ℃, respectively. The loss modulus vs frequency at temperatures of 225 and 235 ℃ are shown in Figs.7 and 8, respectively. It can be seen that the storage moduli of both samples increase with increasing frequency and show solid-like behavior, and similar trends are observed in Figs.7 and 8. Moreover, the storage modulus of EPA6N is larger than that of pure PA6. On a molecular basis, the storage modulus depends on what contour rearrangements can take place within the period of the oscillatory deformation. As the frequency increases, the molecular movement time becomes short, and the movement responses of small- dimension units are dominant. As for EPA6N, some PA6 molecules bond to silica particles, thus resulting in the reduced segmental relaxation, a higher modulus than that of pure PA6, and an increased solid-like behavior of EPA6N.

Fig.5 Storage modulus from oscillatory rheometry at 225 ℃

Fig.6 Storage modulus from oscillatory rheometry at 235 ℃

Fig.7 Loss modulus from oscillatory rheometry at 225 ℃

Fig.8 Loss modulus from oscillatory rheometry at 235 ℃

4 Conclusions

1) The dynamic rheological properties of pure PA6 and novel polyamide 6/silica nanocomposites containing epoxy resins are studied.

2) EPA6N exhibits a solid-like rheological behavior and has higher melt viscosity, storage modulus and loss modulus than pure PA6 because of the small size, large surface of silica particles and the stronger polyamide 6-silica chemical bond formed through the reactions of epoxy resins with end groups of polyamide 6 molecular chains.

Acknowledgements

The authors would like to thank Prof. S. R. LU for the rheology testing.

References

[1] GIANNELIS E P. Polymer layered silicate nanocomposites [J]. Adv Mater, 1993, 8: 29-35.

[2] WEN Y, WILKES G L. Organic/inorganic hybrid network materials by the sol-gel approach [J]. Chem Mater, 1996, 8: 1667-1681.

[3] SHARP K G. Inorganic/organic hybrid materials [J]. Adv Mater, 1998, 10: 1243-1248.

[4] WEI Shan-shan, ZHANG Ping, WANG Xia-yu. Study on the mechanical properties of nylon 6/nano-SiO₂ compositions [J]. Natural Science Journal of Xiangtan University, 2002, 24(4): 42. (in Chinese)

[5] SHEN L, DU Q G, WANG H T, et al. In situ polymerization and characterization of polyamide-6/silica nanocomposites derived from water glass [J]. Polym Int, 2004, 53: 1153-1160.

[6] WANG Xia-yu, ZHAO Cai-xian, ZHANG Ping. Study on dynamic mechanical behavior of PA6/SiO₂ nanocomposites [J]. Natural Science Journal of Xiangtan University, 2005, 27(1): 106-112. (in Chinese)

[7] SALIMA B, ELODIE B L, NATHALIE Z. Silica/polyamide nanocomposite synthesis via an original double emulsification process in miniemulsion [J]. Macromol Rapid Comm, 2005; 26: 1860-1865.

[8] YI Lan-hua, ZHAO Cai-xian, ZHANG Ping, et al. The determination of cyclic oligomers in PA6/SiO₂ nanocomposites by HPLC [J]. Natural Science Journal of Xiangtan University, 2006, 28(4): 70-74. (in Chinese)

[9] ZHAO Cai-xian, ZHANG Ping, WANG Xia-yu, et al. Study on the non-isothermal crystallization behaviors of PA6/silica nano-composites prepared by the sol-gel process [J]. J Mater Sci, 2007, 42: 9083-9099.

[10] ZHAO Cai-xian, ZHANG Ping, WANG Xia-yu, et al. Preparation of PA6/SiO₂ nanocomposites by in situ polymerization [J]. Polymer Materials Science and Engineering, 2007, 23: 218-221. (in Chinese)

[11] ZHANG Ping, WANG Xia-yu, ZHAO Cai-xian. A new method for prepared PA6/EP/silica nanocomposites via in situ polymerization: China Patent, ZL, 200610031604.8 [P]. 2006.

[12] ZHAO Cai-xian, ZHANG Ping, WANG Xia-yu, et al. Study on the non-isothermal crystallization kinetics of novel polyamide 6/silica nanocomposites containing epoxy resins [J]. Polymer Testing, 2008, 27: 412-419.

[13] INCARNATO L, SCARFACO P, ACIERNO D. Rheological behavior of new melt compounded copolyamide nanocomposites [J]. Polymer, 2004, 45: 3487-3496.

[14] DU Miao, ZHENG Qiang, YANG Hong-mei. Dynamic rheological behavior for polymer composites filled with particles [J]. Journal of the Society of Rheology, 2003, 31: 305-311.

[15] RAVISHANKAR K, AYYER A I. Comparative rheological studies of polyamide-6 and its low loaded nanocomposite based on layered silicates [J]. Rheol Acta, 2004, 43: 1435-1528.

[16] VLASVELD D P N, JONG M, BERSEE H E N, et al. The relation between rheological and mechanical properties of PA6 nano- and micro-composites [J]. Polymer, 2005, 46: 10279-10289.

[17] YANG Hong, LI Bo, WANG Ke, et al. Rheology and phase structure of PP/EPDM/SiO₂ ternary composites [J]. Europe Polymer J, 2008, 44: 113-123.

(Edited by CHEN Wei-ping)

Foundation item: Project(07A071) supported by the Scientific Research Foundation of Hunan Provincial Education Department

Received date: 2008-06-25; Accepted date: 2008-08-05

Corresponding author: ZHANG Ping, Professor; Tel: +86-732-8292247; E-mail: zhangp@xtu.edu.cn