Synthesis and characterization of silica nanoparticles and their application as fillers in silica-bismaleimide nanocomposite

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Date
2008
Authors
Padavettan, Vejayakumaran
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Abstract
A series of investigations were carried out to study the formation, growth and methods to control the size of silica particles via sol-gel process using tetraethylorthosilicate (TEOS) as the precursor in basic condition. The formation and growth of silica particles were significantly affected by the NH3 (catalyst) concentration. Lower NH3 concentrations lead to the formation of stable sols while higher NH3 concentrations resulted in bigger, spherical silica particles with sizes varying from 90 - 700 nm. In the first approach, the addition of small amount of ammonium salts (NH4X) produced monodispersed silica particles ranging from 20.5 to 34.1 nm depending on the size and concentration of the anion present in the system. The second approach was conducted to further reduce the silica size by optimizing the reaction parameters such as concentration of TEOS, R (water to TEOS concentration ratio) value, NH3 feed rate and reaction temperature. The optimal reaction conditions successfully produced highly dispersed ultrafine silica nanoparticles with particle size of 7.1 ± 1.9 nm which falls in the primary size range. The freshly synthesized silica nanoparticles was dispersed and dried using a relatively new, simple and cost effective alcohol-dehydration (AD) technique which was able to suppress the agglomeration and improve the dispersion of silica compared to freeze drying (FD) and oven drying (OD) techniques. The processed silica nanoparticles exhibited some interesting size-dependent properties. Significant increase in the specific surface area (SSA) and silanol concentration (δOH) and a more gradual increase in the apparent density (Da) were observed as the particle size was reduced from around 400 to 7 nm. In addition, the decrease in the silanol number (αOH) and Si-O-Si bond angle and the presence of optically active defect sites at smaller particle sizes suggest that the silica structure has been significantly altered at the nanoscale. Chemical modification of silica surface was conducted by grafting epoxide and maleimide groups. The epoxide groups were grafted using 3- glycidyloxypropyltrimethoxysilane (GPTS) while the maleimide groups were grafted by reacting 1,1’-(Methylenedi-4,1-phenelene)bismaleimide (BMI) with silica nanoparticles pre-grafted with amino groups using 3-aminopropyltrimethoxysilane (APTS). Both epoxide and maleimide groups were found covalently bonded to the silica surface and reactive. The loading of functional groups increased with the decrease in the particle size: e.g. 130 nm (1.09 mmol/g) < 20 nm (1.70 mmol/g) < 7 nm (2.04 mmol/g) for the epoxide groups. The matrix for the silica-bismaleimide nanocomposite (SBN) was prepared using a formulation containing BMI and 4,4’-diaminodiphenylmethane (DDM) at 2:1 (BMI/DDM) mol ratio with 0.1 wt.% of dicumylperoxide (DCP) as the curing accelerator. The specified formulation exhibited longer gel-time (208 sec/g) and shorter post-curing time (2 hours) compared to other formulations. The silica nanoparticles (7, 20 and 130 nm) were incorporated into the BMI/DDM matrix using a combination of procedures involving pre-treatments of silica powder in presence of BMI and melt-mixing. The pure silica nanoparticles interacted with the polymer matrix through hydrogen bonding while the surface modified nanoparticles exhibited strong filler-matrix interaction via covalent bonding. Therefore, the surface modified nanoparticles resulted in significant improvements in thermal mechanical properties of SBN such as E’, Tg and Td and also reduction in CTE. The property enhancements were found dependent on the filler concentration, surface functional group and particle size. Overall, the best thermal mechanical properties were obtained for SBN containing 7 nm silica particles (grafted with epoxide groups) at 10.0 wt.%, i.e., E’: 14.1 GPa (at 30 °C), Tg: 300 °C, α1: 28.8 ppm/°C and Td (onset): 451 °C.
Description
PhD
Keywords
Chemical science , Synthesis , Silica nanoparticles , Nanocomposite
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