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