Introduction
Currently, epoxy resins are widely applied to various
engineering fields due to their excellent mechanical and
insulation properties. These resins are reinforced with
various particles or fibers to improve their properties further.
The particle size is currently being decreased to below
the micrometer level to form nanocomposites with superior
mechanical properties.
The mechanical properties of nanocomposites have been
investigated in several previous studies. The effects of the
size and volume fraction of the particles, namely, the
interaction effect between the particles and the epoxy
matrix, have been studied to determine how the properties
of the nanocomposites can be improved. The mechanical
properties of composites are strongly dependent on the
network structure and shape of the silica particles, as found
by Yamamoto et al. [1], Moloney et al. [2–4], and Adachi
et al. [5–7], and on the size, shape, and specific surface of
the silica particles, as clarified by Nakamura et al. [8–10].
In order to further clarify the effect of particle size on the
mechanical properties, properties of the composites filled
with particles having different distributions of particle size
have been investigated. The mechanical properties of these
composites were found to be governed by the distributions
[11–15]. The rapid decrease in the interparticle distance
resulting from their small size has in turn resulted in more
interactions between particles and the epoxy matrix. The
interfacial interaction of particles with the surroundingmatrix has a close relationship with the crosslinking network
morphology in the polymer matrix [16–18]. As a
result, the mechanical properties of nano or micron-particle-reinforced
epoxy composites are considerably dependent
not only on the particle size effect but also on the
intrinsic properties of the epoxy matrix.
An alternative approach to considering the interaction
between the particles and the epoxy matrix resins in composites
is to use epoxy resins with different network structures
as the matrix resins in the composites. Thermosetting
polymer materials with different network structures can be
manufactured by incompletely curing a non-stoichiometric
mixture of pre-polymer and a curing agent. The mechanical
properties of the non-stoichiometrically cured epoxy resins
with and without fillers have been reported in several papers.
Wingard and Beatty [19] reported that glass transition temperature
and secondary glass transition temperature of the
epoxy resins identified from thermo-viscoelastic properties
varied depending on the non-stoichiometrically cured epoxy
resins used. Palmese and McCullough [20] found that the
elastic modulus and glass transition temperature of the epoxy
resin were significantly affected by relatively small variations
in stoichiometry. d’Almeida and Monteiro [21] showed
that composites consisting of glass microspheres in the
epoxy resins fabricated with a hardener-rich formulation had
the best deformation capacity. Vanlandingham et al. [22]
denoted the relation between microstructure and properties
in epoxy resins as a function of epoxy-amine stoichiometry.
Calventus et al. [23] found that the glass transition temperatures
of the cured epoxy resins became highest during
stoichiometric curing. Fang et al. [24] improved the fracture
toughness and flexural strength of nanocomposite-filled
graphene sheets into the epoxy resin by modulating the
stoichiometric ratio around the sheets to construct a hierarchical
structure that can dissipate more strain energy during
fracture. Bignotti et al. [25] reported that epoxy-clay nanocomposites
and the neat epoxy with the same stoichiometric
ratio had basically the same crosslinking density. Garcı´a
et al. [26] clarified the influence of stoichiometry on curing
the epoxy-clay nanocomposites and their viscoelastic properties.
Effects of the incomplete network structures in epoxy
resins have been emerging up to now..
Nano-silica particulate-reinforced epoxy composites are
important for industrial application to packaging materials
in electronic devices, among other things. The mechanical
properties of the composites need to be improved to
increase the reliability of the devices. Therefore, the
interaction effects between network structures in the epoxy
resins and the silica particles on the mechanical properties
must be clarified.
In this study, the mechanical properties of nano-silica
particulate-reinforced epoxy composites with different crosslinking
densities were clarified experimentally to determinthe relation between nano-particles and network structure in
matrix resin. The matrices were prepared by curing with an
excessive mixture of diglycidyl ether of bisphenol A type
epoxy resin to the curing agent (methyl-tetrahydro-phthalic
anhydride) for the stoichiometric condition. The crosslinking
densities and glass transition temperatures