26
1 Introduction to PVA-Based Bionanocomposite Films
ascribed to clay agglomeration resulting from increasing the clay content generally
over 5 wt% associated with non-uniform clay dispersion. Those clay aggregates act
as typical defects of stress-concentration sites for crack initiation [142].
The molecular weight of polymer matrices also has an impact on reinforcement
efficiency and tensile strength of nanocomposites. It has been reported that Young’s
modulus and yield strength of nanocomposites are substantially enhanced with
increasing the molecular weight of polymer matrices [142]. This finding suggests
that increasing the molecular weight of polymer matrices induces higher shear stress
to break up large clay particles into exfoliated platelets. The higher degree of exfoliation inevitably results in effective stress transfer from polymer matrices to nanofillers
with their stronger interfacial bonding.
Other factors to play a key role in enhancing Young’s modulus and tensile strength
of nanocomposites are organic modification of layer silicates and the use of compatibilisers to polymer matrices. Hotta and Paul [171] indicated that increasing the
number of alkali tails for organoclays from one tail to two tails enhanced Young’s
modulus and tensile strength of nanocomposites. Pavlidou and Papaspyrides [142]
also demonstrated that increasing the amount of CAB-MA compatibiliser in cellulose
acetate butyrate (CAB)/CAB-MA-based nanocomposites from 0 to 5 wt% enhanced
Young’s modulus from 3.6 to 3.7 GPa [142].
In polymer/HNT nanocomposite systems, the improvements of elastic modulus
and tensile strength depend primarily on the HNT loading, HNT dispersion and
interfacial adhesion between HNTs and polymer matrices. As indicated in Table 1.3,
increasing the HNT loading generally results in higher reinforcement efficiency, as
well as greater tensile strength of nanocomposites. However, these trends are not
always observed due to the influence of HNT dispersion within polymer matrices.
Although HNTs are more easily dispersed when compared to natural silica and CNTs,
their dispersion can still be challenging, particularly for non-polar polymers [172,
173]. Qiu et al. [16] reported that the tensile strength of PVA/HNT nanocomposites
was decreased by 14.2% when their loading increased from 5 to 10 wt% due to
the HNT agglomeration at 10 wt%. Better dispersion using surface pre-treatment,
including the grafting of silanes or polymers, can yield the improvement of interfacial
bonding (i.e. hydrogen bonding) between HNTs and polymer matrices. It was shown
that the modulus of PVA/HNT nanocomposites was increased by 21% when using
modified HNTs as opposed to unmodified HNTs [14].
1.6.3 Nanomechanical Properties of Nanocomposites
Nanocomposites represent a homogenous material system at the macroscaled level
despite their heterogeneity on the micro- or nanoscale. In general, polymers react with
nanoparticles during a mixing process to yield multiple phases including partially
or fully reacted composites, unreacted components of raw materials and interphase
zones, which have different mechanical properties and chemical properties [174]. The
properties and amount of each phase relative to entire nanocomposites strongly affect
1 Introduction to PVA-Based Bionanocomposite Films
ascribed to clay agglomeration resulting from increasing the clay content generally
over 5 wt% associated with non-uniform clay dispersion. Those clay aggregates act
as typical defects of stress-concentration sites for crack initiation [142].
The molecular weight of polymer matrices also has an impact on reinforcement
efficiency and tensile strength of nanocomposites. It has been reported that Young’s
modulus and yield strength of nanocomposites are substantially enhanced with
increasing the molecular weight of polymer matrices [142]. This finding suggests
that increasing the molecular weight of polymer matrices induces higher shear stress
to break up large clay particles into exfoliated platelets. The higher degree of exfoliation inevitably results in effective stress transfer from polymer matrices to nanofillers
with their stronger interfacial bonding.
Other factors to play a key role in enhancing Young’s modulus and tensile strength
of nanocomposites are organic modification of layer silicates and the use of compatibilisers to polymer matrices. Hotta and Paul [171] indicated that increasing the
number of alkali tails for organoclays from one tail to two tails enhanced Young’s
modulus and tensile strength of nanocomposites. Pavlidou and Papaspyrides [142]
also demonstrated that increasing the amount of CAB-MA compatibiliser in cellulose
acetate butyrate (CAB)/CAB-MA-based nanocomposites from 0 to 5 wt% enhanced
Young’s modulus from 3.6 to 3.7 GPa [142].
In polymer/HNT nanocomposite systems, the improvements of elastic modulus
and tensile strength depend primarily on the HNT loading, HNT dispersion and
interfacial adhesion between HNTs and polymer matrices. As indicated in Table 1.3,
increasing the HNT loading generally results in higher reinforcement efficiency, as
well as greater tensile strength of nanocomposites. However, these trends are not
always observed due to the influence of HNT dispersion within polymer matrices.
Although HNTs are more easily dispersed when compared to natural silica and CNTs,
their dispersion can still be challenging, particularly for non-polar polymers [172,
173]. Qiu et al. [16] reported that the tensile strength of PVA/HNT nanocomposites
was decreased by 14.2% when their loading increased from 5 to 10 wt% due to
the HNT agglomeration at 10 wt%. Better dispersion using surface pre-treatment,
including the grafting of silanes or polymers, can yield the improvement of interfacial
bonding (i.e. hydrogen bonding) between HNTs and polymer matrices. It was shown
that the modulus of PVA/HNT nanocomposites was increased by 21% when using
modified HNTs as opposed to unmodified HNTs [14].
1.6.3 Nanomechanical Properties of Nanocomposites
Nanocomposites represent a homogenous material system at the macroscaled level
despite their heterogeneity on the micro- or nanoscale. In general, polymers react with
nanoparticles during a mixing process to yield multiple phases including partially
or fully reacted composites, unreacted components of raw materials and interphase
zones, which have different mechanical properties and chemical properties [174]. The
properties and amount of each phase relative to entire nanocomposites strongly affect