4.7 Mechanical Properties
101
fraction of interfacial polymers was found to be about 30% in case of nanoparticlebased nanocomposites, as opposed to only 10% for nanotube-based nanocomposites. The second reason in relation to high mechanical performance of PVA/NBC
bionanocomposites could be ascribed to chemical structures of nanofillers in terms of
the amounts and locations of hydroxyl groups in order to control the nanofiller dispersion within bionanocomposites, thus reflecting upon the bonding between polymer
matrices and nanofillers. Pakzad et al. [46] reported that the number and nature of
hydrogen bonds had substantial effect on mechanical properties of nanocomposites.
In case of PVA/3 wt% NBC bionanocomposites, NBCs have highly porous structures
with a large amount of hydroxyl groups located inside these pores when NBCs are
uniformly dispersed. As confirmed by FTIR and XRD results, polymeric chains enter
these pores and form both hydrogen and mechanical bondings. Such two bonding
types can be particularly recognised for NBCs, as compared to Cloisite 30B clays and
HNTs, thus leading to significantly enhanced mechanical properties of nanocomposites [4, 5]. In case of PVA/5 wt% Cloisite 30B clay bionanocomposites in comparison
to PVA/5 wt% HNT bionanocomposites, the strong adhesion of clays in polymer
matrices associated with uniform clay dispersion gave rise to the strong interfacial
bonding between nanoclays and polymer matrices, which thus could predominantly
contribute to the improvement of mechanical properties of nanocomposites.
The dispersion state of nanofillers can also influence mechanical properties
of PVA bionanocomposites. As mentioned earlier, NBCs have better dispersion
state than Cloisite 30B clays and HNTs. The incorporation of NBCs within PVA
matrices produces smoother bionanocomposite films with higher tensile strength
when compared with those of PVA/HNT bionanocomposites and PVA/Cloisite 30B
clay bionanocomposites. The better dispersion state of NBCs improves their interaction with PVA matrices, thus resulting in higher tensile strengths of PVA/NBC
bionanocomposites. On the contrary, increasing the nanofiller content appears to
induce higher surface roughness, as well as lower tensile strengths of nanocomposites, which indicates nanofiller agglomeration apparently has detrimental effect on
the improvement of tensile strength. This is particularly the case for PVA/HNT
bionanocomposites due to poor HNT dispersion state. Conversely, PVA/3 wt%
HNT bionanocomposites yield much higher elongation at break and fracture toughness, as opposed to those of neat PVA, which are different from PVA/3 wt% NBC
bionanocomposites and PVA/5 wt% Cloisite 30B clay bionanocomposites with corresponding lower values. Such results can be clearly explained by two major reasons.
The first reason is ascribed to the number of nanoparticles that depend on the volume
and volume fraction of nanoparticles in nanocomposites. At the same volume fraction, the number of near spherical NBCs is significantly larger than those of tubular
HNTs or platelet-like Cloisite 30B clays. As such, this finding resulted in increasing
the number of available reinforcements for improving matrix rigidity, and then
decreasing fracture toughness [38, 47]. The second reason is related to the mechanism
of fracture toughness including pre-crack effect for the fracture of nanocomposites.
In general, crack deflection and crack pinning are most well-known mechanisms
resulting in an increase in fracture energy [48], and consequently an increase in
fracture toughness of nanocomposites. In both mechanisms mentioned earlier, crack
101
fraction of interfacial polymers was found to be about 30% in case of nanoparticlebased nanocomposites, as opposed to only 10% for nanotube-based nanocomposites. The second reason in relation to high mechanical performance of PVA/NBC
bionanocomposites could be ascribed to chemical structures of nanofillers in terms of
the amounts and locations of hydroxyl groups in order to control the nanofiller dispersion within bionanocomposites, thus reflecting upon the bonding between polymer
matrices and nanofillers. Pakzad et al. [46] reported that the number and nature of
hydrogen bonds had substantial effect on mechanical properties of nanocomposites.
In case of PVA/3 wt% NBC bionanocomposites, NBCs have highly porous structures
with a large amount of hydroxyl groups located inside these pores when NBCs are
uniformly dispersed. As confirmed by FTIR and XRD results, polymeric chains enter
these pores and form both hydrogen and mechanical bondings. Such two bonding
types can be particularly recognised for NBCs, as compared to Cloisite 30B clays and
HNTs, thus leading to significantly enhanced mechanical properties of nanocomposites [4, 5]. In case of PVA/5 wt% Cloisite 30B clay bionanocomposites in comparison
to PVA/5 wt% HNT bionanocomposites, the strong adhesion of clays in polymer
matrices associated with uniform clay dispersion gave rise to the strong interfacial
bonding between nanoclays and polymer matrices, which thus could predominantly
contribute to the improvement of mechanical properties of nanocomposites.
The dispersion state of nanofillers can also influence mechanical properties
of PVA bionanocomposites. As mentioned earlier, NBCs have better dispersion
state than Cloisite 30B clays and HNTs. The incorporation of NBCs within PVA
matrices produces smoother bionanocomposite films with higher tensile strength
when compared with those of PVA/HNT bionanocomposites and PVA/Cloisite 30B
clay bionanocomposites. The better dispersion state of NBCs improves their interaction with PVA matrices, thus resulting in higher tensile strengths of PVA/NBC
bionanocomposites. On the contrary, increasing the nanofiller content appears to
induce higher surface roughness, as well as lower tensile strengths of nanocomposites, which indicates nanofiller agglomeration apparently has detrimental effect on
the improvement of tensile strength. This is particularly the case for PVA/HNT
bionanocomposites due to poor HNT dispersion state. Conversely, PVA/3 wt%
HNT bionanocomposites yield much higher elongation at break and fracture toughness, as opposed to those of neat PVA, which are different from PVA/3 wt% NBC
bionanocomposites and PVA/5 wt% Cloisite 30B clay bionanocomposites with corresponding lower values. Such results can be clearly explained by two major reasons.
The first reason is ascribed to the number of nanoparticles that depend on the volume
and volume fraction of nanoparticles in nanocomposites. At the same volume fraction, the number of near spherical NBCs is significantly larger than those of tubular
HNTs or platelet-like Cloisite 30B clays. As such, this finding resulted in increasing
the number of available reinforcements for improving matrix rigidity, and then
decreasing fracture toughness [38, 47]. The second reason is related to the mechanism
of fracture toughness including pre-crack effect for the fracture of nanocomposites.
In general, crack deflection and crack pinning are most well-known mechanisms
resulting in an increase in fracture energy [48], and consequently an increase in
fracture toughness of nanocomposites. In both mechanisms mentioned earlier, crack