102
4 PVA Bionanocomposite Films with Different Particle …
growth path can increase as long as those cracks reach nanofiller regions and the
reinforcement shape highly affects the amount of crack deviation from their initial
path. Since HNTs have larger lateral dimensions in comparison with near spherical
NBCs, the cracks tend to pass over longer distances in PVA bionanocomposites reinforced with HNTs. Moreover, crack bridging is a well-known fracture mechanism in
nanocomposites reinforced with high-aspect-ratio nanoparticles [25]. An ideal situation in this mechanism occurs when nanotube fillers are still embedded in matrices
while aligned in a perpendicular direction to crack faces. Consequently, their higher
aspect ratios yielded the improvement of fraction toughness when compared with
those of near spherical nanoparticles.
4.8 Fracture Morphology
Figure 4.12 shows typical SEM micrographs of cross-sectional fracture surfaces
for PVA, PVA/HNT bionanocomposites, PVA/Cloisite 30B clay bionanocomposites and PVA/NBC bionanocomposites. It can be clearly seen in Fig. 4.12b, e that
PVA bionanocomposites reinforced with 3 wt% of HNTs and Cloisite 30B clays
reveal much rougher fractured surfaces when compared with those of neat PVA
films, as illustrated in Fig. 4.12a. Moreover, 3 wt% HNTs or Cloisite 30B clays are
distributed uniformly within PVA matrices. The good dispersion of both nanoparticles and strong interaction between clay particles and polymer matrices clearly
contribute to the reinforcing effect, as a result of an increase in both tensile strength
and elastic modulus. Nevertheless, in PVA/HNT bionanocomposite and PVA/Cloisite
30B clay bionanocomposite systems, uniform multi-layered structures have not been
achieved similar to those detected in PVA/3 wt% NBC bionanocomposites illustrated in Fig. 4.12h. Such results are indicative of high NBC dispensability relative to those of HNTs and Cloisite 30B clays along with the highest mechanical
performance. Meanwhile, at the HNT content of 5 wt%, particle–particle interactions
are more favourable than particle–matrix counterparts, as evidenced by more filler
agglomeration in presence of debonding and microvoid effects depicted in Fig. 4.12c.
Such defects in nanocomposite systems give rise to the decrease in tensile strengths
of PVA/HNT bionanocomposites. However, as for PVA/5 wt% Cloisite 30B clay
bionanocomposites, the clay dispersion appears to be still relatively uniform with
presence of small particle agglomeration shown in Fig. 4.12f. With increasing the
nanofiller contents of HNTs and Cloisite 30B clays from 5 to 10 wt%, the fracture
surfaces of bionanocomposites films are altered from ductile characteristic to more
brittle behaviour, as illustrated in Fig. 4.12d, g, respectively. Similar phenomena
were also found in PVA/NBC bionanocomposites in Fig. 4.12j. It is well known that
decreasing surface roughness can make the failure mode of PVA bionanocomposite
films vary from ductile to brittle fracture [49], which is consistent with the reduced
mechanical properties of bionanocomposite films in this study.
4 PVA Bionanocomposite Films with Different Particle …
growth path can increase as long as those cracks reach nanofiller regions and the
reinforcement shape highly affects the amount of crack deviation from their initial
path. Since HNTs have larger lateral dimensions in comparison with near spherical
NBCs, the cracks tend to pass over longer distances in PVA bionanocomposites reinforced with HNTs. Moreover, crack bridging is a well-known fracture mechanism in
nanocomposites reinforced with high-aspect-ratio nanoparticles [25]. An ideal situation in this mechanism occurs when nanotube fillers are still embedded in matrices
while aligned in a perpendicular direction to crack faces. Consequently, their higher
aspect ratios yielded the improvement of fraction toughness when compared with
those of near spherical nanoparticles.
4.8 Fracture Morphology
Figure 4.12 shows typical SEM micrographs of cross-sectional fracture surfaces
for PVA, PVA/HNT bionanocomposites, PVA/Cloisite 30B clay bionanocomposites and PVA/NBC bionanocomposites. It can be clearly seen in Fig. 4.12b, e that
PVA bionanocomposites reinforced with 3 wt% of HNTs and Cloisite 30B clays
reveal much rougher fractured surfaces when compared with those of neat PVA
films, as illustrated in Fig. 4.12a. Moreover, 3 wt% HNTs or Cloisite 30B clays are
distributed uniformly within PVA matrices. The good dispersion of both nanoparticles and strong interaction between clay particles and polymer matrices clearly
contribute to the reinforcing effect, as a result of an increase in both tensile strength
and elastic modulus. Nevertheless, in PVA/HNT bionanocomposite and PVA/Cloisite
30B clay bionanocomposite systems, uniform multi-layered structures have not been
achieved similar to those detected in PVA/3 wt% NBC bionanocomposites illustrated in Fig. 4.12h. Such results are indicative of high NBC dispensability relative to those of HNTs and Cloisite 30B clays along with the highest mechanical
performance. Meanwhile, at the HNT content of 5 wt%, particle–particle interactions
are more favourable than particle–matrix counterparts, as evidenced by more filler
agglomeration in presence of debonding and microvoid effects depicted in Fig. 4.12c.
Such defects in nanocomposite systems give rise to the decrease in tensile strengths
of PVA/HNT bionanocomposites. However, as for PVA/5 wt% Cloisite 30B clay
bionanocomposites, the clay dispersion appears to be still relatively uniform with
presence of small particle agglomeration shown in Fig. 4.12f. With increasing the
nanofiller contents of HNTs and Cloisite 30B clays from 5 to 10 wt%, the fracture
surfaces of bionanocomposites films are altered from ductile characteristic to more
brittle behaviour, as illustrated in Fig. 4.12d, g, respectively. Similar phenomena
were also found in PVA/NBC bionanocomposites in Fig. 4.12j. It is well known that
decreasing surface roughness can make the failure mode of PVA bionanocomposite
films vary from ductile to brittle fracture [49], which is consistent with the reduced
mechanical properties of bionanocomposite films in this study.