4.7 Mechanical Properties
99
much closer interactions between PVA matrices and NBCs via the formation of
mechanical and hydrogen bonds owing to highly porous structures of NBCs. On the
other hand, a different trend for tensile strengths of PVA/NBC bionanocomposites
was revealed from those of PVA/Cloisite 30B clay bionanocomposites and PVA/HNT
bionanocomposites, as shown in Fig. 4.11b. The tensile strength of PVA/HNT
bionanocomposites was improved by 23% with the addition of 3 wt% HNTs relative to that of neat PVA at 70.32 MPa. Nonetheless, a drastic strength-decreasing
tendency occurred with the strength reductions by 3.2 and 13.9% when embedded
with 5 and 10 wt% HNTs, respectively. Such results indicated that the enhancement of tensile strengths for PVA/HNT bionanocomposites depended on effective
stress transfer from PVA matrices to HNTs, mostly resulting from homogeneous
HNT dispersion within PVA matrices. On the contrary, increasing the HNT content
inevitably gave rise to noticeable particle aggregation with more stress concentration sites around HNT agglomerates as a result of potential crack initiation to
deteriorate mechanical performance of such bionanocomposites. With respect to
PVA/Cloisite 30B clay bionanocomposites, their tensile strengths were increased
by 18.8 and 28.4% with the incorporation of 3 and 5 wt% Cloisite 30B clays,
respectively, which was ascribed to more uniform clay dispersion, as well as the
formation of stronger matrix–filler network structures, resulting from increasing
hydrogen bonding between these constituents due to larger clay surface areas [42,
43]. When Cloisite 30B clay content increased up to 10 wt%, the tensile strength
of PVA/Cloisite 30B clay bionanocomposites was decreased by 5.16% as opposed
to that of near PVA. This finding suggested that the aggregation of nanofillers at
high clay content levels could diminish tensile strength. On the contrary, the tensile
strength of PVA/NBC bionanocomposites possessed the initial improvement up to
147.94 MPa (by a maximum level of 110.4%) when the NBC content increased
from 0 to 3 wt%. Beyond 3 wt% NBCs, tensile strengths of such bionanocomposites
tended to decline until they reached the lowest strength levels of 96.34 MPa at the
NBC content of 10 wt% despite being still better than that of neat PVA. Overall, both
tensile moduli and tensile strengths of PVA/NBC bionanocomposites were consistently superior to those of PVA/HNT bionanocomposites and PVA/Cloisite 30B clay
bionanocomposites, thus validating the most effective reinforcement efficiency of
NBCs among all three different nanofillers.
The elongation at break and tensile toughness of PVA/Cloisite 30B clay
bionanocomposites and PVA/NBC bionanocomposites continuously decreased especially beyond the nanofiller content of 3 wt%, Fig. 4.11c, d. The maximum decreasing
levels by approximately 59.5 and 58% in elongation at break were detected for PVA
bionanocomposites reinforced with 10 wt% Cloisite 30B clays and NBCs, respectively. This finding can be associated with the stiffening effect from filler reinforcements of NBCs and Cloisite 30B clays to restrict the movement of PVA molecular
chains, thus resulting in the overall reduction in the flexibility of bionanocomposite
films [44]. As for PVA/HNT bionanocomposites, elongation at break and tensile
toughness were increased by 12.7 and 16.9% with the incorporation of 3 wt% HNTs,
beyond which both of them remarkably diminished until the maximum reductions of
50 and 45.3% took place at the HNT content of 10 wt%, respectively, as opposed to
99
much closer interactions between PVA matrices and NBCs via the formation of
mechanical and hydrogen bonds owing to highly porous structures of NBCs. On the
other hand, a different trend for tensile strengths of PVA/NBC bionanocomposites
was revealed from those of PVA/Cloisite 30B clay bionanocomposites and PVA/HNT
bionanocomposites, as shown in Fig. 4.11b. The tensile strength of PVA/HNT
bionanocomposites was improved by 23% with the addition of 3 wt% HNTs relative to that of neat PVA at 70.32 MPa. Nonetheless, a drastic strength-decreasing
tendency occurred with the strength reductions by 3.2 and 13.9% when embedded
with 5 and 10 wt% HNTs, respectively. Such results indicated that the enhancement of tensile strengths for PVA/HNT bionanocomposites depended on effective
stress transfer from PVA matrices to HNTs, mostly resulting from homogeneous
HNT dispersion within PVA matrices. On the contrary, increasing the HNT content
inevitably gave rise to noticeable particle aggregation with more stress concentration sites around HNT agglomerates as a result of potential crack initiation to
deteriorate mechanical performance of such bionanocomposites. With respect to
PVA/Cloisite 30B clay bionanocomposites, their tensile strengths were increased
by 18.8 and 28.4% with the incorporation of 3 and 5 wt% Cloisite 30B clays,
respectively, which was ascribed to more uniform clay dispersion, as well as the
formation of stronger matrix–filler network structures, resulting from increasing
hydrogen bonding between these constituents due to larger clay surface areas [42,
43]. When Cloisite 30B clay content increased up to 10 wt%, the tensile strength
of PVA/Cloisite 30B clay bionanocomposites was decreased by 5.16% as opposed
to that of near PVA. This finding suggested that the aggregation of nanofillers at
high clay content levels could diminish tensile strength. On the contrary, the tensile
strength of PVA/NBC bionanocomposites possessed the initial improvement up to
147.94 MPa (by a maximum level of 110.4%) when the NBC content increased
from 0 to 3 wt%. Beyond 3 wt% NBCs, tensile strengths of such bionanocomposites
tended to decline until they reached the lowest strength levels of 96.34 MPa at the
NBC content of 10 wt% despite being still better than that of neat PVA. Overall, both
tensile moduli and tensile strengths of PVA/NBC bionanocomposites were consistently superior to those of PVA/HNT bionanocomposites and PVA/Cloisite 30B clay
bionanocomposites, thus validating the most effective reinforcement efficiency of
NBCs among all three different nanofillers.
The elongation at break and tensile toughness of PVA/Cloisite 30B clay
bionanocomposites and PVA/NBC bionanocomposites continuously decreased especially beyond the nanofiller content of 3 wt%, Fig. 4.11c, d. The maximum decreasing
levels by approximately 59.5 and 58% in elongation at break were detected for PVA
bionanocomposites reinforced with 10 wt% Cloisite 30B clays and NBCs, respectively. This finding can be associated with the stiffening effect from filler reinforcements of NBCs and Cloisite 30B clays to restrict the movement of PVA molecular
chains, thus resulting in the overall reduction in the flexibility of bionanocomposite
films [44]. As for PVA/HNT bionanocomposites, elongation at break and tensile
toughness were increased by 12.7 and 16.9% with the incorporation of 3 wt% HNTs,
beyond which both of them remarkably diminished until the maximum reductions of
50 and 45.3% took place at the HNT content of 10 wt%, respectively, as opposed to