5.2 Interphase Characterisation of PVA-Based Bionanocomposites
123
for PVA/NBC bionanocomposites shown in Fig. 3.10c. Such a great difference in
measurements can be ascribed to the following reasons: (i) Though instrumental
parameters have been calibrated prior to AFM measurements, the shape function of
the topmost probe tip may not be accurate enough for the low penetration depth [10,
11]. In this study, the low penetration force has been used, resulting in a low penetration depth in order to minimise the effects of residual stresses, as well as plastic
deformation from the neighbouring indent with the capability of high lateral resolution particularly used for nanointerphases. (ii) Nanosurface properties often vary
from those bulk properties because of the discrepancy in morphological structures
between outer skins on material surfaces and bulk films, as well as the differences in
nanomechanical behaviour and bulk properties [11]. (iii) Despite the data reliability
and repeatability of DMT modulus, its determination using the PFQNM is apparently
distinct from tensile modulus for bulk properties using conventional tensile testing
due to different measurement mechanisms [12].
5.2.3 3D Interphase Dimensions and Modulus
Elastic properties of interphase generally vary when surrounded by individual particles in different sizes and shapes, thus leading to the change of interphase dimensions. Interphase dimensions can be determined according to AFM height and adhesion images of PVA/NBC bionanocomposite samples, depicted in Fig. 5.5a, b, as
well as Fig. 5.5c, d, respectively. As evidently seen in Fig. 5.5a, b, the height of
NBCs is generally much greater than that of PVA. It is clearly shown that a linearly
increasing tendency in height occurs from interphase regions near PVA matrices to
those surrounding NBCs. NBCs tend to be easily distinguished from PVA matrices
due to their different adhesion properties. The tip adhesion to PVA matrices in
PVA/NBC bionanocomposites was determined to be 10.76 ± 3.42 nN, which was
found to be over five-fold greater than that of NBCs at 2.1 ± 0.87 nN. Such a finding
can be associated with the hydrophilic nature of PVA with relatively high adhesion
force when compared with hydrophobic NBCs. Accordingly, interphase thickness is
represented by the scan distances of 16 and 13 nm with a sharp decreasing adhesion
gradient from PVA matrices to NBCs on both sides of selected material regions, as
illustrated in Fig. 5.5d. This result is in good accordance with the values of 13 and
12.5 nm, previously determined for epoxy/graphene nanoplatelet (GNP) composites
and epoxy/graphene oxide (GO) nanocomposites, respectively [13]. A similar procedure was used to detect interphase thickness in PVA/HNT bionanocomposites and
PVA/Cloisite 30B clay bionanocomposites according to height and adhesion profiles
depicted in Figs. 5.6 and 5.7, respectively. In both bionanocomposite systems, the tip
adhesion to PVA matrices was 14.24 ± 2.31 nN, which appeared to be greater than
those for HNTs and Cloisite 30B clays at 8.6 ± 1.08 and 8.4 ± 1.1 nN, respectively.
It was manifested that the tip adhesions to HNTs and Cloisite 30B clays were still
much higher than that of NBCs at 2.1 ± 0.87 nN, resulting from more hydroxyl
groups detected in HNTs and Cloisite 30B clays according to previous FTIR results
123
for PVA/NBC bionanocomposites shown in Fig. 3.10c. Such a great difference in
measurements can be ascribed to the following reasons: (i) Though instrumental
parameters have been calibrated prior to AFM measurements, the shape function of
the topmost probe tip may not be accurate enough for the low penetration depth [10,
11]. In this study, the low penetration force has been used, resulting in a low penetration depth in order to minimise the effects of residual stresses, as well as plastic
deformation from the neighbouring indent with the capability of high lateral resolution particularly used for nanointerphases. (ii) Nanosurface properties often vary
from those bulk properties because of the discrepancy in morphological structures
between outer skins on material surfaces and bulk films, as well as the differences in
nanomechanical behaviour and bulk properties [11]. (iii) Despite the data reliability
and repeatability of DMT modulus, its determination using the PFQNM is apparently
distinct from tensile modulus for bulk properties using conventional tensile testing
due to different measurement mechanisms [12].
5.2.3 3D Interphase Dimensions and Modulus
Elastic properties of interphase generally vary when surrounded by individual particles in different sizes and shapes, thus leading to the change of interphase dimensions. Interphase dimensions can be determined according to AFM height and adhesion images of PVA/NBC bionanocomposite samples, depicted in Fig. 5.5a, b, as
well as Fig. 5.5c, d, respectively. As evidently seen in Fig. 5.5a, b, the height of
NBCs is generally much greater than that of PVA. It is clearly shown that a linearly
increasing tendency in height occurs from interphase regions near PVA matrices to
those surrounding NBCs. NBCs tend to be easily distinguished from PVA matrices
due to their different adhesion properties. The tip adhesion to PVA matrices in
PVA/NBC bionanocomposites was determined to be 10.76 ± 3.42 nN, which was
found to be over five-fold greater than that of NBCs at 2.1 ± 0.87 nN. Such a finding
can be associated with the hydrophilic nature of PVA with relatively high adhesion
force when compared with hydrophobic NBCs. Accordingly, interphase thickness is
represented by the scan distances of 16 and 13 nm with a sharp decreasing adhesion
gradient from PVA matrices to NBCs on both sides of selected material regions, as
illustrated in Fig. 5.5d. This result is in good accordance with the values of 13 and
12.5 nm, previously determined for epoxy/graphene nanoplatelet (GNP) composites
and epoxy/graphene oxide (GO) nanocomposites, respectively [13]. A similar procedure was used to detect interphase thickness in PVA/HNT bionanocomposites and
PVA/Cloisite 30B clay bionanocomposites according to height and adhesion profiles
depicted in Figs. 5.6 and 5.7, respectively. In both bionanocomposite systems, the tip
adhesion to PVA matrices was 14.24 ± 2.31 nN, which appeared to be greater than
those for HNTs and Cloisite 30B clays at 8.6 ± 1.08 and 8.4 ± 1.1 nN, respectively.
It was manifested that the tip adhesions to HNTs and Cloisite 30B clays were still
much higher than that of NBCs at 2.1 ± 0.87 nN, resulting from more hydroxyl
groups detected in HNTs and Cloisite 30B clays according to previous FTIR results