Figure 3 gives the typical tensile stress and strain behavior for neat TPS and its composites with different CNT contents. As seen in the figure, regardless of weight content, addition of MWCNTs was found to give rise to TPS toughness values calculated integrating the area under the stress and strain curves. Table I summarizes the corresponding tensile properties for TPS and its composites. As seen in the table, addition of 1 wt % of MWCNTs increased the TPS tensile strength by 55%, while tensile strength values of neat TPS and its composites with 0.1 and 0.5 wt % of MWCNTs scattered more or less around the same value (0.14 MPa). Note that the composites filled with 0.1 and 0.5 wt % of MWCNTs show relatively low elastic modulus and high elongation values than the neat TPS. It is very well known that the degree of polymer crystallinity may be significantly influential on the polymer mechanical properties, since it affects the extent of the intermolecular secondary bonding. For crystalline regions wherein molecular chains are packed in an ordered arrangement, wide-ranging secondary bonding occurs between adjacent chain segments. These bonds lead to especially polymer tensile modulus to increase significantly with degree of crystallinity. However, our results do not comply with the aforementioned general statement. Previously, XRD findings revealed that TPS crystallinity increases with MWCNTs. However, tensile modulus values of TPS were not proportionally improved by the addition of MWCNTs, except for 1 wt % loading rate. This may be due to the presence of acetic acid in vinegar that may result in formation of starch acetates during melt extrusion, which are widely used in biodegradable packaging foams.2, 11 In this scenario, MWCNT content and distribution characteristics play a pivotal role in the way starch acetates chemically form. In other words, this anomalous behavior may be resulting from preferential or virtually unpredictable distribution characteristics of MWCNTs around the micro-pores within TPS structure with respect to weight content. Recalling XRD findings, we should also take into account the TPS crystallization behavior as well. In polymers, surfaces are known to act as catalysts for the nucleation of crystals. In polymers patterned with pores, as in our case, it is possible that the shape of the pores sizes of which show dependency on CNT weight content can control the kinetics of surface-induced crystal nucleation. We can infer from these expressions that kinetics of crystal nucleation are linked to the way starch acetates occur as a function of CNT weight content, which stimulates CNTs to coalesce around the micro-pores formed. To put answers to these questions, SEM and TEM techniques were further conducted.
Figure 4 (a,b) give the SEM images showing the tensile fracture surfaces of neat TPS and its composites with 1 wt % of MWCNTs, respectively. It is obvious that incorporation of MWCNTs altered TPS fracture surface morphology and failure modes significantly. At 1 wt % loading rate, TPS with CNTs were fractured in a ductile manner although their fracture surfaces look like they are fractured in a brittle manner fracture, whereas neat TPS were fractured in a brittle manner making not a remarkable necking before failed. This is most probably due to modified chemistry of TPS in the presence of MWCNTs at different weight content within TPS. These behaviors are highly proportional to the trend observed in the stress–strain behavior of TPS with and without CNTs as depicted in Figure 3. Figure 5(a) is the high magnification SEM image showing the fracture surface of TPS composites containing 1 wt % of MWCNTs, while Figure 5(b) depicts the SEM image of an intentionally selected region, highlighted with a dashed circle in Figure 5(a), at relatively high magnification. As seen in the figures, porous structure is of characteristics of TPS with MWCNTs. Note that the TPS synthesized in this study possessed porous structure as a result of complex chemical reactions that occur mainly between stearic acid, acetic acid, and glycerol. The evaporation of water contained in starch based materials can also result in undesired bubble formation, which triggers the occurrence of both favorable and unfavorable chemical reactions in the presence of MWCNTs during melt extrusion. More than one of these can be cited as the reason for the TPS to exhibit anomalously low tensile strength and modulus values relative to their equivalences in the literature. Moreover, big white spots in Figure 4(a) refer to undissolved stearic acid particles. More interestingly, in the same figure, MWCNT seem to be accumulating between an individual semigelatinized starch granule and stearic acid particle, which is proportional to the XRD findings. From this point of view, interactions between plasticizers and MWCNTs as well as MWCNT contents are crucial to final TPS properties. In other words, TPS porous structure may, in this case, be highly related to MWCNT content, as stated earlier as our approach. To proceed with this approach, TEM characterization was necessarily needed.
TEM equipped with a HAADF detector was employed to visualize the dispersion state of MWCNTs within TPS matrix. Figure 6(a,b) depict the acquired atomic number (Z)-contrast images of MWCNT modified TPS matrix at different magnifications in STEM-HAADF mode. Note that conventional TEM dark field (DF) images are obtained from the signal emitted by elastic scattering of electrons, whereas STEM-HAADF images come out off the signal emitted by incoherent scattering of electrons at relatively high angles.21, 22 This makes STEM-HAADF capable of offering one to differentiate between the regions of different atomic numbers (Z).21–24 In Figure 6(a,b), dark regions refer to open pores, while gray regions and white spots, highlighted by arrowheads, refer to TPS characteristics structure and tiny MWCNT clusters, respectively. Moreover, it is worthy of mention that each corresponding region shows distinctly different contrast level, despite their similarity in that their own chemical structures are mainly composed of carbon. Properly speaking, in STEM-HAADF mode, the brighter a phase appears under the microscope, the more crystalline this phase is under the specified diffraction condition.23 MWCNT consists of multiple stacked single-walled carbon nanotubes with diameters ranging from 2 to 100 nm shows only one peak in the XRD diagram, as given in Figure 2(a,b). This is due to the arrays of well ordered individual concentric cylinders of MWCNTs that make MWCNT behave like a highly crystalline material. In Figure 6(a,b), the difference between the contrast levels of MWCNTs and TPS matrix are certainly noticeable. TPS matrix with a gray contrast color was considered as a semicrystalline material, which was already confirmed by the XRD findings given in Figure 2(b). In addition, it is highly remarkable that tiny MWCNT clusters are in the majority around the TPS porous structure. Herein the first question that comes to mind is whether the porous structures within TPS matrix result from the highly complicated chemical reactions taking place during complete starch gelatinization or whether they are induced by irradiation during electron beam exposure? For the sake of clarity, the intentionally selected region highlighted in Figure 6(a) was subjected to a continued electron beam exposure at specified time intervals. As a result, it was determined that no remarkable structural change occurred within this region as a function of time, indicating that the porous structures within TPS took place, most likely, because of chemical reactions during starch plasticization. Figure 7(a,b) show HR-TEM images taken from TPS with 1 wt % of MWCNTs at different magnifications. Individual MWCNTs in Figure 7(a) is noticeably visible. Figure 7(b) taken at relatively high magnification, from the white lined circular region apparent on the left side of Figure 7(a) depicts the interaction at the interface among MWCNTs, the plasticizers used and TPS matrix. The contrast level at the interface backs up our approach that MWCNTs accumulate between the plasticizers and the starch matrix. These findings are vastly proportional to those obtained from SEM images [Figure 5(a,b)]. Figure 8 shows the electrical conductivity of neat TPS and its composites containing different content of MWCNTs. It was determined that electrical conductivity of composites increased with increasing content of MWCNTs. In particular, TPS with 1 wt % of MWCNTs exhibited two orders of magnitude higher electrical conductivity value than neat TPS. In contrast to mechanical property enhancement of polymers via CNT, where a tailored interface with the strong adhesion of individual CNT to the surrounding polymer matrix is prerequisite, the electrical conductivity enhancement for polymers is just dependent on percolated pathways of tiny CNT clusters. In other words, small agglomerates accompanied by individual CNTs are highly preferable in terms of polymer electrical conductivity enhancement. Therefore, the quantity and distribution of tiny CNT clusters within a polymer matrix is critical to enhancing the electrical conductivity of a polymer matrix. SEM and STEM images showed that the dispersion characteristic of MWCNTs within TPS was in the form of tiny clusters, which is highly favorable to electrical conductivity enhancement. On the other hand, neat TPS exhibited, unexpectedly, as high electrical conductivity as a common synthetic thermoplastic polymer with a low content of conductive filler. Since starch is hydrophilic, water absorption capability of starch is important to performance of starch based biodegradable polymers. Ma et al.14 showed that the conductivity of the TPS was improved by five orders of magnitude once water content was increased by a factor of six. More importantly, they found that the higher content of MWCNTs within TPS, the less water-dependency TPS electrical conductivity shows. Moreover, since the TPS synthesized in this study possess semiporous structure, they could absorb more water than their bulk counterparts within the same intended time for equilibrium (three weeks prior to property characterization). As a result, due to their high water absorption capability, neat TPS showed almost as high electrical conductivity as TPS with MWCNTs.