*3.4. Mechanical Analysis of TPS, TPS/B and TPS/C Bio-Composites, TPS/B/C Hybrid Bio-Composite Films*

Figure 7a shows the tensile strength of the TPS and TPS bio-composite film with different nano-bentonite (B) and microcrystalline cellulose (C) ratios. Regardless of the types of filler used (single or hybrid), the tensile strength value of the film increased with the incorporation of 5 wt% fillers into the TPS matrix. These results were in accordance with many reviews where phyllosilicate clay and cellulose can improve the TPS composite films' tensile strength due to high compatibility and positive interactions such as hydrogen bonding between the TPS matrix and nanofiller, leading to better stress transfer [31,32]. The unfilled TPS exhibits the lowest tensile strength, which is 4.44 MPa, while the TPS/4B1C hybrid bio-composite film possesses the highest tensile strength (8.52 MPa), showing 92% increment when benchmarked with the unfilled TPS. This signifies that the inclusion of the hybrid B/C fillers at the optimum ratio can bring a positive synergy effect within the TPS matrix and enhance the tensile strength of the film with a more efficient load transfer mechanism. This explanation was in line with the results of Zakuwan et al., in which tensile strength improvement was obtained when cellulose nanocrystal and modified MMT was incorporated into the biopolymer. They claimed that the network forming of nanocellulose and MMT enforcement in a different dimension in the biopolymer matrix can improve bio-composite films' reinforcement capability [33].

**Figure 7.** Tensile properties of TPS, TPS bio-composites and TPS hybrid bio-composite films: (**a**) tensile strength; (**b**) Young's Modulus; (**c**) elongation at break. **Figure 7.** Tensile properties of TPS, TPS bio-composites and TPS hybrid bio-composite films: (**a**) tensile strength; (**b**) Young's Modulus; (**c**) elongation at break.

Based on Figure 7c, the TPS/1B4C hybrid bio-composite film demonstrates the elongation at a break value of 117%, which is 156% higher than the unfilled TPS. This is somewhat impressive as the addition of natural filler or inorganic filler always reduces the elongation at break of the matrix. This is because filler with a stiffer characteristic reduces the flexibility of the biopolymer [9–14]. The results also suggest that the use of single filler (either C alone or B alone) could not allow the film to achieve higher tensile strength than the hybrid fillers 4B1C. However, the use of C resulted in a higher increment in tensile strength as compared to B. As compared to C, B tends to experience a higher degree of agglomeration in the biopolymer matrix due to its platy particles that are easily stacked together to form tactoids. Poor dispersion of tactoids reduces the efficiency of the B filler in reinforcing the matrix. C can better reinforce the TPS due to its fibrous-like particles that can transfer the load more efficiently [34]. However, when used as hybrid filler with the B, C cannot be added in more than 1 wt%. We have postulated that the high content of C in the hybrid filler system can reduce the reinforcing effect because overcrowding of the C particles may occur and inhibit good

dispersion and distribution of the B's nanoplatelets in the matrix. Poorly dispersed hybrid fillers may form a stress concentration point in the TPS matrix and reduce the tensile properties of the TPS [35,36].

The Young's modulus values of the TPS, TPS bio-composites and TPS hybrid biocomposites with different B and C ratios are compared in Figure 7b. It can be seen that the Young's modulus values of all the TPS bio-composites are higher than the unfilled TPS, showing that incorporating single or hybrid fillers into the TPS may improve the stiffness of the film. The unfilled TPS exhibits the lowest Young's modulus, which is 17.1 MPa, while TPS/4B1C hybrid bio-composite demonstrates the highest Young's modulus, which is 42.0 MPa. However, the Young's modulus of the film has slightly decreased to 36.22 MPa when the C loading increased to 2 wt% (3B2C). Moreover, the Young's modulus values of the TPS/2B3C and TPS/1B4C films show no significant difference with the TPS/3B2C, indicating that increases in the C loading in the matrix does not further improve the stiffness of the film. Agglomeration of C particles may occur when added in high content, reducing the matrix–filler interactions [37]. In agreement with the tensile strength data, the Young's modulus of the TPS/5B is lower than the TPS/5C bio-composite film. As mentioned earlier, the platy particles of B can stack together in great numbers, poorly dispersed and distributed in the matrix. This reduces matrix–filler interactions that play the main role in stiffening the matrix.

Based on Figure 7c, the TPS/1B4C hybrid bio-composite film demonstrates the elongation at a break value of 117%, which is 156% higher than the unfilled TPS. This is somewhat impressive as the addition of natural filler or inorganic filler always reduces the elongation at break of the matrix. This is because filler with a stiffer characteristic reduces the flexibility of the biopolymer [9–14].

Tensile toughness is calculated based on the area under the stress–strain curve. The greater the area under the curve, the greater the toughness value will be, showing that the polymer performs greater tensile strength and elongation at break due to a more efficient energy absorption mechanism [38]. Figure 8 illustrates the tensile toughness of the TPS, TPS bio-composite and TPS hybrid bio-composite films. Overall, the results suggest that the film's tensile toughness has been tremendously improved upon incorporation of the hybrid fillers. TPS/4B1C displays the highest value of tensile toughness with an increment of ~338%, when compared with the unfilled TPS. The difference in physical characteristics (size, shape, etc.) and chemistry of B and C fillers might contribute to a greater toughening effect on the film. We are still investigating these factors and perhaps will come out with theory and a mechanism in our next publications after finishing all the analyses. For now, we are postulating that the fibrous particles of C, when dispersed well in the matrix, not only can contribute to good stress transfer mechanism, but will also produce more free volume in the matrix when the film is stretched. Figure 9 illustrates the proposed mechanism. During tensile deformation, the small size of B nanoplatelets and the TPS molecular chains will arrange and stretch according to strain direction. This will produce greater interface bonding between the B and the TPS molecular chains, thus producing a more effective stress transferring mechanism. The bigger and longer C particles are not as mobile as the B nanoplatelets and, therefore, will not exactly follow the strain direction, but rather arrange themselves vertically between the stretched TPS molecular chains. This resulted in the formation of more free volumes in the hybrid bio-composite structure, allowing for greater starch chain mobility (entropy) and conformational freedom. The conformation of the TPS chains will stimulate chain relaxation in the stress concentrated area and allow the matrix to be more flexible and more efficient in absorbing energy through molecular motions. These interactions might responsible for contributing to the toughening effect on the film.

ing effect on the film.

Tensile toughness is calculated based on the area under the stress–strain curve. The greater the area under the curve, the greater the toughness value will be, showing that the polymer performs greater tensile strength and elongation at break due to a more efficient energy absorption mechanism [38]. Figure 8 illustrates the tensile toughness of the TPS, TPS bio-composite and TPS hybrid bio-composite films. Overall, the results suggest that the film's tensile toughness has been tremendously improved upon incorporation of the hybrid fillers. TPS/4B1C displays the highest value of tensile toughness with an increment of ~338%, when compared with the unfilled TPS. The difference in physical characteristics (size, shape, etc.) and chemistry of B and C fillers might contribute to a greater toughening effect on the film. We are still investigating these factors and perhaps will come out with theory and a mechanism in our next publications after finishing all the analyses. For now, we are postulating that the fibrous particles of C, when dispersed well in the matrix, not only can contribute to good stress transfer mechanism, but will also produce more free volume in the matrix when the film is stretched. Figure 9 illustrates the proposed mechanism. During tensile deformation, the small size of B nanoplatelets and the TPS molecular chains will arrange and stretch according to strain direction. This will produce greater interface bonding between the B and the TPS molecular chains, thus producing a more effective stress transferring mechanism. The bigger and longer C particles are not as mobile as the B nanoplatelets and, therefore, will not exactly follow the strain direction, but rather arrange themselves vertically between the stretched TPS molecular chains. This resulted in the formation of more free volumes in the hybrid bio-composite structure, allowing for greater starch chain mobility (entropy) and conformational freedom. The conformation of the TPS chains will stimulate chain relaxation in the stress concentrated area and allow the matrix to be more flexible and more efficient in absorbing energy through molecular motions. These interactions might responsible for contributing to the toughen-

**Figure 8.** Tensile toughness of TPS, TPS bio-composites and TPS hybrid bio-composite films. **Figure 8.** Tensile toughness of TPS, TPS bio-composites and TPS hybrid bio-composite films.

*Polymers* **2021**, *13*, x FOR PEER REVIEW 2 of 22

**Figure 9.** Mechanism of interactions between the TPS and the hybrid fillers during tensile deformation. **Figure 9.** Mechanism of interactions between the TPS and the hybrid fillers during tensile deformation.

Comparative study was done with other TPS bio-composite systems investigated by other researchers. Based on the data obtained from recent literatures, our TPS hybrid biocomposite film's tensile strength is on par with the values reported, while the flexibility of our film is much better than them [13,14,39]. For instance, Fazeli et al. have used cellulose fibers to reinforce the TPS derived from corn. They have applied surface modification on the cellulose fiber using the air plasma treatment to improve the matrix/fiber adhesion in the TPS bio-composite structure. The maximum tensile strength was achieved when the plasma treated cellulose fiber was added in 6 wt%. However, elongation at break value has been dropped significantly from 48.2% to 9.2% only [13]. In another work, Chen et al. have prepared TPS bio-composite films containing oxidized microcrystalline cellulose (MCC) as a reinforcing filler via a hot-compression molding technique. As compared to the original MCC, the oxidized MCC was more capable of enhancing the tensile strength Comparative study was done with other TPS bio-composite systems investigated by other researchers. Based on the data obtained from recent literatures, our TPS hybrid biocomposite film's tensile strength is on par with the values reported, while the flexibility of our film is much better than them [13,14,39]. For instance, Fazeli et al. have used cellulose fibers to reinforce the TPS derived from corn. They have applied surface modification on the cellulose fiber using the air plasma treatment to improve the matrix/fiber adhesion in the TPS bio-composite structure. The maximum tensile strength was achieved when the plasma treated cellulose fiber was added in 6 wt%. However, elongation at break value has been dropped significantly from 48.2% to 9.2% only [13]. In another work, Chen et al. have prepared TPS bio-composite films containing oxidized microcrystalline cellulose (MCC) as a reinforcing filler via a hot-compression molding technique. As compared to the original MCC, the oxidized MCC was more capable of enhancing the tensile strength of the TPS.

of the TPS. However, the tensile strength obtained was 6.61 MPa while the elongation at break was only 40% [14]. In more recent work, Yin et al. have prepared the TPS bio-com-

value of 5.26 MPa. However, the bio-composite only managed to achieve elongation at a break value of 91.60% [39]. Both tensile strength and elongation at break values reported for the TPS bio-composites in the above literatures were lower than the values of our optimum hybrid bio-composite system (TPS/1B4C). As mentioned in the above discussion, the maximum tensile strength and elongation at break of the TPS hybrid bio-composite film in this study were 8.52 MPa and 117%, respectively. Apparently, the improvement in toughness value upon the inclusion of the hybrid fillers was impressive (+338%). The work by Li et al. has shown that they have successfully obtained very high tensile strength starch-based bio-composites when using montmorillonite and cellulose nanofiber as hybrid fillers. However, the elongation at break values of the samples was very low (less than 7%) compared to our hybrid TPS bio-composite films (117%). The above findings proved that that the origin of starch, method of processing and types of filler/hybrid fillers used will determine the obtained tensile strength and elongation at break of the bio-composite film. These are in agreement with review papers wrote by Pérez-Pacheco et al. [11] and Xie et al. [31]. In our case, the unfilled TPS has shown very moderate tensile strength, but the hybrid B/C fillers have played their role in simultaneously enhancing the tensile

However, the tensile strength obtained was 6.61 MPa while the elongation at break was only 40% [14]. In more recent work, Yin et al. have prepared the TPS bio-composites filled with dialdehyde lignocellulose (DLC) in the loading of 0 to 12 wt%. Maximum tensile strength was obtained when 3 wt% DLC was employed as filler with the value of 5.26 MPa. However, the bio-composite only managed to achieve elongation at a break value of 91.60% [39]. Both tensile strength and elongation at break values reported for the TPS bio-composites in the above literatures were lower than the values of our optimum hybrid bio-composite system (TPS/1B4C). As mentioned in the above discussion, the maximum tensile strength and elongation at break of the TPS hybrid bio-composite film in this study were 8.52 MPa and 117%, respectively. Apparently, the improvement in toughness value upon the inclusion of the hybrid fillers was impressive (+338%). The work by Li et al. has shown that they have successfully obtained very high tensile strength starch-based biocomposites when using montmorillonite and cellulose nanofiber as hybrid fillers. However, the elongation at break values of the samples was very low (less than 7%) compared to our hybrid TPS bio-composite films (117%). The above findings proved that that the origin of starch, method of processing and types of filler/hybrid fillers used will determine the obtained tensile strength and elongation at break of the bio-composite film. These are in agreement with review papers wrote by Pérez-Pacheco et al. [11] and Xie et al. [31]. In our case, the unfilled TPS has shown very moderate tensile strength, but the hybrid B/C fillers have played their role in simultaneously enhancing the tensile strength and elongation at break of the TPS matrix. Thus, significant enhancement in the tensile toughness of the film could be observed. This trend is rarely obtained and reported. Nevertheless, we believe that our films that possess good flexibility and toughness will suit for certain applications such as for short term flexible packaging and wrapping plastic.
