*3.6. Fractured Surface Morphology of the TPS Bio-Composite and TPS Hybrid Bio-Composite Films as Observed through Scanning Electron Microscopy (SEM) Analyses*

The tensile fractured surface morphologies of the TPS, TPS bio-composite and TPS hybrid bio-composite were studied by Scanning Electron Microscopy (SEM). Figure 12a shows that the fractured surface of the TPS films was uniform and no residual for the starch granule structure can be observed. The smooth surface indicates that the origin starch granule structure was completely disrupted and broken when starch was plasticized with glycerol and water at a high temperature. The TPS film possesses a smooth surface showing that the unfilled TPS underwent a very low degree of matrix deformation and easily fracture upon the application of a small tensile load/force. Both TPS/5C (Figure 12e)

and TPS/5B (Figure 12b) also show a clean and smooth fracture surface, indicating that both types of the TPS bio-composites have experienced some degree of matrix deformation upon tensile loading, but not much. For the TPS/5B bio-composite film, no image of B filler could be captured due to the very small size of the nano-bentonite. However, for the TPS/5C bio-composite film, the fractured surface shows some aggregation of C (circle in red color) inside the matrix of the TPS due to the strong intramolecular hydrogen bonding between the particles of C. This filler–filler interaction caused aggregation of C in the TPS matrix and prohibited the bio-composite to achieve a higher tensile strength. This was in line with the study of Zhang et al. where the nanocellulose agglomeration phenomenon in the host TPS had caused uneven loading distribution, void formation in the matrix and subsequently led to premature failure [33]. Small voids were observed due to the agglomerated C fiber pullout from the matrix phase. The C fiber was pulled out in the same direction of the tensile force applied. By adding 1% of C as co-filler with B, the fractured surface morphology of the TPS film has changed drastically. The surface roughness increased due to the occurrence of significant matrix deformation. This tallies with the tensile data which indicate that the elongation at break of this TPS/4B1C film was higher than the unfilled TPS and TPS bio-composite with single filler. However, there is no agglomeration and aggregation of C spotted in the SEM images of the TPS/4B1C, indicating that the C particles have been well dispersed inside the TPS matrix due to the good compatibility and interactions with the matrix and B filler. Furthermore, there is no fiber pullout that can be observed in the surface of the TPS/4B1C bio-composite film. This could be attributed to better interface interaction between C and the matrix with the presence of B. The C fiber was well wetted and embedded in the TPS matrix and led to high contact surface area between C and the TPS. In addition, the appearance of a tiny fibril structure bridging the small voids (blue color circle in Figure 12g,h) was noticed, suggesting that the C filler has been vertically oriented to the force applied during the tensile test. A similar structure of nanocellulose bridging in TPS was also observed by Li et al. [40].

the matrix and subsequently led to premature failure [33]. Small voids were observed due to the agglomerated C fiber pullout from the matrix phase. The C fiber was pulled out in the same direction of the tensile force applied. By adding 1% of C as co-filler with B, the fractured surface morphology of the TPS film has changed drastically. The surface roughness increased due to the occurrence of significant matrix deformation. This tallies with the tensile data which indicate that the elongation at break of this TPS/4B1C film was higher than the unfilled TPS and TPS bio-composite with single filler. However, there is no agglomeration and aggregation of C spotted in the SEM images of the TPS/4B1C, indicating that the C particles have been well dispersed inside the TPS matrix due to the good compatibility and interactions with the matrix and B filler. Furthermore, there is no fiber pullout that can be observed in the surface of the TPS/4B1C bio-composite film. This could be attributed to better interface interaction between C and the matrix with the presence of B. The C fiber was well wetted and embedded in the TPS matrix and led to high contact surface area between C and the TPS. In addition, the appearance of a tiny fibril structure bridging the small voids (blue color circle in Figure 12g,h) was noticed, suggesting that the C filler has been vertically oriented to the force applied during the tensile test. A sim-

ilar structure of nanocellulose bridging in TPS was also observed by Li et al. [40].

**Figure 12.** SEM images of the tensile fractured surface of the (**a**,**b**) TPS films, (**c**,**d**) TPS/5B, (**e**,**f**) TPS/5C, (**g**,**h**) and TPS/4B1C at 500× magnification (**left**) and 800× magnification (**right**)**. Figure 12.** SEM images of the tensile fractured surface of the (**a**,**b**) TPS films, (**c**,**d**) TPS/5B, (**e**,**f**) TPS/5C, (**g**,**h**) and TPS/4B1C at 500× magnification (**left**) and 800× magnification (**right**)**.**

#### **4. Conclusions 4. Conclusions**

Microcrystalline cellulose (C) was extracted from the raw OPEFB fiber and nano-bentonite (B) was ultra-sonicated to reduce the large tactoid structure. The changes in structure and morphology of the fillers from their original form after being subjected to chem-Microcrystalline cellulose (C) was extracted from the raw OPEFB fiber and nanobentonite (B) was ultra-sonicated to reduce the large tactoid structure. The changes in structure and morphology of the fillers from their original form after being subjected to

ical treatment and the ultra-sonication process have been witnessed and confirmed through FTIR, XRD and SEM analyses. Both fillers were used to form hybrid fillers in the

hybrid bio-composite films were produced by the casting method and the effects of the OPEFB microcrystalline cellulose (C)/nano-bentonite (B) ratio on the mechanical properties of the resultant TPS bio-composite films were studied. The highest tensile strength, Young's modulus and tensile toughness was achieved when the TPS was incorporated with 4wt% B and 1% of C filler (TPS/4B1C). The use of this hybrid filler system resulted in a more significant enhancement in the tensile properties of the film as compared to the single B or C filler. The results signify that B/C hybrid fillers have brought a positive synergistic effect to the TPS matrix. The good compatibility between C, B and TPS was observed through the FTIR, XRD and SEM analyses. The findings demonstrate that the combination of the OPEFB microcrystalline cellulose and nano-bentonite can produce an efficient hybrid filler system that allows for greater enhancement in the mechanical properties of the TPS-based film for packaging applications. Our work signifies that this hybridization approach is a powerful method to toughen the TPS film. This encourages further exploration on the potential of this hybrid filler system for toughening other bio-polymers/polymers. More importantly, all the ingredients used to form the hybrid TPS biocomposite are environmentally friendly and renewable, thus contributing to the develop-

**Author Contributions:** Conceptualization, A.F.O. and S.A.A.; data curation, D.S.L. and A.A.A.; formal analysis, D.S.L. and A.A.A.; investigation, A.F.O. and D.S.L.; methodology, A.F.O., D.S.L. and I.I.; project administration, A.F.O., S.A.A. and I.I.; software, M.N.A.S. and A.-U.H.; writing original draft, D.S.L.; writing—review and editing, A.F.O., M.N.A.S. and A.-U.H. All authors have

**Funding:** The author would like to acknowledge the support from the Fundamental Research Grant Scheme (FRGS) under a grant no: FRGS/1/2019/TK10/UNIMAP/03/2 from the Ministry of

**Data Availability Statement:** The data presented in this study are available on request from the

**Informed Consent Statement:** Not applicable for studies not involving humans.

ment of sustainable packaging materials.

Education Malaysia.

corresponding author.

read and agreed to the published version of the manuscript.

**Institutional Review Board Statement:** Not applicable.

chemical treatment and the ultra-sonication process have been witnessed and confirmed through FTIR, XRD and SEM analyses. Both fillers were used to form hybrid fillers in the production of the TPS hybrid bio-composite film. The TPS bio-composite films and TPS hybrid bio-composite films were produced by the casting method and the effects of the OPEFB microcrystalline cellulose (C)/nano-bentonite (B) ratio on the mechanical properties of the resultant TPS bio-composite films were studied. The highest tensile strength, Young's modulus and tensile toughness was achieved when the TPS was incorporated with 4 wt% B and 1% of C filler (TPS/4B1C). The use of this hybrid filler system resulted in a more significant enhancement in the tensile properties of the film as compared to the single B or C filler. The results signify that B/C hybrid fillers have brought a positive synergistic effect to the TPS matrix. The good compatibility between C, B and TPS was observed through the FTIR, XRD and SEM analyses. The findings demonstrate that the combination of the OPEFB microcrystalline cellulose and nano-bentonite can produce an efficient hybrid filler system that allows for greater enhancement in the mechanical properties of the TPS-based film for packaging applications. Our work signifies that this hybridization approach is a powerful method to toughen the TPS film. This encourages further exploration on the potential of this hybrid filler system for toughening other bio-polymers/polymers. More importantly, all the ingredients used to form the hybrid TPS bio-composite are environmentally friendly and renewable, thus contributing to the development of sustainable packaging materials.

**Author Contributions:** Conceptualization, A.F.O. and S.A.A.; data curation, D.S.L. and A.A.A.; formal analysis, D.S.L. and A.A.A.; investigation, A.F.O. and D.S.L.; methodology, A.F.O., D.S.L. and I.I.; project administration, A.F.O., S.A.A. and I.I.; software, M.N.A.S. and A.U.-H.; writing—original draft, D.S.L.; writing—review and editing, A.F.O., M.N.A.S. and A.U.-H. All authors have read and agreed to the published version of the manuscript.

**Funding:** The author would like to acknowledge the support from the Fundamental Research Grant Scheme (FRGS) under a grant no: FRGS/1/2019/TK10/UNIMAP/03/2 from the Ministry of Education Malaysia.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable for studies not involving humans.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.
