Extrusion grade biodegradable poly (butylene succinate) (PBS) was purchased from Dongguan Zhanyang Polymer Materials Co., Ltd. (Guangdong, China). The PBS’s physical characteristics were: density, 1.26 g/cm³ at 25°C; molecular weight, 8.0 × 10⁴ g/mol; melting point, 115°C; melt flow rate 22 g/10 min; flexural strength, 40 MPa; and stress at break, 30 MPa. Also, the PBS was spheroidal in shape with a size of 4 × 3 × 2 mm (length × width × height) and porosity of 34.6%. Raw organic baobab (Adansonia digitata) powder (BP) was bought from MRM Nutrition Company (Oceanside, CA, United States). The BP powder was stored in an air-tight container and kept in the dark and dry condition until downstream application.

Before mixing, both PBS granules and BP were vacuum-dried for 24 h at 60 and 37°C, respectively. PBS/BPs at 90/10, 80/20, 70/30, 60/40, and 50/50 wt% were dry-mixed using a laboratory mixer (GM-300, Retsch GmbH, Germany) at 1,000 rpm. The various blends were then added into a batch mixer (CW Brabender, United States) at 110–115°C using a rotor speed of 20 rpm. PBS/BP composites were then extruded using a double-screw extruder (HAAKE PolyLab OS, Thermo Fisher Scientific Inc., Germany) at 110–115°C. Hence, PBS/BP granules at 90/10, 80/20, 70/30, 60/40, and 50/50 wt% ratios were synthesized with the following physical characteristics: length × diameter: 3 × 2 mm (cylindrical shape); density, 1.18 g/cm³ at 25°C; porosity, 28.6%. Thin films were processed under compression from the PBS/BP blends at 130°C and used for viscosity, storage modulus, and loss modulus analyses.

For constructing all the spectra data for data analyses, average values ±standard deviation (SD) were used. Analysis of variance (ANOVA) was used for all measurements, and p ≤ 0.05 was considered statistically significant. Duncan, least significant differences (LSD), and Pearson tests have been calculated to measure the signification among the tested groups.

FTIR technique helps identify the chemical bonds and possible structural modifications that occurred due to blending PBS and BP. As shown in Figure 1, it was observed from the FTIR spectra that the absorption bands assigned to functional groups showed a greater degree of similarity between the pure PBS and the PBS/BP blend at a higher wavenumber range (4,000–1,750 cm⁻¹), with more pronounced differences at lower wavenumber range (1,750–400 cm⁻¹). For both the pure PBS and PBS/BP blends, the absorption bands observed at 3,435 and 2,945 cm⁻¹ could attributes to the elongation vibration of the O−H and asymmetric stretching vibration of −CH₂ − groups, respectively (Zhu et al., 2015; Li et al., 2019).

Moreover, the absorption peaks at 1,722, 1,471, 1,392, 1,160, 1,046, and 748 cm⁻¹ were ascribed to the double bond of carbonyl (C=O), methyl of lignin’s asymmetric bending (−CH₃), C−H deformation in cellulose and hemicellulose, stretching bond of ester (−C−O−C−), primary alcohol C−O bond, and aromatic rings of lignin, respectively (MonikaPal et al., 2018; Qi et al., 2020). Other important peaks obtained are 917 and 805 cm⁻¹ attributed to C−OH bending in the carboxylic group and the cross-linking of the C−C bond, which affects the strength of ester linkages, as reported by MonikaPal et al., 2018. From these results, it was clear that the addition of BP had only slightly modified the pure PBS’s composition while allowing its original functional chemical structure intact. Hence, the new PBS/BP blend would perform the functions of PBS at a greater significance, with other potentially added advantages envisaged.

Figure 2 depicts the representative results of the XRD analysis of the PBS/BP blends and pure PBS. It was found that the PBS/BP blends were a replica of the semi-crystalline PBS as the two sharp peaks (19.4° and 22.5°) presented a decreasing nature of intensity due to an increase in BP’s wt% in the blend. These two characteristics peaks obtained from the XRD spectrum, in addition to 21.6 and 29.2 peaks, were similarly reported by Ye et al. (2017) after analyzing raw PBS and its blend with urea. Hence, the obtained crystalline structure results showed that the blends were just a numeral superposition and a physical mixture of PBS and BP with a reduced pure PBS crystallinity (Sadeghi et al., 2021) suggests faster degradation of the blends than pure PBS.

Table 1 shows the analyzed tensile properties of the pure PBS and the PBS/BP blends. It was observed from the results (Table 1) that pure PBS’s Young modulus was 333 ± 9.22 MPa, which was consistent with the results reported for pure PBS in the previous literature (Delamarche et al., 2020; Jin et al., 2014). Furthermore, increases in Young’s modulus values were recorded as the BP ratios were increased in the blends, as depicted in Table 1. This increase in Young’s modulus values of pure PBS after blending with BP was similarly reported by Yang et al. (2010) after blending PBS with kenaf fibre. However, significant decreases (p < 0.05) were recorded for the yield strength and the maximum strain at break (i.e., fracture strain) as the BP ratios were increased, which were consistent with the results reported in the literature while increasing fibre ratio in PBS (Yang et al., 2010). Hence, these results implied that the higher the BP levels, the faster the degradability of the PBS and less energy required for elongation.

Figure 3 showed detail of elemental contents of the PBS/BP blends using EDS, which revealed significant modification of pure PBS’s original form mainly dominated by carbon (C = 60.08 wt%) and oxygen (O = 39.92 wt%), as depicted in Figure 3A. Meanwhile, Figures 3B–F represents the BP’s level in the PBS/BP blends at 10, 20, 30, 40, and 50 wt%, respectively, with reduced molecular weight compared to pure PBS as it enhances faster degradation (Delamarche et al., 2020). Interestingly, the EDS results (Figures 3B–F) confirmed that the pure PBS was super enriched (after adding BP) by several essential elements, both macro-and-micro nutrients: Na, Mg, P, K, Ca, Cu, and Zn, at varied ratios (Rahul et al., 2015). These essential elements were reported by Singh (2021) to be solely responsible for biomolecule formation, metabolism, and co-factor of enzymes for protein structure stabilization in the cells. Additionally, these macro-and-micro nutrients were proved to be essential in supporting bacterial growth due to adequate nutrition supplementation (Li et al., 2019; Tadda et al., 2021). Besides, it was observed that higher BP content results in higher amounts of essential elements and less carbon and oxygen percent. Hence, faster degradation of PBS/BP will result in higher BP ratios, eventually releasing less carbon into the environment.

Figure 4 showed the SEM micrographs (5,000×), depicting the morphology of PBS/BP blends and pure PBS to ascertain the levels at which their surface structure changes due to BP’s addition. It was observed that pure PBS’s surface morphology was dominated by coarser macro-voids which were unevenly distributed (Figure 4A), compared with other micrographs from PBS/BP blends. Also, the micrographs (Figures 4B–F) showed asymmetric surface structures in which higher BP contents resulted in a reduction in the membrane porosity and cavity size due to the degradation of macro-voids of the pure PBS. When comparing the reduction in surface voids, the decreasing order of the PBS/BP blends’ porosity due to BP addition was: 10 > 20 > 30 > 40 > 50 wt%. For instance, 50 wt% BP depicted in Figure 4F showed a smoother surface structure with fewer surface pores, which is more prone to degradation than 10 wt% BP presented in Figure 4B. Interestingly, our findings from the SEM micrographs were in total agreement with the results reported by Sadeghi et al. (2021), whereby a higher amount of PBS (10 and 20 wt%) revealed an increase in both porosity and instability of pure PCL.

TGA and DSC curves which revealed thermal stability of the pure PBS and PBS/BP blends, were presented in Figure 5. It was observed from Figure 5A that the thermal degradation of pure PBS began at 285°C and achieved 98.7% weight loss at 425°C, which was consistent with a previous study (Hassan et al., 2013). For the PBS/BP blends, it was interesting to note that the degradation took place in three steps: 55 ̴ 200°C; 200 ̴ 330°C; and 330–410°C, with 50 wt% BP blend showing the fastest degradation (Figure 5A). This discovery confirmed that BP’s addition had improved the degradation rate of the pure PBS in ascending order of BP’s percent level in the blends, as clearly supported by the results from the derivative weight curve (DTG) in Figure 5B and that of SEM in Figures 4B–F. However, lower weight loss was observed from the blends than pure PBS due to higher ash content in the PBS/BP blends. This low weight loss was an indication that less carbon was released from the blends, which in practice will amount to reduced carbon pollution when the blends are used in place of pure PBS. Also, it is worth noting that from the DTG curves (Figure 5B), changes were observed in the PBS/BP blends’ results from around 70–345°C, with the most significant change recorded at 243°C. Hence, better environmental management would emerge when the PBS/BP blends are used since their high ash content would add nutritional value to soil, ultimately leading to faster biodegradation and limiting carbon release to the environment.

Furthermore, DSC analysis results for both cooling and heating scans, were presented in Figures 5C,D, respectively. From the results, the melting temperature (T_m) of pure PBS was obtained as 114°C (Figure 5C), which was within the standard T_m for PBS (110–115°C) as reported in the previous studies (Dai and Qiu, 2016; Fenni et al., 2019; Delamarche et al., 2020). It was noted that PBS/BP blends of 90/10 and 50/50 showed a significantly reduced T_m of 94°C (p < 0.05) while other blends showing a T_m of a few degrees less of pure PBS (2–3°C) but with higher heat flow than the pure PBS (Figure 5C). However, for the crystallization temperature (T_c), which results due to cooling of the samples, significant variations were observed with fractionalized crystallization, as depicted in Figure 5D. This phenomenon of fractionated crystallization was a common behavior exhibited by polymer blends due to the heterogeneity of the blend materials (Fenni et al., 2019). Pure PBS had the lowest T_c of 39.5°C, and the lowest heat flow of 5.1 mW/mg, while PBS/BP blend at 90/10 recorded the highest peak (9.4 mW/mg) at T_c of 45.7°C (Figure 5D). In addition, it was also noted from the DSC results that the blends with the highest BP contents (40 and 50 wt%) had entered the endothermic phase at 150°C with other samples showing a lag of 20°C during the endothermic phase change (130°C). Accordingly, these significant variations recorded for T_c have confirmed the changes in intermolecular interactions caused by blending BP with PBS, ultimately ensuring the blends’ degradation at a faster rate.

The storage modulus (E′) and loss modulus (E″) curves which depend on the variations of temperature, were presented in Figures 6A,B, respectively. It was observed that the E′ curves displayed an obvious increasing tendency in ascending order of BP content’s increase in the blends (Figure 6A), which was due to stress transfer from the pure PBS to the BP (Liang et al., 2010). Also, Liang et al. (2010) reported a result consistent with our finding whereby PBS’s E′ increased by more than 100% after blending with kenaf fiber (KF) at 90/10, 80/20, and 70/30 wt% (PBS/KF). It is worth noting that PBS/BP blends at 90/10 wt% showed the least increment of about 25%, while BP content of 20 wt% and above recorded more than 100% increase over the temperature ranges applied. For example, the storage modulus of pure PBS at 40°C was only 302 MPa but increased to 367, 714, 909, and 1,077 MPa for PBS/BP blends at 90/10, 80/20, 70/30, and 60/40, respectively. Such increases might be due to the increase in crystallinity of the PBS/BP blends as supported by the DSC curves explained earlier (Figure 5D).

More so, it was noted that E′ values of both pure PBS and the PBS/BP blends keep decreasing with temperature increase (Figure 6A), which was due to polymer matrix softening (Liang et al., 2010). It was observed that PBS/BP blend at 50/50 was too brittle to allow the test samples (size: 25 mm long, 5 mm wide, and 1 mm thick) to be carved out from the larger thin-film initially produced via compression molding. Therefore, this shows the unsuitability of using the 50/50 wt% combination to fabricate the targeted materials.

For the E″ values shown in Figure 6B, it was observed that both the pure PBS and PBS/BP blends exhibit the same manner displayed by E′ results, whereby the PBS/BP blends recorded higher E″ values than pure PBS. However, the addition of 10 wt% BP had shown a slight increase in E″ values compared to higher increased E″ values recorded in ascending order, after adding 20 wt% BP and above. Therefore, the results of E′ and E″ implied that further addition of BP, which simultaneously decreases the PBS wt%, decreases the blend’s molecular mobility, hence reducing the intermolecular bond friction needed to overcome mechanical loss (Liang et al., 2010). Thus, the blends with higher BP wt% would degrade much faster than those with lower wt%, with however 20 to 30 wt% suggested as the best combination in the PBS/BP blend.

Figure 6C shows the variation of viscosity levels among the PBS/BP blends and pure PBS for varying temperatures. It was observed that pure PBS’s viscosity decreased more rapidly after adding 10 and 20 wt% of BP. However, adding more than 20 wt% of BP had resulted in increased viscosity and irregular pattern of the viscosity curves, which might result from an increase in rigidity of molecular chain and decrease in molecular weight of the blends (Qu et al., 2019). The reduction in viscosity level observed from this study due to an increase in BP and decrease in PBS agreed with a previous study that reported a reduction in viscosity due to a low amount of PBS (Sadeghi et al., 2021).