Tensile properties
The results of the tensile properties are shown in Fig. 2. When comparing the tensile strength of pure PLA before and after annealing, the pure PLA had a higher tensile strength after annealing. The result of the PLA tensile test before annealing was found at 54.2 MPa. The tensile strength was enhanced to 59.9 MPa following the annealing process. This improvement in the tensile strength is due to a polymer recrystallization process that can create microspherical crystal structures during the annealing process [17, 18]. Moreover, the PLA–pectin composite demonstrated that the highest tensile strength was 2.5 times that of pure PLA with 4% pectin added. The increase in impact strength value partly resulted from the recrystallization process, as previously mentioned. This might be related to subsequent chemical interactions between pectin and PLA, especially when the composite contains 4% pectin. On the other hand, the higher levels of pectin addition at 6 and 8% resulted in PLA–pectin composites with lower tensile strength. Nevertheless, the annealing process increases the tensile strength in PLA–pectin composites with pectin contents of 6 and 8% compared to composites without annealing at the same pectin content. The decline in tensile strength at higher pectin contents of 6 and 8% can be affected by the larger grain particles of pectin in the PLA matrix. Li and Favis [19] showed that excessive amounts of compatibilizer higher than 5% w/w can create interface saturation, which does not improve tensile characteristics. In addition, larger spherulites also generate brittleness with low energy absorption during the debonding process following annealing [20, 21].
Tensile strength of pure PLA, PLA–pectin 2%, PLA–pectin 4%, PLA–pectin 6% and PLA–pectin 8% compared between annealing and nonannealing processes
Impact testing
The results of the impact reinforcement before the annealing process are shown in Fig. 3.
Impact strength of pure PLA, PLA–pectin 2%, PLA–pectin 4%, PLA–pectin 6%, and PLA–pectin 8% compared between annealing and nonannealing processes
The impact strength of the PLA–pectin composites was shown to decrease with increasing pectin concentration. The impact strength was reduced to 4.6 KJ/m2 compared to 7.0 KJ/m2 for pure PLA. This might be the effect from the increased level of heterogeneous nucleation between PLA and the pectin matrix that was provided with the gap in the structure and destroyed the composite structure under impact force [22, 23]. In contrast, the PLA–pectin composite improved its impact strength after the annealing process. The recrystallization process plays an important role in improving the impact strength to 11.2 KJ/m2 when compared to nonannealing of pure PLA at 7.0 KJ/m2. The result of 2% pectin addition provided impact strength up to 12.5 KJ/m2 higher than that of annealed bare PLA. The highest impact strength of the composite was found with 4% pectin addition up to 16.1 KJ/m2. The higher impact strength might be related to the improvement of the crystallinity matrix and the spherulitic morphologies of the composite [24]. However, the impact strength decreased when increasing the pectin content up to 6 and 8%. A large number of pectin particles could be created at the larger grain boundaries of spherulites and provide a more brittle composite structure [25].
Morphology
SEM micrographs of the tensile fracture surface of pure and PLA–pectin composites (nonannealing process) are presented in Fig. 4. The surfaces after tensile tests were smooth, with no stack cracked layer appearing, indicating that the pectin had been completely embedded and provided the high interfacial matrix in PLA. These composites would not have formed a crystalline structure without the annealing process (Fig. 4a–e).
SEM micrographs of tensile fractured surfaces for the nonannealing process. a pure PLA, (b) PLA–pectin 2%, (c) PLA–pectin 4%, (d) PLA–pectin 6%, and (e) PLA–pectin 8%
A comparison between the surfaces of pure PLA before and after the annealing process is shown in Fig. 5. The PLA surface exhibits steady crack and propagation after the annealing process (Fig. 5a, b). However, the stack cracked line surface occurred when pectin was added to the PLA matrix (Fig. 5c–f). The fractured surface of the stack after the tensile test revealed that the direction obstructed the crack from the crystalline stack [26,27,28].
SEM micrographs of tensile fractured surfaces of pure PLA before the annealing process. a Pure PLA after the annealing process (b), PLA–pectin 2% after crystallization (c), PLA–pectin 4% after crystallization (d), PLA–pectin 6% after crystallization (e) and PLA–pectin 8% after crystallization (f). Red arrows show the crystalline stack after the annealing process
Thermal stability
TGA can obtain important information on thermal decomposition from polymeric material structures by investigating the weight loss of annealing polymers in a temperature range between 25 and 600 °C [29]. The TGA curves for pure PLA and PLA–pectin composites as a function of temperature are shown in Fig. 6. The thermal stability of pure PLA demonstrated single-step weight loss from 230 to 325 °C. The reason for this is that PLA is hydrophobic, and no initial weight loss from moisture phenomena occurs. The TGA results of PLA–pectin composites at 2 and 4% were demonstrated to be nearly similar to those of pure PLA; however, the thermal stabilities were shifted toward lower temperatures. This information corresponds to other physical properties. The deterioration of the composite material with 6 and 8% pectin occurred in three main stages. The moisture content was eliminated by the initial temperature during the melting process over 100 °C, which was the first step of degradation. Disintegration of the pectin composite was shown in the second step at 50% weight loss. The polysaccharide linkage and major chain structure might be destroyed by the depolymerization process. Evaporative products such as CO2, CO, CH4, and organic compounds were released in the last step at ~30% weight loss. The organic compound composite residue was found in the form of char residue PLA [30]. These several steps of degradation and the volatile residue of the composite indicated that pectin is a significant factor in the thermal stability of this composite. Pectin is involved in strength formation by causing interfacial bonding between PLA and pectin. The overabundant pectin at 6 and 8% demonstrated the rupture of glycosidic linkages, the major compounds of pectin, at ~30% weight reduction. The presence of char residue in the sample following the TGA experiment indicated incomplete degradation.
TGA of PLA–pectin 2%, PLA–pectin 4%, PLA–pectin 6 and PLA–pectin 8%
SR-WAXS
The structural evolution and crystallization of pure PLA and PLA–pectin composites at 2, 4, 6 and 8% w/w were shown in an isothermal crystallization experiment with in situ SR-WAXS measurements at 100 °C. 2D-scattering patterns were recorded every 1 min with 30s of exposure time. An example of a series of scattering patterns elucidating the structural evolution in PLA–pectin in the 4% composite is shown in Fig. 7. After that, the intensities of the scattering patterns of each condition were analyzed by SAXSIT software developed by SLRI. The 1D profile was extracted by radially averaging the intensity from φ = 0° to φ = 270°. Only the plot of crystallization evolution analyzed from the SR-WAXS scattering patterns under the best conditions of the PLA-pectin composite at 4% is shown in Fig. 8 due to space limitations. Furthermore, the evolution of crystallinity for each pectin-added composite was estimated using only data from the peaks at 14.8°, 16.6°, 19.0°, and 22.3°, which were contributed from the α-phase of PLA. The areas under these peaks were fitted with the pseudo-Voigt function and then divided by the total area under all curves and the background of the halo-amorphous peaks fitted with a Gaussian distribution function to elucidate the crystallinities of the biocomposites, as plotted in Fig. 9. The crystallinity of pectin-added composites clearly exhibited an improvement in both the quantity and time of PLA crystallization. While pure PLA crystallized gradually over time, various composites with pectin added crystallized rapidly and saturated in 6–8 mins. While pure PLA and PLA–pectin 2% have not fully crystallized in the experiment, PLA–pectin 4%, 6 and 8% can reach their saturation with crystallization halftime (t1/2) at 5.83, 5.82 and 5.48 min, respectively. The plot also showed that with the increase in pectin added into PLA, a higher level of crystallinity is observed. In addition, the crystal sizes of the PLA–pectin composites have been determined using the Debye–Scherrer equation [31, 32] as in [Eq. 1];
$$tau = frac{{Klambda }}{{beta cos theta }}$$
(1)
where τ is the size of the crystallites, K is the Scherrer constant (K = 0.89), λ is the wavelength of the X-ray in SR-WAXS (0.13776 nm), and β is the full width at half maximum of the peaks. The result showed that the sizes of the crystallites at each PLA–pectin composite can be comparably varying in the range of 12.1–13.0 nm.
Example of a series of scattering patterns elucidating the structural evolution in the PLA–pectin 4% w/w composite at 100 °C. The series is presented at 2 min intervals to emphasize the evolution of the composite
Crystallization evolution was analyzed from the SR-WAXS scattering patterns under the best conditions of the PLA–pectin composite at 4%. The plots analyzed from the WAXS recorded every 1 min at a constant temperature of 100 °C
Development of crystallinity of pectin-added PLA composites vs. time. The crystallization was analyzed from the SR-WAXS experiment
FT-IR
From the synchrotron ATR-FTIR, the average spectra of each group of experiments were determined to represent the FT-IR spectra of pectin, PLA, and PLA–pectin composites at 110 °C (blending temperature) and 180 °C (completely decomposed temperature). The temperature of thermal decomposition was chosen when the composites appeared in dark brown shade. The chemical bonding between PLA and pectin occurred, as seen in the shape, and band peaks from PLA at 1178 and 1127 cm−1 disappeared after blending. The characteristic band of the C=O carbonyl also changed as it broadened (1780–1740 cm−1), indicating that the interaction occurred, which could be from hydrogen bonding with PLA and pectin. The broadening band in PLA–pectin also occurs due to the chemical environments observed by different intermolecular bonding (Fig. 10) [33,34,35]. With the high brightness of SR, we can measure the sample at high spatial resolution with a high signal-to-noise ratio. Therefore, the differences between each sample were clearly elucidated.
Synchrotron-based FT-IR spectrum of 1900–900 cm−1 region of the commercial pectin, pure PLA–pectin 110 and 180 °C, respectively
The peak for the hydroxyl group was exhibited at ~3436 cm−1, which reflected typical water absorption and is shown elsewhere. The pectin region bands situated in the range of 1000–2000 cm−1 were caused by the signal of galacturonic acid. The functional groups of COO− and COOR corresponded to 1735 and 1617 cm−1, respectively (Fig. 10). The characteristic bands of C-O-C vibrations (glycosidic linkages) were observed at 1149 and 1045 cm−1, as mentioned in [36]. Further investigation showed that the heterogeneous distribution of pectin in the mapping showed major components in the composite (Fig. 11). The FT-IR chemical mapping image was analyzed to discover the distribution of pectin by using the characteristic absorption bands of the pectin composite phase. In this research, the absorption bands at 1633 cm−1 were used to characterize the N-H group of pectin in the matrix. The pectin was dispersed over the whole measuring area with a particle size of ~80–100 µm, as seen in Fig. 11. The 4% pectin added to the PLA-pectin composite is consistently and thoroughly distributed in the PLA matrix (Fig. 11b). On the other hand, the excessive amount of pectin caused the agglomeration shown in Fig. 11c and d. Consistent with the morphology results obtained via SEM and the mechanical properties obtained via tensile/impact testing, the influence of pectin dispersion in the PLA matrix plays an important role in the mechanical properties of the PLA–pectin composite. While the pectin was perfectly dispersed in the composite, it induced a perfect crystalline structure. Nevertheless, the excessive amount of pectin composite provided contradicts this result. Oversaturated pectin formed agglomerates and reduced the mechanical characteristics of the composites. This may lead to pectin debonding during mechanical testing or the poor interaction between pectin and PLA in the composites.
2D chemical imaging scanning dispersion of pectin in biopolymer blending, PLA–pectin 2%. a PLA–pectin 4% (b), PLA–pectin 6% (c) and PLA–pectin 8% (d) composites generated from the integrated area under the pectin specific absorption band (N-H group). The color scale is a rainbow scale with blue (low intensity) and pink (high intensity)
