3.1 Characterization and Analysis of PBS, PBS-TME, PBS-TME/PLA
The 1H-NMR spectra of PBS and PBS-TME are shown in Figs. 1(a) and (b), from which it can be seen that, in addition to δ = 1.72, δ = 2.65, and δ = 4.16 (peaks 1, 2, and 3 in the NMR diagrams), which are consistent with the peaks of PBS, the peaks of δ = 4.06, δ = 0.91, δ = 3.68, and δ = 3.45 (peaks 4, 5, 6, and 7 in the figures), which correspond to the proton peaks, methyl proton peaks, and hydroxylated proton peaks directly connected to trimethylol ethane and hydroxylated proton peaks in the molecular chain of PBS, and hydroxylated peaks, respectively, are also present. Peaks 4, 5, 6, and 7), which correspond to the proton peak directly linked to trimethylol ethane, the methyl proton peak and the hydroxyl-linked methylene proton peak, and the hydroxyl peak in the molecular chain of PBS, respectively, the above results indicate that PBS-TME with hydrophilic group hydroxyl group has been successfully prepared. Figure 1(c) shows the infrared plots of PBS and PBS-TME. As can be seen from 1(c), the -OH absorption peak of the chain end of PBS appeared at 3340 cm− 1, the vibrational peak of C-H at 2948 cm− 1, the character-istic peak appeared at 1718 cm− 1 was the telescopic vibrational peak of the C = O double bond in PBS, and the telescopic vibrational absorption peak of
the C-O-C structure of PBS was at 1166 cm− 1 in the PBS plot. In the IR spectra of PBS-TME and its structure, it can be seen that the absorption peak appearing at 3433 cm− 1 is the telescopic vibration peak of the -OH structure, and the intensity of the absorption peak of PBS-TME has become stronger compared with that of the -OH absorption peak of PBS, which moves from 3340cm− 1 to 3433cm− 1.Compared with the -OH absorption peak of PBS, the -OH absorption peak of PBS-TME moves from 3340cm− 1 to 3433cm− 1 and the intensity of the absorption peak becomes stronger. Meanwhile, the telescopic vibrational absorption peaks of C-H, C = O, and C-O-C structures also appeared in the infrared spectrogram of PBS-TME, which likewise proved the successful synthesis of the PBS-TMEmaterial. Figure 1(d) shows the relative molecular weights of PBS-TME and PBS and their distribution coefficients. As can be seen from Fig. 1(d), the relative molecular mass of PBS-TME showed a certain decrease and the dispersion coefficient increased slightly compared with that of PBS, which was due to the introduction of a larger trimethylolysilyl ethane site resistance, and a decrease in the reaction rate and a slow increase in molecular weight when the reaction reached a certain degree, but the distribution coefficient of PBS-TME was still relatively narrow,with the same anisotropic properties.
3.2 XPS analysis of PBS-TME/PLA composites
The XPS test spectra of PBS-TME and PBS-TME/PLA composites and their elemental fractions are shown in Fig. 2, and Table 1 shows the chemicalbonding energy parameters of PBS-TME and PBS-TME/PLA composites. Comb-ined with Fig. 2 and Table 1, it can be seen that the elemental peaks appearing in PBS-TME and PBS-TME/PLA composites are C1s and O1s, which are both consistent with their chemical compositions. In Table 1, the C-C, C-O, and C = O binding energies of PBS-TME/PLA were shifted from D-284.81eV, 286.91eV, and 288.94eV to 284.77eV, 286.82eV, and 288.92eV forPBS-TME/PLA, and the O-C, O = C binding energies of PBS-TME/PLA were shifted from 529.81eV, 531.51eV shifted to 529.78eV, 531.43eV. It indicates that there are intermolecular interaction forces between PBS-TME and PLA in the composites[7], which results in improved composite compatibility.
Table 1
XPS binding energy of the PBS-TME/PLA composite.
| C 1s binding energy /eV | O 1s binding energy /eV |
Key Type | C-C | C-O | C = O | O-C | O = C |
PBS-TME | 284.81 | 286.91 | 288.94 | 529.81 | 531.51 |
PBS-TME/PLA | 284.77 | 286.82 | 288.92 | 529.78 | 531.43 |
3.3 Analysis of crystalline properties of PBS-TME/PLA composites
Figures 3(a)~(h) show the POM maps of PBS-TME/PLA composites, and from Figs. 3(a)~(h), it can be seen that the PBS spherical crystals have a large size with clear boundaries, while the PLA presents an amorphous structure.The introduction of TME increased the randomness of the PBS molecular chain, which led to a decrease in the crystallization ability of PBS-TME, and after the introduction of PLA, the composites showed spherical PLA droplets with smooth surfaces and layered stacking[8], which may be due to the fact that a small amount of cross-linking structure existed within PBS-TME after the composites of PBS-TME and PLA, which made the PLA unable to sufficiently co-crystallize with PBS-TME, and only covered the
surface of PBS-TME in an amorphous form, while PBS-TME could still maintain a certain degree of crystallization ability. This may be due to the factthat after the composite of PBS-TME and PLA, there is a small amount of cross-linking structure within PBS-TME, which prevents PLA from fully co-combining with PBS-TME and can only be covered on the surface of
PBS-TME in an amorphous form, whereas PBS-TME still maintains a certain degree of crystallinity, and PLA, which exhibits the droplet morphology, does not have any obvious non-homogeneous nucleation effect on the crystallization behavior of PBS[9].
The XRD spectra of PBS, PBS-TME and PBS-TME/PLA are shown in Fig. 3(i), from which it can be seen that the diffraction peaks of PBS appeared at 2θ = 19.83°, 2θ = 22.09°, 2θ = 22.85°, and were the diffraction peaks of PBS at the crystal planes of 020, 021, and 110. The diffraction peaks of PBS-TME were at 2θ = 19.49 °, 2θ = 21.85°, and 2θ = 22.48°, respectively, and their diffraction peaks were shifted to different degrees, which was also due to the fact that TME disrupted the PBS molecular chain regularization. The diffraction peaks of the composites at 020, 021 and 110 crystal planes appeared near 2θ = 19°-20°, 2θ = 21°-22°, 2θ = 22°-23°, respectively, which indicated that the com-posites had similar crystallographic patterns with PBS-TME[10], i.e., the introduc-tion of PLA did not change the crystallographic pattern of the PBS-TME crystallites. However, the entanglement of the molecular chains of PLA with those of PBS-TME hindered the migration and alignment of the molecular chains of PBS-TME in the system, which resulted in a weakening of the crystallization ability of the composite PBS-TME/PLA compared to PBS.
Figures 3(j) and (k) show the DSC ramp-up and ramp-down curves of PBS-TME/PLA composites; Table 2 shows the thermodynamic parameters of PBS-TME/PLA composites. Combined with Figs. 3(j), (k) and Table 2, it can be seen that the double melting peaks appeared with the increase of PLA ratio, and the melting peaks of PBS-TME and PLA melting peaks were gradually close to each other with the increase of PLA ratio, and the degree of
crystallinity was gradually decreased, and this conclusion was consistent with
that of XRD.
Table 2
Thermodynamic parameters of PBS-TME/PLA composites.
| PBS-TME/PLA composites |
| PBS-TME | 8:2 | 6:4 | 5:5 | 4:6 | 2:8 |
Tg(PBS−TME)(℃) | -24.53 | -23.76 | -23.52 | -23.16 | -23.36 | -23.81 |
Tm(PBS−TME)(℃) | 97.93 | 93.96 | 92.41 | 94.57 | 96.17 | 97.22 |
∆Hm(PBS−TME)(J/g) | 55.73 | 40.63 | 28.26 | 21.52 | 15.71 | 7.26 |
Tc(PBS−TME)(℃) | 64.42 | 68.89 | 62.09 | 59.11 | 55.90 | - |
Xc(PBS−TME)/% | 27.86 | 25.39 | 23.55 | 21.52 | 19.63 | 18.15 |
3.4 Mechanical properties analysis of PBS-TME/PLA composites
Figures 4(a) and (b) show the TG curves and DTG curves of PBS-TME/PLA composites with different ratios; Table 3 shows the thermal decomposition data table of PBS-TME/PLA composites with different ratios. Combined with Fig. 4(a), (b) and Table 3, it can be seen that the thermal decomposition temperature of the PBS-TME/PLA composite with a ratio of 8:2at 5% decomposition is 293.04℃, which is a certain decrease compared with that of PBS-TME, due to the fact that the PLA in the composite material firstly starts to decompose, and the thermal decomposition temperatures of the composite material at 50% decomposition are 29.92℃ higher than that of PBS-TME, indicating that it can still maintain a good thermal stability at high temperatures. The thermal decomposition temperatures of the composites at 50% decomposition are 29.92℃ higher than those of PBS-TME, indicating that the composites can still maintain good thermal stability at high temperatures. Figure 4(c) shows the mechanical properties of PBS-TME/PLA composites with different ratios, from which it can be seen that the tensile strength of PBS-TME is 13.42MPa and the elongation at break is 21.21%. When the ratio of PBS-TME to PLA is 8:2, the tensile strength of PBS-TME/PLA is enhanced to 15.78MPa, and the elongation at break is 19.56%, which is the best composite mechanical properties. It was shown that the introduction of PLA improved the tensile strength of PBS-TME/PLA composites, but the elongation at break decreased, which was related to the lower elongation at break of PLA material itself[11]. Therefore, biodegradable composites with excellent tensile strength can be prepared by blending PBS-TME with PLA.
Table 3
Thermal decomposition data table for PBS-TME/PLA composites
compound material | Td−5%(℃) | Td−50%(℃) | Td−95%(℃) |
PBS | 308.64 | 366.41 | 412.98 |
PBS-TME | 324.75 | 395.46 | 436.97 |
PBS-TME/PLA 8:2 | 293.04 | 396.33 | 443.24 |
PBS-TME/PLA 6:4 | 285.43 | 382.43 | 446.15 |
PBS-TME/PLA 5:5 | 282.46 | 333.67 | 425.56 |
PBS-TME/PLA 4:6 | 278.35 | 293.44 | 407.00 |
PBS-TME/PLA 2:8 | 270.25 | 288.93 | 347.94 |
PLA | 282.30 | 311.67 | 329.63 |
3.5 Hydrophilicity of PBS-TME/PLA composites
As shown in Fig. 5(a)~(h) for the water contact angle performance test results of PBS-TME and its composites, it can be seen from the figure that the water contact angle of pure PBS is 93.5°, which is a hydrophobic polyester, and through the hydrophilic modification the water contact angle of PBS-TME is reduced to 68.9°, which shows good hydrophilic performance. As shown in Fig. 5(i), the water vapor transmission rate test results of PBS-TME and its PBS-TME/PLA composites are shown, in which the water vapor transmission rate of PBS is 39.94 [g·(m2·d)−1], whereas that of PBS-TME is increased to 56.34 [g·(m2·d)−1], further indicating that the synthesis process of PBS-TME The introduction of the third monomer occurs only linear and branched reactions, successfully exposing -CH2OH to the branched functional
group and forming a functional group. At the same time, the hydroxyl part and water molecules are easy to produce mutual attraction, which makes it easier for water molecules to enter the interior of the material, and improves the water molecule transmission rate.The water vapor transmission rate of the PBS-TME/PLA composites shows a trend of increasing and then decreasing with the increase of the PLA content, and the composites show the island
structure[12] when the ratio of the PBS-TME to the PLA is 8:2, which increases the gap between PBS-TME and PLA to make the water vapor transmission rate further increase. The gap between PBS-TME and PLA increases, which further increases the water vapor transmission rate.
3.6 Degradation properties of PBS-TME/PLA composites
Figure 6 (a) (b) shows the degradation rate of soil supernatant and enzyme degradation rate of different ratios of PBS-TME/PLA composites, from the figure (a), it can be seen that the degradation rate of PBS-TME compared with that of PBS has been significantly improved, which is due to the strong hydrophilicity of PBS-TME, so that very water molecules are more likely to enter the intermolecular chain to make the ester bond hydrolysis in the chain breakage, and the degradation rate is accelerated[13] At the same time, PLA is an aliphatic polyester that is easy to hydrolyze, and the introduction of PLA component makes the composite material decompose firstly and produce gaps, and the water molecules are more likely to penetrate further into the gaps between the materials, so that the area of non-crystalline region inside the
composite system is increasing, thus promoting the degradation of the composite material. From figure (b) it can be seen that the rate of enzymatic degradation of composite PBS-TME/PLA increases with time, this is due to the fact that at the initial stage enzymatic degradation occurs only on the surface, with the increase in time, the enzyme continues to erode the surface of the material so that the contact surface continues to increase the same leads to an increase in the rate of degradation[14].
Figure 7 shows the scanning electron microscope images of the surface of PBS-TME and its composites before and after degradation. Comparing the images before and after degradation, it can be seen that the internal non-crystalline region of the PBS-TME material is firstly degraded after degradation, with a large amount of material peeling off from the surface, and the water molecules are further eroded, which makes the degradation rate accelerated.
Figure 6.Degradation rate profile of PBS-TME/PLA composite soil supernatant (a);Enzymatic degradation rate graph of PBS-TME/PLA composites (b)
3.7 Molecular dynamics simulation of PBS-TME/PLA composites
3.7.1 Blending model and interaction force analysis of PBS-TME/PLA composites
The model of PBS-TME and PLA blending is shown in Fig. 8(a). Observing the figure, it can be seen that when PBS-TME is the matrix and PLA is distributed as the dispersed phase inside PBS-TME, PBS-TME encapsulates the PLA, and the PBS-TME itself is more compactly arranged, which is conducive to maintaining the properties of the material itself; When PLA is the matrix and PBS-TME is the ligand, PLA encapsulates PBS-TME, but the PLA molecular chains are more loosely arranged. Figure 8(b) shows the mixing energy distribution of PBS-TME composites, the two graphs are the binding energy distribution curves of the blending with PBS-TME as matrix and PLA as matrix, respectively, and it can be seen from their mixing energy distribution curves that the Ebs curves are between Ebb and Ess, which indicates that there is a certain degree of compatibility between the matrix and the ligand. Table 4 shows the interaction parameters of PBS-TME composites, the smaller the interaction parameter χ and mixing energy Emix of the materials, the better the compatibility of the materials[15]. From the table, it can be seen that the interaction parameter χ is 18.06 and the mixing energy Emix is 14.77 when PBS-TME is used as the matrix, whereas the interaction parameter χ is 26.28 and the mixing energy Emix is 33.71 when PLA is used as the matrix, so the composites have a better compatibility when PBS-TME is used as the matrix and PLA is used as the dispersed phase.
Table 4
Interaction parameters of composite systems
Sample | χ | Emix(kJ/mol) |
PBS-TME/PLA(PBS-TME as matrix) | 18.06 | 14.77 |
PBS-TME /PLA(PLA as matrix) | 26.28 | 33.71 |
3.7.2 Momentum analysis of PBS-TME/PLA composite dynamics
As shown in Fig. 9 (a1) and (a2) show the kinetic box of the composite system of PBS-TME and PLA during the blending process, (b1) and (b2) show the temperature versus time curves of the blending process of PBS-TME and PLA, and (c1) and (c2) show the energy versus time curves of the blending process of PBS-TME and PLA. While in the blended system with PBS-TME as the matrix, the temperature change of the blended system showed a small fluctuation around 298 K. The blended system was more stable, and the total energy and total bonding energy of the composites were lower, which indicated that the composite system with PBS-TME as the matrix was more stable, and the compatibility between the materials was better.
3.7.3 Analysis of cohesive energy density, solubility parameters and radial distribution function of PBS-TME/PLA composites
The images of the cohesive energy density (CED), dissolution parameters and radial distribution curves of the composite system during the blending process of PBS-TME and PLA are shown in Fig. 10. From Fig. 10 (a) and (b), it can be seen that the CED of the blended material tends to be 3.85×102 J/cm3 when PBS-TME is the matrix and PLA is the dispersed phase, which is significantly higher than that of the blended system with PLA as the matrix; the solubility parameter is also larger than that of the blended system with PLA as the matrix and PBS-TME as the dispersed phase, which further indicates that the composites have better compatibility when PBS-TME is the matrix, This further indicates that the composites are more compatible when PBS-TME is the matrix and PLA is the dispersed phase. As can be seen from Fig. 10 (c), the composite system shows two groups of strong characteristic peaks near 1.05–1.25 nm, and the values of the characteristic peaks g(r) are 8.37 and 3.94 when PBS-TME is used as the matrix, and the values of the characteristic peaks g(r) are 7.57 and 1.56 when PLA is used as the matrix, which indicate that the composite system based on PBS-TME exists a strong intermolecular hydrogen bonding force, and the comprehensive performance is more excellent.
3.8 Analysis of growth and nutrient content of PBS-TME/PLA composite brussels sprouts
Figure 11(a) shows the growth of brussels sprouts with different ratios of PBS-TME/PLA composites; Table 5 shows the effect of different ratios of PBS-TME/PLA composites on the stem length of brussels sprouts. Combined with Fig. 11(a) and Table 5, it can be seen that the leaf rise of bok choy was better when the ratio of PBS-TME/PLA was 8:2, and the stem length of bok choy was 28.6cm after 45 days of growth, which was due to the introduction of hydrophilic groups in PBS-TME, which made it easier for the water molecules in the soil to penetrate into the lower layer of the soil, and the water retention was stronger. Figure 11(b) shows the graphs of the effects of different ratios of PBS-TME/PLA composites on chlorophyll content, soluble protein content, peroxidase activity and vitamin C content in lettuce leaves. From Fig. 11(b1)~(b4), it can be seen that the pre-existing PBS-TME and its composites did not significantly affect the growth of lettuce. With the growth cycle growth of bok choy, when reaching 30 days, the chlorophyll content, soluble protein content, peroxidase activity and vitamin C content of different ratios of composites appeared to be somewhat improved compared with the control group, and after 45 days of growth, when the ratio of PBS-TME/PLA was 8:2, the chlorophyll content, peroxidase activity and vitamin C content of bok choy grown by composites treatment group was the were the highest, indicating that PBS-TME was decomposed into small molecules of butanedioic acid and lactic acid by microorganisms and other microorganisms in the soil, and provided nutrients as a carbon source[16], which could enter into the interior of the brussels sprouts after being absorbed to promote the growth of brussels sprouts.
Table 5
Effect of PBS-TME and its composites on stem length of green vegetables
Time/day | proportions | PBS | 10:0 | 8:2 | 6:4 | 5:5 | 4:6 | 2:8 | PLA |
15 | stem length/cm | 13.8 | 15.2 | 15.2 | 14.6 | 14.5 | 14.2 | 14.3 | 13.8 |
30 | stem length/cm | 19.1 | 20.2 | 19.7 | 20.1 | 19.4 | 19.6 | 19.1 | 18.1 |
45 | stem length/cm | 25.4 | 27.6 | 28.6 | 27.2 | 27.1 | 27.0 | 27.0 | 24.9 |