3.1. Synthesis and Molecular Weight Characterization
The intrinsic viscosity of copolyester was measured by phenol/tetrachloroethane mixture solution (1/1 W/W) and the number mean molecular weight and PDI of copolyester was measured by GPC. The reaction conditions and test results of copolyesters are shown in Table 1.
The esterification reaction is divided into two sections for reaction, in the first section of esterification only add terephthalic acid a monomer, is to 1,10-decyldiamine
and terephthalic acid to fully react. At the same time, the sum of the first and second esterification times is also greater than the time of non-staged esterification to ensure that the esterification reaction is fully cut and complete. The intrinsic viscosity of all polymers is greater than 1, and the number mean molecular weight of all copolyesters is greater than 50000. Compared with C.S., the molecular weight of PBSeT modified has been improved in different degrees, and the trend is similar to the characteristic viscosity. In conclusion, copolyesters have high molecular weight.
Table 1
Synthetic conditionsa and results of the eight poly(butylene sebacate-co-terephthalate)s.
Sample | ΦDAb (mol) | Tes/tesc | Tmp/tmpd | [η]e(dL/g) | GPC |
Mn(g/mol) | Mw(g/mol) | PDI |
PBSeT | 0 | 220/3.5 | 240/6 | 1.09 | 50379 | 64989 | 1.29 |
DA-0.2% | 0.0091 | 180/1.5 + 220/2.5 | 240/5.5 | 1.05 | 53221 | 65994 | 1.24 |
DA-0.4% | 0.0182 | 180/1.5 + 220/2.5 | 240/5 | 1.11 | 53379 | 61920 | 1.16 |
DA-0.6% | 0.0273 | 180/1.5 + 220/2.5 | 240/5 | 1.25 | 60132 | 75165 | 1.25 |
DA-0.8% | 0.0364 | 180/1.5 + 220/2.5 | 240/5.5 | 1.20 | 52219 | 61618 | 1.18 |
DA-1.0% | 0.0455 | 180/1.5 + 220/2.5 | 240/4.5 | 1.13 | 55617 | 61179 | 1.10 |
a: (1) Esterification: TBT, 0.6 mol% based on diacid was used as catalyst; diol/diacid molar ratio 1.2. (2) Polycondensation: additional 0.3 mol% TBT was added. |
b: The moles of 1,10-decyldiamine. |
c: Esterification temperature and time. |
d: Melt polycondensation temperature and time. |
e: Intrinsic viscosity was measured at 25 ℃ using a mixture of phenol-tetrachloroethane with a mass ratio of 1:1 as solvent. |
3.2. Structural Characterization and Hydrogen Bond Formation in PBSeT-Da
1HNMR spectra of copolyester samples obtained by copolymerization with decyldiamine as the fourth monomer are shown in Fig. 1, and the test results of characteristic peaks of hydrogen atoms in the structure are consistent with the expected results. Characteristic absorption peaks a and b are the different absorption peaks of hydrogen splitting in methylene group near and away from ester group in butanediol linked to different units respectively. Both a and b split into four different absorption peaks a1-a4 and b1-b4. Compared with the 1HNMR spectra of unmodified PBSeT copolyester, two new characteristic peaks g and e appeared in PBSET-DA. The characteristic peak of g at δ = 3.75 is the characteristic absorption peak of hydrogen atom in the methylene group adjacent to the amide bond in 1,10-Diaminodecane. The characteristic peak of E at δ = 7.90 is the characteristic absorption peak of hydrogen atom on the amide bond. The e and d characteristic peaks in DA-1% curve were analyzed by MestReNove software. It is found that the area ratio of absorption peak d and absorption peak e of hydrogen atom in benzene ring is 1:0.0096. This indicates that the amide group of 1,10-Diaminodecane, which accounts for 1% of TPA, was successfully incorporated into the main chain of PBSeT copolyester.
The structure of untreated PBSeT and PBSeT with different proportions of decyldiamine were characterized by ATR-FTIR spectroscopy. The result is shown in Fig. 2. All samples had C-O-C stretching vibration and C-O contraction vibration at 1104 cm− 1 and 1216 cm− 1, respectively. The strong signal at 2854 cm− 1 and 2924 cm− 1 is a distinct aliphatic band, both of which testify to the presence of ester groups. Unlike untreated PBSeT, new amide-I and amide-II absorption bands were observed in THE ATR-FTIR spectra of DA (Fig. 2 (C)). The results obtained by ATR-FTIR spectrum were consistent with 1HNMR test results, which further proved that the fourth monomer 1,10-Diaminodecane was introduced into the main chain. The addition of glycerol resulted in a wide hydroxyl absorption band at 3400 cm− 1 in all samples (Fig. 2(B)). It is because the secondary hydroxyl group in glycerol is not easy to participate in the reaction, which is easier to form hydrogen bonds with the hydrogen atom in the amide group, and produce physical cross-linking under this action. The formation of physical crosslinking network is proved.
3.3. Thermal Transition
In order to verify whether the introduction of amide groups and the formation of hydrogen bonds have an effect on the thermal properties of PBSeT copolyesters, DSC and T.G. tests were carried out on the samples. The results of DSC testing the melting and cooling behavior of copolyesters are shown in Fig. 3, and the specific values are shown in Table 2. At the cooling and heating rate of 10℃/min, all samples have crystallization and melting peaks, and all have a single Tg. This shows that the different components of polyester chain segment have good compatibility. During cooling scanning, with the increase of DA content, the maximum temperature Tc corresponding to the exothermic peak produced by crystallization increases continuously, while ΔHc also shows a rising trend.In the crystallization process, the large molecular chain where the benzene ring resides is embedded in the lattice, and multiple chain segments cooperate to complete the crystallization process. The crystallinity of copolyetes generally showed an increasing trend, as shown in Fig. 4, the crystallinity of Xc is 14.97% ~ 16.28%. When the content of sebacediamine was 6%, the crystallinity of PBSeT-Da copolyester reached 16.28%. The possible reason is that the existence of hydrogen bond promotes the synergy between molecular chains and thus improves the crystallization capacity[32].
The melting peak and glass transition temperature appeared in the second heating process of copolyester. The glass transition temperature Tg changed little, while the melting temperature first decreased and then increased. It is worth noting that the cold crystallization temperature is constantly decreasing, and the temperature of combined melting crystallization is constantly increasing, indicating that the crystallization speed of PBSeT copolyester is constantly accelerating with the introduction of DA.
In order to further study the influence of the introduction of DA on the crystallization behavior of PBSeT, the samples were tested by XRD. XRD patterns are shown in Fig. 5. It can be clearly seen from the XRD pattern that PBSeT and PbSET-Da have similar crystal structure, and belong to semi-crystalline polymers with wide diffraction peaks. The XRD patterns of PBSeT were peak-fitting. At 2θ: 16.3°, 17.5°, 20.5°, 23.1° and 24.9°, the characteristic diffraction peaks corresponding to the crystal planes were (011), (010), (101), (100) and (111), respectively. The crystallites of copolyesters grow on different crystal planes, and the peak strength of (100) crystal plane reaches the maximum, which is similar to the crystallization law of PET, proving that the crystallization capacity of PBSeT copolyesters is mainly provided by B.T. unit[33, 34]. The peak area of the spectrum is also consistent with the relative crystallinity obtained by DSC. The crystal plane position of PBSeT did not change significantly, indicating that the addition of decyldiamine did not change the crystal structure of PBSeT.
Table 2
Thermal transition properties of PBSeT(C.S.) and PBSeT-DA.
Sample | cooling scan | second heating scan | ХC% |
Tc | ΔHC | Tg | Tm |
CS | 141.64 | 25.14 | -25.33 | 172.33 | 15.38 |
DA-0.2% | 143.41 | 24.20 | -29 | 169 | 15.28 |
DA-0.4% | 142.23 | 29.02 | -25.67 | 170 | 16.09 |
DA-0.6% | 143.3 | 37.94 | -27.67 | 169.33 | 16.29 |
DA-0.8% | 143.74 | 23.23 | -29.67 | 171 | 14.97 |
DA-1.0% | 139.9 | 28.58 | -31 | 169.67 | 15.45 |
3.3. Thermal Stability
In order to further explore the introduction of amide group and hydrogen bond formation on the thermal stability of PBSeT copolyester. TGA tests were performed on all samples from room temperature to 600℃ in N2 atmosphere. The results are shown in Fig. 5. The decomposition temperature at 5% weight loss, maximum decomposition rate temperature and residual weight at 600℃ are shown in Table 3.
All samples were in a stable state before 250℃, with a weight loss rate of less than 1% and no decomposition. It is concluded that the introduction of sebacediamine does not seriously affect the thermal stability of copolyester, and can meet the requirements of daily use. The main degradation temperatures of all samples were between 350℃ and 430℃. The maximum decomposition rate (TdMax) of the polymer occurs in the narrow temperature range of 404 ~ 407℃. For the temperature at 5% weight loss, the temperature first increases and then decreases, and the temperature decreases to 368℃ at 0.8% DA. The main reason may be that with the addition of decylenediamine, the number of methylene in the long chain of the molecular chain increases, which makes the polymer easy to decompose at high temperature, while the previous increase in decomposition temperature is due to the physical interaction in the hydrogen bond, which improves the anti-decomposition ability of the molecular chain at high temperature. The residual coke content of PBSeT-Da copolymer was also higher than PBSeT in the test temperature range. PBSeT-DA can be safely treated without thermal degradation at temperatures 50 to 60°C above its corresponding Tm
PBSeT(CS) and PBSeT-DA (10 ℃/min, Ar).
Table 3
Characteristic decomposition temperatures of PBSeT(C.S.) and PBSeT-DA copolyesters.
Sample | Td,5 (℃) | Td,max (℃) | Residue at 600 ℃ (%) |
CS | 368.67 | 404 | 2.56 |
DA-0.2% | 369.33 | 406.67 | 3.36 |
DA-0.4% | 371.33 | 406 | 6.30 |
DA-0.6% | 368.66 | 404 | 5.97 |
DA-0.8% | 370 | 403.3 | 5.62 |
DA-1.0% | 368 | 403.3 | 4.83 |
3.4. Mechanical Properties
The mechanical properties of PBSeT and PBSeT-DA were characterized by universal drawing machine. The obtained stress-strain curve and tear test results are shown in Fig. 6. The specific values are shown in Table 4, including Young's modulus, tensile strength, yield strength, tear strength and elongation at break.
Unmodified PBSeT exhibits typical mechanical properties of soft and tough semi-crystalline flexible polymers. The yield strength and elongation at break before fracture are 15.66MPa and 714.55%, respectively. When the amount of DA is less than or equal to 0.4%, the stress-strain curve is still soft and tough, but the elongation at break decreases to 635.67%, and the yield strength and tensile strength increase to 17.38MPa and 20.71MPa, respectively, which are 10.98% and 32.24% higher than those of unmodified PBSeT. It's a material that's going to be strong and tough. When the amount of DA reaches 6%, the elongation at break of the sample plummets to 92.22%, and the yield strength reaches 19.43MPa. When the amount of DA reached 1%, the elongation at break decreased to the lowest 38.30%, and the yield strength and tensile strength reached the maximum 22.03MPa, which increased by 40.68% year-on-year. It is noteworthy that the tensile strength of some samples decreased, presumably because the introduction of decylenediamine brought more long carbon chain structure and reduced molecular chain rigidity. The improvement of PBSeT-Da can be attributed to the introduction of amide groups and the formation of hydrogen bonds, resulting in stronger interactions than the unmodified PBSeT molecular chain. But there are many factors affecting the mechanical properties, such as the increase of crystallinity and molecular weight will improve the mechanical properties.
In the tear test of the sample, the tear strength of unmodified PBSeT is 79.88kN /m, which is far from reaching the standard of industrial production compared with 200KN/m of P.E. However, with the introduction of amide group, the tear strength of samples increased to 86.80-139.61 KN/m. The maximum tear strength reaches 70% of P.E. tear strength, which can meet the standard of normal production and use. Such tearing strength should be the result of the interaction of hydrogen bond to molecular chain and the interaction of long flexible molecular chain.
Table 4
Mechanical properties of PBSeT(CS) and PBSeT-DA.
Sample | E(MPa) | δb(MPa) | δy(MPa) | δT(KN/M) |
CS | 45.38 ± 2.03 | 15.66 ± 0.22 | 15.66 ± 0.22 | 79.87 ± 4.73 |
DA-0.2% | 42.09 ± 1.75 | 20.22 ± 0.56 | 16.55 ± 0.31 | 72.85 ± 6.87 |
DA-0.4% | 53.77 ± 1.32 | 20.71 ± 0.47 | 17.38 ± 0.21 | 86.79 ± 8.53 |
DA-0.6% | 61.85 ± 0.81 | 19.43 ± 0.26 | 19.43 ± 0.26 | 100.69 ± 2.23 |
DA-0.8% | 68.94 ± 0.64 | 20.23 ± 0.33 | 20.23 ± 0.33 | 125.63 ± 4.35 |
DA-1.0% | 79.05 ± 0.91 | 22.03 ± 0.41 | 22.03 ± 0.41 | 139.61 ± 8.14 |
3.5. Hydrophilicity and Water Vapor Barrier Properties
To verify the hydrophilic change of PBSeT copolyester, the water contact Angle was measured at 20℃. The result is shown in Fig. 7. Polyester materials because there are polar ester groups in the large molecular chain segment, so most polyester materials have a certain hydrophilicity, that is, the water contact Angle is about 80°. Unmodified PBSeT copolyesters have a water contact Angle of 80.97°, which is a slightly hydrophilic material. The water contact Angle decreased significantly with the addition of decylenediamine. When the amount of decylenediamine was 1%, the water contact Angle was 62.677°, which decreased by 18.293°. The hydrophilicity of copolyesters was improved obviously. The obvious reason is the introduction of a strongly polar amide group into the PBSeT macromolecular chain, which also proves the successful introduction of the amide group.
The water vapor barrier of polymer samples was tested at 37℃ and 90% relative humidity by cup method. The experimental results are shown in Table 5. The barrier improvement factor (BIF) was defined as C.S./ PBSeT-DA for PBSeT samples. The improvement in air resistance is measured by the size of the BIF. It is not difficult to see from Table 5 that, with the introduction of decyldiamine, the water vapor barrier of different proportions of modified PBSeT copolyets is improved in different degrees. When the addition of 0.6% sebacediamine, the water vapor barrier ability also reached the maximum. At the addition of 0.2%, the water vapor barrier decreased. The main reason is that at this ratio the reduced crystallinity leads to an increase in free volume, making it easier for water vapor molecules to pass through the macromolecular chain. For PBSeT-DA sample with lower relative crystallinity, the increase of vapor barrier is not obvious, which is also the same reason. In addition to the increase of crystallinity, it is speculated that the hydrogen bonding force between the polymer chains makes the polymer chains more easily oriented. The free volume used for gas transport and chain segment movement is reduced, thus improving the water vapor barrier.
Table 5
Water vapor barrier Properties of PBSeT(C.S.) and PBSeT-DA.
Samplea | Pwvb | BIFc |
CS | \(4.70*{10}^{-14}\) | 1 |
DA-0.2% | \(4.72*{10}^{-14}\) | 0.996 |
DA-0.4% | \(4.06*{10}^{-14}\) | 1.158 |
DA-0.6% | \(3.53*{10}^{-14}\) | 1.331 |
DA-0.8% | \(3.96*{10}^{-14}\) | 1.187 |
DA-1.0% | \(3.84*{10}^{-14}\) | 1.224 |
a: water vapor permeation: dish method,38℃, RH = 90% |
b: water vapor permeability coefficient with unit of g·cm·cm− 2·s− 1·Pa− 1). |
c: barrier improvement factor (BIF) is defined as PCS/PPBSeT−DA |
d: water vapor transmission rate, at 38°C, 90% relative humidity. |
3.6. Biodegradation Properties
In this part of the experiment, the biodegradability of PBSeT and its modified product PBSeT-Da was tested by hydrolysis and enzymatic hydrolysis. In order to simulate the degradation of different composts, the hydrolysis experiment was divided into two different PH = 4.4 and PH = 9.6. All degradation tests were incubated in phosphoric acid buffers for 28 days, with buffers replaced every seven days.
The curves of the relative weight of samples changing with time in hydrolysis experiments at PH = 4.4 and PH = 10.6 are shown in Fig. 8.. In acidic environment, the samples showed less mass loss, and only the unmodified PBSeT and DA samples with 0.2% content showed more than 2% mass loss. However, different experimental results were obtained under alkaline conditions, in which only when the addition amount of Da was more than 0.8%, the mass loss was less than 2%. Compared with acidic condition, the mass loss of each proportion was improved. The reason for this may be that the ester group is hydrolyzed into oligomers or shorter polyester segments during polyester hydrolysis[35]. The oligomer of the carboxy-terminated polyester is more easily dissolved and diffused into aqueous solution under alkaline conditions. Hydrolysis is therefore easier under alkaline than acidic conditions[36–38]. After the addition of DA, the hydrolysis capacity of the sample decreased with the increase of the amount of DA, and almost no hydrolysis occurred in the acid and alkali environment when the amount of DA exceeded 0.8%. The main reason is that the introduction of amide group does not lead to hydrolysis reaction, so the macromolecular chain cannot be decomposed into oligomers in aqueous solution, and then cannot be dispersed in the solution.
The enzymolysis experiment is to simulate the degradation of copolyester compost in real soil environment. Figure 8C shows the results of lipase degradation in phosphoric acid buffer culture. The biodegradation ability of aliphatic aromatic copolyesters decreased due to the increase of benzene ring content, so the degradation ability of the copolyester sample with TPA:SeA = 6.5:3.5 was slightly lower than that of the 5:5 copolyester. The mass loss in enzymatic hydrolysis was much higher than that in hydrolysis. Lipase accelerated the hydrolysis of ester groups and led to rapid degradation of copolyesters. The addition of DA leads to the degradation ability decline, mainly the introduction of amide group increases the steric hindrance of molecular chain, so lipase is not easy to act on the corresponding ester group. The degree of crystallinity also affects the degradation ability of copolyesters. When the crystallinity is low, lipase adhesion and enzymatic hydrolysis are more likely to occur enzyme-catalyzed and autocatalyzed hydrolysis reactions. Therefore, after the addition of DA, the increase of crystallinity also greatly weakened the degradation ability, so that the degradation ability was almost lost when the addition of DA reached 1%.