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Materials and Interfaces
Modification of poly(butylene 2,5-furandicarboxylate) with lactic acid for biodegradable copolyesters with good mechanical and barrier properties Han Hu, Ruoyu Zhang, Lei Shi, Wu Bin Ying, Jinggang Wang, and Jin Zhu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02169 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018
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Modification of poly(butylene 2,5-furandicarboxylate) with lactic acid for biodegradable copolyesters with good mechanical and barrier properties Han Hu, 1,2 Ruoyu Zhang, 1* Lei Shi, 1 Wu Bin Ying, 1 Jinggang Wang, 1 Jin Zhu1* 1
Key Laboratory of Bio-based Polymeric Materials Technology and Application of
Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, People’s Republic of China 2
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
China Corresponding authors:
[email protected] (Ruoyu Zhang);
[email protected] (Jin Zhu) Abstract: Fully bio-based poly(butylene 2,5-furandicarboxylate)-co-poly(lactic acid)s (PBFLAs) of lactic acid composition from 10% to 40% are synthesized. They are subjected to molecular, thermal, diffractometric, degradable and mechanical characterizations. Thermogravimetric analysis shows that PBFLA are thermally stable at least 350 °C, much higher than PLA. After the incorporation of lactic acid units, both Tm and crystallization rate decrease with respect to homopolymers. X-ray diffraction measurements indicate that no PLA crystal is present during crystallization. All samples perform better barrier properties with respect to PLA, implying potential application in packaging. These samples behave like semi-crystalline plastic in the tensile tests. PBFLA copolyesters show Young's modulus, maximum tensile strength and breakage elongation higher than 1000 MPa, 38 MPa and 230%, respectively.
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Such comprehensive mechanical behaviors exceed most biodegradable materials. Lastly, biodegradability could be significantly improved when more than 20% of lactic acid units are introduced into homopolymers. Keywords: bio-based; poly(butylene 2,5-furandicarboxylate); barrier properties; hydrolysis degradation; high elastic modulus
1. Introduction The rapid consumption of petroleum-based commodity plastics, which are mostly non-degradable, causes problems like global warming and white pollution1. Many strategies have been tried, such as recycling the used ones and introducing prodegradant additives. Unfortunately, there still lacks a fully effective method2, 3. Adding organic fractions will complicate the recycling of plastics and the use of prodegradants is still in dispute. From the view point of sustainability, the substitution of petrochemical polymers with bio-based and biodegradable alternatives is necessary4-6. Poly(lactic acid) (PLA) could be manufactured from agricultural products and would eventually be degraded into H2O and CO2, is one of the promising biodegradable polymers7-9. However, PLA is brittle and stiff, and further modification is necessary before practical applications. Copolymerization of lactic acid with other aliphatic or/and aromatic monomers is a frequently used strategy to obtain polymers with biodegradability
and
tunable
mechanical
properties.
Poly(butylene
succinate)-co-poly(lactic acid)10, poly(ethylene terephthalate)-co-poly(lactic acid)11-14, poly(propylene
terephthalate)-co-poly(lactic
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acid)15,
poly(butylene
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terephthalate)-co-poly(lactic
acid)16,
17
and
poly(ethylene-co-diethylene
terephthalate)-co-poly(lactic acid)18 have been synthesized and studied. 2,5-Furandicarboxylic acid (FDCA), as derivation of fructose and glucose19-22, is a aromatic monomer which is expected to replace terephthalic acid due to their similar chemical structure22-25. DuPont and DSM refer FDCA as “sleeping giant” and are actively pushing related research26,
27
. Related polyesters have been extensively
studied, such as poly(ethylene furandicarboxylate) (PEF)
23, 28-30
, PPF
31, 32 33
and PBF
34-39
. They demonstrate higher Tg and lower processing temperature compared to their
terephthalic acid based analogues. Unfortunately, above plastics are non-degradable. As has been discussed above, the incorporation of degradable monomers could be a feasible way to balance the degradability and other properties. Sousa et al. firstly synthesized PEF-PLA copolyesters40 and found that 8 mol% of LA units in the copolymer enabled to improve the degradability. Cao et al. reported the copolyesters of FDCA, lactic acid, and ethylene glycol, and their degradation behavior in soil and phosphate buffered solution was evaluated and discussed41. PBF owns balanced thermal and mechanical properties among FDCA based polyesters, and is of great potential in copolymerizing with degradable monomers34, 35. In this work, to modify PBF into degradable copolyesters, PBFLAs were synthesized from FDCA, 1,4-Butanediol and polylactide diols. In the melting polycondensation, the polylactide diols were copolymerized in bulk state with the oligomer of PBF at controlled temperature and vacuum. The influence of LA composition on microstructure, crystallization, diffractometric, mechanical and degradable properties
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were emphasized. It was found that after the incorporation of lactic acid units, both the melting temperature and crystallization rate decreased, but it induces slow crystallization at ambient temperature. Only crystal of PBF is confirmed in PBFLAs. They are thermally stable up to at least 350 °C, thermal degradation process of polylactide
diols
is
non-existing.
The
PBFLA
copolyesters
behave
like
semi-crystalline thermoplastics, the comprehensive mechanical behaviors are comparable with most biodegradable materials. Since until now barrier tests on copolymers of LA and aromatic polyesters were never studied. In this study, the gas barrier properties has been evaluated on PBFLAs, showing much better barrier performances with respect to PLA. Lastly, the hydrolytic degradation behavior of PBFLAs were determined by analyzing weight loss and intrinsic viscosity along with time when exposed to a phosphate buffer solution.
2. Experiments 2.1 Materials. L-lactides (99%) were provided by Purac Co., Ltd. (Netherlands). Three times of recrystallization steps were performed. 1,4-Butanediol (99%), titanium (IV) butoxide (TBT) , tannous octoate (SnOct2) and antimony trioxide (Sb2O3) were bought from Aladdin. Phenol, tetrachloroethane and chloroform were obtained from Sinopharm. FDCA was supplied by Chem target Technologies Co., Ltd. (Mianyang, China). The PLA used (4032D) was purchased from Natureworks. 2.2 Synthetic process of Dimethyl 2, 5-furandicarboxylate (DMFD). The DMFD was synthesized from FDCA and methanol and the procedure was
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described in our previous literature42. 1H NMR and 13C NMR confirmed the chemical structure. 1
H NMR (DMSO-d6): 3.88 (6H); 7.41 (2H).
13
C NMR (DMSO-d6): 52.9 (2C); 119.6 (4C); 146.7 (4C); 158.7 (2C).
2.3 Synthesis of polylactide diols (OLLA). The polylactide diols were synthesized from LA with SnOct2 as catalyst and 1,4-BDO as chain transferring agent. In a N2 atmosphere, LA, 1,4-BDO and SnOct2 were added into a 1L-flask free of O2 and moisture in a feed molar ratio of LA/1,4-BDO=25/1. The amount of SnOct2 is 0.1 wt% with respect to LA. The temperature was controlled at 150 °C for more than 5 h, and then cooled to room temperature. Resulting raw polymer was dissolved in chloroform, isolated by precipitation in 5 fold volume of methanol, and collected on a suction filter. Lastly, the samples were vacuum dried at 50 °C for at least 12 h. 2.4 Synthesis of hydroxyl terminated oligo(butylene furandicarboxylate) (BHBF). 1,4-BDO and DMFD (DMFD/1,4-BDO =1/1.6, mol/mol) were added into a 1L three-necked flask equipped with a mechanical stirring, condenser and N2 inlet, using TBT (0.15 mol% of the DMFD) as catalyst. The reaction was conducted at 180 °C for 4-6 hours until 98% of the theoretical methanol was collected by the condenser. 2.5 Synthetic steps of PBFLAs. The synthetic steps of PBFLAs were performed via polycondensation. Copolymers with LA content from 10% to 40% were prepared by different mole ratios of
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OLLA/BHBF (nLA0 ≈15,28,50 and 80 mol%). Sb2O3 (0.15 wt%) and antioxidant (0.1 wt%) were added. To minimize oligomer sublimation, the pressure of the flask was stepwise decreased to 300 Pa in about 0.5 h. The reaction temperature was controlled at 210 °C to avoid the thermal degradation of OLLA and the polycondensation time was 6 h. After that, the pressure was regained to atmospheric pressure by N2 and light yellow colored products were obtained. 2.6 Characterization methods of the PBFLAs and polylactide diols. The intrinsic viscosities were tested at 25 °C by an Ubbelohde viscometer (diameter 0.792 mm). 0.125 g samples were dissolved in solvents of tetrachloroethane/phenol (1/1, w/w) to achieve a homogeneous solution. Intrinsic viscosity was obtained as eqs. 1-2:
η =
η =
(1)
. .
(2)
Where ηsp is the specific viscosity, C is the concentration, t and t0 are the flow time of solution and pure solvent, respectively. Weight-average molecular weight (Mw) and its dispersity (DI) were measured by GPC (Agilent PL-GPC220) at 40 °C. 10 mg samples were dissolved in solvent of 2-Chlorophenol/chloroform (1/9, v/v) with sample concentration of about 1.0 mg/mL. Chloroform was used as the eluent at flow rate of 1.0 mL/min. The Mw was calculated according to polystyrene standards in the range of 3070-258 000 g/mol. FT-IR spectra were recorded by ATR-FRIR (Agilent Cary 600) with reflection mode and wavenumber ranging from 4000 cm-1 to 500 cm-1. And the chemical structures
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and composition of PBFLAs were confirmed on a Bruker AVIII400 in CF3COOD at 25 °C. The thermal transition behavior was recorded with DSC (PerkinElmer Diamond DSC) under N2 flow of 50 mL/min. The sample was first heated from 0 °C to 200 °C with 10 °C /min ramp, kept at 200 °C for 5 min to erase the thermal history, and subsequently cooled to 0 °C and hold for 5 min. Finally, the second heating run was performed by heating the sample to 200 °C at the same heating rate. The samples for DSC tests were prepared by thermal compression and cold pressing, and then conditioned at room temperature for more than 2 weeks before testing. The thermal stability was determined under N2 atmosphere by a TGA instrument (Mettler-Toledo TGA/DSC). About 5–8 mg of sample was heated from 50 to 800 °C at a rate of 20 °C /min. The X-ray diffraction patterns were recorded with a Bruker AXS D8 Advance, using Cu-Kα radiation (λ=1.541Å) in the scan range from 5 to 60° at 5o min-1. Before the tests, isothermal crystallization at temperature between Tg and Tm (approximately 0.8 Tm) of the copolyesters for 30 min were performed to further increase the crystallinity, showing improved crystal structure. Tensile properties were measured at 25 °C and crosshead speed of 20 mm/min by Instron 5567. Dumbbell-shaped tensile specimens (2 mm width × 0.5 mm thickness) were prepared by thermal compression and cold pressing, and then conditioned at room temperature for more than 2 weeks before testing. Dynamic mechanical analyzer (Q800, TA Instruments) was used to explore dynamic
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mechanical property. Samples were heated from -100 °C to Tm with a heating ramp of 3 °C /min, at a frequency of 1 Hz and an oscillatory amplitude of 5 µm. The samples used for DMA tests were stored under the same condition for the same time as those of tensile tests. The CO2 and O2 barrier properties of PBFLAs were tested at 23 °C by Labthink VAC-V2. The films with surface area of 38.5 cm2 were prepared by melt-press/cold pressing procedures, and then they were annealed at room temperature for at least 2 weeks. The water barrier properties of PBFLAs were studied at 38 °C and 90% RH by Labthink W3-060. The films were prepared by the same method as gas barrier tests with smaller surface area of 33 cm2. Hydrolytic degradation tests were performed in phosphate buffered solution (pH=7.4) at 50 °C. After 10 days of degradation, the films were washed and dried to fixed weight. Also the media was replaced every 10 days. The degradation level was estimated from the percentage of weight loss and the descent of intrinsic viscosity (tested every 20 days), according to the following relationship: Weight loss %! =
w − w × 100% 3! w
Where W 0 is the original weight, and W t is the weight after degradation.
SEM (EVO18, Carl Zeiss) was carried out to observed morphology changes during degradation. All samples were coated with a layer of platinum.
3. Results and discussion 3.1 Synthesis of PBFLA copolyesters. As shown in Scheme 1, PBFLA copolymers with nLA between 10 mol% and 40 mol%
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were prepared using OLLA and BHBF. In comparison, PBF was also synthesized.
Scheme 1. Synthesis of PBFLAs by transesterification and polycondensation reactions. 3.2 The chemical structure and molecular weight of OLLA. The molecular structure of OLLA was confirmed by 1H NMR (Figure S1). Molecular weight and its distribution were tested by GPC, using THF as mobile phase. The Mn, Mw and DI were 3200 g/mol, 4300 g/mol and 1.34, respectively. 3.3 The structures and compositions of PBFLAs and PBF. Chemical structure of the PBFLAs and PBF were confirmed by FTIR (Figure 1) and 1
H NMR (Figure 2). In Figure 1, the absorption bands at 1745 and 1710 cm-1 were
attributed to C=O stretching vibrations of LA units and PBF moieties of PBFLAs, respectively. The intensity of the peak at 1745 cm −1 became weaker with decreasing
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nLA. The structure of 2,5-disubstituted furans could be confirmed by the νC−H of furan at 3120 cm−1, the C=C bond of furan at 1581 and 1519 cm−1. The broad absorption peak at 3435 cm-1 of -OH gradually disappeared during the procedure of copolymerization of OLLA with PBF and the copolymers with relatively high molecular were obtained at the end.
Figure 1. ATR-FTIR spectra of the PBF and PBFLAs. In Figure 2, the 1H NMR spectra of PBFLAs were presented. The segment units were summarized in Figure 2d, and their related peaks were assigned in Figure 2a. These spectra, when compared with those of homopolyester PBF, showed several new resonances. The signals of -CH3- and -CH- in LA units appeared at δ = 5.22−5.39 (d1, d2) and 1.46−1.60 (e1, e2) ppm, respectively. Among these resonances, d1 and e1 were attributed to the F-LA diad, while d2 and e2 corresponded to the LA-LA diad, resulting from the conjugative effect of the furan ring. The peaks at 4.45–4.20 ppm were attributed to -CH2 (b1, b2, b3) of 1,4-BDO covalently linked to furandicarboxylate and/or LA. The middle CH2 in 1,4-BDO unit was at 1.76-1.90
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(ppm) (c1, c2, c3). The molar content of LA (nLA) was determined from the integrations of a1, a2 and d1, d2, as shown in Equation 4. Due to potential depolymerization of OLLA, the nLA in the copolyesters was less than original ratio (nLA0)11, 43.
n)* =
I,- + I, ∗ 100% 4! I,- + I, + I/ + I/-
In Figure 2d, five kinds of possible segmental units were shown. The signals of LBL segments were almost none, and we are not going to discuss it in this work. The number fractions of FBF, FBL, LL and FL segments were proportional to Ib1/4, Ib3/2, Id2 and Id1, respectively. Therefore, number-average sequence length (YLL, YPBF) and degree of randomness (R) were determined from the number fractions of these four segments according to Ref.12, 17, 44, 45 and Equations 5-7:
Y))
I56 1 I,- + 2 I,- + I, = 4 + 7 5! I56 2 I, 2
Y9:;
I5 I56 I/ 1 4 + 2 = 4 + 2 7 6! I56 2 I, 2 1 1 R= + 7! Y)) Y9:;
The YPBF decreased with the increase of nLA. The R of PBFLAs were all close to 1,
indicating a random distribution of every specie. The YLL values, which indicated the probability of finding LA next to another LA unit, increased from 1.18 to 1.55.
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Figure 2. (a)1H NMR spectra of PBF and PBFLAs; (b) enlargement of chemical shifts c, e; (c) enlargement of a, d; (d) chain structures of the PBFLAs. Table 1. Molecular structures of the PBFLAs and PBF.
nLA0b
nLAc
timea (h)
(mol%)
(mol%)
PBFLA40
6
80
38
1.55
2.51
1.04
PBFLA30
6
50
30
1.43
2.79
PBFLA20
6
28
22
1.23
PBFLA10
6
15
10
PBF
6
/
/
Reaction Sample
a
[η] Mw
DI
1.18
79 500
2.2
1.06
1.25
87 600
2.3
3.10
1.07
1.30
96 500
2.2
1.18
9.10
0.96
1.10
73 500
2.0
/
/
1.01
58 000
1.9
YLL
YPBF
R dl/g
: The time of polycondensation. b: Feed ratio of LA units. c: Actual LA content in the
copolyesters. As shown in Table 1, the Mw ranged from 58 000 to 96 500 g/mol, and their DI were between 1.9 and 2.3. In Figure S3, the GPC curves of PBFLAs and PBF showed a
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monomodal distribution of these products, which further suggested that the obtained products were copolymers rather than blends. Correspondingly, the intrinsic viscosities of the samples were at 1.01-1.30 dl/g. These results showed that the molecular weight reached its maximum at PBFLA20, and then decreased with increasing LA units. Nevertheless, samples had relatively high molecular weights. 3.4 Thermal properties of PBFLAs and PBF. The physical features of PBFLAs and PBF were investigated by DSC, TGA, XRD and DMA. 3.4.1 DSC characterization. The thermal transition behaviors were summarized in Figure 3 and Table 2. The first heating curves in Figure 3a could reflect the thermal history of the copolyesters. The samples for DSC were prepared by the compression molding/cooling pressing and stored under room temperature for more than 2 weeks. All the samples showed one or two melting peaks, which meant that they could crystallize in proper conditions. Besides, the melting temperature of PBFLAs decreased with increasing LA content, from 162.2 °C (PBFLA10) to 90.0 °C (PBFLA40). The introduction of the LA units may increase the nucleation activation energy and depress the thickness of the lamellar, which in turn decreased the crystallization rate and the melting point of the crystal. Herein, with the introduction of LA units, the number-average sequence lengths of PBF decreased from 3.10 to 2.51 in PBFLA20-PBFLA40, and the shortened PBF segment repressed the crystallization process. However, because of different crystallization ability, Xc of PBFLAs were discrepant. PBFLA10 and PBF
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showed low Xc, and the reason might be the higher crystallization temperature and poorer chain flexibility. As for PBFLA20-PBFLA40, they had relatively lower melting and crystallization temperatures, which can all be explained by the introduction of LA units. The incorporation of LA unit deteriorated the uniformity of the chain structure and decreased the thickness of the lamellar crystal and the melting point. On the other hand, the LA unit improved the dynamics of the PBF chain and decreased the crystallization temperature. In this work, the samples for the tensile tests and barrier tests had the same history, melt-press/cooling pressing and stored under room temperature for more than 2 weeks. After 2 weeks of storage, the materials reached a stable state, and such treatment could be found in some commercial products too 46. In Figure 3b, no crystallization peak appeared during the cooling procedure for PBFLA20 to 40. In contrary, PBFLA10 and PBF showed crystallization behavior during the same cooling procedure, with 100.9 °C and 14.1 J/g for PBFLA10, and 102.4 °C and 6.0 J/g for PBF, respectively. It indicated that the crystallization ability of PBFLAs became weaker as the increase of LA content. Figure 3c presented the second DSC heating curves. The thermogram showed the melting peaks of PBFLA copolyesters with less than 10% LA units. It is obvious that PBFLA10 had a Tm lower than that of PBF. Other samples didn’t show any cold crystallization or melting behavior in second heating curves. Nevertheless, PBFLA20-PBFLA40 could crystallize under proper conditions, as their first heating scans clearly showed melting behaviors in Figure 3a. Both the asymmetric structure of furan ring and the side chain
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methyl of the lactic acid suppressed the crystallization of the copolymers. Besides, each sample exhibited one Tg, which implied that the thermodynamic compatibility between the two components was quite good. The Tg of copolymers varied in a narrow range, between 36.1 °C of PBF and 38.7 °C of PBFLA30.
Figure 3. DSC curves of (a) 1st heating, (b) cooling and (c) 2nd heating of PLA, PBFLAs and PBF. Table 2. Thermal properties of PLA, PBFLAs and PBF.
Tcc (°C)
1st heating ∆Hcc Tm (J/g) (°C)
∆Hm (J/g)
PLA
110.7
13.6
168.9
PBFLA40
nd
nd
PBFLA30
nd
PBFLA20
nd
Sample
a
Xc (%)
1st cooling ∆Hc Tc (°C) (J/g)
2nd heating Tg Tm ∆Hm (°C) (°C) (J/g)
TGA T5% Td, max (°C) (°C)
20.3
7.1
ndc
nd
61.3
168.0
3.2
334
90.0
22.6
17.5
nd
nd
38.6
nd
nd
nd
111.8
19.7
15.3
nd
nd
38.7
nd
nd
128.6
19.5
15.1
nd
nd
37.3
nd
a
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DMAb Tg d (°C)
XRD Xc d (%)
368
63.0
42.3
347
403
59.2
19.0
nd
349
404
59.1
22.5
nd
356
402
57.8
25.5
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PBFLA10
88.0
25.6
162.2
25.7
1.3
100.9
14.1
37.0
161.5
26.4
360
404
52.1
29.7
PBF
95.6
34.8
168.3
34.8
0
102.4
6.0
36.1
168.8
34.2
376
407
45.0
33.4
a
: The X c was calculated by the equation: Xc=(∆Hm-∆Hcc)/∆H0*100%, and ∆H0=129
J/g for PBF47 and 93.7 J/g for PLA48. b: The peak of α relaxation from DMA curves. c: “nd” represented “not detected”. d: The degree of crystallinity from XRD was calculated by the Jade software. 3.4.2 XRD results of PBF and PBFLAs. Figure 4 showed XRD patterns of PBF homopolyester and PBFLA copolyesters. Each sample was annealed at the temperature of individual’s 0.8 Tm, for 30 min to promote the crystallinity of copolyesters, as can be seen from Table 2. All of the PBFLAs exhibited three main peaks at 2θ=18.1 °, 22.5 ° and 25.0 °, which could be attributed to PBF crystals34, 47. The main peak of the PLA crystal structure appeared at 2θ=16.9 ° and some small peaks exhibited at the 2θ of 15.1 °, 19.4 °, 22.7 °, 25.5 ° and 29.3 ° 49. Therefore, there was no peak that could be assigned to the crystal structure of PLA in current study, indicating that only PBF crystal existed in PBFLAs. It seemed that the copolymerization of LA units didn’t change the crystal structure of PBF, and LA segment could not form crystal. After sufficient heat treatment, Xc calculated from the XRD patterns were larger than the values calculated from the DSC tests. Because these two calculation methods were completely different, we could not directly compare these two values. Nevertheless, the Xc from the XRD increased from 19.0% (PBFLA40) to 33.4% (PBF). The increasing Xc showed that the copolymerzation of LA units weakens the crystallization ability of the copolyesters. Furthermore, Xc of PBFLA10 and PBF
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increased quickly from nearly amorphous to 30%. These two samples crystallized quickly at the temperatures close to the crystallization temperatures, but they kept amorphous when they were stored at room temperature.
Figure 4. XRD patterns of PBFLAs and PBF. 3.4.3 Thermal stability of PBF and PBFLAs. Figure S4a and b showed the TGA and DTG curves under N2 atmosphere. The T5% decreased with the increasing nLA, because PLA had relatively poorer thermal stability than that of aromatic polyesters. Nevertheless, the thermal stability of the PBFLAs still remained at a high level, compared with PLA. All the PBFLAs had similar Td,max, in a small range of 402 to 407 °C, higher than the peak of the PLA (365 °C) for 30 °C. Therefore, in the process of thermal degradation of PBFLAs, no degradation process of PLA long chains was observed. These results also proved that the copolymers were random copolymers of PBF-PLA, not the blends of PLA and PBF oligomers. 3.4.4 DMA tests of PBF and PBFLAs.
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Figure 5a and b demonstrated the dependence of tan δ and storage modulus (E’) on temperature. There were two relaxations on tan δ curves of PBFLAs. The first relaxation (β relaxation) showed at -90 to -20 °C, which involving the reorientation and local motions of the carboxyl groups in the amorphous phase
50
. The second
relaxation (α relaxation) correlated with the glass transition behavior of amorphous phase, which shifted to from 45.0 to 63.0 °C as the increase of LA units. However, the peak temperature of α relaxation was a little bit higher than the glass transition temperature obtained by DSC. Again, no signal of microphase separation was found in DMA tests. For PBFLA10, PLA and PBF, a storage modulus plateau appeared after the α relaxation, which was probably due to the cold crystallization 34. When the LA content was higher than 10 mol%, YPBF became less than 4. As a result, the cold crystallization rate of PBFLA 20 to 40 was very slow and could not be discerned in DMA experiments.
Figure 5. (a) Tan δ and (b) storage modulus curves for PBFLAs, PLA and PBF tested on DMA. 3.5 Mechanical property of PBFLAs.
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Figure 6 showed the stress-strain curves of PBFLAs and PBF. Elastic modulus, stress and elongation at yield point and break point were summarized in Table 3. During the tensile experiment, all the samples behaved like semi-crystalline plastic with three stages, including yielding, neck forming and strain hardening. In the stage of yielding, the stress at yielding point decreased with increasing LA units. Then, thin neck extension occurred between 20% to 140% elongation, without obvious change of stress. In the last stage, strain hardening, the stress increased quickly until the sample broke. Many factors could affect the mechanical behaviors of the polymers, such as crystallinity, molecular weight and chemical structure. Since PBFLAs had similar molecular weights, the degrees of crystallinity and the chemical compositions of the samples were the main factors affecting the mechanical properties. The degree of crystallinity of samples were summarized in Table 2. The Xc of PBF and PBFLA10 were close to 0, showing that the samples were nearly amorphous during the storage. PBFLA20 and PBFLA30 showed Xc about 15% and PBFCL40 showed the highest X c of
17% in this series. Therefore, after 2 weeks’ storage, the crystallinity of samples
increased with the increasing LA contents. In our previous work, thermal history of samples influenced the elastic modulus and elongation at break of the poly(butylene carbonate-co-furandicarboxylate)39. The increasing crystallinity promoted the modulus but decreased the strain at break. However, in this work, different trend appeared. PBF owned the highest modulus (1351 MPa), tensile strength (73.9 MPa) and strain at break (355%) among these samples. With the increase of LA units, the
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elastic modulus decreased from 1351 to 1044 MPa of PBFLA40. The reason for such variation was attributed to the changed chemical composition of the copolyesters. The introduction of LA units destructed of the rigidity and regularity of PBF segments. Since the Tg of the copolymers (36-38 °C) was higher than the room temperature, they were in glass state during mechanical tests. As a result, the stiffness of the molecular chains
had
a
great
influence
on
the
modulus.
But
poly(butylene
carbonate-co-furandicarboxylate)39 copolymers had Tg lower than the room temperature and the crystallization showed different effects. Nevertheless, it was still natural to understand that the tensile strength also decreased with increasing nLA, as the LA unit improved the flexibility of the PBF chains. Besides, another interesting fact is that the copolymers showed relatively high elongation at break and they also slightly decreased with increasing nLA. The thermal effect during the stretching process may lead to such phenomenon
51
. Wojtczak et al.
also monitored the surface temperature of the copolyesters which were stretched at 25 °C 52. The surface temperature could rise to 35-37 °C during the stretching. Because the Tg of pure PBF and PBFLAs were observed in narrow range of 36.1-38.7 °C, it was easy for them to
become more flexible during the stretch process and could undertook large strain. However, with the increase of LA unit, the crystallinity increased, which could easily decrease the elongation at break. In conclusion, PBFLA copolyesters all revealed good mechanical properties. In comparison with PBFLAs, the PBT based analogues, PBT-PLA had both elastic
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modulus and tensile strength less than 10 MPa16. Probably, higher Tg and enhanced intermolecular force by furan rings in PBFLAs promoted the mechanical properties. When compared with other aliphatic and aliphatic-aromatic copolyesters, PBFLAs exhibited comparable or better tensile properties, as shown in Table 3 and Figure 7. In contrast to poly(butylene adipate-co-terephthalate) poly(butylene succinate-co-adipate)
55
53
, poly(ε-caprolactone)
and poly(butylene succinate)
56
54
,
, PBFLAs had
higher strength and elastic modulus. In comparison with PLA51, they had better tensile toughness and higher elongation at break. Importantly, PBFLAs showed better mechanical properties than the linear low density polyethylene
57
, which was one of
the most common packaging materials.
Figure 6. Stress-strain curves of PBFLAs and PBF. Table 3. Mechanical properties of PBFLAs, PBF and some other polymers. Sample
E a (MPa)
σy b (MPa)
εy c (%)
σb d (MPa)
εb e (%)
PBFLA40
1044±12
38.8±0.3
7.8±0.2
30.2±1.2
233±10
PBFLA30
1125±15
42.7±0.4
6.6±0.1
48.1±0.5
286±15
PBFLA20
1180±21
47.8±0.2
6.5±0.3
58.7±0.3
293±11
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a
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PBFLA10
1220±23
52.7±0.6
6.2±0.1
67.0±2.1
311±7
PBF
1351±64
61.0±0.5
3.8±0.2
73.9±1.9
355±8
PBAT f
115±5
8.3±0.2
-
18.9±1.4
800
PCL g
260
-
-
11.3
750
PLA h
3265±29
75.8±2.0
-
-
7.4±1.3
PBS i
432±22
-
-
43.6±1.0
303±15
PBSA j
270
-
-
28
780
LLDPE
270
-
-
12.5
148
: Elastic modulus. b,c: Stress and elongation at yield point. d,e: Stress and elongation at
break point. f: Ecoflex®. g: Mn=120,000 g/mol. h: Natureworks 4032D. i: Mw=162,500 g/mol. j: BIONOLLE® 3001 MD.
Figure 7. Mechanical properties of PBFLAs, PBF and some other polymers. 3.6 Gas and water barrier properties of PBFLAs and PBF. The barrier properties are of great concern for the applications like packaging and films. PLA showed relatively poor barrier properties, which restricted its widespread application in packaging 9. In our previous work, the barrier properties of PLA and PBF blends were tested 51. When the mass content of PBF in the blend reached 30%,
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the barrier properties concerning CO2, O2, and H2O were 6.3, 7.4 and 2 times than those of PLA, respectively. In this work, the barrier properties of PBFLAs and PBF were tested and the data were summarized in Table 4. Obviously, PBF had better superior barrier properties than PLA. As to PBFLAs, with increasing LA content, the barrier properties declined. The CO2 BIFp decreased from 19.6 for PBF to 7.0 for PBFLA40, and the O2 BIFp decreased from 20.8 for PBF to 7.5 for PBFLA40. Still, the O2 and CO2 BIFp for PBFLAs were igher than those of PLA. The test temperature here was 23 °C, which was a frequently adopt temperature for barrier properties evaluation42, 59, and it was lower than the Tg of PBFLAs and PBF. The barrier properties of PBFLAs were closely influenced by their chemical structures. Due to the weak mobility of the chain segment, probably the gas molecules had to follow a zigzag route to diffuse through the matrix. With the increase of LA content, the permeability coefficients of gas and water molecules increased. It seemed that compact stacking of PBF segments could be loosed by LA units and the penetration of small molecules became easier. Even though, PBFLA copolyesters showed much better gas and water barrier properties than the commercial PLA. Table 4. Gas barrier properties of PBFLAs, PBF and PLA. Sample a
CO2 b (barrer)
PLA PBFLA40
BIFp c f
1.0 1.4*10
-1
BIFp
7.0
2.5*10
-1
3.3*10
-2 -2
H2 O e
BIFp
2
(barrer) 1
-1
O2 d
(g·cm/cm ·s·Pa) 1 7.5
1.1*10-13 3.7*10
1
-14
2.5
-14
PBFLA30 PBFLA20
1.0*10 7.7*10-2
10 13
1.9*10 1.7*10-2
12.9 14.4
3.1*10 2.8*10-14
3.6 4.0
PBFLA10 PBF
5.7*10-2 5.1*10-2
17.5 19.6
1.5*10-2 1.2*10-2
18.7 20.8
2.4*10-14 2.3*10-14
4.5 4.8
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a
The barrier tests were performed at pressure of 0.1 MPa.
coefficient.
c
Page 24 of 32
b
CO2 permeability
The BIFp represented the permeability coefficient of PLA divided by
those of samples. 42, 58, 59 d O2 permeability coefficient. e Water vapor transmission rate. f
1 barrer = 10-10 cm3cm/cm2·s·cm Hg.
3.7 Hydrolysis degradation of PBFLAs. Hydrolytic degradation of PBFLAs were performed in phosphate buffered solution at 50 °C. Degradation was induced by the breakage of ester linkages, and then resulted in water soluble oligomeric or monomeric products, which eventually caused weight loss. Figure 8 showed the in vitro hydrolytic degradation of PBFLAs in 80 days. PBF barely showed any degradation until the end of the test. At the same time, PBFLA10 showed weight loss less than 3 %. Because its PBF segments was long. As expected, the LA units accelerated the hydrolysis rates of the copolyesters, just like in other aromatic-aliphatic copolyesters. PBFLA20 reached residue weight of 85 wt% after 80 days and intrinsic viscosity decreased to 0.7 dl/g. PBFLA 30 and 40 showed less residue weight of 78 wt% and 65 wt%, respectively. The intrinsic viscosity sharply decreased to 0.4 and 0.2 dl/g for PBFLA 30 and PBFLA40, respectively. The remarkable decrease in intrinsic viscosity and the obvious weight loss suggested that the copolymers underwent bulk degradation.
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Figure 8. Residue weight (a) and intrinsic viscosity (b) along with the degradation time. The degradation behaviors were further studied by SEM, and the microscale morphologies of the film surfaces were observed (Figure 9). The sample exhibited smooth surface before degradation. After 40 days of the hydrolytic degradation, more abnormalities (for PBFLA20) and irregular holes (for PBFLA30 and 40) were observed, and the holes became deeper with increasing LA units. With increasing the
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degradation time (80 days), the fragments removed from the surface into inside of the film with wider cracks and more cavities (for PBFLA40). It was evident that the film surface was eroded by the hydrolytic degradation. These results clearly suggested that the PBFLAs, even with a small amount of LA units, can be hydrolyzed in PBS solutions at 50 °C.
Figure 9. SEM images of PBFLAs film surface during hydrolytic degradation.
4. Conclusion Novel bio-based and biodegradable copolyesters synthesized by the melt polycondensation of polylactide diols and PBF oligomers were successfully obtained. The copolymerization with LA did not significantly alter the thermal stability of original PBF, a crucial factor during plastic processing. However, the melting temperature and crystallization rate decreased with increasing lactic acid content. The comonomeric units had limited effect on the mechanical properties, showing good modulus and tensile strength (ca. 1125 and 48 MPa, respectively) and better tensile
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toughness (ca. 286%) than PLA. It was impressive to note that the gas and water barrier properties of copolymers were 7.0 and 2.5 times better than the commercial PLA, respectively. As expected, the ester bonds between lactic acid and BF units were easy to be broken by hydrolysis. Meanwhile, the increment of lactic acid content would accelerate the degradation of PBFLAs. The significant decrease in weight and intrinsic viscosity indicated the fracture of polymers during hydrolysis. The new series of PBF-co-PLA polymers own attracting basic properties, which lead to great potential in applications like container and packaging etc. Nevertheless, the control and adjustment of PBFLAs’ crystallization would be interesting and important in the next stage’s research.
Supporting information 1
H-NMR and GPC spectrums of polylactide diols; GPC curves of PBFLAs and PBF;
TGA and DTG scans.
Acknowledgements This work was supported by National Natural Science Foundation of China (51773218) and National Key Research and Development Program of China (2017YFB0303000).
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Bio-based and biodegradable PBFLA copolyesters have better mechanical properties than many other biodegradable materials.
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