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Toward Biobased, Biodegradable and Smart Barrier Packaging Material: Modification of Poly(Neopentyl Glycol 2,5-Furandicarboxylate) with Succinic Acid Han Hu, Ruoyu Zhang, Yanhua Jiang, Lei Shi, Jinggang Wang, Wu Bin Ying, and Jin Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05990 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019
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Toward Biobased, Biodegradable and Smart Barrier Packaging Material: Modification of Poly(Neopentyl Glycol 2,5-Furandicarboxylate) with Succinic Acid Han Hu†,‡, Ruoyu Zhang†*, Yanhua Jiang†, Lei Shi†, Jinggang Wang†, Wu Bin Ying† and Jin Zhu†* †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 ‡University
of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
China
Author Information Corresponding authors: E-mail:
[email protected] (Ruoyu Zhang) E-mail:
[email protected] (Jin Zhu) Email of other authors: Han Hu:
[email protected] Yanhua Jiang:
[email protected] Lei Shi:
[email protected] Jinggang Wang:
[email protected] Wu Bin Ying:
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Abstract: A series of biobased poly(neopentyl glycol succinate/furandicarboxylate) (PNSF) copolyesters were synthesized via a transesterification and melt polycondensation process. The 1H NMR and analysis using the FOX equation suggested the formation of random microstructures. PNSFs displayed poor crystallization capability with a slow crystallization rate. The copolymerization destroyed the spherulite morphology of the homopolyester PNF, and the melting temperature (Tm) also decreased with decreasing neopentyl glycol furandicarboxylate content (mNF). Rheological tests revealed the slight increase in the free volume with the addition of succinic acid. The crystallization induced by thermal annealing enhanced the elastic modulus and tensile strength. PNSF50-PNSF70 possessed modulus and strength greater than 1000 MPa and 27 MPa, respectively, exceeding that of most biodegradable packaging materials. On the other hand, PNSF80 and PNSF90 possessed high modulus of 3.0 GPa and strength of 100 MPa, that were comparable to highperformance engineering plastics. Amorphous PNSFs displayed outstanding gas barrier properties, with reduced CO2 (110×) and O2 permeability (18×) when compared to poly(butylene adipate-co-terephthalate). Interestingly, the gas barrier property was not impacted by succinic acid, because of the steric hindrance of the methyl groups. Key Words: Biobased and degradable polymers; Free volume fraction; High modulus and strength; Smart barrier properties; Green packaging
Introduction In recent decades, increasing demand for environmental protection and sustainability had pushed the packaging industry to search for biobased and biodegradable plastics.1-
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2
The packaging applications contributed over 50% of global biodegradable plastics
consumption, and the demanding for biodegradable plastics in the industry was expected to rise at an annual growth rate of 33% in the period of 2013–2023.3 Nevertheless, compared with the total amount of plastic consumption in the world, the ratio of biodegradable plastics was still very low due to their higher price and worse properties. For example, the barrier property of biodegradable plastics is normally lower than traditional plastics. These properties were critical for food packaging.4-5 Among various renewable polymers, 2,5-furandicarboxylic acid (FDCA) based polyesters had received enormous attentions in recent years.6-8 For instance, poly(ethylene furandicarboxylate) (PEF) was considered to be the most promising biobased alternative to poly(ethylene terephthalate) (PET)9-11 because of its outstanding mechanical and barrier properties.12-14 However, most FDCA based aromatic polyesters were nondegradable. In order to prepare biodegradable polyesters, comonomers like succinic acid,15-16 adipic acid,17 dimethyl carbonate18-19 and poly(ethylene glycol)20-21 were incorporated into aromatic polyesters, and thus endowed the resulted copolyesters with good mechanical performance and acceptable biodegradability. Moreover, Papageorgiou et al.22 reported the synthesis of new FDCA-based polyesters, poly(neopentyl glycolfurandicarboxylate) (PNF), which displayed good barrier performances with relatively high glass transition temperature (70 °C) and spherulites after thermal annealing. Recently, Lotti et al.23 elucidated the underlying mechanisms of mechanical and barrier properties of PNF. Similar to PEF and poly(propylene furandicarboxylate) (PPF),24 PNF possessed high modulus and strength. However, it
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was brittle with less than 5% elongation at break. Therefore, it would be admirable if PNF could be modified to possess good mechanical, excellent barrier properties and biodegradability simultaneously. Succinic acid had become one of the most important biobased platform chemicals because of its wide range of potential applications.25 It has been introduced into poly(butylene terephthalate) (PBT), and the resulting polymer was named as poly(butylene succinate-co-terephthalate) (PBST).26 With a proper composition of succinic acid in the polyester, PBST can demonstrate good biodegradability and mechanical properties.27 Besides, Peng et al. used FDCA to synthesize poly(butylene succinate-co-furandicarboxylate) (PBSF) copolyesters. It was found that PBSF40PBSF60 possessed elastic modulus at ~100 MPa, tensile strength between 10-32 MPa and elongation at break of more than 500%. In addition to the good mechanical properties, it also demonstrated good biodegradability. Moreover, in our previous studies we reported that the biodegradability and tensile ductility of poly(propylene succinate-co-furandicarboxylate) (PPSF) copolymers could be improved by the copolymerization of succinic acid.16 It claimed that succinic acid was an effective comonomer to transform FDCA based polyesters into a completely sustainable polymer with a green origination and end.18, 28-29 In this work, succinic acid was used to convert PNF into poly(neopentyl glycol succinate/furandicarboxylate) (PNSF). PNSFs in wide composition range were synthesized through a transesterification and melt polycondensation method. The molecular structures, thermal properties, crystalline structure, free volume and
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mechanical properties were investigated. In addition, the structures of spherulites were analyzed by small angle laser light scattering (SALS) and polarized optical microscope. The effect of thermal annealing on final properties of copolyesters was further evaluated. The processes of hydrolysis and enzyme degradation of PNSFs were recorded and compared. Based on the comparison with gas barrier performance of PBAT and PET, PNSFs showed the potential applications in the green packaging industry.
Experiments Materials and Characterization. All the materials and characterization methods used in this work were summarized in the Supporting Information. Syntheses of PNSF Copolyesters. Transesterification and melt polycondensation methods were adopted to prepare PNSFs, as illustrated in Scheme 1. The copolyesters were named as PNSFmNF, where mNF % was the molar percent of NF units. In the transesterification step, an 1 L three-necked flask equipped with a mechanical stirrer with torque indicator was placed in a silicone oil bath, and certain amount of DMS (dimethyl succinate), DMFD (dimethyl-2,5-furandicarboxylate) and NPG (neopentyl glycol) in a molar ratio of (NPG/(DMS+DMFD)=1.5/1) were added into the flask. Then Zn(Ac)2 was added as the catalyst. N2 flow was applied and the temperature was set to 180 °C during the reaction. The reaction could continue for 3 to 5 h, until more than 95 wt% of the theoretically calculated amount of methanol was collected. Before the melt polycondensation, Sb2O3 and antioxidant were added into the reactor. The pressure of the system was gradually decreased to 1000 Pa and the temperature was increased to
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220 °C within 30 min. Then the pressure was further decreased to less than 50 Pa and the temperature was maintained at 240 °C. The reaction was continued at this temperature for at least 4 h. Finally, the torque value of the mechanical stirrer remained unchanged, suggesting the end of the polycondensation. The reaction system was recovered to atmospheric pressure by blowing N2 into the flask again, and the target polyester was obtained.
Results and discussion Preparation of PNSFs Copolyesters. PNSF copolyesters were prepared as demonstrated in Scheme 1. For comparison, PNS and PNF homopolyesters were also synthesized.
Scheme 1. Syntheses route of PNSF copolyesters. Structural Characterization and Compositions of PNSFs. FTIR spectra of PNSFs were provided and analyzed in Supporting Information (Figure S1). Figure 1a-1c exhibited the1H NMR spectra of PNSFs, and the obtained attributions of different peaks were summarized in Figure 1d. The chemical shifts of PNF were at δ=7.31 (c), 4.29
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(a4) and 1.12 (b3) ppm, that could be assigned to the CH in the furan ring, methylene (CH2) and methyl (CH3) in the NPG, respectively. For PNS, there was a new peak at δ=2.64 (d), which could be assigned to protons in succinate unit. As for PNSF copolyesters, the peak positions of CH2 (a1-a4) and CH3 (b1-b3) in the NPG units slightly changed when NPG was connected with succinate or furandicarboxylate unit. From the 1H NMR spectra, the copolyester composition (mNF), the number-average sequence length of NS and NF units (Ln,NS and Ln,NF), and the degree of randomness (R) could be calculated by eqs. (equation) 1-4, respectively. The mNF obtained from the 1H
NMR spectra was slightly larger than the mNF,feed because small amount of DMS
would be distilled in the transesterification step. The Ln,NF increased with mNF, and the Ln,NS presented an opposite trend. The R of all the copolyesters was close to 1, suggesting the random sequences connection. 2𝐼𝑐
𝑚𝑁𝐹 = 2𝐼𝑐 + 𝐼𝑑 × 100% 2𝐼𝑎4
𝐿𝑛,𝑁𝐹 = 1 + 𝐼𝑎2 + 𝐼𝑎3 2𝐼𝑎1
𝐿𝑛,𝑁𝑆 = 1 + 𝐼𝑎2 + 𝐼𝑎3 1
1
𝑅 = 𝐿𝑛,𝑁𝐹 + 𝐿𝑛,𝑁𝑆
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(1) (2) (3) (4)
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Figure 1. 1H NMR spectra of PNSFs. (a) Enlargement of chemical shifts a. (b) Enlargement of b. (c) Chain structures of PNSFs. (d) Table 1. Molecular structures of PNSFs. Sample
mNF,need
mNF
Ln,NF
Ln,NS
R
[η] (dL/g)
Mn (g/mol)
DI
PNS PNSF30 PNSF40 PNSF50 PNSF60 PNSF70 PNSF80 PNSF90 PNF
0 30 40 50 60 70 80 90 100
0 37.2 42.9 53.3 60.3 71.4 80.4 90.3 100
/ 1.62 1.83 2.23 2.63 3.57 5.00 9.46 /
/ 2.51 2.10 1.82 1.56 1.34 1.22 1.08 /
/ 1.01 1.02 1.02 1.02 1.01 1.02 1.03 /
1.54 1.52 1.67 1.66 1.54 1.11 0.90 0.91 0.72
9.95×104 1.18×105 1.20×105 1.25×105 1.13×105 7.18×104 6.64×104 6.72×104 6.26×104
2.0 1.9 2.0 1.8 1.8 1.9 1.7 1.6 1.6
The intrinsic viscosity ([η]), number-average molecular weight (Mn) and dispersity (DI) of the PNSFs were summarized in Table 1. The [η] of PNS-PNSF60 was higher
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than 1.5 dL/g, but the viscosity decreased to less than 1 dL/g among PNSF80-PNF. It might imply that the stiffness of the NF segments hindered the growth of the molecular chain. As can be seen from GPC data, Mn of PNSF70-PNF varied from 6.26×104 to 7.18×104 g/mol, and the Mn further increased to 9.95×104 to 1.25×105 g/mol for PNSPNSF60. It can be indicated that high Mn and narrow DI (1.6-2.0) polyesters were successfully synthesized. Thermal Properties of PNSFs. Prior to DSC characterization, the compression molded samples were stored at room temperature for 3 days. DSC traces of these samples were displayed in Figure 2a-2c, and relevant data were summarized in Table S1. As demonstrated in Figure 2a, with such thermal treatment, PNSFs kept almost amorphous, and the DSC experimental results were in accordance with the data of XRD, as it would be elucidated below. PNSF90 and PNF displayed cold crystallization peaks between 140 and 150 °C, and Tm at 186.3 and 202.5 °C, respectively. After storage, a sharp melting peak of PNS was found in the DSC curve, indicating a crystallization procedure had happened. However, there was no crystallization process during the first cooling scan and no cold crystallization phenomenon was found at the second heating scan of PNS. These results revealed the slow crystallization kinetics of PNS. The weak crystallization capability was also observed in PNSF30-PNSF80. A similar phenomenon was found in succinic acid modified PPF too.16 For PNSF90 and PNF, a cold crystallization peak and a melting peak appeared in DSC curves, implying the enhanced crystallization ability. The crystallization ability was apparently affected by the length of the NF sequence. With longer NF sequence, the capability of
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crystallization could be improved. To further explore the crystallization and melting behaviors of PNSFs, controlled thermal annealing was applied. 3h annealing at 45-50 °C, 75-80 °C, and 100-105 °C were employed to PNSF30-PNSF50, PNSF60-PNSF70, and PNSF80, respectively. Besides, PNSF90 and PNF were annealed at their Tc (from Figure 2b) for 3 h. In Figure 2d, the first heating scans of different samples were displayed. PNSFs copolyesters showed one or multiple melting peaks, and cold crystallization processes disappeared in PNSF90 and PNF, indicating good crystallization after thermal annealing. The effect of copolymerization was evident on Tm and melting enthalpy (ΔHm), and both of them gradually decreased as the decrease of mNF. Longer NS segments could induce more defects in the PNF crystals, and would decrease the thickness of lamellar and lower the corresponding Tm. Based on the ΔHm of PNSFs and the ΔHm0 of PNF,22 the crystallinities of these samples were calculated by eqs. 5: Δ𝐻𝑚
𝑋c = Δ𝐻𝑚0 × 100%
(5)
It could be observed that the crystallinity increased from 11.0% of PNSF60 to 29.7% of PNF after thermal annealing. Such result implied that the PNF crystal was dominant in these samples. Similarly, Ln,NF could affect the thickness of lamellar and the Tm of the crystals. Unfortunately, we were unable to calculate the crystallinity of PNSF30PNSF50, because of the crystal structure was new and could not be assigned to any known polymer yet.
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Figure 2. DSC curves of (a) 1st heating, (b) cooling and (c) 2nd heating. (d) 1st heating scans of PNSFs after thermal annealing. Tg of PNSFs monotonously increased from -14.0 °C (PNS) to 70.7 °C (PNF), and their glass transition temperatures were higher than those of PPSF counterparts.16 The high steric resistance of the methyl pendant groups could hinder the rotation of the CC bond and increased the chain rigidity as well as Tg. Furthermore, Tg values and the Fox equation (eqs. 6) were used to examine the thermodynamic compatibility of the two segments:
𝑤𝜔,𝑁𝑆 𝑤𝜔,𝑁𝐹 1 = + 𝑇𝑔 𝑇𝑔,𝑃𝑁𝑆 𝑇𝑔,𝑃𝑁𝐹
(6)
As shown in Figure S2, the Tg data agreed well with the Fox model, confirming thermodynamic compatibility and phase homogeneity in these copolyesters. Finally,
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the dynamic mechanical properties of PNSFs were measured by DMA method (Figure S4) and the results were discussed in the Supporting Information (S7). Thermal Stability of PNSFs. Thermal stability of PNSFs was evaluated by TGA experiments, under N2 and air atmosphere from 50-800 °C at a heating rate of 20 °C /min (Figure S3). The temperature at 5% weight loss (T5%) and at the maximum decomposition rate (Td,max) were summarized in Table S2. T5% of PNS to PNSF60 ranged from 364 to 378 °C under N2 atmosphere, and from 360 to 376 °C under air atmosphere. Additionally, PNSFs with higher mNF (PNSF70-PNF) exhibited relatively better thermal stability, with T5% close to 400 °C under N2 and air atmosphere. Again, PNSF70 to PNF had slightly higher Td,max than those of PNS to PNSF60. As it has been reported, most parts of thermal decomposition in polyesters happened via the βhydrogen bond scission, and only small part was through α-hydrogen scission.30 Obviously, the main chain of PNS but not PNF contained β-hydrogen.23 Consequently, the thermal stability of PNSFs was improved by the increasing content of mNF. These results also demonstrated that PNSFs possessed good thermal stability at processing temperature under both N2 and air atmosphere. Crystal Structures and Morphologies of PNSFs. XRD method was employed to investigate the influence of comonomer content on the crystal structure of PNF. Obviously, PNSFs were in amorphous state before the thermal annealing as shown in Figure 3a, which were in agreement with the results from DSC analysis in Figure 2a. The main diffraction peaks of PNS crystals exhibited at 16.3, 17.6 and 20.4°, and weaker diffraction peaks could also be observed at 18.6, 20.4, 26.1, 29.4, 29.9 and 33.4°.
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In Figure 3b, the diffraction peaks of the annealed samples revealed strong sign of crystallization. The main diffraction peaks of PNF exhibited at 2θ values of 16.4°, 17.8°, 20.4°, 22.0°, 23.3° and 28.0°, which were consistent with the previous reports.22, 31 PNSF60-PNSF90 possessed similar diffraction peaks with those of PNF, suggesting that the PNF crystal structure was not altered by the copolymerized succinate units. As for PNSF30 to PNSF50, new diffraction peaks emerged at 16.1°, 17.2° and 18.5°, which implied the formation of the new crystal structure. The new crystal belonged to neither PNF nor PNS crystals and was required more investigation in the future. The crystallinities of PNSFs samples were calculated based on the XRD curves, and were summarized in Table S1. Clearly, the crystallinity increased after the thermal annealing treatment, and the mNF enhanced the capability of crystallization. Although the crystallinities obtained from DSC and XRD analyses were different, but a similar trend could be found in the two independent experiments.
Figure 3.XRD patterns of PNSFs before (a) and after (b) thermal annealing. The morphological evolution of PNSFs copolyesters during the isothermal crystallization was recorded by a camera fixed on a polarized optical microscopy
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(POM). Usually, small spherulites formed in FDCA based copolyesters,24 especially for samples like PEF9 and PBF32. Tsanaktsis et al. once found relatively large size PNF spherulites with the help of thermal annealing.22 Here, in Figure 4a-4c, the pictures of PNF spherulites were recorded at 10, 20 and 30 min during isothermal annealing at 170 °C. At 10 min, the diameter of PNF spherulites was about 20 μm. At 20 min, the density of spherulites increased but no obvious change in crystal size could be observed. When the annealing time was longer than 20min, the morphology of PNF seemed stable. The changes in the size and the number of crystals were negligible. The real-time observation suggested that the crystallization process of PNF was completed within 20 min at 170 °C. After isothermal crystallization at 150 °C for 30 min, many small size spherulites could be found in Figure 4d for PNSF90. The growth of spherulite was controlled by nucleation and lamellar growth,33 and it seemed that in PNSF90 the addition of NS units could promote the nucleation process but depress the crystal growth capability. The Maltese crosses could be discerned from crystals in PNF and PNSF90 samples, and such phenomenon proved that the structures of PNF spherulites in these two samples were well developed. As for PNSF80 and PNSF70, small crystals formed in the two samples after thermal annealing treatment. These small crystals were not spherulites as they lacked the typical spheroid radiating crystal structure.32 Clearly, the PNF crystal growth capability was suppressed when decreasing mNF. The crystallization rates in PNSF60 and PNSF50 were even slower. After isothermal crystallization for 90 min at 80°C and 50°C respectively, no crystal could be observed. In summary, the PNF spherulites were strongly affected by the content of the
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comonomers.
Figure 4. POM images of PNSF50-PNF after isothermally crystallized. As revealed in Figure 5, the spherulites structure were also studied by small angle laser scattering(SALS) by using samples after POM tests.34 The average radius (r) of spherulites were calculated based on the scattering patterns of PNF and PNSF90 (eqs. 7 and 8):
r=
4.09𝜆 4𝜋sin
(7)
𝜃𝑚 2
𝑑
𝜃𝑚 = tan ―1 𝐿
(8)
where λ was the wavelength of the laser, d was the distance from the center of the scattering pattern to the maximum scattering intensity position, L was the distance from the sample to photographic negative center and θm was the angle between incident light
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and the strongest scattered light. The 2D scattering pattern of PNF was a typical clover leaf structure.35-36 Similarly, PNSF90 also displayed a clover leaf scattering pattern with larger size but weaker light intensity. The size difference between the two scattering patterns indicated that PNF had larger and more perfect spherulites. The average radius of spherulites was summarized in Table S2. The r of PNF spherulites was 9.4 μm, and it decreased to 3.0 μm for those of PNSF90. These numbers coincided with the results based on POM pictures. For PNSFs with mNF less than 80%, no scattering pattern could be observed. It was possible that no spherulite could form in the rest samples.
Figure 5. Hv SALS patterns of PNF (a) and PNSF90 (b). Melt Rheology of PNSFs. Melt flow behaviors of PNSFs and the structural influence on chain dynamics could be manifested by melt rheological tests.37 We measured the storage modulus under six to ten temperatures above the Tm of PNSFs. Time−temperature superposition (TTS) was used to fit the master curves, enabling the data analysis over a big viscoelastic window.38-39 Because the melting temperatures varied a lot among PNF and PNSFs samples, two reference temperatures (Tr) were selected for two kinds of samples, respectively. The Tr of PNSF40-PNSF70 was set at 180 °C (Figure 6a), and for PNSF70-PNF it was 240 °C (Figure 6b). The two master curves displayed acceptable overlap over a large angular frequency zone of 0.01-10000
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rad/s.
Figure 6. Storage modulus master curves of (a) PNSF40-PNSF70 at Tr=180 °C and (b) PNSF70-PNF at Tr =240 °C. The viscoelastic properties of polyesters fitted well with the Williams−Landel−Ferry (WLF) equation (eqs. 9), in which the shift factor (aT) and temperatures were correlated: log (𝑎𝑇) =
― 𝐶1(𝑇 ― 𝑇𝑟) 𝐶2 + (𝑇 ― 𝑇𝑟)
(9)
where the C1 and C2 constants were fitted by TRIOS software, with a fitting degree exceeding 99%. Then, the constants at glass transition temperatures could be calculated by using eqs. 10 and 11. 𝐶𝑔1 =
𝐶1𝐶2 𝐶2 + (𝑇𝑔 ― 𝑇𝑟)
𝐶𝑔2 = 𝐶2 + (𝑇𝑔 ― 𝑇𝑟)
(10) (11)
The results obtained from eqs. 9-11 based on PNSFs samples were summarized in Table 2. The values of C1g and C2g of the PNSFs coincided well with previously reported data for copolyesters.39 Furthermore, eqs. 12 enabled us to calculate the fractional free volume at Tg (fg).
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𝑓𝑔 =
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𝐵
(12)
2.303𝐶𝑔1
Although it would be affected by the fitting procedure, the calculated fractional free volume fg could be taken as a qualitative reference. At Tr of 240 °C, PNSF70-PNSF90 possessed fg slightly greater than that of PNF, confirming the increase of the free volume by the addition of aliphatic diacid. Nevertheless, due to the short chain length of succinic acid, the increase in free volume was not significant in these samples. As for PNSF70-PNSF90, the increase in mNF could not induce greater fg, and such result further proved that the influence of succinic acid on chain mobility and fg was trivial. At Tr of 180 °C, the master curves of storage modulus of PNSF40-PNSF70 were constructed. As it can be seen from Table 2, fg of PNSF40-PNSF60 was larger than that of PNSF70. When compared with fg values in literatures,37, 40-41 fractional free volume of PNSFs was significantly lower than those of many copolyesters. Dennis et.al obtained fg between 0.046 and 0.118 in decahydronaphthalene containing polyesters by melt rheology.37 Lyer et.al calculated the fractional free volume of terephthalate based polyesters in the range of 0.146 to 0.180.41 Low fractional free volumes of PNSFs indicated compact packing of chain segments. It could be speculated that the two side methyl groups in NPG filled in the gaps between molecular chains and increased the packing density. The addition of succinic acid in PNF could improve the polymer chain flexibility, but the fractional free volume changed a little in PNSFs. This might also be attributed to the existence of the two methyl groups. Table 2. WLF parameters andfractional free volume of PNSFs. Sample
Tr(°C)
C1
C2
C1g
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C2g (K)
fg
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PNSF40
180
4.13
219.00
15.41
58.70
0.0282
PNSF50
180
3.90
231.30
11.45
78.80
0.0379
PNSF60
180
4.45
202.10
15.67
57.40
0.0277
PNSF70
180
5.30
170.00
26.81
33.60
0.0162
PNSF70
240
4.02
225.10
31.53
28.70
0.0138
PNSF80
240
4.05
224.10
25.78
35.20
0.0168
PNSF90
240
4.28
211.00
27.45
32.90
0.0158
PNF
240
4.80
189.10
45.84
19.80
0.0095
Mechanical Properties of PNSFs before and after Thermal Annealing. The existence of crystals could strongly affect the mechanical properties. Therefore, it is important to clarify the effect of thermal histories on mechanical properties. As illustrated in Figure 2a and 2d, the crystallinity of PNSFs could be significantly affected by the thermal annealing treatment. The stress-strain curves of PNSFs were elucidated in Figure S5 and the important mechanical properties were summarized in Table 3. It was shown that, after 3 days of storage at room temperature, PNSFs were still in the amorphous state. The experimental temperature (25 °C) was between the glass transition temperatures of PNSF40 (19.7 °C) and PNSF50 (27.5 °C). Therefore, PNSF30 and PNSF40 were in rubbery state and revealed low modulus and strength but high elongation at break. In contrast, the elastic modulus of PNSF50 rapidly increased to more than 700 MPa, which was a typical modulus of plastics. From PNSF50 to PNF, their Tg and storage modulus increased continuously. This was because the polymer chains became more and more rigid with the increasing content of mNF. For example, PNSF90 had elastic modulus as high as 2500 MPa. Interestingly, even with the highest
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Tg, the storage modulus of PNF was lower than those of PNSF80 and PNSF90. The relatively low Mn of PNF could decrease its storage modulus. More importantly, the density of chain entanglement should be taken into consideration. Possessing the highest content of FDCA among all the samples, the rotation and the dynamics of the PNF chains could be significantly restricted. Such restriction in segmental motion could decrease the entanglement density. On the other hand, with the addition of a proper amount of succinic acid, the chain network might gain more entanglement density. The tensile strength also increased continuously with the increasing mNF. However, the crystallized PNSF80-PNF were brittle with an elongation at break of less than 7%, and it could limit their potential applications. After the annealing treatment, the increased crystallinity strengthened the PNSFs samples. The elastic modulus of PNSF30 (52.2 MPa) and PNSF40 (132.6 MPa) became 10 times higher than those before thermal annealing. The tensile strength of the two samples also displayed greater values than those of amorphous samples. From PNSF50 to PNF, the tensile strength and elastic modulus also increased with the increasing mNF. At the same time, the elongation at break declined obviously. In these samples, it was clearly indicated that the crystalline phase acted as the hard domains and further increased the strength of materials. Table 3. Elastic modulus, tensile strength and elongation at break of PNSFs before and after thermal annealing. Sample
Thermal history
E(MPa)
σb (MPa)
εb(%)
PNSF30
No-annealing
3.1±0.1
1.9±0.1
2109±50
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PNSF40
PNSF50
PNSF60
PNSF70
PNSF80
PNSF90
PNF
Annealing
52.2±5.0
8.9±0.3
454±12
No-annealing
13.5±0.5
8.9±1.1
872±62
Annealing
132.6±4.8
15.0±0.1
255±18
No-annealing
701.5±58
16.6±2.0
523±46
Annealing
1173.1±23
27.3±0.2
266±10
No-annealing
786.3±43
32.5±3.4
196.5±7
Annealing
2042.1±42
41.9±1.2
152±18
No-annealing
1542.8±63
55.8±0.7
126.4±5.3
Annealing
2215.5±17
68.8±1.3
76.5±2.7
No-annealing
2123.5±28
70.2±3.5
6.3±0.5
Annealing
2867.6±11
84.8±0.3
5.0±0.2
No-annealing
2526.7±43
88.0±1.2
5.8±0.5
Annealing
3560.6±17
102.5±2.1
3.5±0.2
No-annealing
1976.9±30
68.1±1.5
6.0±0.6
Annealing
2315.1±13
74.5±2.3
4.9±0.3
Barrier Properties of PNSFs. The gas barrier properties are dependent on the intrinsic parameters of the polymer, such as chemical structure and composition, degree of crystallinity, molecular weight and even the thermal and mechanical histories. The analyses of O2, CO2 and H2O’s permeation in amorphous and annealed PNSFs films revealed the influence of chemical structure and crystallinity on the barrier property. The Barrier Improvement Factor (BIFp) was defined as the permeability coefficient in PBAT divided by those in PNSFs and PNF. As shown in Table 4, the gas barrier properties of PNSFs possessed outstanding improvement over commercial PBAT and PLA. Without thermal annealing treatment, the CO2 barrier property could be as efficient as 111.3 times of PBAT, and at least 18.1 times for O2 barrier property. The
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improvement on H2O barrier property was relatively small, but it was still more than 7 times higher. Compared with gas barrier properties of PEF and PET,42-43 PNSFs displayed better performances than PET, and were even comparable with PEF in some aspects. It was interesting to note that the addition of a moderate amount of succinic acid in PNF only slightly changed the gas and water permeability coefficient. Contrary, the chemical composition of PPSFs strongly influenced their permeability to CO2 and O2. 16
As illustrated in Figure 7, the transportation of gas molecules through polymer films
could be divided into three steps: adsorption on the high pressure side, activated diffusion through the film and desorption on the low pressure side. The stable permeability of PNSFs might be explained by the steric hindrance of the side methyl groups in NPG. As reported by Genovese et al., the high barrier performances of PNF could be explained by the polar interaction of the furan ring and the decreased mobility of the polar carbonyl moieties by side methyl groups.44 Although the chain dynamics could be improved by the copolymerization of succinic acid, its chain length was shorter, when compared with the other aliphatic diacids. The steric hindrance of the methyl groups counteracted the increased chain flexibility by succinic acid. As a result, the influence from the addition of succinic acid on the fg of PNSF copolymers was trivial as displayed in Table 2. Besides, the effect of molecular weight on the barrier property shouldn’t be overlooked. The molecular weight of PNSF40-PNSF60 was higher than those of PNSF70-PNF. Higher molecular weight and narrower segmental distribution contributed to the tight alignment of chain segments and hindered the gas
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permeation. The effect of the thermal annealing treatment on the barrier properties was also evaluated. Since the thermal annealing treatment could enhance the crystallinity of polyesters, their permeability dropped accordingly, especially for those with a high content of NF, as shown in Table 4. The relatively high permeability of PNSF40 was due to its relatively low Tg of 19.7 °C. Since the tests were carried out under 23 °C, PNSF40 film was already in the rubbery state and the segmental mobility and free volume was relatively high. As a result, we saw relatively high diffusivity of PNSF40. As for the permeation behavior of H2O, the BIFp values slightly decreased with reducing mNF. The opposite influence of compositions on gas and water barrier property may be attributed to the polar furan rings and the polarity of water molecules. High polarity FDCA unit may adsorb water molecules into the films easier and accelerate the permeation.
Figure 7. Schematic illustration of permeation through PNSF film. Table 4. Gas and water vapor permeability coefficients for PNSFs before and after thermal annealing. Commercialized polymers for packaging materials were listed for comparison.
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Sample [a]
CO2 (barrer) [b]
PBAT PNSF40
5.9 0.053
PNSF40-A[e] PNSF60 PNSF60-A PNSF70 PNSF70-A PNSF90 PNSF90-A PNF PNF-A PLA PEF[f] PEF[g] PET[f]
BIFp
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O2 (barrer) [c]
BIFp
H2O (g·cm/cm2·s·Pa) [d]
BIFp
1 111.3
0.76 0.042
1 18.1
3.52×10-13 4.83×10-14
1 7.3
0.051
115.7
0.040
19.0
4.79×10-14
7.3
0.030 0.027 0.035 0.028 0.036 0.027 0.040 0.026 1.0 0.010 0.026 0.130
196.7 218.5 168.6 210.7 163.9 218.3 147.5 226.9 5.9 590 226.9 45.4
0.024 0.022 0.027 0.024 0.030 0.024 0.035 0.023 0.25 0.011 0.011 0.060
31.7 34.5 28.1 31.7 25.3 31.7 21.7 33.0 3.0 69.1 69.1 12.7
4.78×10-14 4.66×10-14 3.97×10-14 3.70×10-14 3.47×10-14 2.98×10-14 2.58×10-14 2.27×10-14 1.10×10-13 -
7.4 7.6 8.9 9.5 10.1 11.8 13.6 15.5 3.2 -
[a]
The test was performed at low pressure (0.1001 MPa). [b] CO2 permeability coefficient, at 23 °C, 50% relative humidity. 1barrer = 10−10 cm3 cm/cm2 s cmHg. [c] O2 permeability coefficient, at 23 °C, 50% relative humidity. [d] Water vapor transmission rate, at 38 °C, 90% relative humidity. [e] Samples after thermal annealing. [f] Wang et al.42 [g] Burgess et al.43, 45 Degradation of PNSFs. The degradation behavior of polyesters could be influenced by monomer structure,18 molecular weight,46-47 crystallinity,48 hydrophilicity49 and specimen dimensions.50 The 0.5 mm thick films of PNSF30-PNSF70 were used in the degradation tests in PBS solutions with and without Candida antarctica lipase B (CALB) enzymes.51 The weight losses during hydrolysis and enzymatic degradation were displayed in Figure 8a and 8b, respectively. The hydrolysis process mainly happened on ester bonds, and the weight losses of PNSFs were less than 10% after 70 days. The mass loss increased with the decrease of mNF in PNSFs, suggesting that the addition of aliphatic diacids could accelerate the hydrolysis rate. On the other hand, the enzymatic degradation rate was faster. Nevertheless, both
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enzymatic degradation and hydrolysis experiments showed low weight loss (3%-13%). The molecular structure of PNSFs might be the first reason to explain the above results. NPG had higher steric hindrance than that of 1,3-propanediol, 1,4-butanediol and other diols, and prevented the diffusion of water into the bulk materials. As a result, the erosion could only happen on the surface of sample, which slowed down the degradation rate. Secondly, the high molecular weights of PNSFs could decrease the rate of mass loss, as had already been proved by other works.52 Lastly, since the temperature of PBS solution was set at 37 °C, crystallization slowly happened in PNSFs during the experiment and higher crystallinity could also prevent the sample from quick degradation. Despite the slow degradation rate, the biodegradability of PNSFs has been confirmed.
Figure 8. The weight loss curves of PNSFs during hydrolysis (a) and enzymatic degradation (b).
Conclusions High molecular weight PNSF copolyesters were successfully synthesized by transesterification and melt polycondensation method. The random copolyesters possessed excellent thermal stability and each sample had only one Tg which increased
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with decreasing mNF. The copolymerization of succinic acid destroyed the spherulite structure of homopolyester PNF, and the Tm also decreased with decreasing mNF. The side chain methyl had an obvious effect on the free volume, barrier property and biodegradability of PNSFs. The rheological analysis confirmed that the increase in free volume from succinic acid was relatively small. When compared with commercial PBAT, CO2 and O2 barrier properties of PNSFs increased more than 110 times and 18 times, respectively. Thermal annealing significantly changed the thermal and mechanical properties. PNSF50-PNSF70 possessed Young’s modulus and tensile strength greater than 1000 MPa and 27 MPa, respectively, exceeding most biodegradable packaging materials. Biodegradability of PNSF30-PNSF60 was confirmed under hydrolysis and enzymatic degradation circumstances, and the enzymatic degradation possessed a faster rate. With adjustable degradation rate and superior mechanical and barrier properties, PNSFs copolyesters offered a competitive solution for green packaging.
Supporting Information Materials; Characterization methods; Analysis of FTIR spectra; Thermal properties; Fitting of the Fox equation; Thermal stability of PNSFs under air and N2 atmosphere; Analysis of small angle laser scattering; Dynamic mechanical properties; Stress-strain curves; SEM graphs for tensile fracture surfaces.
Author Information Corresponding authors: *E-mail:
[email protected] ACS Paragon Plus Environment
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*E-mail:
[email protected] ORCID Ruoyu Zhang: 0000-0002-3502-8738 Wu Bin Ying: 0000-0002-8768-7428
Conflicts of Interest There was no conflict of interest to declare.
Acknowledgments This work was supported by National Key Research and Development Program of China (2017YFB0303000), National Natural Science Foundation of China (51773218) and Youth Innovation Promotion Association of CAS (2018338). We also thank Mr. Sakil Mahmud (2016 CAS-TWAS Fellow) for his assistance in language modification.
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(1), 19-34, 48. Martino, V. P.; Ruseckaite, R. A.; Jimenez, A.; Averous, L., Correlation between Composition, Structure and Properties of Poly(lactic acid)/Polyadipate-Based NanoBiocomposites. Macromol. Mater. Eng. 2010, 295 (6), 551-558, DOI 10.1002/mame.200900351. 49. Gigli, M.; Negroni, A.; Soccio, M.; Zanaroli, G.; Lotti, N.; Fava, F.; Munari, A., Enzymatic hydrolysis studies on novel eco-friendly aliphatic thiocopolyesters. Polym. Degrad. Stab. 2013, 98 (5), 934-942, DOI 10.1016/j.polymdegradstab.2013.02.019. 50. Woodard, L. N.; Grunlan, M. A., Hydrolytic Degradation and Erosion of Polyester Biomaterials. ACS Macro Lett. 2018, 7 (8), 976-982, DOI 10.1021/acsmacrolett.8b00424. 51. Zhang, M.; Ma, X.-n.; Li, C.-t.; Zhao, D.; Xing, Y.-l.; Qiu, J.-h., A correlation between the degradability of poly(butylene succinate)-based copolyesters and catalytic behavior with Candida antarctica lipase B. RSC Adv. 2017, 7 (68), 43052-43063, DOI 10.1039/c7ra05553f. 52. Saha, S. K.; Tsuji, H., Effects of molecular weight and small amounts of d-lactide units on hydrolytic degradation of poly(l-lactic acid)s. Polym. Degrad. Stab. 2006, 91 (8), 1665-1673, DOI 10.1016/j.polymdegradstab.2005.12.009.
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Table of Contents
Biobased and biodegradable PNSFs copolyesters with smart barrier and mechanical properties have potential application in green packaging.
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