Synthesis and Properties of Biobased Multiblock Polyesters

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Synthesis and properties of bio-based multiblock polyesters containing poly(2,5-furandimethylene succinate) and poly(butylene succinate) blocks Yang Zhang, Ting Li, Zhining Xie, Jiarui Han, Jun Xu, and Bao-Hua Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00516 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 30, 2017

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Industrial & Engineering Chemistry Research

Synthesis and properties of bio-based multiblock polyesters containing poly(2,5-furandimethylene succinate) and poly(butylene succinate) blocks Yang Zhang, Ting Li, Zhining Xie, Jiarui Han, Jun Xu, Baohua Guo*

Key Laboratory of Advanced Materials of Ministry of Education, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

*To whom correspondence should be addressed: Baohua Guo: Department of Chemical Engineering, Tsinghua University, Beijing 100084, China E-mail: [email protected] Tel: +86-10-62784550, Fax: +86-10-62784550

Graphical abstract

Abstract: Novel

bio-based

multiblock

polyesters

ACS Paragon Plus Environment

poly(2,5-furandimethylene


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succinate)-b-poly(butylene succinate) (PFS-PBS) containing PFS and PBS blocks were

synthesized in full composition range via chain-extension reaction of dihydroxyl terminated poly(2,5-furandimethylene succinate) (HO-PFS-OH) and poly(butylene succinate) (HO-PBS-OH) prepolymers. High molecular weight polyesters were obtained and the stability of double bonds in 2, 5-bis(hydroxymethyl)furan was guaranteed via this preparation procedure. The obtained copolyesters were characterized with 1H NMR, GPC, DSC, TGA, POM, WAXD and the mechanical properties were also investigated. No transesterfication reaction happened during chain-extension reaction and the expected multiblock chemical structure was obtained. The PBS block is crystallizable while the PFS block is nearly amorphous. Crystallization temperature and degree of crystallinity gradually decreased with increasing PFS fraction but the melting temperature of crystalline samples showed no sharp reduction (109.4-105.2 ºC). The crystallization rate also decreased with incorporation of PFS during isothermal crystallization but the crystallization mechanism remained unchanged at any composition. Finally, the PFS-PBS copolyesters

exhibit

widely

tunable

mechanical

properties,

ranging

from

semicrystalline thermoplastics to amorphous soft elastomer-like polymers.

Keywords: Bio-based, multiblock, poly(2,5-furandimethylene succinate)-b-poly(butylene succinate), crystallization

1. INTRODUCTION With increasing worldwide concern about shortage of petroleum resources and emission of carbon dioxide, bio-based monomers and polymers have attracted tremendous attention from both academic and industrial fields. Various bio-based polyesters have been extensively studied, including polylactic acid (PLA)1-3, poly(hydroxyalkanoate)s

(PHAs)4,

5

and

poly(butylene

succinate)

(PBS)6-8.

5-hydroxymethylfurfural (HMF) is a promising platform chemical with vast resources


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which can be converted from cellulose, glucose and fructose9, 10. HMF can be further transformed to several monomers of polymers such as 2,5-furandicarboxylic acid (FDCA), 2,5-bis(hydroxymethyl)furan (BHF), 2,5-dihydroxymethyltetrahydrofuran and 2,5-bis(aminomethyl)tetrahydrofuran. Among them, FDCA and BHF are the most studied.

Using

FDCA

as

monomer,

polyesters

including

poly(ethylene

2,5-furandicarboxylate)11, 12, poly(propylene 2,5-furandicarboxylate)13, poly(butylene 2,5-furandicarboxylate)14-16, poly(hexamethylene 2,5-furandicarboxylate)17,

18

and

poly(octylene 2,5-furandicarboxylate)19 were synthesized via polycondensation. In addition, aliphatic random copolyesters incorporating FDCA were also reported, such as

poly(ethylene

succinate-co-ethylene

furandicarboxylate)20,

poly(butylene

succinate-co-butylene furandicarboxylate)21 and poly(butylene adipate-co-butylene furandicarboxylate)22, 23. Compared to FDCA, studies related to polyesters or copolyesters using BHF as monomer are limited. This might be mainly due to the instability of BHF under heating. Usually, temperature above 200 ºC is required during melt polycondensation to obtain high molecular weight polyesters. BHF will decompose before polyesters are formed via melt polycondensation according to the experiments conducted in our lab. Using BHF and various ethyl esters of diacids as monomers, Jiang et al.24 synthesized a series of furan polyesters including poly(2,5-furandimethylene succinate), poly(2,5-furandimethylene glutarate), poly(2,5-furandimethylene adipate), poly(2,5-furandimethylene suberate) with number average molecular weight (Mn) around 2 000 g/mol through enzymatic polymerization utilizing Candida anatarctica Lipase B as catalyst. Low molecular weight PFS25 (Mn = 5100 g/mol) and poly(2,5-furandimethylene succinate-co-butylene succinate)26 (Mn = 4300 ~ 7200 g/mol) were also prepared via solution polymerization at room temperature.

Although PFS is crystallizable, the crystallization rate of PFS is rather low24. Besides its low molecular weight, PFS has poor mechanical properties25, which will restrict its applications. In this paper, high molecular weight multiblock PFS-PBS copolyesters containing PFS and PBS blocks were synthesized via chain-extension reaction. The crystalline PBS blocks can increase the crystallinity of PFS-PBS, which



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is expected to improve the mechanical properties of PFS-PBS copolyesters. Chain-extension reaction was utilized for mainly three reasons. First, high molecular weight polymers can be obtained at relatively low reaction temperature and no vacuum condition is demanded. Second, PFS block can keep stable. No cross-linking of double bonds or decomposition of furan units will happen during chain-extension reaction owing to the mild reaction condition. Third, compared to random copolymers that possess irregular chain structure and the resultant poor thermal properties, the multiblock copolymers with defined block length obtained via chain-extension reaction will show little decrease of melting temperature and lead to balanced performance. The chemical structures, molecular weights, thermal and mechanical properties, crystallization behaviors of PFS-PBS copolyesters were systematically investigated. It was found that PFS-PBS multiblock copolyesters have good thermal and mechanical properties and their properties can be modulated from semicystalline thermalplastics to soft elastomer-like polymers, depending on the molecular compositions.

2. EXPERIMENTAL SECTION 2.1 Materials Succinic acid (purity > 99.5 %) was provided by Anqing Hexing Chemicals Co., Ltd. (China). 1,4-butanediol (purity > 99 %) was purchased from Tianjin Chemical Corporation (China). 2,5-bis(hydroxymethyl)furan (purity ≥ 98 %) was purchased from Annker Organics Corporation (China). 4-dimethylaminopyridine (purity > 98 %), N, N’-diisopropylcarbodiimide (purity > 98 %) and 1,6-hexamethylene diisocyanate (HDI, 99 %) were purchased from Acros Organics (Belgium). Tetra-n-butyl-titanate (TBT, 99 %) was purchased from Aladdin Co., Ltd. (China).Chloroform and other reagents were analytical reagents and purchased from Beijing Chemical Reagent Factory. All reagents were used as received. 2.2 Synthesis of HO-PFS-OH and HO-PBS-OH prepolymers HO-PFS-OH was synthesized according to previous literature25. Briefly, 150 ml


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dehydrated dichloroethane was added to the mixer of 0.1 mol succinic acid, 0.11 mol 2,5-furandimethanol and 0.1 mol 4-dimethylaminopyridine (reaction catalyst) under N2 atmosphere. 0.15 mol N, N’-diisopropylcarbodiimide was gradually added. The reaction lasted for 24 hours at room temperature. Then dichloroethane was evaporated and the product was purified by precipitation from chloroform in excess methanol. The filtered product was dried at 50 ºC under vacuum for at least one day before use. HO-PBS-OH was synthesized as follows: succinic acid (1 mol) and 1,4-butanediol (1.2 mol) were added into a 3-necked flask with 0.5 wt% tetra-n-butyl-titanate and first heated to 140 ºC under nitrogen with stirring. Then temperature was raised to 220 ºC step by step and kept at 220 ºC for 2 hours. The product was dissolved in chloroform and precipitated from methanol. The filtered product was dried at 50 ºC under vacuum for at least one day before use. Poly(2,5-furandimethylene succinate-co-butylene succinate) (PFBS) random copolyester containing 20 mol% BHF was prepared using the same solution polymerization method as HO-PFS-OH. 2.3 Synthesis of PFS-PBS multiblock copolymer Multiblock copolymer PFS-PBS was prepared through chain-extension reaction between HO-PFS-OH and HO-PBS-OH in the presence of HDI. Generally, HO-PFS-OH and HO-PBS-OH were mixed in a 3-necked flask under nitrogen. When the mixture was completely molten at 130 ºC, HDI was dropwise added to the flask with continuous mechanical stirring. The chain-extension reaction lasted for about 1 hour. The product was purified from chloroform with methanol and dried at 50 ºC under vacuum for at least 24 hours before characterizations. The synthetic routes of multiblock copolymer PFS-PBS as well as its prepolymers are illustrated in Scheme 1. The synthesized copolyester are named as PFSα-PBSβ, where α and β is the molar percentage of PFS block and PBS block, respectively. 2.4 Characterization The molecular weights (Mn and Mw) of polyesters were measured at 30 ºC with gel permeation chromatography (GPC) (Viscotek, M302 TDA). Chloroform was used



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as solvent. The flow rate of the eluent was 1mL/min and polystyrene standards were used for calibration. The

1

H NMR spectra were recorded with a NMR spectrometer (JEOL,

ECA-400M), using deuterated chloroform as solvent. Differential scanning calorimetry (DSC) analysis was carried out with a TA equipment (Q 2000). During the nonisothermal crystallization process, the samples were first melted at 160 ºC to eliminate thermal history, then cooled to -50 ºC at a rate of 10 ºC/min, and then reheated to 160 ºC at a rate of 10 ºC/min. For the isothermal crystallization process, samples were melted at 160 ºC for 3 min and quenched to a preset crystallization temperature and maintained at this temperature until the crystallization was complete. The thermal decomposition behavior was recorded with TGA (Q 5000, TA Instruments). The samples were heated from room temperature to 900 ºC at 10 ºC/min under N2 atmosphere. The morphology of spherulites in the samples was observed under an Olympus polarized optical microscope (POM) (BX41) equipped with a Linkam hot stage (T95-HS). Wide-angle X-ray diffraction (WAXD) was measured using a Bruker (D8 Advance) X-ray diffractometer with Cu Kα radiation. Scanning was performed with 2θ from 5º to 40º with a step of 0.01º. The degree of crystallinity (Xc) was calculated from WAXD spectra using the following equation: Xc (%) = 100 × Ic/(Ic + Ia), where Ic is the integral intensities of crystal peaks, Ia is the integral intensity of the amorphous halo. The samples were annealed at room temperature for at least 5 days after being treated at 160 ºC for 5 min. Tensile tests were carried out with a Shimadzu AGX-X test machine with a crosshead speed of 10 mm/min using dumbbell-shaped specimens at ambient temperature. Dumbbell-shaped specimens of 30 mm length, 5 mm width and 0.3 mm thickness were made by hot-pressing at 160 ºC and 30MPa for 2 min. Each measurement was repeated at least five times and the values are averaged.


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Scheme 1. Synthetic routes of multiblock copolyester PFS-PBS.

3. RESULTS AND DISCUSSION 3.1 Synthesis of HO-PFS-OH and HO-PBS-OH prepolymers Due to the instability of BHF under heat, dihydroxyl terminated PFS was prepared

via

solution

polycondensation

under

mild

conditions

with

N,

N’-diisopropylcarbodiimide as a condensation agent. The chemical structure and number average molecular weight (Mn) of HO-PFS-OH were analyzed by 1H NMR spectrum, as shown in Figure 1a. The chemical shifts of CH2 of SA and CH2 linked to furan unit in PFS repeating unit appears at 2.66 ppm (δHs) and 5.04 ppm (δHa), respectively. The signal of =CH-CH= of PFS repeating unit appears at 6.36 ppm (δHf). Protons on terminal groups of molecular chains in prepolymers are identified at 4.58 ppm (δHa’), 6.24 ppm (δHf’’) and 6.33 ppm (δHf’). Mn of HO-PFS-OH can be calculated

according

to

M n (HO− PFS− OH) = 128 +

1

H

NMR

spectrum

by

the

equation:

A5.04 × 210 , where A5.04 and A4.58 represent the integral A4.58

areas of internal and terminal methylene groups in chains, and 210 is the molecular weight of PFS repeating unit. The calculated Mn of resultant HO-PFS-OH is 4620 g/mol. Dihydroxyl end-capped HO-PBS-OH with Mn of 4720 g/mol was prepared



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through melt polycondensation using tetra-n-butyl-titanate as a catalyst. The 1H NMR spectrum of HO-PBS-OH is presented in Figure 1b. Chemical shifts at 2.61 ppm (δHs’), 4.10 ppm (δHb) and 1.69 ppm (δHc) correspond to the three types of CH2 in PBS repeating unit. The peaks at 3.6-3.7 ppm (δHb’) are the signals of protons in terminal groups. Equation M n (HO− PBS− OH) = 90 +

A4.10 ×172 was used to A3.66

calculate Mn of HO-PBS-OH.

Figure 1. 1H NMR spectra of (a) HO-PFS-OH and (b) HO-PBS-OH.

3.2 Synthesis of PFS-PBS multiblock copolymers A series of PFS-PBS samples with PFS content in full composition range was


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prepared through chain-extension reaction using HDI as chain extender. The molecular structure was characterized by 1H NMR. All of the characteristic peaks of PFS-PBS polyesters were identified according to the molecular structure, as shown in Figure S1. Figure 2a shows the 1H NMR spectrum of a representative sample PFS40-PBS60. On one hand, the signals representing terminal hydroxyl groups (δHa’ and δHb’) in PFS and PBS prepolymers disappeared in 1H NMR spectra of PFS-PBS, indicating the complete reaction of terminal hydroxyl groups during chain-extension. On the other hand, new peaks at 3.15 ppm (δHd), 1.48 ppm (δHe) and 1.32 ppm (δHg) corresponding to different CH2 in HDI are observed in 1H NMR spectra of PFS-PBS. Furthermore, the compositions of PFS-PBS were calculated from the integrated area of peaks at 5.03 ppm (δHa) and 4.01 ppm (δHb) and the calculated results are listed in Table 1. The actual composition differs only slightly from the feed ratio. Transesterification between PFS and PBS might occur during chain-extension reaction. If transesterification happened, the block length would decrease, which has influences on the physical properties of copolyesters, including melting temperature and crystallization. Poly(2,5-furandimethylene succinate-co-butylene succinate) (PFBS) random copolyester was prepared and compared with PFS-PBS multiblock copolyester in order to verify whether transesterification happened during chain-extension reaction. The 1H NMR spectra of PFS40-PBS60 and PFBS random copolyester containing 40 mol% FS units are presented in Figure 2. As shown in Figure 2(b), in random copolyester PFBS three types of sequence distributions are possible, depending on the relative position of SA, BHF and BDO: FSF, FSB and BSB. As a result, splitting peaks of protons in succinic acid were identified in the range of 2.55-2.70 ppm (δHs1, δHs2, δHs3, δHs4) in random copolyester. If transesterification occurred between PFS block and PBS block during chain-extension, the FSB sequence would exist in the multiblock PFS-PBS and the corresponding peaks of FSB sequence would be detected in the 1H NMR spectrum of PFS-PBS27. In fact, only two peaks appeared at 2.65 (δHs) ppm and 2.61 (δHs’) ppm, which are attributed to sequence of FSF in PFS block and BSB in PBS block in the multiblock polyester and the peak of FSB was not detected in


1

H NMR spectrum of


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PFS40-PBS60 (Figure 2a). Therefore, it is confirmed that no transesterification occurred and multiblock copolyesters were prepared through chain-extension. The results of molecular weights and PDI of PFS-PBS measured by GPC are listed in Table 1. The molecular weight distribution curve of PFS-PBS polyesters is shown in Figure S2. It is reported that excess HDI would result in allophanate crosslinked structure28 and the molar ratio of –NCO to –OH would affect molecular weight of resultant product28-30. In this work, same molar ratio of –NCO to –OH (1.05:1) was utilized to prepare all the PFS-PBS polyesters. As shown in Table 1, PFS, PBS and PFS-PBS copolyesters after chain-extension have much higher molecular weight than the prepolymers. Therefore, from the above data and analysis, it can be concluded that high molecular weight PFS-PBS polyesters in full composition range were synthesized as designed.

Table1. Compositions and molecular weights of PFS, PBS and PFS-PBS polyesters. Molar ratio of

Molar ratio of

PFS

PBS

Sample

PBS PFS20-PBS80 PFS40-PBS60 PFS60-PBS40 PFS80-PBS20 PFS

Feed

NMR

Feed

NMR

0 20 40 60 80 100

0 19.5 39.8 61.0 80.6 100

100 80 60 40 20 0

100 80.5 60.2 39.0 19.3 0

Mn×10-4(g/mol)

Mw×10-4(g/mol)

PDI

4.51 5.53 4.16 4.94 4.36 4.82

8.30 12.35 7.54 14.02 7.30 9.43

1.84 2.23 1.81 2.83 1.68 1.95


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Figure 2.1H NMR spectra of (a) PFS40-PBS60 multiblock copolyester and (b) PFBS random copolyester containing 40 mol% FS unit.

3.3 Thermal transition behavior For multiblock copolymers, the miscibility of different blocks in the amorphous region is important because it can influence the mechanical properties of materials. Here, the miscibility of PFS-PBS copolyesters was studied using glass transition temperature (Tg), which was measured by DSC. For miscible samples, single Tg is obtained. For partially miscible or immiscible samples, two separate Tg will be observed. The Tg results of PFS-PBS are shown in Figure 3a and Table 2. All the PFS-PBS copolyesters possess single Tg, which increases continuously with the content of PFS block. The Fox equation31 was used to illustrate the composition dependence of Tg. Figure 3b shows the variation of Tg versus composition and it fits



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well with the Fox equation. Therefore, the PFS block and PBS block are miscible in the amorphous region.

Figure 3.(a) DSC curves of PFS-PBS polyesters for glass transition analysis and (b) dependence of glass transition temperature on composition obtained from DSC. The dashed line is calculated from the Fox equation.

The melting and crystallization behavior of PFS-PBS were characterized by DSC and, cooling and second heating scans are shown in Figure 4. The corresponding thermal parameters including melting temperature (Tm), melting enthalpy (∆Hm), crystallization temperature (Tc) and crystallization enthalpy (∆Hc) are summarized in Table 2 and the composition dependence of thermal parameters are illustrated in Figure 5. It can be seen from Figure 4a that the nonisothermal crystallization behavior of PFS-PBS strongly depends on the polyester composition. During the cooling scan， samples with PFS molar fraction of 0-60 % are able to crystallize while PFS80-PBS20 and PFS are amorphous. The PBS block is crystallizable and neat PBS has a Tc of 74.8 ºC and a ∆Hc of 64.6 J/g. As a contrast, the PFS block has poor crystalliability and neat PFS does not demonstrate exothermic peak during cooling scan. All the four crystallizable polyesters show single crystallization peak, which is ascribed to the crystals formed by PBS block. This result indicates that the crystallization of PFS-PBS copolyesters is mainly derived from PBS block. Furthermore, with increasing PFS content in copolyesters composition, the value of Tc


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gradually decreases from 74.8 ºC for neat PBS to 66.5 ºC, 52.9 ºC and 47.2 ºC for PFS20-PBS80,

PFS40-PBS60

and

PFS60-PBS40,

respectively.

During

the

crystallization process, the amorphous PFS blocks are excluded from the crystalline region formed by PBS blocks. Therefore, higher PFS content means more PFS chains to be excluded, leading to the decrement of crystallinity of copolyesters. Similar to the change trend of Tc, with increasing fraction of PFS, the amount of crystalline block PBS decreases, thus leading to the decrease in ∆Hc of copolyester. The ∆Hc gradually reduces from 64.6 J/g to 47.8J/g, 34.8J/g and finally to 22.3J/g for PFS-PBS copolyesters containing PFS fraction of 0-60 mol%. For random copolymers, random copolymerization often leads to sharp decrease of Tm due to the shortened sequence length of the crystalline units. While in block copolymers, the Tm is determined by the block length of crystalline block, which is independent of copolymer composition. During the second heating scan shown in figure 4b, all the four crystalline samples possess single melting peak, which slightly decreases compared to the neat PBS. The value of Tm shifts from 109.4 ºC for neat PBS to 105.2 ºC for copolyesters containing 60 mol% PFS. The limited decrease of Tm is ascribed to the formation of imperfect crystals caused by the interference of increasing amorphous PFS blocks and chain-extender, which are excluded from PBS crystals.

Figure 4. DSC curves of PFS-PBS polyesters during (a) cooling from melt, and (b) subsequent heating process at a rate of 10 ºC /min.



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Figure 5. Composition dependence of (a) Tc and ∆Hc, and (b) Tm and ∆Hm. Table 2. Thermal transition parameters of PFS, PBS and PFS-PBS polyesters. Sample

Tga (ºC)

Tc (ºC)

∆Hc (J/g)

Tm (ºC)

∆Hm (J/g)

Xcb (%)


-39.0 -25.2 -12.2 -3.4 2.7 13.5

74.8 66.5 52.9 47.2 -

64.6 47.8 34.8 18.7 -

109.4 106.5 105.4 105.2 -

57.4 44.5 35.9 20.5 -

59.9 50.1 39.3 32.4 27.2 18.1

a

Obtained from DSC heating scan.

b

Calculated from WAXD spectra.

3.4 Isothermal crystallization In order to investigate the influence of PFS block on the crystallization kinetics of PFS-PBS multiblock polyesters in detail, the isothermal crystallization was carried out in DSC. Here, PFS-PBS samples with PFS molar fraction of 0-60% were studied because PFS80-PBS20 and PFS cannot crystallize during the DSC procedures. Figure 6 and Figure S3 show the plots of the relative crystallinity (Xt) versus crystallization time of different samples at various crystallization temperatures (Tc). For all the samples, more time is required to complete the crystallization process with increased crystallization temperature, indicating the decreased crystallization rate with increasing Tc. Furthermore, it needed more time to accomplish the crystallization with higher PFS content. For example, at the Tc of 82 ºC, it took 4.5, 11.3, 50.1 and 82.0 min to crystallize completely for PBS, PFS20-PBS80, PFS40-PBS60 and PFS60-PBS40, respectively.


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The Avrami equation32, 33 is used to study the isothermal crystallization kinetics, which assumes the relationship of Xt and crystallization time as 1 – Xt = exp(-ktn)

(1)

where n is the Avrami exponent denoting the nucleation mechanism and crystal growth dimension, and k is the crystallization rate constant related to nucleation and crystalline growth rate. The logarithmic form of equation 1 can be written as log[-ln(1 - Xt)] = log k + n log t

(2)

Straight line would be obtained from plot of log[-ln(1 - Xt)] versus log t. Figure 7 and Figure S4 show Avrami plots of PFS-PBS polyesters. A series of parallel straight lines was obtained for all samples at different crystallization temperatures, indicating that the Avrami equation can be suitably used to describe the isothermal crystallization kinetics of PFS-PBS polyesters. The Avrami parameters n and k were obtained from the slopes and intercepts of fitting lines for plots of log[-ln(1 - Xt)] versus log t and they are summarized in Table 3. It can be seen that the n values of the measured samples are in the range of 2.5 to 2.9, suggesting that the crystallization process of PFS-PBS polyesters may correspond to three-dimensional crystal growth from athermal nucleation34 and the amorphous PFS block does not change the crystallization mechanism. Furthermore, crystallization half-time (t0.5) which means the time to achieve 50% of the total crystallinity is calculated using the equation t 0.5 = (

ln 2 1/ n ) k

(3)

And the value of t0.5 can be used to evaluate the overall crystallization rate of polymers. The higher the value of t0.5 is, the lower the overall crystallization rate is. The dependence of the reciprocal of half crystallization time (1/t0.5) on crystallization temperature is shown in Figure S5. It can be seen that t0.5 increases with higher PFS block molar fraction at certain crystallization temperature, suggesting the incorporation of PFS block leads to lower crystallization rates of copolyesters. During the crystallization process, the PBS block can crystallize while the PFS block cannot. As a result, PFS blocks are excluded from the crystalline region of PBS blocks to the



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amorphous phase, thus reducing the crystallization rate of the copolyesters.

Figure 6. Plots of relative crystallinity versus crystallization time at different crystallization temperature for (a) PBS and (b) PFS60-PBS40.

Figure 7. Avrami plots of (a) PBS and (b) PFS60-PBS40. Table 3. Avrami exponents of PFS-PBS polyesters at specified temperatures Sample PBS

PFS20-PBS80

Crystallization temperature (ºC)

n

k (min-n)

t0.5 (min)

82 84 86 88 90 80 82 84

2.73 2.72 2.63 2.65 2.51 2.55 2.49 2.51

9.03 × 10-2 2.15 × 10-2 6.31 × 10-3 1.39 × 10-3 4.85 × 10-4 2.77 × 10-2 1.17 × 10-2 4.04 × 10-3

2.1 3.6 6.0 10.4 18.1 3.5 5.2 7.8


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PFS40-PBS60

PFS60-PBS40

86 88 76 78 80 82 84 74 76 78 80 82

2.49 2.48 2.89 2.86 2.53 2.50 2.48 2.50 2.65 2.69 2.65 2.72

1.39 × 10-3 4.46 × 10-4 2.44 × 10-3 5.37 × 10-4 5.38 × 10-4 2.63 × 10-4 9.74 × 10-5 4.11 × 10-3 7.91 × 10-4 2.86 × 10-4 1.13 × 10-4 2.59 × 10-5

12.1 19.4 7.1 12.2 17.0 23.4 35.8 7.8 12.9 18.1 26.9 42.4

3.5 Spherulite morphology and crystal structure Just as mentioned above, PFS segment has effect on crystallization. Here, the effect of polymer composition on crystalline morphology was studied using POM. The spherulite morphologies of PFS-PBS polyesters containing PFS molar fraction of 0, 20, 40 and 60% formed at the same crystallization temperature of 85 ºC are shown in Figure 8. The spherulites of neat PBS show fibrous morphology and the spherulite diameter is about 200 µm before the spherulites impinge with each other. With the incorporation of PFS blocks, the copolyesters show ring-banded spherulites and the band spacing decreases with increasing content of PFS. In addition, the spherulite density decreases while the spherulite size increases (e.g. about 500 µm for PFS60-PBS40) with higher PFS content in copolyesters, which may be attributed to lower molecular chain regularity and concentration of crystalline PBS blocks.



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Figure 8. Spherulitic morphologies of (a) PBS, (b) PFS20-PBS80, (c) PFS40-PBS60 and (d) PFS60-PBS40 at 85 ºC.

WAXD was measured to study the crystal structure as shown in Figure 9. PBS shows three diffraction peaks at 19.5º, 21.8º and 22.5º. PFS shows six diffraction peaks at 16.8º, 18.6º, 19.7º, 20.3º, 21.3º, 22.4º and 23.4º, which is consistent with the WAXD results of PFS synthesized through enzymatic polymerization24. In PFS-PBS copolymers, PFS20-PBS80, PFS40-PBS60 and PFS60-PBS40 show similar patterns to that of PBS, suggesting that the crystalline are formed by PBS block and the PFS block does not change the crystal structure of PBS. In PFS80-PBS20, the pattern shows characteristic peaks of PFS. The degree of crystallinity (Xc) were calculated from WAXD spectra and the results were listed in Table 2. With the decreasing amount of crystalline block PBS, the Xc of copolymer reduced.


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Figure 9. WAXD patterns of PFS-PBS polyesters.

3.6 Thermal stability The thermal stability of PFS-PBS polyesters was measured with TGA, as presented in Figure 10 and the corresponding DTG curves are shown in Figure S6 in supporting information. The characteristic parameters are listed in Table 4. Neat PBS and PFS degrade in single step at 404.3 ºC and 273.1 ºC, respectively. However, the PFS-PBS copolyesters undergo two-stage thermal degradation. The maximum decomposition temperature of first stage (Td, max1) is between 273-283 ºC, which is attributed to the degradation of PFS blocks. The second stage degradation with a maximum decomposition temperature (Td, max2) at around 400 ºC corresponds to the degradation of PBS blocks. Furthermore, the weight loss at each degradation stage is proportional to the fraction of each kind of block degrading at corresponding stage. The PFS blocks with double bonds in the furan rings are less thermal stable, thus leading to the first stage degradation at a relatively low temperature. In addition, the residual at 900 ºC increases continuously with increasing content of PFS block from 0 % for neat PBS to 10.6 % for neat PFS, which might be caused by the formation of cross-linking structure generated from PFS blocks at high temperature.



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Figure 10. TGA curves of PFS-PBS polyesters at a heating rate of 10 ºC /min under N2 . Table 4. Thermal stability parameters of PFS-PBS polyesters Sample

Td, 5% (ºC)

Td, max1 (ºC)

Td, max2 (ºC)

Residual (%)


342.3 275.6 268.1 266.6 263.9 262.2

282.5 276.8 278.1 275.4 273.9

404.3 405.7 401.5 398.5 393.8 -

0 1.9 3.6 5.2 6.9 10.6

3.7 Mechanical properties Tensile tests were conducted at ambient temperature to characterize the tensile properties including modulus (E), tensile strength (σmax) and elongation at break (εb) of PBS, PFS and PFS-PBS copolyesters (Table 5). The typical stress-strain curves of PFS-PBS polyesters are shown in Figure 11. PBS is a typical semi-crystalline plastic with good tensile properties (E = 410 MPa, σmax = 32.1 MPa, εb = 151 %), showing yielding and stress hardening. With increasing PFS fraction, the tensile properties of PFS-PBS polyesters are composition dependent. The tensile strength gradually reduces from 32.1 MPa for neat PBS to 12.7 MPa for neat PFS while the elongation at break gradually increases from 151 % to 1029 %, which is mainly caused by the reduction of crystallinity. In addition, PFS-PBS copolyesters demonstrate different tensile behaviors with varying


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composition. PFS20-PBS80 and PFS40-PBS60 with relatively high crystallinity exhibit characteristics of semi-crystalline polymers: elastic deformation at low strain, yielding and stress hardening at higher strain, which is caused by the molecular chain orientation, crystallite rearrangement and recrystallization during tensile test. At higher PFS content, the PFS60-PBS40 and PFS80-PBS20 sample exhibit characteristics of amorphous polymers with high-elastic deformation, low tensile modulus (105-127 MPa) and high elongation at break (614-710 %). In addition, PFS-PBS copolymers in this composition range show rebound resilience and some deformation can be reversible after the test force is removed. No yielding or necking was observed. For neat PFS, it possesses low tensile modulus (88 MPa) and high elongation at break (1029 %). In summary, the mechanical properties of PFS-PBS polyesters show composition dependence. The tensile strength and modulus decrease while the elongation at break gradually increases with increasing PFS molar fraction. The copolyesters can be modulated from crystalline thermoplastics with low content of PFS to elastomer-like polymers with high content of PFS. It should be noted that although PFS block is amorphous in the thermal treatment of our experiment, it might be able to crystallize at very low rate (e.g. annealing for long time). Different thermal history may have influence on the mechanical properties of PFS-PBS copolyesters.

Figure 11. Typical stress-strain curves of PFS-PBS polyesters.



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Table 5. Mechanical properties of PFS-PBS polyesters. Sample PBS PFS20-PBS80 PFS40-PBS60 PFS60-PBS40 PFS80-PBS20 PFS

σmax (MPa) 32.1 30.4 14.6 13.4 13. 2 12.7

± ± ± ± ± ±

1.4 5.3 1.7 1.5 2.1 3.4

εb (%)

E (MPa)

151 ± 28 420 ± 38 490 ± 23 614 ± 49 720 ± 54 1029 ± 91

410 ± 4.2 323 ± 1.9 169 ± 2.0 127 ± 2.1 105 ± 3.2 88 ± 1.2

4. CONCLUSIONS A series of high molecular weight (Mw = 73 000-140 200 g/mol) bio-based multiblock polyesters containing PFS and PBS blocks was successfully prepared in a full composition range using chain-extension reaction at mild condition. The number average molecular weight of PFS and PBS prepolymer is 4620 g/mol and 4720 g/mol, respectively. No transesterfication reaction happened during chain-extension reaction and the expected multiblock chemical structure was obtained. The composition of the copolyesters is controlled by the feed ratio of the two blocks. The PBS block has better thermal stability than PFS block and the PFS-PBS copolyesters undergo two separate stages of thermal degradation. The melting temperature of the crystalline samples shows no obvious change because of the constant block length of the crystalline PBS block. However, the degree of crystallinity of copolyesters continuously decreases with increasing PFS fraction, leading to the reduction of Tc and crystallinity. During isothermal crystallization, the overall crystallization rates decreases due to incorporation of PFS while the crystallization mechanism remains unchanged regardless of PFS content. Besides, the mechanical properties of copolyesters can be modulated ranging from semicrystalline thermoplastics to amorphous soft elastomer-like polymers via adjusting chemical composition.

ACKNOWLEDGEMENTS The authors thank the National Natural Science Foundation of China (Grant No. 51673110, 51473085) and the National Basic Research Program of China (973 Program) (Grant No. 2014CB932202) for the financial support of this work.


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Synthesis and Properties of Biobased Multiblock Polyesters

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