CHENG Zhengzai, CHENG Junpeng, CHEN Jun, XIONG Jing, SUN Xin,JIA Ruyan, YUAN Beibei, Gauthier Mario

(1. Research Institute of Fine Organic Chemicals & Organic Materials at School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China; 2. Coal Conversion and New Carbon Materials Hubei Key Laboratory, Wuhan University of Science and Technology, Wuhan 430081, China)

Abstract: Terephthalic acid, 2,5-furandicarboxylic acid and 1,8-octanediol were adopted as monomers and antimony trioxide as catalyst, and poly(terephthalic acid-2,5-furandicarboxylic acid-1,8-octanediol)copolyesters identified as PEOT-x, where x is the mole fraction of furandicarboxylic acid in the samples, were synthesized by direct esterification. The molecular structure of the copolyesters was characterized by FTIR and 1H NMR spectroscopy. Gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were used to characterize the molecular weight, molar mass dispersity, glass transition temperature, and thermal stability of the copolyesters, respectively. The mechanical properties of the samples were also investigated. The number-average molecular weight (Mn) of the samples varies from 9 700-18 800 g/mol, and molar mass dispersity (Đ = Mw/Mn) from 2.15-3.34. The initial decomposition temperature of the copolyesters is in the 332-356 ℃ range, with maximum decomposition rates at 390-410 ℃, while the glass transition temperature (Tg) varies from 0-33 ℃. Mechanical test shows that PEOT-10 has the highest tensile strength, while PEOT-90 has the largest tensile modulus and elongation at break. The experimental results show that these copolyesters can be synthesized with relatively high molecular weights, good thermal stability, and fair mechanical properties, which makes them excellent replacements for commercial polyesters, such as PET,and these properties can be tuned through the relative amounts of biomass monomer 2,5-furandicarboxylic acid used in the reactions.

Key words: 2,5-furandicarboxylic acid; terephthalic acid; melt polymerization; copolyesters

1 Introduction

Polyesters can serve to make fibers and films,mainly in the form of poly(ethylene terephthalate)(PET) and poly(butylene terephthalate) (PBT)[1,2]. The synthesis of PET typically starts from dimethylbenzene,an aromatic petrochemical oxidized to terephthalic acid (TPA), and condensed with ethylene glycol.Since oil resources are expected to become limited,it is important to develop polyesters using biobased monomers to replace completely or partially petrochemical components in these reactions[3-6].2,5-Furandicarboxylic acid (FDCA), which can be obtained from renewable biological feedstocks like plants or crop stalks, should be a good candidate to replace TPA due to its similar structure and properties:both monomers contain cyclic conjugated structures and two carboxyl groups[7-9].

Drewitt and Lincoln first reported the synthesis of polyesters from FDCA in a British patent dating from 1946[10]. In 1958, Hachihama investigated the synthesis of polyesters using FDCA dimethyl ester with ethylene glycol, butanediol, and other aliphatic diols[11].It was shown that the melting point of the ethylene derivative (PEF) was 220-225 ℃, as compared with 163-165 ℃ for the butylene derivative (PBF). In recent years, due to progress in materials characterization and the development of large-scale FDCA synthesis,additional reports about FDCA-based polyesters have emerged. The group of Candini thus compared three different polymerization methods (transesterification,solution and interfacial polymerization) to thoroughly study the condensation of FDCA with seven aliphatic and aromatic diols, and characterized the products by different methods including GPC, FTIR,1H NMR,13C NMR, TGA, and DSC[12,13]. High molecular weight semi-crystalline polyesters resulted from the transesterification reaction between FDCA and aliphatic diols. The physical and chemical properties of PEF were found to be very similar to PET, which makes PEF a good potential replacement for PET.

Considering that the cost of biomass-sourced FDCA is still high, and the use of long chain diols in replacement for ethylene glycol in the synthesis of polyesters is still rare, we examined the synthesis of poly(terephthalic acid-2,5-furandicarboxylic acid-1,8-octanediol) copolyesters (PEOT) by direct esterification of the diacids and 1,8-octanediol, using Sb2O3as catalyst. The molecular structure of the copolyesters was characterized by FTIR and1H NMR spectroscopy,while GPC, DSC, and TGA were used to measure the molecular weight, the glass transition temperature,and the thermal stability of the copolyesters. The mechanical properties of the materials were also compared as a function of their composition.

2 Experimental

2.1 Reagents

Terephthalic acid (purity＞99%) was purchased from the Sinopharm Chemical Reagent Co. 2,5-Furandicarboxylic acid (purity＞99%)was obtained from Shijiazhuang Donglian Nankai Aroma Chemicals Co. 1,8-Octanediol and Sb2O3(analytically pure) were bought from Shanghai Aladdin Biochemical Technology Co. Methyl alcohol, ethyl alcohol, potassium hydroxide, o-cresol, and chloroform(analytically pure) were acquired from Wuhan Shenshi Chemical Reagent Co.

2.2 Characterization

All the newly synthesized materials were characterized and tested within 10 days after preparation.

FTIR spectroscopy: A Bruker VERTEX 70 infrared spectrometer was used to obtain spectra in the 4 000-400 cm-1wavenumber range at a resolution 0.4 cm-1, by acquiring 80 scans with KBr pellets.

1H NMR spectroscopy: A Bruker Avance DMX600 (600 MHz) nuclear magnetic resonance instrument was used with tetramethylsilane (TMS) as internal standard in CDCl3.

DSC and TGA measurements: A Netzsch STA 449 F3 Jupiter instrument was used with a nitrogen flow rate of 40 mL/min. For the DSC measurements,the sample was first heated to 200 ℃ for 5 minutes to erase thermal history, cooled to room temperature,and ramped to 400 ℃ at 10 ℃/min to obtain a heating curve. For the TGA measurements, the temperature was ramped from 25 to 500 ℃ at 10 ℃/min.

GPC measurements: A Viscotek TDAmax Gel chromatography instrument calibrated with polystyrene standards was used to determine the apparent numberaverage (Mn), weight-average molecular weights (Mw)and molar mass dispersity (Đ=Mw/Mn) of the samples synthesized. Either THF or DMF served as mobile phase, at a flow rate of 1.0 mL/min and a temperature of 40 ℃.

Intrinsic viscosity: An Ubbelohde viscometer with a capillary inner diameter of 0.58 mm was used for viscosity measurements for 0.5 mg/mL chloroform solutions of the copolyesters in a 25 ℃ constant temperature oil bath. The average flow times for the solutions (t) and the pure solvent (t0) were used to calculate the intrinsic viscosity [η] (dL/g) of the samples as:

Tensile testing: Samples were prepared as dumbbell-shaped strips by injection moulding, and the tests were carried out according to the ASTM D412a standards. An Instron 5544 mechanical tester with a 500 N load cell was used to test 3-5 samples, to average the data.

2.3 Synthesis of poly(terephthalic acid-2,5-furandicarboxylic acid-1,8-octanediol(PEOT) copolyesters

The synthetic route used is shown in Fig.1. The molar ratio of terephthalic acid to 2,5-furandicarboxylic acid was varied within the range of 0.9 : 0.1 to 0.1 : 0.9 to generate a series of PEOP copolyesters (PEOT-10-PEOT-90) by direct esterification. The molar ratio of diacid to diol was varied from 1 : 1.1 to 1 : 1.2, and the amount of catalyst (Sb2O3) used was 1mol%-2mol% of the diacid. The reaction was first allowed to proceed at 165 ℃ under nitrogen for 8 hours. The temperature was then raised to 240 ℃ and the reaction was continued under vacuum for 3 hours. The product was dissolved in chloroform and methanol was slowly added until precipitation of the polymer stopped. The copolyester sample was recovered by filtration and dried at 70 ℃under vacuum for 6 hours.

Fig.1 Synthesis of PEOT copolyesters by direct esterification. Note that the structure of the copolyesters is expected to be random, in spite of the structure representation used

3 Results and discussion

3.1 FTIR and 1H NMR characterization

All the copolyester samples synthesized were recovered in 81%-84% yield. To confirm their structure,FTIR spectra were obtained for the PEOT-10 - PEOT-90 samples. The different copolyesters yielded very similar infrared absorption peaks (Fig.2). A -CH2-stretching vibration peak is observed at 2925 cm-1.The strong absorption peak at 1 717 cm-1corresponds to the carbonyl stretching vibration peak, together with the peaks at 1 265 and 1 125 cm-1for C-O-C stretching, confirming the presence of ester groups. The furan ring also produces bands at 965, 830, and 766 cm-1for bending vibrations. At last, the presence of an-OH absorption peak (3 200-3 650 cm-1) is far from obvious, again confirming the synthesis of the desired products. Further confirmation of the structure of the copolyesters was obtained by1H nuclear magnetic resonance (NMR) analysis. It can be seen that the NMR spectrum of copolyester PEOT-50, shown in Fig.3 as an example with the peak assignments, has no extraneous peaks apart from the signal atδ7.26 for the residual protons in deuterated chloroform. The two strong absorption peaks atδ8.09 and 7.18 are for the protons on the benzene rings and the furan rings,respectively. The three remaining absorptions are from the 1,8-octanediol fragments.

Fig.2 FTIR spectra for PEOT copolyesters

Fig.3 1H NMR spectrum (600 MHz) for PEOT-50 in CDCl3

The peak atδ4.32 is thus for -OCH2(CH2)6CH2O-,atδ1.76 is for OCH2CH2(CH2)4CH2CH2O-, and atδ1.38 is for -OCH2CH2(CH2)4CH2CH2O-. The peak areas for the different protons correspond to the expected values for a 1 : 1 : 2 ratio of both diacids and the diol,respectively, which further confirms the successful synthesis of the copolyester.

3.2 Molecular weight characterization and thermal properties

To further study structure-performance relationships for the copolyesters, gel permeation chromatography was used to determine their molecular weight (Fig.4). The results obtained from the differential scanning calorimetry and the differential thermal analysis measurements are provided in Fig.5 and Fig.6, respectively. The main parameters of interest determined from these measurements are summarized in Table 1. The GPC yielded apparent (polystyreneequivalent) number-average molecular weights(Mn) ranging from 9 700-18 800 g/mol for all the copolymers, while their molar mass dispersity (Đ=Mw/Mn) is between 2.15-3.34, which was achieved by using a 10mol%-20mol% excess of diol in the reaction, such that direct esterification of the diacid and diol in the first(low temperature) part of the condensation reaction was followed by transesterification with the elimination of excess 1,8-dioctanol in the second (high temperature)part of the reaction. Even though the GPC results are only apparent, they still suggest that polyesters with satisfactory molecular weights were produced. The direct esterification method used in this investigation made the synthesis of linear poly(terephthalic acid-2,5-furandicarboxylic acid-1,8-octanediol) copolymers withMnon the order of 104g/mol simple and easy to achieve using a single metal oxide catalyst. It is interesting to note that the highest molecular weight was achieved among all the copolyesters when equal molar amounts of terephthalic acid and 2,5-furandicarboxylic acid were used. The variations in intrinsic viscosity observed among the PEOT sample series are also consistent with the apparent molecular weights determined by GPC analysis, as [η] is expected to increase with the molecular weight.

Table 1 Summary of GPC and thermal analysis results for PEOT copolyesters

Table 3 Mechanical properties of PEOT-10 - PEOT-90 copolyesters

Fig.4 GPC traces for PEOT copolyesters

Fig.5 TGA curves for PEOT-10 - PEOT-90 copolyesters

Fig.6 DSC curves for PEOT-10 - PEOT-90 copolyesters

According to thermogravimetric analysis, the initial decomposition temperatureTd(when 5%weight loss happens) is 332-356 ℃, the temperature of maximum decomposition rate (Tdm) is 390-410℃, and the thermogravimetric residual mass LR is 18.65%-30.93%. As the content of FDCA increases,Td,Tdm, and the thermogravimetric residue gradually decrease. The reason is that the benzene ring as a closed conjugate system is very stable while the furan ring, though still aromatic, is an oxygen containing five-membered heterocyclic ring with a smaller atomic number, and accordingly less stable. This causes the decrease inTdandTdmwith the increase of FDCA in the copolyester. In addition, theTdandTdmof the PET being 407 ℃ and 440 ℃[14]indicates that the thermal stability of synthesized copolyesters is lower than that of petroleum-based polymer materials. This is because the low number of methylene units in ethylene glycol than in 1,8-octanediol induced a high flexibility in the synthesized polyesters, while low or no crystallization was low or insignificant[15]resulted from the introduction of the third FDCA companent breaking the regularity and symmetry of the chain, therefore the thermal performance was reduced. Every material had only one thermogravimetric degradation step, which means the existence of a long alkyl segment did not change the thermal stability of the ester group.

As binary polyesters, both PET and PEF are crystalline. When a third component is introduced in copolymerization, the regularity and symmetry of the chains are reduced and low or no crystallization can happen. In Fig.6, the DSC traces for the copolyesters,the feature of amorphous aggregate structure is seen in all the polymers. The glass transition temperature(Tg) of the copolyesters was 0-33 ℃. As the FDCA content increased, the glass transition temperature of the copolyesters went up[16]due to the introduction of FDCA leading to reduced flexibility. On the other hand,the FDCA group increased the aromatic heterocyclic content of the macromolecular backbone, the number of rotatable bonds was reduced, and the chain flexibility reduced, causing theTgto increase.

3.3 Mechanical property characterization

To determine the influence of the FDCA monomer on the mechanical properties, tensile tests were done on the materials. From Table 3 for the mechanical properties of copolyesters PEOT-10 - PEOT-90, the tensile modulus (Et) of the copolyesters was 322-382 MPa, tensile strength (σm) was 8.4-19.1 MPa, and the elongation at break (εb) was 37.2%-89.6%. PEOT-10 has the best tensile strength while PEOT-90 had the best tensile modulus and elongation at break. Generally speaking, as the aromatic ring density increased, the tensile modulus and tensile strength increased but elongation at break decreased. The long diol monomer led to a lower tensile modulus and tensile strengths, but a much higher elongation at break than PET.

4 Conclusions

A series of high molecular weight linear poly-(terephthalic acid-2,5-furandicarboxylic acid-1,8-octanediol) copolyesters were synthesized by varying the molar ratios of terephthalic acid and 2,5-furandicarboxylic acid, by a direct melt polymerization method, using terephthalate acid,2,5-furandicarboxylic acid and 1,8-octanediol as monomers, and antimony trioxide as catalyst. The molecular structure of the copolyesters was determined by FTIR and H1NMR spectroscopy. GPC showed that the highest molecular weight was achieved when equal molar of terephthalic acid and 2,5-furandicarboxylic acid. Therefore, when a third component is introduced in the polyester, keeping equal amounts of monomers is an important approach to obtain a copolymer with a high molecular weight. The TG result showed that the copolyesters had very good thermal stability and only one thermogravimetric degradation step, whileTd,Tdm, and the thermogravimetric residue gradually decreased as the FDCA content increased. DSC study results showed that all the copolymers only have a glass transition temperature, and an amorphous structure, an glass transition temperature going up as the FDCA content in the copolyester increase.The mechanical properties of the copolyester were affected by their components. When the FDCA component was introduced, the copolyesters had a much higher elongation at break than PET. To sum up, it is feasible to partially replace terephthalic acid with 2,5-furandicarboxylic acid in the synthesis of aromatic polyesters, and the properties can be turned by adjusting the ratios of terephthalic acid and 2,5-furandicarboxylic acid.