Synthesis, Physical Properties, and Crystal Structure - American

Jan 24, 2013 - University of Bologna, via Selmi 2, 40126. •S Supporting Information. ABSTRACT: This paper describes the synthesis, crystal structure...
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Poly(butylene 2,5-furan dicarboxylate), a Biobased Alternative to PBT: Synthesis, Physical Properties, and Crystal Structure Jianhui Zhu,† Jiali Cai,†,§ Wenchun Xie,† Pin-Hsuan Chen,† Massimo Gazzano,‡ Mariastella Scandola,‡ and Richard A. Gross†,* †

NSF I/UCRC for Biocatalysis & Bioprocessing of Macromolecules, Polytechnic Institute of NYU, Six Metrotech Center, Brooklyn, New York 11201, United States ‡ Chemistry Department “G. Ciamician” and National Consortium of Materials Science and Technology (INSTM, Bologna RU), University of Bologna, via Selmi 2, 40126 S Supporting Information *

ABSTRACT: This paper describes the synthesis, crystal structure, and physicomechanical properties of a biobased polyester prepared from 2,5-furandicarboxylic acid (FDCA) and 1,4-butanediol. Melt-polycondensation experiments were conducted by a two-stage polymerization using titanium tetraisopropoxide (Ti[OiPr]4) as a catalyst. Polymerization conditions (catalyst concentration, reaction time and second stage reaction temperature) were varied to optimize poly(butylene-FDCA), PBF, and molecular weight. A series of PBFs with different Mw were characterized by DSC, TGA, DMTA, X-ray diffraction and tensile testing. Influence of molecular weight and melting/ crystallization enthalpy on PBF material tensile properties was explored. Cold-drawing tensile tests at room temperature for PBF with Mw 16K to 27K showed a brittle-to-ductile transition. When Mw reaches 38K, the Young modulus of PBF remains above 900 MPa, and the elongation at break increases to above 1000%. The mechanical properties, thermal properties and crystal structures of PBF were similar to petroleum derived poly(butylenes-terephthalate), PBT. Fiber diagrams of uniaxially stretched PBF films were collected, indexed, and the unit cell was determined as triclinic (a = 4.78(3) Å, b = 6.03(5) Å, c = 12.3(1) Å, α = 110.1(2)°, β = 121.1(3)°, γ = 100.6(2)°). A crystal structure was derived from this data and final atomic coordinates are reported. We concluded that there is a close similarity of the PBF structure to PBT α- and β-forms.

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chemistry industry.8 On the basis of the above, FDCA might be a suitable replacement for terephthalic acid in engineering plastics such as PET and PBT. Its manufacture would likely be from hydroxymethylfurfural (HMF), an important intermediate for many furan-based monomers. Consequently, improved routes to HMF synthesis with concurrent exploration of HMFderived chemicals such as FDCA are intensely being pursued by academic and industrial scientists.9−14 The synthesis and study of polyesters from FDCA was first investigated by Moore and co-workers15−17 who explored copolymers of FDCA with various diols. Their pioneering work provided the first clues that FDCA-based copolyesters can be synthesized by transesterification. The polyesters they synthesized were black and brittle solids; they determined their structures and thermal transitions. However, due to poor solubility of the FDCA copolyesters which swelled but was insoluble in chloroform, no information on molecular weight and crystallinity were obtained at that time. Gandini and co-

INTRODUCTION Polymers from readily renewable resources, or biobased polymers, continue to draw increased interest from academia and industry. Motivating forces to develop next-generation biobased alternatives to petroleum-derived plastics include environmental pollution, climate change and a finite global supply of fossil fuel that is subject to disruptive price fluctuations.1,2 A recent study predicts the worldwide capacity of biobased plastics will increase from 0.36 Mt in 2007 to 2.33 Mt and 3.45 Mt in 2013 and 2020, respectively.3 Terephthalic acid is used to produce important commodity polyesters such as poly(ethylene terephthalate), PET, and poly(butylene terephthalate), PBT. One vision is to develop biobased terephthalic acid which would enable the manufacture of high biobased content PET and PBT. However, commercially viable routes to terephthalic acid are not yet available and the ultimate success of such efforts is difficult to predict.4−7 The structure of 2,5-furandicarboxylic acid (FDCA) resembles that of terephthalic acid. FDCA was identified by the US department of Energy as one of 12 priority chemicals likely to play an important role in establishing the green © 2013 American Chemical Society

Received: November 9, 2012 Revised: December 27, 2012 Published: January 24, 2013 796

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tetraisopropoxide, Ti(OiPr)4, 100% active content) was purchased from DuPont. 1,4-Butanediol (BD, 99%), purchased from SigmaAldrich, was distilled prior to use. Poly(butylene terephthalate) (PBT, Mw = 47 700, Mw/Mn = 2.4) was synthesized by a two-step polycondensation using Ti(OiPr)4 as a catalyst following a published method.35 If not otherwise specified, chemicals were purchased in the highest available purity and were used as received. Preparation of PBF. In a typical experiment, polymerizations were conducted in 100 mL two-neck round-bottom flasks equipped with a mechanical overhead stirrer, nitrogen inlet and air condenser. FDCA (7.8 g) and 1,4-butanediol (9g) were charged into the reaction flask at a 1 to 3 (FDCA/BDO) mole ratio; a predetermined volume of catalyst solution (10 mg/mL Ti(OiPr)4 in 1,4-butanediol) was transferred into the reaction flask. The system was placed under vacuum (0.1 mmHg), purged with N2 gas, and this cycle was repeated three times. The polymerization process to synthesize PBF includes two stages. During the first stage, the reaction was carried out under N2 gas to form oligomers. The reaction flask was first placed into a preheated 150 °C metal bath for 2 h, then the temperature was increased to 175 °C for 12 h, and finally to 200 °C for 4 h to complete the first stage reaction. Thereafter, the second stage of the polymerization to produce high molecular weight PBF was performed at 200 °C for 8 h at 0.1 mmHg. During both stages of the reaction, stirring was performed using an overhead stirrer at 300 rpm. Mixing of the reactants was accomplished by using a stainless steel shaft with a swivel blade attached to the bottom. After completing reactions, hot PBF products were scraped out from the reaction flask using a spatula and then cooled to room temperature for storage. Products were not fractionated by precipitation or any other method prior to structural, thermal and mechanical analyses. The color of PBF products ranged from light yellow to brown, and the PBFs are soluble in chloroform at low concentration (between 2 and 5 mg/mL). Instrumentation. Nuclear Magnetic Resonance (NMR). Proton (1H) and carbon 13 (13C) NMR spectra were recorded at room temperature on a DPX300 spectrometer (Bruker Instruments, Inc.) at 300 MHz in trifluoroacetic acid-d. Gel Permeation Chromatography (GPC). Molecular weight of PBF samples was determined by gel permeation chromatography (GPC) using a Waters HPLC system equipped with a model 510 pump, model 717 auto sampler and model 486 ultraviolet detector. Waters Empower GPC software (Version 3, Viscotek Corp.) was used for data analysis. Chloroform was used as eluent at a flow rate of 1.0 mL/min. Sample concentrations and injection volumes were 2 mg/mL and 30 μL, respectively. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) were determined based on a calibration curve generated by narrow molecular weight polystyrene standards (Aldrich Chemical Co). Differential Scanning Calorimetry (DSC). DSC measurements were performed using a differential scanning calorimeter (Model 2920, TA Instruments). Temperature calibration was conducted using an indium standard. Measurements were performed under a nitrogen atmosphere at a flow rate of 50 mL/min. Typical parameters for experimental measurements are as follows: After holding at 220 °C for 3 min in order to eliminate the previous thermal history, samples were quenched in liquid nitrogen, and then first heating scans were recorded by heating at 20 °C/min from −10 to +220 °C. Subsequently, samples were held at 220 °C for 3 min and then cooled to −10 °C at 10 °C/ min. Second heating scans were then recorded by reheating at 10 °C/ min to 220 °C. Melting, crystallization temperatures, and the glass transition temperature were obtained using “universal” software of TA Instruments. Thermogravimetric Analysis (TGA). Thermal stability measurements were conducted by a thermogravimetric analyzer (Model 2910, TA Instruments). To obtain TGA trace, the polyester sample (about 3.0 mg) was scanned from 40 to 800 °C at heating rate 20 °C/min under 45.0 mL/min dry nitrogen flow. To further clarify analysis of thermal stability, peak temperature at the derivative of TGA traces was denoted as Td(max) and used as an index to assess the thermal degradation behavior and stabilities of PBF.

workers reported the use of various methods (solution, interfacial polycondensations and melt polytransesterification) for the synthesis of FDCA copolyesters.18−20 Among the various monomers and synthetic pathways they tested to synthesize poly(ethylene furandicarboxylate), PEF, melt transesterification of FDCA with ethylene glycol was preferred. Polymerization was conducted by first synthesizing pure ethylene glycol-FDCA-ethylene glycol trimers and then applying vacuum to grow polymer chains. PEF Mn, determined by mix-solvent SEC, was 22 000. Its glass transition and melting temperatures, determined by DSC, were 75−80 and 210 °C, respectively. The maximum rate of thermal-induced degradation, measured by TGA, occurred at 398 °C. Moreover, PEF crystallinity, characterized by WAXS, showed three sharp diffraction bands at similar positions as those observed for PET. The above studies provided important information on the synthesis and thermal properties of FDCA copolyesters. However, many of these studies focused on comparing and contrasting the synthesis and thermal properties of FDCA copolyesters with differing diols15−17,20 or focused on PEF.18,19 They generally lacked information on the relationship of chain length or crystallinity on thermal or mechanical properties and did not address whether critical chain lengths were reached that provide a sufficient density of entanglements to achieve useful mechanical properties. In condensation polymerizations, organometallic catalysts play an important role in accelerating polymerization rates to produce high molecular weight products.21−23 Currently, metal catalysts based on antimony and germanium dominates industrial condensation polymerization processes. The desire to replace these two catalysts originates from antimony’s negative environmental impact and germanium’s high cost.24,25 Titanium alkoxides are emerging as important polyesterification catalysts due to their high activity, generally accepted view they are environmentally safe and are available at acceptable pricing for performing low-cost industrial processes.26 A number of studies attest to the relatively high activity of titanium alkoxide catalysts for polyesterification reactions.27−32 Investigation of bulk bis(hydroxyethyl) terephthalate polycondensation reactions to synthesize poly(ethylene terephthalate), PET, showed that the relative catalyst activity was as follows: Ti > Sn > Sb > Zn > Pb > Mn.33 This paper describes the synthesis, crystal structure and physicomechanical properties of a biobased polyester prepared from FDCA and 1,4-butanediol. Melt-polycondensation experiments were conducted by a two-stage polymerization using titanium tetraisopropoxide (Ti[OiPr]4) as a catalyst. Polymerization reaction parameters (catalyst amount, reaction time and second stage reaction temperature) were varied to determine conditions for synthesis of PBF with Mw values up to about 60K. A family of poly(1,4-butylene-co-FDCA), PBF, samples, varying in Mw, were prepared and characterized by DSC, TGA, DMTA, X-ray diffraction, and tensile testing. Studies were conducted to determine the influence of molecular weight on PBF melting/crystallization behavior and tensile properties. To assess the potential of replacing PBT with PBF, comparisons of the mechanical and thermal properties of these two polymers were conducted. Finally, the crystal structure of PBF was determined and compared with α and β forms of PBT.34

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EXPERIMENTAL SECTION

Materials. 2,5-Furandicarboxylic acid (FDCA, >99% purity), was purchased from Satachem. Co. Ltd. TYZORR TPT (titanium 797

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Table 1. Thermal and Mechanical Parameters for PBF Determined by DSC and Tensile Testing as a Function of PBF Molecular Weight first heating

GPC results sample no.

Mw × 10−3 (g/mol)

DIa

Tg (°C)b

1 2 3 4 5 6

16 27 38 49 61 65

2.2 2.6 2.4 2.3 2.7 2.8

34 37 38 39 40 39

second heating ΔHm (J/g)c Tm (°C)c Young’s modulus (MPa)d elongation at break (%)d 46 40 39 38 35 34

173 173 173 172 171 172

742 874 919 969 964 959

± ± ± ± ± ±

23 43 48 64 37 58

2.5 506 1184 1105 1108 1055

± ± ± ± ± ±

0.6 82 28 24 108 56

stress at break (MPa)d 5.5 12.1 29.3 28.5 32.9 31.8

± ± ± ± ± ±

1.1 4.2 1.5 1.7 5.2 2.9

DI is the dispersity index that is equal to Mw/Mn. bGlass transition temperature determined from the first heating scan at 20 °C/min after quenching from the melt using liquid nitrogen. cEnthalpy of fusion and peak melting temperatures determined from second heating at 10 °C/min after cooling at 10 °C/min from the melt. dTensile testing parameters under the crosshead speed of 5 mm/min at room temperature.

a

obtained by averaging the data obtained from ≥5 samples. Young’s modulus was measured as the slope of stress−strain curves at strain below 1%, using the linear least-squares method. Elongation and stress at break measurements were determined at the (x,y) coordinates after which the slope of the stress−strain curve became negative.

Dynamic Mechanical Thermal Analysis (DMTA). PBF samples were molded at 200 °C into rectangular bars with dimensions 20 mm (length) × 5 mm (width) × 1.5 mm (thickness), and cooled at room temperature. DMTA measurements were performed in single cantilever bending mode using a TA Instruments RSA3 Rheometer with an LN2 control system. Measurements were performed from −150 to +150 °C at a heating rate and frequency of 3 °C/min and 3 Hz, respectively. TA Orchestrator software was used for acquiring and processing experimental data. Wide Angle X-ray Scattering (WAXS). WAXS measurements were carried out at room temperature with a Bragg/Brentano diffractometer system (XpertPro, PANalytical), equipped with a high efficiency X’Celerator detector and filtered Cu Kα radiation. Data were collected in the range 3.4°-80° counting for 35 s at each 0.05° step. A flat sample holder, 1.5 mm deep, was filled with a PBF fragment previously annealed for 36 h at 95 °C. Fiber X-ray diffraction scans were recorded at room temperature (250 s) by using Mo Kα radiation (Xcalibur, Oxford Diffraction) on a PBF film stretched at 95 °C to 280% elongation using a tensile testing machine (Instron 4465, rate 1.0 mm/ min) and left 30 min at 95 °C. Structure Refinement Procedure. with respect to bond distances and bond angles, the molecule was assumed to be a rigid body. Values of bond distances were chosen from averaged bond lengths in organic compounds.36 The least-squares routine DEBVIN37 was used to refine the crystal structure of the proposed model. In the last refinement cycles, rotation of the molecular chain with respect to cell axes, the cell parameters and the butylenes C−C torsional bonds were refined together with eight noncrystallographic parameters. Tensile Testing. 1. Mw Effect. Dumbbell shaped sample bars with the dimension 9.0 mm (length) × 3.0 mm (neck width) × 1.0 mm (thickness) were prepared by press-molding at 200 °C and subsequent quenching at ambient temperature. These sample bars were subjected to a heating DSC scan. The difference between the melting enthalpy and cold crystallization enthalpy for each sample bar was calculated. The obtained enthalpy values were compared with those of PBF sample 6 below, with different crystallinity values (see Figure S-1a, Supporting Information). From this data the initial crystallinity values for PBF sample bars with different Mw were determined and the corresponding results are shown in Figure S-1b (Supporting Information). 2. Crystallinity Effect. To produce PBF samples of different crystallinity, dumbbell shaped sample bars of the same size as above were prepared by press-molding at 200 °C. Subsequently, quenching of sample 6 was performed under each of the following conditions: (i) at room temperature (sample 6−1); (ii) at 60 °C for 7 min (sample 6−2); (iii) at 60 °C for 25 min (sample 6−3); (iv) from 200 °C to room temperature at 0.1 °C per minute (sample 6−4). 3. Mechanical Studies by Instron Tensile Testing. An Instron 5542 tensile testing machine with a 500 N load cell was used. The crosshead speed was 5 mm/min and the test temperature was 25 °C. Merlin software was used to collect and analyze tensile results (stress was calculated according to the initial cross-sectional area). Values of Young’s modulus, elongation at yield and break and stress at yield were

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RESULTS AND DISCUSSION The 1H NMR (300 MHz, trifluoroacetic acid-d) spectrum of PBF sample 6 (Table 1), the synthesis of which is described below, is shown in Figure 1. All peaks were assigned based on a

Figure 1. 1H NMR spectrum (300 MHz in TFA-d) of poly(butylene furandicarboxylate), PBF.

previously published spectrum of PBF.38 No remarkable features were found in the NMR spectrum and it is consistent with that expected for the PBF structure. All synthesized PBF samples described herein have identical 1H NMR spectra. Synthesis of PBF. Effect of Ti(OiPr)4 Concentration. Studies to determine preferred Ti(OiPr)4 concentration were conducted in two-stage polycondensation reactions. During the first stage, oligomerization was conducted with predetermined quantities of Ti(OiPr)4 and stepwise increased temperature (150 to 200 °C), under N2, for 18 h. Subsequently, in the second stage, the pressure was slowly reduced to below 0.1 mmHg, the temperature was kept at 200 °C, and the reaction was carried out for 8 h. The above starting conditions to assess catalyst concentration were selected based on studies of related polyester synthesis reactions.28−32 Catalyst concentration was 798

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based on the weight percentage of Ti(OiPr)4 relative to FDCA in the monomer feed. Molecular weight and dispersity (DI) were determined on nonfractionated products by GPC. Figure 2a shows the GPC curves of products obtained for catalyst

Figure 3. Effect of second stage reaction temperature on PBF Mw and DI.

65K, and 57K, respectively). Further increases in the secondstage temperature to 220, 240, and 260 °C resulted in decreased PBF Mw (38K, 22K and 11K, respectively). These results suggest that, when the second stage reaction temperature is higher than 220 °C, relative to chain propagation there is an increased rate of reactions leading to bond cleavage due to intrachain ester hydrolysis as well as pyrolysis reactions resulting in the observed decreases in PBF Mw. Indeed, previous reports on the thermal decomposition of PBT indicated that, at temperatures close to 250 °C, PBT undergoes pyrolysis reactions resulting in the formation of carboxylic acid and unsaturated CC bond end groups.39,40 For PBF, it may be that similar pyrolysis reactions are also occurring at reaction temperatures above 220 °C. However, NMR spectra were not recorded on products formed above 220 °C so we do not have experimental data verifying the occurrence of such pyrolysis reactions. DI values of PBFs range from 2.2 to 3.4 for all second-stage temperatures studied. These variations in DI are relatively small and explanations of their origin would be highly speculative. Effect of Reaction Time and Temperature. Results in Figure 3 were based on a fixed 8 h second-stage reaction time. It may be that by conducting polycondensation reactions at elevated reaction temperatures but shorter times (e.g., 240 °C for 2 h), PBF products with relatively higher Mw might result. Hence, experiments herein were conducted to determine relationships between PBF molecular weight as a function of both second-stage reaction temperature and time (see Figure 4). These studies were all performed using 200 ppm Ti(OiPr)4 and fixing the second-stage pressure at 0.1 mmHg. Thus, during second-stage polymerizations conducted at 200, 220, and 240 °C, aliquots were withdrawn at predetermined times and analyzed by GPC. Study of Figure 4 shows that, at 200 °C, as the reaction time increased from 2 to 8 h, there was a regular increase in Mw. Thus, at 2, 4, 6, and 8 h, PBF Mw (DI values in parentheses) values are 32K (2.5), 42K (2.7), 59K (2.9), and 65K (2.8). By 8 h, the product viscosity increased such that the product adhered to the mechanical stirrer blades and mixing was no longer effective. For 2 h reactions, the highest PBF Mw (45K) was reached at 220 °C. Extending the reaction time at 220 °C resulted in small decreases in Mw (e.g., to 38K at 8 h). At 240 °C, as the reaction time increased from 2 to 8 h, Mw decreased from 35K to 22K, with DI remaining at about 2.3. From the above results we conclude that the highest PBF Mw

Figure 2. (a) GPC traces for PBF samples prepared using different Ti(OiPr)4 concentrations (50, 100, 150, 200, 300, 500 ppm); (b) relationship between Ti(OiPr)4 concentration and PBF Mw and DI.

concentrations ranging from 50 to 500 ppm. For catalyst concentrations 150 to 500 ppm, PBF products show symmetrical GPC traces with a negligible fraction of oligomers and no residual monomer. When using 50 and 100 ppm Ti(OiPr)4, the presence of a substantial oligomer fraction was evident. However, peaks corresponding to monomer were not observed. We did not isolate and analyze the oligomer fraction of these products, however, based on the large body of work on related polymers, it is likely they consist of a mixture of cyclics and linear components. Figure 2b shows the influence of Ti(OiPr)4 concentration on Mw and DI of PBF. Mw increased from 9K to 65K as the catalyst concentration increased from 50 to 200 ppm, while DI decreased from 4.2 to about 2.8. Further increase in Ti(OiPr)4 concentration from 200 to 500 ppm gave similar Mw and DI values (about 60K and 2.7, respectively). Effect of Second-Stage Reaction Temperature. Given that reducing PBF viscosity in reactions could lead to increased propagation rates and higher product molecular weight,31 second-stage reaction temperatures focused on those from 190 to 260 °C. The first-stage reaction temperature was gradually increased from 150 to 200 °C over 18 h as described above. On the basis of results in Figure 2, 200 ppm Ti(OiPr)4 was selected as the catalyst concentration. Furthermore, second-stage reaction time and pressure were fixed at 8 h and 0.1 mmHg, respectively. Results in Figure 3 show that at 190, 200, and 210 °C, synthesized PBF Mw remained at about 60K (e.g., 61K, 799

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Figure 4. Effect of reaction time and second-stage temperature on PBF Mw and DI.

was reached for second-stage reactions conducting at 200 °C for 8 h. Furthermore, although 220 and 240 °C give relatively high PBF Mw at short (e.g., 2 h) second-stage reaction times, extending reactions to 8 h results in a higher frequency of PBF chain scission than propagation reactions leading to relatively lower molecular weight PBF products. Thermal Properties. Differential Scanning Calorimetry. Results above illustrate that by changing the catalyst concentration (Figure 2), the second stage reaction temperature (Figure 3) or the second stage reaction time (Figure 4), a series of PBF samples differing in molecular weight can be prepared. These strategies were adopted in order to prepare a series of PBF samples for further thermal and mechanical properties study. Thus, PBF samples with Mw values of 16K, 27K, 38K, 49K, 61K, and 65K (samples 1−6, Table 1) were prepared with similar DI values (2.2−2.8). After quenching melted PBF samples in liquid nitrogen, first heating scans were obtained by heating at 20 °C/min from −10 to +220 °C. First heating scans were used to measure PBF glass transition temperatures. Then, PBF samples were cooled at 10 °C/min to −10 °C and second heating scans were recorded by reheating at 10 °C/min to 220 °C. Parts a and b of Figure 5 display DSC thermograms of first and second heating scans, respectively. Transitions corresponding to glass (Tg), cold crystallization and melting (Tm) were observed for all DSC curves displayed in Figure 5a. The fact that values of crystallization enthalpy (35 ± 4 J/g) and melting enthalpy (ΔHm) are close in value (±3 J/g) for all thermograms implies that liquid nitrogen quenching of samples from the melt gave amorphous materials for first heating experiments. Tg values measured first heating scans are listed in Table 1. Tg values of samples 3−6 with Mw values from 38K to 65K were very similar to each other (39 ± 1 °C) and to the that of PBT (40 °C).41 Lower Tg values of samples 1 and 2 (34 and 37 °C, respectively) may be due to the presence of small quantities of low-molecular weight species that act as plasticizers lowering PBF Tg.42 Figure 5b shows one clear transition in DSC thermograms corresponding to melting. Transitions corresponding to Tg and cold crystallization peaks are weak or not observed since crystallization occurs during cooling cooled at 10 °C/min from +220 °C to −10 °C. Values of ΔHm and Tm measured from melting transitions in second heating thermograms (Figure 5b) are given in Table 1. The Tm of PBF is about 172 °C, approximately 50 °C lower than that of PBT (224 °C).

Figure 5. DSC thermograms recorded for PBF samples 1−6 during (a) first heating and (b) second heating.

Values of ΔHm decreased from 46 to 34 J/g with increasing molecular weight. This trend is consistent with changes in ΔHc (recorded during cooling scans) as a function of PBF Mw. We explain this behavior in ΔHm by increased mobility of lower molecular weight PBF chains that allows them to crystallize to greater extent than correspondingly higher molecular weight PBF during cooling from the melt.43 Thermogravimetric Analysis (TGA). TGA analysis was used to assess the thermal stability of PBF. Figure 6 shows TGA curves of PBF sample 6 (Mw = 65K) and PBT. Both TGA curves have a main weight loss step accounting for ca. 90% of the total sample weight above 400 °C (maximum degradation rate at 428 and 442 °C for PBF and PBT respectively),

Figure 6. TGA traces of PBF sample 6 and PBT. 800

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followed by a minor weight loss step located around 550 °C. These results demonstrate PBF has excellent thermal stability, and it can be safely processed at temperature higher than its melting point. Dynamic Mechanical Thermal Analysis (DMTA). DMTA is an effective technique to provide information on secondary relaxation of polymers including glass transition and crystallization events. PBF samples of differing molecular weight were all investigated by DMTA and similar spectra were obtained. Parts a and b of Figure 7 are representative DMTA

superposition of several processes involving mostly the motion of butanediol group (−O−(CH2)4−O−)45 and carbonyl (−C[O]−O−) residues in the noncrystalline phase.49 Peak temperatures for PBT γ-relaxation from DMTA studies performed by other groups occur at between −85 and −46 °C, depending on the applied frequency.44−49 Mechanical Properties. The effect of molecular weight on the mechanical properties of PBF was investigated by testing the tensile behavior of samples with Mw values from 16K-to65K. Crystallinity of PBF sample bars showed PBF crystallinity decreased with increasing molecular weight. Specifically for Mw values of 16K, 27K, 38K, 49K, 61K, and 65K % crystallinity values are 22.4, 16.0, 15.1, 12.7, 11.0, 8.1%, respectively (see Figure S-1 (Supporting Information) for plots of PBF % crystallinity (χc) versus ΔH and χc versus Mw). The Young’s modulus, elongation at break and stress at break, determined from stress−strain curves, are listed in Table 1 (see Figure S2, Supporting Information for stress−strain curves). With an increase in the molecular weight and decrease in the crystallinity from sample 1 to sample 2, the elongation at break increases significantly and the fracture fashion of PBF changes from brittle-to-ductile. With further increases in Mw, samples exhibit a similar stress−strain curve and PBF samples 3−6 show an elongation at break higher than 1000%. A parallel behavior is shown by the stress at break values that are practically constant (ca. 30 MPa) for samples 3−6 whereas, they are remarkably lower for samples 1 and 2. This behavior may be associated with the dependence on molecular weight on entanglement density. At too low molecular weights the number of entanglements may be insufficient to provide good mechanical properties. Young’s modulus (Table 1) changes little with molecular weight, remaining at about 925 MPa for samples 2−6. Lower molecular weight sample 1 has a relatively lower modulus than samples 2−6, possibly due to sample 1’s lower chain entanglement density relative to those of other samples.51 Excellent agreement was found for mechanical data of PBF samples 3−6 and mechanical data of a commercial PBT (Young’s modulus: 950 ± 104 MPa, stress at break: 38 ± 3 MPa), where analyses for all materials were conducted under the same experimental conditions. Materials generated from sample 6 (Mw = 65K) having different crystal phase content (from 8% to 44%, determined by WAXS) were prepared by varying the annealing conditions as described in the Experimental Section. Tensile testing showed a significant decrease in elongation at break (from >1000% to 7 ± 2%) with increasing crystallinity, whereas elastic modulus and strength changed little (see Figure S3, Supporting Information). The latter effect is consistent with behavior when the amorphous phase is glassy (below Tg). That is, when the amorphous phase is glassy, the presence of crystallinity has a much lower effect on increasing the modulus.52 Also of interest is comparison of PBT and PBF mechanical properties at similar % crystallinity. The crystallinity of PBT sample bar before tensile testing was calculated to be 33.2% from the melting enthalpy (48 J/g) from knowledge of the equilibrium melting enthalpy (144.5 J/g) for PBT that is 100% crystalline.53 Therefore, we also compared the PBT mechanical properties for sample 6−3 that has a % crystallinity value of 37.7 (see Table S3, Supporting Information), close to that of the PBT. The values of tensile parameters of PBF sample 6−3 (Young’s modulus, 1091 ± 45 MPa; stress at break, 35.5 ± 1.9 MPa; elongation at break, 284 ± 93%), are in good agreement

Figure 7. (a) storage modulus, and (b) tan δ, with temperature for PBF sample 6 with Mw = 65K.

thermograms recorded of sample 6 (Mw = 65K) showing variation of storage modulus and tan δ with temperature, respectively. Figure 7a indicates that, for temperatures in the range from −130 to +25 °C, storage modulus decreases slightly with increasing temperature. In other words, as the temperature increases from −130 to +25 °C, the storage modulus decreases by 57%. Increase in the temperature from 25 to 57 °C results in a large decrease in the storage modulus. This occurs due to reaching the sample’s glass transition temperature. Above 57 °C, an increase in the storage modulus occurs due to PBF cold crystallization. Figure 7b shows variation of tan δ with temperature; two relaxation peaks, located at −74 and +39 °C, respectively, are observed. The higher temperature peak is attributed to micro-Brownian motion of PBF chain segments in the amorphous regions, i.e. peak for the transition from glass to rubber.44 Little change in the PBF Tg was observed for the series of different molecular weight PBF samples studied herein. For PBT, the reported Tg from DMTA experiments was between 61 and 82 °C, depending on the applied frequency.44−50 The lower temperature peak in Figure 7b is attributed to γ-relaxation47 which is associated with the 801

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with those of PBT (Young’s modulus, 950 ± 104 MPa; stress at break, 38 ± 3 MPa; elongation at break, 272 ± 71%). Structural Analysis. The PBF powder diffraction profile of a nonoriented sample (Figure 8) is characterized by two strong

Pearson VII curve superimposed to the background line. The comparison between observed and calculated diffraction data which gives a final agreement factor Rwp = 13.8% is displayed in the Figure 8. A sketch of the crystal structure is shown in Figure 10 and the final atomic coordinates are reported in Table 2.

Figure 8. Comparison between calculated (red) and experimental (black) patterns. The dotted line shows the amorphous contribution. The atom labels are presented in the inset.

reflections at 2θ values of 17.70° (d = 5.00 Å) and 24.55° (d = 3.62 Å), and two less intense diffraction peaks at 10.06° (d = 8.78 Å) and 22.23° (d = 3.96 Å). Fiber diagrams of uniaxially stretched PBF films (Figure 9) were collected in order to index Figure 10. Views of the PBF structure.

The c-axis is the fiber axis and the furan moiety is almost parallel to the b,c plane, rotated about 9° around to the b-axis. Table 2. Unit Cell Parameters and Atomic Fractional Coordinates in Space Group P1 of PBF Structure as shown in Figure 8 a = 4.78(3) Å, b = 6.03(5) Å, c = 12.3(1) Å

Figure 9. X-ray fiber patterns of a streched sample taken along the directions shown in the sketch. Indexes hkl of main reflections are reported.

α = 110.1(2)°, β = 121.1(3)°, γ = 100.6(2)°

the main reflections. The facial and lateral projections are identical: few broad reflections are observed where the most intense diffraction spots are on the equator. The pattern collected with the beam parallel to the stretching direction shows some circles with isotropic distributed intensity. After several trials, the fiber pattern was indexed as reported in Figure 9 and the unit cell was determined as triclinic (a = 4.78(3) Å, b = 6.03(5) Å, c = 12.3(1) Å, α = 110.1(2)°, β = 121.1(3)°, γ = 100.6(2)°). The space group was chosen as noncentric, based on the absence of a center of symmetry inside the repeating unit. The molecule was introduced as a rigid body where the furan ring has a fixed planar geometry. The bond lengths were chosen in accordance with the mean data reported in the literature36 and are listed in Table S1, Supporting Information. Torsional bonds inside the butylenic fragment were adjustable parameters. This choice is motivated by the need to maintain a low number of free parameters in consideration of the few observed reflections. The structure was refined from the powder pattern by Rietveld’s method.37 The measured density of 1.3 g/cm3 is in good agreement with that calculated (1.37 g/ cm3) and is consistent with one repeating unit per cell. The contribution of the amorphous fraction was simulated by a

atom

x

y

z

C1 C2 O1 C3 C4 C5 C6 O2 C7 O3 C8 O5 C9 C10 O4

0.2261 0.2150 0.1295 0.1137 0.0249 −0.0443 −0.1098 0.1677 −0.0759 0.0068 −0.1077 −0.1894 −0.2066 −0.0825 −0.0599

−0.0829 −0.2430 −0.1206 −0.2500 −0.1322 0.0767 0.0834 −0.4464 −0.1218 −0.2564 −0.2308 −0.1127 −0.2473 −0.0502 −0.4274

0.5309 0.4001 0.3144 0.1952 0.0998 0.1050 −0.0197 0.1682 −0.0924 −0.0205 −0.2293 −0.3104 −0.4359 −0.4724 −0.2685

There is a close similarity between the structures of PBF and PBT forms α- and β.34 As can be appreciated from the data of Table S2, Supporting Information, the PBF unit cell is reminiscent of PBT ones. Comparison of the PBT structures shows that the two polymorphic forms are different mainly in the butylene group conformation. The conformation of the 802

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PBT butylene group (torsional angles 180°, 66°, 99°, 124°, 148°) is intermediate to those of the α- and β-PBT.

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CONCLUSION In this work, high molecular weight PBFs were successfully synthesized by melt-condensation polymerization catalyzed by Ti(OiPr)4. Thermal and mechanical properties study of PBF showed that it has very good thermal stability as well as impressive strength and ductility. Thus, we consider PBF to be a promising biobased polymer that, with future improvements in process for manufacturing FDCA from sugar, could replace a large fraction of PBT used as an engineering plastic. A twostage polymerization reaction was performed and the experimental conditions were optimized by using Ti(OiPr)4 as catalyst. Tensile testing, DSC, TGA, DMTA, WAXS, and GPC were used to investigate the effect of molecular weight on mechanical, thermal and crystalline material properties. The melting point of PBF was determined to be around 172 °C, and its maximum degradation rate occurs at 428 °C. Furthermore, when the Mw of PBF is ≥38K, sufficient chain entanglements are reached such that the Young modulus and elongation of PBF are above 900 MPa and 1000%, respectively. The crystal structure of PBF was determined as triclinic by WAXS. After comparison of PBF and PBT properties discussed above, we conclude that PBF possesses advantages relative to PBT, such as lower processing temperature and better ductility. However, further physical, mechanical and rheological studies will be needed to provide a more comprehensive understanding of PBF properties. Such work is expected to uncover unique characteristics of PBF that will better define where PBF provides advantages relative to PBT and related petroleumderived materials.

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ASSOCIATED CONTENT

S Supporting Information *

Stress vs strain curves for PBF of varying molecular weight and crystallinity, tabulated data on PBF mechanical properties as a function of crystallinity, bond lengths (Å) in PBF structure, plots for PBF of varying crystallinity displaying relationships between % crystallinity vs ΔH and % crystallinity vs PBF molecular weight, and comparison between cell parameters of PBF and α- and β-PBT. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to financial support from Industrial Members (BASF, Covidien, Firmenich, Novozymes, PepsiCo and Sherwin Williams) of our graduated NSF I/UCRC for Biocatalysis and Bioprocessing of Macromolecules at NYUPOLY. M.G. and M.S. acknowledge partial financial support from MIUR (Italian Ministry for University and Research).

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REFERENCES

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