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Fast crystallization and melting behavior of a long-spaced aliphatic furandicarboxylate bio-based polyester, the poly(dodecylene 2,5-furanoate) Dimitrios G. Papageorgiou, Nathanael Guigo, Vasilios Tsanaktsis, Stylianos Exarhopoulos, Dimitrios N. Bikiaris, Nicolas Sbirrazzuoli, and George Z Papageorgiou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00811 • Publication Date (Web): 21 Apr 2016 Downloaded from http://pubs.acs.org on April 23, 2016

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Industrial & Engineering Chemistry Research

Fast crystallization and melting behavior of a long-spaced aliphatic furandicarboxylate bio-based polyester, the poly(dodecylene 2,5-furanoate)

Dimitrios G. Papageorgiou1, Nathanael Guigo2, Vasilios Tsanaktsis3, Stylianos Exarhopoulos4,5, Dimitrios N. Bikiaris3, Nicolas Sbirrazzuoli2*, George Z. Papageorgiou5*

1

School of Materials and National Graphene Institute, University of Manchester, Oxford

Road, Manchester M13 9PL, United Kingdom 2

Université Nice Sophia Antipolis, CNRS, Laboratoire de Physique de la Matière Condensée

LPMC – UMR 7336, Parc Valrose, 06100 Nice. 3

Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle

University of Thessaloniki, GR-541 24, Thessaloniki, Macedonia, Greece 4

Department of Food Technology, Technological Educational Institute of Thessaloniki, PO

Box 141, GR-57400 Thessaloniki, Greece 5

Chemistry Department, University of Ioannina, P.O. Box 1186, 45110 Ioannina, Greece

Abstract Poly(dodecylene 2,5-furanoate) (PDoF) is a novel alipharomatic polyester which was prepared by combining a long chain glycol as the monomer (1,12-dodecamethylene glycol) and 2,5-furan dicarboxylic acid (FDCA), which can be derived from biomass. A variation of the well-known two-step polycondensation method was applied for the preparation of PDoF. The glass transition temperature of this polyester, which was recorded by using fast scanning calorimetry (FSC) is observed at -5 °C. The melting temperature is about 111 °C while the equilibrium melting temperature was extrapolated through Hoffman-Week plots to 127.3 ± 0.2 °C. New insights in the complex melting behavior were obtained by employing conventional and temperature modulated calorimetry. The crystallization kinetics in isothermal and non-isothermal modes were investigated by means of Avrami, LauritzenHoffman models and model-free kinetics. The thermal stability of PDoF was reduced in 1 ACS Paragon Plus Environment


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comparison with previously studied furanoates, as a result of the flexible macromolecular chains of the material, while the nucleation density was high and the spherulitic size was small.

Keywords: furanoates, 2,5-furandicarboxylic acid, bio-based polymers, crystallization kinetics, thermal properties.

Corresponding Authors: [email protected] (Nicolas Sbirrazzuoli), [email protected] (George Z. Papageorgiou)

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1. Introduction In the recent years, interest and demand in eco-friendly “green” polymers that can be prepared from renewable resources and can be energy recovered or recycled has been constantly growing. 2,5-Furandicarboxylic acid (FDCA) is one of the most promising aromatic monomers, prepared by renewable resources and due to the constant progress in the biorefinery concept there are high hopes that the large scale production of FDCA will lead the way for the massive production of FCDA-based polyesters.1 FDCA has been used as a monomer for the production of poly(ethylene-2,5-furandicarboxylate) (PEF) or poly(butylene 2,5-furan dicarboxylate) (PBF) which are slowly finding their way towards commercial applications as a replacement of their terephthalate homologues. Recent works have reported the advantages of PEF over poly(ethylene terephthalate) (PET) in terms of its mechanical, thermal and barrier properties2-5, while a lot of recent works are dedicated to the furanoate family of materials.6-18 Even though the structure of the FDCA resembles that of terephthalic acid, the dissimilarities in their ring sizes, linearity and polarity attribute different characteristics and physicochemical properties to the final material.19 Burgess et al. have reported an impressive reduction in the permeability of CO2 and O2 for PEF, compared to PET, while the thermal and mechanical properties of the two polymers were at least comparable.2,20,21 Various preparation strategies have been presented in the literature for the synthesis and properties of FCDA-based polyesters. Gomes and Gandini

22

were among the first to use a variety of diols for the

preparation of a series of poly(2,5-furan dicarboxylate)s. However, there are only a few works using long chain glycols as monomers for the preparation of furanoate-based polyesters. This can be of interest for applications where less rigid materials are needed. On that basis, we have combined 1,12-dodecamethylene glycol along with 2,5-furandicarboxylic acid, in order to prepare poly(dodecylene furanoate). 3 ACS Paragon Plus Environment


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The aim of this work was to successfully report the synthesis of a new furanoate-based polyester using an aliphatic diol with 12 methylene groups (-CH2)m in its molecule (m=12, Figure 1a). Initially, the chemical and crystalline structure of the material were assessed by 1

HNMR and WAXD. The thermal transitions of the novel material were extensively evaluated

by differential scanning calorimetry, while its thermal stability was evaluated by thermogravimetric analysis. Fast scanning calorimetry was also utilized for the observation of the glass transition temperature of PDoF. The kinetics of crystallization were studied under various conditions in direct comparison with several furanoate-based materials, while important characteristics of the material such as the equilibrium melting and glass transition temperature, are reported for the first time in literature.

2. Materials & Methods 2.1. Materials 2,5-furan dicarboxylic acid (purum 97 %), 1,12-dodecanediol (1,12-DD) (99%, m.p.=79-81 ºC and b.p.= 189 °C/12 mmHg), and tetrabutyl titanate (TBT) catalyst of analytical grade were purchased from Aldrich Co. All other materials and solvents used were of analytical grade.

2.2. Synthesis of 2,5-dimethylfuran-dicarboxylate (DMFD) A round bottom flask (500 mL) was used to transfer 15.6 g of 2,5-furandicarboxylic acid, 200 mL of anhydrous methanol and 2 mL of concentrated sulfuric acid and the mixture was then refluxed for 5 hours. The filtering and distillation of the excess of methanol proceeded with the use of a disposable Teflon membrane filter. During filtration dimethylester was precipitated as white powder and after cooling 100 mL of distilled water was added. The 4 ACS Paragon Plus Environment

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partial neutralization of the dispersion was performed by adding Na2CO3 5 % w/v during stirring while continuously measuring the pH. The white powder was filtered and the solid was washed several times with distilled water and dried. The isolated white dimethylester was recrystallized with a mixture of 50/50 v/v methanol/water. After cooling 2,5-dimethylfurandicarboxylate (DMFD) was precipitated in the form of white needles. The reaction yield was calculated at 83 %.

2.3. Polyester synthesis PDoF polyester was synthesized by applying a variation of the two-stage melt polycondensation method (esterification and polycondensation) in a glass batch reactor using 1,12-dodecanediol and DMFD.23

2.4. Polyester characterization 2.4.1. Intrinsic viscosity and molecular weight measurements The measurements of the intrinsic viscosity [η] of the prepared polyester were performed using an Ubbelohde viscometer at 30 °C in a mixture of phenol/1,1,2,2-tetrachloroethane (60/40 w/w). In order to obtain a full solution of the sample, it was kept in the mixture of solvents at 90oC for some time. After cooling the solution to ambient temperature, a disposable membrane filter by Teflon was used in order to filter the solution.24 Gel permeation chromatography (GPC) was utilized for the measurement of the number-average molecular weight of PDoF, by using a Waters apparatus (150°C) equipped with three ultrastyragel (103, 104, 105 Å) columns in series and a differential refractometer as detector. Tetrahydrofuran was used as mobile phase at a flow rate 0.5 mL/min at 40 °C. The calibration of the apparatus was executed by using a polystyrene standard with a narrow molecular weight distribution. 5 ACS Paragon Plus Environment


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2.4.2. Nuclear Magnetic Resonance (1H-NMR) A Bruker spectrometer operating at a frequency of 400 MHz for protons was utilized for the 1

H-NMR spectrum of PDoF polyester. Solutions of 5% w/v were prepared using deuterated

trifluoroacetic acid (dTFA). The sweep width was 6 kHz and the number of scans was 10.

2.4.3. Wide angle X-Ray diffraction patterns (WAXD) X-ray diffraction patterns of PDoF were acquired from thin films (10-30 µm) prepared by hot pressing, using a MiniFlex II XRD system from Rigaku Co, with CuKα radiation (λ=0.154 nm), in the 2θ range from 5 to 60 degrees.

2.4.4. Thermogravimetric analysis (TGA) The thermal stability of PDoF was measured using a STA 449C (Netzch-Gerätebau, GmbH, Germany) thermal analyzer. The heating range was from room temperature up to 600 °C, with a heating rate of 10 °C min−1 in N2 (99.9%) constant flow of 30 cm3 min−1.

2.4.5. Differential Scanning Calorimetry (DSC) DSC studies were carried out using a TA Instruments temperature modulated DSC (TA Q2000). Nitrogen gas flow of 50 ml/min was purged into the DSC cell. The mass of the sample was kept around 5 mg in crystallization kinetics tests. The temperature modulated DSC measurements (TMDSC) were performed at a heating rate of 5 °C /min, with temperature modulation amplitude of 1 °C and period of 60 s. The recommendations from the ICTAC committee for collecting thermal analysis data for kinetic computations were followed.25,26 The samples were cooled to 0 °C and then heated at a rate of 20°C/min at temperatures 40 °C higher than the melting temperature (T=160 °C). Thermogravimetric analysis was previously performed to ensure that thermal degradation of the polymer does not


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occur at this temperature. In order to obtain amorphous materials, the samples were held at 160 °C for 5 min, in order to erase any thermal history, before cooling in the DSC with the highest achievable rate.27 Isothermal crystallization investigations at various temperatures below the melting point were carried out after self-nucleation of the polyester sample. Self-nucleation measurements were analogous to the protocol described by Fillon et al.28 The procedure used in this work was very similar with that described in precedent works by Müller et al and our group.29, 16 The measurements, performed on heating after isothermal crystallization, were done at a heating rate of 20 °C/min. The melting enthalpy was estimated to be around 72 Jg-1 at 20 °C/min. Estimations of the melting enthalpy of 100% crystalline PDoF were based on the degree of crystallinity values calculated from the WAXD patterns and DSC data. Errors may arise from insufficient differences in crystallinity or differences in the specimen’s thickness in WAXD measurements, while for DSC data, errors in the weight of the samples and additional crystallization occurring on heating may also lead to inaccurate estimation of the melting enthalpy per gram of sample. Thus, the reported values have to be considered mainly as a first approximation rather than as absolute values.

2.4.6. Fast Scanning Calorimetry Fast Scanning Calorimetry (FSC) experiments were done on a Mettler-Toledo Flash DSC1 equipped with the UFS1 chips. Indium samples were used for calibration. A minuscule PDoF samples was positioned on the sensor after cutting by microtome and was first heated to 160 °C for melting and laying down it uniformly at the sensor surface. A thin layer of PDoF is thus obtained and uniform heating/cooling are expected. In a typical experiment, the PDoF sample was heated to 160 °C and to erase the thermal history, it was maintained at this



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temperature for 30 seconds. Then, the PDoF sample was cooled from 160 °C to -80 °C at different rates, i.e. 0.1 °C/s, 1 °C/s, 10 °C/s, 100 °C/s, 1000 °C/s. In each case, the subsequent heating scans were conducted at 1000 °C/s. A nitrogen flow (~ 30 mL/min) was maintained around the sensor thus ensuring optimal heat diffusion and scanning rate performance. The sample mass on the FSC sensor was estimated from the ratio between the FSC melting enthalpy of crystals and the normalized melting enthalpy obtained by DSC in comparable conditions (i.e. after cooling at 0.1 °C/s). The FSC sample mass was approximately 85 ng.

2.4.7. Polarizing Light microscopy (PLM) A polarizing light optical microscope (Nikon, Optiphot-2) equipped with a Linkam THMS 600 heating stage, a Linkam TP 91 control unit and also a Jenoptic ProgRes C10Plus camera with the Capture Pro 2.1 software was used for PLM observations.

3. Results & Discussion 3.1 Structural Characteristics For the synthesis of the PDoF polyester the procedure that was described in the experimental part was followed. From the intrinsic viscosity measurements a value of IV=0.49 dL/g was determined. Furthermore, GPC measurements showed that the weight average molecular weight was Mw=68965 g/mol and the number average molecular was Mn=39370 g/mol. These values resulted in a dispersity (D) index value of 1.75. The chemical structure of PDoF was confirmed by 1HNMR (Figure 1a) as the characteristic resonances were recorded at: 7.31 δ (2 H,s) for ‘a’ protons, at 4.43 δ (4 H, t) for ‘b’ protons, at 1.80 δ (4 H, q) for ‘c’ protons and at 1.37 δ (16 H, s) for ‘d’ protons. The semicrystalline structure of PDoF can be seen in the diffractogram in Figure 1b. A deconvolution procedure was initially followed, in order to separate the crystalline peaks from the amorphous halo. The degree of crystallinity for this 8 ACS Paragon Plus Environment

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sample which was cooled in ambient conditions from the melt was about 44%. The main diffraction peaks were observed at 9.6o, 17.5o, 21.5o and 23.8o and they are similar to the strong peaks at 21.5o and 23.8o that can be observed in the pattern of polyethylene, which correspond to the reflections from the (100) and (200) crystallographic planes in PE.30

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Figure 1. (a) 1HNMR spectrum of PDoF and (b) X-ray diffractogram of PDoF. 9 ACS Paragon Plus Environment


3.2 Thermal degradation of PDoF The thermal stability of PDoF was studied by means of thermogravimetric analysis (TGA). The mass loss and derivative of mass loss (DTG) curves are presented in Figure 2. It can be seen that the material decomposes in two stages, the first one occurring between 25-400 °C, while the second one represents the end of decomposition at the specific temperature range from 400-600 °C. The slope of the TG curve presents a change at this temperature range which corresponds to a decrease in the degradation rate. The existence of the second stage can be also validated from the small (shoulder) peak in the DTG curve, right after the main peak, indicating a slightly different decomposition step. In most cases, furanoate-based polymers are considerably stable when heated and only exhibit one decomposition step.31 The high chain mobility is attributed to the high number of methylene units and leads subsequently to a decrease in thermal stability.

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Figure 2. Mass loss and differential thermogravimetric curves of PDoF under N2 atmosphere. 10 ACS Paragon Plus Environment

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3.3 Thermal transitions of PDoF The thermal transitions of PDoF were initially assessed by means of conventional DSC (Figure 3a). The sample was heated from ambient temperature up to 175 °C in order to obtain the melting temperature of the sample without erasing its thermal history and it was equal to 111.5 °C. The melting temperature was lower than all other furanoate-based materials studied by our group, such as poly(propylene furanoate)14 and poly(hexamethylene furanoate)17 since PDoF presents more flexible macromolecular chains than the aforementioned materials. Moreover, this temperature is close to the melting temperature of low density polyethylene (LDPE). Then, the sample was quenched, reheated and the melting temperature was observed at 106.6 °C. The absence of glass transition during the heating after quenching in the DSC instrument indicated that the maximal cooling rate available with conventional DSC equipment (80 °C/min) relatively slow, therefore crystallites were formed during the cooling procedure and the sample wasn’t frozen in the glassy state. Therefore, the sample wasn’t completely amorphous. Moreover, a big exothermal thermal event is observed after cooling at 10 °C/min in the range of 40-120 °C and can be attributed to a crystallization process. Temperature-modulated differential scanning calorimetry (TMDSC) was also utilized in order to separate reversing and non-reversing thermal events (Figure 3b). Once again, identification of the glass transition isn’t straightforward from the study of the thermoanalytical curves. Thus, it can be concluded that only very fast cooling rates would provide the appropriate conditions for the observation of the Tg of PDoF. The large peak observed in the nonreversing signal close to 100 °C in the temperature domain of the melting, is an indication of the recrystallization which takes place during the heating procedure, a typical phenomenon for the majority of polyesters and furanoates.14,17



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Figure 3. (a) DSC heating curves of the as received and quenched PDoF sample and DSC cooling curves of the sample cooled at 10 °C/min (b) TMDSC curves for the quenched sample (at 80 °C/min).

To outrun the formation of small crystals during cooling and to observe the glass transition of 100% amorphous PDoF, faster cooling rates, not available with regular DSC and TMDSC, are 12 ACS Paragon Plus Environment

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needed. For this reason, fast scanning calorimetry (FSC) measurements were performed, since rates as fast as 5000 °C/s can be employed to cool the samples. The results from FSC are presented in Figure 4. It can be confirmed that rates in the range of 10 °C/s were not fast enough, since a semicrystalline sample was obtained, as indicated by the presence of an endothermic event in the range of 100-120 °C that can be attributed to the melting of the sample. The critical cooling rate was 100 °C/s, since no melting was observed on the subsequent heating scan and the glass transition temperature of 100% amorphous PDoF was close to -5 °C. This value compares well with that of poly(decylene 2,5-furanoate) (PDeF). When subjected to slower cooling, PDoF is progressively more crystalline. The presence of these crystallites constrains the amorphous phase which is reflected in the progressive shift of the glass transition to higher temperature (Figure 4a). Figure 4b shows that for initially amorphous samples obtained by cooling from the melt at 1000 °C/s, the heating rate of 100 °C/s was also critical for the appearance of cold crystallization, upon subsequent heating. For slower rates, the cold crystallization peak was enhanced and shifted downwards to lower temperatures.



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various rates ranging from 10 to 1000 °C/s (60-60,000 °C/min) after cooling from the melt at 1000 °C/s.

3.4 Melting behavior The melting behavior of the sample was initially studied after isothermal crystallization using conventional DSC. Thus, the sample was crystallized to its maximal crystallinity value at various temperatures ranging from 80 to 105 °C and was subsequently heated at 20 °C/min. The times needed to achieve maximal crystallinity increased with increasing Tc, and ranged from 5 min in case of crystallization at 80 °C to 5.5 hr at 105 °C. The results of this procedure can be seen in Figure 5a. For the various measurements the melting enthalpy is ranging from 61 to 73 J g-1. This corresponds to a crystallinity varying between 38 and 46%. These values are quite close with the value previously reported from X-ray diffraction for the as received samples. Analysis of these curves reveals the high complexity of the melting process. For a crystallization temperature ranging between 80 °C and 87.5 °C, a decrease in the melting temperature with increasing the crystallization temperature is observed. According to the Hoffman-Lauritzen theory, an increase of the crystallization temperature would lead to a decrease in the crystallization rate and thus to an increase in crystalline perfection. Above 80 °C, the crystallization rates become slower with increasing temperature, in agreement with the anti-Arrhenian behavior observed for crystallization temperatures much higher than the glass transition. Thus, higher melting temperatures would be expected with increasing the crystallization temperature (Tc). The evolution of the melting temperature with Tc is more complex here as shown in Figure 5a. Therefore, the existence of additional phenomena has to be considered here.



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At low temperatures (Tc

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