Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Inside PEF: Chain Conformation and Dynamics in Crystalline and Amorphous Domains Catarina F. Araujo,*,† Mariela M. Nolasco,† Paulo J. A. Ribeiro-Claro,† Svemir Rudić,‡ Armando J. D. Silvestre,† Pedro D. Vaz,†,‡ and Andreia F. Sousa*,†,§ †
CICECO − Aveiro Institute of Materials, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal ISIS Neutron & Muon Source, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, U.K. § CEMMPRE, Department of Chemical Engineering, University of Coimbra, 3030-790 Coimbra, Portugal ‡
S Supporting Information *
ABSTRACT: A thorough vibrational spectroscopy and molecular modeling study on poly(ethylene 2,5-furandicarboxylate) (PEF) explores its conformational preferences, in the amorphous and crystalline regions, while clarifying structure−property correlations. Despite the increasing relevance of PEF as a sustainable polymer, some of its unique characteristics are not yet fully understood and benefit from a deeper comprehension of its microstructure and intermolecular bonding. Results show that in the amorphous domains, where intermolecular interactions are weak, PEF chains favor a helical conformation. Prior to crystallization, polymeric chains undergo internal rotations extending their shape in a zigzag patternan energetically unfavorable geometry which is stabilized by C−H···O bonds among adjacent chain segments. The zigzag conformation is the crystalline motif present in the α and β PEF polymorphs. The energy difference among the amorphous and crystalline chains of PEF is higher than in PET poly(ethylene terephthalate) and contributes to PEF’s higher crystallization temperature. The 3D arrangement of PEF chains was probed using inelastic neutron scattering (INS) spectroscopy and periodic DFT calculations. Comparing the INS spectra of PEF with that of poly(ethylene terephthalate) (PET) revealed structure−property correlations. Several low-frequency vibrational modes support the current view that PEF chains are less flexible than those of PET, posing greater resistance to gas penetration and resulting in enhanced barrier properties. The vibrational assignment of PEF’s INS spectrum is a useful guide for future studies on advanced materials based on PEF.
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INTRODUCTION The second half of the 20th century witnessed the rise of oilbased synthetic polymers, whose outstanding properties led to a veritable increase in the standards of living and technological advancement. While the production volume of fossil fuel plastics is still on the rise, their future growth is fraught with complex challenges, such as the depletion of fossil resources, oil price fluctuations, political instability in oil-producing countries, and the environmental impact of plastic waste. The way forward calls for a paradigm shift in polymer production to bring forth a new class of sustainable materials derived from renewable resources which are intended to gradually phase out their petrochemical counterparts.1−8 A successful example of this new paradigm is the replacement of poly(ethylene terephthalate) (PET), a petro-based highperformance polyester widely used as packaging material, by poly(ethylene 2,5-furandicarboxylate) (PEF).9−12 PEF, often designated by the short name “polyethylene furanoate”, is a polyester based of 2,5-furandicarboxylic acid (FDCA) and ethylene glycol (EG) which can be obtained from sugars. PET may also be produced from bio-based monomers,13 although the technology is still in early stages and might prove unfeasible © XXXX American Chemical Society
for scaling up. Meanwhile, PEF has drawn a great deal of interest from food and beverage industries as replacement for PET, not only due to PEF’s low carbon footprint but also owing to its competitive properties. Compared to PET, PEF offers good mechanical performance, comparable thermal stability, and increased barrier propertiesit is 10 times less permeable to oxygen and 20 times less permeable to carbon dioxide. In partnership with Danone and Coca-Cola, Synvina (joint venture of BASF and Avantium) has developed a costeffective process14 to produce PEF and is now implementing its scale-up so that PEF bottles are expected to launch in 2018. It is then prime time to better understand PEF’s structure and properties. A series of studies11 have dealt with its thermal,15−29 mechanical15,17,19,20,22,28,30 and barrier properties,17,23,30−34 kinetics and dynamics of crystallization,21,22,24,25,30,35 and structural characterization.15−18,20,21,23−25,27,28,36,37 A study by Dimitriadis23 and colleagues combined differential scanning calorimetry (DSC) and dielectric spectroscopy to suggest that Received: January 25, 2018 Revised: April 23, 2018
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DOI: 10.1021/acs.macromol.8b00192 Macromolecules XXXX, XXX, XXX−XXX
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In view of completing this vibrational portrait, a sophisticated neutron technique, inelastic neutron scattering (INS), was used, for the first time, to study PEF. The INS spectrum of PEF is analyzed and compared with that of PET in view of finding spectral evidence that justifies differences in their macroscopic properties, with an emphasis on gas permeability. INS is an interesting technique for studying polymers,48−54 as it allows probing the low-frequency region of vibrational spectra. While the internal mode region (200−4000 cm−1) informs upon chain conformation and intermolecular contacts, the lowfrequency vibrational modes stem from cooperative movements of whole chains relative to one another and inform upon their 3D spatial arrangement and collective dynamics. Additional advantages of INS are its pronounced sensitivity to the motions of hydrogen atoms and the absence of selection rulesall vibrational modes are permitted. The assignment of PEF’s INS spectrum herein presented is relevant as a guide for future structural studies on PEFand analogue polymersin confinement55,56 and composite materials,57−59 among other applications.
the microstructure of PEF is best described by a three-phase model: a crystalline fraction, a mobile amorphous fraction where molecular chains are looseand, at their interface, a rigid amorphous fractionwhere the movement of polymeric chains is restricted by the neighboring crystalline segments. The crystalline structure of PEF was recently determined, using Xray fiber diffraction, by Mao et al.,36 who report that uniaxially stretched PEF crystallizes in the P21 space group, with a monoclinic unit cell with density of 1.562 g/cm3. PEF’s repeating unit, depicted in Figure 1, comprises two ethylene
Figure 1. Skeletal formula of poly(ethylene 2,5-furandicarboxylate)’s repeating unit. Dashes delimit the ethylene glycol (EG) moiety and the 2,5-furandicarboxylic acid (FDCA) moiety.
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EXPERIMENTAL SECTION
Synthesis of PEF. First, dimethyl 2,5-dimethylfurandicarboxylate (DMFDC) was synthesized following a previously reported procedure.60 Briefly, DMFDC was prepared by reacting FDCA (30 g, 192.2 mmol) with an excess of methanol (364 mL), under acidic conditions (HCl, 15 mL), at 80 °C for 15 h. The reaction mixture was allowed to cool down. The ensuing white precipitate (25 g, 70%) formed in the mixture, at room temperature, was isolated by filtration and thoroughly washed with cold methanol. PEF was synthesized by an adapted polytransesterification reaction procedure previously reported.61 Briefly, in the first step DMFDC (5.0 g, 23.1 mmol), an excess of EG (3.9 mL, 69.4 mmol), and the tetrabutyl titanate catalyst (1 wt % relative to the total monomers weight) were mixed and heated progressively from 160 up to 190 °C for 5 h and then kept at that maximum temperature under a nitrogen atmosphere for 2 h. Subsequently, in the second step, the reaction was allowed to proceed under vacuum, and the temperature was progressively increased to approximately 210 °C for 1 h and finally kept at that maximum temperature for 2 h. Then, the mixture was purified by dissolving it in hexafluoroisopropanol and pouring the polymer into an excess of cold ethanol (ca. 1 L), filtered, and dried. The ensuing semicrystalline sample is hereinafter designated by “SCPEF” (solvent crystallized). This sample presents an XRD pattern (Figure S9, Supporting Information) corresponding to the polymorphic form labeled β,18 with a crystalline fraction of ca. 60%. Preparation of Amorphous and Cold Crystallized PEF Samples. Additional PEF samples were prepared directly in the differential scanning calorimetry (DSC) device using an adapted protocol, previously reported in the literature.19 DSC scans were carried out in a Setaram DSC92 calorimeter using nitrogen as purging gas (20 mL min−1) and aluminum pans to encapsulate the samples (ca. 5 mg). A quasi-amorphous sample of PEF (AM-PEF) was obtained by heating the sample above its melting temperature (to 250 °C, at 10 °C min−1), maintained at that maximum temperature for 5 min, and then quenched in liquid nitrogen. The XRD pattern of this sample (Figure S9) does not reveal the presence of a crystalline fraction. The cold crystallized sample (CC-PEF) was obtained after a first heating scan up to 250 °C, at 10 °C min−1, maintained at that maximum temperature for 5 min and then quenched in liquid nitrogen to erase the thermal history of PEF. Subsequently, PEF was reheated to three successive isothermal crystallization temperatures of 120.0, 170.0, and 205 °C for a period of time (30, 90, and 60 min, respectively) at 10, 10, and 3 °C min−1, respectively. This sample presents a XRD pattern (Figure S9) corresponding to the polymorphic form labeled α,20 with a crystalline fraction of ca. 60%.
2,5-furandicarboxylate monomers oriented along a 2-fold screw axis. In the unit cell, PEF chains are extended with all ethylene glycol segments (henceforth called “EG”) in the trans conformation and all 2,5-furandicarboxylate moieties (henceforth called “FDCA”) in the syn−syn arrangement (with both carbonyl oxygens pointing in the same direction as the furanic oxygen, hereafter shortly described as “syn”). A recent X-ray diffraction (XRD) study37 suggests that the chain conformation of uniaxially stretched PEF radically differs from other polymorphs (α and β) obtained by cold and solvent induced crystallization. Such claim is disputed in the present study, offering clear spectroscopic evidence that PEF α and β crystals both feature extended synFDCAtransEG chains. Moreover, a thorough investigation of conformational preference for PEF chain segments in the noncrystalline regions is still lacking. Determining which skeletal architecture prevails for polymer strands in the amorphous region is useful for building accurate coarse-grained models of PEF which are then used in mesoscale simulations38−41 to investigate the dynamics and kinetics of crystallization, confinement, gas penetration, fracture, etc. Moreover, its characterization by vibrational spectroscopy techniqueswhich offers a quick and cheap means of quality control in an industrial contexthas been reported15,17,26−28 albeit insufficiently explored. Finding the vibrational fingerprints that identify the amorphous and crystalline regions of PEF constitutes the first step toward realtime monitoring of industrial processes through in-line FTIR and Raman spectroscopy,42−44 as done before in the study of PET.45−47 The present work contributes toward filling these gaps by probing the molecular arrangement of PEF chains in the amorphous and crystalline regions through the lens of vibrational spectroscopy. PEF samples with different polymorphs and similar degrees of crystallinity were studied using infrared and Raman techniques, and the distinct spectral signatures of the amorphous and crystalline regions were identified. The observed spectral differences were then correlated to conformational changes with the aid of ab initio calculations, and a molecular model representative of PEF’s chains in the amorphous segments is proposed. B
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Macromolecules X-ray Diffraction. Quasi-amorphous and semicrystalline (with different polymorphic forms) PEF samples were studied using XRD. XRD powder patterns were collected at room temperature on a Panalytical Empyrean instrument operating with Cu Kα radiation (λ = 1.540 598 0 Å) at 40 kV and 50 mA. Samples were scanned in the 2θ range of 5°−70° with a step size of 0.026° and step time of 67 s. The degree of crystallinity of each PEF sample was estimated using the ratio between the area of crystalline diffraction peaks and the total area (considering in this latter case both crystalline and amorphous contributions). To assess these areas, the diffraction peaks were fitted with a pseudo-Voigt function, and the background was adjusted to a linear segment, provided by HighScore Plus 4.7 software from PANalytical. Spectra Acquisition. The amorphous sample (AM-PEF) and the two semicrystalline samples with different polymorphic forms (SCand CC-PEF) were studied using optical techniques. FTIR-ATR spectra were collected at room temperature on a FT Bruker IFS 55 spectrometer with a Golden Gate ATR accessory using a resolution of 2 cm−1. Raman spectra were measured at room temperature on a Bruker RFS/100S FT-Raman instrument with an Nd:YAG laser and using a resolution of 2 cm−1. The INS spectra of SC-PEF and PET (commercial film, biaxially oriented) were collected using the TOSCA62−64 instrument at the ISIS Neutron and Muon Source of the STFC Rutherford Appleton Laboratory (Chilton, UK). The samples, weighing 2−3 g, were placed inside flat thin-walled aluminum cans which were then mounted perpendicular to the incident beam using a regular TOSCA centered stick. Spectra were collected below 20 K. Samples were “shock-frozen” by quenching in liquid nitrogen before placement in the beam path, therefore preserving the room-temperature morphology of amorphous and crystalline regions. Discrete ab Initio Calculations. Geometry optimizations and vibrational frequency calculations of FDCA and PEF oligomers EG2FDCA, EG3FCDA2, and EG4FDCA3 were computed using the Gaussian 0965 software at the B3LYP level of theory with the 6-311+ +G(d,p) basis set. All the optimized structures were found to be real minima, with no imaginary frequencies. For calculated Raman and infrared spectra, vibrational frequencies were scaled by a factor of 0.967.66 Molecular geometries were rendered using the QuteMol67 software, except for Figure 9 which was rendered using the Mercury 3.868 program. The energy values mentioned throughout the text refer to the electronic energy without zero-point correction. Periodic ab Initio Calculations. Density functional theory (periodic-DFT) calculations under periodic boundary conditions (PBC) were accomplished using the plane wave pseudopotential method as implemented in CASTEP 8.0 code.69,70 All calculations were done using the Perdew−Burke−Ernzerhof (PBE) functional based on the generalized gradient gauge (GGA) approximation.71 The plane-wave cutoff energy was set at 830 eV. Brillouin zone sampling of electronic states was performed on 8 × 4 × 4 Monkhorst−Pack grid. All calculations were done under constant volume with the cell parameters being kept constant during geometry optimization (i.e., only internal coordinates were relaxed). The geometry optimizations used the limited-memory Broyden−Fletcher−Goldfarb−Shanno (LBFGS) algorithm, allowing for an optimized and more intense use of CPU loading resulting in shorter calculation times. Accuracy of the optimization requested residual forces to fall below 0.005 eV A−1. Phonon frequencies were obtained by diagonalization of dynamical matrices calculated using density-functional perturbation theory.72 The finite-displacement phonon calculations were performed at the Γpoint. The calculated atomic displacements in each mode that are part of the CASTEP output enable visualization of the atomic motions and support the assignment of vibrational modes. The simulated inelastic neutron scattering intensities were predicted from the calculated eigenvectors using aCLIMAX, and values were not scaled.73 CASTEP calculations were performed for one PET structure and two PEF conformers. The initial geometries were taken from published data found in the CSD. For PEF the geometries were based on the structures published by Mao and colleagues36 for the synFDCAtransEG tacticity of the polymer. The two structures refer to the
3/12 center-chain staggered (REFCODE: CEJQOA) and to the 5/12 center-chain (REFCODE: CEJQUG) staggered configuration conformers.36 The PET geometry was based on that published by Mak74 (REFCODE: WIMZEX).
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RESULTS AND DISCUSSION Amorphous PEF: How Does It Coil? As stated earlier, PEF crystallizes in extended, zigzag chains36 where the FDCA moiety is in the syn conformation and the EG moiety is in the trans conformation, as shown in Figure 1, yet its architecture in the amorphous domains remains unknown. Polymers tend to crystallize as extended zigzag chains in order to maximize packing efficiency. In the case of PEF, as discussed in another section, the maximization of interchain contacts is an additional factor promoting extended zigzag chains in the crystal. However, in the amorphous regionsas well as in solutionthere is no need for tight packing and intermolecular interactions are relatively weak; therefore, the extended zigzag arrangement is unlikely. In order to increase entropy, in the amorphous domains, polymers tend to adopt a randomly coiled state which follows a statistical distribution of all possible microconformations.75,76 Some conformational patterns are energetically more advantageous and expected to prevail while others are negligibly represented. In order to predict which patterns are energetically favored in the amorphous regions of PEF, we have optimized the geometry of five possible PEF oligomers (Figure 2), in vacuo,
Figure 2. 3D molecular structure of PEF oligomers EG4FDCA3 representing five possible conformations and their relative stability expressed in terms of electronic energy, at the B3LYP/6311+G(d,p) level. antiFDCA and synFDCA indicate respectively whether both carbonyl bonds point away from or in the direction of the furanic oxygen; transEG and gaucheEG indicate respectively a 180° or 60° dihedral angle of the ethylene glycol fragment. C
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Figure 3. Infrared spectra of amorphous (AM-PEF) and semicrystalline (SC-PEF, CC-PEF) samples of PEF, with identification of the bands sensitive to gauche−trans isomerism. Dihedral (O−CH2−CH2−O)gauche ≈ 60°; dihedral (O−CH2−CH2−O)trans ≈ 180°.
A similar approach has been applied previously77−79 to unravel the amorphous structure of PEF’s analogue, PET. PEF and PET are both composed by EG segments with gauche− trans isomerism. The crystalline regions of PET74,80 are like those of PEF, built from stacked extended zigzag chains with all-trans EG segments, giving rise to the bands at 1470 and 1340 cm−1 in the infrared spectrum. Upon melting, trans EG rotates into gauche,78 and these bands disappear.46,78,81−83,77,84,85,47 The 1340 cm−1 band is often used as a probe to monitor the crystallization of PET46 and quantify the crystallinity degree of the final product86a strategy that may be applied in the study of PEF. Hence, it is safe to conclude that in the amorphous melt PEF chains have predominantly gauche EG moieties. However, the residual intensity at 1340 cm−1 in the amorphous PEF spectrum points to the coexistence of a small fraction of trans EG moieties. A crude estimate of the fraction of trans EG form in the amorphous PEF, based on relative band intensities, yields a value of ca. 10%. A similar scenario occurs in the amorphous PET melt, which is estimated to contain 7−14% of trans EG segments.78,87,77 Probing FDCA ConformationSyn or Anti? Considering the molecular models depicted in Figure 2, energetic criteria rules out the synFDCA forms as possible main representatives of the amorphous architecture of PEF, as even the lopsided anti− synFDCA conformation is far from the lowest energy antiFDCA form. To find out experimental support to this conclusion, one can inspect the vibrational modes associated with FDCA. These have no counterpart in PET’s spectra, so conclusions have been drawn from calculations and their comparison with the experimental spectra. As revealed by ab initio calculations (Table S3), the frequencies of two vibrational modesring out-of-plane deformation and CC stretchingvary depending on FDCA’s conformation. For both vibrational modes, the frequency estimated for synFDCA units is consistently lower than that calculated for antiFDCA segments; the infrared spectra of PEF, shown in Figure 4, confirm this ordering. The asymmetrical νCC appears, in SC- and CC-PEF, as a broad band centered at 1577 cm−1 with a shoulder at 1582 cm−1. Likewise, the ring deformation mode, in SC- and CC-PEF, yields a broad band with a maximum at 609 cm−1 and a shoulder at 618 cm−1. The presence of two band components
using ab initio methods. The likelihood of the different chain conformations can be approximated by their electronic energywith the highest energy corresponding to the least stable conformer. Among the four possible chain conformations, the extended synFDCAtransEG arrangement is the least favorable for isolated PEF oligomers. Its polar opposite, the antiFDCAgaucheEG coiled chain, resembling a helix, prevails in vacuo. It is uncertain whether the coiled helix preferred in vacuo is also the one preferred in the amorphous polymer segments a dispute easily settled with the aid of vibrational spectroscopy, as described below. In view of determining which conformational patterns dominate the amorphous regions of semicrystalline PEF, a simple strategy was followed. Three PEF samples were prepared: an amorphous sample (AM-PEF) and two semicrystalline samples with similar degrees of crystallinity, one from solvent crystallization (SC-PEF, 60% crystallinity, β polymorph) and another from cold crystallization (CC-PEF, 60% crystallinity, α polymorph). Their infrared and Raman spectra were collected and compared with those estimated ab initio from the oligomers depicted in Figure 2. By analyzing a few strategic vibrational modes which reveal the conformation of EG (trans or gauche) and FDCA (anti, anti-syn, or syn) segments, one builds a conformational map of PEF’s preferential arrangement in the amorphous melt and crystalline regions. Probing EG ConformationTrans or Gauche? The vibrational modes stemming from PEF’s −CH2− units denounce whether the EG fragment is in the gauche (dihedral (O−CH2−CH2−O) ≈ 60°) or trans (dihedral (O−CH2− CH2−O) ≈ 180°) conformation. The infrared spectra of PEF samples, in the 1300−1500 cm−1 region, are shown in Figure 3. The strong bands at 1340 cm−1 in the spectrum of samples SCand CC-PEF stem from the wagging vibration of CH2 groups in trans EG units. Going from the semicrystalline (SC- and CCPEF) to the amorphous sample (AM-PEF), the intensity of the 1340 cm−1 band drastically decreases while the one at 1370 cm−1, stemming from gauche EG segments, increases. Likewise, the infrared intensity in the 1474−1477 cm−1 region, arising from the deformation of trans CH2 groups, decreases in the amorphous sample, while another appears at 1455 cm−1 due to the presence of the gauche EG conformer. As shown in Table S2, estimated frequency shifts agree well enough with those observed. D
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carbonyl oxygens, depicted in Figure 6, with an average Cring···O distance of 3.7 Å. The infrared and Raman spectra of PEF, herein presented, identify these close contacts as true hydrogen bonds. The vibrational modes of interest, shown in Figure 6, are the in-plane deformation (δ) and the stretching (ν) of furanic C− H and the stretching of the carbonyl CO, as both C−H and CO bonds are sensitive to the formation of hydrogen bonds.88−91 From the amorphous to semicrystalline samples, there is a pronounced intensification in infrared intensity of the νsymCHring mode, along with a red-shift from 3125 to 3118 cm−1, indicative of the formation of C−H···O bonds,88−90 as confirmed by ab initio calculations (Table S4). Further evidence for the formation of C−H···O bonds lies in the δCHring mode, clearly visible in Raman spectra. A single broad band centered at 1020 cm−1 in amorphous PEF (AM-PEF) undergoes a blueshift appearing as the sharp bands at 1045 cm−1 (CC-PEF) and 1038 cm−1 (SC-PEF) in the semicrystalline spectra. The blueshift of the deformation mode is a direct consequence of the increasingly restricted motion of CHring moieties as C−H···O bonds are formed.91 The carbonyl νCO mode is known to be highly sensitive to both intermolecular and intramolecular changes. According to ab initio calculations, the conformational change of the FDCA moiety from syn to anti gives rise to a blue-shift of the νCO mode, while the formation of the intermolecular C− H···O bonds promotes the expected red-shift (Table S4). These competing effects allow the interpretation of the slight changes observed in the νCO band profile. In the amorphous state, νCO gives rise to a broad and highly symmetrical band, which encompasses contributions from a variety of “carbonyl motifs”, from free to H-bonded and from anti to syn, with preponderancy of the former (“free” and “anti”). For the semicrystalline samples, the contribution from the amorphous broad band reduces, and sharper maxima emerge on the νCO band envelope. The maxima suffer small red-shifts as a net result of the opposite effects of anti-to-syn conversion and C−H···O hydrogen bonding. It is worth noticing that both regions in Figure 6 evidence subtle, but meaningful, differences in hydrogen-bonding strength between PEF-SC and PEF-CC samples. In fact, the carbonyl stretching band provides a probe for quick discrimination between PEF polymorphs in highly crystalline samples. The number of C−H···O bonds per chain is maximized among extended synFDCAtransEG strands of PEF chains, so that crystal formation favors extended zigzag chains over coiled helices. Against this claim, one might argue that maximizing packing efficiency is the greater driving force favoring extended chains, while C−H···O contacts play a lesser role. However, the same blueprint is found for 2,5-FDCA, a small, planar molecule, for which the packing efficiency of its syn and anti conformers is similar. In the isolated 2,5-FDCA molecule, the anti conformer is energetically favored (Figure 7a), but the crystal structure92 is built from the less stable syn conformerthe one that enables C−H···O bonding (Figure 7b). In the 2,5-FDCA dimer, each C−H···O bond energy is estimated to be 7.5 kJ/mol from a total stabilization energy of 15 kJ/mol, comfortably compensating for the 2 × 3.2 kJ/mol penalty inherent to the syn conformation. In PEF, C−H···O bonds are expected to be even stronger, according to an ab initio estimate of two hydrogenbonded EG3FCDA2 oligomers, yielding 10 kJ/mol per individual bond.
Figure 4. Infrared spectra of amorphous (AM-PEF) and semicrystalline (SC-PEF, CC-PEF) samples of PEF, with identification of the bands sensitive to syn−anti isomerism. Dihedral (Oring−Cring−C O)syn ≈ 0°; dihedral (Oring−Cring−CO)anti ≈ 180°.
reveals the existence of both synFDCA and antiFDCA moieties, and their relative intensity informs upon which conformer is dominant. In crystalline regions, synFDCA moieties prevail, contributing to the maximum intensities at 1577 and 609 cm−1 in the spectra of semicrystalline PEF. In the amorphous melt, synFDCA moieties lose ground and antiFDCA prevailsas revealed by the higher intensity at 1582 and 618 cm−1 in AM-PEF’s spectrum. Nevertheless, the contribution of syn conformations to the amorphous structure is not less than 10%, reflecting the statistical distribution of all possible conformations expected to be found along the disordered amorphous chains. C−H···O Contacts Favor Extended PEF Chains. As previously suggested, in the absence of intermolecular contacts (in vacuo), PEF strongly prefers a helical conformation. The energy difference between the coiled helix (antiFDCA gaucheEG) and the extended zigzag arrangement (synFDCA transEG) is 28 kJ/mol (Figure 2). Indeed, in the amorphous phase, where the degree of intermolecular bonding is low, helical chain segments are still predominant, although minor fractions of other conformations are present, as attested by infrared spectroscopy. The large energy gap between the helical (amorphous) and the extended (crystalline) PEF chain is a contributing factor to PEF’s higher crystallization temperature (186 °C) relative to PET (152 °C).25 The same rationale can be used to explain why strain-induced crystallization of PEF requires higher stretch ratios than that of PET.30 Before crystallization, PEF undergoes two conformational transitions: the gaucheEG− transEG rotation, increasing energy by 12 kJ/mol, and the antiFDCA−synFDCA transition, resulting in a 16 kJ/mol increment. On the other hand, PET chains only increase their energy by 10 kJ/mol after undergoing the gaucheEG−transEG rotation, since the syn and trans conformations of the CO moieties are pratically isoenergetic.78 The greater ease of chain rearrangement in PET allows crystallization to begin at a lower temperatureor lower stretch ratiosthan that of PEF. It might even seem surprising for PEF to prefer an extended zigzag conformation in the crystal at such high energy penalty. What, then, drives the scales in favor of extended chains when packing into a PEF crystal? Packing efficiency alone tends to favor zigzag conformations, for most polymers. However, in the case of PEF, there is an additional driving factor: the maximization of C−H···O contacts among adjacent chain segments. According to PEF’s crystal structure,36 there are close contacts among the furanic hydrogens and the adjacent E
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Figure 5. Raman spectra of amorphous (AM-PEF) and semicrystalline (SC-PEF, CC-PEF) samples of PEF, highlighting the CH deformation (δCHring) and stretching (νCHring) and CO stretching (νCO) vibrational modes of FDCA moieties.
In addition, the spectra of α and β forms present the same profile for the 609 and 1577 cm−1, compatible with the syn configuration of the FDCA fragment. It is highly unlikely that these identical profiles arise from both the syn and syn−anti configurations proposed for β and α forms, respectively. Another point that benefits from the analysis of the vibrational spectra regards the nature of C−H···O contacts in the polymorphs. Both α and β structures proposed diverge from the nearly symmetric C−H···O alignment reported for the α′ polymorph, which means that both the two CO groups and the two C−H bonds in each FDCA fragment become very distinct. For instance, in the proposed β structure, one carbonyl group is engaged in a very short C−H···O contact (Cring··· Ocarbonyl < 2.4 Å) while the other is free. In the proposed α form, none of the carbonyl groups have C−H bonds in its vicinity, but they diverge in the syn/anti configuration. It is highly unlikely that such distinct carbonyl groups do not translate in larger differences in the νCO mode. In the same way, the disparity of C−H bonds environment in α and β forms is not compatible with the observed vibrational profiles in the νCH stretching region, which are identical for all three forms (α, α′, and β). A more straightforward interpretation assigns the observed νCO and νCH profiles to the same molecular motif: two equivalent C−H bonds and two equivalent CO bonds, engaged in C−H···OC contacts, as reported for the α′ polymorph.36 The aforementioned findings are summarized in Figure 5, depicting the preferred conformation of PEF chains in crystalline (synFDCAtransEG) and amorphous (antiFDCAgaucheEG) regions and listing specific vibrational signatures of each domain. Although it is not possible to draw a defined crystal structure from the vibrational data, it is clear that both α and β polymorphs are built from synFDCAtransEG chains interlocked by C−H···O hydrogen bonds. The molecular structure herein proposed as the most representative of amorphous PEF segments is available as Supporting Information in the form of a .pdb file. This is a basilar point for future works in molecular dynamics and other mesoscale studies, although care must be taken to acknowledge its oversimplicity and incompleteness and adequately compensate for it by optimizing a larger system while allowing the dihedral angles of the polymer backbone sufficient degree of freedom. Structure−Property Relationships through INS. Despite their structural similarity, PEF and PET differ greatly in macroscopic properties such as density, gas permeability, elastic modulus, and melting temperature. These differences arise
The view of C−H···O bonds as drivers of conformational preference is gaining increasing traction.88,93,94 Taylor95 characterized C−H···X (X = O, N, F, Cl) interactions across 600 crystal structures and found that C−H···O contacts are 2.7 times more likely to occur than if packing was random. A DFT study by Jackson96 and his team found C−H···O and C−H···N interactions to act as conformational locks in small molecules and polymers. As the present study demonstrates, PEF can now be added to the list of systems where C−H···O bonds play a pivotal role in molecular packing. Besides driving conformational preferences, the presence of C−H···O contacts is likely to influence PEF’s melting temperature. Thiyagarajan and colleagues16 compared the thermal properties of PEF with those of analogue polyesters prepared with isomers of 2,5-FDCA, namely 3,4-FDCA. While 2,5-PEF melts at 209 °C 3,4-PEF melts at 155 °C. The lower melting temperature of 3,4-PEF is likely due to the absence of C−H···O contacts, which are geometrically unfeasible in this isomer. Therefore, adjacent chain segments in crystalline regions of 3,4-PEF are held together by weaker contacts, requiring less energy to disrupt their ordered arrangement than in the case of C−H···O bound 2,5-PEF sheets. Crystal Structure: What Can Be Learned from Vibrational Spectra. The formation of different polymorphic forms of PEF depending on the preparation methodlabeled α, α′, and βhas been reported previously.18,20 In a recent paper, Maini and colleagues37 identified the existing crystal structure36 (present in uniaxially stretched PEF) as the α′ form20 and propose new crystal structures for the α and β polymorphs from X-ray diffraction data. The crystalline fractions of CC-PEF and SC-PEF samples herein reported correspond to α and β polymorphs, respectively, and the knowledge of their crystal structure is of utmost interest. Unfortunately, the crystal structures proposed by Maini and colleagues37 for the α and β forms are not reliable. Despite the good agreement between the observed and calculated diffraction profiles claimed by the authors,37 the structures lack chemical meaning, as they present unrealistic structural features, such as bond angles around a C(sp3) atom in the 130°−156° range (leading to almost linear O−CH2−C fragments) and CH2−CH2 bond distances larger than 1.8 Å. Moreover, vibrational spectra published by Maini et. al. in their Supporting Information 37 display the undisputed signature of the trans configurationevident in the strong intensities at 1340 and 1477 cm−1while the proposed structures for α and β forms37 present the EG fragment in the syn and gauche configuration, respectively. F
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Figure 6. Molecular model for the anti and syn conformers of 2,5FDCA (a) and of a 2D sheet of the 2,5-FDCA crystal (CCDC Refcode: FURDCA) (b). At the B3LYP/6-311++G(d,p) level, the ΔE(syn−anti) in the isolated molecule is 3.2 kJ/mol. Figure 8. Low-frequency region of the INS spectra of PEF (above, black) and PET (below, black) collected using the TOSCA instrument, at 20 K. Estimated INS intensities for PEF (above, blue) and PET (below, red) as estimated in silico by CASTEP. The vibrational modes that occur in both polymers are assigned.
having been known for decades and confirmed through several X-ray diffraction studies, the same is not the case for PEF. Mao et al.’s recently proposed crystalline model36 of PEFalthough referring to a uniaxially stretched sample and described as the α′-polymorphwas assumed to correspond to the best structure available and used in CASTEP calculations to estimate its INS spectrum. The estimated/observed correspondence is remarkable (Figure 8 and Table S1) despite the experimental INS spectrum of PEF being that of a semicrystalline sample (including the contributions of the amorphous region) while the estimated spectrum assumes a perfect PEF crystal of a different polymorphic form. This results from two concurrent effects. First, the crystalline fraction gives rise to well-defined bands, while the amorphous fraction tends to produce broader features. Second, the vibrations are mainly intramolecular and not affected by packing within the INS resolution. Of course, this is not the case for the low-frequency motions, which are mainly intermolecular and responsive to packing structure. Nevertheless, the agreement between experimental and calculated spectra below ca. 300 cm−1 is good enough to provide a fair assignment of the observed SCPEF bands. The low-frequency vibrational modes of PEF and PET were visualized in order to select those that occur in both polymers. A listing of the most important modes in the region 0−700 cm−1 is provided in Table 1. Describing these low-frequency modes in a succinct manner is no trivial task as the collective motion often results from the concerted movements of many discrete oscillatorsthat is, intermolecular and intramolecular vibrations tend to mix. 98 Therefore, the listed mode descriptions do not reflect their complex and coupled nature, rather focusing on the most relevant contributions to each observed band. CASTEP predicts collectiveor phonon-likemodes to fall under 100 cm−1 for both PEF and PET. The ill-defined band centered at 31 cm−1 in the INS spectrum of PEF results from the concerted bending of the polymeric skeleton resembling the
Figure 7. Short list of PEF’s conformationally sensitive vibrational modes along with 3D models of the most representative molecular arrangement in crystalline and amorphous segments.
since, at the microscopic level, the polymer chains of PEF and PET vibrate following complex patterns characteristic to each, and these patterns determine the macroscopic response of the polymer to inputs such as heat, tension, pressure, etc. The lowfrequency vibrational modes, including lattice modes collective movements of the whole chainand skeletal torsions and deformations are especially relevant for drawing spectrum− structure−property correlations. Low-frequency modes are frequently inaccessible by conventional infrared and Raman techniques but readily observable in INS spectra, as illustrated in Figure S3 comparing PEF’s infrared, Raman, and INS spectra. While the INS spectrum of PEF is novel, originally reported herein, that of PET had already been studied in 1975 by Berghmans et al.97 although poor instrument resolution hampered the spectral analysis performed at the time. Therefore, a new INS spectrum of PET (commercial film, biaxially oriented) was collected for this work, taking advantage of the much improved instrument resolution available nowadays. The INS spectrum of PEF is compared with that of PET, both shown in Figure 8 along with the corresponding estimated spectra, resulting from periodic ab initio (CASTEP) calculations. CASTEP calculations require the definition of a crystalline structure (in fact, calculations are for a perfect crystal) and were based on the crystal structures of either PET or PEF. While PET’s periodic structure is well established,74,80 G
DOI: 10.1021/acs.macromol.8b00192 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. List of Vibrational Modes (Wavenumber, cm−1) That Appear in Both PEF and PETa PEF
PET observed ref 97
assignment
CASTEP
observed
CASTEP
this work
standing wave-like collective mode I standing wave-like collective mode II librational collective mode “seesaw” sym CH2− CH2 torsion sym R-ϕ-R (ring flip) “seesaw” asym CH2−CH2 torsion CH2−CH2 out-of-plane deformation asym R-ϕ-R in-plane deformation asym Cring−(CO)−O in-plane deformation asym R-ϕ-R out-of-plane ring deformation
32
31
25
36
63
73
36
36
89
∼100
72
84
124
∼134
39
36
149
160
118
116
119
159, 185
160, 200
113
116
119
222 263
222 272
178, 216 268
197, 234 278
207
277
290
284
295
335
341
380
385
599
602
408
410
76
416
a
Where feasible, the reported estimated frequencies refer to the wavenumber where INS intensity is predicted to be maximuma value which may result from the sum of more than one contribution. A more comprehensive list is included in the Supporting Information as Table S1.
movement of a standing wave oscillating in a plane normal to a PEF sheet (collective mode I), as depicted in Figure 9a. The analogue mode in PET contributes to the maximum at 36 cm−1. The frequency of collective mode I does not vary much from PEF to PET since the stack of PEF sheets is likely to allow interactions of the π−π type among vertically arranged furan rings,99 similarly to what occurs in PET.100 Upfield, at 73 cm−1 in the INS spectrum of PEF, is a maximum stemming from a similar standing wave-like mode (collective mode II), oscillating in the plane of a PEF sheet, as depicted in Figure 9b. The corresponding mode in PET is another contribution to the maximum observed at 36 cm−1. The reason for this discrepancy is that the collective mode “II” significantly perturbs the C−H···O interactions linking adjacent PEF chains, an aspect that is absent in PET, so that less energy is needed to excite the collective mode “II” in PET than it is for PEF. The unit cell density is another factor influencing collective modes, such as the librational mode found at ca. 100 cm−1 in PEF and 84 cm−1 in PET. This mode consists in a concerted restricted rotation of the whole chain, relative to the unit cell’s longitudinal axis, as depicted in Figure 9c. Higher packing density leads to a higher frequency of the collective librational mode. The unit cell of the PEF crystal36 has a density of 1.56 g/ cm3 while the equivalent for PET74,80 is 1.46 g/cm3; therefore, the higher frequency of PEF’s rotational mode directly stems from its denser packing unit. Above 100 cm−1 and below 200 cm−1 are various complex skeletal modes. Modes below 204 cm−1 are expected to contribute significantly to physical processes that take place at
Figure 9. 3D perspectives of PEF’s crystal structure illustrating the three first collective modes: (a) view of the bc plane showing the standing wave-like collective mode vibrating along the b axis; (b) view of the ac plane showing the standing wave-like collective mode vibrating along the a axis; (c) stereoview of the ab plane showing the librational collective mode (hindered rotation of polymer chain).
room temperature (293 K)101 such as gas permeation, which is most problematic during product storage. Gas permeability directly correlates with chain mobility, which is reflected in several low-frequency modes. One of these modes, the ring flipping of the furan ring, has been identified before through solid-state NMR17 as a hindered motion. In comparison, PET’s phenyl ring rotates freely, facilitating the penetration of gas molecules and contributing to its higher permeability, relative to PEF.31,32 This fact has a huge, positive impact in packaging applications since PEF’s barrier properties are enhanced compared to PET.32 Our INS data are in accordance with PEF’s hindered ring flipping observation described by Burgess and colleagues.17 The ring flipping motion (τ R-ϕ-R) can be described as a concomitant torsion of the COO moiety and the furanic ring around the Cring−C bond, estimated to give rise to the higher frequency component of the broad band at 160 cm−1 in PEF’s INS spectrum. The analogue mode in PET is in the higher frequency side of the 116 cm−1 band, confirming its H
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Macromolecules lower torsional barrier which facilitates PET’s ring flipping motion. Another vibration accompanies ring flipping, undergoing a red-shift of the same magnitude when going from PEF to PET, hinting that it is directly related to chain mobility: the “seesaw” CH2−CH2 mode, which consists of a rotational movement of the whole CH2−CH2 moiety relative to the chain axis. This internal mode is not related to CH2−CH2 torsion (a mode found to be very similar in PET and PEF) but rather with a complex combination of torsion around the Ccarbonyl−Oether bond and ring tilting. In PEF, the highest frequency component of the asymmetrical “seesaw” mode is found at 200 cm−1. Its counterpart in PET lies in the lower frequency side of the 116 cm−1 band. The symmetrical “seesaw” mode follows the same trend. In PEF, “seesaw”sym is the major contribution to the broad band centered at 134 cm−1 while its counterpart in PET is found far downfield, at 36 cm−1. The greater chain rigidity of PEF, compared to PET, stems partly from interlocking C−H··· O hydrogen bonds and partly from the intrinsic hardness conferred by the furanic ring relative to the phenyl moiety. The prime vibrational mode for assessing ring rigidity is the out-ofplane ring deformation (δring), a very intense mode sitting at 602 cm−1 in PEF’s INS spectrum, while the corresponding one for PET is found almost 200 cm−1 downfield, at 410 cm−1.
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nanoparticles, among other relevant materials, through the lens of vibrational spectroscopy.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00192. FTIR-ATR, Raman, and INS spectra of PEF samples; INS spectrum of PET; atom numbering of PEF monomer; figures and tables comparing experimental and calculated vibrational spectra; XRD powder pattern of PEF samples (PDF) Molecular structure proposed as the most representative of amorphous PEF segments (PDB)
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AUTHOR INFORMATION
Corresponding Authors
*(C.F.A.) E-mail
[email protected]. *(A.F.S.) E-mail
[email protected]. ORCID
Catarina F. Araujo: 0000-0002-2753-2651 Mariela M. Nolasco: 0000-0002-4622-5307 Paulo J. A. Ribeiro-Claro: 0000-0001-5171-2153 Armando J. D. Silvestre: 0000-0001-5403-8416 Pedro D. Vaz: 0000-0001-9720-9801
CONCLUSIONS
This study explored the conformation and dynamics of poly(ethylene 2,5-furandicarboxylate), in the crystalline and amorphous regions, using a combination of vibrational spectroscopy and ab initio calculations. In the crystalline region, PEF chains are in the extended zigzag arrangement. This strained synFDCAtransEG conformation is unfeasible for single chains but advantageous in the crystal, since it enables the formation of an extensive array of C−H···O bonds which lock in place adjacent PEF segments. In the amorphous regions, PEF chains tend to coil following a statistical distribution of possible microconformations, the most prevalent of which is the antiFDCAgaucheEG helix. A significant energy difference separates the helical amorphous chain from the extended crystalline motifalmost 3 times the difference found for PET. Consequently, relative to PET, PEF’s crystallization temperature is higher, and strain-induced crystallization requires higher stretch ratios. Vibrational spectroscopy provides a clearer view inside PEF polymorphisma matter of some controversy. Although it is not possible to draw a defined crystal structure from the vibrational data, it is clear that both α and β polymorphs are built from synFDCAtransEG chains, interlocked by C−H···O hydrogen bonds. The packing differences between α and β polymorphs reflect on the carbonyl stretching band profile, which, in this way, provides a probe for quick discrimination between PEF polymorphs in highly crystalline samples. The assignment of PEF’s INS spectrum, a novelty, allowed its comparison with that of PET, revealing distinct low-frequency vibrational profiles which relate to differences in their macroscopic properties. Of particular relevance are those vibrational modes involving the furanic ring (the “ring flipping” mode) and the glycolic moiety (the “seesaw” mode) which reflect PEF’s stiffer polymeric chains, relative to PET, a feature contributing to the lower gas permeability of PEF. The vibrational assignment of PEF herein presented set the grounds for future studies on PEF composites, blends, and
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145FEDER-007679 (FCT ref. UID/CTM/50011/2013), financed by Portuguese funds through the FCT/MEC and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement. FCT and POPH/FSE are gratefully acknowledged for funding a postdoctoral grant to A.F.S. (SFRH/BPD/ 73383/2010). FCT is gratefully acknowledged for funding a PhD grant to C.F.A. (SFRH/BD/129040/2017) and a researcher contract to M.M.N. (IF/01468/2015) under the program IF 2015. The STFC Rutherford Appleton Laboratory is thanked for access to neutron beam facilities. CASTEP calculations were made possible due to the computing resources provided by STFC Scientific Computing Department’s SCARF cluster.
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DOI: 10.1021/acs.macromol.8b00192 Macromolecules XXXX, XXX, XXX−XXX
