Fully Biobased Superpolymers of 2,5-Furandicarboxylic Acid with Different Functional Properties: From Rigid to Flexible, High Performant Packaging Materials

In the present paper, four fully biobased homopolyesters of 2,5-furandicarboxylic acid (2,5-FDCA) with a high molecular weight have been successfully synthesized by two-stage melt polycondensation, starting from the dimethyl ester of 2,5-FDCA and glycols of different lengths (the number of methylene groups ranged from 3 to 6). The synthesized polyesters have been first subjected to an accurate molecular characterization by NMR and gel-permeation chromatography. Afterward, the samples have been successfully processed into free-standing thin films (thickness comprised between 150 to 180 μm) by compression molding. Such films have been characterized from the structural (by wide-angle X-ray scattering and small-angle X-ray scattering), thermal (by differential scanning calorimetry and thermogravimetric analysis), mechanical (by tensile test), and gas barrier (by permeability measurements) point of view. The glycol subunit length was revealed to be the key parameter in determining the kind and fraction of ordered phases developed by the sample during compression molding and subsequent cooling. After storage at room temperature for one month, only the homopolymers containing the glycol subunit with an even number of −CH2– groups (poly(butylene 2,5-furanoate) (PBF) and poly(hexamethylene 2,5-furanoate) (PHF)) were able to develop a three-dimensional ordered crystalline phase in addition to the amorphous one, the other two appearing completely amorphous (poly(propylene 2,5-furanoate (PPF) and poly(pentamethylene 2,5-furanoate) (PPeF)). From X-ray scattering experiments using synchrotron radiation, it was possible to evidence a third phase characterized by a lower degree of order (one- or two-dimensional), called a mesophase, in all the samples under study, its fraction being strictly related to the glycol subunit length: PPeF was found to be the sample with the highest fraction of mesophase followed by PHF. Such a mesophase, together with the amorphous and the eventually present crystalline phase, significantly impacted the mechanical and barrier properties, these last being particularly outstanding for PPeF, the polyester with the highest fraction of mesophase among those synthesized in the present work.


INTRODUCTION
It is now recognized by all, academics, industrialists, politicians and civil society, that the challenges regarding plastics waste treatment are multifaceted and complex and, as numerous studies have indicated, have further worsen with time. In fact, world plastics production is expected to reach 34 billion metric tons by 2050, which will result in a parallel increase of global plastic waste of 70% over current levels. 1,2 It has therefore become urgent and imperative to provide effective solutions in solving or at least mitigating the serious problem of the huge growth of plastic waste worldwide. Plastic waste from food packaging, characterized by short life cycle and difficult and cost-effective recycling, due to contamination with food and also to the multilayer structure necessary to guarantee high barrier performances, contribute massively to these large volumes.
In this context, bioplastics have been proven to be an interesting and promising solution, although continuous innovation and global support are essential in order to fully demonstrate bioplastics socioeconomic benefits and to further challenge the status quo of traditional petroleum-based plastics.
In 2004, the US Department of Energy made known the list of 12 high value-added chemicals obtained from sugars (updated in 2010), 3 among which, we can find 2,5-furandicarboxylic acid (2,. This monomer has attracted the attention of important companies, such as ADM, DuPont, Avantium and BASF, interested in its industrial production, being 2,5-FDCA claimed "a sleeping giant" due to its structural similarity with terephthalic acid employed in the production of important thermoplastic polyesters like poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT).
These glycols can in fact be obtained from renewable sources, making the corresponding polyesters 100% biobased.
The polymers synthesized, processed in form of free-standing thin films, have been characterized from the molecular, structural, thermal, mechanical and barrier point of view in order to establish clear property-structure relationships. according to the procedure previously described. 23,44 Briefly, the mixture was heated to 70 °C under stirring up to dissolution, and then cooled down to room temperature. Then, thionyl chloride (in the same molar amount with respect to the diacid) was added dropwise, and the asobtained solution was heated again to 70 °C under stirring for 3 additional hours. The mixture was then quenched in ice, and dimethyl furan-2,5-dicarboxylate (DMF), which precipitated in form of white floccules during cooling, was repeatedly washed using cold methanol. The so obtained solid was dried at 25°C under vacuum for 8 hours and stored under vacuum before use.
Synthesis of high molecular weight homopolymers. Poly(propylene 2,5-furanoate) (PPF), Poly(butylene 2,5-furanoate) (PBF), Poly(pentametylene 2,5-furanoate) (PPeF) and Poly(hexametylene 2,5-furanoate) (PHF) homopolymers syntheses have been carried out in bulk, starting from DMF, the right diol and the two catalysts TBT and TTIP (200 ppm), in a 250mL 6 stirred glass reactor put in a thermostatted bath, according to the standard polycondensation method. A large molar excess of diol with respect to the ester counterpart was used (Table 1), in order to favour the dissolving of the dimethyl ester. Briefly, in the 1 st stage, carried out at 1 atm under pure N 2 flow, the temperature was raised to 170° -190 °C and kept constant for about 3 hours (Table 1). During this time the 90% of the theoretical amount of methanol was distilled off. At the beginning of the 2 nd stage, the temperature was raised up to 220 -230 °C and pressure was gradually reduced (until about 0,1mbar). The synthesis was carried out until a constant torque value was measured ( Table 1).
As well known, the amount of glycol used as well as the second stage time, depend on several factors, among others: glycol volatility, reactivity, capability to solubilize dimethyl furanoate and system viscosity. In this view, the higher 1,3-PDO excess is directly related to its higher volatility and the consequent need to counter its removal from the reactor. It is worth noticing, the experimental conditions here adopted are in line with other articles 13,45,46 reporting the synthesis of furan-based polyesters.

RESULTS AND DISCUSSION
Synthesis and molecular characterization. The chemical structure of the homopolymers object of the present study is reported in Figure 1. As one can see, the polyesters under investigation are formed by a diacid aromatic sub-unit derived from 2,5-FDCA, present in all the polymers, and by different flexible aliphatic glycol sub-units, whose number of methylene groups ranges from 3 to 6. Moreover, within each repeating unit, we can identify two parts: one polar, coming from the furan ring (= 0.7 D) and the other one apolar, related to the glycol subunit.
The polymers discharged from the reactor looked like slightly colored transparent and filamentous rubber ( Figure S1-A and S1-C). As the sample temperature decreased, polymer textures changed according to their glass transition temperature (below/above room temperature) and their crystallization capability, i.e. they turned opaque if crystallization took place during sample cooling to room temperature (see pictures collected in Figure S1-B and S1-E).
Polymer chemical structures were confirmed by 1 H-NMR analysis. All the spectra were consistent with the theoretical ones, as no extra peaks were found ( Figure S2) PPeF, although amorphous and rubbery at room temperature. This anomalous behavior was previously deeply investigated and ascribed to the presence of a 2D-ordered structure characterized by partially aligned furan rings favored by intermolecular C-H····O bonds. 23 The water contact angle values, correlated to the surface hydrophilicity of the compression molded films, are collected in Table 2. As expected, from the results obtained is evident that the longer the glycol subunit (i.e. the higher the number of methylene groups), the higher the value measured, indicating an increase in the hydrophobicity character passing from PPF (lowest WCA value measured for 3 methylene in the glycol subunit) to PHF (highest WCA value obtained for 6 -CH 2 groups).
Thermal Behavior. The as synthesized as well as the corresponding compression molded films were subjected to calorimetric studies after one month of storage at room temperature, in order to uniform their thermal history, being two of the four polymers under study characterized by a T g below room temperature (PPeF and PHF) (see Table 2). First of all, it has to be remarked that in all cases, no substantial differences in the thermal behavior before and after compression moulding were found. The results obtained on the films are listed in Table 2, while the relative I and II scan DSC curves are reported in Figure 2A and 2B, respectively. 13 The homopolymers containing glycol subunit with an odd number of -CH 2 -groups turned out to be amorphous, being characterized just by the presence of the endothermic glass to rubber transition step occurring at 52 and 13 °C, for PPF and PPeF, respectively. PPF I scan is characterized by the presence of a remarkable enthalpy recovery peak at T g , probably due to the compression molding processing. For the same polymer, once exceeded T g , an exothermic peak followed by an endothermic one at higher temperature is detected, with comparable underlying areas (∆H c  ∆H m ): this behavior can be explained on the basis of the presence of some residual nucleating germs, persisting even after filming, that favor cold crystallization upon heating.
However, in the II DSC scan only the T g step can be noticed. As to PPeF, it shows only the endothermic baseline jump at 13 °C related to glass-to-rubber transition in both I and II scan. On the other hand, the two homopolymers containing glycol subunit with an even number of -CH 2groups show the typical behavior of semicrystalline materials, with a glass transition phenomenon followed by an endothermic melting peak at higher temperature. In the range between T g and T m , PBF shows also an exothermic peak, demonstrating its macromolecular chains are capable of rearranging into an ordered structure upon heating. Anyway, being ∆H c < ∆H m , PBF compression molded film can be considered semicrystalline. The heating II scan curve of PBF recorded after fast cooling of the melt is qualitatively the same as the I scan, the only difference being the crystallization and melting enthalpies, which are equal evidencing the effectiveness of the fast cooling process in quenching PBF film. As concerns PHF, the corresponding I scan DSC curve reveals its semicrystalline nature presenting, together with the T g step at 13 °C, an endothermic peak at 140 °C due to the crystals melting. After rapid cooling from the melt, PHF still shows a remarkable melting peak, which indicates that it was not possible, under the experimental conditions adopted, to quench its chains in the amorphous state. As known, semicrystalline materials show different behavior from the same ones in a completely amorphous state. It is generally assumed that crystalline structure can be considered as a physical cross-link, which limits chain mobility and is responsible for higher T g values. 50,51 In order to avoid the dependence of the glass-to-rubber transition on crystallinity, DSC curves after rapid cooling from the molten state have been analyzed and reported in Figure 2B, and the corresponding data collected in Table 2. Unfortunately, this procedure was not effective for PHF, whose measured T g was higher due to the presence of crystallites after fast cooling from the melt. As it can be seen, T g values regularly decrease by increasing glycolic subunit length, since longer aliphatic segments are more flexible and act as internal plasticizers enhancing the macromolecular chain mobility, except for PHF due to its semicrystalline nature even after fast  crystalline reflection is located at the same q value. 53 In this case, it is plausible that the measured intensity maximum could be consider the result of convolution of both, the 010 crystalline reflection and the peak observed for PPF, PPeF and PHF at the same q value. We have previously reported the presence of this peak in PPeF and its relation to the formation of a mesophase consisting of layers of partially ordered furan rings formed during compression molding and favored by the alignment of the intermolecular C−H···O bonds already present in the as-synthesized material. 23 The results obtained in this paper suggest that the mesophase formation is favored by increasing glycolic sub-unit length, since longer diols are more flexible and therefore can act efficiently as internal plasticizers facilitating the formation of 2D ordered domains during compression molding. Moreover, from the results obtained, i.e. mesophase fraction is higher in PPeF than in PHF, we can evict that the crystallites present in PHF act as constraints inhibiting the mesophase formation As a matter of fact, 2D-or 3D-ordered phase formation are competing, one phase forming at the expense of the other, regardless of whether the mesophase (1D-or 2D-ordered phase) may possibly evolve into the 3D crystalline phase. 1D SAXS intensity profiles presented in Figure S3 were obtained by azimuthal integration of the 2D SAXS patterns through the whole q range, after the subtraction of a pattern in the molten state.
The selected temperatures labelled in Figure 5 Figure 2C, while in Table 2, the temperatures corresponding to initial decomposition (T onset ) and to maximum degradation rate (T max ) are listed. From these data, it is possible to see that all the furan-based polyesters are high thermally stable, with T onset above 364 °C. As one can favored. 54 In addition, PBF, PPeF and PHF are characterized by a lower amount of ester groups for chain unit as compared to PPF, these last being more prone to thermal cleavage. Surprisingly, PPeF appeared to be the most thermally stable among the sample investigated, with a thermal stability similar to PEF. 39  The unexpected highest thermal stability of PPeF could be related to a higher H bonds density that, under the pressure applied during the film preparation, drive the mesophase formation. As a matter of fact, the hydrogen bonds between adjacent macromolecules requires an extra energy to be broken with consequent shift to higher temperatures of the main polymer degradation process.
This result is in line with the study carried out by Tsanaktsis et al. 16 showing lower thermal stability for the higher odd methylene groups containing polymer, poly(heptylene furanoate). Its minor thermal stability could be due to the reduction of the H-bonds-producing furan rings in of poly(heptylene furanoate) with respect to poly(pentamethylene furanoate).
In general, the thermal stability of furan-based polyesters is comparable to their terephthalic counterparts, 13,46 with the exception of PPeF resulting more stable than poly(pentamethylene terephthalate). 55 Mechanical characterization. To evaluate the mechanical properties of the polymers synthesized, tensile tests were carried out on compression molded films, by measuring the variation of stress as a function of the deformation applied. The values of elastic modulus E, stress at break σ B and strain at break ε B are collected in Table 3 and shown in Figure 6.
As to the main factors which affect the mechanical behavior of polymers, chain flexibility (i.e. T g value) and crystallinity degree play a key role in the determination of mechanical response.
According to the data obtained, elastic modulus E and stress at break σ b decrease, while elongation at break ε increases with the diol subunit length.
More in details, PPF shows values of elastic modulus and stress at break similar to those of PBF.
These results are due to the proper balance of two opposite factors: PPF is amorphous with higher T g than PBF which, in turn, is semicrystalline. As to PHF, its lower value of E with respect to PPF and PBF can be explained considering that, although semicrystalline, it is in the rubbery state at room temperature. As concerns the elongation at break, we can associate the lowest value for PPF to its limited macromolecular mobility (PPF shows the highest T g ).
Nevertheless, crystallinity degree has also an effect on the ε B value, the crystalline phase acting as discontinuity points within the polymer matrix. As a matter of fact, if PBF and PHF are compared, one can observe that, despite the presence of a longer aliphatic flexible segment along its macromolecular chain, the more crystalline PHF has an elongation at break three times lower than that of the less crystalline PBF. It is also interesting to notice that, comparing PHF to its 23 chain extended counterpart, 44  Among the furan-based family, PPeF turned out to be the one with the lowest values of E (more than 2 orders of magnitude lower than those of PPF and PBF) and σ B (about 4 times lower), together with an outstanding elongation at break ε B of more than 1000%. In addition, while PPF is characterized by a brittle fracture and both PBF and PHF undergo yielding, PPeF shows the typical elastomeric response, i.e. absence of yielding and almost complete recovery after elongation, as previously reported. 23 The particular mechanical response of PPeF, which at room temperature is in the rubbery amorphous state, has been explained on the basis of the presence of mesophase in the polymeric film, revealed by X-ray diffraction technique.
In conclusion, the results obtained from stress-strain measurements indicate that changing glycol sub-unit length represents an efficient tool to tune the polymer mechanical response, permitting to get rigid as well as elastomeric materials.  Gas permeability studies. Gas barrier ability was checked at 23 °C both to dry O 2 and CO 2 , to evaluate the performance of the materials in view of possible applications in food packaging.
The Gas Transmission Rate (GTR) data together with the corresponding permselectivity ratios are listed in Table 4, with GTR values also graphed in Figure 7. The values for PET and for its furan-based counterpart PEF are also reported for sake of comparison in Table 4 and presented in Figure 7. Lastly, in the same Table,    It is well known that semicrystalline polymers are characterized by the presence of an amorphous phase, which below the T g is in the glassy state, coexisting with a crystalline one. 58 As far as the gas barrier properties are concerned, the glassy state, in comparison with the molten one, exhibits reduced chain mobility, i.e. lower free volume through which gas molecules can diffuse. In addition, the presence of crystals can further limit gas diffusion through the polymeric matrix due to high chain packing of the crystal lattice. 59 Besides the amorphous and crystalline phases, in presence of both rigid and flexible moieties along the macromolecular chain, another kind of ordered phase (1D-or 2D-), generally referred to as mesophase, can also develop, this last being very effective in blocking gas passage, even more than crystalline one. 59 As already observed in literature for propene/ethylene copolymers and for poly(butylene 2,5-thiophenate), the presence of crystalline domains limits the mesophase formation, on the contrary the complete absence of any crystallinity allows the development of  According to the literature, 19,33,36,62 furan moieties are an example of mesogenic groups among the wide family of main chain mesogenic-containing polymer liquid crystals (PLCs), which to date, are the most performing polymeric materials in terms of gas barrier ability.
As to the furan-based polymers, the Wide angle X-ray scattering study described in previous section has evidenced the concomitant presence of the rigid furan ring and a flexible aliphatic segment can lead to the development of this peculiar 2D-ordered phase. The mesophase formation, favoured under the compression moulding process, is driven by the alignment of the inter-chain hydrogen bonds already present in the neat material and seem to be maximized in absence of crystals, as for PPeF film. According to the data reported in Table 4, also GTR values 27 follow an even/odd trend, since the odd -CH 2 -number containing polymers, although amorphous, turned out to be more performant than the even -CH 2 -number containing ones. In addition, GTR PPeF < GTR PPF , conversely from what usually happens for traditional polymers, in which higher values of T g imply better performances. PPeF shows the lowest GTR values among the studied samples, being this polymer the one with the highest fraction of mesophase. Such polymer contains indeed a glycolic subunit flexible enough to facilitate the formation of 2Dordered domains during compression moulding. Moreover, its crystallizing ability is so low that the formation of crystalline phase did not take place, in favour of the 2D-ordered one. Lastly, as stated by the same authors in a previous work, 23 there is no significant separation between mesophase and amorphous regions, both characterized by similar electron density. As regards the materials containing an even number of -CH 2 -groups, PBF and PHF, the former appears to be more performant than the latter, despite the lower fraction of mesophase. The higher GTR value for PHF could be due to several factors: i) higher amount of the so-called disclinations, due to the concomitant presence of 3D-and 2D-ordered domains together with the amorphous ones, the disclinations being channels through which gas molecules can easily diffuse worsening the barrier performances; ii) higher fraction of 3D-ordered phase than the more performant 2D-one; iii) lower free volume fraction in PBF (T g,PBF > T room ) than in PHF (T g,PHF  T room ).
Some interesting property-structure correlations can be also extrapolated by analysing the permselectively ratio values. For this aim, it is worth remembering that the gas transmission rate is the sum of two contributions, one related to the gas diffusion rate and the other to the gas solubility in the polymer matrix. The gas diffusion rate essentially depends on two factors: i) the gas molecules size, i.e. the larger the molecules the higher the diffusion speed (molecular diameter of CO 2 = 3.4 Å, oxygen molecular diameter = 3.1 Å and nitrogen molecular diameter = 28 2.0 Å 63 ); ii) the microstructure density, i.e. the higher the density the lower the diffusion rate. As far as the solubility is concerned, it certainly depends on the chemical affinity between the gas molecules and the polymer matrix: molecules containing polar bonds such as carbon dioxide have greater affinity for polar polymeric matrices, such as those under investigation in the present paper because of the presence of a furan ring. Given these premises, the data reported in Table 4 show a general trend of increase in the permselectively ratio with the length of the glycol subunit, in line with a decrease in the surface hydrophilicity of the films. As a matter of fact, the permselectively ratio is highest for PHF, the density of furan rings responsible of the material polar character being the lowest in this polymer due to the longest glycol subunit. Again, PPeF deviates from the general trend, as it is characterized by a permselectively ratio value even less than 1. Such peculiar result could be explained hypothesizing that the effect of the high density of the film microstructure consistently prevails over that of the polymer matrix polarity.
As to BIF values, it is interesting to notice a bigger increase of CO 2 BIF with respect to O 2 one, although CO 2 molecule, being bigger than oxygen one, is characterized by a faster diffusion rate.
Again, this evidence could be explained considering the higher solubility of CO 2 gas molecules with respect to the non-polar O 2 ones in the polymeric matrix. In fact, the dipolar moment on furan moieties favors the interaction with CO 2 molecules also containing dipoles.
In conclusion, taking into account all the factors affecting gas barrier ability, it is not surprising that PPeF is the most performant polymer among those investigated in the present work. The mobility of macromolecular chains at room temperature favours the formation of mesophase, which is the most efficient ordered microstructure in blocking gas passage, at the expense of the crystalline phase, whose formation is completely inhibited, thus also reducing the amount of disclinations.
Furan-based polymers, presenting very good response both from the mechanical and barrier properties point of view without the necessity of realizing multi component materials, allow an easier recycling. In this view, a mechanical recycling can be envisioned. Nevertheless, previous studies 64 have highlighted the need to avoid high processing temperature for preventing degradation.

CONCLUSION
Four high molecular weight 100% biobased homopolyesters of 2,5-furandicarboxylic acid differing from each other for the glycolic subunit length were successfully synthesized by simple and solvent-free polycondensation process and processed into thin films of at least 11 cm diameter by compression moulding.
By this simple chemical modification, it was possible to favour the selective formation of different ordered phases, i.e. mesophase, preferentially developed by the polyesters containing the longest glycol sub-unit (5 and 6 carbon atom number) and the crystalline one mainly present in the homopolymers containing glycol subunit with an even number of -CH 2 -groups. From the results obtained, it can be evicted, in agreement with what is reported in literature that one type of ordered phase forms at the expense of the other. 60,61 The kind, amount and number of ordered phases in the final materials have a strong impact on the functional properties, the mesophase, when present exclusively, demonstrating to be the phase responsible for a very high thermal stability, a mechanical response typical of elastomeric materials and outstanding barrier properties both to oxygen (small non-polar molecule) and to carbon dioxide (large and dipole-containing molecule).
From the results shown in the present paper, it can be evicted that thanks to a small chemical modification, such as different length of glycol subunit, it is possible to get a plethora of This publication is based upon work from COST Action FUR4Sustain, CA18220, supported by COST (European Cooperation in Science and Technology).

Notes
The authors declare no competing financial interest.