Synergistic interaction of renewable nipagin and eugenol for high performance aromatic copoly(ether ester) materials

Naturally occurring nipagin and eugenol were used as the collaborative starting materials for poly(ether ester) materials. In this study, two series of nipagin and eugenol-derived copoly(ether ester)s, PHN1 1-x E1 x and PHN1 1-x E2 x (x = 0%, 5%, 10%, 15%, 20%), were prepared with renewable 1,6-hexanediol as a comonomer. The nipagin-derived component acts as the renewable surrogate of petroleum-based dimethyl terephthalate (DMT), while the eugenol-derived component acts as the cooperative property modifier of parent homopoly(ether ester) PHN1. 1,6-Hexanediol was chosen as the spacer because of its renewability and short chain to enhance the glass transition temperatures ( T g s) of materials. The molecular weights and chemical structures were confirmed by gel permeation chromatograph (GPC), NMR and FTIR spectroscopies. Thermal and crystalline properties were studied by thermal gravimetric analysis (TGA), differential scanning calorimetric (DSC) and wide-angle X-ray diffraction (WXRD). The tensile assays were conducted to evaluate the mechanical properties. The results suggest that properties of such kind of poly(ether ester)s can be finely tuned by the relative content of two components. Synergistic interaction of two structurally distinctive parts endows the materials with high performance.

widespread applications in fields of liquid crystalline material, 14,15 epoxy resin, [16][17][18] benzoxazine, 19,20 coating, 21,22 and Li-S battery. 23 Because of the multifunctionality (phenol and vinyl) and renewable nature of nipagin and eugenol, our group started to investigate polyester materials starting from them, whose thermal, crystalline, mechanical and degradable properties have been systematically investigated. 24,25 The influence of monomer structure on final properties was also studied. 26 However, the comprehensive performance of prepared polyesters was still improvable, regardless of the renewability of starting materials. Either the glass transition temperatures (Tgs) were not high enough, or the mechanical tough was not ideal.
Consequently, further optimizing the performance of nipagin and eugenol-based polymer materials is particularly important for the development and utilization of such materials.
Considering the structural difference of nipagin and eugenol, the symmetrical nipagin units endow the polymers with high crystallinity and good mechanical strength, but poor flexibility and ductility. 27,28 In contrast, eugenol-derived monomer is highly asymmetrical, which endows the polymers low crystallinity and poor mechanical strength, while their flexibility and ductility are good. 24 Taking advantages of the complementary features of these two building blocks through rational molecular design may give the polymeric materials synergistic effect and excellent final properties. In this work, three nipagin and eugenol-derived diester monomers were firstly synthesized (Supplementary information, Fig. S1-S8), then melt polycondensation between diester monomers and 1,6-hexandiol were carried out. 1,6-Hexandiol was chosen due to its renewability and high boiling point compared with short-chain ethylene glycol and 1,4-butanediol. [29][30][31][32] Meanwhile, because the chain length of 1,6-hexandiol is shorter than 1,10-decanediol, the Tgs of obtained polyesters was enhanced obviously compared with our previously synthesized polyesters with 1,10-decanediol as the spacer. 25   This phenomenon indicates that eugenol-derived units are easier to be oxidized than nipaginderived units.

Synthesis and Structures of the Copoly(ether ester)s
Chemical structures of the copoly(ether ester)s were confirmed by 1 H NMR (Fig. S11-S12), 13 C NMR (Fig. S13-S14) and FTIR spectroscopy ( Fig. S15-S16). Furthermore, their chemical microstructures were studied using the quantitative 13 C NMR spectra taking advantage of the sensitiveness of magnetically different carbon atoms present in backbones towards sequence distributions at the dyed level. [33] In the present study, the methylene carbons adjacent to the alcohol-oxygens were well resolved in the 13 C NMR spectra due to the asymmetrical feature of N1 and E1. They exhibited the difference of head and tail when incorporated into the polymer chains during copolymerizing. However, when N1 was copolymerized with E2, N1 displayed the difference of head and tail while the two ester groups in E2 were equivalent. The possible sequence distributions and splitting situation of methylene carbons in 13 C NMR spectra were depicted in Fig. 2

Thermal Properties
Thermal stabilities of the copoly(ether ester)s were investigated by thermogravimetric analysis (TGA). Weight-loss curves and the corresponding derivative curves are depicted in Fig. 3 and Fig. S18-S20, respectively. Thermal property parameters are summarized in Table 2 and the results suggest that PHN11-xE1x and PHN11-xE2x exhibit comparable thermal stability with the parent PHN1. Despite the content of eugenol-derived composition reached 20%, the temperature at which 5% weight loss (T5%) decreased just about 10 °C for both PHN11-xE1x and PHN11-xE2x relative to PHN1. Furthermore, PHN11-xE1x and PHN11-xE2x exhibited almost the same thermal stability regardless that the ratio of eugenol-derived composition was identical or not. Based on the above results we can conclude that the incorporation of eugenol-derived units   (2)x, whose Td values were hold at about 410 °C and found to be insusceptible to the changes of content in eugenol-derived composition. Furthermore, PHN11-xE1x with eugenol-derived composition below or equal to 10% features a two-step degradation mechanism. However, for PHN11-xE1x with eugenol-derived composition above 10% and the whole series of PHN11-xE2x, single-step degradation is observed. This phenomenon can be explained by the mismatch of conformation of polymer chains at these specific compositions.
Conclusion can be drawn from TGA analysis was that the copoly(ether ester)s feature excellent thermal stabilities, and the incorporation of eugenol-derived component actually has good compatibility with the parent PHN1.
Other thermal properties like glass transition (Tg), melting and crystallization temperatures (Tg, Tm, Tc), together with their corresponding melting and crystallization enthalpies (ΔHm, ΔHc) are critical factors for both injection moulding and engineering applications. Here these properties were studied by differential scanning calorimetric analysis (DSC). The DSC heating traces (2 nd ) are shown in Fig. 4, and the analytical data are gathered in Table 2.

Powder X-ray Diffraction Analysis
Powder wide-angle X-ray diffraction analysis (WXRD) was performed to further verify the DSC results. The WXRD traces are depicted in Fig. 5 and Fig. S21, and the diffraction data are collected in Table 3. All the samples were not able to form well discrete diffraction peaks characteristic of amorphous materials. The scattering pattern for PHN1 is characterized by three reflections at 19.04°, 21.92°, and 25.66°, respectively, which corresponds to the triclinic crystal structure just like displayed by PBT. [34,35] Almost the same diffraction pattern was observed for all the poly(ether ester) materials if the diffraction angles and relative intensities were considered, indicating the  Crystallinity index calculated as the quotient between crystalline area and total area. Crystalline and amorphous areas in the X-ray diffraction pattern were quantified using PeakFit v4.12 software.
crystalline mode of PHN1 was maintained in the copoly(ether ester)s. The crystallinity values can be calculated as the quotient between crystalline area and total area of the diffraction traces.
The copoly(ether ester)s were found to be semi-crystalline and the crystallinities decrease with the increase of eugenol-derived composition. The discrepancy of material attribute from DSC and WXRD analysis can be ascribed to the difference of treatment method. The samples for WXRD were directly from precipitation in methanol, resulting in a certain degree of crystallization. The X-ray diffraction results confirm that the materials still feature slightly crystalline properties, rather than completely amorphous materials. The crystallinities gradually decrease with the insertion of eugenol-derived units.

Stress-Strain Behavior
To study the synergistic effect of nipagin and eugenol-based components on mechanical properties, tensile assays of PHN11-xE1x and PHN11-xE2x copoly(ether ester)s were performed on dumb-bell shaped specimens (12 × 2 × 0.5 mm 3 ), which were prepared by casting the  chloroform solution (0.1 g mL -1 ). The stress-stain curves were depicted in Fig. 6 and the mechanical property data were summarized in Table 4. Young's modulus and tensile strength were found to decrease with the increase of eugenol-derived units in each series of samples. Furthermore, Young's modulus and tensile strength for PHN11-xE1x was slightly higher than those of PHN11-xE2x when the content of eugenol-derived composition was the same. However, the elongations at break firstly decrease and then increase with the content of eugenol-derived composition increasing. For example, elongations at break decreased from 14.5% for PHN1 (the data of PHN1 here are original) [36] to 7.0% for PHN190%E110%, and then increased to 10.0% for PHN180%E120%. The conclusion that can be drawn from tensile assays is that the incorporation of eugenol-derived composition impedes the crystallization of copoly(ether ester)s significantly and thus caused the lower Young's modulus and tensile strength, together with the tunable elongation at break. The harmony synergistic effect of nipagin and eugenolderived composition could result in Young's modulus, tensile strength or elongation at break in a desired application scope.

Discussion
In order to prepare further sustainable and practical polymer materials from naturally occurring biomass, renewable nipagin and eugenol-based aromatic copoly(ether ester)