Aromatic copolyesters with enhanced crystallizability and mechanical properties by adding the renewable nipagin-based composition

Abstract

To improve the crystalline and mechanical properties of bio-based polyester materials, highly symmetrical nipagin-derived dimethyl ester is used for the synthesis of polyesters. Aromatic copolyesters, PDN2_1−xE1_x and PDN2_1−xE2_x (x = 0%, 10%, 20%, 30%, 40%, and 50%), with high crystallinity are synthesized from bio-based 1,10-decanediol, nipagin and eugenol-derived dimethyl esters via a random copolymerization method. The crystallizability of the copolyesters is emphatically investigated by the content of the two asymmetrical eugenol-derived compositions (i.e. E1 and E2). Not only the appearance of the copolyesters, which present a white powder consistently, but also differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WXRD) and isothermal crystallization analyses verify the strong crystallizability of the copolyesters despite the fact that the crystallinity (X_c), melting point (T_m) and corresponding enthalpy (ΔH_m) are found to gradually decrease with the increase in the eugenol-based composition. Furthermore, the tensile assays demonstrate brittle characteristics of the copolyesters and further confirm the strong crystallizability of the nipagin-based PDN2, a potential candidate for replacement of the petroleum-based polyethylene (butylene) terephthalate (PET, PBT).

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Introduction

At present, study on bio-based materials has attracted significant attention due to the increasing demand for reducing the utilization of petrochemicals and taking full advantage of the added-value of agricultural products and other renewable resources.^1–4 Among the naturally occurring resources, plant oils and carbohydrates occupy a position of crucial importance due to their abundant nature reserves and easy availability.^5–9 Furthermore, starch as a renewable resource has already been used as a starting material via microbial fermentation for the synthesis of polyester materials, i.e. polylactic acid (PLA) and polyhydroxybutyrate-valerate (PHBV), which have been industrialized and exhibited extensive applications spanning from short-term packaging plastics to biomedical devices (i.e. implants, sutures and drug encapsulations) due to their biodegradability and biocompatibility.^10,11 For the purpose of broadening the scope of renewable resources, it is still necessary to explore other renewable substituents from naturally occurring resources.^12–16

Nipagin, the methyl ester of p-hydroxybenzoic acid, which is present naturally in campanulaceae and ericaceous plants,¹⁷ has been widely used as an antimicrobial and antiseptic agent in cosmetics, pharmaceuticals and even foods due to the presence of a phenolic hydroxyl group.^18,19 Furthermore, bio-based nipagin has already been used for the synthesis of various multifunctional polymeric materials, such as epoxy resins and liquid crystal materials.^20–23 However, polyester materials from renewable parabens have been rarely reported recently. The major differences between nipagin and plant oils are found to be the structural symmetry and no other needless reactive functional groups, which make nipagin an excellent candidate for the replacement of the petroleum-based dimethyl terephthalate (DMT).

As mentioned in our previous report,²⁴ the eugenol-based dimethyl esters are highly asymmetrical monomers, leading to the so-formed polyesters, which are all amorphous materials with a low Young’s modulus and mechanical strength and far from satisfactory as structural materials. This is largely attributed to the poor structural symmetry and thus causes the inferior crystallizability of such materials. It is well known that the crystallizability is crucial for the maintenance of the mechanical strength and modulus,^25–28 and should be enhanced as much as possible under the premise that the toughness of materials does not decrease significantly.^29,30 In this study, in order to investigate the crystallizability of the nipagin-based polyesters, highly symmetrical nipagin-based dimethyl ester N2 and its counterparts, asymmetrical eugenol-based E1 and E2, were synthesized. Subsequently, two series of aromatic copolyesters, PDN2_1−xE1_x and PDN2_1−xE2_x, were synthesized. Herein, the relationships between the crystalline and mechanical properties and the content of the eugenol-based composition were emphatically studied. Furthermore, the thermal properties of the copolyesters were also investigated.

Experimental section

Chemical reagents and materials

Nipagin (99%), eugenol (99%), methyl thioglycolate (99%), methyl chloroacetate (98%), 2,2-dimethoxy-2-phenylacetophenone (DMPA, 99%) and tetrabutyl titanate (TBT, 99%) were purchased from Sigma-Aldrich (St. Louis, USA). 1,10-Decanediol (98%) was obtained from Aladdin Chemical Reagent Co (Shanghai, China). Other frequently used chemical reagents are of high purity grade and used as received without further purification. Silica gel slices used for thin layer chromatography (TLC) analysis were purchased from Qingdao Haiyang Chemical Co. Ltd (Qingdao, China).

General instrumentation and methods

¹H NMR and ¹³C NMR spectra were recorded in CDCl₃ at 25 °C on a Bruker AVANCE III NMR spectrometer operating at 400 MHz and 100.6 MHz, respectively. Tetramethylsilane was used as the internal reference. Fourier transform infrared (FTIR) spectra were recorded using a Bio-Rad FTS6000 spectrophotometer at 25 °C. Polymer samples were prepared by grinding the polyesters adequately with KBr powder, followed by compressing the mixture to form a pellet. Fourier transform high resolution mass spectra (FTMS) were recorded on a Varian 7.0T FTMS with positive or negative electrospray ionization (±ESI). The molecular weights and dispersity (D) were determined by size exclusion chromatography (SEC, Waters 2414 differential refraction detector) at 35 °C. CHCl₃ and THF were used as the eluents at flow rates of 1.0 mL min⁻¹. The average molecular weights were calibrated against monodisperse polystyrene (PS) standards. Thermogravimetric analysis (TGA) was carried out using a NETZSCH TG209 instrument. In a typical method, the polymer sample was heated from 25 to 800 °C under a nitrogen atmosphere at a rate of 10 °C min⁻¹. The temperature at which 5% weight loss occurred, the temperature at which the maximum degradation rate occurred, and the residual weight (%) at 800 °C were recorded. Differential scanning calorimetry (DSC) was carried out using a Mettler-Toledo DSC Q100 differential scanning calorimeter. The polymer sample was first heated from 25 to 210 °C and then cooled to −30 °C. The glass transition, melting and crystallization temperatures (T_g, T_m, and T_c) and their respective enthalpy (ΔH_m and ΔH_c) were acquired from the second heating run. All runs were carried out at a rate of 10 °C min⁻¹. Indium and zinc were used as the calibration standards for temperature and enthalpy, respectively. The treatment of samples for isothermal crystallization was as follows: the thermal history was eliminated by heating the sample to 50 °C above its T_m and maintained at this temperature for 10 min, and then it was cooled at 50 °C min⁻¹ to the specified crystallization temperature, where it was left to crystallize until saturation. Wide-angle X-ray diffraction (WXRD) patterns were recorded on a D/max-2500 diffractometer using CuKα radiation with a wavelength of 0.1542 nm for powder samples obtained directly from synthesis. Tensile assays were carried out on dumb-bell shaped specimens (12 × 2 × 0.5 mm³) in triplicate at a stretching rate of 50 mm min⁻¹ at 25 °C on a Testometric AX universal strength testing machine. The Young’s modulus, tensile strength, and elongation at the break were obtained by averaging the data from the three parallel tests. Dynamic mechanical analysis (DMA) was carried out using a Mettler Toledo DMA/SDTA861 dynamic mechanical analyser in tension mode with a preloaded force of 0.1 N, and an amplitude of 10 mm, at a fixed frequency of 1 Hz in the −60 to 60 °C range and with a heating rate of 3 °C min⁻¹. The measurements were carried out in triplicate using rectangular specimens (length × width × thickness = 25 × 5 × 0.5 mm³) obtained from casting of chloroform solutions with concentrations of 0.1 g mL⁻¹.

Synthesis of nipagin and eugenol-derived dimethyl esters N2, E1 and E2

The detailed synthetic routes for the preparation of nipagin and eugenol-derived dimethyl esters N1, E1 and E2 can be found in the ESI† of this article (Scheme S1 and Fig. S1–S7†).

General procedure for the preparation of polyesters

PDN2 homopolyester and PDN2_1−xE1_x and PDN2_1−xE2_x (x = 0%, 10%, 20%, 30%, 40%, and 50%) copolyesters were synthesized from a mixture of 1,10-decanediol, N2 and E1 or E2 with the scheduled composition ratios. The polymerization was carried out using a 50 mL Schlenk round-bottom flask equipped with a magnetic stirrer, a nitrogen inlet and a vacuum distillation outlet. The polymerization strategy is exhibited in Scheme 1. A 1 : 1.05 molar ratio of the diester mixture to diol was adopted to guarantee complete reaction of the dimethyl esters. Tetrabutyl titanate (TBT, 0.6% molar relative to diester) was the catalyst of choice. Before transesterification, the apparatus was vented with nitrogen for 10 minutes to ensure that no oxygen remained in the reaction system and to avoid oxidation during polymerization. Transesterification reactions were carried out at 140–180 °C for 3–5 hours under a low nitrogen flow. Polycondensation reactions were then carried out at 180–220 °C for 3–5 hours under a 0.03–0.06 mbar vacuum until the magnetic stirrer got stuck, indicating completion of the polymerization. The reaction mixtures were cooled to room temperature, and simultaneously atmospheric pressure was recovered with nitrogen to prevent degradation of the resulting polyesters. The raw polyester materials were dissolved in a minimum amount of chloroform and precipitated in an excess of methanol in order to remove the unreacted diols and formed oligomers. Finally, the polyesters were collected by filtration, thoroughly washed with methanol, and dried in vacuo.

Scheme 1 Synthetic routes for the preparation of nipagin and eugenol-based PDN2_1−xE1_x and PDN2_1−xE2_x copolyesters.

PDN2 homopolyester: ¹H NMR (400 MHz, CDCl₃): δ 8.46–7.84 (m, 4H; Ar-H), 7.21–6.63 (m, 4H; Ar-H), 4.55–4.17 (m, 4H; –COO–CH₂–), 4.17–3.87 (m, 4H; ArO–CH₂–), 2.14–1.87 (m, 4H; ArOCH₂–CH₂–), 1.86–1.63 (m, 4H; –COOCH₂–CH₂–), 1.61–1.12 (m, 12H; –COO(CH₂)₂–(CH₂)₃–) ppm; ¹³C NMR (100.6 MHz, CDCl₃): δ 166.37 (Ar-CO–), 162.63 (Ar-C), 131.53 (Ar-C), 122.94 (Ar-C), 114.00 (Ar-C), 67.57 (ArO–CH₂–), 64.77 (–COO–CH₂–), 29.41 (–CO–OCH₂–CH₂–(CH₂)₃–), 29.23 (–COO(CH₂)₂–CH₂–(CH₂)₂–), 28.77 (–COO(CH₂)₃–CH₂–CH₂–), 26.02 (–COO(CH₂)₄–CH₂–), 25.84 (ArOCH₂–CH₂–) ppm; FTIR: 2927, 2855, 1706 (C=O), 1604, 1510, 1251, 1170, 1105, 1009, 975, 849, 771, 696 cm⁻¹.

PDN2_1−xE1_x copolyesters: ¹H NMR (400 MHz, CDCl₃): δ 8.06–7.86 (m, (1 − x)·4H; Ar-H), 6.99–6.81 (m, (1 − x)·4H; Ar-H), 6.81–6.59 (m, x·3H; Ar-H), 4.64 (s, x·2H; ArO–CH₂–CO–), 4.37–4.20 (m, (1 − x)·4H; ArCOO–CH₂–), 4.20–4.12 (m, x·4H; –COO–CH₂–), 4.12–3.93 (m, (1 − x)·4H; ArO–CH₂–), 3.84 (s, x·3H; ArO–CH₃), 3.20 (s, x·2H; –S–CH₂–CO–), 2.72–2.53 (m, x·4H; Ar-CH₂–CH₂–CH₂–S–), 2.06–1.92 (m, (1 − x)·4H; ArOCH₂–CH₂–), 1.93–1.82 (m, x·2H; ArCH₂–CH₂–CH₂S–), 1.82–1.66 (m, (1 − x)·4H; ArCOOCH₂–CH₂–), 1.66–1.54 (m, x·4H; –COOCH₂–CH₂–), 1.49–1.06 (m, 24H; –COOCH₂CH₂–(CH₂)₃–) ppm; ¹³C NMR (100.6 MHz, CDCl₃): δ 170.49 (–SCH₂–CO–), 169.17 (ArOCH₂–CO–), 166.24 (Ar-CO–), 162.64 (Ar-C), 149.60 (Ar-C), 145.68 (Ar-C), 135.74 (Ar-C), 131.49 (Ar-C), 122.88 (Ar-C), 120.28 (Ar-C), 114.76 (Ar-C), 114.02 (Ar-C), 112.66 (Ar-C), 67.58 (ArO-CH₂–CH₂–), 66.74 (ArO–CH₂–CO–), 65.32–64.70 (–COO–CH₂–), 55.89 (ArO–CH₃), 34.21 (–S–CH₂–CO–), 33.62 (Ar-CH₂–CH₂CH₂S–), 32.01 (–S–CH₂–CH₂CH₂Ar), 30.57 (ArCH₂–CH₂–SCH₂–), 29.38–25.74 (m, –COOCH₂–(CH₂)₄–), 25.81 (ArOCH₂–CH₂–) ppm; FTIR: 2927, 2854, 1759 (C=O), 1708 (C=O), 1605, 1511, 1468, 1277, 1253, 1121, 1044, 976, 850, 772, 650 cm⁻¹.

PDN2_1−xE2_x copolyesters: ¹H NMR (400 MHz, CDCl₃): δ 8.07–7.87 (m, (1 − x)·4H; Ar-H), 6.97–6.82 (m, (1 − x)·4H; Ar-H), 6.82–6.59 (m, x·6H; Ar-H), 4.38–4.17 (m, (1 − x)·4H; –COO–CH₂–), 4.17–3.93 (m, x·8H + (1 − x)·4H; ArO–CH₂–CH₂– and –COO–CH₂–), 3.81 (s, x·6H; ArO–CH₃), 3.19 (s, x·4H; –S–CH₂–CO–), 2.75–2.52 (m, x·8H; Ar-CH₂–CH₂–CH₂–S–), 2.07–1.93 (m, (1 − x)·4H; ArOCH₂–CH₂–), 1.92–1.81 (m, x·8H; ArCH₂–CH₂–CH₂S– and ArOCH₂–CH₂–), 1.81–1.66 (m, (1 − x)·4H; ArCOOCH₂–CH₂–), 1.66–1.54 (m, x·4H; –COOCH₂–CH₂–), 1.48–1.13 (m, 24H; –COOCH₂CH₂–(CH₂)₃–) ppm; ¹³C NMR (100.6 MHz, CDCl₃): δ 170.53 (–SCH₂–CO–), 166.25 (Ar-CO–), 162.66 (Ar-C), 149.45 (Ar-C), 146.82 (Ar-C), 134.12 (Ar-C), 131.52 (Ar-C), 122.91 (Ar-C), 120.38 (Ar-C), 114.04 (Ar-C), 113.45 (Ar-C), 112.43 (Ar-C), 68.82 (ArO–CH₂–CH₂–), 67.60 (ArO–CH₂–CH₂–), 65.33–64.74 (–COO–CH₂–), 55.94 (ArO–CH₃), 34.22 (–S–CH₂–CO–), 33.65 (Ar-CH₂–CH₂CH₂S–), 32.05 (ArCH₂CH₂–CH₂–S–), 30.69 (ArCH₂–CH₂–CH₂S–), 29.41–26.04 (m, –COOCH₂–(CH₂)₄–), 25.83 (ArOCH₂–CH₂–) ppm; FTIR: 2928, 2854, 1709 (C=O), 1604, 1511, 1468, 1253, 1168, 1041, 1011, 976, 850, 772, 650 cm⁻¹.

Results and discussion

Synthesis and structures of the copolyesters

The bio-based aromatic copolyesters PDN2_1−xE1_x and PDN2_1−xE2_x were synthesized from a mixture of N2, E1 or E2, and 1,10-decanediol with selected composition ratios via a melt polycondensation method. The polymerization strategies can be observed from Scheme 1, and the molecular weights and molar compositions of the resulting polyesters are summarized in Table 1. The copolyesters were characterized by SEC and ¹H NMR, ¹³C NMR, and FTIR spectroscopic analyses (Fig. S8–S15†).

Table 1 Molar composition, molecular weight, polydispersity, isolated yield and state of the PDN2_1−xE1_x and PDN2_1−xE2_x copolyesters

Polyester	Yield	Molar composition				Molecular weight^d			Isolated state
		Feed		Copolyester^c
		XN2	XE	XN2	XE	Mn	Mw	D
a SEC carried out in CHCl3 against PS standards.b SEC carried out in THF against PS standards.c Molar composition determined by integration of the ^1H NMR spectra.d After purification by precipitating from methanol.
PDN2^a	94%	100	0	100	0	21 700	32 100	1.48	White powder
PDN290%E110%^a	92%	90	10	89.5	10.5	14 200	24 200	1.70	White powder
PDN280%E120%^a	88%	80	20	80.6	19.4	12 000	21 400	1.78	White powder
PDN270%E130%^b	86%	70	30	71.4	28.6	15 500	28 100	1.81	White powder
PDN260%E140%^b	85%	60	40	61.3	38.7	16 100	30 700	1.90	White powder
PDN250%E150%^b	82%	50	50	50.4	49.6	15 400	30 700	1.99	White powder
PDN290%E210%^a	93%	90	10	88.4	11.6	15 900	26 600	1.67	White powder
PDN280%E220%^a	89%	80	20	79.2	20.8	14 600	25 300	1.73	White powder
PDN270%E230%^b	86%	70	30	69.9	30.1	17 700	38 100	2.15	White powder
PDN260%E240%^b	85%	60	40	59.9	40.1	16 600	33 600	2.02	White powder
PDN250%E250%^b	83%	50	50	50.1	49.9	19 900	35 500	1.87	White powder

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Chemical microstructures

Quantitative ¹³C NMR spectroscopic analysis was used to study the chemical microstructures of the copolyesters. Apart from those non-protonated aromatic carbons mentioned in previous reports,³¹ the methylene carbons close to the asymmetrical moieties prove to be sensitive to sequence distributions at the dyad level. In this study, the methylene groups adjacent to the hydroxy oxygens prove to be well resolved in the ¹³C NMR spectra due to the inequivalence of the carbonyls in N2 and E1 and E2. It will generate a difference of head and tail displayed by E1 when copolymerized into the polyester chains. However, when N2 was copolymerized with E2, the difference of head and tail will be absent due to the respective two ester groups in N2 and E2 being equivalent. The splitting situations of the methylene groups adjacent to the hydroxy oxygen and the indications of the possible dyads to which they are assigned for the PDN2_1−xE1_x and PDN2_1−xE2_x copolyesters are exhibited in Fig. 1 and S16,† respectively. Six types of dyads (N2-N2, N2-E1H, N2-E1T, E1H-E1H, E1H-E1T, and E1T-E1T) for the PDN2_1−xE1_x copolyesters were formed since the orientation of N2 and E1 incorporated into the polyester chains was adopted in an arbitrary manner. Meanwhile, only three types of dyads (N2-N2, N2-E2, and E2-E2) for PDN2_1−xE2_x are observed. Furthermore, the signal intensities are found to be intimately associated with the composition of the copolyesters. For example, for the PDN2_1−xE1_x copolyesters the signal intensities (β, γ, and δ) corresponding to the eugenol-derived units are gradually enhanced with the increase of the eugenol-based components. It should be noted that the gradually increased signal δ and decreased signal ε are due to the phenoxy-linked methylene carbons in eugenol and the nipagin-derived units, respectively. The chemical microstructure analysis reveals that N2 and E1 and E2 were incorporated into the polyester chains in an arbitrary manner and consequently the nipagin and eugenol-derived copolyesters have completely random microstructures.

Fig. 1 The splitting situations of the methylenes adjacent to the hydroxy oxygens with the indications of the dyads to which they are assigned in the PDN2_1−xE1_x copolyesters.

Thermal properties

To investigate the influence of the insertion of the eugenol-based composition on the thermal stabilities of the copolyesters, thermogravimetric analysis (TGA) was carried out from 25 to 800 °C at a heating rate of 10 °C min⁻¹. The TGA and derivative plots are depicted in Fig. S17–S20,† and thermal data are gathered in Table 2. From the data we can observe that the thermal stabilities of the copolyesters in each series gradually decrease with the increase in the eugenol-based composition, and the decreased degree for PDN2_1−xE1_x seems more obvious than that for the PDN2_1−xE2_x copolyesters when the content of the eugenol-based composition is similar. Nevertheless, even in the most detrimental cases for PDE1 and PDE1 homopolyesters, the temperature for 5% weight loss (T_5%) is still above 347 °C. The data here is original.²⁴ However, the maximum degradation temperature T_d adopts a converse variation trend as T_5%, that is, the decreased values for PDN2_1−xE2_x are more obvious than those for PDN2_1−xE1_x with the gradual incorporation of the eugenol-based composition. Furthermore, the T_d values for the PDN2_1−xE1_x copolyesters seem insusceptible to the composition variation except for PDN2_50%E1_50%. All these results indicate that the nipagin and eugenol-based copolyesters have excellent thermal stabilities which can be comparable to the petroleum-based PET³² and PBT,³³ and completely meet the requirement for use as structural materials.

Table 2 Thermal property parameters of the PDN2_1−xE1_x and PDN2_1−xE2_x copolyesters

Polyester	TGA			DSC^d
	T5%^a (°C)	Td^b (°C)	W^c (%)	Tg (°C)	Tm (°C)	ΔHm (J g^−1)	Tc (°C)	ΔHc (J g^−1)
a Temperature at which the 5% weight loss occurred.b Temperature at which the maximum degradation rate occurred.c Remaining weight at 800 °C.d Glass transition, melting and crystallization temperature (Tg, Tm, and Tc), and corresponding melting and crystallization enthalpy (ΔHm and ΔHc) calculated from the second heating DSC trace of the sample coming directly from precipitation at a heating/cooling rate of 10 °C min^−1.
PDN2	395	424	2.76	22.5	Tm1 = 131.5	ΔHm1 = 5.9	—	—
					Tm2 = 136.8	ΔHm2 = 164.2
PDN290%E110%	385	423	1.70	8.6	Tm1 = 128.5	ΔHm1 = 3.2	—	—
					Tm2 = 132.6	ΔHm2 = 148.2
PDN280%E120%	376	422	1.20	3.4	Tm1 = 122.1	ΔHm1 = 1.8	—	—
					Tm2 = 126.7	ΔHm2 = 126.8
PDN270%E130%	371	422	3.31	−7.5	118.4	96.4	—	—
PDN260%E140%	367	423	8.48	−12.7	106.6	72.6	—	—
PDN250%E150%	353	394	0.90	−15.2	95.6	42.9	—	—
PDN290%E210%	388	422	1.50	7.5	Tm1 = 125.4	ΔHm1 = 2.4	84.2	1.6
					Tm2 = 129.6	ΔHm2 = 142.5
PDN280%E220%	385	424	2.23	2.8	123.4	119.6	74.6	2.8
PDN270%E230%	378	421	5.24	−6.8	112.5	92.5	65.3	4.8
PDN260%E240%	378	413	6.45	−10.6	105.2	71.8	—	—
PDN250%E250%	375	410	11.55	−13.5	93.4	41.9	—	—
PDE1	365	393	8.2	−27.6	—	—	—	—
PDE2	347	381	13.1	−17.3	—	—	—	—

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In order to study the influence of the insertion of the eugenol-based composition on the melting and crystallization behaviour, differential scanning calorimetry (DSC) analysis was carried out from −30 to 210 °C at a heating/cooling rate of 10 °C min⁻¹. The second heating DSC traces are shown in Fig. S21,† and the obtained analytical data are gathered in Table 2. The T_g values are found to present a decreasing trend with the gradual increase in the eugenol-based composition for each of the two series due to the gradual increase in the asymmetrical eugenol-derived units and thus cause more free volumes for segmental motion. Furthermore, PDN2_1−xE1_x has a slightly higher T_g value compared to PDN2_1−xE2_x when the content of the eugenol-based composition is similar, suggesting that E2 features an inferior structural symmetry to E1. This point is also confirmed by the isothermal crystallization analysis in the subsequent section. Furthermore, an almost single melting peak is observed from the second heating traces for all the copolyesters, and the T_m and corresponding ΔH_m values display decreasing trends with the gradual increase in the eugenol-based composition, which is in accordance with the expectations. Furthermore, the melting peaks broaden progressively with the gradual increase in the eugenol-based composition. Similarly, PDN2_1−xE1_x has slightly higher T_m and ΔH_m values compared to those of PDN2_1−xE2_x when the content of the eugenol-based composition is similar. More interestingly, when the content of the eugenol-based composition is below 30% in the series of PDN2_1−xE2_x, the second heating traces exhibit slight cold crystallization peaks, indicating that the PDN2_1−xE2_x copolyesters with a eugenol-based composition below 30% were not able to completely crystallize from molten during the first cooling run. When the temperature reached an appropriate value, the half-baked samples began to crystalize again. The valuable conclusion drawn from DSC analysis is that the PDN2 homopolyester features strong crystallizability despite the fact that the insertion of a eugenol-based composition impedes the crystallization of PDN2 to a certain extent.

Powder X-ray diffraction analysis

To further study the influence of the insertion of the eugenol-based composition on the crystallization of PDN2, powder X-ray diffraction (WXRD) analysis was carried out for the whole range of copolyesters. The powder X-ray diffraction profiles are depicted in Fig. 2, and the corresponding data are gathered in Table S1.† All the copolyesters are able to generate discrete diffraction signals characteristic of semicrystalline materials even if the eugenol-based composition reaches 50%, indicating the strong crystallizability for the PDN2 homopolyester. The diffraction pattern for the PDN2 homopolyester is characterized by five predominant reflections at 14.56°, 16.22°, 17.74°, 19.24° and 24.08°, respectively. Furthermore, almost the same diffraction pattern is shared for all the copolyesters taking both the diffraction angles and relative intensities into consideration, indicating that the crystal structure of PDN2 must be retained in the copolyesters. Given the crystallinity estimated as the quotient between the crystalline area and total area of the diffraction pattern, the crystallinity of the copolyesters gradually decreases with the increase in the eugenol-based composition, and PDN2_1−xE1_x has a slightly higher crystallinity compared to PDN2_1−xE2_x when the content of the eugenol-based composition is similar, which is in accordance with the DSC results. Nevertheless, the crystallinity of PDN2_50%E2_50% still reaches 36%, indicating the strong crystallizability for the PDN2 homopolyester as well.

Fig. 2 Powder WXRD profiles for the PDN2_1−xE1_x and PDN1_1−xE2_x copolyesters.

Isothermal crystallization

The first cooling DSC runs obtained from the molten samples reveal that all the copolyesters are able to crystallize from molten even if the content of the eugenol-based composition reaches 50%. However, the melt crystallization temperature and the respective enthalpy give great differences for the copolyesters with different compositions. Given the importance of crystallization behaviour regarding polymer processing,^34,35 the isothermal crystallization of the PDN2 homopolyester, and the PDN2_90%E1_10%, PDN2_90%E2_10%, PDN2_80%E1_20%, and PDN2_80%E2_20% copolyesters was studied in the 95–115 °C temperature range. Unfortunately, not all of the polyesters performed at the same isothermal crystallization temperature due to the differences in crystallization rates displayed by them and the relatively narrow temperature scopes in which isothermal crystallization evolution can be detected by DSC analysis. Nevertheless, the isothermal crystallization conditions should be selected to be as similar as possible in order to draw useful conclusions. Kinetic data obtained via the Avrami analysis together with the results obtained at the end of the selected crystallization temperature are summarized in Table 3. The evolution of the relative crystallinity, X_t, with crystallization time and the corresponding Avrami plots ln[−ln(1 − X_t)] versus ln(t − t₀) for some illustrative experiments are shown in Fig. 3 and S22.† It is confirmable that the crystallization half-time t_1/2 and the Avrami exponent n increase with the crystallization temperature for each sample. The latter values are in the range of 2.0–3.1, which corresponds to a complex axialitic-spherulitic crystallization behaviour. The double-logarithmic profiles (Fig. 5B) reveal that primary and secondary crystallization took place in the selected time intervals and that the presence of the eugenol-based composition reduces the crystallizability of the PDN2 homopolyester to a certain degree. The early stage of crystallization matched the Avrami model excellently. However, the double-logarithmic plots gradually deviated from the Avrami equation with the progress of crystallization, which was largely attributed to the secondary crystallization that generated the collision of crystals and thus caused the changes of sample volumes. The results demonstrate that the PDN2_80%E1_20% and PDN2_80%E2_20% copolyesters still have rather strong crystallizability. On the other hand, it indicates the strong crystallizability of the PDN2 homopolyester.

Table 3 Isothermal crystallization data of the PDN2 homopolyester, and the PDN2_90%E1_10%, PDN2_90%E2_10%, PDN2_80%E1_20%, and PDN2_80%E2_20% copolyesters

Polyester	Tc^a (°C)	ΔT^a (°C)	t0^b (min)	t1/2^c (min)	n^d	−ln k^e	Tm^f (°C)
a Tc: isothermal crystallization temperature; degree of supercooling: ΔT = Tm − Tc.b Onset crystallization time.c Crystallization half-time.d Avrami exponent.e Kinetic rate constant k (min^−1).f Melting temperature after crystallization.
PDN2	110	27	0.12	0.84	2.13	0.58	136.5
	115	22	0.23	1.43	2.23	0.83	137.6
PDN290%E110%	110	23	0.39	3.56	2.78	3.96	132.9
	115	18	0.55	5.04	3.09	5.03	134.2
PDN280%E120%	95	32	0.24	1.98	2.18	1.95	127.4
	100	27	0.34	3.51	2.23	2.96	128.3
PDN290%E210%	110	20	2.36	10.56	2.02	4.88	131.2
	115	15	3.63	15.09	2.09	5.76	132.3
PDN280%E220%	95	28	0.39	7.68	2.17	4.25	124.2
	100	23	0.55	9.40	2.24	5.41	125.6

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Fig. 3 Isothermal crystallization of PDN2, PDN2_90%E1_10% and PDN2_90%E2_10% at the indicated temperature. Relative crystallinity versus time plots (A) and ln[−ln(1 − X_t)] versus ln(t − t₀) plots (B).

Stress–strain behaviour

In order to further investigate the influence of the incorporation of the eugenol-based composition on the mechanical properties, tensile assays were carried out from the casted films of the copolyesters. The stress–strain curves are exhibited in Fig. 4 and the mechanical property parameters are gathered in Table 4. The Young’s modulus and tensile strength are found to gradually decrease with the increase in the eugenol-based composition and are considerably higher than those of the PDE1 and PDE2 homopolyesters.²⁴ To be specific, the Young’s modulus decreases from 746 MPa for PDN2 to 344 MPa for PDN2_50%E2_50%. Meanwhile, the tensile strength drops about 18.6 MPa from 29.4 MPa for PDN2 to 10.8 MPa for PDN2_50%E2_50%. Furthermore, PDN2_1−xE1_x features a slightly higher Young’s modulus and tensile strength than those of PDN2_1−xE2_x when the content of the eugenol-derived composition is the identical. The reason for these results is largely attributed to the higher crystallizability for the PDN2_1−xE1_x copolyesters. However, the elongation at the break exhibits an increasing trend when the content of the eugenol-based composition increases. Similarly, PDN2_1−xE2_x features a slightly higher elongation at the break than PDN1_1−xE1_x does when the content of the eugenol-based composition is identical. Fortunately, both the Young’s modulus and tensile strength do not decrease significantly and can still be comparable to those of the petroleum-based PET³² and PBT³³ despite the incorporation of the eugenol-based composition. Instead, the eugenol-based composition acts as a toughener in the copolyesters. As a consequence, regarding the mechanical properties the nipagin-based PDN2 has the potential to replace the petroleum-based plastics.

Fig. 4 Stress–strain curves of the PDN2_1−xE1_x and PDN2_1−xE2_x copolyesters.

Table 4 Mechanical and dynamic mechanical properties of the PDN2_1−xE1_x and PDN2_1−xE2_x copolyesters

Polyester	Mechanical properties			Dynamic mechanical properties
	Young’s modulus (Mpa)	Tensile strength (MPa)	Elongation at break (%)	Tg (°C)	Storage modulus (MPa, 20 °C)	tan δmax
PDN2	746 ± 35	29.4 ± 2.1	2.4 ± 0.5	15.6	350 ± 50	0.12
PDN290%E110%	675 ± 28	27.8 ± 2.2	4.2 ± 0.6	10.6	187 ± 36	0.13
PDN280%E120%	608 ± 24	23.2 ± 2.5	5.7 ± 1.2	2.8	125 ± 33	0.14
PDN270%E130%	525 ± 32	19.3 ± 1.8	11.7 ± 1.8	−5.4	86 ± 24	0.17
PDN260%E140%	448 ± 22	14.8 ± 1.4	18.8 ± 2.1	−10.8	69 ± 18	0.21
PDN250%E150%	369 ± 16	12.6 ± 1.2	24.8 ± 2.4	−12.6	56 ± 12	0.25
PDN290%E210%	662 ± 16	26.7 ± 2.2	4.8 ± 0.8	9.2	165 ± 26	0.14
PDN280%E220%	595 ± 15	21.6 ± 2.4	7.6 ± 1.1	2.4	109 ± 22	0.15
PDN270%E230%	503 ± 32	18.4 ± 1.8	14.5 ± 1.7	−5.6	72 ± 17	0.18
PDN260%E240%	429 ± 22	12.6 ± 0.8	19.5 ± 2.5	−11.2	59 ± 11	0.22
PDN250%E250%	344 ± 16	10.8 ± 1.1	25.8 ± 2.8	−15.8	44 ± 13	0.25
PDE1	1.2 ± 0.6	1.0 ± 0.2	844 ± 35	−30.3	52.54	0.15
PDE2	4.0 ± 0.8	2.6 ± 0.3	926 ± 40	−20.7	69.17	0.18
PET	1032 ± 52	45 ± 7	23 ± 5	—	—	—
PBT	841 ± 15	42 ± 5	14 ± 3	—	—	—

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Dynamic mechanical analysis

As structural materials in practical applications, polymers are generally required to suffer from dynamic stress and strain. As a consequence, dynamic viscoelasticity analysis (DMA) is necessary to be carried out in order to explore the relationship between the structure and properties and to further improve the properties of such materials, as appropriate. In this study, DMA was carried out in the −60 to 60 °C temperature range. The storage modulus E′ and dissipation factor tan δ versus temperature plots are depicted in Fig. 5, 6 and S23–S24.† The measured dynamic mechanical data are gathered in Table 4. It can be concluded that the storage modulus and tan δ are intimately related with the composition of the copolyesters. Specifically, with the gradual increase of the incorporated eugenol-derived units, the flexible soft segments increase accordingly. Therefore, storage moduli are progressively decreased with the increasing eugenol-derived segments. Nevertheless, the storage modulus can still be maintained at a rather high level despite half of the nipagin-based N2 units being replaced by the asymmetrical eugenol-based E1 or E2 units, indicating the strong crystallizability of the PDN2 homopolyester. The tan δ values are found to follow an opposite trend to that of the storage modulus due to the fact that internal friction resistance gradually increases with the incorporation of the eugenol-based components. The T_g value is generally defined as the inflection temperature point of the maximum tan δ.³⁵ Consequently, T_g can also be obtained from the tan δ versus temperature plots. It can be explained that the frozen segments begin to move around T_g and thus cause the hysteresis to increase significantly. Furthermore, the T_g values obtained from DMA are almost in accordance with those from DSC analysis. The inconspicuous T_g in the DSC traces becomes obvious in the tan δ versus temperature plots. Storage moduli for PDN2_1−xE1_x are found to be slighter higher than those for PDN2_1−xE2_x when the content of the eugenol-derived components is similar, and at the same time the tan δ values adopt a reverse trend as storage moduli. The reason for this is largely attributed to the higher crystallizability of PDN2_1−xE1_x compared to the PDN2_1−xE2_x copolyesters.

Fig. 5 Storage modulus as a function of temperature for the PDN2_1−xE1_x copolyesters.

Fig. 6 tan δ as a function of temperature for the PDN2_1−xE1_x copolyesters.

Conclusions

In conclusion, the nipagin-based PDN2 is a highly crystalline material. Two series of renewable nipagin and eugenol-based aromatic copolyesters have been developed. The insertion of the eugenol-based composition impedes the crystallization rate of PDN2 significantly. However, up to 36% crystallinity can still be maintained as the asymmetrical eugenol-based composition reaches 50%. The copolyesters exhibit typical characteristics of brittle materials even in the cases of the PDN2_50%E1_50% and PDN2_50%E2_50% copolyesters. The Young’s modulus and tensile strength of PDN2 are found to be 746 MPa and 29.4 MPa, respectively, which can be comparable to the petroleum-based PET and PBT. All of the results verify that the PDN2 homopolyester has strong crystallizability, which is significantly crucial for the maintenance of the mechanical strength and modulus of the materials. Consequently, the nipagin-based polyesters have the potential to replace the currently widely used petroleum-based materials.

Acknowledgements

This work was funded by NSFC (51203079), the Natural Science Foundation of Tianjin (14JCYBJC18100) and PCSIRT (IRT1257).

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Footnote

† Electronic supplementary information (ESI) available: Additional figures and Table S1. See DOI: 10.1039/c6ra02154a

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