Synthesis, Thermal Properties and Decomposition Mechanism of Poly(Ethylene Vanillate) Polyester

polymers

Article Synthesis, Thermal Properties and Decomposition Mechanism of Poly(Ethylene Vanillate) Polyester

Alexandra Zamboulis 1, Lazaros Papadopoulos 1 , Zoi Terzopoulou 1 , Dimitrios N. Bikiaris 1,* , Dimitra Patsiaoura 2, Konstantinos Chrissaﬁs 2, Massimo Gazzano 3 , Nadia Lotti 4 and George Z. Papageorgiou 5,* 1 Laboratory of Chemistry and Technology of Polymers and Dyes, Department of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Macedonia, Greece; [email protected] (A.Z.); [email protected] (L.P.); [email protected] (Z.T.) 2 Solid State Physics Department, School of Physics, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece; [email protected] (D.P.); hrisaﬁ[email protected] (K.C.) 3 Organic Synthesis and Photoreactivity Institute, ISOF-CNR, Via Gobetti 101, 40129 Bologna, Italy; [email protected] 4 Civil, Chemical, Environmental and Materials Engineering Department, University of Bologna, via Terracini 28, 40131 Bologna, Italy; [email protected] 5 Chemistry Department, University of Ioannina, P.O. Box 1186, 45110 Ioannina, Greece * Correspondence: [email protected] (D.N.B.); [email protected] (G.Z.P.); Tel.: +30-2310-997812 (D.N.B.)

Received: 16 September 2019; Accepted: 9 October 2019; Published: 14 October 2019

Abstract: Plastics are perceived as modern and versatile materials, but their use is linked to numerous environmental issues as their production is based on ﬁnite raw materials (petroleum or natural gas). Additionally, their low biodegradability results in the accumulation of microplastics. As a result, there is extensive interest in the production of new, environmentally friendly, bio-based and biodegradable polymers. In this context, poly(ethylene vanillate) (PEV) has a great potential as a potentially bio-based alternative to poly(ethylene terephthalate); however, it has not yet been extensively studied. In the present work, the preparation of PEV is reported. The enthalpy and the entropy of fusion of the pure crystalline PEV have been estimated for the ﬁrst time. Additionally, the equilibrium melting temperature has also been calculated. Furthermore, the isothermal and non-isothermal crystallization behavior are reported in detail, and new insights on the thermal stability and degradation mechanism of PEV are given.

Keywords: poly(ethylene vanillate); synthesis; thermal properties; crystallization; thermal stability; decomposition mechanism

1. Introduction Plastics are perceived as modern and versatile materials in the 21st century, and imagining our life without them seems impossible. Those materials provide many advantages for today’s society, fostering and enabling its sustainability [1]. However, plastics are also polluting materials. Indeed, their disposable character and short-term applications in combination with a low or absent biodegradability have led to the accumulation of microplastics worldwide, causing important environmental issues [2]. Additionally, their production depends on petroleum and natural gas, which are ﬁnite raw materials. As a result, environmental concerns have been raised both for the resources used and their end-life options [3]. In a quest towards greener materials, many natural feedstocks have been explored for the production of monomers that can be used to synthesize bio-based polymers, mainly cellulose, lignin and polysaccharides [4]. Therefore, environmentally friendly materials, bio-based and biodegradable, are intensively sought by the academic, as well as the industrial community to replace traditional polymers.

Polymers 2019, 11, 1672; doi:10.3390/polym11101672 www.mdpi.com/journal/polymers Polymers 2019, 11, 1672 2 of 27

The replacement of fossil fuels for the production of monomers by alternative inexpensive and renewable starting materials, such as cellulose, starch, lignin, proteins and vegetable oils is a great challenge. The idea of producing polymers from renewable resources is not new. However, there is always a drawback, due to their relatively high cost in comparison with their petrochemical homologues. The cost of bio-based chemicals as building blocks is expected to decrease soon, and the production of platform chemicals and monomers following the concept of the biorefinery seems to be an answer. Biomass-derived monomers are generally classified on the basis of their natural molecular biomass origins into: (i) Oxygen-rich monomers, including carboxylic acids, polyols, dianhydroalditols, and furans; (ii) hydrocarbon-rich monomers, such as vegetable oils, fatty acids, terpenes, terpenoids and resin acids; (iii) hydrocarbon monomers, such as bio-ethene, bio-propene, bio-isoprene and bio-butene; and (iv) non-hydrocarbon monomers, i.e., carbon dioxide and carbon monoxide. Replacing the widely used, petroleum-derived plastic poly(ethylene terephthalate) (PET) with a bio-based counterpart is an intriguing case study. PET is a widely used polymer, especially for short-term packaging applications. Along with the recycling of PET [5], the search for its bio-based alternatives has attracted considerable interest. Poly(ethylene furanoate) (PEF), and 2,5-furandicarboxylate polyesters are, in general, some of the most promising alternatives which have been extensively studied [6]. The reasons behind that interest are the facts that PEF exhibits superior thermal stability, a lower melting temperature, significantly lower O2 and CO2 permeability than PET, as well as excellent mechanical properties and good processability. Therefore, a series of studies have been performed concerning furan polyesters, focusing on key parameters, such as catalysis [7], solid state polymerization [8–10], copolymerization [11,12] and nanocomposite materials [13,14]. However, a very important factor that has to be taken into consideration is the high cost linked to the production of 2,5-furandicarboxylic acid [15]. It is a major drawback that hampers the further development of this class of polyesters, and therefore, other routes towards bio-based PET alternatives need to be also explored. An interesting alternative bio-based building block which has recently regained attention is vanillic acid or 4-hydroxy-3-methoxybenzoic acid. Vanillic acid can be produced by oxidizing vanillin, which can be isolated from lignin [16,17]. Lignin is found in plant biomass; it is the second most abundant organic polymer in nature (after cellulose) and is a unique source of aromatic building blocks. It is not a coincidence that according to the BIOSPRI tender "Study on support to R&I policy in the area of bio-based products (BBPs) and services", implemented by the University of Bologna and the Fraunhofer ISI, seven of the top 20 selected bio-based products are derived from lignin [18]. Vanillin production is dominated by the catechol–guaiacol process, which is petroleum-dependent. However, Borregaard, the second largest vanillin producer worldwide, produces vanillin from lignin by oxidation (15% of the global vanillin production). In parallel, progress in lignin depolymerization [19] and the biotechnological production of vanillin [20,21], are expected to bring about more sustainable processes for vanillin production, rendering vanillin a top-priority renewable building block [22]. Poly(ethylene vanillate) (PEV) can be prepared from vanillic acid and 2-chloroethanol. As 2-chloroethanol can be produced from ethylene glycol, which can originate from biomass, PEV is considered a bio-based polymer. Due to its analogous aliphatic/aromatic structure, and also similar mechanical and thermal properties, PEV is considered a potential alternative to PET. However, despite its great potential, it has not yet been widely investigated. Mialon et al. investigated a series of polyesters derived from 4-hydroxybenzoic acid, vanillic acid and syringic acid (4-hydroxy-3,4-dimethoxybenzoic acid) [23]. The aromatic acids were modified with ω-chloro-alcohols on the aromatic hydroxyl group to afford hydroxy-carboxylic acids, which were further polymerized in the presence of Sb2O3 as a catalyst. Poly(alkylene vanillate)s exhibited higher melting temperatures (Tm) than the corresponding poly(alkylene 4-hydroxybenzoate)s and poly(alkylene syringate)s and a decrease in the glass transition temperature (Tg) was observed with the increasing length of alkylene chain. The polymerization of vanillic acid was unsuccessful, yielding a low-molecular weight, insoluble material. More recently, Gioia et al. reported a one-pot synthesis of PEV from vanillic acid in the presence of ethylene carbonate, catalyzed by dibutyltin oxide [24]. The average molecular weight of the polymers obtained in the Polymers 2019, 11, 1672 3 of 27

1 latter work was slightly lower than in the work of Mialon et al. (4700 vs 5390 g mol− ), but the melting temperature rather higher (264 ◦C vs 239 ◦C). Gioia et al. copolymerized vanillic acid with ε-caprolactone [24] and ricinoleic acid [25], while Nguyen et al. have reported the copolymerization of 4-hydroxyethylvanillic acid with ε-caprolactone and l-lactide [26]. Apart from PEV, other vanillic polymers have been synthesized, as the aromatic units tend to increase the mechanical properties and the Tg of the resulting polymers [27–31]. Finally, vanillic acid has also been incorporated in thermotropic liquid crystalline polyesters [32–36]. PEV is a new bio-based polyester for promising applications mainly as a packaging material. However, extensive studies on its thermal properties have not yet been reported. In the present study, PEV was synthesized via the melt polycondensation of 4-(2-hydroxyethoxy)-3-methoxybenzoic acid or 4-hydroxyethylvanillic acid. The thermodynamic properties, such as the enthalpy and entropy of fusion of 100% crystalline PEV and the equilibrium melting temperature were estimated and are reported for the first time. The multiple melting behavior was also investigated. The isothermal crystallization from the melt, as well as the non-isothermal crystallization from the glass and from the melt, were studied using differential scanning calorimetry (DSC), polarized light microscopy (PLM), and wide-angle X-ray diffraction (WAXD). Furthermore, thermogravimetric analysis (TGA) and pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) measurements were also conducted, giving new insights about the thermal stability of PEV and its decomposition mechanism.

2. Materials and Methods

2.1. Materials

Vanillic acid (VA, purum 97%), 2-chloroethanol (>99%), titanium butoxide (Ti(OBu)4) antimony trioxide (Sb2O3 99.99%) catalyst were purchased from Aldrich Co. (Chemie GmbH, Steinheim, Germany). The purchased monomers have a petrochemical origin.

2.2. Synthesis of 4-(2-Hydroxyethoxy)-3-Methoxybenzoic Acid 16.0 g (0.095 mol) of vanillic acid and 2.85 g (0.019 mol) of sodium iodide were dissolved in aqueous sodium hydroxide (15.2 g (3.81 mol) in 70 mL of water); 11.49 g (0.14 mol) of chloroethanol dissolved in 140 mL of ethanol and degassed by N2 bubbling were added dropwise at 100 ◦C. The reacting mixture was refluxed, and 3.8 g of chloroethanol were added every 24 h. After 4 days, the reacting mixture was concentrated, and the residue was dissolved in water. The aqueous solution was washed with ether and acidified with HCl(aq.). The solid that precipitated was isolated by filtration and purified by 1 recrystallisation in ethanol to afford the product in 54% yield. H NMR (500 MHz, DMSO-d6, δ): 12.64 (s, 1H), 7.54 (dd, J = 8.5, 2.0 Hz, 1H), 7.45 (d, J = 2.0 Hz, 1H), 7.04 (d, J = 8.5 Hz, 1H), 4.89 (s, 1H), 4.04 13 (t, J = 4.9 Hz, 2H), 3.80 (s, 3H), 3.74 (t, J = 4.9 Hz, 2H). C NMR (126 MHz, DMSO-d6, δ): 167.1, 152.1, 148.4, 123.2, 122.9, 112.1, 111.9, 70.2, 59.4, 55.4.

2.3. Synthesis of Poly(Ethylene Vanillate) (PEV) PEV was prepared by a two-stage melt polycondensation procedure in a glass batch reactor [6,37]. According to this, in the first esterification step, 8 g of 4-(2-hydroxyethoxy)-3-methoxybenzoic acid, which was produced and purified previously, were added into the reaction tube of the polycondensation reactor. Ti(OBu)4 (TBT) 400 ppm or Sb2O3 was charged, and the apparatus containing the reagents was evacuated several times and filled with nitrogen in order to remove the existing oxygen. The reagents were heated at 190 ◦C under argon flow (50 mL/min) for 2 h. In the second step of polycondensation, vacuum (5.0 Pa) was applied slowly over a period of time of about 15 min, and the temperature was gradually increased to 240 ◦C, while stirring speed was also increased from 350 rpm to 720 rpm. The reaction continued at this temperature for 2 h. Then, the temperature was gradually increased to 250 ◦C and 260 ◦C for 1h each step. After the polycondensation reaction was completed, the polyesters were removed from the reactor, milled and Polymers 2019, 11, 1672 4 of 27 washed with methanol. The polycondensation was followed by solid state polymerization (SSP), that was performed under vacuum in a glass batch reactor. The milled polymer obtained after the melt polycondensation was heated under vacuum for 5 h, at 230 ◦C for 30 min, at 240 ◦C for 1h, at 250 ◦C for 1.5 h and at 255 ◦C for 2 h.

2.4. Polyester Characterization

2.4.1. Intrinsic Viscosity Measurement Intrinsic viscosity [η] measurements were performed with an Ubbelohde viscometer (Schott Gerate GMBH, Hofheim, Germany) at 25 ◦C in a mixture of phenol and tetrachloroethane (60/40, w/w). The samples were maintained in the above mixture of solvents at 60 ◦C for 20 min to achieve complete dissolution. The intrinsic viscosity of polyester was calculated using the Solomon–Ciuta Equation (1) of a single point measurement: 1 h n t t oi 2 2 t ln t 1 [η] = 0 − 0 − , (1) c where c is the concentration of the solution; t, the ﬂow time of solution and t0 the ﬂow time of pure solvent. For each sample, three measurements were conducted, and the average value was calculated.

2.4.2. Wide Angle X-ray Diﬀraction Patterns (WAXD) X-ray diﬀraction measurements of the samples were performed using a MiniFlex II XRD system from Rigaku Co. (Rigaku Co., Tokyo, Japan), with CuKα radiation (λ = 0.154 nm) in the angle (2θ) range from 5 to 65 degrees.

2.4.3. Fourier Transformed-Infrared Spectroscopy (FTIR) FTIR spectra were obtained using a Perkin–Elmer FTIR spectrometer (Perkin Elmer, Waltham, 1 MA, USA), model Spectrum One, in absorbance mode and in the spectral region of 500–4000 cm− 1 using a resolution of 4 cm− and 64 co-added scans.

2.4.4. Nuclear Magnetic Resonance (NMR)

NMR spectra were recorded in deuterated dimethylsulfoxide (DMSO-d6) or triﬂuoroacetic acid (TFA-d), on an Agilent 500 spectrometer (Agilent Technologies, Santa Clara, CA, USA), at room temperature. Spectra in TFA-d were recorded in the presence of a DMSO probe Spectra were internally referenced with tetramethylsilane (TMS) and calibrated using the residual solvent peaks.

2.4.5. Differential Scanning Calorimetry (DSC) Thermal analysis studies were carried out using a using Perkin Elmer Diamond DSC (PerkinElmer Corporation, Waltham, MA, USA) updated to DSC 8500 level, combined with an Intracooler IIP cooling system. Samples of about 5 mg were used. The samples were first heated at 20 ◦C/min to a temperature 40 ◦C above the melting peak temperature, to erase the previous thermal history. To record the glass transition, the samples were first melt-quenched on a frozen metal plate and then were put in the DSC cell. In general, heating scans of the samples were conducted at 20 ◦C/min. To study the crystallization on cooling from the melt, scans at different rates were performed. Isothermal crystallization experiments of the polymers at various temperatures below the melting point were performed after self-nucleation (SN) of the polyester sample. The SN procedure leads to enhanced crystallization rates, so that it allows crystallizations to be performed even in case of very small supercoolings, that is at temperatures close to the melting temperature. As a result, crystallizations were tested in a wide temperature range. Self-nucleation measurements were performed in analogy to the procedure described by Fillon et al. [38]. The protocol used was very similar with that described by Müller et al. [39], and can be summarized as follows: (a) Melting of the sample at 40 ◦C above the Polymers 2019, 11, 1672 5 of 27

observed melting point for 5 min to erase any previous thermal history; (b) cooling at 10 ◦C/min to a reference temperature and crystallization, to create a “standard” thermal history; (c) partial melting by heating at 20 ◦C/min up to a “self-nucleation temperature”, Ts which differed for the various polymers; and (d) thermal conditioning at Ts for 5 min. Depending on the Ts, the crystalline polyester will be completely molten, only self-nucleated or self-nucleated and annealed. If Ts is sufficiently high, no self-nuclei or crystal fragments can remain. At intermediate Ts values, the sample is almost completely molten, but some small crystal fragments or crystal memory effects remain, which can act as self-nuclei during a subsequent cooling from Ts. Finally, if Ts is too low, the crystals will only be partially molten, and the remaining crystals will undergo annealing during the 5 min at Ts, while the molten crystals will be self-nucleated during the following cooling; (e) cooling scan from Ts at 20 ◦C/min to the crystallization temperature (Tc), where the effects of the previous thermal treatment will be reflected on isothermal crystallization; (f) heating scan at 20 ◦C/min to 40 ◦C above the melting point, where the effects of the thermal history will be apparent on the melting signal. Experiments were performed to check that the sample did not crystallize during the cooling to Tc and that a full crystallization exothermic peak was recorded at Tc. In heating scans after isothermal crystallization, the standard heating rate was 20 ◦C/min. In some specific cases, heating experiments at different rates after isothermal crystallization was also carried out to better understand the melting behavior of the polyester; and for PEV Ts was 265 ◦C. Tests showed that at higher Ts values, the polyester was completely molten. In contrast, annealing was observed at temperatures lower than 265 ◦C. To investigate the non-isothermal crystallization of the polyesters from the melt, the samples were first heated to 310 ◦C for 1 min and then the samples were cooled from the melt under a wide range of cooling rates, from 5 to 20 ◦C/min. To study the cold-crystallization behavior the samples were first melt-quenched and then heated at the predetermined heating rate (5, 10, 15, 20 ◦C/min).

2.4.6. Polarizing Light Microscopy (PLM) A polarizing light microscope (Nikon, Optiphot-2, Melville, NY, USA) equipped with a Linkam THMS 600 heating stage (Linkam Scientiﬁc Instruments Ltd., Surrey, UK), a Linkam TP 91 control unit and also a Jenoptic ProgRes C10Plus camera (Jenoptik Optical Systems GmbH, Jena, Germany) were used for PLM observations.

2.4.7. Thermogravimetric Analysis (TGA) Thermogravimetric analysis was performed with a SETARAM SETSYS TG-DTA 16/18 instrument (Setaram instrumentation, Lyon, France). The samples (8 0.2 mg) were placed in alumina crucibles, ± while a blank measurement was performed and subsequently was subtracted by the experimental curve, in order to eliminate the buoyancy eﬀect. PEV samples were heated from ambient temperature up to 550 ◦C in a 50 mL/min N2 ﬂow at the following heating rates—5, 10, 15, and 20 ◦C/min. Continuous recording of both sample temperature and sample weight was carried out.

2.4.8. Pyrolysis-Gas Chromatography-Mass Spectroscopy (Py-GC/MS) For Py-GC/MS analysis of the polyesters, a very small amount of each material is “dropped” initially into the “Double-Shot” EGA/PY-3030D Pyrolyzer (Frontier Laboratories Ltd., Fukushima Japan) using a CGS-1050Ex (Japan), carrier gas selector. For pyrolysis analysis (ﬂash pyrolysis), each sample was placed into the sample cup, which afterwards fell free into the pyrolyzer furnace. The pre-selected pyrolysis temperatures were 330 ◦C, 420 ◦C and 500 ◦C the GC oven temperature was heated from 50 to 300 ◦C at 20 ◦C/min. Those two temperatures were selected based on the TGA pyrogram and represent the sample prior and after thermal decomposition. Sample vapors generated in the furnace were split (at a ratio of 1/50), a portion moved to the column at a ﬂow rate of 1 mL/min, pressure 53.6 kPa and the remaining portion exited the system via the vent. The pyrolyzates were separated using temperature-programmed capillary column of a Shimadzu QP-2010 Ultra Plus (Shimadzu, Kyoto, Japan) gas chromatogram and analyzed by the mass spectrometer MS-QP2010SE of Shimadzu (Japan) PolymersPolymers 20192019,, 1111,, 1672x FOR PEER REVIEW 66 of 2727

of Shimadzu (Japan) use 70 eV. Ultra-ALLOY® metal capillary column from Frontier Laboratories useLTD 70 (Fukushima, eV. Ultra-ALLOY Japan)® wasmetal used capillary containing column 5% fromdiphenyl Frontier and Laboratories95% dimethylpolysiloxane LTD (Fukushima, stationary Japan) wasphase, used column containing length 5% 30 diphenyl m and column and 95% ID dimethylpolysiloxane 0.25 mm. For the mass stationary spectrometer phase,column the following length 30conditions m and column were used: ID 0.25 Ion mm. source For heater the mass 200 spectrometer°C, interface temperature the following 300 conditions °C, vacuum were 10 used:-4–100 IonPa, -4 0 sourcem/z range heater 10–500 200 ◦ amuC, interface and scan temperature speed 10,000. 300 ◦TheC, vacuum chromatogram 10 –10 andPa, m spectra/z range retrieved 10–500 amu by each and scanexperiment speed 10,000. were subject The chromatogram to further interpretation and spectra retrievedthrough Shimadzu by each experiment and Frontier were post-run subject tosoftware. further interpretation through Shimadzu and Frontier post-run software. 2.4.9. Nanoindentation 2.4.9. Nanoindentation PEV was assessed through nanoindentation tests in order to measure its modulus and hardness. In instrumentedPEV was assessed indentation through tests, nanoindentation the load is measur testsed in as order a function to measure of penetration its modulus depth. and Such hardness. tests Inenable instrumented local variations indentation of modulus tests, the and load hardness is measured to be as measured a function precisely of penetration [40–43]. depth. In the Such current tests enablework, localthe indentations variations of moduluswere conducted and hardness using toa bedynamic measured ultra-micro-hardness precisely [40–43]. In tester the current (DUH-211; work, theShimadzu indentations Co., Kyoto, were conductedJapan) fitted using with a a dynamic triangular ultra-micro-hardness pyramid indenter tip tester (Berkovich (DUH-211; indenter). Shimadzu The Co.,indentations Kyoto, Japan) made fitted on the with surface a triangular of PEV pyramidfilm (about indenter 1 mm tipdepth (Berkovich prepared indenter). at 270 °C) The appeared indentations as an madeequilateral on the triangle. surface ofTen PEV measurements film (about 1 were mm depthconducted prepared on each at 270 sample,◦C) appeared which aswere an equilateralpurposely triangle.scattered Ten on the measurements surface. After were contact conducted of the indenter on each with sample, the whichsurface, were this purposelywas driven scattered into the surface on the surface.until a peak After load contact of 500 of themN indenter was reached. with the The surface, peak thisload was was driven held for into 3 thes (in surface order untilto minimize a peak load the ofeffect 500 mNof viscoelastic was reached. deformation The peak loadof the was specim helden, for 3notably s (in order creep, to minimizeon property the measurements) effect of viscoelastic and deformationthen the indenter of the was specimen, unloaded, notably to a load creep, of onzero. property measurements) and then the indenter was unloaded, to a load of zero. 3. Results and Discussion 3. Results and Discussion

3.1.3.1. Synthesis and StructuralStructural Characterization of PEV 4-(2-hydroxyethoxy)-3-methoxybenzoic acid was synthesized from vanillic acid and 2-chloro-1- 4-(2-Hydroxyethoxy)-3-methoxybenzoic acid was synthesized from vanillic acid and 2-chloroethanol via a Williamson reaction, according to the literature [23]. PEV was synthesized via the two- 1-ethanol via a Williamson reaction, according to the literature [23]. PEV was synthesized via the step polycondensation method, according to the reaction procedure presented in Scheme 1. The two-step polycondensation method, according to the reaction procedure presented in Scheme1. received material was solid, and the color was light yellow. The intrinsic viscosity values of the The received material was solid, and the color was light yellow. The intrinsic viscosity values of the prepared polyesters were 0.28 and 0.32 g/dL using TBT and Sb2O3 as catalysts, respectively. After the prepared polyesters were 0.28 and 0.32 g/dL using TBT and Sb2O3 as catalysts, respectively. After the SSP procedure, their intrinsic viscosity values were increased to 0.34 and 0.38 g/dL, respectively. SSP procedure, their intrinsic viscosity values were increased to 0.34 and 0.38 g/dL, respectively.

SchemeScheme 1.1. Synthesis of poly(ethylene vanillate) (PEV).(PEV). The synthesized PEV was characterized by FT-IR and NMR, and both methods conﬁrmed the The synthesized PEV was characterized by FT-IR and NMR, and both methods confirmed the successful polymerization of 4-(2-hydroxyethoxy)-3-methoxybenzoic acid. In the IR spectra, a decrease successful polymerization of 4-(2-hydroxyethoxy)-3-methoxybenzoic acid. In the IR spectra, a in the OH band at 3400 cm 1 is clearly visible (Figure1). Even more strikingly, the bands at 2533 decrease in the OH band at −3400 cm−1 is clearly visible (Figure 1). Even more strikingly, the bands at and 2626 cm 1, attributed to the OH groups bound to COOH moieties through H-bonds [44], were 2533 and 2626− cm−1, attributed to the OH groups bound to COOH moieties through H-bonds [44], diminished in the spectra of the polymers, indicating successful polymerization. Finally, the shift of the were diminished in the spectra of the polymers, indicating successful polymerization. Finally, the band corresponding to the C=O bond from 1681 cm 1 in the monomer to 1713 cm 1 in the polymers shift of the band corresponding to the C=O bond from− 1681 cm−1 in the monomer −to 1713 cm−1 in the further conﬁrmed those observations. In PEV, after SSP, the peak attributed to –OH groups is further polymers further confirmed those observations. In PEV, after SSP, the peak attributed to –OH groups reduced, due to their reaction with –COOH end groups and molecular weight increase. is further reduced, due to their reaction with –COOH end groups and molecular weight increase.

Polymers 2019, 11, 1672 7 of 27 Polymers 2019, 11, x FOR PEER REVIEW 7 of 27 Polymers 2019, 11, x FOR PEER REVIEW 7 of 27

PEV after SSP PEV after SSP

PEV before SSP PEV before SSP Absorbance

Absorbance Monomer Monomer

4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000-1 1500 1000 500 Wavenumber (cm -1) Wavenumber (cm )

FigureFigure 1. 1. FT-IRFT-IR spectra spectra of of 4-(2-hydroxyethoxy)-3-metho 4-(2-hydroxyethoxy)-3-methoxybenzoicxybenzoic acid acid and and the the obtained obtained polymers, polymers, Figure 1. FT-IR spectra of 4-(2-hydroxyethoxy)-3-methoxybenzoic acid and the obtained polymers, beforebefore and and after after solid solid state state polymerization polymerization (SSP). (SSP). before and after solid state polymerization (SSP). SimilarlySimilarly to to FT-IR, FT-IR, the the NMR NMR spectra spectra showed showed th thatat a a successful successful polymerization polymerization was was carried carried out out Similarly to FT-IR, the NMR spectra showed that a successful polymerization was carried out (Figure(Figure 2).2). This This was was mainly inferred by the shift ofof thethe methylenemethylene protonsprotons ofof thethe O–CHO–CH2–CH–CH22–OH–OH (Figure 2). This was mainly inferred by the shift of the methylene protons of the O–CH2–CH2–OH segmentsegment from from 4.09 4.09 and and 4.20 4.20 ppm ppm to to 4.40 4.40 and and 4.68 4.68 ppm, ppm, as as a a result result of of the the esterification esteriﬁcation of of the the hydroxyl hydroxyl segment from 4.09 and 4.20 ppm to 4.40 and 4.68 ppm, as a result13 of the esterification of the hydroxyl group.group. Corresponding Corresponding changes changes are are also also observed observed in in the the 13CC spectra, spectra, and, and, in in addition, addition, the the peak peak group. Corresponding changes are also observed in the 13C spectra, and, in addition, the peak correspondingcorresponding to to the COOHCOOH carboncarbon at at 171.8 171.8 ppm ppm disappeared disappeared and and was was replaced replaced by aby peak a peak at 168.1 at 168.1 ppm corresponding to the COOH carbon at 171.8 ppm disappeared and was replaced by a peak at 168.1 ppmcorresponding corresponding to the to carbon the carbon of the of ester the ester moieties. moieties. ppm corresponding to the carbon of the ester moieties.

* *

Monomer Monomer

Monomer * * Monomer * *

Polymer Polymer

864180 160 140 120 100 80 60 40 20 864ppm 180 160 140 120ppm 100 80 60 40 20 ppm ppm (a) (b) (a) (b) FigureFigure 2. 2. (a(a) )1H,1H, and and (b (b) )1313CC NMR NMR spectra spectra of of 4-(2-hydroxyethoxy 4-(2-hydroxyethoxy)-3-methoxybenzoic)-3-methoxybenzoic acid acid and and PEV. PEV. Figure 2. (a) 1H, and (b) 13C NMR spectra of 4-(2-hydroxyethoxy)-3-methoxybenzoic acid and PEV. TheThe solvent solvent peaks peaks have have been been removed removed for for clarity. clarity. Peaks Peaks indicated indicated with with an an asterisk asterisk (*) (*) are are due due to to The solvent peaks have been removed for clarity. Peaks indicated with an asterisk (*) are due to reactionsreactions of of the the monomer monomer in in the the presen presencece of of trifluoroacetic triﬂuoroacetic acid acid (TFA-d). (TFA-d). reactions of the monomer in the presence of trifluoroacetic acid (TFA-d). 3.2. Thermal Properties and Crystallization Behavior of PEV 3.2. Thermal Properties and Crystallization Behavior of PEV 3.2. Thermal Properties and Crystallization Behavior of PEV The synthesized PEV samples were studied with respect to their thermal transitions with DSC The synthesized PEV samples were studied with respect to their thermal transitions with DSC (FigureThe3 ).synthesized The low molecular PEV samples weight were of studied the as-prepared with respect polyester to their synthesized thermal transitions with TBT with catalyst DSC (Figure 3). The low molecular weight of the as-prepared polyester synthesized with TBT catalyst after (Figureafter solid 3). The state low polymerization molecular weight showed of the a meltingas-prepa peakred polyester temperature synthesizedTm = 261 with◦C (FigureTBT catalyst3a). After after solid state polymerization showed a melting peak temperature Tm = 261 °C (Figure 3a). After solidquenching, state polymerization the amorphous showed sample showeda melting a Tpeakg = 75 temperature◦C and a sharp Tm = cold261 crystallization°C (Figure 3a). peak After at quenching, the amorphous sample showed a Tg = 75 °C and a sharp cold crystallization peak at Tcc = quenching,T = 107 C. theOn amorphous cooling at sample 10 C/min, showed a broad a Tg peak= 75 °C appeared and a sharp at 145 coldC. crystallization On the contrary, peak the at T PEVcc = 107cc °C. On◦ cooling at 10 °C/min,◦ a broad peak appeared at 145 °C. On the◦ contrary, the PEV sample 107sample °C. On synthesized cooling at using 10 °C/min, Sb2O3 as a broad catalyst peak did notappeared crystallize at 145 at all°C. onOn cooling. the contrary, It also showedthe PEV asample higher synthesized using Sb2O3 as catalyst did not crystallize at all on cooling. It also showed a higher Tg = synthesizedTg = 83 ◦C and using a higher Sb2O3 cold-crystallization as catalyst did not temperaturecrystallize atT allcc =on132 cooling.◦C. These It also values showed are comparable a higher Tg to= 83 °C and a higher cold-crystallization temperature Tcc = 132 °C. These values are comparable to those 83 °C and a higher cold-crystallization temperature Tcc = 132 °C. These values are comparable to those

Polymers 2019, 11, 1672 8 of 27 Polymers 2019, 11, x FOR PEER REVIEW 8 of 27 forthose PET for (T PETg = 80 ( T°Cg =and80 ◦TCm = and 252T °C),m = 252while◦C), the while Tm is thehigherTm isthan higher that than for PEF that (T forg = PEF 87 °C (T gand= 87 Tm◦ C= 230 and °C)Tm [6].= 230 ◦C) [6].

a PEV SSP TBT o b T =261oC Tm=261 C PEV SSP Sb2O3 m 2 W/g 1W/g As received T =259oC m As received Quenched o o T =251 C w (W/g) Endo Up m Tg=75 C

o Quenched Tg=83 C o Tcc=107 C o Tcc=132 C Cooling at 10oC/min o o Cooling at 10 C/min Normalized Heat Flo Normalized Heat Flow (W/g) Endo Up Tc=145 C 0 25 50 75 100 125 150 175 200 225 250 275 300 325 0 25 50 75 100 125 150 175 200 225 250 275 300 325 o o Temperature ( C) Temperature ( C) FigureFigure 3. 3. DifferentialDiﬀerential scanning scanning calorimetry calorimetry (DSC) (DSC) scans scans for: for: (a ()a )PEV PEV prepared prepared using using TBT TBT catalyst catalyst and and

(b(b) )PEV PEV prepared prepared using using Sb Sb2O2O3 3catalyst.catalyst.

T TheThe melting melting temperature temperature of of the the cold-crystallized cold-crystallized sample sample prepared prepared with Sb 2OO33 waswas Tmm == 251251 °C,◦C, T muchmuch lower lower than than that that for for the the cold-crystalli cold-crystallizedzed sample sample prepared prepared with with the the use use of of TBT TBT ( (Tmm == 259259 °C).◦C). Finally,Finally, the the enthalpy enthalpy of of melting melting and and crystallization crystallization was was lower lower in in the the case case of of the the higher higher molecular molecular weightweight sample. sample. TheThe cold-crystallization cold-crystallization behavior behavior of ofthe the polyesters polyesters was wasexamined examined at different at different heating heating rates after rates theirafter melt-quenching. their melt-quenching. Both samples Both samples are cold-crystal are cold-crystallizedlized under all under the heating all the rates heating applied rates (Figure applied T 4a,b).(Figure However,4a,b). However, the lower the molecular lower molecular weight weightsample samplealways alwaysshowed showed a lower a lowerTcc and acc largerand a largerfinal enthalpy,final enthalpy, which which also means also means a larger a larger final degree final degree of crystallinity. of crystallinity. As can As be can seen be seenin Figure in Figure 4c,d,4 thec,d, lowerthe lower molecular molecular weight weight sample sample crystallized crystallized even up evenon cooling upon cooling at the fastest at the rate fastest (20 rate°C/min), (20 ◦ Cwhilst/min), thewhilst higher the molecular higher molecular weight PEV weight did PEV not didcrystallize not crystallize at all even at allwhen even cooling when coolingwith the with slower the rate slower of 5rate °C/min. of 5 ◦InC /general,min. In general,the main the obstacle main for obstacle PEV’s for crystallization PEV’s crystallization proved to proved be poor to nucleation be poor nucleation density. Thisdensity. behavior This is behavior different is from different that of from PET that which of PET always which crystallizes always crystallizes upon cooling upon from cooling the melt from even the atmelt fast evenrates. at It fast is rather rates. similar It is rather to that similar of PEF, to that which of PEF,also whichshows alsoslow shows crystallization slow crystallization [6]. [6].

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a b o o 20 C/min 20 C/min PEV Sb2O3 PEV TBT

15oC/min 15oC/min

10oC/min 10oC/min

5oC/min 5oC/min 1W/g 3 W/g Up End (W/g) Flow Heat Normalized Normalized Flow (W/g) End Up Heat 0 50 100 150 200 250 300 0 50 100 150 200 250 300 o o Temperature ( C) Temperature ( C) c 20oC/min d 1 W/g

20oC/min 5 W/g 15oC/min

15oC/min

10oC/min 10oC/min

o o 5 C/min Normalized Flow (W/g) End Heat Up 5 C/min Normalized End Up Flow (W/g) Heat

75 100 125 150 175 200 225 250 275 300 75 100 125 150 175 200 225 250 275 300 o o Temperature ( C) Temperature ( C) FigureFigure 4. ((aa,,bb)) heating heating scans scans at at different diﬀerent rates rates for for melt melt-quenched-quenched low low and and higher higher molecular molecular sample, sample,

respectively,respectively, (c,d) cooling scans at didifferentﬀerent ratesrates forfor thethe lowlow andand higherhigher MMww sample, respectively.

TheThe isothermal isothermal crystallization crystallization from from the the melt melt was was studied studied for for both samples at at low temperatures (large(large supercoolings), because,because, inin general, general, the the process process was was slow. slow. Slow Slow crystallization crystallization was was observed observed for forthe the high high molecular molecular weight weight sample, sample, while while under under low orlow even or moderateeven moderate supercoolings supercoolings both the both higher the and the lower Mw samples could not crystallize fast (Figure5a,b). In accordance with the previously higher and the lower Mw samples could not crystallize fast (Figure 5a,b). In accordance with the discussed ﬁndings, the crystallization of the higher Mw sample was much slower, as can be concluded previously discussed findings, the crystallization of the higher Mw sample was much slower, as can begiven concluded the large given diﬀ erencesthe large in differences the crystallization in the crys half-timestallization presented half-times in presented Figure5c. in In Figure general, 5c. the In general,crystallization the crystallization half-times increased half-times exponentially increased exponentially with temperature, with temperature, as was expected. as was expected.

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a b PEV TBT

120oC 130oC 120oC 140oC 130oC 150oC 140oC 160oC 150oC 1 W/g 170oC 160oC 180oC 170oC 190oC 0.5 W/g 180oC 200oC Normalized Heat Flow (W/g) End Up Normalized Heat Flow (W/g) End Up

0 5 10 15 20 25 30 0 40 80 120 160 200 Time (min) Time (min) c 100 PEV SSP Sb O 2 3 PEV SSP TBT 80

20 Crystallization half-time (min) half-time Crystallization

0 110 120 130 140 150 160 170 180 190 200 210 Temperature (oC)

Figure 5. (a(a) )and and ( (bb)) isothermal isothermal crystallization crystallization peaks peaks for for the the low low and and higher Mw PEVPEV samples, samples, respectively,respectively, and (c)) crystallizationcrystallization half-timeshalf-times vsvs temperature.temperature.

To estimate the enthalpy of fusion fusion of of the the pure pure cr crystallineystalline PEV, PEV, a a series series of of samples samples were were prepared. prepared. First, the sample was melt-quenched. The The WAXD WAXD pattern pattern and and the the DSC DSC thermogram thermogram were were recorded. recorded. Afterwards, it was heated to 80 °C◦C for 2 min, and the WAXD pattern and DSC thermogram were recorded again again after after cooling cooling to toroom room temperature. temperature. The sample The sample was heated was heated several several times to times an always to an higheralways temperature higher temperature for a specific for a speciﬁc amount amount of time of to time achieve to achieve some someadditional additional crystallization. crystallization. The crystallinityThe crystallinity values values were were calculated calculated from from the WAXD the WAXD patterns patterns along along with with the relative the relative areas areas under under the crystallinethe crystalline peaks, peaks, Ac, andAc, the and amorphous the amorphous background, background, Aam, usingAam, Equation using Equation (2), according (2), according to Hay toet al.Hay [45]: et al. [45]: 1 Aam − Xc = 1 + 𝐴 (2) 𝑋 = (1+Ac ) (2) 𝐴 The WAXD patterns of the tested samples are presented in Figure6a, and the heat of fusion valuesThe determined WAXD patterns with DSC of the was tested plotted samples against are the pres degreeented of crystallinityin Figure 6a, determined and the heat with of WAXDfusion values determined with DSC was plotted against the degree of crystallinity determined with WAXD (Figure6b). Extrapolation to 100% degree of crystallinity resulted in a value of ∆Hm = 166 16 J/g. (Figure 6b). Extrapolation to 100% degree of crystallinity resulted in a value of ΔΗm = 166 ±± 16 J/g. Although the value calculated by applying the group contributions method is ∆Hm = 104 J/g [46], Althoughthe value the of 166value J/g calculated seems much by applyi moreng reasonable, the group ascontributions PEV samples method give inis Δ generalΗm = 104 high J/g [46], melting the valueenthalpy of 166 values J/g seems of about much 70–100 more J/ greasonable, even after as cold-crystallization, PEV samples give andin general this applies high melting especially enthalpy for the values of about 70–100 J/g even after cold-crystallization, and this applies especially for the low Mw low Mw sample. sample.

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180 a b180 a b 160 ΔH o=166.1J/g 160 o m ΔHm =166.1J/g As received 140 As receivedo 230o C 30 min 140 230 Co 30 min 210o C 30 min 210 Co 30 min 120 200o C 30 min 120 200 Co 30 min 180o C 10 min 180 Co 10 min 100 150o C 10 min 100 150 Co 10 min 125o C 10 min 125 C o10 min 80 115o C 5 min 80 115 Co 5 min 110o C 5 min 110 Co 5 min

Intensity (a.u.) Intensity 105o C 5 min 60 Intensity (a.u.) Intensity 105 Co 5 min 60 100o C 5 min 100 Co 5 min o 95 C 5 min (J/g) fusion of Enthlpy 40

95 Co 5 min (J/g) fusion of Enthlpy 40 85o C 5 min 85 Co 5 min 80o C 2 min 80 C 2 min 2020 QuenchedQuenched 0 0 5 101520253035404550 101520253035404550 00 102030405060708090100 102030405060708090100 Degree of Crystallinity (%) AngleAngle 2θ 2θ (deg) (deg) Degree of Crystallinity (%) Figure 6. 6. (((aaa))) wide-anglewide-angle Wide-angle X-ray X-ray X-ray diffraction diffraction diﬀraction (WAXD) (WAXD) (WAXD) pattern pattern patternss fors for for the the the PEV PEV PEV specimens specimens specimens after after after annealing annealing annealing at at at thethe indicated indicated conditionsconditions and and ( b( b) )enthalpy enthalpy of of fusion fusion vs vs degree degree of of crystallinity crystallinity plot. plot.

The isothermally crystallizedcrystallized samplessamples showed showed multiple multiple melting melting behaviors behaviors upon upon subsequent subsequent heating, as as can can bebe seenseen inin FigureFigure 7a, 77a,a, for forfor the thethe low lowlow M Mw wPEV. PEV.PEV. An An exothermic exothermic recrystallization recrystallization peak peak was was observed aboveabove 200200200◦ C.°C.°C. The TheThe samples samplessamples crystallized crystallized crystallized at theat at the lowthe low temperaturelow temperature temperature region, region, region, below below 180below◦ C,180 showed180 °C, °C, showeddual melting, dual withmelting,melting, the recrystallization withwith thethe recrystallizationrecrystallization peak before peak ﬁnalpeak melting.before before final Thefinal samplesmelting. melting. thatThe The crystallized samples samples that above that crystallized180 ◦C showed above triple 180180 melting. °C °C showed showed For triple triple the higher melting. melting.Mw For Fortriple the the higher melting higher M wasMw triplew triple observed melting melting above was was observed 160 observed◦C (Figure above above7 b). 160Furthermore, °C (Figure the 7b).7b). recrystallization Furthermore,Furthermore, the the exotherm recrystallization recrystallization is not so exotherm profound,exotherm is is slowernot not so so profound, than profound, for the slower lowslower Mthanw thansample for for w thetheand low masked Mw sample bysample the melting. andand maskedmasked In any byby case, the the itmelting. melting. seems that In In any the any multiplecase, case, it itseems melting seems that behaviorthat the the multiple multiple is associated melting melting with behaviorpartial melting, is associated recrystallization withwith partial partial and melting, melting, ﬁnal melting.recrystallization recrystallization and and final final melting. melting.

a PEV TBT b PEV Sb O a b PEV Sb2 3O o PEV TBT 2 3 180 C 180oC 1 W/g o 1 W/g 200 Co 1 W/g 200 C 1 W/g 160oC 180oC 160oC 180oC o 160 Co 160 C o 140 C o 140oC 140 C 140oC 120oC 120oC 120oC 120oC Normalized Heat Flow (W/g) End Up Normalized Heat Flow (W/g) End Up Normalized Heat Flow (W/g) End Up Normalized Heat Flow (W/g) End Up 100 120 140 160 180 200 220 240 260 280 300 100 120 140 160 180 200 220 240 260 280 300 100 120 140 160 180 200 220o 240 260 280 300 Temperature ( C) 100 120 140 160Temperature 180 200 (oC) 220 240 260 280 300 o Temperature ( C) Temperature (oC) Figure 7. Subsequent DSC heating scans after isothermal melt-crystallization at the indicated

Figuretemperatures 7. SubsequentSubsequent for (a) low DSCM DSCw and heating heating (b) higher scans scans M wafter afterPEV samples.isothermal isothermal melt-crystallization melt-crystallization at at the the indicated temperaturestemperatures for ( a) low Mw andand ( (bb)) higher higher MMww PEVPEV samples. samples. As stated above, the stage which determines PEV’s crystallization rates is nucleation, which is very Asslow stated at temperatures above,above, thethe stage stage above which which 180 determines determines°C. To investigate PEV’s PEV’s crystallization crystallization the melting rates behaviorrates is nucleation, is nucleation, after isothermal which which is very is verycrystallizationslow atslow temperatures at attemperatures high abovetemperatures, 180above◦C. Tocrystallizations180 investigate °C. To investigate the were melting performed the behavior melting after after self-nucleation. behavior isothermal after crystallization As isothermal can be crystallizationseenat high in Figure temperatures, 8a,at highin the crystallizations temperatures, DSC heating traces werecrystallizations performedfor the low were afterMw sample performed self-nucleation. after afterself-nucleated Asself-nucleation. can be crystallization seen in As Figure can 8bea, seenatin temperatures the in DSC Figure heating 8a, above in tracesthe 225 DSC for°C, heating themultiple low tracesM meltingw sample for thepeaks after low appear. self-nucleatedMw sample For samples after crystallization self-nucleated crystallized at atcrystallization temperatures Tc < 237.5 at°Cabove temperatures triple 225 melting◦C, multiple above peaks 225 meltingare °C,observed. multiple peaks Crystallization appear. melting For peaks samples at appear.235.5 crystallized < ForTc < samples245at °CT cresults <245 Tc in °C< dual245 give °C melting a resultssingle upon metingin dual subsequent peak.melting uponMultipleheating. subsequent melting Finally, samplesis heating. observed crystallized Fina in DSClly, samples traces at T cof> crystallizedthermoplastics245 ◦C give at a singleTafterc > 245isothermal meting °C give peak. crystallization a single Multiple meting melting[47,48] peak. is Multipleandobserved more melting inoften DSC for tracesis low observed M ofw thermoplastics polymers in DSC [49]. traces afterIt has of isothermal thermoplasticsbeen attributed crystallization toafter the isothermal melting [47,48 of] andcrystalscrystallization more of often different for[47,48] low andstabilityMw polymersmore (dual often [ 49 morphologyfor]. Itlow has M beenw polymers mechanism) attributed [49]. to theandIt has melting the been me ofattributedlting, crystals re-crystallization, ofto ditheﬀerent melting stability ofre-melting crystals (dualmorphology ofprocess different stability(reorganizationmechanism) (dual and morphologymechanism) the melting, [50].mechanism) re-crystallization, Since exclusive and the re-melting melting melting, ofprocess the re-crystallization, originally (reorganization formed re-melting crystals mechanism) is processvery [50 ]. (reorganizationdifficult to be observed, mechanism) the origin [50]. of Since the multiple exclusive melting melting peaks of appearingthe originally in DSC formed curves crystals of polymers is very difficult to be observed, the origin of the multiple melting peaks appearing in DSC curves of polymers

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Polymers 2019, 11, x FOR PEER REVIEW 12 of 27 Since exclusive melting of the originally formed crystals is very diﬃcult to be observed, the origin isof thestill multiplecontroversially melting peaksdiscussed. appearing Recent in DSCworks curves utilizing of polymers fast chip is stillcalorimetry controversially allow discussed.a better understandingRecent works utilizing of melting fast [51,52]. chip calorimetry allow a better understanding of melting [51,52].

a PEV b

4 W/g 240oC 250oC 247.5oC 3 W/g 245oC 235oC 242.5oC 240oC 230oC 237.5oC 235oC 225oC 232.5oC o 230 C o III o 220 C 227.5 C II o I 225 C 215oC Normalized Heat Flow (W/g) End Up End (W/g) Flow Heat Normalized Normalized Heat Flow (W/g) End Up

220 230 240 250 260 270 280 290 220 240 260 280 o Temperature (oC) Temperature ( C)

FigureFigure 8. SubsequentSubsequent DSC heating scansscans atat 2020◦ °C/minC/min after after self-nucleated self-nucleated isothermal isothermal crystallization crystallization at atthe the indicated indicated temperatures temperatures for for (a )(a lower) lowerM wMandw and (b ()b higher) higherM Mw wPEV PEV sample. sample.

ToTo investigate thethe meltingmelting behavior behavior of of PEV PEV in in depth, depth, the the higher higherMw Msamplew sample was was also also studied, studied, and andindicative indicative heating heating scans scans are shownare shown in Figure in Figure8b. Since8b. Since this this sample sample crystalized crystalized very very slowly slowly despite despite the theprolonged prolonged crystallization crystallization times, times, the the heat heat of fusionof fusi wason was rather rather low low or ator least at least much much smaller smaller than than the thecorresponding corresponding values values for thefor the low lowMw Msample.w sample. In anyIn any case, case, multiple multiple melting melting behavior behavior was was observed observed for forthe the higher higherMw Msamplew sample too. too. To To overcome overcome uncertainty, uncertainty, the the melting melting behavior behavior of ofthe the lowlow MMww PEV after after crystallizationcrystallization at at elevated elevated temperatures temperatures and and particularly particularly the the medium medium melting melting temperature temperature peak peak was was usedused for for the the estimation estimation of of the the equi equilibriumlibrium melting temperature of PEV.

3.2.1.3.2.1. Evaluation Evaluation of of the the Equilibrium Equilibrium Melting Melting Temperature Temperature 0 The equilibrium melting temperature (Tm ) is an important thermodynamic parameter for The equilibrium melting temperature (𝑇 ) is an important thermodynamic parameter for polymericpolymeric materials. materials. The The Hoffman Hoffman and and Weeks Weeks method [53] [53] is is one one of of the the most most commonly commonly used used for for the the 0 evaluation of Tm . In the Hoffman and Weeks method, the measured melting points of the samples evaluation of 𝑇 . In the Hoffman and Weeks method, the measured melting points of the samples crystallizedcrystallized at at different different temperatures temperatures are are plotte plottedd against the crystallization temperature temperature and and the the 0 extrapolation of the linear fit to the Tc = Tm line, the Tm can be estimated as the intersection point. extrapolation of the linear fit to the Tc = Tm line, the 𝑇 can be estimated as the intersection point. The modelThe model is described is described by the by Equation the Equation (3): (3): 1 ! 𝑇 0 1 TC 𝑇Tm == 𝑇T11 − ++ (3)(3) m −𝛽β 𝛽 β where Tm is the observed melting temperature of a crystal formed at a temperature Tc, while β is the where Tm is the observed melting temperature of a crystal formed at a temperature Tc, while β is the * thickening parameter equal to Lc/Lc* . The thickening parameter β indicates the ratio of the thickness thickening parameter equal to Lc/Lc . The thickening parameter β indicates the ratio of the thickness of * of the mature crystallites Lc to that of the initial ones *Lc [53]. the mature crystallites Lc to that of the initial ones Lc [53]. The Hoffman and Weeks plot for PEV was constructed for the PEV TBT, and it is presented in The Hoffman and Weeks plot for PEV was constructed for the PEV TBT, and it is presented Figure 9. The calculated value after the extrapolation was 𝑇 0= 301.4 °C. We used the group in Figure9. The calculated value after the extrapolation was Tm = 301.4 ◦C. We used the group contributions to calculate the enthalpy and entropy of fusion [46]. Given that in equilibrium 𝑇 = contributions to calculate the enthalpy and entropy of fusion [46]. Given that in equilibrium T0 = ∆m m ∆Sm and using the calculated values ΔHm = 20 KJ/mol = 103.6 J/g and ΔSm = 35 J/(mol K) = 0.1814 J/(g and using the calculated values ∆Hm = 20 KJ/mol = 103.6 J/g and ∆Sm = 35 J/(mol K) = 0.1814 J/(g K) a 0 K)value a value of T mof= 𝑇298.4 = 298.4◦C was °C found,was found, very very close close to the to experimentally the experimentally estimated estimated one. one.

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310 320 o o Tm =301.4 C 310300 o o Tm =301.4 C C)

o 300290 C) o 290280 280 270

270 260 PEV 260 PEV 250 250 240 240MeltingTemperature ( MeltingTemperature ( 230 230

220220 220220 240 240 260 260 280 280 300 300 320 320 o CrystallizationCrystallization Temperature Temperature (oC) ( C) FigureFigureFigure 9. Hoffman-Weeks 9. 9. Hoffman-WeeksHoﬀman-Weeks plot for plot plot PEV. for for PEV. PEV.

3.2.2.3.2.2.3.2.2. SpheruliticSpherulitic Morphology Morphology Morphology and and and Spherulite Spherulite Spherulite Growth Growth Growth Rates Rates The spherulitic morphology of PEV was studied using PLM. As can be seen in Figure 10, TheThe spheruliticspherulitic morphology morphology of of PEV PEV was was studied studied using using PLM. AsPLM. can As be seencan inbe Figure seen 10in , coarseningFigure 10, coarsening was evidenced with increasing crystallization temperature. A clear difference can be coarseningwas evidenced was with evidenced increasing with crystallization increasing crystall temperature.ization Atemperature. clear diﬀerence A clear can bedifference observed can above be observed above 235 °C. The spherulite growth rates were measured from the increase in the 235 ◦C. The spherulite growth rates were measured from the increase in the spherulite with time at observedspherulite abovewith time 235 at various°C. The temperatures. spherulite Figuregrowth 10 showsrates werethe variation measured of the from spherulite the growthincrease in the various temperatures. Figure 10 shows the variation of the spherulite growth rate with temperature. spheruliterate with temperature. with time at A variousbell-shaped temperatures. curve can be Figure seen as 10the shows measurements the variation were performed of the spherulite over a growth A bell-shaped curve can be seen as the measurements were performed over a wide temperature range. ratewide with temperature temperature. range. A bell-shaped curve can be seen as the measurements were performed over a wide temperature range.

150 °C 165 °C 180 °C 210 °C

150(a) °C (b165) °C (c) 180 °C (d) 210 °C

(a) (b) (c) (d)

225 °C 235 °C 245 °C 250 °C

(e) (f) (g) (h)

Figure225 10. 10. Polarized °CPolarized light light microscopy microscopy (PLM)235 (PLM) micrographs°C micrographs of PEV ofspherulites PEV spherulites245 at °C various at temperatures: various temperatures: 250 °C (a) 150 °C, (b) 165 °C, (c) 180 °C, (d) 210 °C, (e) 225 °C, (f) 235 °C, (g) 245 °C and (h) 250 °C. (a) 150 ◦C, (b) 165 ◦C, (c) 180 ◦C, (d) 210 ◦C, (e) 225 ◦C, (f) 235 ◦C, (g) 245 ◦C and (h) 250 ◦C. (e) (f) (g) (h) 3.2.3.3.2.3. Application of of Secondary Secondary Nucleation Nucleation Theory Theory Using Using the Spherulitic the Spherulitic Growth Growth Rates Rates Figure 10. Polarized light microscopy (PLM) micrographs of PEV spherulites at various temperatures: The spherulitic growth rate G data from isothermal crystallization can be analyzed with the (Thea) 150 spherulitic °C, (b) 165 °C, growth (c) 180 rate °C, (Gd)data 210 °C, from (e) 225 isothermal °C, (f) 235 crystallization °C, (g) 245 °C and can (h) be 250 analyzed °C. with the Lauritzen-HoLauritzen-Hoffmanﬀman secondary secondary nucleation nucleation theory theory [54]. In [54 the]. Inparticular the particular theory, theory,G is a functionG is a functionof the of the isothermal crystallization temperature, given as follows: 3.2.3.isothermal Application crystallization of Secondary temperature, Nucleation given Theory as follows: Using the Spherulitic Growth Rates " # " # The spherulitic growth rate G data from isothermalU∗ crystallizationKg can be analyzed with the G = G0 exp exp (4) Lauritzen-Hoffman secondary nucleation th−eoryR(TC [54].T In) the particularTc(∆T) f theory, G is a function of the isothermal crystallization temperature, given as follows:− ∞

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∗ 𝑈 𝐾 𝐺 = 𝐺 𝑒𝑥𝑝 − 𝑒𝑥𝑝 (4) where G0 is the pre-exponential factor.𝑅(𝑇 The first−𝑇 exponential) 𝑇(∆𝑇) term𝑓 is associated with the impact of wherethe di Gff0usion is the processpre-exponential to the growth factor. rate,The first while exponent the secondial term exponential is associated term with describes the impact the of nucleation the * diffusionprocess. processU is the to activation the growth energy rate, while for molecular the second diff exponentialusion across term the interfacialdescribes the boundary nucleation between process.melt and U* crystals, is the activation and T isenergy the temperature for molecular below diffusion which across diffusion the interfacial stops. Kg is boundary a nucleation between parameter, ∞ 0 meltwhile and∆ Tcrystals,is the degree and T∞ of is undercoolingthe temperature (∆ belowT = Tm whichTc) diffusion and f is a stops. correction Kg is a factor nucleation which parameter, is close to unity 0 − whileat high ΔT temperatures is the degree andof undercooling is given as f (=ΔΤ2T =c /T(Tmm −+ TTc)c ).[and54 ]f Theis a correction equilibrium factor melting which point is close of PEV to was 0 * unityset equal at high to 301.4temperatures◦C, while and the is glassgiven transition as f = 2Tc/( wasTm + set Tc) equal.[54] The to 75 equilibrium◦C, while themelting values pointU of= 6285PEV J/mol * wasand setT equal= (T gto 301.430) K°C, were while also the used glass [ 54transition]. was set equal to 75 °C, while the values U = 6285 ∞ ∞ − J/mol andPrior T to = ( calculatingTg – 30) K were the also nucleation used [54]. parameter Kg, the double logarithmic transformation of EquationPrior to (4) calculating is taken: the nucleation parameter Kg, the double logarithmic transformation of Equation (4) is taken: U Kg ( ) + ∗ = ( ) ln G ∗ ln G0 (5) R𝑈(TC T ) 𝐾− Tc(∆T) f ln(𝐺) + − =∞ ln(𝐺 ) − (5) 𝑅(𝑇 −𝑇 ) 𝑇 (∆𝑇)𝑓 The plot of the left-hand side of Equation (5) versus 1/Tc(∆T) can be fitted with a straight line, andThe the plot slope of andthe left-hand intercept side of this of Equation line give (5) the versus nucleation 1/Tc(Δ constantT) can be andfitted the with pre-exponential a straight line, factor, andrespectively. the slope and The intercept critical of breakpoints this line give in the the nu graph,cleation which constant can and be identifiedthe pre-exponential by the change factor, in the respectively.slope of the The line, critical indicate breakpoints regime transitions in the graph, accompanied which can be by identified morphological by the change changes in the of theslope formed ofcrystals. the line, Theindicate calculated regime valuestransitions of the accompanie spheruliticd by growth morphological rate versus changes temperature of the formed were crystals. used, and the TheLauritzen-Ho calculated valuesffman plotof the was spherulitic constructed growth for PEVrate versus (Figure temperature 11). As it can were be seen used, in and the the graph, Lauritzen- a breakpoint Hoffman plot was constructed for PEV (Figure 11). As it can be seen in the graph, a breakpoint appears at 238 ◦C, corresponding to regime I to regime II transition. A second breakpoint appears appears at 238 °C, corresponding to regime I to regime II transition. A second breakpoint appears at at about 165 ◦C. This corresponds to the regime II to regime III transition. These regime transitions aboutwere 165 verified °C. This by corresponds the transitions to the in regime the spherulitic II to regime morphologies III transition. observedThese regime with transitions PLM, as were discussed verified by the transitions in the spherulitic morphologies observed with PLM,5 2 as discussed above.5 2 above. The calculated nucleation parameter values were KgI = 2.45 10 K , KgII = 1.19 10 K and 5 2 5 2 The calculated nucleation5 2 parameter values were KgI =5 2.45 × 10 K5 , KgII ×= 1.19 × 10 K and KgIII× = 2.74 KgIII = 2.74 10 K . Moreover, KgI/KgII = 2.45 10 /1.19 10 = 2.06 that is very close to the expected × 105 K2. Moreover,× KgI/KgII = 2.45 × 105/1.19 × 105 =× 2.06 that is× very close to the expected KgI/KgII ratio K K K K 5 5 K gI/ gII ratio value 2, while gIII5 / gII = 2.745 10 /1.19 10 = 2.30. For comparison, the gII value5 was value 2, while KgIII5/KgII = 2.74 × 10 /1.19 × 10 = 2.30.× For comparison,× the KgII value was found 2.505 ×2 10 found 2.50 10 by Hay [45]. For poly(propylene terephthalate) (PPT)5 2 a KgII = 1.47 10 K [55] and by Hay [45]. ×For poly(propylene terephthalate)5 2 (PPT) a KgII = 1.475 ×2 10 K [55] and for PBT× the values for PBT the values KgII = 0.53 10 K and KgIII = 1.86 10 K have been reported [56]. KgII = 0.53 × 105 K2 and KgIII = 1.86× × 105 K2 have been reported× [56].

4 7.5

a 5 b K =2.74*10 7.0 gIII

6.5 3 K =1.19*105 6.0 gII T=165oC 5.5 T=238oC

5.0 /R(T-Too))

2 * 4.5

4.0 (LnG+U Growth rate (ìm/sec) rate Growth

1 3.5 5 K =2.45*10 gI 3.0

140 150 160 170 180 190 200 210 220 230 240 250 260 0.000015 0.000020 0.000025 0.000030 0.000035 0.000040 0.000045 Temperature (oC) (1/T(ΔT)f) FigureFigure 11. 11. (a)( Spherulitea) Spherulite growth growth rates rates vs temperaturevs temperature and ( andb) Lauritzen-Hoffman (b) Lauritzen-Hoﬀ plotman for plot PEV. for PEV.

3.3.3.3. Thermal Thermal Degradation Degradation Kinetics Kinetics TheThe kinetics kinetics of thethe thermal thermal degradation degradation of theof PEVthe PEV polymer polymer with higherwith higher crystallinity crystallinity (synthesized (synthesizedwith TBT catalyst) with TBT were catalyst) studied were with studied TGA. Initially, with TGA. a heating Initially, rate a of heating 10 ◦C/min rate in of nitrogen 10 °C/min atmosphere in nitrogenwas applied, atmosphere in order was to make applied, a preliminary in order evaluationto make a of preliminary the thermal degradationevaluation of process, the thermal before fully degradationmonitoring process, it by means before of kineticfully monitoring analysis. Asit by it canmeans be seen of kinetic in Figure analysis. 12a, the As thermalit can be decomposition seen in Figure 12a, the thermal decomposition of PEV is a one-step procedure, while its maximum of PEV is a one-step procedure, while its maximum decomposition rate is achieved at ~420 ◦C, decompositioncorresponding rate to the is achieved peak of the at derivative~420 °C, corresp of theonding TGA curve. to the Furthermore,peak of the derivative the rest key of parametersthe TGA for the evaluation of TGA measurements, which are the temperatures corresponding to 1%, 5% and 10% Polymers 2019, 11, x FOR PEER REVIEW 15 of 27 Polymers 2019, 11, 1672 15 of 27 curve. Furthermore, the rest key parameters for the evaluation of TGA measurements, which are the temperatures corresponding to 1%, 5% and 10% mass loss, are recorded at 327 °C, 373 °C and 388 °C, respectively.mass loss, are Finally, recorded according at 327 ◦C, to 373 Figure◦C and 12a, 388 PEV◦C, respectively.is not fully degraded Finally, according up to 550 to °C, Figure exhibiting 12a, PEV a residualis not fully mass degraded of ~11% up at tothat 550 specific◦C, exhibiting temperature. a residual mass of ~11% at that speciﬁc temperature.

(b) 100 (a) 100

80 80

60 60

dTG

40 Mass (%)

Mass (%) 40 5 °C/min 10 °C/min 20 15 °C/min 20 20 °C/min

0 100 200 300 400 500 300 350 400 450 500 Temperature (°C) Temperature (°C)

Figure 12. (a) Mass loss curve and dTG of PEV at 10 ◦C/min in N2,(b) Mass loss curves of PEV under Figure 12. (a) Mass loss curve and dTG of PEV at 10 °C/min in N2, (b) Mass loss curves of PEV under multiple heating rates. multiple heating rates. Kinetic analysis of the thermal degradation process was carried out, in order to further investigate Kinetic analysis of the thermal degradation process was carried out, in order to further the thermal decomposition of PEV. According to ICTAC [57], measurements under different heating investigate the thermal decomposition of PEV. According to ICTAC [57], measurements under rates must be conducted to configure the rates of the procedure as a function of different variables and different heating rates must be conducted to configure the rates of the procedure as a function of to investigate the degradation mechanisms thoroughly. Therefore, four heating rates were employed different variables and to investigate the degradation mechanisms thoroughly. Therefore, four in the present study, namely, 5, 10, 15 and 20 ◦C/min (Figure 12a). heating rates were employed in the present study, namely, 5, 10, 15 and 20 °C/min (Figure 12a). 3.3.1. Isoconversional Kinetics 3.3.1. Isoconversional Kinetics Isoconversional (or model-free) methods of thermally induced processes provide reliable Isoconversional (or model-free) methods of thermally induced processes provide reliable estimations concerning the activation energy (Eα) values and furthermore they yield useful indications estimationsregarding the concerning model fitting the part activation of the kinetic energy study. (Eα) Accordingvalues and to thefurthermore isoconversional they methods,yield useful the indicationsreaction rate regarding during a the thermally model stimulatedfitting part processof the kinetic at the study. constant According extent of to conversion the isoconversional (α) is only methods,dependent the on reaction temperature rate during [58], and a thermally no assumption stimul ofated the process reaction at models the constant is required. extent The of conversion term extent (ofα) conversionis only dependent (α) which on temperature is mentioned [58], above, and refers no assumption to the mass of the alteration reaction when models thermogravimetric is required. The termmeasurements extent ofare conversion employed, (α and) which it is determined is mentioned as the above, ratioof refers the ongoing to the massmass change alteration (∆m )when to the thermogravimetric measurements are employed, and it is determined as the ratio of the ongoing mass entire mass loss (∆mtot) which occurs during the total process: change (Δm) to the entire mass loss (Δmtot) which occurs during the total process: m0 m ∆m α = 𝑚− – 𝑚 = ∆𝑚 (6) 𝛼 m = 0 m f = ∆mtot (6) 𝑚− – 𝑚 ∆𝑚 InIn the the current current study, study, the the differential differential method prop proposedosed by Friedman [59] [59] and the the integral integral method method proposed by Ozawa, Flynn, and and Wall Wall (OFW) [60–62] [60–62] are applied, in order order to determine determine the the activation activation energyenergy values as a function of α (0.05 < αα << 0.95).0.95). TheThe corresponding corresponding equation describing Friedman’s theory is the following:  𝑑𝛼!  𝐸  dα  E ln𝑙𝑛β 𝛽  = =ln[ 𝑙𝑛f (α𝑓)(𝛼)A ]𝐴 – α (7)(7)  i 𝑑𝑇 , α 𝑅𝑇, dT α,i − RTα,i where I is ascribed to the individual heating rates and Tα,I to the temperature at which α is reached where I is ascribed to the individual heating rates and Tα,I to the temperature at which α is reached within that ith heating rate, whereas, the Eα values are estimated from the slope of the plot of within that ith heating rate, whereas, the Eα values are estimated from the slope of the plot of ln[βi(dα/dT)α,i] against (1/Tα,i) [63]. ln[β (dα/dT) ] against (1/T )[63]. iAccordingly,α,i the correspondingα,i equation describing OFW’s theory is the following:

𝐸 𝑙𝑛(𝛽) = 𝐶𝑜𝑛𝑠𝑡– − 1.0516 (8) 𝑅𝑇,

Polymers 2019, 11, 1672 16 of 27

Accordingly, the corresponding equation describing OFW’s theory is the following: ! E ln( ) = Const α Polymers 2019, 11, x FOR PEER REVIEW βi – 1.0516 16 of 27(8) − RTα,i wherewhere the the EEα αvaluesvalues are are determined determined from from the the slope slope of of the the pl plotot of of the the left left part part of of Equation Equation (8) (8) against against (1/(1T/Tα,iα),i. ). TheThe results results of of both both methods methods are are displayed displayed in in Figure Figure 13, 13 ,where where the the dependence dependence of of the the EEαα valuesvalues onon the the extent extent of of conversion conversion can can be beseen. seen. The The differ diﬀenceerence between between the theactivation activation energies energies calculated calculated by theby two the twomethods methods can canbe attributed be attributed to todifferent diﬀerent factors, factors, such such asas baseline baseline stability stability and and improper improper integrationintegration [64,65]. [64,65].

240 Friedman OFW

210

180 Avtivation Energy (kJ/mol)

150

0.0 0.2 0.4 0.6 0.8 1.0 Degree of conversion α

Figure 13. Dependence of the eﬀective activation energy (Eα) on the degree of conversion (α) according Figureto Friedman’s 13. Dependence and Ozawa, of Flynn,the effective and Wall’s activation (OFW’s) energy methods. (Eα) on the degree of conversion (α) according to Friedman’s and Ozawa, Flynn, and Wall’s (OFW’s) methods.

According to Figure 13, the Eα α dependency of both methods can be divided in two regions. − At theAccording first one to (α Figure< 0.9), 13, the the activation Eα – α dependency energy values of both are steadily methods increasing can be divided upon αinaugmentation, two regions. Atthough the first the one numerical (α < 0.9), di fftheerences activation are not energy notable. values On thearecontrary, steadily atincreasing high α values upon (α augmentation,0,95) there is a →→ thoughrapid increasethe numerical in the differencesEα values with are no increasingt notable. degree On the of contrary, conversion. at high The α existencevalues (α of0,95) two ditherefferent is aregions rapid increase implies in the the existence Eα values of twowith or increasing more mechanisms degree of that conversion. should be The applied existence in order of two to successfully different regionsfit theexperimental implies the dataexistence in the model-fittingof two or more part ofme thechanisms kinetic analysis.that should Generally, be applied in case in of importantorder to successfullydifferences observedfit the experimental in the Eα values data in upon the αmodel-fiincreasingtting (more part of than the 10% kinetic [66]), analysis. the model Generally, fitting part in caseof the of important thermogravimetric differences kinetic observed analysis in the is moreEα values complex upon and α increasing more than (more onemechanisms than 10% [66]), may the be modelrequired fitting to be part applied, of the in thermogravimetric order to describe the kinetic decomposition analysis is ofmore the materials.complex and Thereafter, more than the greatone mechanismsdivergence inmay the be activation required to energy be applied, values in indicates order to thedescribe existence the decomposition of more complicated of the materials. processes, Thereafter,where diff erentthe great stages divergence of the thermal in the decomposition activation energy are governed values byindicates different the mechanisms. existence of more complicated processes, where different stages of the thermal decomposition are governed by different3.3.2. Model mechanisms. Fitting Kinetics This part of the kinetic analysis of thermal degradation deals with the theoretical calculation 3.3.2. Model Fitting Kinetics of the kinetic triplet by using different models which correspond to various reaction mechanisms. In theThis case part of of a single-stepthe kinetic analysis process, of this thermal triplet degradation is composed deals by the with activation the theoretical energy calculationEα (in kJ/mol), of 1 thethe kinetic pre-exponential triplet by using factor differentA (in s models− ) and which the reaction correspond model to variousf (α). That reac specifiction mechanisms. study is applied In the caseconcurrently of a single-step to the process, experimental this triplet curves is composed which have by the been activation measured energy at E diαff (inerent kJ/mol), heating the pre- rates. exponentialFurthermore, factor the A complementary (in s−1) and the characterreaction model of the f( isoconversionalα). That specific andstudy the ismodel-fitting applied concurrently methods toprovides the experimental a criterion curves for the which selection have of been the measur most suitableed at different model, whichheating is rates. the achievement Furthermore, of the the complementaryshortest deviation character of Eα values of the ofisoconversional both the aforementioned and the model-fitting methods [63 methods]. provides a criterion for the selection of the most suitable model, which is the achievement of the shortest deviation of Eα values of both the aforementioned methods [63]. Initially, the decomposition of the sample is simulated by a single mechanism. In case the experimental data are poorly fitted, or the results of the fitting procedure are not in agreement with the results of the model free part of the kinetic analysis, combinations of more mechanisms should be applied. Sixteen reaction models were adopted for the single-step decomposition of PEV, with the

Polymers 2019, 11, 1672 17 of 27

Initially, the decomposition of the sample is simulated by a single mechanism. In case the experimental data are poorly fitted, or the results of the fitting procedure are not in agreement with the results of the model free part of the kinetic analysis, combinations of more mechanisms should Polymers 2019, 11, x FOR PEER REVIEW 17 of 27 be applied. Sixteen reaction models were adopted for the single-step decomposition of PEV, with resultsthe results of the of fitting the fitting procedures procedures according according to the to Cn the andCn Fnand modelsFn models –the –themost mostused usedmodels models in the in literaturethe literature describing describing polymers’ polymers’ degradation-being degradation-being reported reported in Figure in Figure 14a,b 14 anda,b andTable Table 1. It1 must. It must be mentionedbe mentioned that Cn that isCn an nis-th an ordern-th ordermodel model with autocatalysi with autocatalysis,s, represented represented by the equation: by the equation: f(α) = (1−αf ()αn ) n (1+= K(1cat.αX)), where(1+Kcat K.Xcat), is where ascribedKcat tois the ascribed autocatalysis to the autocatalysisrate constant and rate X constant to the reactants. and X to Accordingly, the reactants. − n Accordingly, Fn is an n-th order model, represented by the equation:n f (α) = (1 α) . Concerning the Fn is an n-th order model, represented by the equation: f(α) = (1−α) . Concerning the− single mechanism fittingsingle of mechanism PEV Figure fitting 14a,b, of both PEV the Figure fittings 14a,b, are both rather the poor, fittings since are ratherthe last poor, part sinceof the the simulations last part of presentsthe simulations great deviation presents between great deviation theoretical between and expe theoreticalrimental andvalues. experimental Furthermore, values. the values Furthermore, of the activationthe values energy of the activationcalculated energy by the calculated single-mecha by thenism single-mechanism model-fitting kinetics, model-fitting namely, kinetics, 166 and namely, 178 kJ/mol166 and for 178 the kJCn/mol and for Fn themodels,Cn and respectivelyFn models, (Table respectively 1), are not (Table representative1), are not of representative the corresponding of the valuescorresponding calculated values at the calculated isoconversional at the isoconversional kinetic study on kinetic the whole study extent on the of whole conversion extent ofα Figure conversion 13. Thus,α Figure although 13. Thus, correlation although coeffi correlationcients are coe ffiquitecients acceptable, are quite acceptable,the single themechanism single mechanism fitting of fittingPEV neitherof PEV is neither in full is inagreement full agreement with the with isoconvers the isoconversionalional part partof the of thekineti kineticc analysis, analysis, nor nor provides provides acceptableacceptable fitting fitting quality. quality.

100 100 (a) Cn (b) Fn

80 80

60 60 Mass(%) Mass(%) 40 40 5 °C/min 5 °C/min 10 °C/min 10 °C/min 15 °C/min 15 °C/min 20 °C/min 20 °C/min 20 20 fitted data fitted data

300 350 400 450 500 300 350 400 450 500 Temperature (°C) Temperature (°C)

100 100 (c) Cn-Cn (c)(d) Cn-CnFn-Fn

80 80

60 60 Mass (%) Mass Mass (%) 40 5 °C/min 40 5 °C/min 10 °C/min 10 °C/min 15 °C/min 15 °C/min 20 °C/min 20 °C/min 20 fitted data 20 fitted data

300 350 400 450 500 300 350 400 450 500 Temperature(°C) Temperature (°C)

Figure 14. Mass loss (%) and fitting curves of PEV for different heating rates β = 5, 10, 15 and 20 ◦C/min Figurein nitrogen 14. Mass atmosphere loss (%) and (a) fitting using curves the Cn ofmodel, PEV for (b) different using the heatingFn model, rates (βc) = using 5, 10, 15 the andCn–Cn 20 °C/minmodel, in(d nitrogen) using the atmosphereFn–Fn model. (a) using the Cn model, (b) using the Fn model, (c) using the Cn–Cn model, (d) using the Fn–Fn model. Therefore, combinations of more theoretical models are required. Two-step mechanisms have been employed, with theTable assumption 1. Results that of the model-fitting mechanisms method are consecutive. for PEV. The combinations which yielded the best fitting quality, as well 1 Single as the Mechanism higher correlation coe 2 Consecutivefficient values Mechanisms for PEV, were the n-th orderMechanism with autocatalysis in bothCn mechanisms (Cn–CnFn ) (FigureCn–Cn 14c) and the n-th Fn–Fnorder in both Fn–Fn mechanisms ( ) (Figure 14d).1st The Mechanism corresponding kinetic parameters are summarized in Table1, with the correlation coefficients beingCn high in both cases.Fn Cn Fn log A1(s−1) 10.00 11.16 10.09 10.09 E1 (kJ/mol) 166 178 162 162 log Kcat1 0.81 −15.47 2nd Mechanism Cn Fn log A2(s−1) - - 14.58 14.22 E2 (kJ/mol) - - 207 202 log Kcat2 - - −6.99

Polymers 2019, 11, 1672 18 of 27

Table 1. Results of the model-ﬁtting method for PEV.

1 Single Mechanism 2 Consecutive Mechanisms Mechanism Cn Fn Cn–Cn Fn–Fn 1st Mechanism Polymers 2019, 11, x FOR PEER REVIEW Cn Fn Cn Fn 18 of 27 1 log A1(s− ) 10.00 11.16 10.09 10.09 Correlation CoefficientE1 (kJ/mol) 0.99972166 0.99897 178 162 0.99994 162 0.99994 log Kcat1 0.81 15.47 − Therefore, combinations of more 2ndtheoretical Mechanism models are required. Two-step mechanisms have been employed, with the assumption that the mechanisms are Cnconsecutive. TheFn combinations which 1 yielded the best fittinglog quality,A2(s− ) as well as -the higher -correlation 14.58 coefficient 14.22values for PEV, were the E (kJ/mol) - - 207 202 n-th order with autocatalysis2 in both mechanisms (Cn–Cn) (Figure 14c) and the n-th order in both log Kcat2 -- 6.99 mechanisms (Fn–Fn) (Figure 14d). The corresponding kinetic −parameters are summarized in Table 1, Correlation Coefficient 0.99972 0.99897 0.99994 0.99994 with the correlation coefficients being high in both cases. The values of the activation energy calculated by both models are in the proximity of the correspondingThe values values of the calculated activation during energy calculatedthe isoconversional by both modelskinetics are study. in the Though, proximity since of the calculatedcorresponding logK valuesat values calculated in the duringCn–Cn themodel isoconversional are −15.47 kineticsand −6.99 study. (great Though, negative since numbers), the calculated and therefore,logKat values the inparameter the Cn– CnKcatmodel is almost are zero,15.47 according and 6.99 to (greatthe equations negative describing numbers), the and Cn therefore, and Fn − − models,the parameter the CnK kineticcat is almost model zero, coincides according with to the the Fn equations model [63]. describing Thus, taking the Cn intoand considerationFn models, the thisCn remark,kinetic model it is concluded coincides that with the the modelFn model which [63 best]. Thus, describes taking the into thermal consideration degradation this of remark, PEV is itthe is consecutiveconcluded that Fn– theFn model. model which best describes the thermal degradation of PEV is the consecutive Fn–Fn model. 3.4. Thermal Degradation Mechanism 3.4. Thermal Degradation Mechanism Py–GC/MS is a valuable analytic method that allows the in-depth study of the degradation mechanismsPy–GC/ MSof ispolymers a valuable and analytic other complex method thatmatrices allows [67]. the in-depthTo gain studyinsight of into the degradationthe specific degradationmechanisms ofmechanism polymers andof PEV, other complexthe PEV matricespolymer [67 with]. To higher gain insight crystallinity into the specific(TBT catalyst) degradation was subjectedmechanism to of pyrolysis PEV, the under PEV polymer He atmosphere with higher at 330 crystallinity °C, 420 °C (TBT and catalyst) 500 °C. wasThese subjected temperatures to pyrolysis were selectedunder He from atmosphere the TGA at 330data◦C, and 420 roughly◦C and 500corre◦C.sponded These temperatures to the beginning, were selected middle from and the end TGA of degradation.data and roughly The correspondedrecorded total to ion the chromatographs beginning, middle (TICs) and endare presented of degradation. in Figure The recorded15. The main total degradationion chromatographs products (TICs) were are identified presented through in Figure their 15 mass. The spectra main degradation (not presented products for brevity) were identified and are summarizedthrough their in mass Table spectra 2. (not presented for brevity) and are summarized in Table2.

500 oC

monomer

vanillic acid

420 oC monomer

330 oC Relative intensity

monomer

0 5 10 15 20 25 Ret.Time (min)

Figure 15. Total Ion Chromatographs of PEV after pyrolysis at 330 ◦C, 420 ◦C, and 500 ◦C. Figure 15. Total Ion Chromatographs of PEV after pyrolysis at 330 °C, 420 °C, and 500 °C.

Table 2. Identified thermal degradation products of PEV after pyrolysis at different temperatures. The retention times of the larger peaks are denoted in bold.

Rt (min) MW (amu) Possible Compound 330 420 500 0.62 0.18 0.33 28, 44 CO/CO2 1.22 1.22 1.23 28, 44 CO/CO2

Polymers 2019, 11, 1672 19 of 27

Table 2. Identiﬁed thermal degradation products of PEV after pyrolysis at diﬀerent temperatures. The retention times of the larger peaks are denoted in bold.

Rt (min) MW (amu) Possible Compound 330 420 500

Polymers0.62 2019, 0.1811, x FOR PEER 0.33 REVIEW 28, 44 CO/CO2 19 of 27 Polymers 2019, 11, x FOR PEER REVIEW 19 of 27 PolymersPolymers 20192019,,, 1111,,, xxx FORFORFOR PEERPEERPEER REVIEWREVIEWREVIEW 1919 ofof 2727 1.22 1.22 1.23 28, 44 CO/CO2

- 6.31 6.33 124 - 6.31 6.33 124 -- 6.316.31 6.336.33 124 124

guaiacol guaiacolguaiacol

- 7.02 7.04 150 -- -7.027.02 7.02 7.047.04 7.04 150 150 150

1-methoxy-2-(vinyloxy)benzene 1-methoxy-2-(vinyloxy)benzene1-methoxy-2-(vinyloxy)benzene1-methoxy-2-(vinyloxy)benzene

- 7.33 7.32 136 -- -7.337.33 7.33 7.327.32 7.32 136 136 136

m-Anisaldehyde m-Anisaldehydem-Anisaldehydem-Anisaldehyde

- 10.61 10.65 168 -- -10.6110.61 10.61 10.6510.65 10.65 168 168 168

vanillic acid vanillicvanillicvanillic acidacid

11.99 12.26 12.23 182 11.9911.9911.99 12.2612.26 12.26 12.2312.23 12.23 182 182 182

methyl 4-hydroxy-3-methoxybenzoate methylmethylmethyl 4-hydroxy-3-methoxybenzoate4-hydroxy-3-methoxybenzoate 4-hydroxy-3-methoxybenzoate

- 12.60 12.67 194 -- -12.6012.60 12.60 12.6712.67 12.67 194 194 194

3-methoxy-4-(vinyloxy)benzoic acid 3-methoxy-4-(vinyloxy)benzoic3-methoxy-4-(vinyloxy)benzoic3-methoxy-4-(vinyloxy)benzoic acid acidacid

- 12.66 12.69 208 -- -12.6612.66 12.66 12.6912.69 12.69 208 208 208

methyl 3-methoxy-4-(vinyloxy)benzoate methylmethylmethyl 3-methoxy-4-(vinyloxy)benzoate3-methoxy-4-(vinyloxy)benzoate 3-methoxy-4-(vinyloxy)benzoate

4-ethoxy-3-methoxybenzoic acid 4-ethoxy-3-metho4-ethoxy-3-methoxybenzoicxybenzoic acid acid 4-ethoxy-3-metho4-ethoxy-3-methoorxybenzoic xybenzoic acidacid - -12.97 12.97 12.99 12.99 196 196 or -- 12.9712.97 12.9912.99 196 196 oror

methylmethyl 3,4-dimethoxybenzoate 3,4-dimethoxybenzoate methylmethyl 3,4-dimethoxybenzoate3,4-dimethoxybenzoate

14.60 13.73 13.68 210 14.60 13.73 13.68 210 14.6014.60 13.7313.73 13.6813.68 210 210 4-ethoxy-3-methoxybenzoic acid 4-ethoxy-3-methoxybenzoic acid 4-ethoxy-3-metho4-ethoxy-3-methoOrxybenzoicxybenzoic acidacid OrOr

Polymers 2019, 11, x FOR PEER REVIEW 19 of 27

- 6.31 6.33 124

guaiacol

- 7.02 7.04 150

1-methoxy-2-(vinyloxy)benzene

- 7.33 7.32 136

m-Anisaldehyde

- 10.61 10.65 168

vanillic acid

11.99 12.26 12.23 182

methyl 4-hydroxy-3-methoxybenzoate

- 12.60 12.67 194

3-methoxy-4-(vinyloxy)benzoic acid

- 12.66 12.69 208

methyl 3-methoxy-4-(vinyloxy)benzoate

4-ethoxy-3-methoxybenzoic acid Polymers 2019, 11, 1672 20 of 27 - 12.97 12.99 196 or

Table 2. Cont.

Rt (min) MW (amu) Possible Compound 330 420 500 methyl 3,4-dimethoxybenzoate

14.60 13.73 13.68 210

PolymersPolymersPolymers 2019 20192019,,,,, , 11 1111,,,,, , x xxxxx FOR FORFORFORFORFOR PEER PEERPEERPEERPEERPEER REVIEW REVIEWREVIEWREVIEWREVIEWREVIEW 202020 ofofof 272727 4-ethoxy-3-metho4-ethoxy-3-methoxybenzoicxybenzoic acid acid 14.60 13.73 13.68 210 OrOr

ethylethylethylethyl 3,4-dimethoxybenzoate3,4-dimethoxybenzoate3,4-dimethoxybenzoate 3,4-dimethoxybenzoate

------13.9613.9613.96 13.96 208208208 208

vinyl 3,4-dimethoxybenzoate vinylvinyl 3,4-dimethoxybenzoate3,4-dimethoxybenzoate

--- -14.1314.1314.13 14.13 14.1514.1514.15 14.15 206 206 206 206

vinylvinylvinyl 3-hydroxy-4-(vinyloxy)benzoate3-hydroxy-4-(vinyloxy)benzoate 3-hydroxy-4-(vinyloxy)benzoate

--- -15.1215.1215.12 15.12 15.0215.0215.02 15.02 222 222 222 222

ethylethylethylethyl 3-methoxy-4-(vinyloxy)benzoate3-methoxy-4-(vinyloxy)benzoate3-methoxy-4-(vinyloxy)benzoate 3-methoxy-4-(vinyloxy)benzoate

15.8615.8615.8615.86 16.1116.1116.11 16.11 16.1216.1216.12 16.12 212 212 212 212

4-(2-hydroxyethoxy)-3-methoxybenzoic4-(2-hydroxyethoxy)-3-methoxybenzoic4-(2-hydroxyethoxy)-3-methoxybenzoic4-(2-hydroxyethoxy)-3-methoxybenzoic acid acidacidacid

15.9815.9815.9815.98 16.1616.1616.16 16.16 16.1716.1716.17 16.17 226 226 226 226

methylmethylmethyl 4-(2-hydroxyethoxy)-3-methoxybenzoate4-(2-hydroxyethoxy)-3-methoxybenzoate 4-(2-hydroxyethoxy)-3-methoxybenzoate

17.2217.2217.2217.22 17.3917.3917.39 17.39 17.3917.3917.39 17.39 224 224 224 224

ethylethylethylethyl 4-ethoxy-3-m4-ethoxy-3-m4-ethoxy-3-m 4-ethoxy-3-methoxybenzoateethoxybenzoateethoxybenzoateethoxybenzoate

19.1219.1219.1219.12 19.1319.1319.13 19.13 19.2019.2019.20 19.20 256 256 256 256

2-hydroxyethyl2-hydroxyethyl2-hydroxyethyl2-hydroxyethyl 4-(2-hydroxy4-(2-hydroxy4-(2-hydroxy 4-(2-hydroxyethoxy)-3-methoxybenzoateethoxy)-3-methoxybenzoateethoxy)-3-methoxybenzoateethoxy)-3-methoxybenzoate

22.2522.2522.25 22.3722.3722.37 22.4922.4922.49 318 318 318

2-(2-methoxyphenoxy)ethyl2-(2-methoxyphenoxy)ethyl2-(2-methoxyphenoxy)ethyl 4-4-4-hydroxy-3-methoxybenzoatehydroxy-3-methoxybenzoate

--- 22.4322.4322.43 22.5322.5322.53 344 344 344

2-(2-methoxyphenoxy)ethyl2-(2-methoxyphenoxy)ethyl2-(2-methoxyphenoxy)ethyl 3-methoxy-4-(vinyloxy)benzoate3-methoxy-4-(vinyloxy)benzoate3-methoxy-4-(vinyloxy)benzoate

24.7424.74 24.7724.77 24.7924.79 388 388 24.7424.74 24.7724.77 24.7924.79 388 388 3-methoxy-4-(2-((3-methoxy-4-3-methoxy-4-(2-((3-methoxy-4-3-methoxy-4-(2-((3-methoxy-4- (vinyloxy)benzoyl)oxy)ethoxy)benzoic(vinyloxy)benzoyl)oxy)ethoxy)benzoic(vinyloxy)benzoyl)oxy)ethoxy)benzoic acidacidacid

PolymersPolymers 20192019,,, 1111,,, xxx FORFORFOR PEERPEERPEER REVIEWREVIEWREVIEW 2020 ofof 2727

ethylethyl 3,4-dimethoxybenzoate3,4-dimethoxybenzoate

-- -- 13.9613.96 208208

vinylvinyl 3,4-dimethoxybenzoate3,4-dimethoxybenzoate

-- 14.1314.13 14.1514.15 206 206

vinylvinyl 3-hydroxy-4-(vinyloxy)benzoate3-hydroxy-4-(vinyloxy)benzoate

-- 15.1215.12 15.0215.02 222 222

ethylethyl 3-methoxy-4-(vinyloxy)benzoate3-methoxy-4-(vinyloxy)benzoate

15.8615.86 16.1116.11 16.1216.12 212 212

4-(2-hydroxyethoxy)-3-methoxybenzoic4-(2-hydroxyethoxy)-3-methoxybenzoic acidacid

15.9815.98 16.1616.16 16.1716.17 226 226

methylmethyl 4-(2-hydroxyethoxy)-3-methoxybenzoate4-(2-hydroxyethoxy)-3-methoxybenzoate

17.2217.22 17.3917.39 17.3917.39 224 224 Polymers 2019, 11, 1672 21 of 27 ethylethyl 4-ethoxy-3-m4-ethoxy-3-methoxybenzoateethoxybenzoate

Table 2. Cont. 19.1219.12 19.1319.13 19.2019.20 256 256 Rt (min) MW (amu) Possible Compound 330 420 500 2-hydroxyethyl2-hydroxyethyl 4-(2-hydroxy4-(2-hydroxyethoxy)-3-methoxybenzoateethoxy)-3-methoxybenzoate

22.2522.2522.25 22.3722.37 22.37 22.4922.49 22.49 318 318 318

2-(2-methoxyphenoxy)ethyl2-(2-methoxyphenoxy)ethyl2-(2-methoxyphenoxy)ethyl 4- 4-hydroxy-3-methoxybenzoate4-hydroxy-3-methoxybenzoatehydroxy-3-methoxybenzoate

-- -22.4322.43 22.43 22.5322.53 22.53 344 344 344

2-(2-methoxyphenoxy)ethyl2-(2-methoxyphenoxy)ethyl2-(2-methoxyphenoxy)ethyl 3-methoxy-4-(vinyloxy)benzoate 3-methoxy-4-(vinyloxy)benzoate

24.74 24.77 24.79 388 24.74 24.77 24.79 388 24.7424.74 24.7724.77 24.7924.79 388 388 3-methoxy-4-(2-((3-methoxy-4-(vinyloxy)benzoyl)oxy)ethoxy)benzoic3-methoxy-4-(2-((3-methoxy-4-3-methoxy-4-(2-((3-methoxy-4- acid (vinyloxy)benzoyl)oxy)ethoxy)benzoic(vinyloxy)benzoyl)oxy)ethoxy)benzoic acidacid Polyesters with β–hydrogen atoms degrade predominantly by heterolytic β–scission, with the transfer of the hydrogen atom from the β–alkoxy carbon atom to the ester carbonyl that results in the formation of carboxyl and vinyl end groups [11,68–72]. Homolytic pathways have also been identified, including acyl-oxygen and alkyl-oxygen bond scission leading to alkyl, hydroxyl and aldehyde-capped compounds. Homolysis occurs in a smaller extent, compared to β–scission and is believed to be promoted by higher pyrolysis temperatures (>300 ◦C) [70,71]. PEV, as a polyester with a β–hydrogen to the ester bond is expected to degrade in a similar manner, but it has a peculiarity; it contains an ether bond in para position to the ester group that is expected to result in a complex degradation mechanism because of the presence of two characteristic groups on its structure. Polymers based on vanillic acid are expected to share common features in their degradation mechanism with woody biomass, which involves homolysis, mainly of the C-O bond, and concerted routes [73–77]. Ethers and subsequently polyethers degrade through radical processes with mechanisms that include homolytic cleavage, disproportionation and abstraction of the formed radicals [78,79]. Therefore, a multiple mechanism that will include heterolysis and homolysis is expected to take place during the thermal degradation of PEV. The evolution of degradation products during the pyrolysis of PEV is clearly temperature- dependent (Figure 12). At 330 ◦C, only a handful of compounds are detected, as evidenced by the simple chromatograph recorded. These compounds correspond to the degradation reactions that require less thermal energy to take place, providing insight into the main degradation mechanism. The number and the complexity of the peaks increase with the increase of pyrolysis temperature, leading to a final complex pattern that proves the evolution of a plethora of compounds, suggesting a complicated degradation pattern. Five main peaks were recorded in Rt = 15.86 min, 15.98 min, 17.22 min, 19.12 min and 24.74 min and were identified as the monomer 4-(2-hydroxyethoxy)-3-methoxybenzoic acid, ethyl 4-ethoxy-3-methoxybenzoate, methyl 4-(2-hydroxyethoxy)-3-methoxybenzoate, the methyl ester of the monomer 2-hydroxyethyl 4-(2-hydroxyethoxy)-3-methoxybenzoate and 3-methoxy-4-(2-((3-methoxy-4-(vinyloxy)benzoyl)oxy)ethoxy)benzoic acid. Their chemical structures are presented in Table2. The nature of these compounds that contain hydroxyl, carboxyl, vinyl and methoxy end groups suggests a series of different, consequent scission reactions. More specifically, compounds containing the –CH2CH2OH and –COOH end groups of the polymer that were subjected to either C–O or C–C homolytic scission on the other end are detected, along with a β–scission derivative with m/z = 388 amu. In higher temperatures, the end groups of all compounds remain the same, but the overall amount of degradation products increases as a result of more extensive chain scission. Additional degradation Polymers 2019, 11, 1672 22 of 27 products were also detected, such as guaiacol and vanillic acid, both high added value compounds derived by the pyrolysis of biomass [80]. Noticeably, methyl 4-hydroxy-3-methoxybenzoate eluted in Rt 12 min in relatively large quantities, both at 420 C and 500 C, along with additional products ≈ ◦ ◦ with an end methoxy group, indicating extensive homolysis of the usually stable C–C bond. It appears that heterolysis and homolysis of C–O and C–C bonds occurred simultaneously with the heterolytic scission of the ester linkage, in all pyrolysis temperatures. This hypothesis is further supported by the detection of both CO and CO2, which can be released after acyl-oxygen and alkyl-oxygen homolysis, respectively. The extensive homolysis could be attributed to the present of the o-methoxy group of the monomer that has been found to reduce the neighboring bond dissociation [74,75]. Additionally, the unusual extensive scission of the C–C bond could be attributed to the stabilization of the –CH2· free radicals by the lone pairs of their adjacent oxygens. The complexity of the degradation process concluded by Py/GC-MS is reflected in the involved pathways (Figure A1 in AppendixA) and also supported by the isoconversional and model fitting kinetics results.

3.5. Nanoindentation The loading-unloading indentation curve of PEV presented a creep phenomenon at the peak force of 500 mN (Figure A2). The curve does not show any discontinuities or steps which proves no cracks were formed during the measurement. The mechanical performances of PEV are summarized in Table3. The indentation depth at the peak load was 14.25 0.26, the indentation hardness 177.81 8.37 N/mm2 ± ± and the elastic modulus was calculated 1506.00 96.19 N/mm2, which is close to that of polypropylene. ± The hardness measured for PEV lies in the same range as PET, since amorphous and annealed PET have hardness values of about 120 N/mm2 and up to 200 N/mm2, respectively [81,82].

Table 3. Maximum displacement, indentation hardness and elastic modulus of PEV calculated by nanoindentation tests.

Max Displacement (µm) Indentation Hardness (N/mm2) Elastic Modulus (N/mm2) 14.25 0.26 177.81 8.37 1506.00 96.19 ± ± ± Experiments were repeated 5 times.

4. Conclusions In the present work, the preparation of poly(ethylene vanillate) using two different catalysts via a two-step polycondensation and followed by a solid-state polymerization step was reported. Intrinsic viscosities of 0.34 g/dL (TBT catalyst) and 0.38 g/dL (Sb2O3 catalyst) were measured for the final polymers. The polymers were characterized by DSC, WAXS and PLM. The melting, glass transition and cold-crystallization temperatures were determined and found comparable to PET. The higher molecular weight polymer (prepared with Sb2O3) exhibited a lower crystallinity. A slower crystallization was observed compared to PET, attributed to a poor nucleation density. ∆Hm was estimated to be 166 16 J/g ± and the equilibrium melting temperature 301.4 ◦C. The spherulitic morphology was observed with PLM; and the spherulite growth rates were measured at different temperatures and the Lauritzen-Hoffman plot constructed. A two-step mechanism was observed for the thermal degradation of PEV, which was best fitted by the Fn–Fn model (n-th order model). The thermal degradation mechanism was studied by Py/GC-MS: heterolytic scission of the ester linkages occurring in parallel to the heterolysis and homolysis of C–C and C–O bonds was evidenced. Finally, nanoindentation measurements evidenced an indentation depth at peak load of 14.25 0.26 µm, an indentation hardness of 177.81 8.37 N/mm2 ± ± and an elastic modulus of 1506.00 96.19 N/mm2. All these data show that PEV is a promising polymer, ± with characteristics similar to PET, and it will be further investigated in our future work.

cracks were formed during the measurement. The mechanical performances of PEV are summarized in Table 3. The indentation depth at the peak load was 14.25 ± 0.26, the indentation hardness 177.81 ± 8.37 N/mm2 and the elastic modulus was calculated 1506.00 ± 96.19 N/mm2, which is close to that of polypropylene. The hardness measured for PEV lies in the same range as PET, since amorphous and annealed PET have hardness values of about 120 N/mm2 and up to 200 N/mm2, respectively [81,82].

Table 3. Maximum displacement, indentation hardness and elastic modulus of PEV calculated by nanoindentation tests.

Max Displacement (μm) Indentation Hardness (N/mm2) Elastic Modulus (N/mm2) 14.25 ± 0.26 177.81 ± 8.37 1506.00 ± 96.19 Experiments were repeated 5 times.

4. Conclusions In the present work, the preparation of poly(ethylene vanillate) using two different catalysts via a two-step polycondensation and followed by a solid-state polymerization step was reported. Intrinsic viscosities of 0.34 g/dL (TBT catalyst) and 0.38 g/dL (Sb2O3 catalyst) were measured for the final polymers. The polymers were characterized by DSC, WAXS and PLM. The melting, glass transition and cold-crystallization temperatures were determined and found comparable to PET. The higher molecular weight polymer (prepared with Sb2O3) exhibited a lower crystallinity. A slower crystallization was observed compared to PET, attributed to a poor nucleation density. ΔHm was estimated to be 166 ± 16 J/g and the equilibrium melting temperature 301.4 °C. The spherulitic morphology was observed with PLM; and the spherulite growth rates were measured at different temperatures and the Lauritzen-Hoffman plot constructed. A two-step mechanism was observed for the thermal degradation of PEV, which was best fitted by the Fn–Fn model (n-th order model). The thermal degradation mechanism was studied by Py/GC-MS: heterolytic scission of the ester linkages occurring in parallel to the heterolysis and homolysis of C–C and C–O bonds was evidenced. Finally, nanoindentation measurements evidenced an indentation depth at peak load of 14.25 ± 0.26 μm, an indentation hardness of 177.81 ± 8.37 N/mm2 and an elastic modulus of 1506.00 ± 96.19 N/mm2. All these data show that PEV is a promising polymer, with characteristics similar to PET, and it will be further investigated in our future work. Polymers 2019, 11, 1672 23 of 27

Author Contributions: Investigation and material synthesis, A.Z. and L.P., investigation of thermal properties L.P., Z.T. and D.P., crystal structure M.G., Supervision, writing—original draft D.N.B., K.C., N.L. and G.Z.P. Acknowledgments: Authors would like to thank Dimitrios Tzetzis of School of Science and Technology, InternationalAcknowledgments: Hellenic Authors University, would Central like Macedonia, to thank Dimitrios Greece for Tzet mechanicalzis of School properties of Science measurements. and Technology, ConﬂictsInternational of Interest: HellenicThe University, authors declare Central no Macedonia, conﬂict of Greece interest. for mechanical properties measurements. Conflicts of Interest: The authors declare no conflict of interest. Appendix A Appendix A

Polymers 2019, 11, x FOR PEER REVIEWFigureFigure A1.A1. PossiblePossible degradation pathways pathways of of PEV. PEV. 23 of 27

500

400

loading (elastic-plastic) 300

Force (mN) 200 unloading (elastic)

100

0 0246810121416 Depth (um)

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