Effects of Composition and Sequence of Ethylene-Vinyl Acetate

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Effects of Composition and Sequence of Ethylene-Vinyl Acetate Copolymers on Their Alcoholysis and Oxygen Barrier Property of Alcoholyzed Copolymers Xiaoxian Xue,† Li Tian,† Song Hong,‡ Shu Zhang,*,† and Yixian Wu*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China Center for Instrumental Analysis, Beijing University of Chemical Technology, Beijing, 100029, P. R. China

‡

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S Supporting Information *

ABSTRACT: A highly effective alcoholysis of ethylene-vinyl acetate (EVA) copolymers has been achieved to prepare ethylene-vinyl alcohol (EVOH) copolymers with various ethylene contents (30.2−49.5 mol %) having ca. 100% of alcoholysis degree even at a very low catalyst dosage. The kinetics show that the alcoholysis rate strongly depends on the composition and sequence of the EVA copolymer. The alcoholysis rate decreases with increasing E content and decreases in the order of VAcVAcVAc > VAcVAcE > EVAcE triad sequence. The formation of densely packed regions in EVOH from VVV, EVE, and EEE triad sequences along macromolecular chains via hydrogen bonds and crystallization makes a great contribution to inhibit oxygen transmission and thus to improve the barrier property. To the best of our knowledge, this is the first example to reveal the effect of sequence on alcoholysis of EVA and the oxygen barrier property of EVOH, which is profitable for macromolecular engineering of high performance EVOH copolymers.

1. INTRODUCTION The inter- and intramolecular hydrogen bonds in poly(vinyl alcohol) (PVA) give birth to an improvement of molecular accumulation and to a decrease of gas diffusion coefficient, but to a limited temperature window for polymer processing. Ethylene-vinyl alcohol (EVOH) copolymers with an improved processing property have been brought forward by introducing hydrophobic ethylene segments into PVA macromolecular chains. EVOH products with ethylene (E) contents of 25−45 mol % are valuable in packaging, automotive fuel tanks, and polymer blend applications due to their excellent properties in gas barrier, smell maintainability, oil resistance, thermal stability, durability, mechanical strength, and regeneration.1−4 Recently, the application fields of EVOH copolymers have been extended to battery separator,5 hemodialysis membrane,6,7 drug carrier,8 and ionic polymer metal composite.9 In addition, chemically modified EVOH copolymers10−17 are expected to be utilized in antimicrobial biomaterials,14 single ion polymer electrolytes,15 direct fuel cell membranes,16 and ionic actuators.17 EVOH copolymers can be synthesized by the saponification of ethylene-vinyl acetate (EVA) copolymers at very high catalyst (e.g., NaOH) concentration in the presence of water or alcoholysis of EVA copolymers at relatively high catalyst (e.g., NaOH, NaOCH3) concentration in the absence of water.18 Alcoholysis degree (DA) is recognized as one of the most important criteria for the properties of EVOH © XXXX American Chemical Society

copolymers, such as thermal stabilities, mechanical properties, oxygen barrier property, etc.19 The DA of EVOH copolymers with an E content of 88 mol % could only reach a low level of 20% at a low catalyst dosage (ncatalyst) of 0.06.18 The catalyst dosage (ncatalyst) is expressed by the molar ratio of sodium methoxide (NaOCH3) to vinyl acetate (VAc) units along EVA macromolecular chain. In order to obtain EVOH copolymers with DA of 99.5%, high catalyst dosage (ncatalyst = 1.8) and long reaction time (t = 5−6 h) were demanded for the hydrolysis of an EVA copolymer with an E content of 82 mol % in toluene using NaOH as the catalyst.20 The catalyst dosage was also set at a high level (nNaOCH3 > 1.06) for the synthesis of an EVOH copolymer (E content = 82 mol %) with DA > 99%.21 To the best of our knowledge, the alcoholysis kinetics of EVA copolymers with E content less than 82 mol % in methanol has not been reported. Therefore, a large amount of catalyst was normally required for the complete alcoholysis of EVA copolymers.21 Experimental efforts to understand the fundamental effect of composition structure (e.g., E content, DA) on the oxygen barrier property and to verify the theoretical generalizations have been confronted. For example, the oxygen barrier Received: December 18, 2018 Revised: February 19, 2019 Accepted: February 20, 2019

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DOI: 10.1021/acs.iecr.8b06260 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research property of the EVOH copolymer with an E content of 44 mol % was only 20% of that of the EVOH copolymer with an E content of 32 mol %.22 The EVOH copolymer with a crystallization degree of 70% has a much better oxygen barrier property than that with a crystallization degree of 58%, since the crystals in the EVOH copolymer impel the diffusive path of small molecular gas.23 However, the effect of sequence of EVOH copolymers on their barrier property has not been reported until now. In order to synthesize EVOH copolymers with excellent oxygen barrier property, it is worthy to establish a highly efficient approach for the preparation of EVOH copolymers at low catalyst dosage. It is also important to investigate the effects of composition and sequence on the oxygen barrier property of EVOH copolymers. Herein, we directly focus on the highly efficient alcoholysis of EVA copolymers with E content ranging from 30.2 to 49.5 mol % in a methanol solution after the free radical copolymerization of ethylene and vinyl acetate which is normally conducted in methanol. A novel and feasible measurement of DA by attenuated total reflectionFourier transform infrared (ATR-FTIR) characterization was developed to directly monitor the alcoholysis process of an EVA copolymer and to further investigate the alcoholysis kinetics. The effect of structural sequence on the alcoholysis of EVA copolymers was also investigated. A variable-temperature Fourier-transform infrared (VT-FTIR) method was utilized to verify the dissociation of hydrogen-bonded O−H bonds in EVOH copolymers. Moreover, the FTIR method was used to study the relationship between the intensity of hydrogenbonded O−H bonds and oxygen barrier property of EVOH copolymers. The effect of structural sequence on the oxygen barrier property was finally established.

byproduct (methyl acetate) was removed by distillation during the reaction, and methanol was simultaneously added into the reaction solution to keep the polymer concentration almost unchanged. 2.3. Characterization of EVA Copolymers and Alcoholyzed EVA Copolymers. Number-average molecular weights (Mn), weight-average molecular weights (Mw), and polydispersity index (PDI, Mw/Mn) of EVA copolymers were determined by gel permeation chromatography (GPC) at 30 °C using a Waters 1515-2410 system equipped with Waters RI 2410, UV 2489 detectors and four Waters styragel HT 3-4-5-6 columns (Milford, MA). THF was used as a solvent, and the flow rate of the mobile phase (THF) was 1.0 mL·min−1. The calibration curve was obtained with a polystyrene standard. The resulting solution of alcoholyzed EVA copolymer in methanol was taken out from the reaction system during the alcoholysis process and dropped onto water to form the alcoholyzed EVA copolymer film with a thickness around 10 μm. The film was quickly dried at 80 °C until a constant weight. ATR-FTIR analysis was directly performed on the above film surface by a Nicolet Nexus 6700 FTIR spectrometer with a resolution of 4 cm−1 and 64 scans at room temperature. The scans ranged from 4000 to 650 cm−1 in wavenumber. VT-FTIR spectra were recorded on a Nicolet Nexus 6700 FTIR spectrometer with an environmental chamber. The films with thicknesses of 8−10 μm (according to IR transparency) were prepared by hot-pressing EVOH copolymers at a temperature of 200 °C and a pressure of 10 MPa for 10 min. The films were cooled at room temperature and then heated at 80 °C for 16 h in a vacuum for drying and annealing before tests. The samples were heated to 150 °C at a 10 °C increment (accuracy: 0.1 °C) and allowed to equilibrate for 5 min before collecting the spectra. The films were characterized on BaF2 windows in the range from 4000 to 650 cm−1 with a resolution of 4 cm−1 and 32 scans. The chemical structures of EVA copolymers and the alcoholyzed EVA copolymers were characterized by 1H NMR characterization using a Bruker AV 400 spectrometer and by 13C NMR characterization using a Bruker AV 600 spectrometer at 25 °C. Chemical shifts were referenced to tetramethylsilane (TMS) as an internal standard and given in ppm (δ = 0.0 ppm, 1H NMR, 13C NMR). The alcoholyzed EVA copolymers with relatively high DA (≥38%) were dissolved in 0.5 mL of DMSO-d6 at 100 °C (30 mg for 1H NMR characterization and 100 mg for 13C NMR characterization). EVA copolymers or alcoholyzed EVA copolymers with low DA (≤33%) were dissolved in 0.5 mL of CDCl3 at 40 °C for 1H NMR characterization. Here, 100 mg of EVOH copolymers was dissolved in DMSO-d6 at 100 °C for 48 h to enhance the signal and reduce the signal-to-noise ratio. The quantitative 13C NMR (scan times: 2400) was utilized to determinate the triad fractions. The DA of alcoholyzed EVA (E-VAc-V) copolymers could be determined by 1H NMR characterization according to eq 1.

2. EXPERIMENTAL SECTION 2.1. Materials. Methanol (CH3OH, A.R., supplied by Beijing Chemical Plant) was distilled before use. Sodium hydroxide (NaOH, A.R., purchased from Sinopharm Chemical Reagent Co. Ltd.) and tetrahydrofuran (THF, A.R., supplied by Beijing Chemical Plant) were used without further purification. Deuterodimethyl sulfoxide (DMSO-d6) and deuterotrichloromethane (CDCl3) were purchased from Sinopharm Chemical Reagent Co. Ltd. Azobis(isobutyronitrile) (AIBN) was purified by recrystallization prior to use, and sodium methoxide (NaOCH3) in a methanol solution (0.05 M) was prepared by dissolving NaOH in methanol via ultrasound method. EVA copolymers with various E contents (30.2−49.5 mol %) were synthesized via free radical copolymerization of ethylene and vinyl acetate in methanol using AIBN as an initiator. 2.2. Procedure of Alcoholysis Reaction. Here, 25 g of EVA copolymer and 75 g of methanol were added into a 250 mL three-necked flask equipped with a mechanical stirring bar. After all the EVA copolymer was dissolved in methanol at 65 °C, a certain amount of NaOCH3 in a methanol solution (0.05 M) was added into the reactor. Then, 20% of the NaOCH3 solution in methanol was first added into the EVA solution in methanol to catalyze the alcoholysis for 0.5 h. Then, 60% of the NaOCH3 solution was further added into the reaction system to continue the alcoholysis for 2 h. Finally, 20% of the NaOCH3 solution was added into the reaction system to complete the alcoholysis for 0.5 h. The alcoholysis of the EVA copolymer was conducted using NaOCH3 as a catalyst in boiling methanol at 65 °C. The mixture of methanol and

AVAc,E‐VAc−V /A CH2 ,E ‐VAc‐V zyz ji zz × 100% DA,NMR = jjjj1 − j AVAc,EVA /A CH2 ,EVA zz k {

(1)

where AVAc,EVA and AVAc,E‑VAc‑V are the total integral values of characteristic resonances, which are assigned to the protons in methyl groups (−CH3−) for EVA copolymers and alcoholyzed EVA copolymers, respectively (δ = 2.0 ppm in both DMSO-d6 and CDCl3); ACH2,EVA and ACH2,E‑VAc‑V are the total integral B


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Scheme 1. Synthetic Mechanism of EVOH Copolymers via Alcoholysis of EVA Copolymers Catalyzed by NaOCH3 in Methanol

reorientation was around 80 °C.24 The Tm value of the EVOH copolymer was obtained from the first heating DSC curve from 0 to 200 °C at a heating rate of 10 °C/min after heat treatment. Transmission electron microscope (TEM) characterization was performed on a HT7700 electron microscope with an acceleration voltage of 100 kV (Hitachi, Japan). The samples were prepared by dropping the EVOH copolymer/DMSO solution (0.05 mg/mL) onto copper grids (400 meshes) coated with carbon films. The polymer films were heated at 80 °C for 16 h in a vacuum for drying and annealing. High resolution transmission electron microscope (HRTEM) characterization was performed on a HT3010 electron microscope operating at 300 kV (Hitachi, Japan). For better observation of the EVOH copolymer, the surface of the EVOH copolymer membrane (annealed at 80 °C for 16 h) was stained with ruthenium tetroxide by the immersion of a membrane into ruthenium trichloride solution and oxidization using potassium permanganate. The resulting EVOH copolymer films were embedded in an epoxy resin for immobilization and then sliced into ultrathin sections at −55 °C. The samples were removed to copper grids (400 meshes) coated with carbon films for HR-TEM characterization. EVOH copolymer films around 130 μm for the test of the oxygen transmission rate (OTR) value were prepared by hotpressing an EVOH copolymer at a pressure of 2 MPa and temperature of 200 °C for 5 min. The films were cooled at room temperature and annealed at 80 °C for 8 h in order to achieve complete crystallization, since the temperature for unit reorientation was around 80 °C.24 The OTR values of EVOH copolymer films were measured according to Chinese Standard GB1038-2000 using a VAC-V2 film permeability testing machine (Jinan Languang Mechanical and Electrical Technology Co, Ltd., China) at 23 °C.

values of peaks assigned to the protons in methylene (−CH2−) groups for EVA copolymers and alcoholyzed EVA copolymers, respectively (δ = 1.2−1.9 ppm in CDCl3, δ = 1.2−1.7 ppm in DMSO-d6). The fractions of VVV, VVE(EVV), EVE, EEE, VEE(EEV), and VEV triads (f triads) could be determined according to 13C NMR characterization using eq 2−7). fVVV =

A p1 A p1 + A p2 + A p3 + Ak1/2 + Ak2 + Ak3

× 100% (2)

fVVE =

A p2 A p1 + A p2 + A p3 + Ak1/2 + Ak2 + Ak3

× 100% (3)

fEVE =

A p3 A p1 + A p2 + A p3 + Ak1/2 + Ak2 + Ak3

× 100% (4)

fEEE =

A p1 + A p2

Ak1/2 × 100% + A p3 + Ak1/2 + Ak2 + Ak3 (5)

fVEE =

A p1 + A p2 + A p3

Ak2 × 100% + Ak1/2 + Ak2 + Ak3 (6)

fVEV =

Ak3 × 100% A p1 + A p2 + A p3 + Ak1/2 + Ak2 + Ak3 (7)

where f VVV + f VVE + f EVE + f EEE + f VEE + f VEV = 100%. Ap1 is the total area of peaks of the carbon atoms in methyne (−CH−) groups in the middle vinyl alcohol units of rrVVV (∼64.0 ppm), mrVVV (∼65.9 ppm), and mmVVV (∼67.9 ppm) triads; Ap2 is the total area of peaks of the carbon atoms in methyne (−CH−) groups in the middle vinyl alcohol units of rEVV (∼66.7 ppm) and mEVV triads (∼68.9 ppm); Ap3 is the area of peak of the carbon atoms in methyne (−CH−) groups in the middle vinyl alcohol units of EVE triad at δ of ∼69.7 ppm; Ak1, Ak2m and Ak3 are the area of peaks of the carbon atoms of methylene (−CH2−) groups in the middle ethylene units of EEE (∼29.3 ppm), VEE (∼25.4 ppm), and VEV (∼21.6 ppm) triads, respectively. The thermal properties of EVOH copolymers were examined using differential scanning calorimeter (DSC, Q200, TA Instruments, New Castle, DE) equipped with a liquid nitrogen cooling accessory. A nitrogen atmosphere was utilized to prevent oxidation degradation. A small amount (∼10 mg) of sample was laid in an aluminum pan. EVOH samples were heated at 100 °C for 12 h to normalize heattreatment before the DSC test since the temperature for unit

3. RESULTS AND DISCUSSION 3.1. Highly Efficient Alcoholysis of EVA Copolymers. The preparation of EVOH copolymers via alcoholysis of EVA copolymers was conducted in methanol using NaOCH3 as a catalyst, and the synthetic mechanism is displayed in Scheme 1. The critical factors of the DA of the resulting alcoholyzed EVA copolymers mainly include catalyst dosage, solvent, removal rate of byproduct (methyl acetate), reaction temperature, and reaction time. It can be seen from Scheme 1 that the equilibrium of alcoholysis of acetate groups exists in the reaction system, and thus, it is difficult to fully convert acetate groups into hydroxyl groups. The alcoholysis reaction equilibrium (Scheme 1) would be shifted to the left if the byproduct (methyl acetate) increased in the reaction system; thus, byproduct is unfavorable to improve the DA of alcoholyzed EVA copolymers. Moreover, the alcoholysis of the byproduct occurs in the presence of a catalyst, leading to extra consumption of the catalyst and thus C


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conducted in methanol using NaOCH3 as a catalyst. The alcoholyzed EVA copolymers obtained at different reaction times were characterized by ATR-FTIR, and the corresponding FTIR spectra are displayed in Figure 2(a). For comparison, all the FTIR spectra were normalized by the absorbance of peaks at 2858 cm−1 (antisymmetric stretching vibration of methylene groups) and 2931 cm−1 (symmetric stretching vibration of methylene groups) since the methylene groups in polymer chains stayed unchanged throughout the alcoholysis reaction. It can be clearly observed that all the intensities of characteristic peaks for the CO stretching vibration (1735 cm−1) and symmetric stretching vibration (1020 cm−1) in acetate groups gradually decreased with prolonged reaction time. Meanwhile, the intensity of the characteristic bond at 3350 cm−1 for the −OH stretching vibration increased with prolonged reaction time, illustrating that the side acetate groups in EVA chains were gradually converted into hydroxyl groups. In order to normalize the absorbance of CO vibration, UAc was defined as the ratio of the absorbance of CO vibration to the total absorbance of methylene deformation at 2858 and 2931 cm−1, A1735/(A2858 + A2931). The relationship between UAc and reaction time is shown in Figure 2(b). It can be observed that UAc decreased quickly with increasing reaction time within the first 10 min and then decreased gradually with further prolonging the time. The DA of alcoholyzed EVA copolymers was determined by 1 H NMR characterization according to eq 1, and UAc as a function of DA,NMR (DA determined by 1H NMR characterization) is presented in Figure 3. It can be clearly seen that UAc exhibits a good linear relationship (eq 8, shown in Figure 3) with DA,NMR when DA,NMR is in the range from 4% to 92%; thus, DA can be calculated according to eq 8 which is obtained from the linearity. Several EVA copolymers with various molecular weights (18−59 kg·mol−1) and E contents (37−46 mol %) were alcoholyzed in methanol, and the resulting alcoholyzed EVA copolymers with a DA,NMR of 23%−88% were obtained. The DA,FTIR values of the alcoholyzed EVA copolymers were calculated according to eq 8. The relationship between DA,FTIR and DA,NMR for the alcoholyzed EVA copolymers is displayed in Figure 4. It can be clearly seen that DA,FTIR is coincident with DA,NMR, indicating that DA data obtained via ATR-FTIR characterization are reliable. Thus, ATR-FTIR characterization can be utilized to determine the DA of alcoholyzed EVA copolymers quickly and quantitatively; in other words, a convenient and reliable method has been developed to determine DA by ATR-FTIR characterization. ATR-FTIR characterization was further utilized to investigate the alcoholysis kinetics of the EVA copolymer with an E content of 38.3 mol % at different nNaOCH3 values ranging from 0.01 to 0.15 within 30 min. The DA,FTIR values of the resulting alcoholyzed EVA copolymers as a function of reaction time at different nNaOCH3 values are given in Figure 5(a). The DA,FTIR of alcoholyzed EVA copolymers increased gradually at low catalyst dosage (nNaOCH3 = 0.01 and 0.05) with prolonging reaction time. The DA could reach relatively high levels of 81% within 30 min even at very low catalyst dosage (nNaOCH3 = 0.01), which was much lower than the reported data.18 It is noticeable that the alcoholysis processed rapidly at high catalyst dosage (nNaOCH3 = 0.10 and 0.15). The DA data of the resulting alcoholyzed EVA copolymers obtained by alcoholysis with nNaOCH3 values of 0.05 and 0.15 were almost the same at 30 min.

to a decrease in catalyst efficiency (as shown in Figure S1). Rapid alcoholysis of EVA copolymers took place at the beginning stage, leading to the formation of a large amount of byproduct which inhibited the further effective alcoholysis of EVA copolymers (as shown in Figure S2). It is beneficial for acceleration of alcoholysis, improvement of DA, and decrease in catalyst dosage by continuous removal of byproduct out of the reaction system during alcoholysis. The mixtures of byproduct and methanol were removed via distillation for all the following alcoholysis reactions in this research. The DA of alcoholyzed EVA copolymers throughout alcoholysis are normally obtained via off-line determination according to nuclear magnetic resonance (NMR).18,25−30 The alcoholyzed EVA copolymers with DA ≥ 38% have good solubility in DMSO-d6, while EVA copolymers and alcoholyzed EVA copolymers with DA ≤ 33% have good solubility in CDCl3. Thus, CDCl3 and DMSO-d6 were selected as solvents for 1H NMR characterization and 13C NMR characterization of EVA copolymers and alcoholyzed EVA copolymers with various DA. The representative 1H NMR spectra of the alcoholyzed EVA copolymer with a DA of 83% in DMSO-d6 and EVA copolymer in CDCl3 are shown in Figure 1.

Figure 1. Representative 1H NMR spectra of alcoholyzed EVA copolymer with DA of 83% in DMSO-d6 and EVA copolymer in CDCl3. EVA copolymer in CDCl3 (ppm): 1.2 (g, CH2-CH2-CH2); 1.5, 1.8, and 1.9 (c, -CH2-CHOAc); 2.0 (e, CH3-C = O); 4.75 (d, CHOAc). Alcoholyzed EVA copolymer in DMSO-d6 (ppm): 1.2 (g, CH2-CH2-CH2); 1.3 (f, HOCH-CH2-CHOH); 1.4 (f, HOCH-CH2CHOAc); 1.7 (c, CH2-CHOAc); 4.9 (a4, −OH in VAcV); 4.7 (a1, -OH in mmVVV triad); 4.5 (a2, -OH in mrVVV and mVVE triads); 4.2 (a3, -OH in rrVVV, rVVE, and EVE triads); 3.9 (b1, CH(OH) in mmVVV, mrVVV, and rrVVV triads); 3.6 (b2, CH(OH) in mVVE and rVVE triads); 3.4 (b3, CH(OH) in EVE triad, H2O).

The alcoholysis degree of alcoholyzed EVA copolymers with E contents more than 88 mol % was determined by FTIR via evaluating the ratio of the absorbance for carbonyl stretch (1730 cm−l) to that for methylene deformation (720 cm−l) in ethylene units −(CH2)n− (n ≥ 4).30 However, the characteristic peaks at 720 cm−1 are very weak in FTIR spectra of EVA copolymers and alcoholyzed EVA copolymers with E contents less than 49.5 mol % in our research. Consequently, it is necessary to develop a novel and convenient method for the quantitative determination of DA. The alcoholysis of the EVA copolymer (EVA-371) with an E content of 37.1 mol % and Mn,EVA of 18 kg·mol−1 was D


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Figure 2. (a) ATR-FTIR spectra of EVA copolymer (EVA-371) and resulting alcoholyzed EVA copolymers. (b) UAc as a function of alcoholysis time. EVA-371: E content = 37.1 mol %, Mn,EVA = 18 kg·mol−1, PDI = 2.7, nNaOCH3 = 0.05.

The alcoholysis of several EVA copolymers with similar E contents but different molecular weights was carried out to investigate the effect of catalyst dosage on initial alcoholysis rate (Ri0) and DA,FTIR of alcoholyzed EVA copolymers. Here, in order to describe the reaction rate at the initial stage of alcoholysis, Ri0 is defined as the ratio of ΔDA to Δt (80%) at the first stage, and an excessive catalyst is unnecessary for the alcoholysis at the first stage. On the basis of the above observation, a highly effective alcoholysis of EVA copolymers via sequential addition of catalyst was further achieved to synthesize EVOH copolymers with extremely high DA of ca. 100% even at low catalyst dosage (nNaOCH3 = 0.05), which is only around 5% of reported data for complete alcoholysis in toluen.18 The sequential addition of catalyst is beneficial to improve the catalyst efficiency and to decrease the total catalyst dosage. 3.2. Effects of Composition and Sequence of EVA Copolymers on Alcoholysis. The composition and structural sequence of EVA copolymers are important factors in the alcoholysis process. In order to investigate the effect of E content on the alcoholysis reaction, EVA copolymers with various E contents from 30.2 to 49.5 mol % were alcoholyzed under the same reaction conditions, and the results are shown in Figure 6. The DA of the resulting alcoholyzed EVA (E-VAcV) copolymers decreased linearly with an increase in E content of EVA copolymers. The DA of the E-VAc-V copolymer with an E content of 30.2 mol % was over 3 times of that of the E-VAcV copolymer with an E content of 49.5 mol %. The results illustrate that an increase in ethylene segments along macromolecular chains leads to a decrease in both alcoholysis rate and DA under the same alcoholysis reaction conditions. The representative 1H NMR spectra of the alcoholyzed copolymers with an E content of 35.8 mol % obtained at different reaction times are shown in Figure 7(a). The peak areas of hydrogen atoms in the middle hydroxyl groups (−OH) of VVV (b1, δ = 3.8 ppm) and VVE (b2, δ = 3.6 ppm)

Figure 3. Linear relationship between UAc obtained by ATR-FTIR characterization and DA,NMR obtained by 1H NMR characterization. EVA-370: E content = 37 mol %, Mn,EVA = 54 kg·mol−1, PDI = 2.6. □: nNaOCH3 = 0.005; Δ: nNaOCH3 = 0.015; ○: nNaOCH3 = 0.025. Other conditions are the same as in Figure 2.

Figure 4. Coincidence between DA,FTIR and DA,NMR of the alcoholyzed EVA copolymers. ○: E content = 40.2 mol %, Mn,EVA = 54 kg·mol−1, PDI = 2.6; Δ: E content = 39.9 mol %, Mn,EVA = 59 kg·mol−1, PDI = 2.0; □: E content = 37.4 mol %, Mn,EVA = 18 kg·mol−1, PDI = 2.7; ◊: E content = 45.7 mol %, Mn,EVA = 38 kg·mol−1, PDI = 2.5. Other conditions are the same as Figure 2. E


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Figure 5. (a) DA,FTIR of alcoholyzed EVA copolymers as a function of reaction time at different nNaOCH3. EVA copolymer (EVA-383): E content = 38.3 mol %, Mn,EVA = 11 kg·mol−1, PDI = 3.5. (b) Effect of nNaOCH3 on DA,FTIR of alcoholyzed EVA copolymers at reaction time t = 30 min (left axis) and the initial alcoholysis rate (right axis). □, ×: EVA-383; ○, Δ: E content = 37.4 mol %, Mn,EVA = 18 kg·mol−1, PDI = 2.7. Other conditions are same as Figure 2.

increased, suggesting that acetate groups were converted into hydroxyl groups. The peaks belonging to CH(OCOCH3)CH2CHOH (a4, δ = 4.9 ppm) could be clearly observed from 1 H NMR spectra of alcoholyzed EVA copolymers with DA less than 87.2%, while that peak was absent in the spectra of alcoholyzed EVA copolymers with higher DA. It illustrates that the alcoholysis of acetate groups in the VAcV diad completes when the DA of alcoholyzed EVA copolymers reaches 87.2%, which is different from the results for alcoholysis of EVA copolymer with an E content of 88 mol %.18 The alcoholysis of EVA copolymers with E contents of 38.8 and 40.2 mol % was further carried out to investigate the effect of triad sequence in EVA copolymer chains on the alcoholysis reaction. The conversions of acetate groups (i.e., yield of hydroxyl groups) in the alcoholyzed EVA copolymers obtained at different reaction times are displayed in Figure 7(b). The conversion of acetate groups in different triads of VAcVAcVAc, VAcVAcE, and EVAcE in EVA copolymers to corresponding hydroxyl groups is expressed as the ratio of peak areas of protons in the middle hydroxyl groups of VVV, VVE, and EVE triads in alcoholyzed EVA copolymers to those in EVOH copolymers with DA of 100%, respectively. Those characteristic

Figure 6. Effect of E content on DA of E-VAc-V copolymers obtained by alcoholysis of EVA copolymers with E contents varied from 30.2 to 49.5 mol %.

increased with an increase in DA, while the areas of peaks belonging to CH(OAc) (d, δ = 4.8 ppm) decreased as DA

Figure 7. (a) Representative 1H NMR spectra of alcoholyzed EVA copolymers with different DA obtained throughout alcoholysis: (1) DA = 50%, (2) DA = 73%, (3) DA = 87%, (4) DA = 91%, (5) DA = 100%. (b) Effect of DA on yield of hydroxyl groups in different triads for alcoholyzed EVA copolymers with E contents of 38.8 and 40.2 mol %. F


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EVE, mEVV, mmVVV, rEVV, mrVVV, and rrVVV triads, respectively. The characteristic signals at δ of 45.1, 38.2, and 37.4 ppm (j) belong to the carbon atoms in methylene (−CH2−) groups in vinyl alcohol units. The characteristic signals around 29.3 ppm (k1), 25.4 ppm (k2), and 21.6 ppm (k3) present for the carbon atoms of triads with methylene (−CH2−) groups in ethylene units, which are in the middle of EEE, VEE, and VEV triads, respectively. The fractions of VVV, VVE (EVV), EVE, VEV, VEE (EEV), and EEE triads can be calculated using eqs 2−7) on the basis of 13C NMR characterization, respectively. 3.3. Effects of Composition, Alcoholysis Degree, and Structural Sequence on the Oxygen Barrier Property of EVOH Copolymers. The oxygen barrier property of EVOH copolymers is normally evaluated by the oxygen transmission rate (OTR) value, and the EVOH copolymer with a low OTR value exhibits a good oxygen barrier property. EVOH copolymers with different OTR values were selected and divided into three series according to their molecular weights. The effect of the E content of EVOH copolymers on the OTR value is displayed in Figure 9. It can be seen that the OTR

peak areas are normalized by the total peak area of protons in methylene (−CH2−) groups at 1.2−1.9 ppm. For the alcoholysis of an EVA copolymer with an E content of 40.2 mol %, the formation of hydroxyl groups in the VVV triad was almost completed when the DA of the alcoholyzed EVA copolymer was 74%. The alcoholysis of acetate groups in the VAcVAcVAc triad is extremely rapid, and acetate groups are fully alcoholyzed at the first stage due to the neighbor effect, in which as soon as the transformation from VAc to VOH takes place the neighboring VAc units are transformed more readily.31−34 The formation rate of VVE triad increased linearly with increasing DA, indicating that the alcoholysis of acetate groups in VAcVAcE triad takes place throughout the reaction. The formation rate of the EVE triad increased smoothly at the beginning and increased dramatically after the DA of the alcoholyzed EVA copolymer was over 87%. A high proportion of acetate groups in the EVAcE triad were alcoholyzed at the final stage. It can be observed from Figure 7(b) that the alcoholysis rate of acetate groups in different triads (VAcVAcVAc, VAcVAcE, EVAcE) of an EVA copolymer with an E content of 38.8 mol % was similar to that of an EVA copolymer with an E content of 40.2 mol %. Therefore, the alcoholysis rate of EVA copolymers is obviously dependent on their structural sequences, i.e., decreases in the order of VAcVAcVAc > VAcVAcE > EVAcE triad sequence. The complete alcoholysis of EVA copolymers with various E contents in the range from 30.2 to 49.5 mol % has been achieved to produce the corresponding EVOH copolymers with DA of ca. 100% even at very low catalyst dosage (nNaOCH3 = 0.05) via combination of sequential catalyst addition with continuous removal of byproduct within 3 h. The representative 13C NMR spectrum of an EVOH copolymer with an E content of 37.0 mol % is displayed in Figure 8. The absence of

Figure 9. Effect of E content on OTR values of EVOH copolymers for series with similar Mn,EVA and PDI: EVOH-370, Mn,EVA = 30 kg· mol−1, PDI = 1.7; EVOH-495, Mn,EVA = 31 kg·mol−1, PDI = 1.9; EVOH-369, Mn,EVA = 54 kg·mol−1, PDI = 2.3; EVOH-447, Mn,EVA = 45 kg·mol−1, PDI = 2.2; EVOH-383, Mn,EVA = 68 kg·mol−1, PDI = 1.8; EVOH-444, Mn,EVA = 68 kg·mol−1, PDI = 1.6. The DA values of all the above EVOH copolymer samples are ca. 100%.

value of the EVOH copolymer increased with increasing E content for every series. For series I (M-30PDI-18) with a Mn,EVA of around 30 kg·mol−1 and PDI of around 1.8, the OTR value of EVOH-495 with an E content of 49.5 mol % was 6.33 times larger than that of EVOH-370 with an E content of 37.0 mol %. For series II (M-50PDI-22) with Mn,EVA of around 50 kg·mol−1 and PDI of around 2.2, the OTR value of EVOH-447 with an E content of 44.7 mol % was 6.15 times as large as that of EVOH-369 with an E content of 36.9 mol %. EVOH copolymers with low E contents have good oxygen barrier property due to the increase in inter/intra molecular hydrogen bonds and cohesive energy. To identify the presence of hydrogen-bonded O−H groups, the variable-temperature FTIR (VT-FTIR) was performed on the EVOH copolymer. As shown in Figure 10(a), the broad bands at 3000−3650 cm−1 are assigned to the O−H stretching bands which experience a wide range of hydrogen bond strengths due to the interaction of inter/inner macro-

Figure 8. Representative 13C NMR spectrum of EVOH copolymers with E content of 37.0 mol % and DA,NMR of ca.100%.

signals belonging to acetate groups at 14.0 and 120.0 ppm in the 13C NMR spectrum indicates that EVOH copolymers with DA of ca. 100% could be obtained by the complete alcoholysis of EVA copolymers. It has been reported that the composition and sequence of EVOH copolymers can be usually characterized by 1H NMR and 13C NMR.35−37 As shown in Figure 8, the characteristic signals around 69.7 ppm (p3), 68.9 ppm (p2), 67.9 ppm (p1), 66.7 ppm (p2), 65.9 ppm (p1), and 64.0 ppm (p1) are assigned to the carbon atoms of triads with methyne (−CH−) groups in the vinyl alcohol units, which are in the middle of G


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Figure 10. (a) Variable-temperature FTIR (VT-FTIR) spectra of EVOH copolymer in the O−H stretching region (2400−3800 cm−1) obtained with increasing temperature from 30 to 150 °C at an interval of 10 °C. (b) FTIR spectra of EVOH-370 with E content of 37.0 mol % and EVOH495 with E content of 49.5 mol %. The DA values of all the above EVOH copolymer samples are ca. 100%.

molecules.38−41 The dissociation of hydrogen bonds could be observed from the upshift of the O−H stretching band and reduction in intensity when the temperature increased from 30 to 150 °C. It has been reported that the absorbance at around 3310 cm−1 is assigned to the O−H stretching of hydrogenbonded O−H groups, v(O-Hb), in V units.38 The peaks assigned to the stretching vibration of methylene groups at 2858 and 2931 cm−1, v(CH2), did not reveal any appreciable changes with temperature. Therefore, v(CH2) can be selected as a reference for the evaluation of intensities of v(O-Hb) at around 3310 cm−1. The normalized absorbance of v(O-Hb) (Ab‑OH) is defined as the ratio of the absorbance of v(O-Hb) at 3310 cm−1 to the total absorbance of vs(CH2) at 2858 cm−1 and vas(CH2) at 2931 cm−1, abbreviated as A3310/(A2858 + A2931). EVOH copolymers with E contents of 37.0 and 49.5 mol % were characterized by FTIR at 20 °C, and the spectra are displayed in Figure 10(b). It can be seen that EVOH-370 with a lower E content of 37.0 mol % presents much larger Ab‑OH, illustrating that the intensity of hydrogen bonds in EVOH copolymers with low E contents is much larger than that in a EVOH copolymer with a high E content. Moreover, the OTR value of EVOH-370 with a low E content of 37.0 mol % is only 13.5% of that of EVOH-495 with an E content of 49.5 mol %. The pronounced increment in the oxygen barrier property is caused by the increase in hydrogen bonds. Consequently, EVOH copolymers with lower E contents have better oxygen barrier property due to hydrogen bonds and cohesive energy. The effect of DA on the oxygen barrier property of the alcoholyzed EVA copolymers was investigated by taking the EVA copolymer as a control. The relationship between DA of the corresponding copolymers and OTR values is shown in Figure 11. The OTR values of the EVA copolymer and the alcoholyzed EVA copolymer with a DA of 58% were determined to be 3.89 × 10−14 and 5.92 × 10−15 cm3·cm· cm−2·s−1·Pa−1 respectively, illustrating that the oxygen barrier property can be improved as DA increases. The OTR values of the alcoholyzed EVA copolymer decreased to 6.32 × 10−16 cm3·cm·cm−2·s−1·Pa−1 when DA increased to 89%. The OTR value of EVOH copolymer with DA of ca. 100% was dramatically dropped to 6.17 × 10−17 cm3·cm·cm−2·s−1·Pa−1, which was only 37% of that of alcoholyzed EVA copolymers with DA of 97%. We come to the conclusion that the oxygen barrier property is remarkably affected by even a small amount

Figure 11. Relationship between DA,NMR and OTR values of alcoholyzed EVA copolymers.

of residual acetate groups in the alcoholyzed EVA copolymers. The residual acetate groups reduce chain segment mobility, but in turn result in the formation of a lattice defect and the generation of a large fractional free volume, which provide a pathway for oxygen.32 In order to achieve EVOH copolymers with excellent oxygen barrier property, it is essential to prepare EVOH copolymers with a DA of ca. 100%. EVOH samples with various E contents were characterized by DSC in order to investigate the influence of E content on crystallization behavior. Figure 12(a) summarizes the representative DSC curves of EVOH copolymers with different E contents. The crystallization enthalpy (ΔHm) and melting temperature (Tm) of EVOH copolymers were obtained from the thermogram analysis, and the effect of E content on ΔHm and Tm is displayed in Figure 12(b). It can be clearly seen that Tm and ΔHm of EVOH copolymers with different E contents decreased approximately linearly with an increase in E content due to the formation of hydrogen bonds. EVOH copolymers with low E contents possess large amounts of crystalline regions mainly formed by repeating V units, resulting in high Tm and ΔHm.42,43 Thus, an increase in E content leads to a decrease in the amount of hydrogen bonds and an increase in mobility of molecular chain. EVOH copolymers with low E contents possess superior oxygen barrier property due to strong molecular cohesion formed by inter/intra-molecular hydrogen bonds. The first H


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Figure 12. (a) DSC curves of EVOH copolymers with different E contents varied from 30.2 to 47.1 mol %. (b) Effects of E content on melting enthalpy (ΔHm) and melting temperature (Tm). The DA values of all the EVOH copolymer samples are ca. 100%.

pivotal role in the enhancement of the oxygen barrier property of EVOH copolymers. Our approach here is to obtain the characteristic micromorphology of EVOH copolymers with different OTR values. Transmission electron microscope (TEM) and high resolution transmission electron microscope (HR-TEM) characterizations were performed on EVOH copolymers to visualize the aggregations in EVOH copolymers. As shown in Figure 14, the dark-contrast stripes in TEM images correspond to the crystalline regions in EVOH copolymers, while the lightcontrast stripes represent the amorphous regions. It can be seen that both of EVOH-370 and EVOH-400 possess similar bicontinuous phase separation. As shown in Figure 14(a), EVOH-370 with a lower OTR value of 2.73 × 10−17 cm3·cm· cm−2·s−1·Pa−1 possessed more distinct phase-separated morphology with average domains spacing size of ca. 25 nm. The phase separation with average domains spacing size of ca. 31 nm to create relatively large free volumes which provide pathway for oxygen diffusion can be observed in the TEM image of EVOH-400 (Figure 14(b)), leading to an increase in OTR value, 6.17 × 10−17 cm3·cm·cm−2·s−1·Pa−1. Ultrathin film of EVOH-370 was performed on HR-TEM characterization, and the result is displayed in Figure 14(c). The uniform micromorphology of EVOH-370 in the HR-TEM image (Figure 14 (c)) is in agreement with the observation in TEM (Figure 14 (a)), suggesting that EVOH copolymers with high oxygen barrier property possess dense microphase separation. Thus, EVOH copolymers with densely packed microphase separations formed by well-distributed and small-

step of crystallization in EVOH copolymers is the formation of hydrogen bonds, which begins among the V repeating units.24 As shown in Figure 13, EVOH copolymers with a high

Figure 13. Schematic diagram of microphase separation formed by crystalline regions and amorphous regions.

crystallization degree have excellent oxygen barrier property since the diffusion paths of small molecular gas are impelled by crystalline regions, while the transport of penetrant molecules through polymer materials occurs in the free volumes in amorphous regions.44,45 Generally, hydrogen bonds play a

Figure 14. (a) TEM image of EVOH-370 with OTR value of 2.73 × 10−17 cm3·cm·cm−2·s−1·Pa−1. (b) TEM image of EVOH-400 with OTR value of 11.00 × 10−17 cm·cm·cm−2·s−1·Pa−1. (c) HR-TEM image of EVOH-370. EVOH-370: E = 37.0 mol %, Mn,EVA = 30 kg·mol−1, PDI = 1.7, f VVV = 25.1%, f VVE = 28.7%, f EVE = 8.2%, f EEV = 16.2%, f VEV = 13.3%; EVOH-400: E = 40.0 mol %, Mn,EVA = 43 kg·mol−1, PDI = 3.4, f VVV = 23.0%, f VVE = 27.0%, f EVE = 8.1%, f EEV = 17.5%, f VEV = 14.2%. The DA values of all the above EVOH copolymer samples are ca. 100%. I


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Figure 15. (a) Effect of Ab‑OH on OTR values of EVOH copolymers with similar molecular weight but different triad fractions. (b) Effect of triad fractions on Ab‑OH (■: f EVE, ▼: f VEV, ▲: f EEV, ●: f VVE, ⧫: f VVV); EVOH-392: E content = 39.2 mol %, Mn,EVA = 39 kg·mol−1, PDI = 3.2; EVOH386: E content = 38.6 mol %, Mn,EVA = 40 kg·mol−1, PDI = 3.1; EVOH-370: E content = 37.0 mol %, Mn,EVA = 30 kg·mol−1, PDI = 1.6; EVOH-361: E content = 36.1 mol %, Mn,EVA = 31 kg·mol−1, PDI = 2.6; EVOH-369: E content = 36.9 mol %, Mn,EVA = 54 kg·mol−1, PDI = 2.3; EVOH-377: E content = 37.7 mol %, Mn,EVA = 55 kg·mol−1, PDI = 2.4. The DA values of all the above EVOH copolymer samples are ca. 100%.

4. CONCLUSIONS

sized crystalline regions and amorphous regions have excellent oxygen barrier property. In order to investigate the effect of triad fractions on the oxygen barrier property, EVOH copolymers with similar E contents, Mn,EVA, and PDI but different triad fractions were selected and divided into three series for comparison. Each series is identified by the average E content and Mn,EVA, i. e., series IV (E-390M-40: EVOH-392; EVOH-386), series V (E365M-30: EVOH-370; EVOH-361), and series VI (E-373M55: EVOH-369; EVOH-377). As shown in Figure 15(a), the OTR values of EVOH copolymers decreased with increasing Ab‑OH in each series, indicating that the formation of hydrogen bonds is beneficial for the improvement of the oxygen barrier property. The effect of triad fractions on Ab‑OH of EVOH copolymers is displayed in Figure 15(b). The difference of Ab‑OH for each series of EVOH copolymers comes from the variance of triad fraction under similar structural parameters of E content, molecular weight, and PDI. For series IV (E-390M40), the Ab‑OH increased from 0.677 to 0.688 with the increase in f EVE from 7.1% to 10.3% and the decrease in f VEV from 14.5% to 13.8%. For series V (E-365M-30), Ab‑OH increased from 0.657 to 0.710 when f VVV of the EVOH copolymer increased from 22.5% to 25.1%, and f EEV decreased from 17.6% to 16.2%. For series VI (E-373M-55), Ab‑OH increased from 0.661 to 0.680 with increasing f EVE from 9.1% to 9.9% and decreasing f VEV from 15.0% to 11.9%. The schematic diagram of microphase separation formed by crystalline regions and amorphous regions in EVOH copolymers is given in Figure 13. The neighbored V units, i.e., V units in the VVV triad sequence, contribute to the formation of crystals and result in an improvement in oxygen barrier property. The V units with an orderly arrangement along the macromolecular chains, i.e., hydroxyl groups in the EVE triad sequence, form hydrogen bonds in amorphous regions and lead to the well-arranged and small-sized microphase separation. The densely packed and well-distributed microphase separation provides tortuous paths for the diffusion of oxygen molecular and prevents oxygen from diffusion. Thus, EVOH copolymers with high f VVV and f EVE, and low f EEV and f VEV would have excellent oxygen barrier property.

A novel convenient and reliable method was developed for the quantitative determination of DA on the basis of ATR-FTIR characterization and is utilized in alcoholysis kinetics. A small amount of catalyst (nNaOCH3 = 0.01) is actually sufficient to get relatively high DA (>80%) at the first stage, and excessive catalyst is unnecessary for alcoholysis. On the basis of kinetics, EVOH copolymers with DA of ca. 100% and different E contents of 30.2−49.5 mol % can be synthesized efficiently via combination of a sequential addition of catalyst with continuous removal of byproduct during the alcoholysis reaction. The alcoholysis rate is dependent on the composition and sequence of EVA copolymers. The increase in E content of the EVA copolymer leads to decreases in both reaction rate and DA of the resulting EVOH copolymer. The alcoholysis rate of acetate groups in different triads decreases in the order of VAcVAcVAc > VAcVAcE > EVAcE. The oxygen barrier property of the EVOH copolymer is remarkably affected by their E contents and DA. The EVOH copolymer with a low E content and high DA has as a good oxygen barrier property due to the increases in inter/intra molecular hydrogen bonds and cohesive energy. Furthermore, the oxygen barrier property of EVOH copolymers is improved by the formation of densely packed regions from the neighbored V units (VVV), orderly arranged V units (EVE), and EEE triad sequences along macromolecular chains via hydrogen bonds and crystallization. EVOH copolymers with high f VVV and f EVE, and low f EEV and f VEV would have excellent oxygen barrier property. EVOH copolymers with excellent oxygen barrier property have a great potential to be applied in the fields of packaging and polymer blends.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b06260. Information as mentioned in the text. (PDF) J


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S. Zhang). *E-mail: [email protected] (Y. Wu). ORCID

Yixian Wu: 0000-0002-3482-4564 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial supports from the National Nature Science Foundation of China (51521062) and China Petroleum & Chemical Corporation (SINOPEC 213054) are greatly appreciated.

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ABBREVIATIONS EVA = ethylene-vinyl acetate EVOH = ethylene-vinyl alcohol NaOCH3 = sodium methoxide VAc = vinyl acetate DMSO-d6 = deuterodimethyl sulfoxide CH3OH = methanol THF = tetrahydrofuran CDCl3 = deuterotrichloromethane AIBN = azobis(isobutyronitrile) OTR = oxygen transmission rate

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