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isopropanol solution
Accepted Manuscript NMR study of ring opening reaction of epoxidized natural rubber in presence of potassium hydroxide/isopropanol solution Omar S. Dahham, Rosniza Hamzah, Mohamad Abu Bakar, Nik Noriman Zulkepli, Saad S. Dahham, Sam Sung Ting PII:
S0142-9418(16)31346-0
DOI:
10.1016/j.polymertesting.2017.01.006
Reference:
POTE 4892
To appear in:
Polymer Testing
Received Date: 2 December 2016 Accepted Date: 6 January 2017
Please cite this article as: O.S. Dahham, R. Hamzah, M.A. Bakar, N.N. Zulkepli, S.S. Dahham, S.S. Ting, NMR study of ring opening reaction of epoxidized natural rubber in presence of potassium hydroxide/isopropanol solution, Polymer Testing (2017), doi: 10.1016/j.polymertesting.2017.01.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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NMR study of ring opening reaction of epoxidized natural rubber in presence of potassium hydroxide/isopropanol solution
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Omar S. Dahham1a, Rosniza Hamzah1b, Mohamad Abu Bakar2c, Nik Noriman Zulkepli1d, Saad S. Dahham3e, Sam Sung Ting4f 1
Center of Excellence Geopolymer and Green Technology, Faculty of Engineering Technology, Universiti Malaysia Perlis, 02100 Padang Besar, Perlis, Malaysia
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Nanoscience Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
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EMAN Research and Testing Laboratory, School of Pharmaceutical Sciences, Universiti Sains Malaysia, 11800 Minden, Pulau Pinang, Malaysia
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School of Bioprocess Engineering, Universiti Malaysia Perlis (UniMAP), Kompleks Pengajian Jejawi 3, 02600 Arau, Perlis, Malaysia a
[email protected], [email protected], [email protected], d [email protected], [email protected], [email protected] Correspondence to: Name: Rosniza Hamzah E-mail: [email protected]
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Abstract: The ring opening reaction of epoxidized natural rubber (ENR-50) in the presence of potassium hydroxide/isopropanol solution was studied using NMR, and it’s thermal characteristic was investigated using TG/DTG and DSC. 1H-NMR showed that 16.9% of epoxide units was ring-opened in treated ENR-50, which was also supported by quantitative FTIR spectroscopy. 13C-NMR proved the location of alkyl group (isopropyl) in the polymer chain of treated ENR-50. The attachment location of isopropyl occurred at both most (↑) and least (↓) hindered carbons of the epoxide. 2D-NMR was used to identify and scrutinize the triad assignment of treated ENR-50. The TG/DTG results presented three decomposition steps at 190-331, 331521 and 521-706 °C due to the existence of mixtures of polymer chains i.e. ringopened and intact epoxide of ENR-50, which also led to increase in Tg of treated ENR-50 at 13.2°C compared with purified ENR-50 at -17.7°C. Keywords: epoxidized natural rubber, ring opening reaction, epoxide, nuclear magnetic resonance, thermal characteristic. 1.Introduction Nuclear magnetic resonance spectroscopy (NMR) techniques are extensively used for polymeric materials characterization either in solid or liquid phase. 1D NMR provides structural explanation on monomer units and reactive groups in the polymer chain. 2D NMR gives detail structural arrangements on the f monomer units in the polymer chain. The 1D NMR technique or both 1D NMR and 2D NMR techniques were
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applied previously to epoxidized natural rubber (ENR), such as ENR25 [1], ENR50 [2-6], ENR75 [3] and other ENR type [7,8]. Nevertheless, most of these researches didn’t give complete structural studies because of the overlapping of signals arising from the random distribution of isoprene (C) and epoxidized isoprene (E) monomers [9,10]. A triad of ENR is made up of three possible sequences of C and E units. All the possible ENR-50 triad sequences are shown in Figure 1. For nomenclature purposes, the common structure and the carbon atoms numbering of the E and C units in ENR-50 is shown in Scheme 1. In a triad, the neighbouring units may dictate the 1 H and 13C environments of the middle unit. It is the middle unit that will reveal the triad structure. For example, an ENR chain fragment may be comprised of C unit attached to an E unit which in turn is attached to another E unit. Thus, this triad sequence is denoted as CEE. Triad comprising of similar unit is only denoted by a single unit. Triad CCC is denoted as C and triad EEE is denoted as E. The methyl of the middle unit in the CEE and CCC triads are denoted as CE10E and C5, respectively, where the superscript refers to the (methyl) carbon (C) number in the ENR-50 structure, as shown in Scheme 1. This nomenclature also applies to the proton in the ENR-50 structure. Prior to the advent of the triad sequence, previous researches have only ascertained the 1H chemical shifts assignments of methine proton of E and C units but sporadically to either the methyl and/or methylene protons of E and C units of the ENR [5-8,11,13]. Furst and Pretsch have reported the use of computer simulation to predict chemical shifts of these protons and their triad assignments [10]. On the other hand, Gelling has assigned the triad sequence based on 13C-NMR chemical shifts for ENR-20 [12]. However, the assignment deals with certain triad sequences only and the position of carbon represented by the triad sequences was found to be inconsistent. Consequently, Saito et al. have successfully interpreted the 13C-NMR spectra for the ENR related compounds and assigned their triad sequence [13]. The manipulating of ENR pH below or above 7 can play an important role to catalyse the epoxide ring opening reaction and provides crosslinking among polymer chains. The existence of acid accelerates the addition of most nucleophiles to the epoxide carbon atoms, which in turn led to ring opening reaction [14]. The reaction of ring opening in the presence of acid produces an alcohol group along with formyl ester. Furthermore, acid could catalyse the epoxide ring opening and produce ether crosslinking [9]. In the common base, a nucleophile attacks the less substituted carbon of the epoxide causing the β-opening at the epoxide ring [15]. In rubber processing via vulcanization, the excess of base will cause premature curing and accelerate the reaction. This premature curing is due to the epoxide ring opening reaction under basic condition [16,17]. Our previous work has studied the influence of acetic acid as a mild acidic condition on the ring opening reaction of epoxidized natural rubber using 1D and 2D NMR supported with thermal analyses (TG/DTG and DSC). The 1H-NMR result showed that 19.56% of epoxide ring opening took place in the acid treated ENR-50 and 13CNMR suggested the acetate group location in the polymer chains. Furthermore, the
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alkyl groups attached to the ring-opened epoxide unites have improved the thermal characteristics of the acid treated ENR-50 [18]. The main objective of our current work is to introduce a comprehensive study of ring opening ENR-50 in the presence of potassium hydroxide/isopropanol solution. This study included FTIR, 1D and 2D NMR in addition to TG/DTG and DSC for thermal characteristics study. The rubber type used in this work is a low molecular weight ENR-50 which was obtained by a sol-gel technique. This work differs from other stated works in regard of the epoxidation degree and physical form of epoxidized natural rubber [13,19]. 2. Experimental 2.1. Materials
2.2. Preparative Procedure 2.2.1. ENR-50 Purification
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Toluene, chloroform, n-hexane, 2-propanol, IPA (Systerm, Malaysia), Potassium hydroxide, 85.0%, KOH (Q Rec Asia, Malaysia), Deuterated chloroform and CDCl3 (Fluka Chemicals, Switzerland) were all purchased commercially and used without additional treatment unless otherwise stated. ENR-50 was obtained from the Rubber Research Institute, Kuala Lumpur - Malaysia.
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20.00 g of ENR-50 was swelled in 400 mL of chloroform and stirred for 24 h at 25°C. A cotton gauze pack was used to filter the solution and separate the low molecular weight ENR-50 from the high molecular weight ENR-50. The low molecular weight ENR-50 was precipitated in n hexane and then stirred manually using a glass rod. The white precipitate stuck on the glass rod was moved to a Teflon petri dish. The samples were dried using vacuum oven at 50 °C for 48 h. The dried samples represented the purified ENR-50. Treated ENR-50
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A 0.21 M (14.09 mg mL-1) KOH/IPA solution was prepared by dissolving 1.41 g (21.35 x 10-3 mol) of KOH pellets in 100 mL IPA. In a typical preparation, about 200 mg (1.32 x 10-3 mol) of purified ENR-50 was dissolved in 40 mL toluene with stirring. The pH of the solution was adjusted to pH 12 by addition of a 10 mL of KOH stock solution. The mixture was then refluxed at 85 °C for 3 hours. It was then left to cool to room temperature. The reaction mixture was washed with 150 mL of water until the washing attained a neutral pH. On separation, the organic layer was cast into Teflon dishes followed by drying in a vacuum oven at 80 °C for 24 hours. 2.3. Measurements and Characterization Techniques The NMR spectra were obtained using a Bruker Avance 500 MHz instrument in CDCl3 at 25 °C. A sample was swelled for 4 days in a NMR tube before conducting tests. About 10mg of prepared solution was used for 1H Spectroscopy and 50 mg was
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2.4. Theoretical Treatment
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used for the 13C-NMR, heteronuclear multiple bond coherence (HMBC), heteronuclear multiple quantum coherence (HMQC) and correlation spectroscopy (COSY) analyses. The spectra respective range and scan number for 1H and for 13CNMR measurements were 15–0 ppm with 16 scans and 200–0 ppm with 8,000 scans, respectively. A Perkin-Elmer 2000-FTIR spectrometer was utilized with single beam transmittance onto a film of sample on zinc selenide (ZnSe) window in the wavelength range of 4,000–600 cm−1. The FTIR samples were prepared using 5 mL of chloroform to swell 100.00 mg of the samples. After sample swelling, the solution was cast into a 3.0 cm x 1.0 cm x 0.5 cm Teflon mould and air dried. The vacuum oven was used at 50 °C and 1 hour for further drying and the sample then transferred onto the ZnSe window. A thermal gravimetric (TG) analyser, model Perkin-Elmer TGA7 was used to assess the samples’ thermal stability. A 10mg sample was heated from 30°C to 900°C at 20 °C min-1 under nitrogen atmosphere. Differential scanning calorimetry (DSC) was done using a Perkin-Elmer Pyris6 instrument. About 10. mg sample was sealed in an aluminium pan. The thermograms were recorded over a temperature range of -50 to 140 °C at a heating rate of 20 °C min-1 under nitrogen atmosphere. First, the sample was heated from -50 to 140 °C and held at 140 °C for 1 minute. Then, it was quenched to -50 °C and held for another 5 minutes. The final step was the reheating from -50 to 140 °C at the same heating rate.
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In 1H-NMR, the amount of epoxide before and after reaction, and the percentage of ring opening in a sample was determined using Equation (1) [1,2,12]. I2.7 is the integrated area of the methine proton of epoxidized isoprene and I5.2 is the integrated area of the methine proton of isoprene. In Equation (1), I2.7 is chosen because it is the initial reactive site for the reaction and, for purified ENR-50, the integrated area of this peak is one proton. Typically, the integrated area at I2.7 after reaction is less than the I2.7 before reaction, while I5.2 remains similar before and after the reaction. This is because methine proton of isoprene is not involved in this reaction. In Equation (2), the percentage of ring opening in the sample is a deduction of the percentage of epoxide in purified ENR-50 with the percentage of epoxide left in the ring opened sample after reaction. The semi quantitative analysis of FTIR was based on the peak area of epoxide and methyl functional groups. The percentage of epoxide in the sample was calculated using Equation (3). In equation (1), Nmethyl is the methyl normalized peak area, A epoxide sample is the epoxide functional group peak area and A epoxide ENR-50 is the peak area of respective functional group stretching peak in purified ENR-50. The percentage of epoxide in the sample was calculated using Equation (1) [5]. % Epoxide in sample = I2.7 * 100% / I2.7 * I5.2
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ACCEPTED MANUSCRIPT % Ring opening in sample = (% epoxide in purified ENR-50 - % epoxide in sample) * 100% / % epoxide in purified ENR-50
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% poxide = Nmethyl * Aepoxide sample * 100% / Aepoxide ENR-50
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3.1. NMR 1
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3.1.1. H- and C-NMR Spectroscopy
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3. Results and Discussion
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The 1H- and 13C-NMR spectra of treated ENR-50 are shown in Figure 2, and the respective chemical shifts are tabulated in columns 1 and 2 of Table 1. From Figure 1(a), chemical shifts of C2 at δ 5.13-5.17 and E7 at δ 2.73 ppm are similar to the purified ENR-50 [20]. However, the integral of E7 (m, 0.797H) is slightly decreased (viz. 0.203H) as compared to C2 (m, 1.000H). This small decrement is due to 16.9 % of ring opening reaction (Equation 2). The region of hydroxyl proton of E7 and E6 is shown at δ 3.00-4.20 ppm (br s, 0.122H) and δ 4.70-5.00 ppm (br s, 0.122H), respectively [1,2]. This shows that the reaction has taken place at most hindered carbon and less hindered carbon positions abbreviated as (↑) and (↓), respectively. The reason for broadness for these peaks is fast proton exchange between hydroxyl group and the solvent in the system [21-23]. The peak at δ 1.31 ppm (Figure 1, a) is a mixture of methyl protons of E10 of the ENR-50 and methyl protons of isopropyl (IP) E21 (Scheme 2). The peak at δ 1.57 ppm indicates the methylene protons at (↑) and (↓) positions for E8, EE8C, CE8E, CE8C and at (↓) position for CE9C, EE9C. It also belongs to the methylene protons of E8, EE8C, CE8E, CE8C, CE9C and EE9C of the ENR-50. The peak at δ 1.69 ppm represents methylene protons at (↑) and (↓) positions for E9, CE9E as well as methylene protons of E9, CE9E and methyl protons of C5 of the purified ENR-50. The methylene protons of C3, CC3E, C4, EC4C and CC4E, EC4E, EC3C, EC3E are located at δ 2.06 and 2.18 ppm respectively. Typically, the peak at δ 2.73 ppm belongs to the methine proton of E7 which remains intact in the polymer chains. However, it also represents the methine proton at (↑) and (↓) positions for E7, CE7E, EE7C, CE7C as well as methine proton of IP E20 (Scheme 2). The 13C-NMR spectrum of treated ENR-50 is shown in Figure 1 (b) and tabulated in column 2 of Table 1. The presence of IP group within the ENR-50 structure is evidenced via methine carbon of IP (E20) and methyl carbon of IP (E21). The chemical shift of methyl carbon E21(↑) and methine carbon E20(↓) is at δ 22.2 and δ 73.7 ppm respectively. As shown earlier, the (↑) position produces ether carbon E6 and the (↓) produces ether carbon E7. The ether carbon E6 shows a single peak at δ 84.5 ppm while the ether carbon E7 shows 2 peaks at δ 77.7 and 86.5 ppm. Both of the hydroxyl carbons E6, E7 are at a similar chemical shift i.e δ 77.7 ppm, regardless of the (↑) or (↓) positions. This is due to the similar electron environment for the respective hydroxyl. The
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3.1.2
HMQC
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methine carbon E7, which is not involved in ring opening reaction, remains similar to the purified ENR-50 at δ 64.5 ppm. The methyl carbon E10 is represented by 3 peaks at δ 22.2, 23.3 and 23.9 ppm. The peak at δ 23.3 ppm is when attachment of methyl is at (↑), while at δ 23.9 ppm it is at (↓). The methyl carbon E10 which is not involved in the ring opening reaction remains at δ 22.2 ppm. The methylene carbon E8 shows 6 peaks at δ 29.7, 32.0, 33.1, 35.0, 36.5 and 41.0 ppm. The peak at δ 32.0, 35.0, 36.5 ppm belongs to E8 and EE8C. The methylene at (↓) gives a peak at δ 35.0 ppm. The attachment at (↑) produces two peaks at δ 32.0, 36.5 ppm. This is due to the additional ring opened epoxide of L and R units of these triads. The peak at δ 41.0 ppm corresponds to triads CE8E and CE8C due to the attachment at (↑) and (↓) positions. The peak at δ 29.7 and 33.1 ppm represents the methylene of EE8C, E8 and CE8C, CE8E of the ENR-50, respectively. The methylene carbon E9 shows 6 peaks at δ 23.3, 23.9, 24.6, 26.2, 27.0 and 29.7 ppm. Both peaks at δ 23.9 and 29.7 ppm represent the methylene at (↓) for CE9C and EE9C. Similarly, peaks at δ 23.3 and 26.2 ppm belong to methylene at (↓) of E9 and CE9E. The methylene at (↑) produces a peak at δ 27.0 ppm for these triads. The peak at δ 27.0 ppm belongs to EE9C and CE9C while for E9 and CE9E it is located at δ 24.6 ppm respectively. The methylene carbon of C3 shows 2 peaks at δ 26.2 and 28.7 ppm. The peak at δ 26.2 ppm corresponds to the C3 and CC3E while the peak at δ 28.7 ppm is for EC3C and EC3E triads. Similarly, the peak due the methylene carbon of CC4E and EC4E is at δ 23.9 ppm and δ 26.2 ppm for C4 and EC4C triads, respectively. Finally, the quaternary carbon C1 and methine carbon C2 remains at δ 134.7 and 125.1 ppm respectively.
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Figure 3 shows HMQC spectra of treated ENR-50, and the assignments are tabulated in Table 1 (3rd column). The HMQC spectra are used to verify the ring opening product and the attachment of IP from its contour correlation. The correlation of methyl of IP (E21) is shown at the most upfield region in the spectra and similar to the methyl of E10 for purified ENR-50. This is followed sequentially by methyl at (↑) and (↓) positions of E10. The correlation of methylene E8 and E9 which remains intact and ring opened in the polymer chains shows that E9 occurs at a slightly more upfield region than E8. Particularly, the correlation of methylene E9 is arranged accordingly due to the (↓), the middle unit which is not ring opened, and the (↑), while the correlation of methylene E8 is arranged accordingly due to the (↑), the middle unit which is not ring opened, and the (↓). The methine E7 of the middle unit remains similar to ENR-50 as it is not ring opened. The correlation of methine of IP (E20) is overlapped with methine E7 and ether E7 in 1 H-NMR spectrum. However, E20 is located at a more upfield region than ether E7 in 13 C-NMR spectrum. The ether E7 has more electron density than methine E7. The
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hydroxyl E7 is a 3° substituted carbon as compared to hydroxyl E6 which is a 2° substituted carbon. Thus, the correlation between hydroxyl E7 is located at a more upfield region than hydroxyl E6. Typically, the correlation of methylene of C4 is positioned at a slightly more upfield region than the methylene of C3, even although the ring opening reaction does not apply in C units. The correlation of the quaternary carbon of C1 and methine carbon of C2 are shown at δ 134.7 and 125.1 ppm, respectively. HMBC
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Figure 4 shows HMBC spectra of treated ENR-50, and the assignments of the middle unit of the triad sequence are tabulated in Table 1 (4th column). The attachment of IP at the ring opened epoxide is further scrutinized in HMBC spectra via 1H- and 13CNMR correlation. The HMBC spectra show correlation of methyl of IP (E21), methine of IP (E20) and ether E7. The correlations of the hydroxyl E6 and E7 are not obtained in the spectra due to the intramolecular hydrogen bond of the similar polymer chains, intermolecular hydrogen bond amongst the polymers chains, as well as the rapid exchange of the hydroxyl with the solvent [22]. The correlation of the methylene of E8, E9 at (↑) or (↓) showed slightly different correlation. Typically, the methylene of E8 (↓) and E9 (↓) showed correlation at a downfield region as compared to the methylene of E8 (↑) and E9 (↑). The correlation of methyl of E10, methylene of C3, C4, quaternary carbon of C1 and methine carbon of C2 is similar to the ENR-50 [20].
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The COSY of treated ENR-50 is shown in Figure 5, and the respective data are tabulated in Table 1 (5th and 6th column). The COSY spectra show correlation at δ 1.30, 1.57, 1.69, 2.06, 2.18, 2.73 and 5.13-5.17 ppm. The signal at δ 1.30 ppm correlates with the methine proton of IP (E20) at δ 2.73 ppm of the middle unit within the triad sequence. The signal at δ 1.57 ppm correlates with the signal at δ 1.69, 2.18 and 2.73 ppm. The signal of methylene protons of E9 at δ 1.69 is correlated with the methylene protons of ring opened E8, EE8C, and CE8E within same unit of triad sequences. The signal of methylene protons of C3 and C4 at δ 2.18 ppm correlates with the methylene protons of ring opened CE9C, EE9C and CE8E, CE8C, respectively, within same unit of triad sequences. The signal of methine proton of E7 at δ 2.73 ppm correlates with the methylene protons of ring opened CE9C, EE9C of the middle unit. The signal that represents the methylene protons of epoxide ring opened E9 and CE9E at δ 1.69 ppm is correlated with the signal at δ 1.57 and 2.73 ppm. The methylene protons of E8 at δ 1.57 ppm correlates with the methine proton of E7 at δ 2.73 ppm of the middle unit of the triad sequence. The methylene proton of C3, CC3E, C4 and EC4C at δ 2.06 ppm correlates with the signal at δ 2.06 ppm and methine proton of C2 at δ 5.13-5.17 ppm. The correlation at δ 2.06 ppm is within the same unit, and at δ 5.13-5.17 ppm is in the middle unit of the triad sequence. The signal at δ 2.18 ppm correlates with signals at δ 1.57 and 5.13-5.17 ppm. The methylene protons of E8 at δ 1.57 ppm correlates with
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FTIR Spectroscopy
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the methylene protons of epoxide ring opened EC4E, CC4E, EC3E, EC3C at δ 1.57 ppm. The methine proton of C2 at δ 5.13-5.17 ppm also correlates with methylene protons of EC4E and CC4E of the middle unit of triad sequence. The signal at δ 2.73 ppm correlates with the signal at δ 1.30, 1.57, 1.69 and 2.18 ppm. The signal of methyl protons of IP (E21) at δ 1.30 ppm correlates with methine proton of IP (E20). The proton signal of methylene of E9 at δ 1.57 ppm correlates with methine proton of epoxide ring opened EE7C and CE7C of the middle unit of triad sequence. The signal of methylene protons of E9 at δ 1.69 ppm correlates with methine proton of epoxide ring opened E7 and CE7E triads. The signal of methylene protons of E8 at δ 2.18 ppm correlates with methine proton of EE7C and CE7C triads. The signal at δ 5.13-5.17 ppm correlates with the signal at δ 2.06 and 2.18 ppm. The signal of methylene protons of C4 at δ 2.06 ppm is correlated with methine proton of the middle unit of triad C2, EC2C and CC2E, while signal of methylene protons of C4 at δ 2.18 ppm correlates with methine proton of the middle unit of triad EC2E and CC2E.
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Figure 6 shows FTIR spectra of pure and treated ENR-50. The FTIR spectra of ring opened ENR-50 via KOH/IP solution show the characteristic of ENR-50 and OH peaks. The additional ether peak is shown for treated ENR-50. In Figure 6 (b), the characteristic of ENR-50, OH and ether peaks are observed. The OH peak is at 3490 cm-1 includes the intermolecular and intramolecular hydrogen bonds due to hydroxyl attached at (↑) and (↓) positions. The ether peaks at 1150, 1113 cm-1 corresponds to asymmetric and symmetric C-O-C stretching vibrations, respectively. The epoxide peak remains at 877 cm-1. Comparison of the treated ENR-50 spectra (Figure 6, b) to the purified ENR-50 spectrum (Figure 6, a), indicates a decrease in the intensity of epoxide peak after the reaction [24]. The reaction extent is calculated using equation 3. The peak of methyl at 1378 cm-1 was used as the internal standard for the current study [5]. According to the approach of semi-quantitative FTIR, out of the original epoxide ring, 17.1% of the ring opening reaction has occurred in treated ENR-50. This proves the previous results of 1H-NMR at 16.9% for treated ENR-50. 3.3 3.3.1
Thermal Analysis TG/DTG The TG and DTG results of purified and treated ENR-50 are shown in Figure 7. TG and DTG thermograms of purified ENR-50 shows a single step degradation range and maximum temperature degradation (Tmax) at 331-493 °C and 408 °C respectively [25,26], while treated ENR-50 shows 3 steps of degradations over the temperature range studied. The temperature ranges for treated ENR-50 are at 190-331, 331-521, 521-706 °C with Tmax at 256, 441 and 628 °C, respectively. The treated ENR-50 first step represents water molecules removal that originates from the ring open structure of ENR-50 i.e. hydroxyl groups. In theory, the hydroxyl amount in treated ENR-50 is 10.49 %wt while the experimental amount is 11.8 %wt, indicating that part of the hydroxyl groups degrade through the second stage of degradation. The
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second stage represents the polymer chains pyrolysis including the breakdown of the side and main chains of polymer such as IP groups [27,28]. This type of degradation occurs at a wider range of temperature compared with purified ENR-50. This could be due to the existence of polymer chain mixture i.e ring-opened and intact epoxide of ENR-50. The third stage is the carbonaceous materials rearrangement from the polymer chains degradation as an aromatic carbon which is later present as char residue [27,29]. Based on these temperature ranges, it shows that the thermal profile of treated ENR-50 is more stable than pure ENR-50. This is due to the alkyl group that is attached to the carbons of the ring opened epoxide, which in turn led to improvement in the thermal stability of treated ENR-50 more than pure ENR-50. The char residue of ENR-50 and treated ENR-50 are 0.07 and 1.55 %wt, respectively. This may be related to the percentage of the ring opening that is available in alkali as compared to pure ENR-50, as mention earlier in 1H-NMR. DSC
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DSC results of purified and treated ENR-50 are shown in Figure 8. The purified and treated ENR-50 show a single glass transition (Tg) at -17.7 and 13.2 °C respectively. The Tg of treated ENR-50 was higher than purified ENR-50 due to the presence of ring opened epoxide. Typically, a single Tg in the polymer indicates good miscibility among the intact and ring opened epoxide in the ENR-50 [30,31]. This is attributed to the similar polymer backbone of the ring-opened product. The IP group attached to the ring-opened epoxide produces a rigid polymer at higher temperature than pure ENR-50. The presence of IP group in treated ENR-50 has restricted the polymer chains motion via filling more holes and voids among the polymer chains. Further, the presence of hydrogen bonds in the polymer contributes to this phenomenon [32].
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The epoxide ring opening reaction of ENR-50 with KOH/IPA solution suggested the attachment of isopropyl group via ether link and hydroxyl groups formation in the polymer chains. The reactions of treated ENR-50 produced a mixture of ring-opened products at most (↑) and least (↓) positions of the epoxide carbon, and were effectively assigned using 2D-NMR techniques. Moreover, the thermal stability of treated ENR-50 was improved. This is agreement with the DSC profiles where isopropyl group is able to restrict motion of the polymer chains in ENR-50. 5. Acknowledgements: The authors would like to acknowledge the Fundamental Research Grant Scheme (FRGS) Phase 1/2015 (9003-00523) under Ministry of Higher Education, Malaysia.
References
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1. J. Jeerupun, J. Wootthikanokkhan, P. Phinyocheep. Effects of Epoxidation Content of ENR on Morphology and Mechanical Properties of Natural Rubber Blended PVC. Macromol Symp 2004;216(1):281. 2. S.Y Lee, A. Hassan, I.K.P Tan, K. Terakawa, N. Ichikawa, S.N. Gan. Reaction of Palm Oil Based mcl-PHAs with Epoxidized Natural Rubber. J Appl Polym Sci 2010;115(4):2039. 3. D.R Burfield, K.L Lim, K.S Law. Epoxidation of Natural Rubber Lattices: Methods of Preparation and Properties of Modified Rubbers. J App Polym Sci 1984;29(5):1661. 4. I.R Gelling. Epoxidized Natural Rubber. J.C Salamone, (ed). Polymeric Materials Encyclopedia, Volume 3 (D-E). New York: CRC Press, Boca Raton, Florida, pp. 2192. 1996 5. S. Bhattacharjee, A.K Bhowmick, B.N Avasthi. Hydrogenation of Epoxidized Natural Rubber in the Presence of Palladium Acetate Catalyst. Polymer 1993; 34(24):5168. 6. S.N Gan, A.H Ziana. Partial Conversion of Epoxide Groups to Diols in Epoxidized Natural Rubber. Polymer 1997;38(8):1953. 7. D. Derouet, J.C Brosse, A. Challioui. Alcoholysis of Epoxidized Polyisoprene by Direct Opening of Oxirane Rings with Alcohol Derivatives. 1. Modelization of the Reaction. Eur Polym J 2001;37(7):1315. 8. D. Derouet, J.C. Brosse, A. Challioui. Alcoholysis of Epoxidized Polyisoprene by Direct Opening of Oxirane Rings with Alcohol Derivatives. 2. Study on Epoxidized 1,4-Polyisoprene. Eur Polym J 2001;37(7):1327. 9. J.H Bradbury, M.C.S Perera. Epoxidation of Natural Rubber Studied by NMR Spectroscopy. J App Polym Sci 1985;30(8):3347. 10. A. Furst, E. Pretsch. A Computer Program for the Prediction of 13C-NMR Chemical Shifts of Organic Compounds. Anal Chim Acta 1990;229:17. 11. S.F Thames, S. Gupta. Synthesis and Characterization of Pendent Hydroxy Fluoroesters of Secondary High Molecular Weight Guayule Rubber. J Appl Polym Sci 1997; 63(8):1077. 12. I.R Gelling. Modification of Natural Rubber with Peracetic Acid. Rubber Chem Technol 1985; 58(1):86. 13. T. Saito, W. Klinklai, S. Kawahara. Characterization of Epoxidized Natural Rubber by 2D NMR Spectroscopy. Polymer 2007;48(3):750. 14. P.Y Bruice. Organic Chemistry. 3rd ed. Chapter 11: Reactions at sp3 Hybridized Carbon III: Substitution and Elimination Reactions of Compounds with Leaving Groups Other Than Halogen. Organometallic Compounds. New Jersey: Prentice Hall. pp. 445-448. 2001 15. Z. Grobelny, A. Stolarzewicz, B. Morejko-Buz, A. Maercker, S. Krompiec, T. Bieg. Regioselectivity of the Ring Opening in the Reaction of Phenyloxirane, (Phenylmethyl)oxirane and (2-Phenylethyl)oxirane with K-, K+ (15-crown-5)2. J Organomet Chem 2003;672(1):43. 16. C.S.L Baker, I.R Gelling, R. Newell. Epoxidized Natural Rubber. Rubber Chem Technol 1984;58(1):67. 17. C.S.L Baker, I.R Gelling. Epoxidized Natural Rubber – A New Synthetic Polymer. Rubber World 1985;191(6):15. 18. R. Hamzah, M. A Bakar, O.S Dahham, N.N Zulkepli, S.S Dahham. , A structural study of epoxidized natural rubber (ENR-50) ring opening under mild acidic condition. J Appl Polym Sci 2016;133(43): doi: 10.1002/app.44123.
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19. P.T Nghia, N. Siripitakchai, W. Klinklai, T. Saito, Y. Yamamoto, S. Kawahara J Appl Polym Sci 2008;108(1):393. 20. R. Hamzah, M.A Bakar, M. Khairuddean, I.A Mohammed, R. Adnan. A structural study of epoxidized natural rubber (ENR-50) and its cyclic dithiocarbonate derivative using NMR spectroscopy techniques. Molecules 2012;17(9):10974. 21. P. Charisiadis, V. Exarchou, A.N Troganis, I.P Gerothanassis. Exploring the “Forgotten” –OH NMR Spectral Region in Natural Products. Chem Commun 2010;46(20):3589. 22. V. Exarchou, A. Troganis, I.P Gerothanassis, M. Tsimidou, D. Boskou. Do Strong Intramolecular Hydrogen Bonds Persist in Aqueous Solution? Variable Temperature Gradient 1H, 1H-13C GE-HSQC and GE-HMBC NMR Studies of Flavonols and Flavones in Organic and Aqueous Mixtures. Tetrahedron 2002; 58(37):7423. 23. R.S Macomber. A Complete Introduction to Modern NMR Spectroscopy. John Wiley & Sons, Inc., New York, NY, USA; Chapter 6, pp. 77. 1998 24. N.Z Noriman, H. Ismail, A.A Rashid. Characterization of styrene butadiene rubber/recycled acrylonitrile-butadiene rubber (SBR/NBRr) blends: the effects of epoxidized natural rubber (ENR-50) as a compatibilizer. Polym Test 2010; 29(2):200. 25. S.D Li, Y. Chen, J. Zhou, P.S Li, C.S Zhu, M.L Lin. Study on the Thermal Degradation of Epoxidized Natural Rubber. J Appl Polym Sci 1998;67(13):2207. 26. S. George, K.T Varughese, S. Thomas. Thermal and Crystallization Behaviour of Isotactic Polypropylene/Nitrile Rubber Blends. Polymer 2000;41(14):5485. 27. J.H Flynn. Handbook of Thermal Analysis and Calorimetry. Vol. 3: Application to Polymers and Plastics. Chapter 14: Polymer Degradation. (S.Z.D Cheng, Ed.) USA: Elsevier Science B.V., p. 593. 2002 28. V.S Mathew, S.C George, J. Parameswaranpillai, S. Thomas. Epoxidized natural rubber/epoxy blends: phase morphology and thermomechanical properties. J Appl Polym Sci 2014;131(4): doi: 10.1002/app.39906. 29. M.N Radhakrishnan-Nair, G.V Thomas, M.R.G Gopinathan Nair. Thermogravimetric Analysis of PVC/ELNR Blends. Polym Degrad Stab 2007;92(2):189. 30. D.J Burlett, M.B Altman. Handbook of Thermal Analysis and Calorimetry. Vol. 3: Application to Polymers and Plastics. Chapter 13: Thermal Analysis and Calorimetry of Elastomers. (S.Z.D Cheng, Ed.) USA: Elsevier Science B.V., pp. 521, 537, 539-540. 2002 31. A. Salehabadi, M. Abu Bakar, N.H.H Abu Bakar. Effects of Organo-Modified Nanoclay on the Thermal and Bulk Structural Properties of Poly(3hydroxybutyrate)-Epoxidized Natural Rubber Blends: Formation of MultiComponents Biobased Nanohybrids, Materials 2014;7(6):4508. 32. M.M Coleman, P.C Painter. Hydrogen Bonded Polymer Blends. Prog Polym Sci 1995;20(1):1.
Figure Captions
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Scheme 2
The possible triad sequences of epoxidized natural rubber (ENR-50) The general structure and the numbering of carbon atom of C and E units in epoxidized natural rubber (ENR-50) employed in this work Reaction of isopropyl alcohol/ potassium hydroxide with epoxidized isoprene and its carbon numbering (a) 1H- and (b) 13C-NMR spectra of treated epoxidized natural rubber (ENR-50) in deuterated chloroform (CDCl3) (a) heteronuclear multiple quantum coherence (HMQC) spectra of treated epoxidized natural rubber (ENR-50) and (b) enlargement of the box region in (a) (a) heteronuclear multiple bond coherence (HMBC spectra of treated epoxidized natural rubber (ENR-50) and (b,c) enlargement of the box region in (a) Correlation spectroscopy (COSY) spectra of treated epoxidized natural rubber (ENR-50) Fourier transform infrared spectroscopy (FTIR) spectra of (a) purified epoxidized natural rubber (ENR-50) and (b) treated epoxidized natural rubber (ENR-50) (a) Thermogravimetric (TG) and (b) derivative thermogravimetric (DTG) thermograms of (i) purified epoxidized natural rubber (ENR-50) and (ii) treated epoxidized natural rubber (ENR-50) Differential scanning calorimetry (DSC) thermograms of (a) purified epoxidized natural rubber (ENR-50) and (b) treated epoxidized natural rubber (ENR-50)
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Table 1: 1H-, 13C-NMR chemical shifts and HMQC, HMBC and COSY spin coupling correlations of treated ENR-50. 1 13 H C HMQC HMBC coupling COSY coupling chemical chemical correlation correlation shift, shift, Triad assignments Middle unit, δ (ppm) Middle Within δ (ppm) δ (ppm) unit, same δ (ppm) unit, δ (ppm) 10 8 6 1.30 22.2 E 29.7 (E ), 60.8 (E ), 64.5 None None (E7) 22.2 {E21, EE21C, 73.7 (E20) CE21E, CE21C} (↑& ↓) 23.3 {E10, EE10C, 32.0, 35.0, 41.0 (E8), 77.7 10 10 CE E, CE C} (↑) (E7), 84.5 (E6) 23.9 {E10, EE10C, 36.5, 41.0 (E8), 77.7 (E6), 10 10 CE E, CE C} (↓) 86.5 (E7) 1.57 23.9 {CE9C, EE9C} (↓) 77.7 (E6), 86.5 (E7) 2.73 (E7) 2.18 (C3) 27.0 CE9C, EE9C 60.8 (E6), 64.5 (E7) 8 8 29.7 EE C, E 22.2 (E10), 60.8 (E6), 64.5 None 1.69 (E7) (E9) 29.7 { CE9C, EE9C} (↓) 77.7 (E7). 84.5 (E6) 2.73 (E7) 2.18 (C3) 8 8 10 7 32.0 {E , EE C} (↓) 23.3 (E ), 77.7 (E ), 84.5 None 1.69 (E6) (E9) 33.1 CE8C, CE8E 22.2 (E10), 60.8 (E6), 64.5 2.18 (E7) (C4) 1.69 35.0 {E8, EE8C} (↑) 23.3 (E10), 77.7 (E7), 84.5 (E9) (E6) 36.5 {E8, EE8C} (↓) 23.9 (E10), 77.7 (E6), 86.5 (E7) 41.0 {CE8E, CE8C} (↑ 23.3, 23.9 (E10), 77.7 (E7), 2.18 & ↓) 84.5 (E6) (C4) 1.69 23.3 C5 32.0 (C3), 125.1 (C2), 134.7 None (C1) 23.3 {E9, CE9E} (↓) 77.7 (E6), 86.5 (E7) 2.73 (E7) 1.57 (E8) 24.6 CE9E, E9 60.8 (E6), 64.5 (E7) 9 9 26.2 {E , CE E} (↓) 77.7 (E6), 86.5 (E7) 27.0 {E9, CE9E} (↑) 77.7 (E7), 84.5 (E6) 3 3 2.06 26.2 C , CC E 23.3 (C5), 125.1 (C2), 134.7 None 2.06 (C1) (C4) 26.2 C4, EC4C 125.1 (C2), 134.7 (C1) 5.13-5.17 (C2) 4 4 2 1 2.18 23.9 CC E, EC E 125.1 (C ), 134.7 (C ) 1.57 (E8) 28.7 EC3C, EC3E 23.3 (C5), 125.1 (C2), 134.7 None
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CE7E (↓)
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CE7C (↓)
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{E7, CE7E} (↓)
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{E7, CE7E, EE7C, CE7C} (↑) {E6, EE6C, CE6E, CE6C} (↓) C2
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C1
23.3 (E10), 27.0 (E9), 32.0, 35.0, 41.0 (E8), 84.5 (E6) 23.3 (E10), 29.7 (E9), 32.0, 35.0, 41.0 (E8), 84.5 (E6) 23.3 (E9, E10), 36.5 (E8), 77.7 (E6) 23.3 (E9), 23.9 (E10), 36.5 (E8), 77.7 (E6) 23.3 (E9), 23.9 (E10), 41.0 (E8), 77.7 (E6) 23.9 (E9, E10), 41.0 (E8), 77.7 (E6) 23.9 (E10), 26.2 (E9), 36.5, 41.0 (E8), 77.7 (E6) None
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3.004.20 4.705.00 5.135.17 -
E6 {E6, EE6C, CE6E, CE6C} (↑) E7
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None
1.30 (E21)
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23.3 (C5), 26.2 (C4), 32.0 (C3), 134.7 (C1) -
1.69 (E9) 1.57 (E9)
1.57 (E8) 2.18 (C4)
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1.57 (E8)
1.57 (E9)
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2.18 (C3) 1.57 (E8) None
2.06 (C4)
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1.69 (E9)
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S0142-9418(16)31346-0
DOI:
10.1016/j.polymertesting.2017.01.006
Reference:
POTE 4892
To appear in:
Polymer Testing
Received Date: 2 December 2016 Accepted Date: 6 January 2017
Please cite this article as: O.S. Dahham, R. Hamzah, M.A. Bakar, N.N. Zulkepli, S.S. Dahham, S.S. Ting, NMR study of ring opening reaction of epoxidized natural rubber in presence of potassium hydroxide/isopropanol solution, Polymer Testing (2017), doi: 10.1016/j.polymertesting.2017.01.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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NMR study of ring opening reaction of epoxidized natural rubber in presence of potassium hydroxide/isopropanol solution
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Omar S. Dahham1a, Rosniza Hamzah1b, Mohamad Abu Bakar2c, Nik Noriman Zulkepli1d, Saad S. Dahham3e, Sam Sung Ting4f 1
Center of Excellence Geopolymer and Green Technology, Faculty of Engineering Technology, Universiti Malaysia Perlis, 02100 Padang Besar, Perlis, Malaysia
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Nanoscience Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
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EMAN Research and Testing Laboratory, School of Pharmaceutical Sciences, Universiti Sains Malaysia, 11800 Minden, Pulau Pinang, Malaysia
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School of Bioprocess Engineering, Universiti Malaysia Perlis (UniMAP), Kompleks Pengajian Jejawi 3, 02600 Arau, Perlis, Malaysia a
[email protected], [email protected], [email protected], d [email protected], [email protected], [email protected] Correspondence to: Name: Rosniza Hamzah E-mail: [email protected]
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Abstract: The ring opening reaction of epoxidized natural rubber (ENR-50) in the presence of potassium hydroxide/isopropanol solution was studied using NMR, and it’s thermal characteristic was investigated using TG/DTG and DSC. 1H-NMR showed that 16.9% of epoxide units was ring-opened in treated ENR-50, which was also supported by quantitative FTIR spectroscopy. 13C-NMR proved the location of alkyl group (isopropyl) in the polymer chain of treated ENR-50. The attachment location of isopropyl occurred at both most (↑) and least (↓) hindered carbons of the epoxide. 2D-NMR was used to identify and scrutinize the triad assignment of treated ENR-50. The TG/DTG results presented three decomposition steps at 190-331, 331521 and 521-706 °C due to the existence of mixtures of polymer chains i.e. ringopened and intact epoxide of ENR-50, which also led to increase in Tg of treated ENR-50 at 13.2°C compared with purified ENR-50 at -17.7°C. Keywords: epoxidized natural rubber, ring opening reaction, epoxide, nuclear magnetic resonance, thermal characteristic. 1.Introduction Nuclear magnetic resonance spectroscopy (NMR) techniques are extensively used for polymeric materials characterization either in solid or liquid phase. 1D NMR provides structural explanation on monomer units and reactive groups in the polymer chain. 2D NMR gives detail structural arrangements on the f monomer units in the polymer chain. The 1D NMR technique or both 1D NMR and 2D NMR techniques were
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applied previously to epoxidized natural rubber (ENR), such as ENR25 [1], ENR50 [2-6], ENR75 [3] and other ENR type [7,8]. Nevertheless, most of these researches didn’t give complete structural studies because of the overlapping of signals arising from the random distribution of isoprene (C) and epoxidized isoprene (E) monomers [9,10]. A triad of ENR is made up of three possible sequences of C and E units. All the possible ENR-50 triad sequences are shown in Figure 1. For nomenclature purposes, the common structure and the carbon atoms numbering of the E and C units in ENR-50 is shown in Scheme 1. In a triad, the neighbouring units may dictate the 1 H and 13C environments of the middle unit. It is the middle unit that will reveal the triad structure. For example, an ENR chain fragment may be comprised of C unit attached to an E unit which in turn is attached to another E unit. Thus, this triad sequence is denoted as CEE. Triad comprising of similar unit is only denoted by a single unit. Triad CCC is denoted as C and triad EEE is denoted as E. The methyl of the middle unit in the CEE and CCC triads are denoted as CE10E and C5, respectively, where the superscript refers to the (methyl) carbon (C) number in the ENR-50 structure, as shown in Scheme 1. This nomenclature also applies to the proton in the ENR-50 structure. Prior to the advent of the triad sequence, previous researches have only ascertained the 1H chemical shifts assignments of methine proton of E and C units but sporadically to either the methyl and/or methylene protons of E and C units of the ENR [5-8,11,13]. Furst and Pretsch have reported the use of computer simulation to predict chemical shifts of these protons and their triad assignments [10]. On the other hand, Gelling has assigned the triad sequence based on 13C-NMR chemical shifts for ENR-20 [12]. However, the assignment deals with certain triad sequences only and the position of carbon represented by the triad sequences was found to be inconsistent. Consequently, Saito et al. have successfully interpreted the 13C-NMR spectra for the ENR related compounds and assigned their triad sequence [13]. The manipulating of ENR pH below or above 7 can play an important role to catalyse the epoxide ring opening reaction and provides crosslinking among polymer chains. The existence of acid accelerates the addition of most nucleophiles to the epoxide carbon atoms, which in turn led to ring opening reaction [14]. The reaction of ring opening in the presence of acid produces an alcohol group along with formyl ester. Furthermore, acid could catalyse the epoxide ring opening and produce ether crosslinking [9]. In the common base, a nucleophile attacks the less substituted carbon of the epoxide causing the β-opening at the epoxide ring [15]. In rubber processing via vulcanization, the excess of base will cause premature curing and accelerate the reaction. This premature curing is due to the epoxide ring opening reaction under basic condition [16,17]. Our previous work has studied the influence of acetic acid as a mild acidic condition on the ring opening reaction of epoxidized natural rubber using 1D and 2D NMR supported with thermal analyses (TG/DTG and DSC). The 1H-NMR result showed that 19.56% of epoxide ring opening took place in the acid treated ENR-50 and 13CNMR suggested the acetate group location in the polymer chains. Furthermore, the
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alkyl groups attached to the ring-opened epoxide unites have improved the thermal characteristics of the acid treated ENR-50 [18]. The main objective of our current work is to introduce a comprehensive study of ring opening ENR-50 in the presence of potassium hydroxide/isopropanol solution. This study included FTIR, 1D and 2D NMR in addition to TG/DTG and DSC for thermal characteristics study. The rubber type used in this work is a low molecular weight ENR-50 which was obtained by a sol-gel technique. This work differs from other stated works in regard of the epoxidation degree and physical form of epoxidized natural rubber [13,19]. 2. Experimental 2.1. Materials
2.2. Preparative Procedure 2.2.1. ENR-50 Purification
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Toluene, chloroform, n-hexane, 2-propanol, IPA (Systerm, Malaysia), Potassium hydroxide, 85.0%, KOH (Q Rec Asia, Malaysia), Deuterated chloroform and CDCl3 (Fluka Chemicals, Switzerland) were all purchased commercially and used without additional treatment unless otherwise stated. ENR-50 was obtained from the Rubber Research Institute, Kuala Lumpur - Malaysia.
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20.00 g of ENR-50 was swelled in 400 mL of chloroform and stirred for 24 h at 25°C. A cotton gauze pack was used to filter the solution and separate the low molecular weight ENR-50 from the high molecular weight ENR-50. The low molecular weight ENR-50 was precipitated in n hexane and then stirred manually using a glass rod. The white precipitate stuck on the glass rod was moved to a Teflon petri dish. The samples were dried using vacuum oven at 50 °C for 48 h. The dried samples represented the purified ENR-50. Treated ENR-50
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A 0.21 M (14.09 mg mL-1) KOH/IPA solution was prepared by dissolving 1.41 g (21.35 x 10-3 mol) of KOH pellets in 100 mL IPA. In a typical preparation, about 200 mg (1.32 x 10-3 mol) of purified ENR-50 was dissolved in 40 mL toluene with stirring. The pH of the solution was adjusted to pH 12 by addition of a 10 mL of KOH stock solution. The mixture was then refluxed at 85 °C for 3 hours. It was then left to cool to room temperature. The reaction mixture was washed with 150 mL of water until the washing attained a neutral pH. On separation, the organic layer was cast into Teflon dishes followed by drying in a vacuum oven at 80 °C for 24 hours. 2.3. Measurements and Characterization Techniques The NMR spectra were obtained using a Bruker Avance 500 MHz instrument in CDCl3 at 25 °C. A sample was swelled for 4 days in a NMR tube before conducting tests. About 10mg of prepared solution was used for 1H Spectroscopy and 50 mg was
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2.4. Theoretical Treatment
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used for the 13C-NMR, heteronuclear multiple bond coherence (HMBC), heteronuclear multiple quantum coherence (HMQC) and correlation spectroscopy (COSY) analyses. The spectra respective range and scan number for 1H and for 13CNMR measurements were 15–0 ppm with 16 scans and 200–0 ppm with 8,000 scans, respectively. A Perkin-Elmer 2000-FTIR spectrometer was utilized with single beam transmittance onto a film of sample on zinc selenide (ZnSe) window in the wavelength range of 4,000–600 cm−1. The FTIR samples were prepared using 5 mL of chloroform to swell 100.00 mg of the samples. After sample swelling, the solution was cast into a 3.0 cm x 1.0 cm x 0.5 cm Teflon mould and air dried. The vacuum oven was used at 50 °C and 1 hour for further drying and the sample then transferred onto the ZnSe window. A thermal gravimetric (TG) analyser, model Perkin-Elmer TGA7 was used to assess the samples’ thermal stability. A 10mg sample was heated from 30°C to 900°C at 20 °C min-1 under nitrogen atmosphere. Differential scanning calorimetry (DSC) was done using a Perkin-Elmer Pyris6 instrument. About 10. mg sample was sealed in an aluminium pan. The thermograms were recorded over a temperature range of -50 to 140 °C at a heating rate of 20 °C min-1 under nitrogen atmosphere. First, the sample was heated from -50 to 140 °C and held at 140 °C for 1 minute. Then, it was quenched to -50 °C and held for another 5 minutes. The final step was the reheating from -50 to 140 °C at the same heating rate.
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In 1H-NMR, the amount of epoxide before and after reaction, and the percentage of ring opening in a sample was determined using Equation (1) [1,2,12]. I2.7 is the integrated area of the methine proton of epoxidized isoprene and I5.2 is the integrated area of the methine proton of isoprene. In Equation (1), I2.7 is chosen because it is the initial reactive site for the reaction and, for purified ENR-50, the integrated area of this peak is one proton. Typically, the integrated area at I2.7 after reaction is less than the I2.7 before reaction, while I5.2 remains similar before and after the reaction. This is because methine proton of isoprene is not involved in this reaction. In Equation (2), the percentage of ring opening in the sample is a deduction of the percentage of epoxide in purified ENR-50 with the percentage of epoxide left in the ring opened sample after reaction. The semi quantitative analysis of FTIR was based on the peak area of epoxide and methyl functional groups. The percentage of epoxide in the sample was calculated using Equation (3). In equation (1), Nmethyl is the methyl normalized peak area, A epoxide sample is the epoxide functional group peak area and A epoxide ENR-50 is the peak area of respective functional group stretching peak in purified ENR-50. The percentage of epoxide in the sample was calculated using Equation (1) [5]. % Epoxide in sample = I2.7 * 100% / I2.7 * I5.2
(1)
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(2)
% poxide = Nmethyl * Aepoxide sample * 100% / Aepoxide ENR-50
(3)
3.1. NMR 1
13
3.1.1. H- and C-NMR Spectroscopy
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3. Results and Discussion
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The 1H- and 13C-NMR spectra of treated ENR-50 are shown in Figure 2, and the respective chemical shifts are tabulated in columns 1 and 2 of Table 1. From Figure 1(a), chemical shifts of C2 at δ 5.13-5.17 and E7 at δ 2.73 ppm are similar to the purified ENR-50 [20]. However, the integral of E7 (m, 0.797H) is slightly decreased (viz. 0.203H) as compared to C2 (m, 1.000H). This small decrement is due to 16.9 % of ring opening reaction (Equation 2). The region of hydroxyl proton of E7 and E6 is shown at δ 3.00-4.20 ppm (br s, 0.122H) and δ 4.70-5.00 ppm (br s, 0.122H), respectively [1,2]. This shows that the reaction has taken place at most hindered carbon and less hindered carbon positions abbreviated as (↑) and (↓), respectively. The reason for broadness for these peaks is fast proton exchange between hydroxyl group and the solvent in the system [21-23]. The peak at δ 1.31 ppm (Figure 1, a) is a mixture of methyl protons of E10 of the ENR-50 and methyl protons of isopropyl (IP) E21 (Scheme 2). The peak at δ 1.57 ppm indicates the methylene protons at (↑) and (↓) positions for E8, EE8C, CE8E, CE8C and at (↓) position for CE9C, EE9C. It also belongs to the methylene protons of E8, EE8C, CE8E, CE8C, CE9C and EE9C of the ENR-50. The peak at δ 1.69 ppm represents methylene protons at (↑) and (↓) positions for E9, CE9E as well as methylene protons of E9, CE9E and methyl protons of C5 of the purified ENR-50. The methylene protons of C3, CC3E, C4, EC4C and CC4E, EC4E, EC3C, EC3E are located at δ 2.06 and 2.18 ppm respectively. Typically, the peak at δ 2.73 ppm belongs to the methine proton of E7 which remains intact in the polymer chains. However, it also represents the methine proton at (↑) and (↓) positions for E7, CE7E, EE7C, CE7C as well as methine proton of IP E20 (Scheme 2). The 13C-NMR spectrum of treated ENR-50 is shown in Figure 1 (b) and tabulated in column 2 of Table 1. The presence of IP group within the ENR-50 structure is evidenced via methine carbon of IP (E20) and methyl carbon of IP (E21). The chemical shift of methyl carbon E21(↑) and methine carbon E20(↓) is at δ 22.2 and δ 73.7 ppm respectively. As shown earlier, the (↑) position produces ether carbon E6 and the (↓) produces ether carbon E7. The ether carbon E6 shows a single peak at δ 84.5 ppm while the ether carbon E7 shows 2 peaks at δ 77.7 and 86.5 ppm. Both of the hydroxyl carbons E6, E7 are at a similar chemical shift i.e δ 77.7 ppm, regardless of the (↑) or (↓) positions. This is due to the similar electron environment for the respective hydroxyl. The
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methine carbon E7, which is not involved in ring opening reaction, remains similar to the purified ENR-50 at δ 64.5 ppm. The methyl carbon E10 is represented by 3 peaks at δ 22.2, 23.3 and 23.9 ppm. The peak at δ 23.3 ppm is when attachment of methyl is at (↑), while at δ 23.9 ppm it is at (↓). The methyl carbon E10 which is not involved in the ring opening reaction remains at δ 22.2 ppm. The methylene carbon E8 shows 6 peaks at δ 29.7, 32.0, 33.1, 35.0, 36.5 and 41.0 ppm. The peak at δ 32.0, 35.0, 36.5 ppm belongs to E8 and EE8C. The methylene at (↓) gives a peak at δ 35.0 ppm. The attachment at (↑) produces two peaks at δ 32.0, 36.5 ppm. This is due to the additional ring opened epoxide of L and R units of these triads. The peak at δ 41.0 ppm corresponds to triads CE8E and CE8C due to the attachment at (↑) and (↓) positions. The peak at δ 29.7 and 33.1 ppm represents the methylene of EE8C, E8 and CE8C, CE8E of the ENR-50, respectively. The methylene carbon E9 shows 6 peaks at δ 23.3, 23.9, 24.6, 26.2, 27.0 and 29.7 ppm. Both peaks at δ 23.9 and 29.7 ppm represent the methylene at (↓) for CE9C and EE9C. Similarly, peaks at δ 23.3 and 26.2 ppm belong to methylene at (↓) of E9 and CE9E. The methylene at (↑) produces a peak at δ 27.0 ppm for these triads. The peak at δ 27.0 ppm belongs to EE9C and CE9C while for E9 and CE9E it is located at δ 24.6 ppm respectively. The methylene carbon of C3 shows 2 peaks at δ 26.2 and 28.7 ppm. The peak at δ 26.2 ppm corresponds to the C3 and CC3E while the peak at δ 28.7 ppm is for EC3C and EC3E triads. Similarly, the peak due the methylene carbon of CC4E and EC4E is at δ 23.9 ppm and δ 26.2 ppm for C4 and EC4C triads, respectively. Finally, the quaternary carbon C1 and methine carbon C2 remains at δ 134.7 and 125.1 ppm respectively.
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Figure 3 shows HMQC spectra of treated ENR-50, and the assignments are tabulated in Table 1 (3rd column). The HMQC spectra are used to verify the ring opening product and the attachment of IP from its contour correlation. The correlation of methyl of IP (E21) is shown at the most upfield region in the spectra and similar to the methyl of E10 for purified ENR-50. This is followed sequentially by methyl at (↑) and (↓) positions of E10. The correlation of methylene E8 and E9 which remains intact and ring opened in the polymer chains shows that E9 occurs at a slightly more upfield region than E8. Particularly, the correlation of methylene E9 is arranged accordingly due to the (↓), the middle unit which is not ring opened, and the (↑), while the correlation of methylene E8 is arranged accordingly due to the (↑), the middle unit which is not ring opened, and the (↓). The methine E7 of the middle unit remains similar to ENR-50 as it is not ring opened. The correlation of methine of IP (E20) is overlapped with methine E7 and ether E7 in 1 H-NMR spectrum. However, E20 is located at a more upfield region than ether E7 in 13 C-NMR spectrum. The ether E7 has more electron density than methine E7. The
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hydroxyl E7 is a 3° substituted carbon as compared to hydroxyl E6 which is a 2° substituted carbon. Thus, the correlation between hydroxyl E7 is located at a more upfield region than hydroxyl E6. Typically, the correlation of methylene of C4 is positioned at a slightly more upfield region than the methylene of C3, even although the ring opening reaction does not apply in C units. The correlation of the quaternary carbon of C1 and methine carbon of C2 are shown at δ 134.7 and 125.1 ppm, respectively. HMBC
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Figure 4 shows HMBC spectra of treated ENR-50, and the assignments of the middle unit of the triad sequence are tabulated in Table 1 (4th column). The attachment of IP at the ring opened epoxide is further scrutinized in HMBC spectra via 1H- and 13CNMR correlation. The HMBC spectra show correlation of methyl of IP (E21), methine of IP (E20) and ether E7. The correlations of the hydroxyl E6 and E7 are not obtained in the spectra due to the intramolecular hydrogen bond of the similar polymer chains, intermolecular hydrogen bond amongst the polymers chains, as well as the rapid exchange of the hydroxyl with the solvent [22]. The correlation of the methylene of E8, E9 at (↑) or (↓) showed slightly different correlation. Typically, the methylene of E8 (↓) and E9 (↓) showed correlation at a downfield region as compared to the methylene of E8 (↑) and E9 (↑). The correlation of methyl of E10, methylene of C3, C4, quaternary carbon of C1 and methine carbon of C2 is similar to the ENR-50 [20].
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The COSY of treated ENR-50 is shown in Figure 5, and the respective data are tabulated in Table 1 (5th and 6th column). The COSY spectra show correlation at δ 1.30, 1.57, 1.69, 2.06, 2.18, 2.73 and 5.13-5.17 ppm. The signal at δ 1.30 ppm correlates with the methine proton of IP (E20) at δ 2.73 ppm of the middle unit within the triad sequence. The signal at δ 1.57 ppm correlates with the signal at δ 1.69, 2.18 and 2.73 ppm. The signal of methylene protons of E9 at δ 1.69 is correlated with the methylene protons of ring opened E8, EE8C, and CE8E within same unit of triad sequences. The signal of methylene protons of C3 and C4 at δ 2.18 ppm correlates with the methylene protons of ring opened CE9C, EE9C and CE8E, CE8C, respectively, within same unit of triad sequences. The signal of methine proton of E7 at δ 2.73 ppm correlates with the methylene protons of ring opened CE9C, EE9C of the middle unit. The signal that represents the methylene protons of epoxide ring opened E9 and CE9E at δ 1.69 ppm is correlated with the signal at δ 1.57 and 2.73 ppm. The methylene protons of E8 at δ 1.57 ppm correlates with the methine proton of E7 at δ 2.73 ppm of the middle unit of the triad sequence. The methylene proton of C3, CC3E, C4 and EC4C at δ 2.06 ppm correlates with the signal at δ 2.06 ppm and methine proton of C2 at δ 5.13-5.17 ppm. The correlation at δ 2.06 ppm is within the same unit, and at δ 5.13-5.17 ppm is in the middle unit of the triad sequence. The signal at δ 2.18 ppm correlates with signals at δ 1.57 and 5.13-5.17 ppm. The methylene protons of E8 at δ 1.57 ppm correlates with
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the methylene protons of epoxide ring opened EC4E, CC4E, EC3E, EC3C at δ 1.57 ppm. The methine proton of C2 at δ 5.13-5.17 ppm also correlates with methylene protons of EC4E and CC4E of the middle unit of triad sequence. The signal at δ 2.73 ppm correlates with the signal at δ 1.30, 1.57, 1.69 and 2.18 ppm. The signal of methyl protons of IP (E21) at δ 1.30 ppm correlates with methine proton of IP (E20). The proton signal of methylene of E9 at δ 1.57 ppm correlates with methine proton of epoxide ring opened EE7C and CE7C of the middle unit of triad sequence. The signal of methylene protons of E9 at δ 1.69 ppm correlates with methine proton of epoxide ring opened E7 and CE7E triads. The signal of methylene protons of E8 at δ 2.18 ppm correlates with methine proton of EE7C and CE7C triads. The signal at δ 5.13-5.17 ppm correlates with the signal at δ 2.06 and 2.18 ppm. The signal of methylene protons of C4 at δ 2.06 ppm is correlated with methine proton of the middle unit of triad C2, EC2C and CC2E, while signal of methylene protons of C4 at δ 2.18 ppm correlates with methine proton of the middle unit of triad EC2E and CC2E.
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Figure 6 shows FTIR spectra of pure and treated ENR-50. The FTIR spectra of ring opened ENR-50 via KOH/IP solution show the characteristic of ENR-50 and OH peaks. The additional ether peak is shown for treated ENR-50. In Figure 6 (b), the characteristic of ENR-50, OH and ether peaks are observed. The OH peak is at 3490 cm-1 includes the intermolecular and intramolecular hydrogen bonds due to hydroxyl attached at (↑) and (↓) positions. The ether peaks at 1150, 1113 cm-1 corresponds to asymmetric and symmetric C-O-C stretching vibrations, respectively. The epoxide peak remains at 877 cm-1. Comparison of the treated ENR-50 spectra (Figure 6, b) to the purified ENR-50 spectrum (Figure 6, a), indicates a decrease in the intensity of epoxide peak after the reaction [24]. The reaction extent is calculated using equation 3. The peak of methyl at 1378 cm-1 was used as the internal standard for the current study [5]. According to the approach of semi-quantitative FTIR, out of the original epoxide ring, 17.1% of the ring opening reaction has occurred in treated ENR-50. This proves the previous results of 1H-NMR at 16.9% for treated ENR-50. 3.3 3.3.1
Thermal Analysis TG/DTG The TG and DTG results of purified and treated ENR-50 are shown in Figure 7. TG and DTG thermograms of purified ENR-50 shows a single step degradation range and maximum temperature degradation (Tmax) at 331-493 °C and 408 °C respectively [25,26], while treated ENR-50 shows 3 steps of degradations over the temperature range studied. The temperature ranges for treated ENR-50 are at 190-331, 331-521, 521-706 °C with Tmax at 256, 441 and 628 °C, respectively. The treated ENR-50 first step represents water molecules removal that originates from the ring open structure of ENR-50 i.e. hydroxyl groups. In theory, the hydroxyl amount in treated ENR-50 is 10.49 %wt while the experimental amount is 11.8 %wt, indicating that part of the hydroxyl groups degrade through the second stage of degradation. The
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second stage represents the polymer chains pyrolysis including the breakdown of the side and main chains of polymer such as IP groups [27,28]. This type of degradation occurs at a wider range of temperature compared with purified ENR-50. This could be due to the existence of polymer chain mixture i.e ring-opened and intact epoxide of ENR-50. The third stage is the carbonaceous materials rearrangement from the polymer chains degradation as an aromatic carbon which is later present as char residue [27,29]. Based on these temperature ranges, it shows that the thermal profile of treated ENR-50 is more stable than pure ENR-50. This is due to the alkyl group that is attached to the carbons of the ring opened epoxide, which in turn led to improvement in the thermal stability of treated ENR-50 more than pure ENR-50. The char residue of ENR-50 and treated ENR-50 are 0.07 and 1.55 %wt, respectively. This may be related to the percentage of the ring opening that is available in alkali as compared to pure ENR-50, as mention earlier in 1H-NMR. DSC
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DSC results of purified and treated ENR-50 are shown in Figure 8. The purified and treated ENR-50 show a single glass transition (Tg) at -17.7 and 13.2 °C respectively. The Tg of treated ENR-50 was higher than purified ENR-50 due to the presence of ring opened epoxide. Typically, a single Tg in the polymer indicates good miscibility among the intact and ring opened epoxide in the ENR-50 [30,31]. This is attributed to the similar polymer backbone of the ring-opened product. The IP group attached to the ring-opened epoxide produces a rigid polymer at higher temperature than pure ENR-50. The presence of IP group in treated ENR-50 has restricted the polymer chains motion via filling more holes and voids among the polymer chains. Further, the presence of hydrogen bonds in the polymer contributes to this phenomenon [32].
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The epoxide ring opening reaction of ENR-50 with KOH/IPA solution suggested the attachment of isopropyl group via ether link and hydroxyl groups formation in the polymer chains. The reactions of treated ENR-50 produced a mixture of ring-opened products at most (↑) and least (↓) positions of the epoxide carbon, and were effectively assigned using 2D-NMR techniques. Moreover, the thermal stability of treated ENR-50 was improved. This is agreement with the DSC profiles where isopropyl group is able to restrict motion of the polymer chains in ENR-50. 5. Acknowledgements: The authors would like to acknowledge the Fundamental Research Grant Scheme (FRGS) Phase 1/2015 (9003-00523) under Ministry of Higher Education, Malaysia.
References
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1. J. Jeerupun, J. Wootthikanokkhan, P. Phinyocheep. Effects of Epoxidation Content of ENR on Morphology and Mechanical Properties of Natural Rubber Blended PVC. Macromol Symp 2004;216(1):281. 2. S.Y Lee, A. Hassan, I.K.P Tan, K. Terakawa, N. Ichikawa, S.N. Gan. Reaction of Palm Oil Based mcl-PHAs with Epoxidized Natural Rubber. J Appl Polym Sci 2010;115(4):2039. 3. D.R Burfield, K.L Lim, K.S Law. Epoxidation of Natural Rubber Lattices: Methods of Preparation and Properties of Modified Rubbers. J App Polym Sci 1984;29(5):1661. 4. I.R Gelling. Epoxidized Natural Rubber. J.C Salamone, (ed). Polymeric Materials Encyclopedia, Volume 3 (D-E). New York: CRC Press, Boca Raton, Florida, pp. 2192. 1996 5. S. Bhattacharjee, A.K Bhowmick, B.N Avasthi. Hydrogenation of Epoxidized Natural Rubber in the Presence of Palladium Acetate Catalyst. Polymer 1993; 34(24):5168. 6. S.N Gan, A.H Ziana. Partial Conversion of Epoxide Groups to Diols in Epoxidized Natural Rubber. Polymer 1997;38(8):1953. 7. D. Derouet, J.C Brosse, A. Challioui. Alcoholysis of Epoxidized Polyisoprene by Direct Opening of Oxirane Rings with Alcohol Derivatives. 1. Modelization of the Reaction. Eur Polym J 2001;37(7):1315. 8. D. Derouet, J.C. Brosse, A. Challioui. Alcoholysis of Epoxidized Polyisoprene by Direct Opening of Oxirane Rings with Alcohol Derivatives. 2. Study on Epoxidized 1,4-Polyisoprene. Eur Polym J 2001;37(7):1327. 9. J.H Bradbury, M.C.S Perera. Epoxidation of Natural Rubber Studied by NMR Spectroscopy. J App Polym Sci 1985;30(8):3347. 10. A. Furst, E. Pretsch. A Computer Program for the Prediction of 13C-NMR Chemical Shifts of Organic Compounds. Anal Chim Acta 1990;229:17. 11. S.F Thames, S. Gupta. Synthesis and Characterization of Pendent Hydroxy Fluoroesters of Secondary High Molecular Weight Guayule Rubber. J Appl Polym Sci 1997; 63(8):1077. 12. I.R Gelling. Modification of Natural Rubber with Peracetic Acid. Rubber Chem Technol 1985; 58(1):86. 13. T. Saito, W. Klinklai, S. Kawahara. Characterization of Epoxidized Natural Rubber by 2D NMR Spectroscopy. Polymer 2007;48(3):750. 14. P.Y Bruice. Organic Chemistry. 3rd ed. Chapter 11: Reactions at sp3 Hybridized Carbon III: Substitution and Elimination Reactions of Compounds with Leaving Groups Other Than Halogen. Organometallic Compounds. New Jersey: Prentice Hall. pp. 445-448. 2001 15. Z. Grobelny, A. Stolarzewicz, B. Morejko-Buz, A. Maercker, S. Krompiec, T. Bieg. Regioselectivity of the Ring Opening in the Reaction of Phenyloxirane, (Phenylmethyl)oxirane and (2-Phenylethyl)oxirane with K-, K+ (15-crown-5)2. J Organomet Chem 2003;672(1):43. 16. C.S.L Baker, I.R Gelling, R. Newell. Epoxidized Natural Rubber. Rubber Chem Technol 1984;58(1):67. 17. C.S.L Baker, I.R Gelling. Epoxidized Natural Rubber – A New Synthetic Polymer. Rubber World 1985;191(6):15. 18. R. Hamzah, M. A Bakar, O.S Dahham, N.N Zulkepli, S.S Dahham. , A structural study of epoxidized natural rubber (ENR-50) ring opening under mild acidic condition. J Appl Polym Sci 2016;133(43): doi: 10.1002/app.44123.
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19. P.T Nghia, N. Siripitakchai, W. Klinklai, T. Saito, Y. Yamamoto, S. Kawahara J Appl Polym Sci 2008;108(1):393. 20. R. Hamzah, M.A Bakar, M. Khairuddean, I.A Mohammed, R. Adnan. A structural study of epoxidized natural rubber (ENR-50) and its cyclic dithiocarbonate derivative using NMR spectroscopy techniques. Molecules 2012;17(9):10974. 21. P. Charisiadis, V. Exarchou, A.N Troganis, I.P Gerothanassis. Exploring the “Forgotten” –OH NMR Spectral Region in Natural Products. Chem Commun 2010;46(20):3589. 22. V. Exarchou, A. Troganis, I.P Gerothanassis, M. Tsimidou, D. Boskou. Do Strong Intramolecular Hydrogen Bonds Persist in Aqueous Solution? Variable Temperature Gradient 1H, 1H-13C GE-HSQC and GE-HMBC NMR Studies of Flavonols and Flavones in Organic and Aqueous Mixtures. Tetrahedron 2002; 58(37):7423. 23. R.S Macomber. A Complete Introduction to Modern NMR Spectroscopy. John Wiley & Sons, Inc., New York, NY, USA; Chapter 6, pp. 77. 1998 24. N.Z Noriman, H. Ismail, A.A Rashid. Characterization of styrene butadiene rubber/recycled acrylonitrile-butadiene rubber (SBR/NBRr) blends: the effects of epoxidized natural rubber (ENR-50) as a compatibilizer. Polym Test 2010; 29(2):200. 25. S.D Li, Y. Chen, J. Zhou, P.S Li, C.S Zhu, M.L Lin. Study on the Thermal Degradation of Epoxidized Natural Rubber. J Appl Polym Sci 1998;67(13):2207. 26. S. George, K.T Varughese, S. Thomas. Thermal and Crystallization Behaviour of Isotactic Polypropylene/Nitrile Rubber Blends. Polymer 2000;41(14):5485. 27. J.H Flynn. Handbook of Thermal Analysis and Calorimetry. Vol. 3: Application to Polymers and Plastics. Chapter 14: Polymer Degradation. (S.Z.D Cheng, Ed.) USA: Elsevier Science B.V., p. 593. 2002 28. V.S Mathew, S.C George, J. Parameswaranpillai, S. Thomas. Epoxidized natural rubber/epoxy blends: phase morphology and thermomechanical properties. J Appl Polym Sci 2014;131(4): doi: 10.1002/app.39906. 29. M.N Radhakrishnan-Nair, G.V Thomas, M.R.G Gopinathan Nair. Thermogravimetric Analysis of PVC/ELNR Blends. Polym Degrad Stab 2007;92(2):189. 30. D.J Burlett, M.B Altman. Handbook of Thermal Analysis and Calorimetry. Vol. 3: Application to Polymers and Plastics. Chapter 13: Thermal Analysis and Calorimetry of Elastomers. (S.Z.D Cheng, Ed.) USA: Elsevier Science B.V., pp. 521, 537, 539-540. 2002 31. A. Salehabadi, M. Abu Bakar, N.H.H Abu Bakar. Effects of Organo-Modified Nanoclay on the Thermal and Bulk Structural Properties of Poly(3hydroxybutyrate)-Epoxidized Natural Rubber Blends: Formation of MultiComponents Biobased Nanohybrids, Materials 2014;7(6):4508. 32. M.M Coleman, P.C Painter. Hydrogen Bonded Polymer Blends. Prog Polym Sci 1995;20(1):1.
Figure Captions
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The possible triad sequences of epoxidized natural rubber (ENR-50) The general structure and the numbering of carbon atom of C and E units in epoxidized natural rubber (ENR-50) employed in this work Reaction of isopropyl alcohol/ potassium hydroxide with epoxidized isoprene and its carbon numbering (a) 1H- and (b) 13C-NMR spectra of treated epoxidized natural rubber (ENR-50) in deuterated chloroform (CDCl3) (a) heteronuclear multiple quantum coherence (HMQC) spectra of treated epoxidized natural rubber (ENR-50) and (b) enlargement of the box region in (a) (a) heteronuclear multiple bond coherence (HMBC spectra of treated epoxidized natural rubber (ENR-50) and (b,c) enlargement of the box region in (a) Correlation spectroscopy (COSY) spectra of treated epoxidized natural rubber (ENR-50) Fourier transform infrared spectroscopy (FTIR) spectra of (a) purified epoxidized natural rubber (ENR-50) and (b) treated epoxidized natural rubber (ENR-50) (a) Thermogravimetric (TG) and (b) derivative thermogravimetric (DTG) thermograms of (i) purified epoxidized natural rubber (ENR-50) and (ii) treated epoxidized natural rubber (ENR-50) Differential scanning calorimetry (DSC) thermograms of (a) purified epoxidized natural rubber (ENR-50) and (b) treated epoxidized natural rubber (ENR-50)
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Table 1: 1H-, 13C-NMR chemical shifts and HMQC, HMBC and COSY spin coupling correlations of treated ENR-50. 1 13 H C HMQC HMBC coupling COSY coupling chemical chemical correlation correlation shift, shift, Triad assignments Middle unit, δ (ppm) Middle Within δ (ppm) δ (ppm) unit, same δ (ppm) unit, δ (ppm) 10 8 6 1.30 22.2 E 29.7 (E ), 60.8 (E ), 64.5 None None (E7) 22.2 {E21, EE21C, 73.7 (E20) CE21E, CE21C} (↑& ↓) 23.3 {E10, EE10C, 32.0, 35.0, 41.0 (E8), 77.7 10 10 CE E, CE C} (↑) (E7), 84.5 (E6) 23.9 {E10, EE10C, 36.5, 41.0 (E8), 77.7 (E6), 10 10 CE E, CE C} (↓) 86.5 (E7) 1.57 23.9 {CE9C, EE9C} (↓) 77.7 (E6), 86.5 (E7) 2.73 (E7) 2.18 (C3) 27.0 CE9C, EE9C 60.8 (E6), 64.5 (E7) 8 8 29.7 EE C, E 22.2 (E10), 60.8 (E6), 64.5 None 1.69 (E7) (E9) 29.7 { CE9C, EE9C} (↓) 77.7 (E7). 84.5 (E6) 2.73 (E7) 2.18 (C3) 8 8 10 7 32.0 {E , EE C} (↓) 23.3 (E ), 77.7 (E ), 84.5 None 1.69 (E6) (E9) 33.1 CE8C, CE8E 22.2 (E10), 60.8 (E6), 64.5 2.18 (E7) (C4) 1.69 35.0 {E8, EE8C} (↑) 23.3 (E10), 77.7 (E7), 84.5 (E9) (E6) 36.5 {E8, EE8C} (↓) 23.9 (E10), 77.7 (E6), 86.5 (E7) 41.0 {CE8E, CE8C} (↑ 23.3, 23.9 (E10), 77.7 (E7), 2.18 & ↓) 84.5 (E6) (C4) 1.69 23.3 C5 32.0 (C3), 125.1 (C2), 134.7 None (C1) 23.3 {E9, CE9E} (↓) 77.7 (E6), 86.5 (E7) 2.73 (E7) 1.57 (E8) 24.6 CE9E, E9 60.8 (E6), 64.5 (E7) 9 9 26.2 {E , CE E} (↓) 77.7 (E6), 86.5 (E7) 27.0 {E9, CE9E} (↑) 77.7 (E7), 84.5 (E6) 3 3 2.06 26.2 C , CC E 23.3 (C5), 125.1 (C2), 134.7 None 2.06 (C1) (C4) 26.2 C4, EC4C 125.1 (C2), 134.7 (C1) 5.13-5.17 (C2) 4 4 2 1 2.18 23.9 CC E, EC E 125.1 (C ), 134.7 (C ) 1.57 (E8) 28.7 EC3C, EC3E 23.3 (C5), 125.1 (C2), 134.7 None
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23.3 (E10), 27.0 (E9), 32.0, 35.0, 41.0 (E8), 84.5 (E6) 23.3 (E10), 29.7 (E9), 32.0, 35.0, 41.0 (E8), 84.5 (E6) 23.3 (E9, E10), 36.5 (E8), 77.7 (E6) 23.3 (E9), 23.9 (E10), 36.5 (E8), 77.7 (E6) 23.3 (E9), 23.9 (E10), 41.0 (E8), 77.7 (E6) 23.9 (E9, E10), 41.0 (E8), 77.7 (E6) 23.9 (E10), 26.2 (E9), 36.5, 41.0 (E8), 77.7 (E6) None
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1.69 (E9) 1.57 (E9)
1.57 (E8) 2.18 (C4)
1.69 (E9)
1.57 (E8)
1.57 (E9)
None
2.18 (C3) 1.57 (E8) None
2.06 (C4)
None
-
-
1.69 (E9)
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT