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Characterization of xylan-graft-polycaprolactone copolymers prepared in ionic liquid Xue-Qin Zhang, Ming-Jie Chen, Hui-Hui Wang, Chuan-fu Liu, Aiping Zhang, and Runcang Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01323 • Publication Date (Web): 04 Jun 2015 Downloaded from http://pubs.acs.org on June 15, 2015

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Industrial & Engineering Chemistry Research

Characterization of xylan-graft-polycaprolactone copolymers prepared in ionic liquid Xueqin Zhang,† Mingjie Chen,† Huihui Wang,† Chuanfu Liu,*,† Aiping Zhang,‡ and Runcang Sun†,§

†

State Key Laboratory of Pulp and Paper Engineering, South China University of

Technology, Guangzhou 510640, China ‡

Institute of New Energy and New Material, Guangdong Key Laboratory for

Innovative Development and Utilization of Forest Plant Germplasm, South China Agricultural University, Guangzhou 510640, China §

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University,

Beijing 100083, China

*

Corresponding author.

Tel: 86-20-87111735. Fax: 86-20-87111861. E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT In this study, xylan-graft-polycaprolactone (xylan-g-PCL) with different degree of substitution (DS) and degree of polymerization (DP) were prepared by homogeneous ring-opening graft polymerization (ROGP) of ε-caprolactone (ε-CL) onto xylan in ionic

liquid

(IL)

1-allyl-3-methylimidazolium

chloride

([Amim]Cl)

using

4-dimethylaminopyridine (DMAP) as catalyst. FT-IR, 1D (1H- and 13C-NMR) and 2D NMR (1H-13C HSQC and HMBC) spectra provided the evidence of the occurrence of ROGP reaction. 1H-1H COSY confirmed the correct assignment of the proton signals. HSQC also indicated that 38.78% and 61.22% of PCL side chains were attached to C2 and C3 positions of AXU, respectively. Xylan-g-PCL copolymers were further characterized by TGA/DTG, SEM and XRD. The results indicated that the thermal stability of xylan increased upon the DMAP-catalyzed ROGP reaction in [Amim]Cl, and the surface morphologies were significantly changed with increased DS. KEYWORDS: HSQC, HMBC, xylan, ε-caprolactone, ROGP, [Amim]Cl

2


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1. INTRODUCTION The typical compositions of lignocellulosic biomass are 35-50% cellulose, 20-50% hemicelluloses and 5-30% lignin.1,2 As one of the three main components of lignocellulosic biomass, hemicelluloses, the heterogeneous polymers with degree of polymerization (DP) 80-200, have long been neglected and underutilized from an application point of view. In the last few decades, the application of hemicelluloses began to receive attention in many industries.3,4 Typically, hemicelluloses contain pentose (xylose and arabinose), hexoses (mannose, glucose, and galactose), and sugar acids (acetic, ferulic and p-coumaric acids). One of the most abundant hemicelluloses is xylan.3,5,6 In many plants, xylans are heteropolysaccharides with homopolymeric backbone chains of 1,4-linked β-D-xylopyranose units.7 Besides xylose, xylan may also

contain

O-acetyl,

α-L-arabinofuranosyl,

α-1,2-linked

glucuronic,

or

4-O-methlglucuronic acid substituents.4

Many attempts have been made to improve the physic-chemical properties of hemicelluloses, exploit their novel functions and broaden their applications.5 These methods include etherification, esterification, oxidation and grafting.4,8,9 Due to the excellent biodegradability, biocompatibility and permeability, considerable attention has been paid to aliphatic polyesters from lactones and lactides, among which poly (ε-caprolactone) (PCL) is especially interesting as potential biodegradable candidate matrixes in biocomposites.10-14 PCL is hydrophobic and aliphatic polyester with excellent biocompatibility, low immunogenicity, non-toxicity, good mechanical and thermoplastic properties.15-17 It is a tough, crystalline polymer with low melting point 3



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(Tm) of 60 oC, glass transition temperature (Tg) of -60 oC and good compatibility with many polymers and organic compounds. Ring opening graft polymerization (ROGP) of cyclic monomers is a well-established method to produce polymers with controlled molecular weight and molecular weight distribution.8,18-20 According to our previous research, xylan-graft-PCL (xylan-g-PCL) copolymers with low degree of substitution (DS) in the range of 0.03-0.39 and DP only one were obtained in dual-component dimethyl sulfoxide (DMSO)/LiCl without any additional catalysts.21 In the present study, to increase the DS and DP of xylan-g-PCL copolymers, the ROGP reaction of ε-caprolactone (ε-CL) onto xylan was investigated in IL [Amim]Cl with 4-dimethylaminopyridine (DMAP) as catalyst. The physical and chemical properties of obtained copolymers highly depend on how and where substituents are located along the polymer chains.22-24 Thus, accurate and complete structure characterization of copolymers is of interest. Nuclear magnetic resonance (NMR) is an effective characterization tool indispensable for analyses in the field of organic chemistry.23 Based on many published researches, cellulose, starch and chitosan grafted PCL copolymers have been synthesized. The structure of these copolymers was only characterized by the one-dimensional (1D)

1

H- and

13

C-NMR.11,20,25 However, 1D measurement may not provide sufficient resolution in

the case of a complicated mixture, such as correct assignment of signals and accurate reaction sites. Two-dimensional (2D) NMR spectroscopy allows one to bypass signal overlapping by spreading the peaks along two orthogonal dimensions, proving precious information in terms of structural elucidation.26 In this study, the structure of 4


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xylan-g-PCL copolymers was characterized with 1D (1H- and

13

C-NMR) and 2D

NMR (1H-1H correlation spectroscopy (COSY), 1H-13C heteronuclear single-quantum correlation (HSQC) and heteronuclear multiple-bond correlation (HMBC)) to prove the direct evidence of the occurrence of the ROGP reaction. The physicochemical properties of the obtained graft copolymers were also characterized with FT-IR, X-ray diffraction (XRD), and scanning electron microscopy (SEM) as well as thermal analysis.

2. MATERIALS AND METHODS Materials. Xylan with a xylose content of over 85% (isolated from sugarcane bagasse) was provided by Yuan-Ye Biological Technology Co., Ltd. (Shanghai, China). The average molar mass of xylan was 49027 g mol-1, determined by GPC using a PL aquagel-OH 50 column according to the published method.27 [Amim]Cl with purity of 99.0% was supplied by Cheng-Jie Chemical Co., Ltd. (Shanghai, China), and dried in vacuum for 48 h at 70 oC before used. The commercial PCL homopolymer (average Mn 10000), ε-CL with 99.5% purity and DMAP with 99% purity were purchased from Aladdin Reagent Co. (Shanghai, China). Other chemicals were supplied by Guangzhou Chemical Reagent Factory (Guangdong, China). All chemicals and reagents except [Amim]Cl were of analytical-reagent grade and directly used without further purification. Synthesis of xylan-g-PCL copolymers via ROGP in [Amim]Cl. The preparation of xylan-g-PCL copolymers in [Amim]Cl was performed according to the following 5



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procedure: xylan (0.33 g, 5 mmol of hydroxyl group in xylan) was added into [Amim]Cl (10 g) in a 50 mL dried three-neck flask. The mixture was stirred at 80 oC for 2 h in the nitrogen atmosphere to achieve a homogenous solution. Then the required quantities of ε-CL [molar ratios of ε-CL to anhydroxylose units (AXU) in xylan] and DMAP were slowly added portionwise into the xylan solution, and the ROGP reaction was carried out under the protection of nitrogen with vigorous agitation. After the required time, the resulting solution was cooled to room temperature and poured into 150 mL ethanol. The solid residues were filtered out, washed carefully with 150 mL ethanol for three times, and suspended in dichloromethane (CH2Cl2) with magnetic agitation at room temperature for 24 h (thrice, total 72 h) to remove the homopolymers of ε-CL. The final product was dried in vacuum for 48 h at 50 oC. The detailed conditions of ROGP reaction are listed in Table 1. Based on the regenerated xylan after the dissolution in [Amim]Cl, the subsequent agitation for 24 h without the addition of ε-CL, and the subsequent suspension in CH2Cl2, the weight gains of xylan-g-PCL copolymers were in the range of 3.85%-42.31%. The detailed structural factors of the copolymers were more reliably determined by NMR. Solubility of xylan-g-PCL copolymers. The solubility of xylan-g-PCL copolymers and the commercial PCL homopolymer in common solvents including acetone, tetrahydrofuran (THF), CH2Cl2, trichloromethane (CHCl3), DMSO and H2O was examined. A certain amount of samples (10 mg) was added into the 0.5 mL solvent and stirred at room temperature for 12 h. After the required time, the results 6


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were recorded. Characterization. FT-IR spectra of unmodified xylan and xylan-g-PCL copolymers were recorded on a Bruker spectrophotometer (Tensor 27, Germany) from a KBr disc containing 1% (w/w) finely ground samples in the range 4000-400 cm-1. The 1H-NMR, 1H-1H COSY, 13C-NMR, 1H-13C HSQC and 1H-13C HMBC spectra of unmodified xylan, xylan-g-PCL copolymers and the commercial PCL homopolymer were recorded from 40 mg samples in 0.5 mL DMSO-d6 on a Bruker Avance III 400 M (1H-NMR, 1H-1H COSY,

13

C-NMR, and 1H-13C HSQC) and HD

600 M (1H-13C HMBC) spectrometer (Germany) with a 5 mm multinuclear probe. For the 1H-NMR analysis, the detailed collecting and processing parameters were listed as follows: number of scans, 16; receiver gain, 140; acquisition time, 4.0894 s; relaxation delay, 1.0 s; pulse width, 9.0 s; spectrometer frequency, 400.13 MHz; and spectral width, 8012.8 Hz. For the COSY analysis, the detailed collecting and processing parameters were listed as follows: number of scans, 8; receiver gain, 447; acquisition time, 0.4588 s; relaxation delay, 2.0 s; pulse width, 9.0 s; spectrometer frequency, 400.13/400.13 MHz; and spectral width, 4000.0/4000.0 Hz. For the 13

C-NMR analysis, the detailed collecting and processing parameters were listed as

follows: number of scans, 1932; receiver gain, 1440; acquisition time, 1.2583 s; relaxation delay, 1.5 s; pulse width, 12.0 s; spectrometer frequency, 100.61 MHz; and spectral width, 26041.7 Hz. For the HSQC analysis, the detailed collecting and processing parameters were listed as follows: number of scans, 28; receiver gain, 2050; acquisition time, 0.0639 s; relaxation delay, 2.0 s; pulse width, 8.5 s; 7



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spectrometer frequency, 400.13/100.61 MHz; and spectral width, 8012.8/20161.3 Hz. For the HMBC analysis, the detailed collecting and processing parameters were listed as follows: number of scans, 128; receiver gain, 187; acquisition time, 0.3338 s; relaxation delay, 2.0 s; pulse width, 11.0 s; spectrometer frequency, 600.17/150.91 MHz; and spectral width, 3067.5/33557.0 Hz. The DS and DP of xylan-g-PCL copolymers were calculated from the integration of the resonances assigned to characteristic signals in 1H-NMR spectra. The thermal stability of the samples was performed by using themogravimetric analysis

(TGA)

and

derivative

thermogravimetry

(DTG)

on

a

Q500

theromgravimetric analyzer (TA, USA). The apparatus was continually flushed with nitrogen. The sample between 9 and 11 mg was heated from 30 oC to 600 oC at a heating rate of 10 oC/min. The surface morphology was examined by SEM on a field emission microscopy (LEO 1530 VP, LEO, Germany). The samples were prepared by casting few solids onto a mica sheet followed by gold-plating. XRD was determined on a D/Max-III X-ray diffractometer (Rigaku, Japan) equipped with the high-intensity monochromatic nickel-filtered Cu Kα1 radiation (λ=0.154 nm). The operating voltage and current were 40 kV and 40 mA, respectively. Data were collected with diffraction angle 2θ ranging from 5 to 60° with a step size of 0.04° and time per step of 0.2 s at room temperature.

3. RESULTS AND DISCUSSION 8


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Effects of reaction conditions on ROGP reaction. A series of xylan-g-PCL copolymers were prepared in [Amim]Cl with DMAP as catalyst by ROGP reaction of ε-CL onto xylan, as shown in Figure 1A. The effects of reaction conditions, including reaction temperature, the molar ratio of ε-CL to AXU, the dosage of DMAP catalyst and reaction time, on DS and DP of xylan-g-PCL copolymers were discussed, as summarized in Table 1. As shown in Table 1, with the increment of DMAP dosage from 1% to 3%, the DS and DP of xylan-g-PCL copolymers increased from 0.26 to 0.56 and 1.23 to 1.46, respectively, indicating the efficient catalysis of DMAP for ROGP reaction. The DMAP-catalyzed ROGP reaction of ε-CL onto xylan is considered to be a nucleophilic substitution reaction.28,29 The proposed mechanism is illustrated in Figure 1B. In the ROGP reaction, ε-CL reacts with DMAP to form intermediate 1 due to the attack of the nucleophilic nitrogen in DMAP to electron-deficient carbon of carbonyl group in ε-CL. This intermediate can more easily attack hydroxyl groups of xylan than ε-CL. Because of the tendency of hydroxyl oxygen of xylan to link with carbonyl group at the presence of electron-sufficient oxygen, the formed intermediate 2 can easily cleave to DMAP and xylan-g-CL 3. DMAP enters into the next cycle of catalyzed reaction to perform ROGP reaction. Because of the presence of hydroxyl groups on the attached side chain, xylan-g-CL 3 allows the further attachment of ε-CL similar as xylan, resulting in the formation of xylan-g-PCL copolymers 4. Similarly, the homopolymers of ε-CL are easily obtained via this DMAP-catalyzed ROGP reaction. Therefore, it is necessary for the products to be post-treated with CH2Cl2 to 9



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remove the homopolymers. Further increment of DMAP dosage from 3% to 5% did not result in any increase in DS, and the DP of xylan-g-PCL copolymers increased from 1.46 to 1.68, which indicated that excess catalyst had no positive effect on the attachment of PCL onto xylan, only helpful for the growth of PCL chains. The increase of reaction temperature from 80 to 120 oC resulted in an increase in DS from 0.04 to 0.41, and DP from 1.11 to 1.46, respectively, which was probably due to the favourable effect of reaction temperature on the motion and collision of molecules. Holding reaction temperature at 120 oC, increasing the molar ratio of ε-CL to AXU from 1:1 to 8:1, 12:1, and 20:1, DS increased from 0.02 to 0.65 and 0.88 and then decreased to 0.79, while DP increased from 1.00 to 1.47, 1.43, and 1.57, respectively. The difference of DS from DP was probably due to the competitive ROGP reaction and homopolymerization of ε-CL at high ε-CL dosage. Keeping reaction temperature at 120 oC, the molar ratio of ε-CL to AXU at 4:1, and catalyst dosage at 2%, the DS and DP of xylan-g-PCL copolymers reached 0.06 and 1.23 in 3 h, 0.14 and 1.39 within 9 h, 0.17 and 1.47 within 12 h, and 0.41 and 1.46 with 24 h, suggesting the positive effect of reaction time on ROGP reaction due to the increased molecular collision with the improved duration. Further improving reaction time from 24 to 48 h did not result in the increase in DS and DP, indicating that the chemical equilibrium of the reversible ROGP reaction was reached within 24 h under the selected conditions. The effects of reaction parameters on DS and DP of xylan-g-PCL copolymers showed that the homogeneous ROGP reaction with DMAP as catalyst in [Amim]Cl 10


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under the selected conditions effectively attached PCL onto xylan to prepare xylan derivatives with DS 0.02–0.88 and DP 1.00-1.68. However, these values were much lower

than

those

of

cellulose-g-PCL

prepared

in

another

IL

1-butyl-3-methylimidazolium chloride ([Bmim]Cl) with DMAP as catalyst at 130 o

C.11 The higher DS (1.87-2.41) and DP (2.84-3.05) of cellulose-g-PCL was probably

due to the higher temperature in [Bmim]Cl and the linear macromolecular structure of cellulose with more hydroxyl groups available, which allowed for more side chains attached on the biopolymer. In our previous study, xylan-g-PCL copolymers with DS 0.03-0.39 and DP only one were prepared in DMSO/LiCl without any additional catalysts.21 Comparatively, the reaction efficiency was noticeably improved in [Amim]Cl with DMAP as catalyst. As a result, the DS increased to 0.02-0.88, and more importantly, the DP of the attached PCL side chains increased to 1.00-1.68, indicating the potential application of novel green solvent IL in the utilization of biopolymers. The solubility of xylan-g-PCL copolymers and the commercial PCL homopolymer in common organic solvents and water was also examined, as listed in Table 1. As expected, xylan-g-PCL copolymers with different DS could be easily dissolved in DMSO, providing the further structural characterization of the xylan derivatives with NMR. They were insoluble in other organic solvents including acetone, CH2Cl2, CHCl3 and THF, whereas could be swollen, dissolved or undissolved in water, depending on DS under the selected conditions to a large extent. It sounds unreasonable that hydrophilic but insoluble xylan became soluble graft copolymers 11



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after the attachment of hydrophobic PCL side chain. However, considering the extensive presence of hydrogen bonds in xylan, the solubility of xylan derivatives depends on the contents of hydrophilic and hydrophobic groups as well as hydrogen bond network.15,30 The attachment of side chains onto xylan led to the increased hydrophobic groups with the significant disruption of hydrogen bonds between xylan macromolecules. Therefore, the copolymers with DS lower than 0.14 were only swollen but not soluble in water due to the extensively present hydrogen bonds, whereas those with DS in the range of 0.14–0.56 were soluble because of the increased disruption of hydrogen bonds. However, the copolymers with higher DS (0.65-0.88) were insoluble to obtain the emulsions of the copolymers, due to the increased hydrophobic groups in xylan derivatives. The changed solubility of xylan-g-PCL copolymers suggested that the xylan derivatives could be tailored according to the application conditions. In addition, the commercial PCL homopolymer could be easily dissolved in DMSO, acetone, CH2Cl2, and CHCl3, but insoluble in water. Compared with PCL homopolymer, the decreased solubility of xylan-g-PCL copolymers in these organic solvents was probably due to the hydrophilic nature of xylan. The different solubility of PCL homopolymer from xylan-g-PCL copolymers provided the successful removal of PCL homopolymer by the extraction with CH2Cl2 at room temperature for 24 h (thrice, total 72 h).

FT-IR spectra. The FT-IR spectra of unmodified xylan and xylan-g-PCL copolymer samples 5 (DS of 0.41) and 8 (DS of 0.88) are shown in Figure 2. The 12


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characteristic bands at 3451, 2878, 1734, 1632, 1044, and 896 cm−1 were previously reported.31-33 The spectra of xylan-g-PCL copolymer samples provided evidence of ROGP reaction by the changes of two important absorbances compared with that of unmodified xylan. After the ROGP reaction, the presence of the new band at 2922 cm−1 for CH2 stretching and the noticeably increased intensity of the absorbance at 1734 cm−1 for C=O stretching indicated that new groups containing methylene and carbonyl were attached onto xylan, suggesting the occurrence of the reaction in Figure 1A.30 In addition, the intensities of these two bands increased from sample 5 to sample 8 are corresponding to the increase of DS as shown in Table 1.

1D and 2D NMR. To further confirm the ROPG reaction onto xylan, the characterization of unmodified xylan (Figure not shown), xylan-g-PCL copolymers and the commercial PCL homopolymer were investigated using 1H-NMR, 1H-1H COSY, 13C-NMR, 1H-13C HSQC, and 1H-13C HMBC spectroscopies, as illustrated in Figure 3. In the 1H-NMR spectra (Figure 3A-3B), the proton signals in xylan-g-PCL copolymer sample 8 at 3.02, 3.13, 3.25, 3.49, 3.85 and 4.26 ppm are assigned to H2, H5a, H3, H4, H5e, and H1 of xylan backbone, as reported in the previous publication.19 The signals in the range of 4.95–5.41 ppm relate to the protons from the hydroxyl groups in AXU.34 Besides the proton signals similar to unmodified xylan,21 two new signals for the protons at substituted C2 and C3 positions appeared at 4.51 and 4.80 ppm, respectively, suggesting the occurrence of the reaction onto AXU. More 13



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importantly, the strong signals at 2.25 ppm (-COCH2-, a), 1.52 ppm (-CH2-, b and d),1.41 (-CH2-, d’, end unit), 1.30 ppm (-CH2-, c), 3.45 ppm (-CH2OH, e’, end unit) and 3.98 ppm (-CH2O-, e) are attributed to the methylene proton in PCL side chains.35 The proton from hydroxyl group on methylene at e’ position (e’-OH) gives the signal at 4.39 ppm. As expected, no signals for the protons from carboxyl acid were present in Figure 3A, implying the absence of homopolymers of ε-CL and the attachment of PCL onto xylan to form ester. Comparatively, the

1

H-NMR spectrum of the

commercial PCL homopolymer (Figure 3B) provided the common methylene proton signals in repeating unit at a-e positions. The signals at 1.30, 2.27 and 3.90 ppm originate from methylene protons at c, a, and e positions, respectively, and the proton signals at b and d positions were overlapped at 1.54 ppm. The changes of primary proton signals in 1H-NMR spectra indicated that the reaction in Figure 1A occurred, which was in agreement with the results from FT-IR analysis. To confirm the assignment of the proton signals, the 1H-1H COSY spectrum of xylan-g-PCL copolymer sample 8 was collected, as illustrated in Figure 3C. To clearly present the primary signals of the attached PCL side chains, the spectrum is shown at higher contour level and the primary signals and their cross-correlations are not shown. The strong cross-correlations for a/b, b/c, c/d’, d’/e’, e’/e’-OH of PCL side chains were clearly observed, indicating the high content of end unit. In addition, the moderate cross-correlations at δH/δH of 3.98/1.55 and 1.55/3.98 for d/e indicated the presence of PCL repeating unit. Comparatively, the cross-correlations for repeating unit almost disappeared in the 1H-1H COSY spectra of xylan-g-PCL copolymers with 14


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DP only 1 prepared in DMSO/LiCl,21 suggesting the increased DP of xylan-g-PCL copolymers prepared in the present study. Therefore, the further calculation of DP and DS of the attached PCL side chain was necessary. Theoretically, due to the special structure of ε-CL, there are the equal quantities of methylene protons at different positions from a to e in the attached PCL group. Compared with the repeating unit of PCL, free hydroxyl group is present in the end unit, resulting in the different signals at end methylene proton position in PCL end unit (e’) and repeating unit (e). Based on the typical proton signal from PCL side chains (a, e and e’ positions) and AXU in xylan (H1), the DS and DP of xylan-g-PCL copolymers was estimated with the integral area of the resonances for the corresponding protons according to the following equation: DS =

CLTerminal I e ' / 2 ( I a − I e ) / 2 = = AXU I H1 I H1

DP =

I ( e + e ') CLTotal Ia = = CLTerminal I e' Ia − Ie

where DS is the degree of substitution of PCL, DP is the degree of polymerization of PCL, AXU is anhydroxylose unit, CLTerminal is the end unit of PCL, CLTotal is the total units of PCL, 2 is two protons in each methylene group, Ie, Ie’ and Ia are the integral area of the resonances of the corresponding methylene protons at e, e’, and a positions of PCL, and IH1 is the integral area of the resonance assigned to H1 of AXU. The DS and DP values estimated from 1H-NMR are listed in Table 1. The results indicated that the xylan derivatives with DS 0.02–0.88 and DP 1.00-1.68 were obtained under the selected conditions. 15



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The

13

Page 16 of 39

C-NMR spectra of xylan-g-PCL copolymer sample 8 and the commercial

PCL homopolymer are shown in Figure 3D and 3E. In Figure 3D, the five primary carbon signals previously reported at 101.8, 75.5, 74.1, 72.7 and 63.3 ppm are assigned to C1, C4, C3, C2 and C5 of AXU, respectively, in (1-4)-linked β-D-xylan residues,34 which was similar to that of unmodified xylan (Figure not shown). The signal at 172.1 ppm relates to the carbonyl carbon at f position in the PCL side chains. More importantly, the methylene carbons at a, b, c, d’, and e’ positions in the PCL end unit gave the signals at 33.7, 24.3, 25.0, 32.3, and 60.1 ppm, respectively. The similar results were reported in DMSO/LiCl.21 In addition, the methylene carbon signals at e and d positions from PCL repeating units were overlapped with C5 of AXU and that at d’ position of PCL end unit, respectively. Comparatively, the carbon signals at 23.8, 24.8, 27.7, 33.2, 63.4 and 172.3 ppm are assigned to b, c, d, a, e and f in the repeating units of PCL homopolymer in Figure 3E,20 which confirmed the overlapping of the signals from PCL repeating units with other carbon signals. The 1H- and

13

C-NMR

comparative analysis of xylan-g-PCL copolymer with unmodified xylan and the commercial PCL homopolymer confirmed the attachment of PCL side chains onto xylan, corresponding to FT-IR analysis, which suggested the successful ROGP reaction in [Amim]Cl illustrated in Figure 1A. HSQC is a versatile tool for qualitative and quantitative analysis of chemical structures, which could provide more detailed information of signals overlapped in 1

H- and

13

C-NMR spectra. To better understand the chemical structure of graft

copolymers, Figure 3F shows the HSQC spectrum of xylan-g-PCL copolymer sample 16


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8, and the different correlations from xylan backbone and PCL side chains are exhibited in Blue colour and Red colour, respectively. The spectrum is illustrated at a relatively low contour level to exhibit the primary correlations both unsubstituted and substituted. The strong correlations at δC/δH 33.7/2.25, 24.3/1.52, 25.0/1.30, 32.3/1.41, 32.5/1.48, 61.0/3.37 and 63.3/3.99 ppm are associated with Ca-Ha, Cb-Hb, Cc-Hc, Cd’-Hd’, Cd-Hd, Ce’-He’ and Ce-He, respectively, indicated that the PCL side chains were successfully attached onto xylan. Clearly, the strong correlations in carbohydrate region at δC/δH 101.6/4.27, 72.7/3.02, 74.1/3.23, 75.2/3.58, 63.2/3.86, and 63.2/3.18 ppm are attributed to C1-H1, C2-H2, C3-H3, C4-H4, C5e-H5e and C5a-H5a in AXU of xylan, respectively. The C1-H1 (A1), C2-H2 (A2), C3-H3 (A3), C4-H4 (A4), and C5-H5 (A5) in α-L-arabinofuranosyl residues in xylan gives the weak correlations at δC/δH 99.3/4.50, 78.5/3.62, 80.8/3.81, 86.4/3.96, and 62.2/3.46 ppm, respectively. More importantly, the correlations at δC/δH 72.7/4.51 and 74.1/4.81 for substituted C2-H2 (2’ in Figure 3F) and substituted C3-H3 (3’ in Figure 3F),36 respectively, provided the possible quantitative estimation of ROGP reaction occurred at C2 and C3 positions. Clearly, more PCL side chains were attached to C3 position than to C2 position. The integrated resonances for substituted and unsubstituted C2-H2 and C3-H3 indicated that 38.78% and 61.22% of PCL side chains were attached to C2 and C3 positions of AXU, respectively. HMBC could give correlations between carbons and protons that are separated by two, three, and, sometimes in conjugated systems, four bonds. Theoretical, the positions of AXU attached with PCL side chains could be detected with HMBC. 17



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Therefore, to further confirm the attachment of PCL side chains onto xylan and the correct assignment of the primary signals of xylan-g-PCL copolymers, the HMBC spectrum of xylan-g-PCL copolymer sample 8 is illustrated in Figure 3G. To better understand the spectrum, the primary correlations from the different segments are shown in Blue colour for xylan backbone, Red colour for PCL side chain, and Green colour for cross-correlations between xylan and PCL. Expectedly, the Green correlations at δC/δH 172.1/4.81 (Cf-H3’) and 172.1/4.51 ppm (Cf-H2’) in the bottom left corner are the cross-correlations between carbonyl carbon (C=O, f carbon) in PCL side chains and the protons at the substituted C3 and C2 position (3’ and 2’ protons, respectively). The presence of these cross-correlations confirmed the chemical bonding between PCL side chain and xylan, providing the direct evidence of the attachment of PCL onto xylan. Clearly, more PCL side chains were attached to C3 position than to C2 position, corresponding to the results from HSQC. In addition, the correlations at δC/δH 172.1/4.00 (Cf-He), 172.1/2.25 (Cf-Ha), and 172.1/1.52 ppm (Cf-Hb) are attributed to the cross-correlations between carbonyl carbon and methylene protons in PCL chains. The presence of Cf-He cross-correlation implied that DP of the attached PCL side chains was higher than 1. Clearly, compared with Cf-Ha and Cf-Hb cross-correlations, the drastically decreased intensity of the Cf-He cross-correlation suggested the short length of PCL side chains. These observations were in agreement with the low DP determined from

1

H-NMR. Besides the

cross-correlations between f carbon with methylene protons in PCL chains, the primary cross-correlations were detected in Red colour at δC/δH 24.1/2.25 (Cb-Ha), 18


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24.1/1.52 (Cb-Hd), 24.1/1.41 (Cb-Hd’), 24.1/1.30 (Cb-Hc), 25.3/2.25 (Cc-Ha), 25.3/1.52 (Cc-Hb,d), 25.3/1.41 (Cc-Hd’), 25.3/3.38 (Cc-He’), 25.3/4.00 (Cc-He), 32.0/1.52 (Cd,d’-Hb), 32.0/1.30 (Cd,d’-Hc), 32.0/3.38 (Cd’-He’), 34.0/1.51 (Ca-Hb), 34.0/1.30 (Ca-Hc), 60.7/1.41 (Ce’-Hd’), 60.7/1.30 (Ce’-Hc), and 63.6/1.55 ppm (Ce-Hd), confirming the correct assignments of primary carbon and proton signals in PCL side chains. In addition, the primary cross-correlations from AXU in xylan were also observed in Blue colour at 101.9/2.94 (C1-H2), 101.9/3.10 (C1-H5), 101.9/3.25 (C1-H3), 100.3/4.55 (C1-H2’), 76.1/3.26 (C4-H3), 76.1/4.39 (C4-H2’), 76.1/4.83 (C4-H3’), 74.0/3.53 (C3-H4), 74.0/3.11 (C3-H5), 74.0/2.93 (C3-H2), 72.4/3.26 (C2-H3), 72.0/4.83 (C2-H3’), 60.3/3.53 (C5-H4), and 60.3/3.26 ppm (C5-H3). The cross-correlations at δC/δH 101.9/3.54 (C1-H4) and 76.1/4.30 ppm (C4-H1) are associated with the linked xylose unit by β-1,4 linkage. The other cross-correlations from xylan were not detected under the selected contour level. Based on the combined 1D and 2D NMR analyses, PCL side chains were confirmed to be chemically bonded to xylan at C2 and C3 position with the ratio of 38.78% to 61.22%.

Thermal analysis. Thermal stability is a significant indicator of thermal properties and the associated kinetics for the chemical modification. It supplies a measurement of weight loss of the samples on the basis of time and temperature. In the present study, the decomposition pattern and the thermal stability of xylan, xylan-g-PCL copolymers with different DS, and the commercial PCL homopolymer 19



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were investigated by TGA in the temperature range from 30 to 600 oC under nitrogen atmosphere (Figure 4). According to the TGA curves, the weight loss in the examined temperature range can be divided into three stages: minor weight loss below 150 oC, substantial weight loss, and the subsequent marginal weight loss at high temperature. The minor weight loss below 150 oC at the first stage was attributed to the removal of absorbed moisture. Clearly, xylan-g-PCL copolymers comparatively contained less moisture than unmodified xylan and more moisture than the commercial PCL homopolymer, which was due to the hydrophilic and hydrophobic nature of xylan and PCL side chains, respectively.19 At the second stage, the substantial weight loss was due to the decomposition of biopolymers. Unmodified xylan began to decompose at 195 oC, while xylan-g-PCL copolymer samples 17 and 8 started to decompose at 202 and 205 oC, respectively, indicating the increased thermal stability after ROGP reaction. However, the commercial PCL homopolymer began to decompose at about 300 oC, which was undoubtedly due to its high DP and crystalline structure. At 50% weight loss, the decomposition temperature occurred at 280 oC for unmodified xylan, 270 oC for sample 17 and 250 oC for sample 8, implying the decreased thermal stability after ROGP reaction, which was inconsistent with the trend of the initial degradation. In comparison with the commercial PCL homopolymer (about 375 oC), the decomposition temperature at 50% weight loss of xylan-g-PCL copolymers was much low, which was probably due to the much lower DP of the attached PCL side chains onto xylan than the commercial PCL homopolymer. To further explore the decomposition process and elucidate the possible differences between the 20


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decomposition behaviors, DTG curves are also illustrated in Figure 4. DTG represents the degradation rate and can be used for the comparison of the thermal stability between the samples. The primary peak in DTG curves showed the maximum degradation rate at 280 oC for unmodified xylan, 245 oC for sample 17, 265 o

C for sample 8, and 380 oC for PCL homopolymer. This information indicated that

the ROGP reaction of ε-CL onto xylan led to the decreased thermal stability of xylan and PCL. Moreover, there were two peaks present in DTG curves of xylan-g-PCL copolymer samples, corresponding to the degradation of xylan residues (the former strong one) and PCL side chains (the latter small one). The details in DTG curves should be further explored to elucidate the changed trend of thermal stability in TGA curves. Compared with xylan-g-PCL copolymers, there was a decomposition shoulder in DTG curves of unmodified xylan prior to the primary decomposition peak. This small decomposition shoulder was probably due to the decomposition of low molecular weight fractions in xylan. This difference of the decomposition shoulder in DTG curves indicated that the low molecular weight fractions were unrecovered in [Amim]Cl due to its excellent solubility, which was probably responsible for the contradictory behavior in the substantial weight loss stage. At the last stage, the pyrolysis residues at 600 oC were 30% for xylan, higher than 18% and 15% for samples 8 and 17, respectively. Pyrolysis residues are believed to be inorganic salts that cannot further be decomposed. Due to the excellent solubility of [Amim]Cl, inorganic salts were only partly regenerated from [Amim]Cl, resulting in the relatively high content of inorganic salts in unmodified xylan. The TGA/DTG 21



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analyses indicated that the thermal stability of xylan decreased upon ROGP reaction in [Amim]Cl. SEM analysis. To investigate the changes in surface morphology of xylan upon ROGP reaction, a series of comparative SEM observations of unmodified xylan and xylan-g-PCL copolymer samples 5, 10, 11 and 16 are illustrated in Figure 5. Unmodified xylan exhibited rough and irregular surface with different size of agglomerates due to the extensively present hydrogen bonds. After ROGP in [Amim]Cl, the agglomerates were destroyed upon the dissolution, modification and regeneration, and the morphology became to relatively less rough, compact, and highly porous surface with the increased DS. These changes were probably due to the attached PCL side chains onto xylan, which was consistent with the changes of the solubility in water. XRD analysis. The crystal structure of xylan and xylan-g-PCL copolymers was studied and the XRD patterns are shown in Figure 6. The similar diffraction patterns with one primary diffraction peak at 18.5o-21.5o were obtained. This strong diffraction peak slightly shifted to higher 2θ direction from unmodified xylan to xylan-g-PCL copolymer samples. This phenomenon indicated that the crystallization structure of xylan was only slightly influenced by the attached PCL side chains.

4. CONCLUSION In conclusion, the ROGP of ε-CL onto xylan were accomplished in [Amim]Cl with DMAP as a catalyst. The DS of xylan-g-PCL copolymers and the DP of the 22


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attached PCL side chains were in the range of 0.02–0.88 and 1.00-1.68, respectively. The FT-IR and NMR analyses provided more evidences of the chemical attachment of PCL side chains onto xylan. HSQC analysis indicated that 38.78% and 61.22% of PCL side chains were attached to C2 and C3 positions of AXU, respectively. The thermal stability of xylan decreased upon DMAP-catalyzed ROGP reaction in [Amim]Cl. Considering the good biodegradability of xylan and PCL, this kind of xylan derivative has potential application as environmentally friendly and biodegradable materials such as edible package films.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (31170550, 31170555), Program for New Century Excellent Talents in University (NCET-11-0154), the Fundamental Research Funds for the Central Universities (2014ZG0046), and the National Program for Support of Top-notch Young Professionals.

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Figure Captions

Figure 1. The grafting copolymerization of PCL onto xylan backbone in [Amim]Cl with DMAP as catalyst (A) and the possible mechanism (B).

Figure 2. FT-IR spectra of unmodified xylan, and xylan-g-PCL copolymer samples 5 and 8.

Figure 3. 1H-NMR, 1H-1H COSY, 13C-NMR, HSQC and HMBC spectra of xylan-g-PCL copolymer sample 8 (A, C, D, F, and G) and the commercial PCL homopolymer (B and E).

Figure 4. TGA/DTG curves of xylan, xylan-g-PCL copolymer samples 8 and 17, and the commercial PCL homopolymer.

Figure 5. SEM images of unmodified xylan (A), xylan-g-PCL copolymer samples 16 (DS=0.17, B), 10 (DS=0.26, C), 5 (DS=0.41, D) and 11 (DS=0.56, E).

Figure 6. XRD curves of unmodified xylan and xylan-g-PCL copolymer samples 5, 9 and 11.

29



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Table 1. The DS, DP and solubility of xylan derivatives obtained under various conditions in [Amim]Cl with DMAP as catalyst. Solubility

Sample No

Temp (oC)

Time (h)

Catalyst wt%

ε-CL /AXU

DS

1

80

24

2%

4:1

0.04

2

90

24

2%

4:1

3

100

24

2%

4

110

24

5

120

6

a

DP

b

DMSO

H2 O

Acetone

CH2Cl2

CHCl3

THF

1.11

++c

+d

-e

-

-

-

0.06

1.18

++

+

-

-

-

-

4:1

0.09

1.35

++

+

-

-

-

-

2%

4:1

0.30

1.37

++

++

-

-

-

-

24

2%

4:1

0.41

1.46

++

++

-

-

-

-

120

24

2%

1:1

0.02

1.00

++

+

-

-

-

-

7

120

24

2%

8:1

0.65

1.47

++

-

-

-

-

-

8

120

24

2%

12:1

0.88

1.43

++

-

-

-

-

-

9

120

24

2%

20:1

0.79

1.57

++

-

-

-

-

-

10

120

24

1%

4:1

0.26

1.23

++

++

-

-

-

-

11

120

24

3%

4:1

0.56

1.46

++

++

-

-

-

-

12

120

24

4%

4:1

0.55

1.66

++

++

-

-

-

-

13

120

24

5%

4:1

0.56

1.68

++

++

-

-

-

-

14

120

3

2%

4:1

0.06

1.23

++

+

-

-

-

-

15

120

9

2%

4:1

0.14

1.39

++

++

-

-

-

-

16

120

12

2%

4:1

0.17

1.47

++

++

-

-

-

-

17

120

48

2%

4:1

0.42

1.45

++

++

-

-

-

-

++

-

++

++

++

++

PCL a

The degree of substitution of xylan-g-PCL copolymers, calculated by 1H-NMR; bThe degree of

polymerization of PCL side chains, calculated by 1H-NMR; cRepresenting soluble; dRepresenting swelling; eRepresenting insoluble.

30


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(A)

(B) Figure 1. The grafting copolymerization of PCL onto xylan backbone in [Amim]Cl with DMAP as catalyst (A) and the possible mechanism (B).

31



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Figure 2. FT-IR spectra of unmodified xylan, and xylan-g-PCL copolymer samples 5 and 8.

32


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33



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Figure 3. 1H-NMR, 1H-1H COSY,

13

C-NMR, HSQC and HMBC spectra of

xylan-g-PCL copolymer sample 8 (A, C, D, F, and G) and the commercial PCL homopolymer (B and E).

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Figure 4. TGA/DTG curves of xylan, xylan-g-PCL copolymer samples 8 and 17, and the commercial PCL homopolymer.

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A

B

C

D

E Figure 5. SEM images of unmodified xylan (A), xylan-g-PCL copolymer samples 16 (DS=0.17, B), 10 (DS=0.26, C), 5 (DS=0.41, D) and 11 (DS=0.56, E).

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Figure 6. XRD curves of unmodified xylan and xylan-g-PCL copolymer samples 5, 9 and 11.

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Characterization of Xylan-graft-Polycaprolactone ... - ACS Publications

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