International Journal of Biological Macromolecules 67 (2014) 483–489
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Integration of lignin and acrylic monomers towards grafted copolymers by free radical polymerization Xiaohuan Liu a,b,c,d , Yuzhi Xu a,b,c,d , Juan Yu a,b,c,d , Shouhai Li a,b,c,d , Jifu Wang a,b,c,d,∗ , Chunpeng Wang a,b,c,d,∗ , Fuxiang Chu a,b,c,d,∗ a
Institute of Chemical Industry of Forestry Products, CAF, Nanjing, China Key Laboratory of Biomass Energy and Material, Nanjing, Jiangsu Province, China c National Engineering Laboratory for Biomass Chemical Utilization, Nanjing, China d Key and Open Laboratory on Forest Chemical Engineering, SFA, Nanjing 210042, China b
a r t i c l e
i n f o
Article history: Received 9 January 2014 Received in revised form 5 March 2014 Accepted 7 April 2014 Available online 15 April 2014 Keywords: Biobutanol lignin (BBL) Graft polymerization Acrylic monomer Hydrophobicity UV absorption
a b s t r a c t Three kinds of acrylic monomers (2,2,3,4,4,4-hexafluorobutyl methacrylate (HFBMA), methyl methacrylate (MMA) and butyl acrylate (BA)) were utilized to modify the lignin (BBL) by “grafting from” free radical polymerization (FRP), respectively. Calcium chloride/hydrogen peroxide (CaCl2 /H2 O2 ) was used as initiator. Effects of monomer type and concentration, initiator concentration and polymerization time on grafting from BBL were studied. Grafting of poly (acrylic monomers) onto BBL was verified by the following characterizations and this synthesis method was found to be high efficient and selective for grafting polymerization of BBL. The presence of the BBL moiety in the backbone also resulted in higher glass transition temperature compared with the homopolymer of each monomer, and some modified copolymers also improved its thermal stability. All modifications made BBL more hydrophobic and the static contact angles of these modified copolymers were above 80◦ . XPS analysis revealed that the surface of these modified BBL copolymers were dominated by acrylate monomer moiety. Additionally, the BBLg-PBA copolymers can be used as dispersion modifiers in PLA-based materials to enhance UV absorption. © 2014 Elsevier B.V. All rights reserved.
1. Introduction With the depletion of fossil oil, a major feedstock for polymers, and environmental awareness, a global interest has been gradually shifted towards the development of renewable biobased polymers and composites derived from natural resources [1–6]. Lignin, as a class of biodegradable, sustainable and inexpensive natural amorphous hydrophobic branched heteropolymer second only to cellulose in natural abundance, has been widely used for various applications [7–10]. However, the main intrinsic properties of lignin such as self-aggregate due to intermolecular association from hydrogen bondings and – stacking of aromatic groups [11], result in the poor dispersion in many thermoplastic polymers which has negatively impacted the mechanical properties of the
∗ Corresponding authors at: National Engineering Laboratory for Biomass Chemical Utilization China, Nanjing 210042, China. Tel.: +86 10 62889300; fax: +86 10 62889013. E-mail addresses: [email protected] (J. Wang), [email protected] (C. Wang), [email protected] (F. Chu). http://dx.doi.org/10.1016/j.ijbiomac.2014.04.005 0141-8130/© 2014 Elsevier B.V. All rights reserved.
resulting composite materials [12,13]. In an attempt to improve the miscibility properties of the lignin, extensive studies have been published about modifying lignin such as esterification [14], etherification [15], etc. Biobutanol lignin (BBL), a “waste” from the biobutanol industries, which is prepared from cornstalk or wheat straw usually by a combination of chemical and enzymatic treatments. BBL is regarded as a novel lignin with low molecular weight and high chemical activity than lignosulfonate or kraft lignin [16]. A variety of studies have focused on the kraft lignin which was recovered by pulping and paper industry. However, relatively few researches about the grafting of enzymatic hydrolysis lignin that obtained from co-product of biofuel production are little until now. Graft polymerization is one of the key techniques to covalently modify the surface of lignin by introduction of various functional polymers onto the side chains of lignin, resulting in greatly enhanced mechanical, thermal resistance and UV absorption properties of the host polymers. Chung and co-workers have synthesized lignin-g-PLA by ring-opening polymerization, which was found to be better dispersed in poly(lactic acid) thermoplastic materials, while compared to the unmodified lignin [4]. Lignin was also found to be grafted with polymers by growing polymer chains
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from radical sites on the lignin backbone (grafting from) [17], by introducing vinylic groups to lignin and copolymerizing the resulting macromonomer with a small molecular weight comonomer (grafting through) [18] or by attaching previously formed polymer chains to the lignin backbone (grafting onto) [19]. Presently, there are three major methods of grafting polymerization: free radical polymerization [19], ring-opening polymerization [4] and living radical polymerization [2,20]. Among of these graft polymerization methods, free radical polymerization (FRP) provides a simple method to modify lignin and lignin derivatives. Significant efforts have been devoted to the grafting polymerization of different kind of monomers (e.g., styrene [21], n-butyl methacrylate (BMA), methyl methacrylate (MMA) [22], poly(lactic acid) (PLA) [4], vinyl acetate (VAc) [23] and acrylic acid (AA) [17] from lignin backbones by FRP. These grafted copolymers were found to have excellent properties including hydrophobicity, elasticity, thermal resistance and UV absorption, which could find potential applications such as compatibilizers for blends of thermoplastics materials and UV blockers that can potentially be applied in the biocompatible packaging for extending shelf life of light sensitive products. Among the initiators for FRP, initiator (CaCl2 –H2 O2 ) has been thoroughly tested and was a selective method for grafting of lignin [21,22]. The reaction initiates free-radical grafting phenylethylene onto lignin by a redox reaction (CaCl2 -H2 O2 ) were firstly reported by Meister et al. [21]. The poly(1ignin-g-(1-phenylethy1ene)) products were formed with almost 100% grafting efficiency for lignin, and the temperature of the reaction was low. Previous lignin grafting studies have dealt with single monomers only. However, reports about carrying out an extensive comparison of different monomers to graft lignin are rare. Herein, in this context, we prepared three kinds of grafted lignin copolymers through “grafting from” FRP with the aim to make BBL more hydrophobic and miscible with thermoplastics or thermosetting materials. In order to better understand the grafting of BBL with acrylic monomers, the influence of a various factors like reaction time, monomer concentration, initiator concentration on the graft copolymer formation were studied. The chemical composition, molecular weight, thermal properties, hydrophobicity and UV absorption were also explored. 2. Experimental 2.1. Materials Lignin (BBL) used in the presented work was a byproduct of biomass-derived butanol production, and were purchased from songyuan bairui bio-polyos Co. Ltd., China in power form. The main characteristics of BBL are the C9 formula (C9 H8.160 O2.996 N0.094 S0.006 (OCH3 )0.594 ), a lignin content of 92.5% (w w−1 ), the methoxyl groups content of 3.24 mmol g−1 , the total hydroxyl of lignin is 5.62 mmol g−1 , a weight-average molecular weight of 876 g mol−1 , and a polydispersity of 1.6 [24]. 2,2,3,4,4,4-Hexafluorobutyl methacrylate (HFBMA), methyl methacrylate (MMA) and butyl acrylate (BA) were purchased from Aladdin Industrial Inc. and used as received. Dimethyl sulfoxide (DMSO), anhydrous calcium (CaCl2 ), hydrogen peroxide (H2 O2 ), and hydrochloric acid (HCl) were purchased from Nanjing Chemical Reagent Co., Ltd. and used as received without further purification.
2.3. Nuclear magnetic resonance spectroscopy (NMR) analysis Macromolecular structure characterization of BBL and BBL grafted copolymers was recorded under ambient temperature on a Bruker Avance 500 MHz spectrometer using DMSO-d6 and tetramethylsilane (TMS) as the corresponding solvent internal standard. 2.4. X-ray photoelectron spectroscopy (XPS) analysis X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5000 Versa Probe XPS spectrometer (ESCALAB 250 US Thermo Electron Co). All samples were analyzed using a microfocused, monochromated Al K␣ X-ray source. All spectra were referenced to the C1s peak assumed to originate from the surface hydrocarbon contamination at 284.6 eV binding energy controlled by means of the well-known photo electron peaks of metallic Cu, Ag, and Au. 2.5. Thermogravimetric analysis (TGA) Thermogravimetric analyses was performed using a TGA Q500 (TA Instruments) instrument. About 4 mg of each sample was scanned from 30 to 800 ◦ C at a heating rate of 10 ◦ C min−1 under nitrogen gas at a flow rate of 100 mL min−1 . 2.6. Differential scanning calorimetry (DSC) analysis Differential scanning calorimetry (DSC) was run on Perkin Elmer Diamond DSC instrument. Approximately 3–4 mg sample was enclosed in an aluminum pan. The temperature was increased at a rate of 20 ◦ C min−1 , and the data were obtained from the second heating scan. 2.7. Gel permeation chromatography (GPC) analysis Gel permeation chromatography (GPC HPLC Pump VE1122, Dual channel in Line Solvent degasser VE7510, Malvern) was used to measure molecular weight and molecular weight distribution of grafted copolymers. HPLC grade THF was employed as the eluent at a flow rate of 1.0 mL min−1 , and each sample concentration was about 3.0–5.0 mg mL−1 . The polymer solutions were filtered (0.45 m, PEFT Syringes Filters, Millipore, USA). The average molecular weight and dispersity were calculated by using a calibration curve which was obtained by polystyrene (PSt). 2.8. Water contact angle measurement The contact angle measurement was carried out by a DSA100 (KRUSS, Germany) instrument at 50% relative humidity and at 25 ◦ C, using 10 L droplets of deionized water. 2.9. Scanning electron microscopy (SEM) analysis The compatibility of the modified BBL and unmodified BBL samples with the PLA matrix was analyzed with a Hitachi S3400 scanning electron microscope scanning electron microscope (Hitachi Limited, Japan), respectively. The fractured surfaces of samples were observed using the SEM. 2.10. Optical properties of PLA–BBL composites
2.2. Fourier transform infrared spectroscopy (FTIR) analysis FTIR spectra of samples were recorded on a Fourier transform infrared spectrometer in a range of wave numbers from 4000 to 400 cm−1 , using attenuated total reflection Fourier transform infrared (ATR-FTIR) method on a Nicolet (USA)IS10 instrument.
A total of 1.0 g PLA was blended with unmodified BBL (12.5 mg) and BBL-g-PBA copolymer (100 mg) in chloroform (20 mL) at room temperature for 2 h [4]. Afterwards, the mixture solution of BBL/PLA and BBL-g-PBA/PLA was deposited (200 L) on quartz slides (4.50 cm−2 ) by micropipettes, respectively. The slides were
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dried under a vacuum until constant weight. Each dried coating was stored in a desiccator until spectroscopically characterized. Optical properties of PLA, PLA/BBL and PLA/BBL-g-PBA coating films were recorded on a UV Spectrophotometer (MAPADA UV-1800PC) in single-beam mode, using the same cleaned quartz slide as reference.
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where A2 is the area of the peak 2, A1 stands for the area of the peak 1. 3. Results and discussion 3.1. Synthesis of BBL graft copolymer via free radical polymerization (FRP)
2.11. Graft copolymerization The copolymerization reactions were performed by first adding calcium chloride (1.0 g) and dimethyl sulfoxide solvent (20 mL) to a dry 50 mL Schlenk flask, and then stirred until dissolved. Then BBL (1.0 g) was added in the solvent and stirred for 20 min. Finally the methyl methacrylate (MMA) (e.g., 4 g) and the H2 O2 (e.g., 1.0 mL) to the solvent. After 5 min of stirring, the Schlenk flask was immersed in a silicone oil bath set at 40 ◦ C at setting time (e.g., 24 h). The reaction was terminated by addition of inhibitor (4-methoxyphenol) (e.g., 4 mg). The details of experimental conditions for grafting reactions are listed in Table 1. After finishing the grafting reactions, the resulting solution was added dropwise to a 20-fold excess of diluted hydrochloric acid solution (pH = 3.0∼3.5), and the precipitated copolymer was filtered, repeatedly washed with water [21]. The final product was dried in a vacuum oven at 80 ◦ C for 48 h. The BBL-g-PMMA was obtained. The synthesis of BBL grafted copolymers (BBL-g-PBA and BBL-g-PHFBMA) was similar to the procedure described above. The percentage of grafting efficiency was calculated using the following equation. G(%) =
W2 − W1 × 100% W0
(1)
where W2 is the total weight solid mass recovered form grafting reactions, W1 is the weight of BBL used, W0 stands for the weight of monomers used in the grafting reactions. In order to know whether homopolymerization occurs, the blank experiment was prepared (poly(MMA)), which was tested exactly under the same experimental conditions without BBL. After reactions finishing, the sample was withdrawn from the flask with degassed syringes to determine the monomer conversion by 1 H NMR. The conversion of MMA was calculated using the following equation, according to the ratio of NMR integral areas of the peak 2 to peak 1 in 1 H NMR spectrum of sample. C (%) =
A 2 A1 × 3
− 1 × 100%
(2)
Table 1 displays that the influence of reaction time, monomer concentration, initiator concentration on the graft copolymer formation. The graft yield, molecular weight and molecular weight distribution of each products were also characterized. The data of SELS-g-PMMA [22] and Lignin-g-VAc [23] from literatures were showed in Table 1 for comparison. It should be pointed out that gel permeation chromatography (GPC) was used to determinate the molecular weight and molecular weight distribution of all copolymers. However, the signal of each BBL-g-PHFBMA sample was too weak to show up in the RI detector. This was because that most of the samples were absorbed by chromatographic columns due to the high-polarity of fluorine, in which fluorine content was 37.1 wt.% in BBL-g-PHFBMA4 copolymer (from Table 3). As shown in Table 1, molecular weight of the grafted copolymers varied widely with the type of monomers. For MMA and BA, the decrease in the amount both resulted in the lower molecular weight of the corresponding grafted copolymers. It was observed that the weight-average molecular weight of BBL-g-PBA3 (113,194 g mol−1 ) was highest in the whole BBL-g-PBA samples, implying that the lower chloride ion content of the reaction produced fewer grafting sites and longer grafted chains on the product. This was because that chloride atom was the active agent in producing the free radical on BBL while a hydroperoxide was the source of redox energy in the grafting reaction [25]. Increasing the polymerization time from 24 h (Table 1 sample BBL-g-PMMA1, 84.3%) to 48 h (Table 1 sample BBL-g-PMMA4, 94.8%) caused an increase in the graft yield and graft efficiency for MMA. Thus, more graft polymerization still occurred between 24 h and 48 h, but there was no increase in the graft yield for BA and HFBMA. Additionally, all the grafted copolymer possessed the high graft efficiency (>80%). It is worth mentioning that the yield of BBL-g-PMMA copolymer in our study (94.8 wt.%) was obviously higher than that of SELS-g-PMMA copolymer (0.12 wt.%) obtained from the steamexploded lignin copolymerized with MMA [22], although they were
Table 1 Polymerization parameters and gravimetrically determined conversion, graft yield, molecular weight and molecular weight distribution of each products.a Sample
Monomer (g)
CaCl2 (g)
t (h)
Yield (g/(wt. %)
BBL-g-PMMA1 BBL-g-PMMA2 BBL-g-PMMA3 BBL-g-PMMA4 c PMMA d SELS-g-PMMA e Lignin-g-VAc BBL-g-PBA1 BBL-g-PBA2 BBL-g-PBA3 BBL-g-PBA4 BBL-g-PHFBMA1 BBL-g-PHFBMA2 BBL-g-PHFBMA3 BBL-g-PHFBMA4
8 4 8 8 8 1.803 – 8 4 8 8 8 4 8 8
1 1 0.5 1 1 0.203 – 1 1 0.5 1 1 1 0.5 1
24 24 24 48 24 48 – 24 24 24 48 24 24 24 48
7.75/84.3 4.57/89.3 7.51/81.4 8.58/94.8 0 0.012/0.7 –/60 8.16/89.5 4.57/89.3 8.57/94.6 8.08/88.5 7.84/85.5 4.59/89.8 7.92/86.5 7.92/86.5
a b c d e
b
Mw (g mol−1 )
33,912 15,693 22,817 26,621 – – – 75,265 30,491 113,194 79,667 – – – –
Fixed conditions: 1.0 g BBL, 1.0 mL H2 O2 , 20 mL DMSO and T = 40 ◦ C. Determined by GPC. Blank experiment: PMMA homopolymer (produced under the same reaction conditions used for the production of BBL-g-PMMA1). SELS-g-PMMA is the graft copolymer of steam explosion lignin with methyl methacrylate. The data is from Bonini et al. [22]. Lignin-g-VAc is the graft copolymer of lignin with vinyl acetate. The date is form Panesar et al. [23].
b
Mn (g mol−1 )
11,469 5445 7801 10,031 – – – 13,868 10,838 23,399 14,926 – – – –
PDI 2.96 2.88 2.92 2.65 – – – 5.43 2.81 4.84 5.34 – – – –
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Fig. 2. FTIR spectra of BBL and BBL grafted copolymers.
Fig. 1. (A) 1 H NMR spectrum of poly(MMA)(blank experiment) in DMSO-d6 and (B) GPC traces of BBL-g-PMMA1 and poly(MMA)(blank experiment).
both initiated by the same (H2 O2 /CaCl2 ) complex system. This was probably because that the structural characteristics of BBL were different from that of steam-exploded lignin. Meanwhile, the (H2 O2 /CaCl2 ) initiator system in our study showed higher initiation efficiency (94.8 wt.%) than the initiator potassium persulfate (KPS) in the earlier report (60 wt.%) [23]. In order to see the role of BBL in the polymerization, a polymerization in the absence of BBL was run by placing 20 mL of dimethyl sulfoxide in a 50 mL conical flask containing 1.0 g of calcium chloride and 8.0 g of MMA (sample PMMA in Table 1). During the procedure of separation, no precipitation was obtained by the addition of diluted hydrochloric acid solution resulting into zero yield after filtration. Meanwhile, the conversion of MMA into Poly(MMA) in the presence of CaCl2 and H2 O2 with same condition described in Section 2.11 was also determined by 1 H NMR. As shown in Fig. 1A, the ratio of the integral area of the characteristic peak corresponding to the methyl protons ( CH3 ) (peak 1, around at 3.76 ppm) and the vinyl proton ( CH CH2 ) (peak 2, around at 6.15 ppm) was almost totally consistent with theoretical value (1:3), implying the MMA didn’t polymerize at that case. Meanwhile, there was no GPC signal of Poly(MMA) sample obtained from blank experiment during the whole test process (Fig. 1B), implying that the graft copolymerization was dominant. This behaviour could be explained that the radicals are not situated
on small molecules but on the surface of BBL [26]. Meister et al. [21] has been reported the possible mechanism of the reaction. Firstly, a hydroperoxide-chloride ion complex was formed by reaction of the hydroperoxide and chloride ion. Then, the chlorine abstracted hydrogen from lignin to form the free-radical site on the natural backbone and initiate polymerization. The results showed that this synthesis method was found to be selective for grafting. The BBL-g-PMMA (Table 1, sample BBL-g-PMMA4), BBL-g-PBA (Table 1, sample BBL-g-PBA4), and BBL-g-PHFBMA (Table 1, sample BBL-g-PHFBMA4) will be used for characterization of all properties described in later sections of this paper. Fig. 2 shows the FTIR spectra of the original BBL, BBL-g-PMMA, BBL-g-PBA and BBL-g-PHFBMA. In the spectra of BBL composites (BBL-g-PMMA, BBL-g-PBA), two strong and narrow absorption peaks shown at 1731 cm−1 ( COO ) and 1146 (C O) confirms the presence of the acrylate functionality. For BBL-g-PHFBMA, the appearance of two new peaks at 1288 cm−1 and 1176 cm−1 was attributed to the characteristic absorbance of CF2 [27]. The Further structural confirmations of three grafted copolymers were obtained by 1 H NMR spectra in Fig. 3. All peaks corresponding to vinyl protons of monomers completely disappeared. The characteristic chemical shifts, including the peaks occurring at 0.9–1.2 ppm and 3.5–4.2 ppm, respectively, corresponding to methyl protons
Fig. 3.
1
H NMR spectra of BBL and BBL grafted copolymers (DMSO-d6 ).
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Fig. 5. DSC curves for modified BBL composites.
Fig. 4. (A) TGA and (B) DTG curves for BBL and modified BBL composites.
( CH3 ) and OCH2 in the polymer backbone, respectively. The peaks at 4.55 ppm and 5.97 ppm were assigned to CHF , CH2 F [27], respectively. The 1 H NMR and FTIR spectra confirmed that the three grafted copolymers were successfully synthesized. 3.2. Thermal properties of BBL grafted copolymers The influence of grafted chains on the thermostability of the modified BBL composites was investigated with TGA. The temperature at 5% weight loss (Td,5 , from TGA curves (Fig. 4A)) and temperature at maximum weight loss rate (Td ,max , from DTG curves (Fig. 4B)) of BBL occurred at 230 ◦ C and 363 ◦ C, respectively. A significant increase in Td ,5 was observed for samples grafted with PMMA and PBA, by about 26 and 106 ◦ C, respectively. For other modifications, Td,5 , decreased compared with the unmodified BBL. A more important finding is that the synthesized BBL-g-PMMA, BBL-g-PBA and BBL-g-PHFBMA composites showed much higher Td ,max (392, 398 and 383 ◦ C (the second stage), respectively) than those of BBL (363 ◦ C), which implied that the BBL backbones have been successfully wrapped by PMMA, PBA or PFBMA grafting chains [29]. For the BBL-g-PHFBMA, it was found that the decomposition has two stages. The temperature of the first weight loss was about 274 ◦ C and the second plateau was about at 383 ◦ C. The weight loss for the first stage may be associated with end groups [28], and the second stage was probably attributed to the cleavage of part of the side chain and degradation of the main chain. These results were similar with some previous reports [30,31]. The thermal properties of BBL and BBL grafted copolymers (BBL, BBL-g-PMMA, BBL-g-PBA, and BBL-g-PHFBMA) were characterized by DSC (Fig. 5). Table 2 summarizes experimental glass transition temperature of graft copolymers, and glass transition temperature
of acrylic homopolymers from literature reference values. The Tg of original BBL was difficult to be observed, probably because the lignin was an inhomogeneous natural polymer and has random distribution of functional groups [32]. However, with acrylic polymers as side chains of BBL, it was obviously observed that Tg of all BBL grafted composites were identified. The observed temperatures were all higher than the Tg values of the homopolymers reported in literature. This was because that the incorporation of BBL into these polymeric materials affected their thermal properties. The BBL segment, which possesses condensed rigid phenolic moieties restricted the thermal mobility of lignin molecules, contributed to the increased Tg of copolymers. 3.3. Hydrophobicity of BBL grafted copolymers Lignin is considered as a hydrophilic natural biopolymer because of the existence of large number of aliphatic and aromatic hydroxyl groups [35]. The incorporation of acrylate units into the lignin backbone is expected to be enhanced in hydrophobicity, with the aim to extend the application of lignin to a variety of fields. Static contact angle measurement was employed to investigate the hydrophobicity of these modified BBL composites. The films on glass substrates were prepared by drop-casting THF solutions of these materials (BBL, BBL-g-PMMA, BBL-g-PBA and BBL-g-PHFBMA). Fig. 6 shows the images of water contact angle measurement. For the raw material BBL, the contact angle is 73◦ . After the introduction of acrylic polymers into the backbone of BBL, all contact angles increased to >80◦ , suggesting that the hydrophobicity of acrylic monomer was successfully imparted into BBL composites, particularly on the surface.
Table 2 Experimental glass transition temperatures of BBL graft copolymers, homopolymer from literature reference values, TGA values measured the temperature at 5% weight loss (Td,5 ) and maximum weight loss decomposition temperature (Td,max ). Sample
Tg (grafted) (◦ C)
BBL BBL-g-PMMA BBL-g-PBA BBL-g-PHFBMA
–a 121.6 −43.6 45.8
a b c d
Tg (homopolymer) (◦ C) 150b 105c −54c 41d
Tg was not observed. Data from Chung et al. [4]. Data from Andrews and Grulke [33]. Data from Hussain et al. [34].
Td,5 (◦ C)
Td,max (◦ C)
230 256 336 222
363 392 398 274, 383
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Fig. 6. Contact angle images of water droplet on the films of both BBL and modified BBL composites: (a) BBL: 73◦ , (b) BBL-g-PMMA: 81◦ , (d) BBL-g-PBA: 101◦ and (e) BBL-g-PHFBMA: 106◦ .
Fig. 7. (A) XPS wide scans from the BBL and BBL-g-PHFBMA, (B) deconvolution of C1s signals for the BBL and (C) deconvolution of C1s signals for the BBL-g-PHFBMA.
To further confirm the above hypothesis, XPS analysis was employed to reveal the surface elemental compositions of BBL and BBL-g-PHFBMA. In the wide scanning spectra for BBL and BBL-gPHFBMA (Fig. 7A), the characteristic peaks corresponding to C1s and O1s appeared at about 286 eV, 532 eV associated with BBL, respectively. A new peak at 688 eV, was corresponding to the carbon atoms of CF2 . As shown in Fig. 7B, the C 1s XPS spectra of the raw material BBL was de-convoluted into three peaks: 284.6 eV corresponding to C1s of surface groups of nonoxidized carbon (C H, C C,C C), 286.6 eV corresponding to C1s of surface groups of carbon with single bond to oxygen (C O, C O C), and 289 eV corresponding to C1s of surface groups of carbon with double bonds to oxygen (O C O), respectively [20,36]. After long
PFBMA chains grafted onto the backbone of BBL (Fig. 7C, BBLg-PHFBMA), the peaks at 289 eV increased, while the peaks at 286.6 decreased, which was due to the introduction of O C O bonds from ester group structure onto the BBL surface. Particularly, two new peaks at the carbon atoms of (C1s at 290.6 eV) and (C1s at 293.4 eV) groups attributed to CF2 , CF3 were also observed, respectively [27]. The relative atomic compositions of carbon with different oxidation levels of BBL and BBL-g-PHFBMA were calculated and presented in Table 3. The content of carbon with double bonds to oxygen (O C O, 289 eV) group increased from 6.14% (BBL) to 24.41% (BBL-g-PHFBMA). At the same time, the content of alcohol (C OH, 286.6 eV) groups decreased from 23.20% (BBL) to 10.34% (BBL-g-PHFBMA). Additionally, the contents of new
Table 3 XPS analysis of BBL and BBL-g-PHFBMA. Surface elemental composition (%) C Sample BBL BBL-g-PHFBMA
78.4 51.9
O 21.6 11.0
F 0 37.1
Surface chemical groups (%) CF3 293.4 (eV) 0 11.13
CF2 290.6 (eV) 0 11.05
O C O
C O
C C, C H
289 (eV) 6.14 24.41
286.6 (eV) 23.20 10.34
284.6 (eV) 70.66 43.09
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BBL composites was confirmed by TGA. The enhancement of hydrophobicity was demonstrated by the water contact angle study and was further confirmed by XPS. The synthesized BBL-g-PBA copolymers can be utilized as dispersion modifiers in PLA-based biobased composite materials, improving UV absorbance. This study may open a simple avenue to improve the compatibility of BBL with synthetic polymers, removing one obstacle to strong lignin-reinforced composites. Acknowledgements We would like to acknowledge support from Forestry Public Sector Research Fund of State Forestry Administration of China (201304606) and National Natural Science Foundation of China (31200447). References Fig. 8. UV–vis spectrum for PLA, and PLA-lignin composites with unmodified BBL and BBL-g-PBA copolymers. (Inset) SEM morphology of the fractured surface of the modified BBL samples and unmodified BBL in PLA matrix (a) BBL-g-PBA/PLA and (b) BBL/PLA. The amounts of BBL in the PLA films with unmodified BBL and with BBL-g-PLA copolymers are both about 1.0 wt.%.
groups appeared in BBL-g-PHFBMA are 11.05% (CF2 , 290.6 eV) and 11.13% (CF2 , 293.4 eV), respectively. All these results were clearly accounted for the hydrophobicity enhancement in these modified BBL composites. 3.4. Optical properties of PLA composites with BBL-g-PBA copolymer It was reported that lignin-g-PLA copolymer synthesized by ring-opening polymerization can be better dispersed in poly(lactic acid) to enhance UV absorption [4]. The grafted copolymer, PBA, is hydrophobic and expected to enhance the interface adhesion between BBL and PLA. Furthermore, PBA has a low glass-transition temperature (Tg ) and a soft chain, a core/shell structure can thereby be formed after grafting PBA onto BBL [37]. Thus, BBL-g-PBA copolymers was chose to investigate the light barrier property of PLA films with unmodified BBL and BBL-g-PBA copolymers (Fig. 8). The PLA film shows no ultraviolet (UV) light transmission in the lower range of UV-C (100–230 nm). By adding a low amount of BBL (about 1.0 wt.%) with PBA grafted on the surface into the PLA films, the film (BBL-g-PBA/PLA) showed UV light barrier properties, which block nearly 80% UV-B and half of UV-A (315–400 nm). The unmodified BBL did not significantly reduce the UV light transmission because of the poor dispersion in the PLA materials. As shown in Fig. 8 (inset), the BBL-g-PBA is uniformly dispersed in the PLA matrix compared to that of the individual BBL, implying that BBL-g-PBA dispersed well in the PLA matrix. Thus, the BBL-g-PBA copolymer could act as a well-dispersed UV blocker that can potentially be applied in the biocompatible packaging for extending shelf life of light sensitive products [4]. 4. Conclusions In conclusion, three different BBL graft copolymers were achieved by “grafting from” free radical polymerization in DMSO solvent containing calcium chloride and hydrogen peroxide (CaCl2 /H2 O2 ). The method was both efficient and selective: graft copolymerization was dominant almost no homopolymerization. The glass transition temperature of BBL graft copolymers was higher than the homopolymer of each monomer due to the introduction of the BBL. Increasing thermal stability of some modified
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Integration of lignin and acrylic monomers towards grafted copolymers by free radical polymerization Xiaohuan Liu a,b,c,d , Yuzhi Xu a,b,c,d , Juan Yu a,b,c,d , Shouhai Li a,b,c,d , Jifu Wang a,b,c,d,∗ , Chunpeng Wang a,b,c,d,∗ , Fuxiang Chu a,b,c,d,∗ a
Institute of Chemical Industry of Forestry Products, CAF, Nanjing, China Key Laboratory of Biomass Energy and Material, Nanjing, Jiangsu Province, China c National Engineering Laboratory for Biomass Chemical Utilization, Nanjing, China d Key and Open Laboratory on Forest Chemical Engineering, SFA, Nanjing 210042, China b
a r t i c l e
i n f o
Article history: Received 9 January 2014 Received in revised form 5 March 2014 Accepted 7 April 2014 Available online 15 April 2014 Keywords: Biobutanol lignin (BBL) Graft polymerization Acrylic monomer Hydrophobicity UV absorption
a b s t r a c t Three kinds of acrylic monomers (2,2,3,4,4,4-hexafluorobutyl methacrylate (HFBMA), methyl methacrylate (MMA) and butyl acrylate (BA)) were utilized to modify the lignin (BBL) by “grafting from” free radical polymerization (FRP), respectively. Calcium chloride/hydrogen peroxide (CaCl2 /H2 O2 ) was used as initiator. Effects of monomer type and concentration, initiator concentration and polymerization time on grafting from BBL were studied. Grafting of poly (acrylic monomers) onto BBL was verified by the following characterizations and this synthesis method was found to be high efficient and selective for grafting polymerization of BBL. The presence of the BBL moiety in the backbone also resulted in higher glass transition temperature compared with the homopolymer of each monomer, and some modified copolymers also improved its thermal stability. All modifications made BBL more hydrophobic and the static contact angles of these modified copolymers were above 80◦ . XPS analysis revealed that the surface of these modified BBL copolymers were dominated by acrylate monomer moiety. Additionally, the BBLg-PBA copolymers can be used as dispersion modifiers in PLA-based materials to enhance UV absorption. © 2014 Elsevier B.V. All rights reserved.
1. Introduction With the depletion of fossil oil, a major feedstock for polymers, and environmental awareness, a global interest has been gradually shifted towards the development of renewable biobased polymers and composites derived from natural resources [1–6]. Lignin, as a class of biodegradable, sustainable and inexpensive natural amorphous hydrophobic branched heteropolymer second only to cellulose in natural abundance, has been widely used for various applications [7–10]. However, the main intrinsic properties of lignin such as self-aggregate due to intermolecular association from hydrogen bondings and – stacking of aromatic groups [11], result in the poor dispersion in many thermoplastic polymers which has negatively impacted the mechanical properties of the
∗ Corresponding authors at: National Engineering Laboratory for Biomass Chemical Utilization China, Nanjing 210042, China. Tel.: +86 10 62889300; fax: +86 10 62889013. E-mail addresses: [email protected] (J. Wang), [email protected] (C. Wang), [email protected] (F. Chu). http://dx.doi.org/10.1016/j.ijbiomac.2014.04.005 0141-8130/© 2014 Elsevier B.V. All rights reserved.
resulting composite materials [12,13]. In an attempt to improve the miscibility properties of the lignin, extensive studies have been published about modifying lignin such as esterification [14], etherification [15], etc. Biobutanol lignin (BBL), a “waste” from the biobutanol industries, which is prepared from cornstalk or wheat straw usually by a combination of chemical and enzymatic treatments. BBL is regarded as a novel lignin with low molecular weight and high chemical activity than lignosulfonate or kraft lignin [16]. A variety of studies have focused on the kraft lignin which was recovered by pulping and paper industry. However, relatively few researches about the grafting of enzymatic hydrolysis lignin that obtained from co-product of biofuel production are little until now. Graft polymerization is one of the key techniques to covalently modify the surface of lignin by introduction of various functional polymers onto the side chains of lignin, resulting in greatly enhanced mechanical, thermal resistance and UV absorption properties of the host polymers. Chung and co-workers have synthesized lignin-g-PLA by ring-opening polymerization, which was found to be better dispersed in poly(lactic acid) thermoplastic materials, while compared to the unmodified lignin [4]. Lignin was also found to be grafted with polymers by growing polymer chains
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from radical sites on the lignin backbone (grafting from) [17], by introducing vinylic groups to lignin and copolymerizing the resulting macromonomer with a small molecular weight comonomer (grafting through) [18] or by attaching previously formed polymer chains to the lignin backbone (grafting onto) [19]. Presently, there are three major methods of grafting polymerization: free radical polymerization [19], ring-opening polymerization [4] and living radical polymerization [2,20]. Among of these graft polymerization methods, free radical polymerization (FRP) provides a simple method to modify lignin and lignin derivatives. Significant efforts have been devoted to the grafting polymerization of different kind of monomers (e.g., styrene [21], n-butyl methacrylate (BMA), methyl methacrylate (MMA) [22], poly(lactic acid) (PLA) [4], vinyl acetate (VAc) [23] and acrylic acid (AA) [17] from lignin backbones by FRP. These grafted copolymers were found to have excellent properties including hydrophobicity, elasticity, thermal resistance and UV absorption, which could find potential applications such as compatibilizers for blends of thermoplastics materials and UV blockers that can potentially be applied in the biocompatible packaging for extending shelf life of light sensitive products. Among the initiators for FRP, initiator (CaCl2 –H2 O2 ) has been thoroughly tested and was a selective method for grafting of lignin [21,22]. The reaction initiates free-radical grafting phenylethylene onto lignin by a redox reaction (CaCl2 -H2 O2 ) were firstly reported by Meister et al. [21]. The poly(1ignin-g-(1-phenylethy1ene)) products were formed with almost 100% grafting efficiency for lignin, and the temperature of the reaction was low. Previous lignin grafting studies have dealt with single monomers only. However, reports about carrying out an extensive comparison of different monomers to graft lignin are rare. Herein, in this context, we prepared three kinds of grafted lignin copolymers through “grafting from” FRP with the aim to make BBL more hydrophobic and miscible with thermoplastics or thermosetting materials. In order to better understand the grafting of BBL with acrylic monomers, the influence of a various factors like reaction time, monomer concentration, initiator concentration on the graft copolymer formation were studied. The chemical composition, molecular weight, thermal properties, hydrophobicity and UV absorption were also explored. 2. Experimental 2.1. Materials Lignin (BBL) used in the presented work was a byproduct of biomass-derived butanol production, and were purchased from songyuan bairui bio-polyos Co. Ltd., China in power form. The main characteristics of BBL are the C9 formula (C9 H8.160 O2.996 N0.094 S0.006 (OCH3 )0.594 ), a lignin content of 92.5% (w w−1 ), the methoxyl groups content of 3.24 mmol g−1 , the total hydroxyl of lignin is 5.62 mmol g−1 , a weight-average molecular weight of 876 g mol−1 , and a polydispersity of 1.6 [24]. 2,2,3,4,4,4-Hexafluorobutyl methacrylate (HFBMA), methyl methacrylate (MMA) and butyl acrylate (BA) were purchased from Aladdin Industrial Inc. and used as received. Dimethyl sulfoxide (DMSO), anhydrous calcium (CaCl2 ), hydrogen peroxide (H2 O2 ), and hydrochloric acid (HCl) were purchased from Nanjing Chemical Reagent Co., Ltd. and used as received without further purification.
2.3. Nuclear magnetic resonance spectroscopy (NMR) analysis Macromolecular structure characterization of BBL and BBL grafted copolymers was recorded under ambient temperature on a Bruker Avance 500 MHz spectrometer using DMSO-d6 and tetramethylsilane (TMS) as the corresponding solvent internal standard. 2.4. X-ray photoelectron spectroscopy (XPS) analysis X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5000 Versa Probe XPS spectrometer (ESCALAB 250 US Thermo Electron Co). All samples were analyzed using a microfocused, monochromated Al K␣ X-ray source. All spectra were referenced to the C1s peak assumed to originate from the surface hydrocarbon contamination at 284.6 eV binding energy controlled by means of the well-known photo electron peaks of metallic Cu, Ag, and Au. 2.5. Thermogravimetric analysis (TGA) Thermogravimetric analyses was performed using a TGA Q500 (TA Instruments) instrument. About 4 mg of each sample was scanned from 30 to 800 ◦ C at a heating rate of 10 ◦ C min−1 under nitrogen gas at a flow rate of 100 mL min−1 . 2.6. Differential scanning calorimetry (DSC) analysis Differential scanning calorimetry (DSC) was run on Perkin Elmer Diamond DSC instrument. Approximately 3–4 mg sample was enclosed in an aluminum pan. The temperature was increased at a rate of 20 ◦ C min−1 , and the data were obtained from the second heating scan. 2.7. Gel permeation chromatography (GPC) analysis Gel permeation chromatography (GPC HPLC Pump VE1122, Dual channel in Line Solvent degasser VE7510, Malvern) was used to measure molecular weight and molecular weight distribution of grafted copolymers. HPLC grade THF was employed as the eluent at a flow rate of 1.0 mL min−1 , and each sample concentration was about 3.0–5.0 mg mL−1 . The polymer solutions were filtered (0.45 m, PEFT Syringes Filters, Millipore, USA). The average molecular weight and dispersity were calculated by using a calibration curve which was obtained by polystyrene (PSt). 2.8. Water contact angle measurement The contact angle measurement was carried out by a DSA100 (KRUSS, Germany) instrument at 50% relative humidity and at 25 ◦ C, using 10 L droplets of deionized water. 2.9. Scanning electron microscopy (SEM) analysis The compatibility of the modified BBL and unmodified BBL samples with the PLA matrix was analyzed with a Hitachi S3400 scanning electron microscope scanning electron microscope (Hitachi Limited, Japan), respectively. The fractured surfaces of samples were observed using the SEM. 2.10. Optical properties of PLA–BBL composites
2.2. Fourier transform infrared spectroscopy (FTIR) analysis FTIR spectra of samples were recorded on a Fourier transform infrared spectrometer in a range of wave numbers from 4000 to 400 cm−1 , using attenuated total reflection Fourier transform infrared (ATR-FTIR) method on a Nicolet (USA)IS10 instrument.
A total of 1.0 g PLA was blended with unmodified BBL (12.5 mg) and BBL-g-PBA copolymer (100 mg) in chloroform (20 mL) at room temperature for 2 h [4]. Afterwards, the mixture solution of BBL/PLA and BBL-g-PBA/PLA was deposited (200 L) on quartz slides (4.50 cm−2 ) by micropipettes, respectively. The slides were
X. Liu et al. / International Journal of Biological Macromolecules 67 (2014) 483–489
dried under a vacuum until constant weight. Each dried coating was stored in a desiccator until spectroscopically characterized. Optical properties of PLA, PLA/BBL and PLA/BBL-g-PBA coating films were recorded on a UV Spectrophotometer (MAPADA UV-1800PC) in single-beam mode, using the same cleaned quartz slide as reference.
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where A2 is the area of the peak 2, A1 stands for the area of the peak 1. 3. Results and discussion 3.1. Synthesis of BBL graft copolymer via free radical polymerization (FRP)
2.11. Graft copolymerization The copolymerization reactions were performed by first adding calcium chloride (1.0 g) and dimethyl sulfoxide solvent (20 mL) to a dry 50 mL Schlenk flask, and then stirred until dissolved. Then BBL (1.0 g) was added in the solvent and stirred for 20 min. Finally the methyl methacrylate (MMA) (e.g., 4 g) and the H2 O2 (e.g., 1.0 mL) to the solvent. After 5 min of stirring, the Schlenk flask was immersed in a silicone oil bath set at 40 ◦ C at setting time (e.g., 24 h). The reaction was terminated by addition of inhibitor (4-methoxyphenol) (e.g., 4 mg). The details of experimental conditions for grafting reactions are listed in Table 1. After finishing the grafting reactions, the resulting solution was added dropwise to a 20-fold excess of diluted hydrochloric acid solution (pH = 3.0∼3.5), and the precipitated copolymer was filtered, repeatedly washed with water [21]. The final product was dried in a vacuum oven at 80 ◦ C for 48 h. The BBL-g-PMMA was obtained. The synthesis of BBL grafted copolymers (BBL-g-PBA and BBL-g-PHFBMA) was similar to the procedure described above. The percentage of grafting efficiency was calculated using the following equation. G(%) =
W2 − W1 × 100% W0
(1)
where W2 is the total weight solid mass recovered form grafting reactions, W1 is the weight of BBL used, W0 stands for the weight of monomers used in the grafting reactions. In order to know whether homopolymerization occurs, the blank experiment was prepared (poly(MMA)), which was tested exactly under the same experimental conditions without BBL. After reactions finishing, the sample was withdrawn from the flask with degassed syringes to determine the monomer conversion by 1 H NMR. The conversion of MMA was calculated using the following equation, according to the ratio of NMR integral areas of the peak 2 to peak 1 in 1 H NMR spectrum of sample. C (%) =
A 2 A1 × 3
− 1 × 100%
(2)
Table 1 displays that the influence of reaction time, monomer concentration, initiator concentration on the graft copolymer formation. The graft yield, molecular weight and molecular weight distribution of each products were also characterized. The data of SELS-g-PMMA [22] and Lignin-g-VAc [23] from literatures were showed in Table 1 for comparison. It should be pointed out that gel permeation chromatography (GPC) was used to determinate the molecular weight and molecular weight distribution of all copolymers. However, the signal of each BBL-g-PHFBMA sample was too weak to show up in the RI detector. This was because that most of the samples were absorbed by chromatographic columns due to the high-polarity of fluorine, in which fluorine content was 37.1 wt.% in BBL-g-PHFBMA4 copolymer (from Table 3). As shown in Table 1, molecular weight of the grafted copolymers varied widely with the type of monomers. For MMA and BA, the decrease in the amount both resulted in the lower molecular weight of the corresponding grafted copolymers. It was observed that the weight-average molecular weight of BBL-g-PBA3 (113,194 g mol−1 ) was highest in the whole BBL-g-PBA samples, implying that the lower chloride ion content of the reaction produced fewer grafting sites and longer grafted chains on the product. This was because that chloride atom was the active agent in producing the free radical on BBL while a hydroperoxide was the source of redox energy in the grafting reaction [25]. Increasing the polymerization time from 24 h (Table 1 sample BBL-g-PMMA1, 84.3%) to 48 h (Table 1 sample BBL-g-PMMA4, 94.8%) caused an increase in the graft yield and graft efficiency for MMA. Thus, more graft polymerization still occurred between 24 h and 48 h, but there was no increase in the graft yield for BA and HFBMA. Additionally, all the grafted copolymer possessed the high graft efficiency (>80%). It is worth mentioning that the yield of BBL-g-PMMA copolymer in our study (94.8 wt.%) was obviously higher than that of SELS-g-PMMA copolymer (0.12 wt.%) obtained from the steamexploded lignin copolymerized with MMA [22], although they were
Table 1 Polymerization parameters and gravimetrically determined conversion, graft yield, molecular weight and molecular weight distribution of each products.a Sample
Monomer (g)
CaCl2 (g)
t (h)
Yield (g/(wt. %)
BBL-g-PMMA1 BBL-g-PMMA2 BBL-g-PMMA3 BBL-g-PMMA4 c PMMA d SELS-g-PMMA e Lignin-g-VAc BBL-g-PBA1 BBL-g-PBA2 BBL-g-PBA3 BBL-g-PBA4 BBL-g-PHFBMA1 BBL-g-PHFBMA2 BBL-g-PHFBMA3 BBL-g-PHFBMA4
8 4 8 8 8 1.803 – 8 4 8 8 8 4 8 8
1 1 0.5 1 1 0.203 – 1 1 0.5 1 1 1 0.5 1
24 24 24 48 24 48 – 24 24 24 48 24 24 24 48
7.75/84.3 4.57/89.3 7.51/81.4 8.58/94.8 0 0.012/0.7 –/60 8.16/89.5 4.57/89.3 8.57/94.6 8.08/88.5 7.84/85.5 4.59/89.8 7.92/86.5 7.92/86.5
a b c d e
b
Mw (g mol−1 )
33,912 15,693 22,817 26,621 – – – 75,265 30,491 113,194 79,667 – – – –
Fixed conditions: 1.0 g BBL, 1.0 mL H2 O2 , 20 mL DMSO and T = 40 ◦ C. Determined by GPC. Blank experiment: PMMA homopolymer (produced under the same reaction conditions used for the production of BBL-g-PMMA1). SELS-g-PMMA is the graft copolymer of steam explosion lignin with methyl methacrylate. The data is from Bonini et al. [22]. Lignin-g-VAc is the graft copolymer of lignin with vinyl acetate. The date is form Panesar et al. [23].
b
Mn (g mol−1 )
11,469 5445 7801 10,031 – – – 13,868 10,838 23,399 14,926 – – – –
PDI 2.96 2.88 2.92 2.65 – – – 5.43 2.81 4.84 5.34 – – – –
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Fig. 2. FTIR spectra of BBL and BBL grafted copolymers.
Fig. 1. (A) 1 H NMR spectrum of poly(MMA)(blank experiment) in DMSO-d6 and (B) GPC traces of BBL-g-PMMA1 and poly(MMA)(blank experiment).
both initiated by the same (H2 O2 /CaCl2 ) complex system. This was probably because that the structural characteristics of BBL were different from that of steam-exploded lignin. Meanwhile, the (H2 O2 /CaCl2 ) initiator system in our study showed higher initiation efficiency (94.8 wt.%) than the initiator potassium persulfate (KPS) in the earlier report (60 wt.%) [23]. In order to see the role of BBL in the polymerization, a polymerization in the absence of BBL was run by placing 20 mL of dimethyl sulfoxide in a 50 mL conical flask containing 1.0 g of calcium chloride and 8.0 g of MMA (sample PMMA in Table 1). During the procedure of separation, no precipitation was obtained by the addition of diluted hydrochloric acid solution resulting into zero yield after filtration. Meanwhile, the conversion of MMA into Poly(MMA) in the presence of CaCl2 and H2 O2 with same condition described in Section 2.11 was also determined by 1 H NMR. As shown in Fig. 1A, the ratio of the integral area of the characteristic peak corresponding to the methyl protons ( CH3 ) (peak 1, around at 3.76 ppm) and the vinyl proton ( CH CH2 ) (peak 2, around at 6.15 ppm) was almost totally consistent with theoretical value (1:3), implying the MMA didn’t polymerize at that case. Meanwhile, there was no GPC signal of Poly(MMA) sample obtained from blank experiment during the whole test process (Fig. 1B), implying that the graft copolymerization was dominant. This behaviour could be explained that the radicals are not situated
on small molecules but on the surface of BBL [26]. Meister et al. [21] has been reported the possible mechanism of the reaction. Firstly, a hydroperoxide-chloride ion complex was formed by reaction of the hydroperoxide and chloride ion. Then, the chlorine abstracted hydrogen from lignin to form the free-radical site on the natural backbone and initiate polymerization. The results showed that this synthesis method was found to be selective for grafting. The BBL-g-PMMA (Table 1, sample BBL-g-PMMA4), BBL-g-PBA (Table 1, sample BBL-g-PBA4), and BBL-g-PHFBMA (Table 1, sample BBL-g-PHFBMA4) will be used for characterization of all properties described in later sections of this paper. Fig. 2 shows the FTIR spectra of the original BBL, BBL-g-PMMA, BBL-g-PBA and BBL-g-PHFBMA. In the spectra of BBL composites (BBL-g-PMMA, BBL-g-PBA), two strong and narrow absorption peaks shown at 1731 cm−1 ( COO ) and 1146 (C O) confirms the presence of the acrylate functionality. For BBL-g-PHFBMA, the appearance of two new peaks at 1288 cm−1 and 1176 cm−1 was attributed to the characteristic absorbance of CF2 [27]. The Further structural confirmations of three grafted copolymers were obtained by 1 H NMR spectra in Fig. 3. All peaks corresponding to vinyl protons of monomers completely disappeared. The characteristic chemical shifts, including the peaks occurring at 0.9–1.2 ppm and 3.5–4.2 ppm, respectively, corresponding to methyl protons
Fig. 3.
1
H NMR spectra of BBL and BBL grafted copolymers (DMSO-d6 ).
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Fig. 5. DSC curves for modified BBL composites.
Fig. 4. (A) TGA and (B) DTG curves for BBL and modified BBL composites.
( CH3 ) and OCH2 in the polymer backbone, respectively. The peaks at 4.55 ppm and 5.97 ppm were assigned to CHF , CH2 F [27], respectively. The 1 H NMR and FTIR spectra confirmed that the three grafted copolymers were successfully synthesized. 3.2. Thermal properties of BBL grafted copolymers The influence of grafted chains on the thermostability of the modified BBL composites was investigated with TGA. The temperature at 5% weight loss (Td,5 , from TGA curves (Fig. 4A)) and temperature at maximum weight loss rate (Td ,max , from DTG curves (Fig. 4B)) of BBL occurred at 230 ◦ C and 363 ◦ C, respectively. A significant increase in Td ,5 was observed for samples grafted with PMMA and PBA, by about 26 and 106 ◦ C, respectively. For other modifications, Td,5 , decreased compared with the unmodified BBL. A more important finding is that the synthesized BBL-g-PMMA, BBL-g-PBA and BBL-g-PHFBMA composites showed much higher Td ,max (392, 398 and 383 ◦ C (the second stage), respectively) than those of BBL (363 ◦ C), which implied that the BBL backbones have been successfully wrapped by PMMA, PBA or PFBMA grafting chains [29]. For the BBL-g-PHFBMA, it was found that the decomposition has two stages. The temperature of the first weight loss was about 274 ◦ C and the second plateau was about at 383 ◦ C. The weight loss for the first stage may be associated with end groups [28], and the second stage was probably attributed to the cleavage of part of the side chain and degradation of the main chain. These results were similar with some previous reports [30,31]. The thermal properties of BBL and BBL grafted copolymers (BBL, BBL-g-PMMA, BBL-g-PBA, and BBL-g-PHFBMA) were characterized by DSC (Fig. 5). Table 2 summarizes experimental glass transition temperature of graft copolymers, and glass transition temperature
of acrylic homopolymers from literature reference values. The Tg of original BBL was difficult to be observed, probably because the lignin was an inhomogeneous natural polymer and has random distribution of functional groups [32]. However, with acrylic polymers as side chains of BBL, it was obviously observed that Tg of all BBL grafted composites were identified. The observed temperatures were all higher than the Tg values of the homopolymers reported in literature. This was because that the incorporation of BBL into these polymeric materials affected their thermal properties. The BBL segment, which possesses condensed rigid phenolic moieties restricted the thermal mobility of lignin molecules, contributed to the increased Tg of copolymers. 3.3. Hydrophobicity of BBL grafted copolymers Lignin is considered as a hydrophilic natural biopolymer because of the existence of large number of aliphatic and aromatic hydroxyl groups [35]. The incorporation of acrylate units into the lignin backbone is expected to be enhanced in hydrophobicity, with the aim to extend the application of lignin to a variety of fields. Static contact angle measurement was employed to investigate the hydrophobicity of these modified BBL composites. The films on glass substrates were prepared by drop-casting THF solutions of these materials (BBL, BBL-g-PMMA, BBL-g-PBA and BBL-g-PHFBMA). Fig. 6 shows the images of water contact angle measurement. For the raw material BBL, the contact angle is 73◦ . After the introduction of acrylic polymers into the backbone of BBL, all contact angles increased to >80◦ , suggesting that the hydrophobicity of acrylic monomer was successfully imparted into BBL composites, particularly on the surface.
Table 2 Experimental glass transition temperatures of BBL graft copolymers, homopolymer from literature reference values, TGA values measured the temperature at 5% weight loss (Td,5 ) and maximum weight loss decomposition temperature (Td,max ). Sample
Tg (grafted) (◦ C)
BBL BBL-g-PMMA BBL-g-PBA BBL-g-PHFBMA
–a 121.6 −43.6 45.8
a b c d
Tg (homopolymer) (◦ C) 150b 105c −54c 41d
Tg was not observed. Data from Chung et al. [4]. Data from Andrews and Grulke [33]. Data from Hussain et al. [34].
Td,5 (◦ C)
Td,max (◦ C)
230 256 336 222
363 392 398 274, 383
488
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Fig. 6. Contact angle images of water droplet on the films of both BBL and modified BBL composites: (a) BBL: 73◦ , (b) BBL-g-PMMA: 81◦ , (d) BBL-g-PBA: 101◦ and (e) BBL-g-PHFBMA: 106◦ .
Fig. 7. (A) XPS wide scans from the BBL and BBL-g-PHFBMA, (B) deconvolution of C1s signals for the BBL and (C) deconvolution of C1s signals for the BBL-g-PHFBMA.
To further confirm the above hypothesis, XPS analysis was employed to reveal the surface elemental compositions of BBL and BBL-g-PHFBMA. In the wide scanning spectra for BBL and BBL-gPHFBMA (Fig. 7A), the characteristic peaks corresponding to C1s and O1s appeared at about 286 eV, 532 eV associated with BBL, respectively. A new peak at 688 eV, was corresponding to the carbon atoms of CF2 . As shown in Fig. 7B, the C 1s XPS spectra of the raw material BBL was de-convoluted into three peaks: 284.6 eV corresponding to C1s of surface groups of nonoxidized carbon (C H, C C,C C), 286.6 eV corresponding to C1s of surface groups of carbon with single bond to oxygen (C O, C O C), and 289 eV corresponding to C1s of surface groups of carbon with double bonds to oxygen (O C O), respectively [20,36]. After long
PFBMA chains grafted onto the backbone of BBL (Fig. 7C, BBLg-PHFBMA), the peaks at 289 eV increased, while the peaks at 286.6 decreased, which was due to the introduction of O C O bonds from ester group structure onto the BBL surface. Particularly, two new peaks at the carbon atoms of (C1s at 290.6 eV) and (C1s at 293.4 eV) groups attributed to CF2 , CF3 were also observed, respectively [27]. The relative atomic compositions of carbon with different oxidation levels of BBL and BBL-g-PHFBMA were calculated and presented in Table 3. The content of carbon with double bonds to oxygen (O C O, 289 eV) group increased from 6.14% (BBL) to 24.41% (BBL-g-PHFBMA). At the same time, the content of alcohol (C OH, 286.6 eV) groups decreased from 23.20% (BBL) to 10.34% (BBL-g-PHFBMA). Additionally, the contents of new
Table 3 XPS analysis of BBL and BBL-g-PHFBMA. Surface elemental composition (%) C Sample BBL BBL-g-PHFBMA
78.4 51.9
O 21.6 11.0
F 0 37.1
Surface chemical groups (%) CF3 293.4 (eV) 0 11.13
CF2 290.6 (eV) 0 11.05
O C O
C O
C C, C H
289 (eV) 6.14 24.41
286.6 (eV) 23.20 10.34
284.6 (eV) 70.66 43.09
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BBL composites was confirmed by TGA. The enhancement of hydrophobicity was demonstrated by the water contact angle study and was further confirmed by XPS. The synthesized BBL-g-PBA copolymers can be utilized as dispersion modifiers in PLA-based biobased composite materials, improving UV absorbance. This study may open a simple avenue to improve the compatibility of BBL with synthetic polymers, removing one obstacle to strong lignin-reinforced composites. Acknowledgements We would like to acknowledge support from Forestry Public Sector Research Fund of State Forestry Administration of China (201304606) and National Natural Science Foundation of China (31200447). References Fig. 8. UV–vis spectrum for PLA, and PLA-lignin composites with unmodified BBL and BBL-g-PBA copolymers. (Inset) SEM morphology of the fractured surface of the modified BBL samples and unmodified BBL in PLA matrix (a) BBL-g-PBA/PLA and (b) BBL/PLA. The amounts of BBL in the PLA films with unmodified BBL and with BBL-g-PLA copolymers are both about 1.0 wt.%.
groups appeared in BBL-g-PHFBMA are 11.05% (CF2 , 290.6 eV) and 11.13% (CF2 , 293.4 eV), respectively. All these results were clearly accounted for the hydrophobicity enhancement in these modified BBL composites. 3.4. Optical properties of PLA composites with BBL-g-PBA copolymer It was reported that lignin-g-PLA copolymer synthesized by ring-opening polymerization can be better dispersed in poly(lactic acid) to enhance UV absorption [4]. The grafted copolymer, PBA, is hydrophobic and expected to enhance the interface adhesion between BBL and PLA. Furthermore, PBA has a low glass-transition temperature (Tg ) and a soft chain, a core/shell structure can thereby be formed after grafting PBA onto BBL [37]. Thus, BBL-g-PBA copolymers was chose to investigate the light barrier property of PLA films with unmodified BBL and BBL-g-PBA copolymers (Fig. 8). The PLA film shows no ultraviolet (UV) light transmission in the lower range of UV-C (100–230 nm). By adding a low amount of BBL (about 1.0 wt.%) with PBA grafted on the surface into the PLA films, the film (BBL-g-PBA/PLA) showed UV light barrier properties, which block nearly 80% UV-B and half of UV-A (315–400 nm). The unmodified BBL did not significantly reduce the UV light transmission because of the poor dispersion in the PLA materials. As shown in Fig. 8 (inset), the BBL-g-PBA is uniformly dispersed in the PLA matrix compared to that of the individual BBL, implying that BBL-g-PBA dispersed well in the PLA matrix. Thus, the BBL-g-PBA copolymer could act as a well-dispersed UV blocker that can potentially be applied in the biocompatible packaging for extending shelf life of light sensitive products [4]. 4. Conclusions In conclusion, three different BBL graft copolymers were achieved by “grafting from” free radical polymerization in DMSO solvent containing calcium chloride and hydrogen peroxide (CaCl2 /H2 O2 ). The method was both efficient and selective: graft copolymerization was dominant almost no homopolymerization. The glass transition temperature of BBL graft copolymers was higher than the homopolymer of each monomer due to the introduction of the BBL. Increasing thermal stability of some modified
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