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Graft Polymerization of Acrylic Monomers onto Lignin with CaCl2-H2O2 as Initiator: Preparation, Mechanism, Characterization, and Application in Poly(Lactic Acid) Enmin Zong, Xiaohuan Liu, Lina Liu, Jifu Wang, Pingan Song, Zhongqing Ma, Jie Ding, and Shenyuan Fu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02599 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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ACS Sustainable Chemistry & Engineering

Graft Polymerization of Acrylic Monomers onto Lignin with CaCl2H2O2 as Initiator: Preparation, Mechanism, Characterization, and Application in Poly(Lactic Acid) Enmin Zong†, Xiaohuan Liu,*,‡ Lina Liu‡, Jifu Wang £, Pingan Song,*,‡,§ Zhongqing Ma‡, Jie Ding¶, Shenyuan Fu‡ †

Zhejiang Provincial Key Laboratory of Plant Evolutionary Ecology and Conservation, Taizhou

University, 1139 Shifu Street, Taizhou 318000, PR China ‡

School of Engineering, Zhejiang A & F University, 666 Wusu Street, Hangzhou 311300, PR

China §

Center for Future Materials, University of Southern Queensland, West Street, Toowoomba, 4350,

Australia £

Jiangsu Key Lab. of Biomass Energy and Material, Institute of Chemical Industry of Forest

Products, CAF, 16 Suojin Fifth Village, Nanjing 210042, PR China ¶

Yuhang District Agro-product Monitoring Center in Hangzhou City, 162 Yisheng Road,

Hangzhou,311199, PR China

*Corresponding authors should be addressed at: [email protected] (X.H. Liu) and [email protected] (P.A. Song)

ACS Paragon Plus Environment

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ABSTRACT: Effective utilization of abundant industrial lignin has growingly attracted much attention besides potential environmental issues. Although chemical graft polymerization modification is one facile strategy for extending its applications, it remains an intractable challenge to select highly efficient initiation system. We herein have attempt to understand the CaCl2-H2O2 system in initiating the graft polymerization of acrylic monomers onto acetic acid lignin (AAL) and biobutanol lignin (BBL). The initiation system is found to be highly efficient and selective, as proved by the successful graft of polyacrylates onto them, and a possible mechanism is also proposed. Thermal analysis show that the graft modification results in a higher glass transition temperature and higher thermal stability of lignin. The graft modification make both AAL and BBL become more hydrophobic than before modification. Moreover, adding a small amount of lignin-graft-polyacrylate can considerably improve the UV blocking capability in addition to the reinforcing effect on polylactic acid. This work offers a novel, highly effective and selective free radical initiation system for functionalizing lignin. KEYWORDS: Industrial lignin, Initiation system, Graft polymerization, Poly(lactic acid)


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INTRODUCTION Lignin, typically composed of 18-25% hardwoods and of 25-35% softwoods,1 is a highly branched aromatic polymer with nine carbon units derived from coumaryl, coniferyl and syringyl alcohols.2 It is now considered as the main aromatic renewable resource with attractive properties, including high thermal stability, biodegradability, antimicrobial, antioxidation, and UV absorption properties.3-4 As is well-known, lignin has a negative effect on pulp and paper industry as well as lignocellulosic biofule industry, thus needing to be removed during the process.5-6 It is estimated that about 50 million tons of lignocellulosic wastes that contained the rich lignin came from the lignocellulosic biofules, pulp and paper industry annually in the world. 7

There has been an increase in the utilization of lignocellulosic waste residue, due to its low-

cost, plentiful and renewable advantages.8 Biobutanol lignin (BBL), a "lignocellulosic waste residues", is recovered from the biobutanol industries.9 BBL is usually obtained by the pretreatment and enzymatic hydrolysis steps, and retained high chemical activity. Acetic acid lignin (AAL), another kind of lignocellulosic waste residue, is recovered from the acetic acid pulping.10 AAL exhibits many advantages including no sulfur, lower ash content, less structural change, low condensed structure and higher purity, which extend its scope for producing high value-added materials. The development of value-added products from lignocellulosic waste residues plays a role in improving the economics of producing liquid fuels and pulping from biomass.11-13 Unfortunately, until now the large amount of lignin is basically used for generating energy. Only a minority of lignin is used to produce valuable chemicals14-15 or materials such as dispersants, adhesives, surfactants and antioxidants.10,

16

The abundance and environmental

friendliness enable lignin to be promising feedstocks for fabricating high value-added


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biopolymers.17-20 In particular, the increasing environment pollution issues have driven the development of renewable materials to replace non-renewable petroleum-based synthetic polymers.4 One facile method for utilizing natural lignin is preparing biocomposites by blending lignin with green materials such as PLA.19, 21 It is predicted that the incorporation of lignin can offset the defects of pure PLA such as high cost, brittleness, poor UV light barrier and poor thermal stability properties.22-23 However, the direct use of lignin as filler in PLA usually negatively impacts the mechanical properties of the resulting bio-composites due to the poor dispersion of lignin in the host material. This is mainly attributed to self-aggregation caused by hydrogen bonding and π-π stacking of aromatic groups.24 Additionally, it is reported that the dispersion of lignin becomes worse with the decreasing molecular weight.20 To overcome disadvantages mentioned above, the copolymerization of lignin with vinyl monomers has been reported to increase the molecular weight as well as the number of functional groups to lignin, by which lignin can disperse better in the PLA host. Free-radical polymerization (FRP) is a well-known method to copolymerize lignin with vinyl monomers to functionalize lignin.25 Because of cost effective and facile advantages, graft copolymerization can endow lignin with desirable properties by selecting different functional monomers.7 The grafted copolymers has been currently synthesized by three main strategies: grafting-onto, grafting-from and grafting-through.26 Among them, grafting-from is the most efficiently and widely strategy to functionalize lignin. So far, many efforts have been devoted to graft different kinds of vinyl monomers such as styrene,27 sodium acrylate, methyl methacrylate, acrylamide,28 acrylonitrile, n-butyl methacrylate,29 vinyl acetate7 and acrylic acid20 onto lignin backbones by “graft from” Free-radical polymerization. These grafted copolymers exhibited some impressive properties, such as thermal resistance and UV absorption, which could find


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potential applications such as compatibilisers for the blends of thermoplastics materials and UV blockers for biocomposites.21 Currently, the graft copolymerization reaction between lignin and vinyl monomers have been proved to be successfully initiated by FeCl2-H2O2, K2S2O8-NaS2O3, H2O2 and K2S2O8-(NH4)2Fe (SO4)2·6H2O.7,20,30-33 However, the homopolymers normally form during the graft copolymerization reaction. Based on the previous reports published by others27 and us,25 it is worth emphasizing that the calcium chloride-hydrogen peroxide (CaCl2-H2O2) initiator system exhibits a highly selective method for grafting acrylic monomer onto lignin. Our previous work showed that the homopolymer (Poly (MMA)) was found not to form when MMA was grafted onto lignin using the CaCl2-H2O2 as the initiator.25 In this work, we aim to prepare AAL and BBL-graft-polyacrylate through "grafting from" FRP using CaCl2-H2O2 as the initiator to improve their hydrophobicity and miscibility with PLA (Scheme 1). Another objective of this work is to preliminarily clarify the copolymerization mechanism of lignin with acrylic monomer. In addition, we also investigate the properties changes (chemical composition, molecular weight, thermal properties, hydrophobicity, and morphology) of AAL and BBL after graft copolymerization. Then, as-prepared lignin-graftpolyacrylate is added into PLA to evaluate their applications and both mechanical and UV adsorption properties of the PLA biocomposites are significantly improved. This study also intends to evaluate the effect of AAL and BBL addition on UV adsorption and mechanical properties of PLA. EXPERIMENTAL SECTION Materials. Acetic acid lignin (AAL) powders were purchased from Konglong paint Co. Ltd., China, biobutal lignin (BBL) was purchased from Songyuan Bairui Bio-polyos Co. Ltd., China. PLA was purchased from Nature Works LCC. N-butyl methacrylate (BMA), methyl


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methacrylate (MMA), ethyl methacrylate (EMA), benzyl methacrylate (BZMA) and styrene (St) were purchased from the Aladdin Industrial Corporation. Dimethyl sulfoxide (DMSO), N, NDimethylformamide (DMF), Dimethylacetamide (DMAC), hydrogen peroxide (H2O2), magnesium chloride

(MgCl2),

calcium

chloride

(CaCl2),

sodium chloride

(NaCl),

potassium chloride (KCl) and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent co., Ltd, China. All the chemicals were used without any additional purification. Characterizations. Fourier Transform Infrared Spectra (FTIR) spectra of samples were recorded on a Nicolet (USA)IS10 instrument by the KBr pellet pressing method in a range of wavenumbers from 4000 to 400 cm-1 for a total scan times of 64 . The 1H NMR analysis was performed on a Bruker AVANCE 400 NMR spectrometer using CDCl3 as the solvent. Thermogravimetric

analyses

(TGA)

was

carried

out

using

a

thermal

gravimetric

NETZSCH TG 209F1 Iris instrument with a heating rate of 10 °C min-1 under nitrogen gas condition. Differential scanning calorimetry (DSC) was performed on a TA Instruments Q2000 DSC under a nitrogen atmosphere. About 6-7 mg of each sample was tested over a temperature range from 0 to 150 °C at a rate of 20 °C min-1 in flowing nitrogen. The glass transition temperature (Tg) was obtained from the second heating scan. Gel permeation chromatography (GPC) data were obtained from a Malvern Viscotek 3580 System equipped with a Viscotek GPC2502

refractive

detector.

Mono-dispersed

polystyrene

(PSt)

was

used

as the calibration standard using HPLC grade THF as mobile phase at a flow rate of 1.0 mL min1

to generate the calibration curve. The sample concentration was approximately 3.0-5.0 mg mL-

1

. Prior to test, the polymer solutions were filtered through a micro-filter with a 0.45 µm PEFT

Syringes Filters. The water contact angle measurement was carried out on a DSA100 instrument


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at three different points on the sample surface in a conditioned room. Scanning electron microscope (SEM) was collected on a Hitachi S-4800 SEM at an accelerating voltage of 15000 V. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Thermo ESCALAB 250 spectrometer. The characteristic X-rays focus on the elements of C and O in this study. All spectra were referenced to the C1s peak of adventitious carbon at 284.6 eV. Mechanical property was measured by using a SANS universal testing machine (CMT 6000) with a test rate of 10 mm min L-1. Five replicates were tested for each sample. Transmittance spectra were recorded using on a MAPADA UV-1800PC UV Spectrophotometer in the UV-visible region (200-800 nm). Synthesis of Lignin-graft-Polyacrylate. According to the fabrication method as our previous report,25 typically a certain amount of calcium chloride was added into a dry 50 mL flask with 20 mL of dimethyl sulfoxide, and the mixture was stirred for 10 min until completely dissolved. Then 1.0 g AAL was added in the mixture and keep stirring for 20 min. Finally, the n-butyl methacrylate (BMA) and 1.0 mL H2O2 were added into the above mixture. After 5 min of stirring, the schlenk flask was heated to 50 °C. The reaction was terminated via adding a small amount

of

4-methoxyphenol.

The

details

of

experimental

conditions

for

grafting

copolymerization reactions are listed in Table 1. After the reaction was complete, the resulting solution was slowly added to an excessive diluted hydrochloric acid solution. The precipitate was filtered and washed with water, and dried in a vacuum oven at 60 °C for 24 h. The resulting product (AAL-g-PBMA) was obtained. The synthesis of AAL-g-PMMA, BBL-g-PBZMA and BBL-g-PEMA was similar to the procedure described above. The conversion of monomer was determined using the following equation (1).34

Conversion (%) =

W2 - W1 × 100% W0

(1)


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Where W2 is the amount of solid mass recovered from grafting reactions, W1 is the weight of lignin used in the experiments and W0 is the amount of monomers used in the experiments. Additionally, control experiments (Poly(MMA), Poly(BMA), Poly(EMA) and Poly(BZMA)) were also prepared under the same experimental procedure as stated without of AAL and BBL to investigate whether homopolymerization occurred. After reactions finishing, each sample was withdrawn to determine the monomer conversion by 1H NMR. The results exhibited that the four monomers were no conversion by comparing between the experimental value of characteristic peaks ratio and the theoretical value of characteristic peaks ratio (data not shown).25 Composites Fabrication. The AAL grafted copolymer/PLA and BBL grafted copolymer/PLA composites were prepared by melt-blended using a Thermo Haaker mixerat 185 oC with a rotor speed 60 rpm. The loading level of polyacrylate-graft-lignin was set as 1wt% in PLA. The mixing time was 10 min for each sample. Then, all samples were obtained by hot-pressed at 30 MPa using the above melt-blended composites. RESULTS AND DISCUSSION Table 1 Characterization of Industrial Lignin. Two lignin wastes (AAL and BBL) are used as the raw materials in this work. The main chemical composition of AAL and BBL was showed in the Table 1. The element compositions of AAL were 58.37 wt. % C, 39.35 wt. % O, 1.54 wt. % H and 0.74 wt. % N. Moreover, BBL displayed 65.16 wt. % of C, 28.94 wt. % of O, 3.99 wt. % H and 0.96 wt. % N and 0.95 wt. % S. It could be seen that the higher carbon content in BBL suggested more content of aromatic ring. The AAL contains 89.6 wt.% lignin, 1.2 wt% ash, and a high hydroxyl content of 7.17 mmol g-1 whereas BBL mainly contains 92.5 wt. % lignin, 1.5


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wt.% ash, and a hydroxyl content of 5.62 mmol g-1. Apparently, it can be concluded that the lignocellulosic wastes of AAL and BBL are of low ash and high purity. Meanwhile, both AAL and BBL contain low sulfur, confirming the high purity of lignin, which was suitable as a raw material for developing high value-added products. In addition, the molecular weight of AAL (2353 g mol-1) is much higher than BBL (876 g mol-1), indicating that less structural changes occur during acetic acid pulping procedure.10 Additionally, the high hydroxyl content of AAL and BBL allows for their reactivity or chemical modification. Figure 1 Synthesis of Lignin-graft-Polyacrylate. In order to maximize the monomer conversion in the grafting reaction, the effect of solvents (DMF, DMAC and DMSO) and salts (MgCl2, CaCl2, KCl and NaCl) on the monomer conversion were investigated. The results are listed in Figure 1(a) and Figure 1(b) respectively. Evidently, a high monomer conversion (> 85 %) can be achieved in all three solvents and the conversion reaches the maximum value for DMSO. Thus, DMSO was chosen as the reaction media for the following experiments. Interestingly, the MMA obtains a high conversion in DMAC, which is different from the others’ report.27 This was probably due to the structural difference of lignin and different monomers. As shown in Figure 1(b), the next series of experiments focused upon the effect of cationic species on the monomer conversion in the grafting reaction. The results showed that cationic species had a limited effect on the monomer conversion and all of them lead to high monomer conversions. The calcium chloride was the preferred co-initiator. Therefore, DMSO was chosen as the solvent for the next experiments. Table 2


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Table 2 summarized the detailed data of the monomer conversion, weight average molar mass, and polydispersity of the grafted copolymers. The molecular weight of grafted copolymers strongly depends on the type of monomers. All monomers possess high grafting conversion (> 85 %) via (H2O2/CaCl2) initiator system in our study, which is better than others from copolymerization reaction between lignin and vinyl monomer using the K2S2O8-(NH4)2Fe (SO4)2·6H2O (60%) 7, or FeCl2-H2O2 (59.5%) as initiator 30. In particular, it is worth mentioning that these traces of lignin grafted copolymers were found to be narrow and monomodal as shown in Figure 1c, indicating that as-synthesized copolymers were pure graft copolymers. Interestingly, it is found that the homopolymers (Poly (MMA), Poly (BMA), Poly (BZMA) and Poly (EMA)) are not obtained in this initiator system, implying that there are no homopolymerization occurring during the grafting polymerization. The results show that this polymerization method was found to be selective for grafting. GPC results display that the increase of the amount of monomers (e.g. AAL-g-PMMA2: 24867 g mol-1) result in the higher molecular weight of the corresponding grafted copolymers (e.g. AAL-g-PMMA1: 36594 g mol1

). Meanwhile, the deviation of GPC molecular weight of the different monomer values are

observed, which might be due to the different hydrodynamic behaviors between the PSt standards (conventional calibration) and the grafted polymers.36 It is found that increasing the copolymerization time from 6 h (e.g. AAL-g-PMMA4: 86.5 %) to 12 h (e.g. AAL-g-PMMA1: 88.1%) causes a slightly increase in the graft conversion for each monomer. It is reasonable to conclude that the graft polymerization mainly takes place between 6 h and 12 h. Additionally, monitoring the conversion of different monomers during the polymerization reaction is shown in Figure S1, it can be clearly seen that most monomers exhibited low conversion in 1 hour because


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of the induction period, but the conversion increases quickly in next 3 hours in the acceleration period, followed by a progressive slowing down until a plateau is achieved in 12 h. Figure 2 Table 3 Mechanism for Graft Polymerization. There has been a standpoint on the role that the lignin plays in its graft polymerization with different vinyl monomers such as vinyl acetate (VAc) or acrylic acid (AA). The presence of lignin is considered to be able to accelerate the polymerization of the vinyl monomers because of the existence of phenolic groups which is proved to be act as active sites in the grafting polymerization.7,37 To determine if phenolic groups in the lignin molecules affect the copolymerization in our work, AAL is first acetylated by using pyridine-acetic anhydride (1:1, v/v) at 25 oC for 72 h for protecting the phenolic and aliphatic hydroxyl groups in the AAL molecules, and then reacts with MMA (Table 3, entry 1). The monomer conversion is found to be 90.2 %, indicating that MMA is successfully grafted onto AAL in the absence of phenolic hydroxyl groups. The unexpected result is quite different from some previous investigations.7,37 Moreover, styrene (St) is chosen as the monomer to homopolymerize in the absence of AAL (Table 3, entry 2). Surprisingly, poly(styrene) homopolymer or PS is obtained with a conversion as low as 19%, which is different from other blank experiments where EMA or PBZMA or BMA or MMA acts as the monomer. Additionally, the grafting of St on the AAL was also tested (Table 3, entry 3), and the conversion of styrene was 72.5%, demonstrating that the presence of lignin can accelerate the polymerization reaction. The GPC trace of AAL-g-poly (St) (Figure 1d) obviously exhibits a high molecular weight (a shoulder on GPC curve), which is most likely attributed to the presence of a small amount of free


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PSt chains, suggesting that the PS homopolymer simultaneously generates when the PS chains are grafted onto the AAL.38 Furthermore, to better understand the role of lignin in the copolymerization with monomers, the conversion of MMA grafting onto different content of AAL are determined and presented in Table 3. It should be pointed out that once the amount of lignin was increased from 0.4 g to 1.0 g, the conversion of MMA increases from 15% to 88.1%, which is due to the higher chance of collision occurring between AAL and MMA at a higher AAL content. This behavior can be explained that the radical is inclined on the macromolecule AAL but not on micromolecular MMA, which indicates that the AAL is very important in this complex initiator system (CaCl2H2O2).39 Meanwhile, no conversion of MMA is observed when CaCl2 (g) or H2O2 is applied separately (Table 3, entry 8 and entry 9), indicating that the initiator is formed to be a complex between CaCl2 and H2O2.27 Based on the results of our experiment, the proposed mechanism on the grafting copolymerization lignin with MMA is proposed. As depicted in Figure 2, in the initiating system containing H2O2 and Cl-, a concerted reaction maybe occur to form a hydroperoxide-chloride ion complex (reaction (1)),27 which is able to abstract hydrogen from lignin to form lignin macromolecular radicals (reaction (2)). The high concentration of benzylic sites in lignin are more susceptible to hydrogen abstraction and thus support active grafting sites.11 Additionally, the formed hydroxyl radicals can react with chlorine iron to form chloride atom rapidly (reaction (3)), suggesting that the homolymerization does not happen.40 The formed chloride atom can also abstract hydrogen from lignin to form lignin macroradicals (reaction (4)). It is assumed that once the lignin macromolecular radicals generate, they can initiate MMA monomer to perform the grafting polymerization (reaction (5)), by which the PMMA chains anchor onto the lignin


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(reaction (6)). Finally, the termination of these grafted chains occur by disproportionation, coupling or chain transfer between living radicals or radicals and macromolecules.7 Figure 3

Structural Characterization of Lignin-graft-Polyacrylate. FTIR spectra are usually employed to identify the structural change of lignin before and after grafting modification.17 As shown in Figure 3a, the band at near 3329 cm-1 is due to the hydroxyl groups of the AAL. The stretching bands appear at 1600, 1511 and 1451 cm-1 ascribed to aromatic rings, and the peaks at 1232 cm-1 ascribed to phenolic C-O groups. It is found that the stretching bands at 1734 and 1720 cm-1 are attributed to the acetyl groups due to the acetylation of phenolic and aliphatic hydroxyl groups in AAL.38 Importantly, two conspicuous peaks that observed at 1727 cm-1 (COO-) and 1143 cm-1 (C-O) indicate the presence of the acrylate functionality after grafting,25 confirming that the AAL-g-PBMA and AAL-g-PMMA aresuccessfully prepared. As shown in the FTIR spectrum of BBL (Figure 3b), the absorption bands locate at 3400-3500 cm-1 correspond to aromatic and aliphatic hydroxyl groups, 1600, 1511 and 1451 cm-1 correspond to aromatic rings.18 The grafted copolymers BBL-g-PEMA and BBL-g-PBZMA exhibit two new peaks at 1727 and 1141 cm−1, demonstrating the existence of carbonyl group of ester. These confirmed that the four grafted copolymers are successfully synthesized by an efficient initiator system, CaCl2-H2O2. Besides, 1H NMR is also employed to characterize the grafted copolymers. The results are showed in Figure 3c. It can be seen that the aromatic and methoxy protons signal peaks of AAL and BBL appeared at 6.0-8.0 ppm and 3.5-4.2 ppm, respectively.13 For AAL, the 1H NMR


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spectrum clearly exhibited chemical shifts at around 1.8-2.4 ppm, which is attributed to the acetyl protons signal peaks, agreeing well with the of FTIR results. After grafting modification, the vinyl protons of monomers (around 6.0-7.0 ppm) are not observed. The characteristic chemical shifts, particularly at 0.8-2.1 ppm and 3.5-5.0 ppm, respectively correspond to methyl/methylene/methyne protons (≡CH/=CH2/-CH3) and methyl/methylene protons lined to O atoms (-OCH2-/-OCH3) in the polymer backbone. It is worth noting that protons of -OCH2shifted to 5.0 ppm in BBL-g-PBZMA compared to the others due to the existence of aromatic ring. Therefore, 1H NMR and FT-IR results verifies that the ALL and BBL grafted copolymers are successfully synthesized. Figure 4 Table 4 Thermal Properties of Lignin-graft-Polyacrylate. Thermal stability of the ALL, BBL and their grafted copolymers are investigated using thermogravimetric analysis (TGA).42,43 Figure 4a shows the TGA curves for AAL, BBL and their grafted copolymers under nitrogen atmosphere. The relevant thermal decomposition data including Ti and char residue are listed in Table 4. As it can be seen, the Ti of AAL occurs at 219 °C, which was as same as to that observed in BBL. Besides, the char residue of BBL (45.2 wt. %) was slightly higher than that of AAL (41.9 wt. %) due to the higher content of aromatic ring in BBL than that of AAL, as confirmed by the higher content of carbon in BBL according to the results of Table 1. The Ti of AAL-g-PMMA and AAL-g-PBMA occur at 225 °C and 241 °C, respectively, both of which are higher than that of AAL, showing its enhanced thermal stability after grafting modification. Additionally, AAL and BBL both give higher char residues than that of their grafted copolymers. This is because the


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char residue is created mainly from the aromatic framework of lignin, which is similar to our previous report.44 DSC analysis provide important information with regard to the chains mobility of the synthesized copolymers.38 The curves are showed in Figure 4b and the corresponding results are summarized in Table 4. Normally, the Tg of lignin is usually difficult to clearly observed because of its irregular structure. Lignin shows a relatively high Tg, because the natural condensed structure and strong intermolecular hydrogen bonding interactions restrict the mobility of lignin molecules chains.19 In this case, Tg of BBL has not been clearly identified due to the heterogeneity, as reported in our previous work.25 However, it was found that AAL obviously showed an identifiable Tg approximately at 125.2 oC, due to the more homogeneous and higher weight average molar mass than BBL. Besides, after grafting modification, it can be seen that Tg of AAL-g-PMMA (110 oC), AAL-g-PBMA (32.1 oC), BBL-g-PBZMA (57.9 oC) and BBL-gPEMA (75.0 oC) are higher than those of the theoretical values of homopolymers. This is because that the incorporation of rigid lignin restrict the molecular movement of the grafted polymers chains, thus leading to higher Tgs. Similar observations was reported in the previous work.25 The DSC curves for the grafted copolymers exceptionally show one distinct Tg, indicating the successful synthesis of grafted copolymers.36 Surface Features of Lignin-graft-Polyacrylate. Water contact angle measurement is carried out in order to study the change in the hydrophobicity of lignin before and after graft modifications. The contact angles of the AAL, BBL and their grafted copolymers films are measured and the results are listed in Figure 4c. The films on glass substrates are prepared by drop-casting THF, CHCl3 or DMF solutions of these materials (AAL, AAL-g-PBMA, AAL-gPMMA, BBL, BBL-g-PEMA and BBL-g-PBZMA). The AAL film show a contact angle of 44o


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(±4), and the AAL-g-PBMA, AAL-g-PMMA are more hydrophobic with contact angles of 105o (±4) and 90o (±3), which should be due to the introduction of acrylic polymers (Figure 4c). Moreover, the larger contact angles of AAL-g-PBMA than AAL-g-PMMA are likely due to the longer alkane carbon chains of BMA. As shown in Figure 4d, the contact angle of BBL (73o (±5)) is higher than that of AAL, which is because of the lower total hydroxyl content in BBL, in good agreement with the results in Table 1. After the introduction of acrylic polymers into the backbone of BBL, the contact angle of BBL-g-PBZMA increases to 85o (±5), and that of BBL-gPEMA increased to 95o (±4), suggesting that the hydrophobicity of grafting polymers chains is mainly on the surface of the composites. Furthermore, to understand whether the grafted polymers chains cover on the AAL and BBL surfaces, X-ray photoelectron spectroscopy (XPS) analysis is used to provide usefully information with respect to the surface elemental composition and structure groups of AAL, BBL and their grafted copolymers. The corresponding results are listed and presented in Table 5. It can be seen that the experimental atom ratios of C/O of grafted copolymers were 2.8 (AAL-g-PMMA), 3.8 (AAL-g-PBMA), 2.5 (BBL-g-PEMA) and 4.0 (BBL-g-PBZMA), respectively, all of which areclose to the theoretical atom ratios of C/O of the corresponding homopolymers. For instance, the (AAL-g-PBMA) is very close to the theoretical value obtained by the homopolymer (Poly (BMA)) (4.0), confirming that the surface of Poly (BMA) chains was covered onto the BBL. It is found that the O-C=O groups content of AAL-g-PMMA (26%) and AAL-g-PBMA (9.4%) are both higher than that of AAL (2.9%). Meanwhile, the O-C=O groups content of BBL-g-PBMA (26%) also increases as compared with the AAL. However, the O-C=O groups content of BBL-g-PBZMA (2.0%) decreases relative to that of BBL (6.1%). The above results can explain why the hydrophobicity of AAL and BBL coating films increases as a result of polymer chains grafting.


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Morphology of Lignin-graft-Polyacrylate. Scanning electron microscopy (SEM) is employed to observe the changes in surface morphology after grafting modification. Figure 5 shows the images of AAL, AAL-g-PMMA, AAL-g-PBMA, BBL, BBL-g-PBZMA, and BBL-gPEMA, respectively. It can be seen that a lager difference in the microstructure of AAL and BBL, compared to the grafted AAL and grafted BBL samples. This demonstrates that the structure undergoes significant changes during the graft copolymerization process. It is clearly observed that the AAL and BBL exhibited an irregular and rough surface with different sizes of agglomerates and blocks (Figure 5a and Figure 5b), which is in good agreement with literatures.30,34 After grafting modification, the morphologies of AAL-g-PMMA (Figure 5b), AAL-g-PBMA (Figure 5c), and BBL-g-PBZMA (Figure 5e) copolymers turn relatively smooth. Besides, BBL-g-PEMA forms microsphere structure (Figure 5f). The above changes after grafting indicate that the surface is covered with the large amount of grafted polymer chains, which is consistent with the XPS results. Applications of Lignin-graft-Polyacrylate in PLA. Most plastics are hydrocarbon-based materials, which usually need addition of UV blocking agent in applications. It is reported that the lignin can provide excellent UV-blocking properties due to the presence of the basic phenylpropane units and the introduced phenolic hydroxyl groups.2, 4, 21 In addition, the grafted polymers are hydrophobic and expected to enhance the interface adhesion between lignin and PLA. Thus, all the lignin grafted copolymers are used to investigate the light barrier property of PLA with lignin grafted copolymers (Figure 6a). The color of the samples changed deep yellowish compared to the pure PLA (Figure 6a (inset)). All the PLA/lignin grafted copolymers blend films exhibited higher UV blocking function than pure PLA. As shown in Figure 6, it is clearly seen that the 85% of UV at a wavelength higher than 280 nm can pass through the pure


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PLA film. Nevertheless, the PLA blend films based on lignin grafted copolymers can nearly block 100% of UV-C (100-280 nm) and UV-B (280-315 nm) and over 80% of UV-A (315-360 nm), which is because of the good dispersion of AAL grafted copolymers and BBL grafted copolymers in PLA materials. Effects of AAL, AAL-g-PMMA, AAL-g-PBMA, BBL-g-PBZMA and BBL-g-PEMA blending on the mechanical properties of PLA are also evaluated. The tensile strength, elastic modulus and elongation at break of AAL-g-PMMA/PLA and AAL-g-PBMA/PLA, BBL-gPBZMA/PLA and BBL-g-PBZMA/PLA composites are shown in Figure 6b, 6c, and 6d. The PLA displays a tensile strength of 61.6 MPa, elastic modulus of 1.26 GPa and elongation at break of 9.2%. The addition of grafted copolymers has a limited effect on the mechanical properties of PLA. For instance, upon adding 1.0 wt % of AAL-g-PMMA, AAL-g-PBMA, BBLg-PBZMA and BBL-g-PEMA, the tensile strength of PLA blends slightly increases to 61.7 MPa, 57.0 MPa, 63.3 MPa, and 62.4 MPa, respectively as compared with 59.4 MPa for unmodified AAL. Once the loading level increases to 10%, most PLA blends shows a decrease compared with that of 1.0% loading. However, the tensile strength of PLA/AAL-g-PMMA, PLA/AAL-gPBMA, PLA/BBL-g-PBZMA and PLA/BBL-g-PEMA are much higher than that of PLA/AAL due to improved interfacial compatibility. The Elastic modulus data show a slight decrease for all PLA composites with lignin. Unexpectedly, the elongation of the blend PLA/AAL-g-PBMA is significantly increased by more than 3 times to around 28% in comparison with the 9.2% elongation of the pure PLA, and 4 times as compared with 6.7% for unmodified AAL, respectively (Figure 6d). This is most likely due to the long side groups on the PBMA leading to better toughening effect on PLA than other polyacrylates. Such mechanical improvements are


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primarily because of the reinforcement effect of modified lignin and good interfacial compatibility with the PLA matrix.45 As shown in Figure 7, SEM images clearly show largely improved interfacial adhesion and dispersion for two kinds of lignin after graft modification with acrylates.45 Lignin particles (with size of 0.1-1.0 µm for AAL, and 0.1-0.4 µm for BBL) can be easily observed within the PLA matrix, as indicated by red arrows (Figure 7a & 7d). Moreover, some small voids are found on the fracture surface of PLA/AAL (Figure 7a), which is due to the dropping off of the lignin phase during the mechanical failure because of poor interfacial interactions. For PLA/AAL-g-PMMA and PLA/AAL-g-PBMA, the size of lignin domains basically is reduced below 100 nm due to the improved interfacial bonding and dispersion (Figure 7b & 7c). Similarly, both BBL-gPBZMA and BBL-g-PEMA also show significantly smaller phase sizes within the PLA matrix because of better interfacial compatibility than the pure BBL. Therefore, the improved mechanical performances of PLA biocomposites based on the polyacrylate-graft-lignin are mainly attributed to the enhanced interfacial interactions between modified lignin and PLA. Therefore, the addition of AAL grafted copolymers or BBL graft copolymers not only improves the UV blocking capability, but also enhances the mechanical property of the pure PLA. CONCLUSIONS In this work, four kinds of AAL and BBL graft copolymers have been successfully prepared by "grafting from" free radical polymerization using calcium chloride and hydrogen peroxide (CaCl2-H2O2) as the initiation system. Four acrylate monomers can be efficiently grafted onto the backbone of AAL or BBL apart from St and the graft copolymerization is dominant without homopolymerization. MMA can also be chemically grafted onto AAL in the absence of phenolic and aliphatic hydroxyl groups. Lignin plays an important role in this complex initiator system


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(CaCl2-H2O2). No conversion of MMA was observed if CaCl2 or H2O2 is used individually. The Tg of graft copolymer is higher than that of the homopolymer of each monomer due to the incorporation of the rigid lignin. Meanwhile, the presence of polymer chains increases the thermal stability and hydrophobicity of two kinds of industrial lignin. The lignin-graftpolyacrylate copolymers enable PLA to show improved UV-blocking capability and mechanical performances only at very low loading levels. This work provides a new strategy for comprehensive utilization of industrial lignin as functional fillers for biopolymers like PLA. Supporting Information Effects of polymerization time on conversion curves for AAL-g-PMMA, BBL-g-PEMA, AALg-PBMA, and BBL-g-PBZMA. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. * E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Jiangsu Key Lab. of Biomass Energy and Material Foundation of China (Grant No. JSBEM201703); the Postdoctoral Science Foundation of China (Grant No.


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2017M612000); the Natural Science Foundation of Zhejiang Province (LQ18B070001); the Scientific Research Foundation of Zhejiang A & F University (Grant No. 2055210012); the National Natural Science Foundation of China (Grant No. 51303162 and 51628302); and Australia Research Council (ARC) Industrial Transformation Training Centre (Grant No. IC170100032). REFERENCES (1) Tekin, K.; Karagöz, S.; Bektaş, S. A review of hydrothermal biomass processing. Renew. Sust. Energ. Rev. 2014, 40, 673-687. DOI: 10.1016/j.rser.2014.07216. (2) Jairam, S.; Bucklin, R.; Correll, M.; Sakthivel, T. S.; Seal, S.; Truett, J.; Tong, Z. UV resistance of polystyrene co-butyl acrylate (PSBA) encapsulated lignin–saponite nanohybrid composite film. Mater. Design 2016, 90, 151-156. DOI: 10.1016/j.matdes.2015.10.118. (3) Thakur, V. K.; Thakur, M. K.; Raghavan, P.; Kessler, M. R. Progress in Green Polymer Composites from Lignin for Multifunctional Applications: A Review. ACS Sustainable Chem. Eng. 2014, 2, 1072-1092. DOI: 10.1021/sc500087z. (4) Ren, W.; Pan, X.; Wang, G.; Cheng, W.; Liu, Y. Dodecylated lignin-g-PLA for effective toughening of PLA. Green Chem. 2016, 18, 5008-5014. DOI: 10.1039/c6gc01341d. (5) Li, H.; McDonald, A. G. Fractionation and characterization of industrial lignins. Ind. Crop. Prod. 2014, 62, 67-76. DOI: 10.1016/j.indcrop.2014.08.013. (6) Zeng, Y.; Zhao, S.; Yang, S.; Ding, S.-Y. Lignin plays a negative role in the biochemical process for producing lignocellulosic biofuels. Curr. Opin. Biotech. 2014, 27, 38-45. DOI: 10.1016/j.copbio.2013.09.008.


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Caption of Figures Scheme 1. Illustration of the representation for the grafting copolymerization of lignin with acrylic monomers and their preliminary applications in PLA. Figure 1. a) Effects of solvents and b) cationic species on monomer conversion in the grafting polymerization reaction, GPC traces of c) AAL-g-PMMA, AAL-g-PBMA, BBL-g-PEMA and BBL-g-PBZMA and d) AAL-g-Poly (styrene). Figure 2. Representation for the proposed initiation mechanism for grafting MMA onto lignin. Figure 3. FTIR spectra of a) the pristine AAL, AAL-g-PMMA and AAL-g-PBMA, and b) pristine BBL, BBL-g-PEMA and BBL-g-PBZMA, and c) their corresponding 1H NMR spectra. Figure 4. a) TGA and b) DSC curves of AAL, AAL-g-PMMA, AAL-g-PBMA, BBL, BBL-gPEMA and BBL-g-PBZMA, water contact angles of c) AAL, AAL-g-PMMA, and AAL-gPBMA, and d) BBL, BBL-g-PEMA and BBL-g-PBZMA. Figure 5. SEM images for a) AAL, b) AAL-g-PMMA, c) AAL-g-PBMA, d) BBL, e) BBL-gPBZMA, and f) BBL-g-PEMA. Figure 6. a) UV-visible transmittance spectra for 1) PLA and its biocomposites based on ligningraft-polyacrylate: 2) AAL-g-PBMA, 3) AAL-g-PMMA, 4) BBL-g-PEMA and 5) BBL-gPBZMA. Inserted are digital photos of corresponding PLA and its biocomposites. b) Tensile strength, c) elastic modulus, and d) elongation at break for 1) PLA and its biocomposites based on lignin-graft-polyacrylate: 2) PLA/AAL, 3) AAL-g-PBMA, 4) AAL-g-PMMA, 5) BBL-gPEMA and 6) BBL-g-PBZMA.


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Figure 7 SEM images for the fracture surface a) PLA/AAL, b) PLA/AAL-g-PBMA, c) PLA/ AAL-g-PMMA, d) PLA/BBL, e) PLA/BBL-g-PEMA, and f) PLA/BBL-g-PBZMA after rapid tensile measurements. The loading level of lignin or modified lignin in PLA for all samples is 10%.


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Scheme 1

Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Figure 7


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Table 1. Physicochemical characteristics of two kinds of industrial lignin wastes. Parameter (%)

AAL

BBL

C (wt%)

58.37

65.16

H (wt%)

1.54

3.99

O (wt%)

39.35

28.94

N (wt%)

0.74

0.96

S (wt%)

0

0.95

89.6

92.5

1.2

1.5

a

Total lignin (wt%)

a

Ash (wt%)

a

-1

Total-OH (mmol g )

7.17

5.62

a

Phenolic-OH (mmol g-1)

4.57

2.95

Mw (g mol-1)

2353

876

PDI

1.92

1.60

a

The data was from the literature.32


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Table 2. Reaction conditions and experimental results for the preparation of AAL and BBL grafted polyacrylate copolymers

Run AAL-g-PMMA1 AAL-g-PMMA2 AAL-g-PMMA3 AAL-g-PMMA4 Poly(MMA) a Lignin-g-VAc b Lignin-g-PSt AAL-g-PBMA1 AAL-g-PBMA2 AAL-g-PBMA3 AAL-g-PBMA4 Poly(BMA) BBL-gPBZMA1 BBL-gPBZMA2 BBL-gPBZMA3 BBL-gPBZMA4 Poly(BZMA) BBL-g-PEMA1 BBL-g-PEMA2 BBL-g-PEMA3 BBL-g-PEMA4 Poly(EMA)

88.1 86.9 gel 86.5 0 60 (grafting) 59.5 (grafting) 90.9 87.2 93.2 90.1 0

Mw (g mol1 ) 36594 24867 42178 46565 29156 47721 41675 -

Mn (g mol1 ) 14543 6102 18053 27934 19295 27535 24658 -

2.51 4.08 2.34 1.667 1.511 1.733 1.690 -

12

95.4

21214

10781

1.968

1.0

12

92.1

13189

6522

2.022

8

0.5

12

96.4

30862

15040

2.052

8

1.0

6

90.6

32855

16570

1.983

8 8 4 8 8 8

1.0 1.0 1.0 0.5 1.0 1.0

12 12 12 12 6 12

0 88.1 87.2 93.9 88.0 0

48972 36594 54389 45674 -

25430 22890 31234 24567 -

1.93 1.60 1.74 1.86 -

Monomer (g)

CaCl2 (g)

t (h)

Conversion (%)

8 4 8 8 8 5.58 2.083 8 4 8 8 8

1.0 1.0 0.5 1.0 1.0 1.0 1.0 0.5 1.0 1.0

12 12 12 6 12 48 12 12 12 6 12

8

1.0

4

a

Lignin-g-VAc is the graft copolymer of lignin with vinyl acetate. 7

b

Lignin-g-PSt is the graft copolymer of dealkaline lignin with styrene. 30


PDI

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Table 3. Effects of AAL, CaCl2 and H2O2 content on conversion of monomer in the grafting reaction. Run AAL MMA (g) CaCl2 (g) H2O2 (mL) Conversion (%) 1

a

1.0

8

1.0

0

90.2

2

0

b

8

1.0

1.0

19

3

1.0

b

8

1.0

1.0

72.5

4

0

8

1.0

1.0

0

5

0.4

8

1.0

1.0

15

6

0.8

8

1.0

1.0

86.5

7

1.0

8

1.0

1.0

88.1

8

1.0

8

0

1.0

0

9

1.0

8

1.0

0

0

a

Using acetylated AAL instead of AAL

b

The monomer is styrene (St).

Table 4. Thermal degradation parameters and glass transition temperatures (Tg) of AAL and BBL as well as their graft copolymers. a

Samples

a b

Ti (oC) Tg (oC)

b

Char residue (wt.%)

AAL

219

125.2

41.5

AAL-g-PMMA

225

110

13.4

AAL-g-PBMA

241

32.1

9.8

BBL

219

-

45.2

BBL-g-PEMA

253

75.0

11.3

BBL-g-PBZMA

235

57.9

9.4

Ti refers to temperature at which the 5 wt.% of weight loss occurs. The char residue refers to the char obtained at 800 °C.


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Table 5. XPS peak assignments and atom compositions for the surface of AAL, BBL and their grafted copolymers

a

No.

Element composition (%)

Assignment

C

O

C/O

AAL

74 25

3.0

-

42

56

2.9

AAL-g-PMMA

74 26

2.8

2.5

58

16

26

AAL-g-PBMA

79 21

3.8

4.0

78

12

9.4

BBL

78 22

3.5

-

71

23

6.1

BBL-g-PEMA

71 29

2.5

3.0

70

20

10

BBL-g-PBZMA 80 20

4.0

5.5

87

11

2.0

a

Chemical groups (%)

C/Ohomo C-C,C-H C-O O-C=O

C/Ohomo is determined from the atom ratio of C/O of each monomer.


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For Table of Contents Use Only

Acrylic monomers have been grafted onto industrial lignin initiated by CaCl2-H2O2 to prepare modified lignin as green fillers for polylactide (PLA).


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Graft Polymerization of Acrylic Monomers onto ... - ACS Publications

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