Article pubs.acs.org/Macromolecules
Self-Healing Properties of Lignin-Containing Nanocomposite: Synthesis of Lignin-graf t-poly(5-acetylaminopentyl acrylate) via RAFT and Click Chemistry Hailing Liu and Hoyong Chung* Department of Chemical and Biomedical Engineering, Florida State University, Tallahassee, Florida 32310, United States
ABSTRACT: Lignin can be an important source of synthetic commodity materials owing to its abundance in nature and low production cost. The current use of lignin as a raw material, however, is very limited and focused only on cheap and poorly defined nonfunctional materials. Herein we report a new lignin-containing functional polymer, lignin-graf t-poly(5acetylaminopentyl acrylate) (lignin-graf t-PAA), which has been prepared by the covalent linkage of chemically modified lignin with PAA, which is an end-group functionalized polymer. This work makes two significant advances in the study of lignincontaining polymers: (1) lignin-graf t-PAA is the first example of lignin being modified by a polymer with sophisticated structure, and (2) lignin-graf t-PAA shows a special performance, autonomic self-healing properties, which have not yet been seen in lignincontaining polymers. The key synthetic step in this process utilizes a copper-catalyzed azide−alkyne cycloaddition or “click” reaction in order to join together the lignin and polymer moieties. The polymer, PAA, was itself prepared via reversible addition− fragmentation chain transfer (RAFT) polymerization of monomers containing multiple hydrogen-bonding sites on their pendants in the form of acetylamino functional groups. The selected RAFT agents also resulted in a polymer with an azide group at its terminus, which is necessary for the desired click reaction. Separately, biomass lignin was chemically modified by 5-hexynoic acid to introduce an alkyne functionality onto lignin. The azide terminus of the polymer was joined to the alkyne group of lignin to form a covalent bond. The lignin-graf t-PAA possesses a well-dispersed multiphase nanostructure, a rigid lignin phase and a soft-PAA phase, which has a rubber-like flexibility. The mechanical properties of the newly synthesized lignin-graf t-PAA can be readily controlled by the mass ratio of lignin and polymer during synthesis. In this study, the mass ratio was varied by either polymer length or weight percentage of lignin. It was revealed that a lignin-graf t-PAA composite with 15−20 wt % lignin demonstrated the most optimal rubber-like, flexible property. Thanks to the high degree of hydrogen bonding from the acetylamino functionalities, lignin-graf t-PAA also showed self-healing properties, recovering up to 93% of its original maximum stress before fracture.
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INTRODUCTION
dimensional structure with many functionalities and covalent linkages. For example, some of the most important C−O linkages in lignin are β-O-4, α-O-4, and 4-O-5, and C−C links are β-5, 5-5, β-1, and β-β linkage.2 Other common functional groups found in lignin include methoxy, phenolic hydroxyl, aliphatic hydroxyl, and other carbonyl groups. The unique blends of functional groups combined with a networked structure, diverse set of chemical linkages, and generally poor
Lignin is the second most abundant plant-based biopolymer in the world after cellulose and is therefore a sustainable raw material.1−3 Because of its physical strength and durability, lignin is often found as a structural component of cell walls of many plants, allowing them to grow tall as well as protecting them from external attacks from microorganisms and insects. Additionally, lignin also comprises one of the main structural components of the vascular system of many plants, where its hydrophobic nature aids in the transport of water. Chemically, lignin demonstrates a random dendritic network that is composed of phenylpropane groups creating a complex three© XXXX American Chemical Society
Received: May 16, 2016 Revised: September 6, 2016
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DOI: 10.1021/acs.macromol.6b01028 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Synthesis of Lignin-graf t-poly(5-acetylaminopentyl acrylate) (PAA): (a) Schematic Diagram of the Structure of Lignin-graf t-PAA Showing Three Important Components of the Lignin−Polymer Nanocomposite;a (b) Synthesis of Monomer, 5-Acetylaminopentyl Acrylate; (c) Polymerization of Monomer via Reversible Addition−Fragmentation Chain Transfer (RAFT); (d) Alkynylation of Natural Lignin;b (e) Copper-Catalyzed Click Reaction of Polymer and Alkynylated Ligninc
a
Although lignin is not a spherical shape, the spherical shape presents complex chemical structure of lignin. bHydroxyl groups at lignin were functionalized with 5-hexynoic acid to have alkyne groups on lignin. cThe spherical symbol of lignin is an simplified structure of natural lignin, an actual lignin chemical structure is shown in (a). The 5-hexyonic acid reacts to both aliphatic hydroxyl groups and phenolic hydroxyl groups of lignin.
solubility in organic solvents means that lignin and its derivatives are notoriously difficult to isolate and characterize. Advanced applications of lignin necessitate modification of the chemical structure, which is a complex task due to the generally inert and complex nature of lignin. There are primarily two actively studied lignin modification methods: catalytic cleavage of lignin4,5 and polymeric modification of lignin.3,6 The catalytic cleavage of the chemical structure of lignin is important because lignin contains a large amount of aromatic groups that can be used as a substitute for petroleumbased aromatic fine chemicals. While research into the catalytic cleavage of lignin has seen progress recently, these works mostly focus on model compounds rather than raw lignin. By comparison, polymeric modification of lignin can produce practical and useful materials. The first method of polymeric
modification involves the integration of lignin and petroleumbased polymers via covalent bonds. The second method involves the physical mixing (blending) of petroleum-based polymers and raw lignin with heat and pressure. The physical mixing method is convenient for production but suffers from limited control and reproducibility. Most importantly, it is hard to expect advanced properties from the blended lignin polymers due to phase separation and limited compatibilities of polymer/lignin combinations.3,6 Typical petroleum-based polymers for the polymeric modification of lignin are polyurethane, phenol formaldehyde, phenol epoxy, polyester, polyolefin, starch, poly(ethylene oxide), poly(vinyl chloride), poly(vinyl acetate), polystyrene, poly(vinyl alcohol), and poly(acrylamide). As a specific example, lignin-based polyurethane can be synthesized by B
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Figure 1. 1H NMR spectra of (a) poly(5-acetylaminopentyl acrylate), (b) 5-acetylaminopentyl acrylate, and (c) 5-acetamido-1-pentanol. All samples were analyzed in DMSO-d6.
lifetime of the polymers in question while reducing the detrimental effect of impacts from surrounding environments, including mechanical and thermal stress.28,29 In particular, autonomic healing polymers have advantages over other polymer healing methods because the healing process requires neither severe external energy nor external chemical healing agents.30 Guan et al. recently reported a new autonomic healing polymer which does not require any external stimulus to trigger the polymer healing processa phenomenon resulting from the materials microstructure. The microstructure of this polymer contains two phases: a rigid phase of polystyrene and a “soft” phase from acetylamino polymer. The combination of these two phases yielded a microphasial dispersion of hard/ glassy domains in a soft rubbery matrix. The soft polymer phase provided a strong hydrogen-bonding environment which induced autonomic self-healing. Guan et al.’s novel polymer has two important features: a multiphasial microstructure and a strong hydrogen-bonding environment created by the polymer.31 These two features were leveraged herein for the presented lignin-containing nanocomposite research. The high density of aromatic groups of lignin, as shown in the chemical structure of Scheme 1a, affords the same kind of hard properties as previous work involving polystyrene and should function in a similar manner. In this context, lignin can be an excellent example of the “hard” moiety of these multiphase nanocomposites. The polymers that comprise the “soft” matrix can be precisely prepared by common synthetic procedures to yield diverse properties. The covalent integration of a lignin with well-defined polymers has the potential to open a new class of advanced functional materials, owing to the flexibility in design and synthesis of the polymer. These lignin-containing polymers possess structural features which offer a novel types of nanocomposite that display highly regular microstructures and diverse enhanced bulk materials.32,33 Herein the polymer nanocomposite has a nanometer scale (10−9 m) reinforcing
urethane linkage between the hydroxyl groups of lignin and petroleum-based diisocyanates.7−15 Here, the role of lignin is as a source of polyols in conventional polyurethane synthesis. The lignin-based polyurethane is an example of a step-growth polymerization method to integrate lignin and petroleum-based polymers. The development of synthetic integration methods between petroleum-based vinyl polymers and lignin is crucial from a commercial standpoint. These vinyl polymers are a group of polymers synthesized from vinyl monomers via chain-growth polymerizations. Recently reported graft-from copolymerization (in graft-from copolymerization, side chains are polymerized from initiator sites which are located on a polymer backbone) using controlled/living radical polymerization (CRP)10,11 has gained attention as a useful polymeric modification method16−19 because the CRP can readily and precisely control molecular weight of vinyl polymers while possessing a large polar functional groups tolerance toward the vinyl monomers.20,21 The second lignin−vinyl polymer integration method is graft-onto copolymerization.16,22,23 This method has several advantages stemming from the use of experimentally simple click chemistry, resulting in high yields an extended scope, the freedom to use easily removable solvents, and byproducts which can be easily removed without complex purification methods.22,24,25 In the graft-onto copolymerization method, synthetic polymers and chemically modified lignin are first prepared separately. Next, a chemically active polymer and lignin, both possessing chemically active sites, can be covalently linked. Both graft-from and -onto copolymerization methods are very useful to produce chemically well-defined lignin-based polymers. The polymeric modification of lignin is welldiscussed in recently published review papers.26,27 Self-healing polymers have the capability to heal damage on bulk structures via an external stimulus or spontaneous healing (autonomic healing). The self-healing polymer gains significant attention because it can substantially increase the durability and C
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Macromolecules fillers in a matrix of polymers. The polymer nanocomposite presents combined synergic properties from two different materials.34,35 As an initial example of advanced functional lignin/polymer-based materials, this work presents a lignincontaining autonomic self-healing polymer that contains hydrogen bond inducing acetylamino groups as polymer pendants. This work discusses the synthesis and characterization of the new lignin-containing polymer as well as investigations into the self-healing and mechanical properties of the resulting material.
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progress of polymerization was monitored by characterizing aliquots at 0 min, 30 min, 1 h 30 min, 3 h, and 4 h. The final conversion was 89% at 4 h of polymerization. The synthesized polymer was recovered by precipitation in cold ether. The crude solid polymer was further purified by dissolving in methanol and precipitate in cold ether three times. The obtained polymer was dried under vacuum overnight at ambient temperature. Finally, the obtained polymer was a yellow sticky solid. The chemical structure of poly(5-acetylaminopentyl acrylate) (PAA) was determined by 1H NMR in DMSO-d6 as shown in Figure 1. The 1H NMR of PAA was demonstrated together with 1H NMR spectra of a monomer, 5-acetamido-1-pentanol, and a monomer precursor, 5-acetamido-1-pentanol. Alkyne Functionalization of Lignin. Lignin was purchased from TCI America (Product number L0045, softwood lignin) and used after repeated washing with aqueous 2 M hydogen chloride solution.37 Two grams of lignin, 5-hexynoic acid (2 mL, 0.018 mmol), and N,N′dicyclohexylcarbodiimide (3.8 mg, 0.018 mmol) were added into 50 mL of DMF. Separately prepared 4-(dimethylamino)pyridine (0.28 g, 0.002 mmol) in 4 mL of DMF solution was then slowly added to the reaction mixture. The dark reaction mixture was stirred at room temperature for 48 h followed by vacuum filteration to remove formed white solids. The residue dark solution was condensed by rotovap and then precipitated in aqueous HCl solution (pH = 1). The yielded slurry-like solution was vacuum filtered to collect a brown solid. The obtained solid product was dried in a vacuum oven for 3 h. The product was further purified by dissolving it in DCM and precipitating from hexane. The product was isolated by vacuum filteration and then dried in vacuo overnight. Finally, obtained alkyne functionalized lignin product was a brown powder. The chemical structure was determined by 1H NMR in DMSO-d6 as shown in Figure 2 compared with pristine lignin. Another 1H NMR of 5-hexynoic acid is presented in Figure 2 to confirm the introduction of alkyne group on the lignin.
EXPERIMENTAL SECTION
Materials. All chemicals were purchased from Sigma-Aldrich Ltd. and TCI America and used without further purification unless otherwise stated. All organic solvents were degassed with bubbling dry nitrogen gas in the presence of molecular sieves. Column chromatography was performed with silica gel 60, 215−400 mesh from Alfa Aesar. Thin layer chromatography (TLC) was carried out on glass silica plates obtained from Agela Technologies. Instrumentation. Nuclear magnetic resonance experiments (1H and 13C) were carried out on a Bruker Avance 400 MHz spectrometer. All spectra were internally referenced to tetramethylsilane or to residual solvent signals. Polymer molecular weights and dispersities were determined on an Agilent−Wyatt combination gel permeation chromatography (GPC) instrument containing three successive Agilent PLgel Mixed C columns, an Agilent 1260 infinity series pump, degasser, and autosampler. The Wyatt detection unit hosts a Dawn Heleos 8+8-angle light scattering detector and Optilab TrEX refractive index detector. The FT-IR spectra were obtained from a PerkinElmer spectrum 100 FT-IR spectrometer. Synthesis of 5-Acetamido-1-pentanol. Acetic anhydride (3 mL, 32 mmol) was added in 60 mL of ethyl acetate solution containing 5amino-1-pentanol (3 g, 29.1 mmol) and stirred for 2 h in a nitrogen atmosphere and room temperature. After stirring, the solution was vacuum-filtered to remove solid particles.36 The resulting colorless transparent solution was concentrated by a rotary evaporator. The synthesized 5-acetanido-1-pentanol was colorless transparent, and the yield was 99%. The prepared 5-acetanido-1-pentanol was determined by 1H NMR in deuterated dimethyl sulfoxide (DMSO-d6) as shown in Figure 1. Synthesis of 5-Acetylaminopentyl Acrylate Monomer. 5Acetamido-1-pentanol (4.8 mg, 33.1 mmol) and acrylic acid (3.41 mL, 49.65 mmol) were added into 90 mL of dry dichloromethane (DCM), followed by mixing N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (10.47 mg, 1.65 eq. 54.62 mmol) and N,N-diisopropylethylamine (9.51 mL, 1.65 eq. 54.62 mmol). After stirring at room temperature under a nitrogen atmosphere for 24 h, another 90 mL of DCM was added. The diluted solution was washed with 1 M of aqueous sodium hydroxide solution, 1 M of aqueous hydrochloride solution, saturated aqueous sodium bicarbonate solution, and brine sequentially. The yielded yellow organic layer was dried over MgSO4 and concentrated by a rotary evaporator. The obtain product was a pale yellow viscous liquid, and the yield was 82%. The prepared monomer, 5-acetylaminopentyl acrylate, was determined by 1H NMR in deuterated dimethyl sulfoxide (DMSO-d6) as shown in Figure 1. Polymerization of 5-Acetylaminopentyl Acrylate via RAFT. A radical initiator, 2,2′-azobis(2-methylpropionitrile) (AIBN), was recrystallized over methanol prior to polymerization. For RAFT, AIBN (4.1 mg, 0.025 mmol) and stir bar were added in a Schlenk flask and sealed followed by evacuation and nitrogen refill three times. Then 5-acetylaminopentyl acrylate monomer (2 mg, 10 mmol), 2(dodecylthiocarbonothioylthio)-2-methylpropionic acid 3-azido-1propanol ester, RAFT agent (42.3 μL, 0.1 mmol), 7 mL of N,Ndimethylformamide (DMF), and 1 μL of 2,3,4,5,6-pentafluorobenzaldehyde (internal standard for NMR characterization) are added to an AIBN containing Schlenk flask. The reaction mixture was degassed with dry nitrogen for 20 min prior to heating at 75 °C. A
Figure 2. 1H NMR spectra of (a) prestine lignin and (b) 5-hexynoic acid functionalized lignin. Proton appearance shows the successful functionalization of lignin with alkyne; all samples were analyzed in DMSO-d6. Click Reaction, Copper-Catalyzed Alkyne−Azide Cycloaddition, between Alkyne Functionalized Lignin and PAA (Synthesis of 10 wt % Lignin Containing Lignin-graf t-PAA). In a Schlenk flask, all solid reagents, 0.1 g of alkyne functionalized lignin, copper(I) bromide (4.7 mg, 0.033 mmol), and stir bar are placed and sealed. Next, the flask was evacuated and backfill with dry nitrogen repeatly for three times. Other liquid reagents, 0.9 g of PAA, 15 mL of degassed DMF, and 6.5 μL of N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), are separately degassed with dry nitrogen gas, and then they are added to a reaction vessel. After overnight stirring at room temperature, the reaction solution was exposed to air to stop the reaction. The resulting soulution was filtered through neutral alumina column to remove copper. The filtrate was concentrated and D
DOI: 10.1021/acs.macromol.6b01028 Macromolecules XXXX, XXX, XXX−XXX
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precipitated in cold ethyl acetate. The obtained solid product was dried for 2 days at 70 °C in a vacuum oven. The click reaction, coppercatalyzed alkyne−azide cycloaddition, was ensured by monitoring zide as shown in FT-IR spectra (Figure 3).
Article
RESULTS AND DISCUSSION Overview of Lignin-graf t-Synthetic Polymer Nanocomposite Synthesis. An illustrated example of the newly synthesized lignin-containing polymer is demonstrated in Scheme 1a. The nanocomposite structure has three components: lignin, functional polymer arm, and a covalent linkage between them. We envisioned the synthesis of this lignin/ polymer composite to require three general steps. The first step is the modification of lignin (Scheme 1a,d) via carbodiimidemediated esterification to yield a lignin derivative with terminal alkyne moieties. The second concerns the synthesis of functional polymer (Scheme 1a−c), which was accomplished via RAFT polymerization to yield polymers with acetylamino pendant groups which will provide the desired autonomic healing properties in the final lignin-containing polymer nanocomposites. The third and final step is to unite the two separate fragments into a single polymer, which is easily accomplished by copper-catalyzed azide/alkyne cycloaddition or “click” chemistry between the alkynes of lignin and the azide end groups of the polymers, which result from the chosen RAFT agent (Scheme 1a,e). It is our hypothesis that the highly modular nature of this general scheme will lead to a variety of functional materials with advanced properties that can be easily prepared. Finally, this report investigates the structure/function relationships of the desired nanocomposites by systematically varying the mass ratio of lignin/polymer, polymer length, and the polymer density on lignin in order to gain a better understanding of bulk properties of our system. Synthesis of Lignin-graf t-poly(5-acetylaminopentyl acrylate) (Lignin-graf t-PAA). The syntheses of the desired monomer, 5-acetylaminopentyl acrylate, and the desired polymer, poly(5-acetylaminopentyl acrylate) (PAA), are depicted in Scheme 1b,c. The secondary amide of the acetylamino moiety of the polymer is known to generate strong and reversible hydrogen bonding, and it is this hydrogen bonding which serve as the driving force behind the self-healing properties of these polymers.31,39 The pentyl group of the monomer separates the acetylamino functionality from the vinyl group of the acrylate, resulting in a “soft” polymer with a low glass transition temperature. It is this property which results in the formation of a soft domain in the final, lignincontaining, multiphase polymer matrix, while the lignin serves as a hard domain. The desired monomer was prepared by a two-step reaction sequence as shown in Scheme 1b. Beginning from 5-acetamido1-pentanol, the primary amine was acrylated. The presence and integration of a characteristic acetyl group signal at 1.92 ppm in the 1H NMR spectrum (Figure 1c) were used to confirm the presence of acetylamino moiety in the eventual monomer. Carbodiimide-mediated esterification of the resulting secondary amide produced the desired 5-acetylaminopentyl acrylate (Figure 1b), which shows characteristic methylene peak at 4.10 ppm. The prepared 5-acetylaminopentyl acrylate monomer was polymerized via RAFT, using a RAFT reagent which terminates in an azido group (Scheme 1c). Although ATRP of 5acetylaminopentyl acrylate has been reported,31,39 the RAFT polymerization of 5-acetylaminopentyl acrylate has not been demonstrated. In RAFT, polymer chain length (and thus the molecular weight of polymer) can be conveniently controlled by the ratio of monomer to RAFT agent.40,41 Entry 1 in Table 1, for example, utilizes a 20/1 molar ratio of monomer to RAFT
Figure 3. FT-IR spectra of (a) azide terminal PAA; the azide appears at 2110 cm−1, (b) lignin-graft-PAA showing that the azide is consumed during the click reaction, and (c) lignin. The spectra in (a) and (b) were obtained from DP 20 (Table 1, entry 1) of PAA. Transmission Electron Microscopy (TEM). All TEM images were obtained using a JEM-ARM200CF (JEOL Ltd., Tokyo, Japan) instrument. A TEM grid (PELLA 01829) was purchased from TED PELLA, INC. Before sampling, Formvar on TEM grid was removed to get clean images. The TEM grid was washed prior to polymer sampling. Briefly, the grid was immerged into chloroform with facing a darker side down. After 10 s, the grid was removed from the chloroform followed by drying in the ambient air. The grid was further dried in the 80 °C oven for an additional 30 min. Next, polymer sample was prepared in pyridine with 1 mg/mL concentration for sampling on a TEM grid. The prepared polymer solution in pyridine was dropped on a dark side of the grid. Note that only one drop of polymer solution was enough for imaging. The polymer sampled Grid was dried at ambient temperature and further dried in an 80 °C oven for 30 min. The TEM imaging of the prepared lignin-graf t-PAA samples did not require further staining because the original color of lignin is dark, which can be shown in the TEM image. Tensile Test. Mechanical properties were measured by a Shimadzu tensile-compression tester (model: EZ-LX) equipped with a force transducer (Interface Ltd. model: SM-200 N-168). Typically, a synthesized rectangular shape of polymer sample was clamped at one end and pulled at a constant rate (100 mm/min) of elongation at the other clamped end. All tests were performed at room temperature, 22 °C. Each measurement were repeated at least three times, yielding average and standard deviation. Young’s modulus was determined from the slope at the initial range of the stress−strain plot following Hookean behavior. For ductile polymers, the Hookean behavior occurs before 1% of strain in general.38 The measured maximum tensile strength was the maximum stress value of the obtained stress− strain plot. Energy-to-break was determined from the area under stress−strain plot. Sample Damage and Healing Process. Specimens were cut into two separate pieces by a razor blade and then brought back into contact with cutting face gently for 10 s. The whole self-healing occurred at room temperature without heat, light stimulants, and addition of solvent and/or any additives. The tensile tests were conducted the same as the previously mentioned method to assess healed mechanical properties. E
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peaks is the result of several factors, including the wide distribution of molecular weight from natural lginin molecules, the random polymeric nature of the substance itself with mixtures of both phenolic and aliphatic hydroxyl groups, and because this reaction represents only partial conversion of the hydroxyl groups of lignin to hexynoic esters. This type of specturm is to be expected based upon literature precedents,7,16,17 and clearly shows the presence of terminal alkynes on the lignin structure. The complicated nature of these spectra does, however, necessitiate that other spectroscopic characterization and property measurements should be performed comprehensively in order to precisely characterize the synthesized lignin-containing polymeric materials. In this context, as a part of comprehensive analysis of lignin-graf tPAA, FT-IR spectroscopy, transmission electron microscopy (TEM), and mechenical property test results will be discussed in the latter part of this report. The PAA polymer and lignin were then joined by triazole formation in a copper(I)-catalyzed azide−alkyne cycloaddition reaction as depicted in Scheme 1e, with the reaction progress monitored by FT-IR, as shown in Figure 3. An absorption band of typical azide appears at 2110 cm−1 for the polymer before the reaction (Figure 3a). The signal disappeared during the course of the reaction (Figure 3b), showing full conversion of azido-terminated polymer. Generally an azide signal is somewhat more intense and in this case is likely weaker due to its terminal position in the polymer; however, this stretching frequency has been used in similar situation in the past for the identification of azide moieties in chemical compounds.43−45 The results obtained by both NMR and FT-IR indicates that the general “graft onto” method is viable for integrating separately prepared lignin and polymer components. More importantly, this method could prove useful for the grafting of any number of petroleum-based or advanced polymers onto natural lignin to produce materials with a variety of properties. Properties of Lignin-graft-PAA. The “graft onto” method just described allows for precise control and tuning of the mechanical properties of the resulting nanocomposite. The polymer chain length is easily controlled for modification of the dynamic hydrogen bonding properties of the soft domain, and the hard domain is tuned simply be altering the weight fraction of lignin in the final material. Other important properties, such as viscoelasticity and the thermal properties, of lignincontaining polymers were recently studied by Epps et al.46 For accurate structure−solid property studies of lignin-graf tPAA, two factors were varied: the polymer chain length (Table 1) and lignin weight fractions, adjusted at 10%, 15%, 20%, and 25%. As a note on nomenclature, please note that these values represents the weight percentage of lignin in lignin-graf t-PAA, not the mole ratios. For example, 100 g of 15 wt % lignin-graf tPAA would contain 15 g of lignin and 85 g of PAA polymer. In previous work, it has been shown that the available modified functional groups, alkynes, on lignin are sufficient enough to integrate a very large amount of azide terminal polymers.16 Specifically, the lignin typically has 4.48 × 10−3 mmol of chemically available sites (functionalized hydroxyl groups, alkyne) per milligram of material. The concentration of functional group was characterized by 1H NMR with using internal standard 2,3,4,5,6-pentafluorobenzaldehyde (PFB) which appears at 10.10 ppm, which is not shown in Figure 2. These experiments revealed a concentration of alkyne functionalities on lignin roughly corresponds to 112 chemically available sites per lignin assuming a lignin molecular weight of
Table 1. List of Prepared Azido-Terminal Poly(5acetylaminopentyl acrylate) entry 1 2 3 4 5
[M]/ [RAFT]/[I] 20/1/0.25 100/1/0.25 200/1/0.25 300/1/0.25 500/1/0.25
final conv (%) (1H NMR)
NMR Mna (1H NMR, g/mol)
MALS Mn (MALS, g/mol)
PDI
85 86 84 88 81
× × × × ×
× × × × ×
1.08 1.29 1.33 1.37 1.49
6.12 2.43 3.15 3.54 4.55
3
10 104 104 104 104
7.57 3.74 4.05 3.84 4.81
3
10 104 104 104 104
a NMR molecular weight was calculated based on monomer conversions determined by 1H NMR.
agent, by concentration. Thus, the target degree of polymerization (DP) of entry 1 is 20. The molecular weights of the prepared polymers were determined by two methods: 1H NMR and GPC-MALS. Conversion of monomer, 5-acetylaminopentyl acrylate, was measured by the disappearance of the vinyl peak at 5.8, 6.1, and 6.4 ppm. The conversion presented in Table 1 is the final conversion measured as the polymerization stops. As displayed in Table 1, the RAFT polymerization of 5-acetylaminopentyl acrylate demonstrated high control over a diverse range of polymer molecular weights, with an average conversion of 85% and molecular weight distributions, represented as a polydispersity index (PDI), generally at or below 1.3. The PDI value is seen to rise above 1.3 in entries 4 and 5 to 1.37 and 1.49, respectively, which is presumably due to of high concentrations of monomers in the reaction mixture relative to RAFT agent which promotes biradical termination event of the propagating polymer chains during polymerization.41,42 The prepared polymer was characterized by 1H NMR. The lack of vinyl peaks at 5.8, 6.1, and 6.4 ppm in Figure 1a indicates that the 5-acetylaminopentyl acrylate monomer had been fully consumed during the course of the reaction. The integration of terminal peaks and the methyl groups of the aliphatic chain next to the trithiocarbonate moiety at 0.86 ppm (Figure 1a, inset) were used to determine NMR numberaverage molecular weights. The number-average molecular weight, Mn, based on monomer conversions determined by 1H NMR of prepared polymers is displayed in Table 1. Several different lengths of PAA were prepared in this manner, with low PDI values. As a result of the RAFT agent, each polymer also contains a terminal azido group which will be used later to form triazole linkages to alkynylated lignin via copper-catalyzed click reactions. The lignin coupling partner for our polymers was created by the functionalization of a portion of the natural hydroxyl groups already present in the lignin structure so as to introduce alkynes, as shown in Scheme 1d. This was accomplished simply by carbodiimide-mediated esterification using 5-hexynoic acid in the presence of N,N′-dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridine (DMAP). The introduction of 5hexynoic acid fragments onto natural lignin is depicted spectroscopically in Figure 2. The 1H NMR spectrum of natural lignin shows broad peaks between 3.00 and 4.20 ppm due to the random polymeric nature of lignin, composed of a variety of phenylpropane groups (Figure 2a). The addition of 5hexynoic acid to lignin adds atypical peaks to the spectrum, including a signal representing the proton of the terminal alkyne around 2.80 ppm. Additionally, three broad peaks representing methylene protons are present around 2.20, 1.65, and 1.20 ppm (Figure 2b). The lack of definition in the NMR F
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another is fairly regular and approximately 20 nm according to scale bar in the inset. The lignin-graf t-PAA represented by the images in Figure 4b,c was prepared in order to view the effect that lignin weight percent has on the nanocomposite dispersion. Figure 4b contains 15 wt % lignin in lignin-graf t-PAA, and Figure 4c contains 20 wt % lignin in lignin-graf t-PAA, both with polymer arm lengths of DP 500. Both sets of conditions result in polymer matrixes that are uniformly dispersed, though the lignin clusters at 20 wt % are significantly larger. Just as for the lignin clusters in Figure 4a, however, the individual lignin molecules are uniformly spaced. The origin of the difference in lignin cluster size relative to lignin weight percent is still under investigation; however, the key discovery here is that ligningraf t-PAA shows uniform dispersion regardless of polymer arm length or the weight percentage of lignin, within the limits that we have tested. This allows for the mechanical properties of lignin-graf t-PAA to be precisely controlled by manipulation of these two variables. The results of more detailed mechanical property studies are discussed in the next section. The solid state mechanical properties of variously prepared lignin-graf t-PAAs were measured by static tensile tests (Figure 5). The goal of these studies is to deduce the effect that both
25,000 g/mol according to previous study of lignin modification via esterification.16 Because of the generally high modulus of lignin, the modulus of lignin-graf t-polymers was expected to increase as the weight percentage of lignin is increased. The first set of experiments to test the mechanical properties of the prepared nanocomposite used the same weight fraction of lignin while altering the chain lengths of PAA polymer. Short chain polymers prepared via RAFT require a larger amount of RAFT agent relative to monomer (see Table 1, entries 1 and 2), which constitutes the largest share of the overall production cost of the polymer. For this reason, polymer chains of DP 300 (Table 1, entry 4) and DP 500 (Table 1, entry 5) were selected for use in the following mechanical property studies (Table 1, entries 4 and 5, respectively). The lignin−polymer nanocomposites were successfully prepared by the graft-onto method and analyzed by TEM in Figure 4. The dark spots in the images are lignin, and the
Figure 4. Transmission electron microscopy (TEM) images of ligningraft-PAA. Lignin (dark spots in images) is uniformly dispersed in images (brighter background in images). (a) DP 20, 10 wt % lignin containing lignin-graft-PAA. (b) DP 500, 15 wt % lignin containing lignin-graft-PAA. (c) DP 500, 20 wt % lignin containing lignin-graftPAA. The scale bar is 200 nm. The scale bar in the inset of (a) is 20 nm.
Figure 5. A sample of lignin-graft-PAA undergoing a static tensile test: (a) side view; (b) front view.
lignin weight percent and polymer arm length has upon the mechanical properties of bulk nanocomposite and in doing so tease out the optimal composition of lignin-graft-PAA for many different applications. This is another example of the benefit of the graft-onto approach, as this modular method allows for nanocomposites with a range of mechanical properties to be produced quickly by a single well-developed synthetic method. For the mechanical property studies, polymer arm lengths of DP 300 and DP 500 were used to create lignin-graf t-PAA with lignin weight percent of 10, 15, 20, and 25 wt %. During preparation, the weight ratio of lignin in the composite is controlled very simply by changing the ratio of lignin to polymer during the click reaction. It has already been shown that the functionalized lignin has more than enough reactive sites for the integration of polymers.16 Figure 6c shows 25 wt % lignin containing lignin-graf t-PAA with DP 500 polymer. The synthesized polymer was very brittle and yielded pieces which shattered, as shown in Figure 6c.
dispersion of these spots is observed to be uniform in all cases, indicating that lignin and PAA polymer are highly compatible in lignin-graf t-PAA, as no significant microscale phase separation is observed. A homogeneous dispersion of lignin in the polymer matrix is very important to the overall mechanical properties of lignin-graf t-PAA. As is generally accepted about polymer nanocomposites, the increased surface-to-volume ratio creates a large interfacial area between the “hard” and “soft” domains within the polymer matrix.34,47,48 The homogeneity displayed by lignin-graf t-PAA yielded stable and reproducible mechanical properties as presented in the later part of the report. Additionally, the inset of Figure 4a shows a cluster of small black particles, i.e., lignin. Those black particles keep specific distances each other because of PAA occupation between adjacent lignin molecules. The distance between one lignin and G
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materials (Figure 5). The samples of lignin-graf t-PAA were prepared in a silicone mold to give uniformly rectangular shapes (see Figure 6a,b) and then dried in a vacuum oven to remove any remaining solvents as well as any cavities or voids which may cause unexpected failure during the tensile testing. A small piece of the lignin-graf t-PAA sample was dissolved in a deuterated solvent and analyzed by 1H NMR to determine if the organic solvents had indeed been removed. Tensile deformation of the dry and faultless samples was recorded as stress−strain plots, as depicted in Figure 7. Figure 7a demonstrates the stress−strain plot of 10, 15, and 20 wt % lignin containing lignin-graf t-PAA with DP 500 polymer. Varying the lignin concentration in the nanocomposite showed that a higher lignin content results in superior mechanical properties including Young’s modulus, maximum tensile strength, and energy-to-break (Tables 2 and 3). An increase in mechanical properties accompanying an increase in lignin concentration makes sense as lignin serves as the hard domain of these lignin−polymer nanocomposites; therefore, an increase in the hard domain would be expected to result in a toughening of the nanocomposites itself, up to the point where it becomes brittle. According to generally accepted explanations, the stress initially concentrates to an equator of the rigid domain during mechanical deformation. Then the stress concentration led triaxial stress around the rigid domain, followed by partial debonding at the rigid domain−polymer interface. The resulting voids cause shear yield and/or crazing around every rigid domain throughout the overall volume of the material. As a consequence, the nanocomposite can absorb a large amount of energy during deformation and is reinforced.47,49 Important requirements for this mechanism are small-sized rigid domains (less than 5 μm) and homogeneity of the rigid domain distribution in the polymer
Figure 6. Images of synthesized lignin-graf t-PAAs: (a) 20 wt % ligningraft-PAA, DP 500; (b) 20 wt % lignin-graf t-PAA, DP 500 in a mold; (c) 25 wt % lignin-graft-PAA, DP 500 and shattered samples due to brittleness of this lignin concentration.
Another important property of lignin-graf t-PAA is its selfhealing ability, which originates in the strong hydrogen-bonding interactions between PAA polymers. The rubber-like flexibility of the material is essential to promote these contacts for selfhealing; therefore, 25 wt % lignin containing lignin-graf t-PAA was omitted from further mechanical property studies, as Figure 6c shows it to be very brittle. This observation is additionally important because it presents for us a threshold in lignin concentration above which the composite becomes too brittle for this kind of use. This tough-to-brittle transition occurs between 20 and 25 wt % of lignin,48 a value which will prove useful when designing lignin-graf t-PAA with specific applications in mind. Unlike 25 wt %, 10, 15, and 20 wt % lignin containing lignin-graf t-PAA were tougher and presented a rubber-like flexibility as shown in Figure 6a,b. A Shimadzu model EZ-LX static tensile tester with force transducer (interface, model: SM-200 N-168) was used to more precisely measure the mechanical properties of the developed
Figure 7. Stress−strain plots of 15%, 20%, and 25% lignin containing lignin-graft-PAA, DP 500, and DP 300. (a) Comparison of 15%, 20%, and 25% lignin weight ratio with DP 500. (b) Comparison of different polymer lengths, DP 300, and DP 500 under constant lignin weight ratio of 15%. (c) Comparison of different polymer lengths, DP 300 and DP 500, under the constant lignin weight ratio 10%. H
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Macromolecules Table 2. Mechanical Properties of 10 and 15 wt % Lignin Containing Lignin-graf t-PAA, DP 500, and DP 300a sample DP DP DP DP
300, 500, 300, 500,
10% 10% 15% 15%
Young’s modulus (MPa) 5.87 3.76 20.43 29.16
± ± ± ±
3.00 0.16 1.25 6.44
max tensile strength (MPa) ± ± ± ±
0.57 0.66 1.66 1.78
0.05 0.07 0.17 0.30
strain at break 12.67 19.18 5.61 11.36
± ± ± ±
1.97 3.39 1.49 0.59
energy-to-break (MPa) 2.75 3.96 6.61 10.99
± ± ± ±
0.10 1.52 0.79 0.96
a
Two different polymer lengths, DP 300 and DP 500, in lignin-graft-PAA were compared to study effect of length on mechanical properties of prepared lignin-graf t-PAAs.
Table 3. Mechanical Properties of 10, 15, and 20 wt % Lignin Containing Lignin-graf t-PAA with Constant DP 500a sample
Young’s modulus (MPa)
max tensile strength (MPa)
strain at break
energy-to-break (MPa)
DP 500, 10% DP 500, 15% DP 500, 20%
3.76 ± 0.16 29.16 ± 6.44 72.75 ± 2.50
0.66 ± 0.07 1.78 ± 0.30 2.87 ± 0.17
19.18 ± 3.39 11.36 ± 0.59 12.14 ± 0.96
3.96 ± 1.52 10.99 ± 0.96 21.56 ± 2.39
a
Various lignin weight ratio, 10, 15, and 20 wt % containing lignin-graft-PAA was tested under constant polymer length, DP 500, to determine the effect that lignin weight ratio has on mechanical properties of final products. Values of DP 500 and 10% and 500 and 15% were reused from Table 2 for comparison.
shown in Figure 8. The severed sections of polymer sample were then rejoined and pressed gently for 10 s at ambient
matrix. As discussed in reference to the TEM images, ligningraf t-PAA sufficiently satisfies all of these requirements. Having explored the relationship between lignin content and bulk mechanical properties, we then turned our attention to the effect of polymer length in lignin-graf t-PAA. Figures 7b and 7c show stress−strain plots with PAA of DP 300 and 500 under constant lignin weight ratios. In both cases of 10 and 15 wt % lignin, samples with polymers of DP 500 showed superior mechanical properties (Tables 2 and 3). One plausible explanation for these observations is that the different polymer lengths result in different levels of entanglement. The entanglement of polymer chains would have an obvious impact on the bulk mechanical properties of the lignin-containing polymers.33,50 There are a few theories to explain the effect of polymer entanglements by De Gennes51 and Rouse-Bueche.52 In both theories, the molecular weight of the polymer in question is the decisive factor for controlling mechanical properties. Hence, we reasonably assume that the reinforced mechanical properties of higher molecular weight lignin-graf tPAA are due to enhanced polymer entanglements. To sum up, we have explored two methods to control the mechanical properties of lignin-graf t-PAAs: First, we can increase the weight ratio of lignin to enhance the mechanical properties of the product, with a maximum at 20 wt % in order to maintain the desired rubber-like properties. Second, the overall mechanical properties of the lignin-graf t-PAA can be strengthened by including longer polymer chains where bulk polymer entanglements serve to increase useful mechanical properties. Self-Healing Studies. The PAA polymers used to prepare lignin-graf t-PAA were designed for their hydrogen-bonding characteristics resulting from the presence of the acetylamino moiety. This property results in robust and spontaneous attracting between proximal PAA polymer chains,31 and in the bulk materials this results in the spontaneous, or autonomic, healing of broken nanocomposite without the input of heat, light, and additional monomers, catalysts, and/or solvents.53,54 Thus, autonomic self-healing is yet another attractive feature of the described lignin-graft-PAAs which has been incorporated by design utilizing the modular graft-onto synthetic approach. The efficiency of autonomic healing was judged by the same tensile testing to which the original nanocomposite strips were subjected. The materials were served with a razor blade, as
Figure 8. Self-healing test of 20 wt % containing lignin-graft-PAA (PAA DP 500); the original undamaged material (left) was cut into two pieces (middle). Complete healing was observed after adjoining the pieces at the interface. The self-healing performed under ambient conditions (23 °C and 55% humidity).
temperature. Next, the rejoined sample allowed to heal for 1 day (24 h). After the healing process, it was found that the rejoined lignin-graf t-PAAs were healed (Figure 8). The sample chosen for this experiment was 20 wt % containing lignin-graf tPAA, with polymer length of DP500 because this sample demonstrated the highest Young’s modulus and energy-tobreak according to the mechanical property tests of undamaged samples (sample DP 500, 20%, in Table 3). The mechanical properties of the healed lignin-graf t-PAAs are displayed in Figure 9 and Table 4. According to the stress− strain plot in Figure 9, the maximum stress endured was 93% of the stress from undamaged samples. This high level of maximum stress recovery in healed polymer samples was repeatedly observed, with relatively low standard deviation (Table 4). The other tested mechanical properties showed moderate healing capability. For example, Young’s modulus revealed 55% of recovery, while energy-to-break yielded 60% of recovery. Overall, dynamic hydrogen bonding between acetylamino moieties of the polymer was efficient for inducing autonomic self-healing of lignin-graft-PAAs. These autonomic self-healing properties will be useful in developing a new avenue I
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composites, suggesting that natural lignin could be an excellent substitute for conventional inorganic fillers.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (H.C.). Funding
This work was supported by the Florida State University Energy and Materials Hiring initiative and FSU Department of Chemical and Biomedical Engineering. Notes
The authors declare no competing financial interest.
Figure 9. Stress−strain plots of 20 wt % lignin containing lignin-graftPAA, DP 500 at room temperature; an undamaged sample of ligningraft-PAA was measured before being cut (undamaged), and the sample was then cut into two separate pieces and rejoined and allowed to heal for 1 day before being tested again.
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ACKNOWLEDGMENTS The authors thank Prof. Newell R. Washburn and Dr. Brian Ondrusek for helpful discussions.
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of materials which possess enhanced durability and extended material lifetime.
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REFERENCES
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CONCLUSION These results details the synthesis of a new natural lignincontaining polymer which possesses easily controllable mechanical properties and self-healing properties. The synthesis of these lignin-graf t-PAAs occurs via a three-step synthetic plan: preparation of polymers, functionalization of raw lignin with terminal alkynes, and a copper-catalyzed azide−alkyne cycloaddition reaction between the azides of the polymer and the alkynes of lignin. The successful synthesis yields a welldefined morphological structure in the bulk nanocomposite matrix which has a lignin-based hard domains and PAA-based soft domains. The overall mechanical properties of lignin-graf tPAA were easily controlled simply by adjusting the weight ratio of lignin and PAA during the click reaction. In particular, it was found that there is brittle-to-tough transition point at 20 wt % lignin, above which the bulk material become brittle. While holding the weight percent of lignin constant, longer PAA chains showed superior mechanical properties (maximum tensile strength, energy-to-break, and Young’s modulus), likely due to polymer entanglement. Hydrogen bonding, induced by the acetylamino moieties on the polymers, led directly to autonomic healing properties in bulk lignin-graf t-PAA. On the basis of static tensile tests, the severed lignin-graf t-PAA samples showed 93% of maximum strength recovery after the selfhealing process, relative to the maximum strength of undamaged samples. The developed lignin-graf t-PAA is a new example of lignin containing extraordinary functional material. The modular synthetic methods used to prepare lignin-graf tPAA can be further applied to modify lignin for producing various functional materials for advanced materials beyond mere cheap and low-quality commodity materials, simply by changing the polymer which is grafted onto lignin. The new nanocomposite, lignin-graf t-PAA, follows generally accepted mechanical property enhancement mechanisms of nano-
Table 4. Mechanical and Self-Healing Properties of 20 wt % Lignin Containing Lignin-graf t-PAA, DP 500 entry
Young’s modulus (MPa)
max tensile strength (MPa)
strain at break
strength at break (MPa)
energy-to-break (MPa)
undamaged 1 day self-healing
72.75 ± 2.50 39.39 ± 10.47
2.87 ± 0.17 2.69 ± 0.42
12.14 ± 0.96 6.07 ± 2.21
0.79 ± 0.24 1.94 ± 0.47
21.56 ± 2.39 13.07 ± 5.17
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