Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/ascecg
Synthesis and Thermorheological Analysis of Biobased Lignin-graf tpoly(lactide) Copolymers and Their Blends Love-Ese Chile,†,‡ Samuel J. Kaser,† Savvas G. Hatzikiriakos,*,‡ and Parisa Mehrkhodavandi*,† †
Department of Chemistry and ‡Department of Chemical and Biological Engineering, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada S Supporting Information *
ABSTRACT: Despite numerous accounts of biobased composite materials through blending and copolymerization of lignin and other polymers, there are no systematic studies connecting the synthetic methodology, molecular structure, and polymer topology with the rheological properties of these materials. In this report lignin-graf t-poly(lactide) copolymers are synthesized via three routes (indium and organocatalyzed “graft-from” methods as well as a “graft-to” method) and the resulting reaction products (shown to include linear PLAs, cyclic PLAs, and star-shaped lignin-graf t-PLA copolymers) are investigated using chemical and rheological methods. The topology of the products of the graft-from methods is affected by the initial lignin concentration; polymerizations with low lignin loading generate cyclic PLAs, which can be identified by 10-fold lower viscosities compared to linear PLAs of the same molecular weight. Under higher lignin loadings, star-shaped lignin-graf t-PLA copolymers are formed which show viscosities 2 orders of magnitude lower than those of comparable linear PLAs. Rheological studies show that cyclic PLAs lack a well-defined rubber plateau, whereas star-shaped lignin-graf t-PLAs lack a significant G′ to G′′ cross-over. The rheological results coupled with thermogravimetric analysis give an indication to the structure of star-shaped lignin-graftPLA copolymers, which are estimated to contain a small lignin core surrounded by PLA segments with molecular weights from 2.0 to 20 kg mol−1. KEYWORDS: Biomass, Green composites, PLA, Blends, Graft copolymers, Lignin, Cyclic polymers, Star polymers, Rheology
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INTRODUCTION
poor stress transfer resulting from insufficient compatibilization between lignin and the polymer matrix.2,3,5,6,17 Graft copolymerization is one way to improve the adhesion of two incompatible phases, and researchers have successfully grafted18 many synthetic19−25 and bioderived polymers to lignin.26,27 Although an exciting variety of bioderived and compostable materials have been synthesized,18,19,26,28,29 many of the recent reports of these lignin graft copolymers focus on determining the mechanical response of composites29,30 or on characterizing specific behavior (e.g., wettability23 or antioxidant26 behavior). Graft copolymers can be generated through two major strategies: grafting to and grafting from (Scheme 1). For
After cellulose, lignin is the second most abundant polymer found in biomass. Millions of tons of kraft lignin are produced annually by the pulp and paper industry through sulfur delignification processes.1,2 Recently, there has been a surge of interest in valorizing this cheap and renewable source of polymeric material.3−5 Lignin is used as a filler in a variety of green composites6,7 to improve the properties of other bioderived and biodegradable polymers such as poly(lactic acid) (PLA).8,9 Thus, unmodified lignin/polymer blends and their polymer properties have been a significant field of investigation.10−16 In general, lignin−polymer blends have shown greater thermal stability at high lignin loading; however, the overall mechanical properties are often diminished or remain unchanged compared to the native polymer. This depreciation of composite properties has been attributed to © XXXX American Chemical Society
Received: August 18, 2017 Revised: January 5, 2018
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DOI: 10.1021/acssuschemeng.7b02866 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis for Lignin Graft Copolymers Using Graft-from and Graft-to Strategies
31
P{1H} NMR spectroscopy can be used to characterize the hydroxyl groups in lignin.49 Reaction of lignin with 2-chloro4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) in the presence of a base such as pyridine phosphitylates the lignin hydroxyl groups.50 The P-derivatized lignins can be compared to an internal standard (cyclohexanol) to quantify the concentration of hydroxy species, [OH]lig in mmol g−1 (Table 1, Figure S1).
example, to achieve star-shaped lignin copolymers, isocyanatefunctionalized poly(ε-caprolactone) (PCL) can be utilized in a “graft-to” approach to synthesize lignin-graf t-PCL copolymers.31 Lignin-graf t-PCL copolymers with varying PCL segment molecular weights (MW) have been synthesized by controlling the CL/OH ratio. However, at high CL:OH ratios, linear PCL chains were also generated.20 Sattely et al. synthesized kraft lignin-graf t-PLA copolymers using a “graftfrom” approach through organocatalyzed ring-opening polymerization (ROP) of lactide (LA).30 PCL19 and PLA−lignin32 composites with UV-resistant and antioxidant properties can also be synthesized. Importantly, Liu et al. showed that a high concentration of hydroxy groups constrained the chain growth of grafted PLA, while selective alkylation of phenolic hydroxy (100%) and carboxylic (70%) OH (COOH) groups allowed for the synthesis of graft copolymers with the highest reported PLA segment MWs (up to 28 kg mol−1).33 Recent reviews18,34 have underscored that despite these advances in lignin-PLA graft copolymerization, the following key issues remain unresolved: (a) the accurate determination of lignin integration and the influence of unreacted lignin on polymer properties of grafts and (b) the topology and rheology of the graft copolymers. In our study of indium-35−43 and zincbased44−47 complexes for the living ROP of cyclic esters, we developed strategies for the synthesis of highly controlled star block copolymers through immortal polymerization.45,48 In this work, we utilize these strategies to synthesize lignin-graf t-PLA copolymers using three different methods: (1) graft-from lignin using an InCl3/NEt3 catalyst system, (2) graft-from lignin using organocatalyst 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), and (3) graft-to lignin using traditional methodologies (Scheme 1). We elucidate the structure, topology, and viscoelastic properties of the reaction products and explore the incorporation of lignin and the effect on the melt rheology of the graft copolymers. We also study the impact of the interaction between lignin and the matrix in a series of copolymer blends.
Table 1. Hydroxy Group Content for Indulin AT Kraft (IAK) Lignin and Alkali Kraft (AK) Lignin (in mmol g−1)a type of OH
indulin AT kraft lignin (IAK)b
alkali kraft lignin (AK)b
alkyl OH (150−146 ppm) phenolic OH (144−138 ppm) COOH (136−134 ppm) total [OH]lig Mw (D̵ )c
7.5(0.9) 14(1) 1.5(0.2) 23(3) 6500(9.9)
5.0(0.9) 9.1(0.7) 2.3(0.2) 16(1) 14 200(8.1)
a
Calculated from 31P{1 H} NMR spectra of 2-chloro-4,4,5,5tetramethyl-1,3,2-dioxaphospholane derivatized lignin in CHCl3/ pyridine. Internal standard at 145.2 ppm (cyclohexanol, 0.011 mmol g−1). Reference peak 132.2 ppm (phosphitylated water). bHydroxyl (OH) group content in units of mmol g−1. Standard errors are in parentheses. cMolecular weights, Mw, in g mol−1. Dispersities, D̵ , are in parentheses.
A series of lignin-graf t-PLA copolymers can be synthesized via graft-to and graft-from approaches (Scheme 1). Crucially, graft copolymers are rigorously purified via multiple centrifugation/precipitation cycles to remove the maximum amount of excess lignin (see below). As a result, only soluble copolymers were isolated and reaction products with very high lignin content are excluded from this analysis. Copolymers can be characterized by 1H, 1H−1H COSY (CDCl3, 25 °C), and 31 1 P{ H} NMR spectroscopy (CHCl3:pyridine, 25 °C) as well as by IR spectroscopy and gel permeation chromatography (GPC) (Figures S2−10). PLA-lignin grafts are soluble; thus, it is possible to observe broad 1H NMR signals for lignin aromatic (∼7 ppm) and methoxy (∼4 ppm) protons (Figure 1).19,25,28,30 The average molecular weights of PLA segments are estimated from the ratio of polymer to chain-end (4.36 ppm) signals. Changing the lignin mol % generates graft copolymers with variable PLA segment MWs.20,30,33 Impor-
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RESULTS AND DISCUSSION Synthesis and Characterization of Lignin-graf t-PLA Copolymers. Two commercially available grades of kraft lignin are used in this study: Indulin AT kraft (IAK) lignin from ingenvity (Mw = 6500 g mol−1; D̵ = 9.9) and alkali kraft (AK) lignin from Sigma−Aldrich (Mw = 14 200 g mol−1; D̵ = 8.1). B
DOI: 10.1021/acssuschemeng.7b02866 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. 1H NMR spectra (CDCl3, 25 °C, 400 MHz) for polymers with different wt % of IAK lignin, generated from Method 1.
studies show that in these systems Mn describes the graft segment MW on a randomly selected core, whereas Mw describes the average MW of a randomly selected segment.53 The relative molecular weights of graft copolymers were determined by triple-detector GPC via universal calibration. The graft copolymers in this study display high dispersities; as such, we find Mw,GPC to be their most appropriate description. PLA segment MWs, Mw,GPC, were higher than the theoretical values and corroborated the 6% initiation of hydroxy groups on lignin (Figures S8 and S9). The discrepancy between the NMR segment MW and the GPC segment MW implies that there is no symmetrical PLA growth from hydroxy moieties during polymerization. Reports on lignin copolymers by Sattely,30 Liu,33 and Kai and Loh29,32 show similar inconsistencies between NMR and GPC molecular weight data. The universal calibration used for GPC analysis relates intrinsic viscosity with hydrodynamic volume; this approximation only gives a relative molecular weight and does not impart any real structural information for polymers with complex topologies. Coupling these analyses with rheological analysis can aid in identifying and interpreting product mixtures (see below). The FTIR spectra for a representative lignin-graf t-PLA copolymer (Table 2, entry 4) display a characteristic broad OH stretch at around 3300 cm−1 resulting from the large number of phenolic and alkyl OH groups within the lignin framework (Figure S10). PLA has a carbonyl stretch at 1738 cm−1 as well as C−H stretches at 2959 and 3001 cm−1. The spectrum for the graft copolymer shows a carbonyl peak at 1744 cm−1, which has shifted to higher wavenumber compared to native PLA. For analogous PCL-lignin grafts, this hypsochromic shift has been attributed to hydrogen bonding between PCL and phenolic moieties on lignin.20 In order to gauge the effect of lignin source on copolymer formation, analogous experiments were conducted using alkali kraft (AK) lignin from Sigma−Aldrich (Table 2, entries 6−11). AK lignin yields grafts with larger PLA segments, and segment MWs are less dependent on the [LA]:[OH]lig ratio (Table 2, entry 3 vs 10). A comparison of the 31P{1H} NMR spectra of AK and IAK shows that IAK has a higher concentration of hydroxy groups than AK, suggesting there are more active sites
tantly, the observed molecular weights of PLA segments are higher than the theoretical lengths calculated from the [LA]: [OH]lig ratio, suggesting incomplete activation (∼6%) of all of the OH moieties on lignin (Figure 2, Figure S6).
Figure 2. Experimental PLA segment MW (NMR) vs mol % of lignin (blue squares, both IAK and AK lignin) and theoretical segment MW (black circles, Mn,theo = MLA × ([LA]/([OH]lig + [InCl3])) × conversion) for graft-from copolymers generated using Method 1. Difference in slope between the theoretical and the experimental segment MWs was used to determine that 6% of the OH groups on lignin was activated during polymerization (corrected theoretical segment MW, red triangles).
Accurate molecular weight determination of lignin copolymers is nontrivial due to the disorder of the lignin core.51 Hydrogen-bonding interactions between moieties within the structure of lignin make it a very hard and impermeable segment.31 Functionalizing lignin has been shown to increase the softness of lignin by disrupting these interactions. Lignin functionalized with short-chain polymers has been reported to be star shaped.20,31,52 Recent theory on star polymers with cross-linked cores suggests that more specific information can be obtained from experimental molecular weight data. These C
DOI: 10.1021/acssuschemeng.7b02866 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Table 2. Polymerization Data from Lignin-graf t-poly(lactide)s Formed via Method 1 (InCl3/NEt3)a
1 2f 3f 4f 5f 6 7g 8g 9g 10g 11g
[LA]:[InCl3]
mol % OHlig
560 580 570 570 570 820 720 600 720 1100 670
0 6 11 24 38 0 1 4 8 12 20
[LA]:[OH]lig 6.1 3.1 1.2 0.61 40 8.4 4.2 2.7 1.6
conv. (%)b 98 96 93 90 91 93 87 79 81 64 70
theoretical PLA segment MW (g mol−1)c
average PLA segment MW (g mol−1)d
829 415 154 80
7370 3810 1200 713
3190 1050 564 354 159
15 200 17 300 13 500 5680 5890
Mn,GPC (g mol−1)e
Mw,GPC (g mol−1)e
D̵ d
71 300 46 600 509 000 158 000 92 800 44 500 52 800 14 400 12 600 10 000 32 600
80 600 56 300 563 000 221 000 108 000 62 600 76 000 16 300 15 200 12 500 36 300
1.13 1.21 1.11 1.39 1.16 1.41 1.44 1.13 1.20 1.25 1.11
Reactions were carried out in toluene at 120 °C for 48 h. bLactide conversion determined by 1H NMR spectroscopy. cTheoretical segment MW = MLA × ([LA]/([OH]lig + [InCl3])) dCalculated from integration of the polymer and chain end methine protons multiplied by the molecular weight of LA. eRelative molecular weights were determined by triple-detector GPC (gel permeation chromatography) via universal calibration (THF 4 mg mL−1, flow rate = 0.5 mL min−1, dn/dc = 0.040 mL g −1). fIAK = Indulin AT kraft lignin (Mw = 6500 g mol−1 (9.9); [OH]lig = 23 mmol g−1). gAK = alkali kraft lignin (Mw = 14200 g mol−1 (8.1); [OH]lig = 16 mmol g−1). a
from which PLA can grow, leading to higher lignin content and smaller PLA segments (Table 1). This is an interesting observation as there can be significant variance in graft copolymer formation even within the same class of lignin. Therefore, for our rheological studies (see below) we discuss grafts generated from both classes of lignin. Awareness of the possibility of lignin contamination in graft copolymers is important for further studies of these materials as it has a strong impact on flow and mechanical properties of the resulting materials. If not removed adequately, unreacted lignin forms a composite material with the PLA-lignin graft copolymers. A comparison of the 31P{1H} NMR signals of a phosphitylated lignin-PLA copolymer with unreacted lignin shows the presence of alkyl and carboxylic OH groups indicative of the formation of PLA (Figure S2).30,49 A significant concentration of phenolic OH groups, likely from unreacted lignin, is also observed. The 31P{1H} NMR spectra obtained from purified graft copolymers lack phenolic moieties, implying that this material has PLA attached to the hydroxy moieties on lignin (Figure S3). Lignin has a distinctive thermal degradation profile but does not fully degrade below 500 °C. Thus, the lignin content of the copolymers can be estimated by examining the ash content after thermal exposure.54 TGA analysis of purified and crude copolymer samples shows higher lignin ash content in the crude copolymer (Figure 3). The melt rheology of samples before purification shows an 80% increase in complex viscosity compared to the pure copolymer, indicating that lignin is acting as filler and further modifying the properties of the copolymer (Figure S14). Impact of Synthetic Route on Copolymerization. In Method 1, we polymerized racemic lactide (rac-LA) with the InCl3/NEt3 catalyst system reported by Hillmyer and Tolman55,56 in conjunction with various amounts of indulin AT (IAK) lignin as the alcoholic initiator (Table 2, entries 1−5). Rac-LA was stirred in toluene with InCl3/NEt3 and 1−38 mol% of lignin at 120 °C for 48 h. GPC traces for samples with low lignin loading (high [LA]:[OH]lig) showed long tails in the low molecular weight region, while samples at higher lignin loading (low [LA]:[OH]lig) gave peaks with high molecular weight shoulders (Figure S7). Similar multimodal peak shapes have been reported for both lignin-graf t-PMMA19,57 and lignin-graf tPCL20,31 copolymers. The graft copolymers elute more slowly than linear chains of similar molecular weight, which is
Figure 3. Thermogravimetric analysis traces for lignin-graft-PLA copolymers before and after removal of excess lignin (synthesized using Method 2 and IAK lignin).
indicative of polymers with different chain architectures (Figure S15). In order to compare our results with those of Sattely et al. directly (Method 2), we used organocatalyst triazabicyclodecene (TBD) (Scheme 1) to produce a second family of graftfrom copolymers from the reaction of rac-LA, 1 wt % TBD, and 1−35 mol% of indulin AT kraft (IAK) lignin at 130 °C (Table 3).30 The polymers generated from this reaction showed the expected PLA segment mass dependence on lignin-OH mol% and high dispersities, indicative of lignin incorporation.20 In Method 3, a graft-to synthetic strategy54 was used to synthesize a final family of lignin-graf t-PLA copolymers (Scheme 1). Monodispersed linear PLAs of various molecular weights were synthesized via the immortal ROP of lactide using the dinuclear indium complex [(NNO)InCl]2(μ-Cl)(μ-OEt) (A)58 and MeOH. These were reacted with oxalyl chloride to generate chloro-terminated PLA prepolymers (PLA-Cl). A solution of PLA-Cl in DMF was added to deprotonated IAK to form the PLA-lignin copolymers (Table 4). The [K2CO3]:[OH]lig ratio, potentially indicative of the degree of deprotonation of lignin, does not have an impact on the formation of graft copolymers. Reactions with nearly 10D
DOI: 10.1021/acssuschemeng.7b02866 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Table 3. Polymerization Data from Lignin-graf t-PLAs Formed via Method 2 (TBD)a
1 2 3 4 5
mol % lignin-OHb
conv. (%)c
[LA]:[OH]ligd
theoretical PLA segment MW (g mol−1)e
average PLA segment MW (g mol−1)f
Mn,GPC (g mol−1)g
Mw,GPC (g mol−1)g
D̵ g
0 5 9 20 33
94 86 80 75 69
1.3 2.3 5.7 12
640 330 130 59
11 200 13 200 8050 5110
31 800 36 700 34 000 34 400 33 700
50 200 54 800 65 200 59 000 59 500
1.58 1.48 1.92 1.72 1.77
Reactions were carried out in the melt at 130 °C for 3−4 h. bIndulin AT kraft (IAK) lignin (Mw = 6500 g mol−1 (9.9)). cMonomer conversion determined by 1H NMR spectroscopy. d[OH]lig = 23 mmol g−1 determined via 31P{1H} NMR spectroscopy.30,49 eTheoretical PLA segment MW = MLA × ([LA]/([OH]lig + [TBD])). fCalculated from integration of the polymer and chain end methine protons multiplied by the molecular weight of lactide. gRelative molecular weights were determined by triple detection GPC (gel permeation chromatography) via universal calibration (THF 4 mg mL−1, flow rate = 0.5 mL min−1, dn/dc = 0.040 mL g−1). a
Table 4. Polymerization Data from Lignin-graf t-poly(lactide)s Formed Using IAK Lignin via Method 3 (graft-to)a 1 2 3 4 5 6 7 8 9
PLA-Cl Mw (g mol−1)b
[K2CO3]:[OH]ligc
wt % lignind
average PLA segment MW (g mol−1)e
Mn,GPC (g mol−1)f
Mw,GPC (g mol−1)f
D̵ f
18 200 18 200 46 400 46 400 46 400 154 000 154 000 154 000 32 300
0.158 1.01 0.988 0.158 0.158 G′ (Figure 8). At medium to high frequencies, the storage and loss moduli become almost equivalent, G′ ≈ G′′, implying gel-like response. The terminal zone was reached at very low frequencies, where LVE moduli display the characteristic slopes of G′ ∝ ω2 and G″∝ ω. Another interesting feature of the master curves is the lack of a rubber plateau in the storage modulus at high deformation frequencies. A similar result was observed for 3- and 6-arm PHBs, which showed that increasing the arm length from 3 to 6 reduced the intensity of the rubber plateau.45 The loss of the rubber plateau in the graft copolymers is likely due to the large number of short PLA segments growing from the various hydroxyl groups in the lignin structure. These results indicate that the PLA graft segments are arranged such that they have few intermolecular chain entanglements, thus reducing the ability of the polymer to form a rubbery network. It is hypothesized that increasing the PLA graft segment MW would allow for thermoplastic behavior; however, there are currently very few reports of lignin-graf t-PLA copolymers with segment molecular weights of more than 30 kg mol−1.33 Thermal transitions were probed using temperature ramp experiments at constant deformation strain (2%) and frequency (0.5 Hz). Both cyclic and star lignin-graf t-PLAs showed glass transitions at ∼45 °C. At lower temperatures, G′ dominates and thus copolymers display glassy behavior. Above 45 °C the polymers display gel-like behavior up to ∼65 °C after which fluid response dominates (Figure S22). The dynamics of star/ring blends are currently underexplored in the literature, though some hypotheses can be made by extrapolating from studies of cyclic/linear70,80,81,83 and star/ linear19,31,48,57,78,79,82,84,85 blends. Star graft copolymers with short graft segments are likely to have lower viscosity if contaminated with cyclic PLAs as they will not form intermolecular interactions. Graft copolymers with long chain segments could retard cycle deformation through threading interactions; however, this could be balanced by star arm retraction relaxation modes. From the results presented, we estimate the graft copolymers generated by Methods 1 and 2 to have small lignin cores with short PLA-grafted segments.
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CONCLUSIONS Lignin-graf t-PLAs were successfully synthesized via three synthetic strategies. The highest lignin incorporation was achieved using Method 3, a graft-to approach. The organocatalyst, TBD (Method 2), gave the highest lignin incorporation of the graft-from syntheses; however, rigorous purification is necessary to remove unreacted lignin from grafts in both Methods 1 and 2. Unreacted lignin causes errors in predicting melt properties. We showed that avoiding very low lignin concentrations ([OH]lig) is important in avoiding the formation of cyclic PLA byproducts during polymerization. Cyclic PLAs were characterized by their lower intrinsic and zero-shear viscosities compared to linear PLAs of the same GPC molecular weight. The melt rheology of cyclic PLAs generated via Methods 1 and 2 showed shorter terminal relaxation compared to linear PLAs. The presence of linear contaminants in these samples impacted the rubbery plateau region evidenced by an uncharacteristic cross-over between G′ and G′′. We investigated the melt rheology of star-shaped lignin-graf tPLAs which were generated at high [OH]lig using Methods 1 and 2. The lignin within the copolymer structure causes a deviation from linear structure evidenced by a decrease in their Mark−Houwink slope and their 2-fold decrease in zero-shear viscosity compared to linear PLAs. The loss modulus dominates over the storage modulus over the whole linear viscoelastic (LVE) regime, indicating that the star grafts in this study have short PLA segments which are unable to form intermolecular entanglements. It is predicted that by further increasing the graft segment MW, thermoplastic behavior could be imparted to the material. However, this would require careful control of [OH]lig, potentially through prealkylation of lignin. As the graft copolymers can be processed at relatively low temperatures (
