Sustainable Hydrogels Based on Lignin-Methacrylate Copolymers

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Sustainable hydrogels based on lignin-methacrylate copolymers with enhanced water retention and tunable material properties Kalavathy Rajan, Jeffrey K Mann, Eldon English, David P. Harper, Danielle Julie Carrier, Timothy G. Rials, Nicole Labbé, and Stephen C. Chmely Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b00282 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Sustainable hydrogels based on lignin-methacrylate copolymers with enhanced water retention and tunable material properties Kalavathy Rajan [1],[2], Jeffrey K. Mann [1], Eldon English [1],[3], David P. Harper [1], Danielle Julie Carrier [2], Timothy G. Rials [1], Nicole Labbé [1] and Stephen C. Chmely[1]* [1]

Center for Renewable Carbon, The University of Tennessee Institute of Agriculture,

Knoxville, TN 37996, USA [2]

Department of Biosystems Engineering and Soil Science, The University of Tennessee

Institute of Agriculture, Knoxville, TN 37996, USA [3]

Faculty of Sciences, Halls Middle School, 4317 E. Emory Road, Knoxville, TN 37938, USA

KEYWORDS lignin hydrogel, random copolymer, hydrogel, (2-hydroxyethyl) methacrylate, hybrid poplar lignin

ABSTRACT Synthesizing lignin-based copolymers would valorize a major co-product stream from pulp and paper mills and bio-refineries, as well as reduce the dependence on petrochemicalbased consumer goods. In this study, we used organosolv lignin isolated from hybrid poplar (Populus trichocarpa ✕ P. deltoides) to generate lignin-containing methacrylate hydrogels. The

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copolymer hydrogels were synthesized by first grafting 2-hydroxyethyl methacrylate (HEMA) onto lignin (OSLH) via esterification, and then by free radical polymerization of OSLH with excess HEMA. The copolymer hydrogels were prepared with different stoichiometric ratios of OSLH (e.g. 0, 10, 20 and 40 wt%) with respect to HEMA. Copolymerization with OSLH led to an increase in cross-linking density, which in turn enhanced the hydrogel’s material properties; we report up to 39% improvement in water retention, 20% increase in thermo-stability and up to 3 order increase in magnitude of storage modulus (G’). The copolymer’s properties, such as water retention and glass transition temperature, could be tuned by altering the percent functionalization of lignin OH-groups and the ratio of OSLH to HEMA.

Introduction Concern over the long-term use of fossil-derived carbon has increased the need for sustainable and renewable feedstocks. Lignin, which accounts for 15-30% of the renewable lignocellulosic feedstock, is a logical alternative feedstock for manufacturing materials and chemicals.1 Only a very small percentage (1-2%) of the 70 million tons of lignin generated in pulp and paper mills has been reportedly used for manufacturing specialty chemicals, while the rest is consumed as a low-value fuel.2 Advances in lignin valorization have generated several functional lignin-based polymers and industrially relevant chemicals that integrate both the intrinsic features of lignin as well as additional properties of the polymers.3,

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Valorization of lignin would increase the

economic feasibility and sustainability of bio-based consumer goods, thereby becoming a driving force for a bio-based economy.5 Lignin, a major co-product of bio-refineries, is a complex polymer of phenylpropane precursors, including coniferyl, sinapyl, and p-coumaryl alcohol. The properties of lignin vary depending on the biomass source and isolation technique.6,

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On a commercial scale, lignin is


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fractionated from biomass using various pulping processes. These conversion platforms mainly target the alkyl-aryl ether linkages (e.g. the ubiquitous β-O-4 linkage) to cleave and isolate lignin from plant cell wall components.1 Recently developed fractionation techniques like the organosolv process can afford lignin fragments with low molecular weight (MW) that are free of sulfur (encountered in Kraft lignin) and biological debris (encountered in acid-pretreated biorefinery lignin).8 These fragments demonstrate a great potential in polymer chemistry for use as polymer additives, fillers, reinforcements, or cross-linkers, or as a source of macro-monomers for new polymeric applications.1, 2 Preliminary studies to develop synthetic blends of lignin model compounds (e.g. guaiacol and vanillin) with co-monomers such as acrylates and vinyl ester resins have demonstrated the potential for controlled nano-assembly of copolymers that mimic the properties of parent homopolymer.9, 10 Clearly, suitably modified lignin that is isolated from various biomass types can be successfully used to synthesize new soft materials or replace existing non-renewable polymers.10, 11 Introduction of new reactive sites on lignin by OH-group functionalization affords the means to graft additional co-monomers onto the lignin macro-monomer and form new copolymers.2 There are emerging prospects for synthesizing lignin-containing copolymers using commonplace materials such as polyacrylamide and poly(methyl methacrylate).12 In one such study, lignin isolated from eucalyptus wood via acetic acid hydrolysis was grafted with acrylamide monomers by hydrogen abstraction from its reactive sites.13 In another study, bio-oil made from softwood lignin was functionalized by appending methacrylate moieties to its OH groups; the modified bio-oil was later copolymerized with methacrylic anhydride using reversible addition-fragmentation chain transfer (RAFT) polymerization.14 However, in the


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majority of examples to date, lignin-containing copolymers employ very little lignin (as low as 0.5% by wt.), and increasing the weight ratio of lignin in functional bio-based materials remains a significant challenge.12 Methacrylate polymers such as pHEMA [poly(2-hydroxyethyl methacrylate)] have been widely used in biomedical applications due to their biocompatibility and versatility in polymer fabrication. Cross-linked hydrogels containing pHEMA can swell and retain significant quantities of water within their networked structure. These gels have been commonly used in tissue regeneration, medical implants, and soft contact lenses.15 Bio-based hydrogels have been previously synthesized using lignin as the macro-initiator and ethylene glycol and acrylamide as the monomer units.11, 13 Biodegradable hydrogels based on pHEMA have also been synthesized, where methacrylate-functionalized spruce hemicellulose was mixed with an excess of HEMA and polymerized using a thermally-activated radical initiator.16,

17

The hemicellulose-pHEMA

hydrogel exhibited tunable and sustained release of the absorbed solution, which in turn was exploited to make designer seed coatings.18 In this study, we have synthesized and characterized random copolymers of lignin and pHEMA. Using the esterification reagent reported by Ranucci et al.,19 we modified the OH groups of lignin and then bulk polymerized the modified lignin with HEMA to produce copolymer hydrogels that contained as much as 40 wt% lignin. We tested water retention properties of the as-formed gels, as well as characterized other material properties using an array of thermal, physico-chemical, and dynamic rheological experiments in an effort to uncover fundamental physical and chemical properties of these lignin-containing hydrogels. We will demonstrate below that incorporating lignin into pHEMA hydrogels improves their water retention, thermal stability, and material strength. Lignin-containing hydrogels thus have diverse


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material functions, such that their potential applications could be expanded, but not limited to, enzyme immobilization, filtration membranes, seed coatings, and many others. Materials and methods General considerations Organosolv lignin (OSL) was fractionated from hybrid poplar (Populus trichocarpa ✕ P. deltoides) woodchips using an organosolv process as described elsewhere.8 Reagents including 1,1’-carbonyldiimidazole (CDI), 2,2’-azobis-(2-methylpropionitrile) (AIBN), endo-N-hydroxy5-norbornene-2,3-dicarboximide, triethylamine, and N,N’-dimethylformamide (DMF) were purchased from Sigma-Aldrich (St. Louis, MO). Anhydrous ethyl ether, acetyl bromide, and pyridine were purchased from ThermoFisher Scientific (Waltham, MA). Deuterated chloroform (CDCl3) was purchased from Cambridge Isotope Laboratories, Inc. (Tewksbury, MA). The phosphorylating reagent for lignin OH group analysis, 2-chloro-4,4,5,5-tetramethyl- 1,3,2dioxaphospholane (TMDP), was purchased from Santa Cruz Biotechnology Inc. (Dallas, TX) and was stored in a desiccator to protect it from atmospheric moisture. The monomer 2hydroxyethyl methacrylate (HEMA) was purchased from Acros Organics (Pittsburgh, PA). All chemicals were used as received except for HEMA, which contained G > H-OH groups.

Figure 1. (a) 31P NMR spectra of lignin, before (top) and after modification (bottom) of its OH groups with HEMA-Im. Chemical shifts for lignin OH groups were assigned as described by Balakshin and Capanema.20 (b) Percent modification of OSL aliphatic (black ), syringyl (red ●), guaiacyl (blue ▲), p-hydroxyphenyl (pink ▼), and carboxyl (green ◆) OH groups at various H: L ratios. Characterization of OSLH using DSC showed that the glass transition temperatures (Tg) of OSLH samples (Table 1) were lower than that of lignin (147.1 °C), which is common in cases of appending low molecular weight functional groups. Furthermore, FTIR spectroscopy of OSL and OSLH (Figure 2) showed a decrease in intensity of bands corresponding to valence vibrations of O–H (3459 cm-1), which we attribute to methacrylation of lignin OH groups.24 As shown in Figure 2, the FTIR spectra of OSLH also displayed an increase in intensity of valence vibrations


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of C=O (1735 and 1751 cm-1)24 and asymmetric stretching of C–O–C (1175 cm-1),23 which we ascribe to the presence of HEMA–lignin covalent linkages. The OSLH spectra additionally displayed a higher C–O stretching of G-lignin25 corresponding to 1259 cm-1 and a significant decrease in C-H deformation of S-lignin25 corresponding to 1123 cm-1. A decrease in signal intensity of syringyl lignin could be due to a higher HEMA substitution in S-OH groups (corroborated by 31P NMR data) and possible steric hindrances to S-ring stretching.

Figure 2. Fourier transform infrared (FTIR) spectra of lignin, before (OSL) and after modification (OSLH) with HEMA-Im. The inset shows an expanded view of the fingerprint region. Copolymers synthesis and characterization Lindblad et al.,16 reported that 10-30% of functionalization was optimal for hemicelluloseHEMA hydrogel synthesis. Accordingly, we employed OSL-23H as the macro-monomer for synthesis of OSLH-HEMA copolymer hydrogels. Unlike its homopolymer, each copolymer formed a stable gel with DMF, the solvent used for synthesis, and at a faster rate. The gelation time was significantly reduced for all OSLH-HEMA copolymers, ranging from 4-6 h, which we


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attribute to the formation of cross-linked gel networks. Originally, we tested different stoichiometric ratios of OSLH-to-HEMA, from 10 to 80%, but based on preliminary material analysis (see below), we chose the OSLH-10, -20 and -40 copolymers for detailed characterization. We used principal component analysis (PCA) of the FTIR spectra of OSLH-HEMA xerogels to reveal what chemical changes OSLH imparts to the OSLH-HEMA copolymers. The results of PCA showed a significant difference between the control, OSLH-10, -20 and -40 copolymers (Figure 3a). As expected, presence of OSLH significantly affected the spectral absorbance of the copolymer hydrogels. Principal component-1 (PC-1) accounted for 90% of the observed variance and its corresponding loadings plot is given in Figure 3b. It was shown that the OSLH-10 copolymer had significantly higher conjugated C=O stretch corresponding to 1727 cm-1, which we ascribe to the higher presence of HEMA side-chains. There was also an increase in νC–O–C asymmetric stretching at 1151 cm-1 and C–O deformation at 1075 cm-1. We once again ascribe these increases to higher degree of polymerization of HEMA in OSLH-10 copolymer as opposed to OSLH-20 and -40 copolymers. Alternatively, we attribute the lower modified lignin content of OSLH-10 copolymer to the lower display of aromatic C–H deformation of S-lignin corresponding to 1011 cm-1 and other spectral bands that are associated with C–C, C–O, and C=O stretching of lignin.


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Figure 3. (a) Principal component analysis (PCA) scores plot for FTIR spectra of pHEMA (red ) and OSLH-10 (blue ●), OSLH-20 (yellow ▲), and OSLH-40 (green ▼) copolymer hydrogels. (b) PC1 loadings plot for FTIR spectra of OSLH-HEMA copolymers. Lignin hydrogel properties Thermal properties. The random copolymers of OSLH and HEMA displayed a single Tg value and when plotted as a function of lignin content, the Tg -curve exhibited a convex trend as shown in Figure 4. Glass transition temperature values are useful to derive information on polymer chain stiffness and energetics, which in turn can be translated to material strength and other physical properties.31 As per an empirical rule obtained by Hirooka and Kato,32 if there exists a convex relationship between the Tg values and the composition of a random comonomer, then the corresponding alternating copolymer (of lignin and HEMA) would be expected to have a Tg lying above the convex curve. Based on this observation and further evidences provided by Daimon et al.,33 we propose that the internal plasticization effect provided by the HEMA side-chains could have been considerably decreased by the stiffness of copolymer backbones provided by the lignin macromere units, thus resulting in a convex Tg curve (Figure


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4a). Therefore, increase in proportion of OSLH in the copolymers would result in corresponding overall increase of polymer stiffness. The Tg of hydrogels is also dependent on the degree of cross-linking;34 previous reports have also shown that the Tg values of pHEMA-based hydrogels increased with the increase in concentration of cross-linking agents such as trimethacrylate.35 In the case of OSLH-HEMA copolymers, the functional groups of modified lignin may serve as cross-linking sites for adjacent HEMA polymer chains, thereby affecting the copolymer’s Tg values. It is therefore possible to tune the properties of the copolymer hydrogels by varying the stoichiometric ratio of HEMA to OSLH.

Figure 4. (a) Plot of typical glass transition temperatures (Tg) of copolymers versus modified lignin (OSLH) content. (b) Differential thermo-gravimetric (DTG) curves of pHEMA (red) and OSLH-10 (blue), -20 (yellow), and -40 (green) copolymer xerogels. (c) Plot of the amount of water retained by the swollen hydrogels over time. The color scheme is identical to (b). The thermal stability of OSLH-HEMA xerogels was also evaluated by thermogravimetric analysis (TGA) from 30 to 600 °C, under nitrogen flow. The degradation onset temperature, the temperature at which maximum rate of weight loss occurs (Tmax), and the residual weight of each


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sample after TGA analyses are summarized in Table 2. Increase in OSLH content led to an apparent increase in residual sample weight of the copolymers, with the highest value of 15% recorded for OSLH-40 xerogel. While the onset temperatures were similar for all samples, the Tmax temperatures were significantly higher for the OSLH-10, -20 and -40 xerogels. We also observed that the copolymers exhibited two-step degradation when compared to pHEMA in the first-order derivative of TGA thermogram (DTG) (Figure 4b). Based on the DTG data of control, we attribute the first stage of copolymer decomposition to the depolymerization of pHEMA, whereas the second stage may correspond to the decomposition of OSLH. Evidently, crosslinking of HEMA and OSLH imparted better thermal stability; kinetic studies of pHEMA decomposition have shown that, a higher activation energy possessed by the OSLH-HEMA cross-linkages could be the reason for the relative increase in resistance to thermal decomposition.36 Table 2: Thermal properties of copolymer xerogels consisting of OSLH and HEMA. Thermogravimetric analysisa Name

% OSLH

Degradation onset (°C)

Tmax (°C)

Weight loss (%)

Tg (°C)a,b

pHEMA

0

239(9)

342(3)

99(1)

85(1)

OSLH-10

10

250(7)

431(1)

96(1)

95(2)

OSLH-20

20

250(5)

432(2)

92(2)

110(2)

OSLH-40

40

260(10)

431(1)

85(1)

121(4)

a

Means and standard deviations are provided for N = 3.

b

Measured using DSC.

Hydrogel swelling and shrinkage. The control and OSLH-HEMA hydrogels displayed swelling when soaked in water; measurements based on time indicated that equilibrium weights were achieved after 10 minutes for the control and OSLH-10 hydrogels, whereas the OSLH-20


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and -40 hydrogels attained equilibrium after 15 minutes. Apart from the OSLH-40 hydrogel, there was no significant difference in equilibrium swelling ratio between the control and copolymer hydrogels; the average water absorption capacity was 105 ± 10% of the dry gel weight. We attribute the significant decrease in equilibrium-swelling ratio of the OSLH-40 hydrogel to the increase in crosslinking density, which has been reported to affect water mobility within the polymer network.37 Properties such as solvent diffusion in a hydrogel network play an important role in developing applications for copolymer hydrogels. These results show that it is possible to alter the hydrophobicity of copolymer hydrogels by controlling the OSLH content.

Table 3. OSLH-HEMA hydrogel crosslink density based on water swelling behavior.

Copolymer

V2,sa

Specific volume '̅ (cm3/g)

Mc,sb

Cross-link density, ρc (mol/cm3)

OSLH-10

0.14

1.17

1406

9.53 × 10-4

OSLH-20

0.10

0.96

533

1.96 × 10-3

OSLH-40

0.12

0.91

493

1.62 × 10-3

a

Final swollen equilibrium polymer volume fraction

b

Average molecular weight per cross-linking unit

The copolymer hydrogels also showed less shrinkage after reaching the equilibrium waterswelling ratio, which was significantly superior to the control. Figure 4c depicts the weight percent of water retained within the control and OSLH-HEMA hydrogels over a period of 960 min. At the end of testing period, OSLH-40 hydrogel displayed 39% improvement in water retention when compared to pHEMA. Improvement in hydrogel stability and reduction in hydrogel shrinkage were attributed to the increase in cross-linking between adjacent HEMA


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polymer chains. Based on the equilibrium gel swelling ratios, the average molecular weight per cross-linking unit (Mc,s) for different OSLH-HEMA hydrogels could be estimated (eqn. 1) and the computed results are summarized in Table 3. The effect of hydrophobic interactions between OSLH and water was omitted, because studying multiple unknown variables involved in χ parameter estimation would exceed the scope of this study. Moreover, the molecular make-up of OSLH and pHEMA are quite different, thus the cross-linking behavior during copolymerization of OSLH and HEMA will be random. Therefore, at best, the Mc,s values will provide a coarse average estimate of the cross-linking density of the copolymer hydrogels.38 There was a seemingly exponential increase in cross-link density between OSLH-10, -20 and -40 hydrogels, indicating that the feed ratio of OSLH-to-HEMA is important for controlling the hydrogel material properties. In case of branched and cross-linked copolymers like OSLH-HEMA, addition of multifunctional lignin cores would apparently lead to increase in cross-linking between the HEMA polymer branches. Dynamic rheological behavior. The hydrogels, swollen to equilibrium weight in water, were subjected to dynamic rheological testing in order to evaluate the material behavior in shear. The linear viscoelastic strain cut-offs for control, OSLH-10 and -20 hydrogels were at 0.6, 0.3, and 0.15%, respectively (Figure 5a). In case of OSLH-40, the linear viscoelastic region occurred below 0.1% strain (data not shown), which conveys that its heavily cross-linked structure lent itself a solid-like behavior under the tested conditions, and hence was not included with the other hydrogels displaying comparable viscoelastic deformations. The shear storage modulus (G’) of OSLH-10 and -20 hydrogels displayed a 1.7- and 2.6- fold increase in magnitude, respectively, when compared to the control, indicating that the increase in proportion of OSLH lent rigidity and structural stability. Increase in cross-linking of OSLH-10 and -20 copolymers might be


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responsible for the observed corresponding increase of G’, which in turn can be correlated to the reduction in hydrogel shrinkage. Apart from chemical crosslinking, hydrogen bonding and hydrophobic associations also play an important role in improving the mechanical strength of hydrogels. Grafting a large hydrophobic macro-monomer to the pHEMA network could contribute to stronger hydrophobic associations and improved copolymer hydrogel stiffness.39 The loss factors, which are the ratio of viscous (G”) and elastic (G’) portions of the viscoelastic deformation, demonstrate a significantly higher frequency-dependence for pHEMA than the OSLH-HEMA hydrogels (Figure 5b). The control hydrogel exhibited a higher viscous response at lower angular frequency and later stiffened to provide a more elastic response at higher angular frequencies. Therefore, we infer that the hydrogel network of pHEMA collapsed under higher frequencies. The G” and G’ moduli of OSLH-10 and -20 copolymers were relatively frequency-independent and the hydrogels exhibited a solid-like behavior. Also, the elastic modulus component (G’) was significantly larger than the viscous modulus component (G”), which is characteristic of a well-developed and stable cross-linked polymer system.40 This shows that, addition of OSLH and the resultant physical and chemical cross-linking influenced the hydrogel stability. Overall, modulating the amount of OSLH is essential to optimize the material strength as well as the hydrogel’s water retention capacity; OSLH content at ≤ 20% stabilizes the copolymer network and therefore, enhances its water retention capacity.


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Figure 5. (a) Stress sweep at ω= 0.1 and (b) Frequency sweep of water-swollen pHEMA (red ) and OSLH-10 (blue ●) and -20 (yellow ▲) copolymer hydrogels, at 25 °C. Conclusion Lignin from bio-refinery product streams could thus be effectively utilized for the synthesis of sustainable and eco-friendly polymeric materials. Functional copolymers containing up to 40% by weight of lignin were synthesized using a methacrylate-modified lignin and HEMA as comonomers. The copolymer hydrogels exhibited enhanced water retention, material strength and thermal stability. Copolymerization with modified lignin afforded a densely cross-linked and stable network system that led to the observed improvement in hydrogel properties. Finally, by modulating the percent functionalization of lignin or the stoichiometric ratio of HEMA-to-lignin it is possible to tune the copolymer’s material properties.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel: +1 (865) 946-1106.


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This project was supported by funds from the Southeastern Sun Grant Center and the US Department of Transportation, Research and Innovative Technology Administration DTOS5907-G-00050. T.G.R., N.L., and S.C.C. also acknowledge the Southeastern Partnership for Integrated Biomass Supply Systems (IBSS), which is supported by AFRI 2011-68005-30410 from USDA NIFA. D.P.H., N.L., and S.C.C. also acknowledge support from the USDA National Institute of Food and Agriculture, Hatch Project 1012359.

ACKNOWLEDGMENT The authors wish to thank Prof. Brian Long (UTK Chemistry) for helpful discussions and input and Ms. Katrina Pangilinan (UTK Polymer Characterization Laboratory) for help with GPC data collection and analysis. REFERENCES 1.

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FOR TABLE OF CONTENTS USE ONLY

Sustainable hydrogels based on lignin-methacrylate copolymers with enhanced water retention and tunable material properties


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Figure 1. (a) 31P NMR spectra of lignin, before (top) and after functionalization (bottom) of its OH groups with HEMA-Im. Chemical shifts for lignin OH groups were assigned as described by Balakshin and Capanema.18 (b) Plot of degree of modification of OSL aliphatic (black ν), syringyl (red ●), guaiacyl (blue ▲), p-hydroxyphenyl (pink ▼), and COOH (green ◆) OH groups versus various H:L ratios. 190x76mm (300 x 300 DPI)



Figure 2. Fourier transform infrared (FTIR) spectra of lignin, before (OSL) and after modification (OSLH) with HEMA-Im. (Inset: expanded view of the fingerprint region) 202x141mm (300 x 300 DPI)


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Figure 3. (a) Principal component analysis (PCA) scores plot for FTIR spectra of 100% pHEMA (red ν) and OSLH-HEMA graft copolymers containing 90% (blue ●), 80% (yellow ▲), and 60% (green ▼) HEMA. (b) PCA loadings plot for FTIR spectra of pHEMA and OSLH based graft copolymers. 190x76mm (300 x 300 DPI)



Figure 4. (a) Plot of typical glass transition temperatures (Tg) of copolymers versus modified lignin (OSLH) content. (b) Differential thermo-gravimetric (DTG) curves of 100% pHEMA (red) and OSLH-HEMA graft copolymer xerogels containing 10% (blue), 20% (yellow), and 40% (green) OSLH. (c) Plot of the amount of water lost by the hydrogels over time. The color scheme is identical to (b). 190x76mm (300 x 300 DPI)


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Figure 5. (a) Stress sweep at ω= 0.1 and (b) Frequency sweep of water-swollen pHEMA (red ν) and OSLHHEMA copolymer hydrogels containing 10% (blue ●) and 20% (yellow ▲) lignin, at 25 °C. 177x76mm (300 x 300 DPI)


Sustainable Hydrogels Based on Lignin-Methacrylate Copolymers

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