Amphiphilic Copolymers Derived from Butanosolv Lignin and Acrylamide: Synthesis, Properties in Water Solution, and Potential Applications

University of Groningen

Amphiphilic Copolymers Derived from Butanosolv Lignin and Acrylamide

Migliore, Nicola; Zijlstra, Douwe S.; Kooten, Theo G. Van; Deuss, Peter J.; Raffa, Patrizio

ACS Applied Polymer Materials DOI:

10.1021/acsapm.0c01006

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Migliore, N., Zijlstra, D. S., Kooten, T. G. V., Deuss, P. J., & Raffa, P. (2020). Amphiphilic Copolymers Derived from Butanosolv Lignin and Acrylamide: Synthesis, Properties in Water Solution, and Potential Applications. ACS Applied Polymer Materials, 2(12), 5705-5715. https://doi.org/10.1021/acsapm.0c01006

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Amphiphilic Copolymers Derived from Butanosolv Lignin and

Acrylamide: Synthesis, Properties in Water Solution, and Potential

Applications

Nicola Migliore, Douwe S. Zijlstra, Theo G. Van Kooten, Peter J. Deuss, and Patrizio Raffa

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ABSTRACT: In this work, a series of amphiphilic lignin-acrylamide copolymers was synthetized via a“grafting from” approach using α-butoxylated organosolv lignin. This lignin is obtained in high yield via a mild organosolv extraction with butanol and contains a well-defined modified β-O-4 structure that allows for site-selective modification of the primary alcohol in the γ-position. The modified lignin was then used as a precursor of amphiphilic copolymers by reaction with acrylamide, either via free radical polymerization or via atom transfer radical polymerization after converting the lignin into a suitable macroinitiator. The effect of the synthetic method and acrylamide/lignin ratio on the final properties was studied and compared. Relevant solution properties, in particular, shear viscosity and interfacial and surface tension, showed that different synthetic methods and polymer compositions allow a tuning of the solution behavior toward specific potential applications, such as emulsion stabilization or enhanced oil recovery. Furthermore, it was preliminarily shown that the obtained polymers may potentially display low cytotoxicity, further increasing the possibilities for applications.

KEYWORDS: amphiphilic polymers, organosolv lignin, bio-based materials, emulsifiers, enhanced oil recovery, ATRP

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Due to the need of a more sustainable economy, the synthesis of new polymeric materials from bio-based sources is attracting great interest in recent years. The synthesis of such polymeric materials can be accomplished either by the chemical modification of biomaterials such as starch,1−4 biopolymers such as chitosan5−7and alginate,8−10or by the polymerization of bio-derived monomers, such as monosaccharides, terpenes, fatty acids, and many others.11−20 These synthetic (co)-polymers can potentially be used in diverse application areas such as biomedicine, health care, and agriculture.21Bio-based sources such as carbohydrates or lignin can also be used to make amphiphilic polymers.22,23 The use of lignocellulosic materials is particularly attractive, as it enables the valorization of inedible plant waste. Lignin is the second most abundant natural polymer after cellulose; it currently has limited industrial applications, and it is typically burned as residual low-value fuel in many industries. Due to its hydrophobic nature, it can potentially be used to synthetize amphiphilic copolymers if coupled with hydrophilic moieties.

Some examples of lignin-based amphiphilic copolymers are reported in the literature; lignin is typically coupled with monomers containing highly hydrophilic functionalities, such as hydroxyl, carboxyl, amides, or PEG units.8,20,24,25 These systems have been prepared and studied for applications such as drug delivery26or water purification.27An interesting class of these amphiphilic copolymers is lignin-g-polyacrylamide that are particularly interesting for applications such as emulsion

stabilization and particle dispersion.27−29 However, lignin precursors used in the previous work are obtained by the traditional Kraft process, which affords a material characterized by an ill-defined molecular architecture and complex product structures with scarce reproducibility and final properties strongly varying from batch to batch.30 The use of well-characterizable lignin that closer resembles the structure of native lignin in terms of linking motifs, in particular, theβ-aryl linking motif, is the key to obtain reproducible materials and it enables a study of structure−property relationships in lignin-derived synthetic polymers.

Most applied fractionation methods, such as the mentioned Kraft process, focus on the complete removal of lignin from the cellulose fraction, but due to the combination of chemical reactants and harsh conditions, these yield condensed lignin with a highly complex irregular structure compared to the original one.31−34Milder methods can yield a more structurally defined lignin with a higher resemblance to the native structure.35−39 One of the emerging strategies is mild organosolv extraction, where an organic solvent, in many Received: September 10, 2020

Accepted: November 23, 2020

Published: December 3, 2020

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cases in combination with water and an acid, allows for the extraction of lignin with a highβ-aryl content.39−41Especially, in combination with n-butanol, high retention of the β-O-4 motif and low degradation can be achieved due to in situ selective alkoxylation of the benzylicα-hydroxyl in the β-O-4 structure, which enhances solubility in the extraction liquid.42−44 This butanosolv lignin can be then converted into an amphiphilic polymer by direct incorporation into polyacrylamide via free radical polymerization, or into more defined structures via ATRP. Of course, structures with different complexity and properties can be expected, as schematically represented inScheme 1.

The interest in amphiphilic polymer is wide. For example, they possess interesting rheological properties in water solution,45,46 which can be exploited in many applications, such as the production of hydrogels, or for enhanced oil recovery (EOR).47−50 Moreover, analogously to their low molecular weight counterparts, amphiphilic polymers have the ability to self-assemble into micellar aggregates and to adsorb at the oil/water interface and generate highly stable emulsions.51−54

The self-assembly of amphiphilic polymers into nano- or microstructures has another important consequence, as it offers the possibility to use those systems as biomimetic nano-capsules for drug or gene delivery. It is important that those polymers display low cytotoxicity when intended for biomedical applications; therefore, naturally derived building blocks, such as lignin, can offer an advantage.55−58

This work explores the preparation of a unique set of lignin-g-polyacrylamide copolymers, starting from a butanosolv lignin with a relatively well-defined chemical structure, which is highly soluble in organic solvents. Here, in contrast with a previous work using Kraft lignin, the unique structure of the lignin starting material allows for efficient modification to form a macroinitiator, which can be used in an ATRP process to generate amphiphilic polymers (Scheme 1). Moreover, an extensive characterization of solution properties was per-formed. Various synthetic methods and different acrylamide/ lignin molar ratios were used in order to have a broader overview into the structure−property relationships of those

polymers. Our attention was focused on rheology, surface, and interfacial tension in water solutions, as they are of importance for many potential applications such as enhanced oil recovery50 and emulsion stabilization. Finally, toxicity tests on lignin-based polymer solutions and lignin-lignin-based polymers were performed in order to evaluate if those polymers can be potentially used in biological environments.

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Materials. Walnut powder (produced in our lab from walnuts shell), acrylamide (AM, Sigma-Aldrich, for electrophorensis,≥99%), α-bromoisobutyryl bromide (BIBB, Sigma-Aldrich, 98%), pentam-ethyldiethylenetriamine (PMDETA, Aldrich, 99%), hydrazine mono-hydrate (Sigma-Aldrich, 98%), 2,2′-azobis(2-methylpropionitrile) (AIBN, Sigma-Aldrich, 98%), methanol (MeOH, Sigma-Aldrich 99.8%), tetrahydrofuran, (THF, Sigma-Aldrich, ≥99.9%), diethyl ether (Sigma-Aldrich, anhydrous,≥ 99.7%), N,N-dimethylformamide (Sigma-Aldrich, anhydrous, 99.8%), n-hexane (Macron, 99%), acetic acid (Sigma-Aldrich, ACS reagent,≥99.7%), ethanol (denatured with about 1% methyl ethyl ketone for analysis EMSURE), ethyl acetate (Sigma-Aldrich, anhydrous, 99.8%), butanol (Sigma-Aldrich, for molecular biology, ≥99%), hydrochloric acid (HCl, Sigma-Aldrich, ACS reagent 37%)), sodium sulfate (Na2SO4, Sigma-Aldrich,

anhydrous), and CuBr2(Sigma-Aldrich, 99%) were used as received.

CuBr (Sigma-Aldrich,≥ 98%) was stirred in glacial acetic acid for at least 5 h and thenfiltered, washed with acetic acid, ethanol and ethyl acetate, and dried under vacuum before use. For the cytotoxicity test, Dulbecco’s Modified Eagle Medium (DMEM) was used, containing 4.5 g/L D-glucose, GlutaMax, 100 U/mL penicillin, 100 ug/mL streptomycin, and 10% Fetal Bovine Serum. Dimethyl sulfoxide (DMSO, Sigma-Aldrich,≥ 99%) was deoxygenated by bubbling with argon for at least 40 min before use. Dimethyl sulfoxide-d6

(DMSO-d6, anhydrous, 99.9 atom % D, Sigma-Aldrich) and deuterium oxide

(D2O, Sigma-Aldrich 99.9 atom%D) were used as deuterated solvents

for NMR studies.

Synthesis. Preparation of Butoxylated Lignin (LigBu) from Ground Walnut Shells. Walnut shells were pretreated as described in the literature to generatefine, extractive-free, walnut powder.59A total of 30 g of extract-free walnut powder was placed in a 500 mL round bottomflask. A total of 240 mL n-butanol, 60 mL water, and 18 mL of HCl (12 M) solution were added to theflask. The mixture was heated at 120°C for 24 h, after which it was allowed to cool down to room temperature. The mixture was filtered, and the filtrate was concentrated in vacuum. The obtained solid was redissolved in a Scheme 1. Different Ways to Synthetize Lignin-Based Materials Used in This Work: Grafting from (Blue) and Free Radical Polymerization (Green)

minimal amount of acetone and added to 450 mL water to which an additional 50 mL of a saturated aqueous Na2SO4solution was added

to further improve the precipitation of the lignin product. The precipitated lignin product was collected byfiltration and air dried overnight to yield α-butoxylated lignin (8.7 g, 62% extraction efficiency based on Klason lignin after correction of alcohol incorporation).

Synthesis of Butoxylated Lignin-Bromide (LigBr) Macro Initiator. A total of 7 g of butoxylated lignin was dissolved in 140 mL of anhydrous DMF in a 500 mL three neck bottomflask, under argon. Based on the equivalent weight of the repeating unit of lignin (238.3 g/mol, calculation based on the S/G/H ratio of the lignin, structure in

Scheme 2), an equimolar amount (6.7 g; 0.029 mol) of 2-bromo-2-methylpropionyl bromide (BIBB) was dissolved in 100 mL of anhydrous DMF and then added dropwise into a stirred lignin solution, previously cooled to 0 °C, under argon. The reaction mixture was constantly purged with argon via a needle through a rubber septum in order to remove the by-product (HBr) from the reaction. The solution was stirred overnight at room temperature and then the mixture was added dropwise to an excess of MilliQ water to precipitate a solid product. The precipitate of LigBr was recovered by filtration and dried for 5 h by exposition to the air and then in the oven at 60°C overnight and then characterized by FT-IR, NMR and GPC.

ATRP of Acrylamide on Butoxylated Lignin-Bromide Initiator. Lignin-bromide-g-polyacrylamide (LigBr-g-PAM) was synthesized via ATRP as follows: LigBr (1 meq monomeric units), CuBr (1 mmol), acrylamide (50 mmol), and deoxygenated DMSO (25 mL) were introduced under argon in a 100 mL round-bottomedflask equipped with a magnetic stirring bar and a reflux condenser, previously purged with argon for 40 min. The apparatus was put in an oil bath set to a temperature of 60 °C. After 1 min, PMEDTA (2 mmol) was introduced under argon. After 24 h, the reaction was stopped by cooling down, introducing air, and diluting with around 20 mL of DMSO. The DMSO solution was precipitated in a 20-fold excess of diethyl ether. The crude product was dissolved in MilliQ water and dialyzed, using Spectra membrane 3,5 KDa, against MilliQ water in order to remove the CuBr and other low-molecular weight impurities. The external water was changed at least three times over a period of 2 days, and then the content of the bag was dried at 60 °C under vacuum for 24 h. A total of 1.3 g of a dark brownish powder was obtained. The molecular weight was determined by GPC, and the effective ratio between the lignin and acrylamide was determined gravimetrically.

Synthesis of Butoxylated Lignin-Bromide-g-Acrylamide (LigBr-g-PAM) via ARGET ATRP. The butoxylated lignin-bromide-g-acrylamide polymers were synthesized via ARGET ATRP as follows: LigBr (1 mmol), CuBr2 (0,005 mmol), acrylamide (35 mmol - 80 mmol),

PMEDTA (0.05 mmol), and deoxygenated DMSO (25 mL) were introduced under argon in a 100 mL round-bottomedflask, equipped with a magnetic stirring bar and a reflux condenser, previously purged with argon for 40 min. The apparatus was put in an oil bath set to a temperature of 60°C (Scheme 3). After 1 min, the Hydrazine (0.4 mmol) was introduced under argon. After 24 h, the reaction was stopped by cooling down, introducing air, and diluting with around 20 mL of DMSO. The DMSO solution was precipitated in a 20-fold excess of diethyl ether then drying at 60°C under vacuum for 24 h. A total of 1.4 g of LigBr-g-PAM ARGET1 and 1.6 g of LigBr-g-PAM ARGET2 were obtained as brownish powder. The molecular weight was determined by GPC, and the effective ratio between the lignin and acrylamide was determined gravimetrically.

Synthesis of Butoxylated Lignin-g-Acrylamide (LigBu-g-PAM) via Free Radical Polymerization. The butoxylated lignin-g-acrylamide polymers via free radical polymerization were synthesized according the following procedure. Butoxylated lignin (1 mmol), acrylamide (35−80 mmol), AIBN (1.5 wt %), and deoxygenated DMSO (24 mL) were introduced under argon in a 100 mL round-bottomedflask equipped with a magnetic stirring bar and a reflux condenser, previously purged with nitrogen for 40 min. The apparatus was put in an oil bath set to a temperature of 65°C. After 6 h, the reaction was stopped, due to the high viscosity, and cooled down, introducing air and diluting with around 20 mL of DMSO. The organic solution was precipitated in a 20-fold excess of diethyl ether. The precipitate was washed with THF and dried overnight at 60°C, affording 2.2 g of a light brownish solid. The molecular weight was determined by GPC, and the effective ratio between the lignin and acrylamide was determined gravimetrically.

Characterization. The polymers were characterized by the following analysis. The 2D HSQC and H-NMR spectra were recorded on a Varian Mercury Plus 600 MHz spectrometer, using DMSO-d6as the solvent. The infrared (IR) spectra were recorded by

FT-IR with a Shimadzu IR-Tracer-100 with a golden gate diamond ATR sample unit in the range of 4000 to 500 cm−1at a resolution of 4 cm−1 averaged over 64 scans. The molecular weights of all the polymers were measured by GPC analysis. The measurements were performed using an SEC system consisting of an isocratic pump, auto sampler without temperature regulation, online degasser, 0.2 μm inline filter, refractive index detector (G1362A 1260 RID Agilent Scheme 2. Synthesis of Butoxylated Lignin-Bromide (LigBr) Macro Initiator

Scheme 3. Synthesis of Butoxylated Lignin-Bromide-g-Acrylamide (LigBr-g-AAM) via Traditional ATRP (1) and ARGET Variant (2)

Technologies), viscometer (ETA-2010 PSS, Mainz), and MALLS detector (SLD 7000, PSS, Mainz). The samples were injected with a flow rate of 0.5 mL min−1_{into an MZ Super-FG 100 SEC column and}

two PFG SEC columns 300 and 4000. The columns were held at 80 °C, and the detectors were held at 60 °C (Visco) and 45 °C (RI). A standard pullulan kit (PSS, Mainz, Germany) with molecular weights from 342 to 805,000 Da was employed to generate a calibration curve. The data were processed with the WinGPC Unity software (PSS, Mainz). The samples were dissolved in DMSO-LiBr (0.05 M), also used as eluent, at a concentration of 2 mg/mL by overnight stirring at room temperature. All samples werefiltered through a 0.20 μm PTFE membrane before injected. Due to the unavailability of values of dn/ dc and Mark−Houwink fit parameters for our systems, it was not possible to obtain absolute values for molecular weight. This is made particularly difficult, if not impossible, by the fact that the polymers are not uniform in structure and chemical composition.

The rheological measurements were performed with an HAAKE Mars III (Thermo Scientific, Waltham, MA, USA) rheometer equipped with a cone−plate geometry (diameter 60 mm, angle 2°) using 2 mL of solution. Solution viscosity was measured as a function of shear rate (0.1 to 1750 s−1, T = 20°C), using a trap for the solvent in order to avoid water evaporation during the measurements and be careful to not trap air during the loading of the solutions on the plate, so that no rest time was needed. Solutions with concentration between 5.0 and 10.0 wt % were prepared by dissolving the polymers in MilliQ water followed by stirring for at least 10 h before the measurement in order to get homogeneous solutions. Surface tension was measured with an OCA 15EC tensiometer from Dataphysics, using the pendant drop method. Interfacial tension was measured with an SVT20−Spinning drop video tensiometer from Dataphysics. The polymer solutions were analyzed by cryo-TEM in order to evaluate micelles formation. The samples were examined in an FEI T20 electron microscope operating at 200 keV. Images were recorded on a slow scan CCD camera. A drop of the copolymer solution was placed on a glow discharged plain carbon-coated 400 mesh copper grid. Dead endflow cells experiments were performed using a 2D cell, consisting of an aluminum bottom and a transparent plastic top cover with size-varied chambers to consecutively simulate dead end pores in oil reservoirs. The experimental procedure and the setup are described

in a previous work.60The image analysis was performed using Adobe Photoshop CS6 as reported by Raffa et al.60_{The cytotoxicity of the}

two lignin-based polymers was evaluated as follows. The polymer solutions were tested with macrophages (J774) and fibroblasts (L929). Solutions were added to the growth medium (DMEM) in a ratio of 1:9, i.e., 10% of thefinal liquid consisted of the dissolved polymer and 90% consisted of the medium. Cells were cultured for 24 h in 12-well plates. Then, the polymer-containing medium was added and cells were exposed to the polymer for another 24 h. In order to create sterile polymer solutions, the received solutions were scheduled to be filtered over a sterile 0.45 μm filter, which led to almost immediate blockage of thefilter probably due to the dimensions of the micelles that the polymers form. Therefore, centrifugation at 13.000 g was performed. The supernatants were kept for further testing, with a suspected infection of LigBu-g-PAM AIBN2. Instead, LigBr-g-PAM ARGET1 gave a more colloidal-like solution.

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Synthesis of Butoxylated Lignin-g-Acrylamide (LigBu-g-PAM) and Butoxylated Lignin-Bromide-g-Acrylamide (LigBr-g-PAM) Polymers. Living ization (ATRP, ARGET-ATRP) and free radical polymer-ization were chosen as methods to synthetize polymers whit different architectures. Additionally, the acrylamide/lignin ratio was varied, in order to tune the amphiphilic character of the systems studied.

For this study, butanosolv lignin was extracted from walnut shells under specific mild conditions to retain the dominant β-O-4 backbone. Walnut shells, a waste material from the food industry, are particularly rich in lignin (40.3% lignin content as determined by the Klason method), which make them an interesting starting material for lignin extraction. The extraction was performed at mild organosolv extraction conditions (120°C), leading to lignin with a very high β-O-4 content (7β-O-4 per 100 aromatic units as compared to around <10 per 100 for technical organosolv) at relatively good extraction efficiency (∼50%).59 Due to the use of a relatively Figure 1.2D HSQC spectra (d6-DMSO) of the aliphatic region (left, [(1H; 0−2.3 ppm)(13C; 0−40 ppm)]) and the oxygenated aliphatic region

high concentration of acid in combination with n-butanol, most of theβ-O-4 motifs are butoxylated at the α-position (63 out the 74 β-O-4 linkages, Figure 1), which was shown to facilitate the efficient extraction of lignin and prevent C−C bond formation via condensation reactions and thus keeping the amount of phenolic groups low.44Noγ-modification of the dominantβ-O-4 linking motif was observed as no shift of the γ-signal was observed by 2D HSQC spectroscopy and 31

P-NMR spectroscopy clearly showed the presence of aliphatic −OH groups in the isolated lignin structure (Figure S1). The absence of γ-alkoxylation is in accordance with previous alcohol incorporation experiments performed on model compounds.44 This selective modification at the α-position also allows for targeted modification of the primary alcohol in the γ-position of the β-O-4 motif.42,61 For this reason, this butoxylated lignin is particularly suitable for the synthesis of lignin-based copolymers using a controlled polymerization approach, where the hydroxyl functionality can be easily converted by reaction with BIBB to form a suitable multifunctional ATRP macroinitiator (Scheme 2).

HSQC NMR analysis of the obtained precipitated material from a reaction of BIBB with LigBu at 0°C clearly showed the signal at (1.75, 29.8) ppm, which corresponds to the two introduced methyl groups of the macro initiator (Figure 1, left). Additionally, NMR analysis shows that no significant degradation of the butoxylatedβ-O-4 linking motif occurred under the applied reaction conditions as can be seen in the oxygenated aliphatic region (Figure 1, oxygenated aliphatic region) from which can be calculated that a total of 72β-O-4 linking motifs per 100 aromatic units remain of which 62 are butoxylated. A reaction with BIBB was also performed on a C3 lignin model compound, which confirmed that the modifica-tion does indeed not cause significant shifts in the signals for theβ-O-4 motif (see Supporting Information,Figures S2 and S3).

The FTIR spectra of the LigBr displays a new signal at 1722 cm−1 associated to CO stretching of the α-halogen ester,1 enough to confirm the NMR analysis (Figure 2).

With the goal of obtaining a well-defined branched polymer structure similar to the one reported in the literature,33,62,63 LigBr was then used as macroinitiator for traditional ATRP

(Scheme 3, reaction 1) and ARGET ATRP (Scheme 3, reaction 2). The use of the ARGET variant of the ATRP process presents the advantage over traditional ATRP of using a much lower amount of metal catalyst,64 which avoids long purification procedures to obtain a Cu-free polymer, making the process more appealing from a potential industrial point of view. In order to prove that in the ATRP and ARGET processes, the AM chains were grown on the Br site, affording the desired graft structure, a blank experiment was carried out replicating ARGET and ATRP conditions but using pristine LigBu as the macroinitiator. No polymerization occurred in this case, and LigBu was recovered unaltered, proving that no free radical polymerization is active in these conditions, and therefore excluding the formation of acrylamide homopol-ymers.

Free radical polymerization of acrylamide in the presence of LigBu was also performed using a thermal initiator since the polymer obtained from this method has a much more irregular structure and in principle will present rather different solution properties. Lignin is known to be a radical scavenger;65,66 therefore, chain transfer reactions are expected to occur during AM radical polymerization, causing random incorporation of lignin units in the growing polymer chains and uncontrolled branching and cross-linking (Scheme 1).

All the copolymers were isolated as brown/brownish powders, upon precipitation in diethyl ether and drying, and characterized by 2D HSQC NMR, FT-IR, and GPC (Figure S4, Figure S5, and Figure 3, respectively). The conversion of the monomer for each reaction was calculated gravimetrically (Table 1).

The GPC chromatogram (Figure 3) shows that the polydispersity of the obtained polymers is extremely high, especially for controlled polymerization. However, it must be considered that the starting lignin already possess broad molecular weight distribution, which remains constant upon chain extension. Numerical values of molecular weight and PDI must be considered only as relative values, considering that the calibration of the GPC is made with narrow and linear standards of pullulan, structurally and chemically very different Figure 2. FT-IR spectra comparison between pristine LigBu and

LigBu modified with BIBB (LigBr).

Figure 3. GPC chromatograph comparison between the pristine LigBu, modified LigBu with Br, the copolymers obtained via controlled copolymerization (LigBr-g-PAM ATRP, LigBr-g-PAM ARGET1 and LigBr-g-PAM ARGET2) and reference copolymers produced by radical polymerization (LigBu-g-PAM AIBN1 and LigBu-g-PAM AIBN2).

from our systems. For comparison, it can be also noticed here that the used DMSO/pullulan standard method provides slightly higher Mnand PDI for the starting lignin, with respect

to typically reported THF/polystyrene standards,44as can be seen by comparing Figure 3 to Figure S6 in the Supporting Information. The choice of DMSO as eluent in our case was imposed by the solubility of the copolymers.

Most of the produced copolymers present broad and non-monomodal peaks, suggesting that the process is not controlled. However, no significant peak of residual macro-initiator is visible and this let presume that the chain extension of the lignin was always achieved.

Comparing the different ATRP synthetic methods, the ARGET variation gives better results in terms of conversion of the monomer, compared to the traditional ATRP. Kinetic experiments on the ARGET polymerization were carried out in order to verify the effective livingness of the polymerization (see Supporting Information,Figure S7). It is evidenced that the amount of radical formed during the reaction is not constant overtime. Together with the non-monomodal and large molecular weight distribution, this indicates that the polymerization is hardly controlled (Figure S7). However, it is still possible to say that the mechanism involved is not through free radical, as confirmed by the blank experiment (seeTable 1) and by the fact that conversion and molecular weight keep increasing with time. Assuming that the lignin macroinitiator reacts completely, the GPC traces give an indication of the lig/ AM molar ratio. Namely, the polymers synthetized via ATRP methods have a higher relative amount of lignin, so they are less hydrophilic than the ones prepared by free radical polymerization. However, it must be considered that for the free radical polymerization, a significant amount of poly-acrylamide homopolymer was most likely formed, which was not possible to separate from the copolymer.

As anticipated, the structure obtained by free radical polymerization is expected to be rather different than that of the other polymers, with the presence of random branching and cross-linking, and uncontrolled incorporation of several lignin units in the chains. (Scheme 1).

Water Solubility. The main focus of this work was to evaluate these new amphiphilic copolymers for applications in water solution. Therefore, the solubility of the obtained products was evaluated by dissolution in deionized water. All polymers proved to be water soluble up to concentration as high as 10 wt %. However, while the solutions obtained by dissolving the polymers synthetized by the living

polymer-ization mechanism (ATRP and ARGET) are completely transparent, those obtained from free radical polymerization are cloudy (Figure S8) and after 3 weeks, a small amount of precipitate appears on the bottom. The precipitate is probably unreacted LigBu that was kept in solution by hydrophobic interaction with other chains of LigBu-g-PAM; however, after a long period of time, this LigBu precipitated. This presence of unreacted LigBu could slightly affect the actual ratio LigBu/ AM present in the polymers making the estimated one slightly higher than the real one.

Solution Properties. It is known in the literature that the different polymer architecture can lead to a different assembly in solution, and consequently, a different rheological behavior can appear.47,67 For this reason, the rheological behavior in solution of all copolymers synthetized in this work was studied in some detail.

Shear viscosity was measured for solutions at concentrations from 0.5 to 10 wt %. The results for the more concentrated solutions, which have the highest viscosity values, are displayed inFigure 4. Measurements at lower concentration are reported in the Supporting Information (Figure S9). Higher viscosities seem to be mostly associated to higher molecular weight. LigBu-g-PAM AIBN1 solution has the highest values at 10 wt % and display remarkable viscosity values already at a Table 1. Molecular Characterization of Studied Polymersd

GPC (kDa)

synthesis sample feed AM/Lig (meq ratio) M_n M_w D conv (%)b _{AM/Lig (meq ratio)}c

LigBu 2.18 11.44 5.22

LigBr 2.47 17.21 6.95

ATRP LigBr-g-PAM ATRP 50 3.12 22.15 7.10 29 14.5

ARGET LigBr-g-PAM ARGET1 80 15.41 92.41 6.00 42 33.5

ARGET LigBr-g-PAM ARGET2 35 9.67 75.28 7.78 79 27.5

free radical LigBu-g-PAM AIBN1 80 33.61 241.38 7.18 63 50.5

free radical LigBu-g-PAM AIBN2 35 6.35 125.98 19.8 62 21.5

ARGETa LigBu-g-PAM TEST 80 nd nd nd 0

a_{Experiment was carried out to show that without modification of the lignin (LigBu) with BIBB, the polymerization does not happen.} b_Conversion weight of polymers obtained ₁₀₀

weight of initial monomers used

= × .cBased on conversion.dNumber-average molecular weight (Mn), and dispersity (D) were obtained

by GPC analysis, and the conversions were obtained by gravimetric analyses.

Figure 4.Comparison of viscosity as function of shear rate of lignin-based polymers synthetized in this work at a concentration of 10 wt %.

concentration of 2.5 wt % (Figure S9c). Polymers with Mw

values below 100 kDa display a Newtonian behavior up to 5 wt % concentration; however, at a polymer concentration of 10 wt %, the solution obtained with the polymers at higher Mware

slightly shear thinning in the frequency range investigated, as typically observed for other branched polyacrylamide sys-tems.60 Although it is not possible to isolate effects of the molecular weight and AM/Lig molar ratio, as both values vary, we can make a general observation that viscosity increases with molecular weight of the polymer, which can be logically expected (seeTable 1for polymers structure characterization). To characterize the amphiphilic behavior of the prepared polymers, surface tension of water solutions was measured in the range 0.065 wt % (0.014 mM) to 10 wt % (0.003 M). The measured values (Figure 5a) are coherent with what usually is observed for polymeric surfactants.47

As expected, the values measured are significantly higher compared to traditional low molecular weight surfactants.68,69 The ratio between hydrophilic and hydrophobic moieties in the polymer seems to display an important effect on the surface

activity of those systems. Indeed, the LigBr-b-PAM ATRP, which has the higher hydrophobic content (lower AM/Lig ratio, see Table 1), shows a more pronounced decrease in surface tension. Indeed, polymers with lower HLB (hydro-philic−lypophilic balance) should adsorb more at the interface, with a resulting sharper decrease of the surface tension value. It is worth mentioning that the values are plotted as a function of mM concentration in order to compare polymers at a constant number of polymer chains present in solution. However, the molecular weight might also play a relevant role in the dynamic of adsorption at the interface. As the more hydrophilic polymers, which are less surface active, are also the ones with higher molecular weight, definitive conclusions cannot be drawn. Analogous behavior and very similar trend are observed for the interfacial tension between water and n-hexane (Figure 5b), measured with a spinning drop tensiometer.

Cryo-TEM was carried out in order to verify the presence of micelles in the polymer solutions. The analysis was carried out at 2.5 wt %, because according to the data collected from interfacial and surface tension, at this concentration, value Figure 5.(a) Surface tension versus different concentrations of lignin-based polymers and (b) interfacial tension versus different concentrations of lignin-based polymers.

aggregation should have taken place, at least for LigBr-g-PAM ATRP. The obtained pictures (Figure S10) suggest the presence of spherical micelles for the LigBr-g-PAM ATRP (Figure S10d). For the other lignin derivate polymers, not clearly visible aggregates are shown, and the pictures suggest worm-like aggregates, but definitive conclusions cannot be drawn. For LigBr-g-PAM ARGET1 (Figure S10e), the presence of aggregates is not evidenced.

Application Testing of Lignin-Based Amphiphilic Copolymers. After measuring relevant properties and showing that the polymer structure indeed affects them in different ways, we moved to evaluating the possibility of the practical use of our polymers in industrial applications. With this purpose, we tested some of our polymer samples in emulsion stabilization experiments (Figure S11 and relative discussion), oil recovery in modelflow cells (Figure S12and relative discussion), and for their cytotoxicity (Table S1,

Figures S13−S15, and relative discussion), the latter relevant for potential biomedical applications, such as drug delivery or tissue engineering. It should be stressed out that this investigation was not completely systematic and intended to be very preliminary. The experiments, for the interested readers, are described in detail in theSupporting Information. We report here the mainfindings.

All studied lignin derivatives are effective as emulsion stabilizers, showing good stability of o/w emulsions of n-hexane in water (Figure S11). This is particularly true for polymers with a lower AM/Lig ratio and therefore more hydrophobic. The polymers that proved to be the better viscosifiers, corresponding to those prepared by free radical polymerization, were tested in flow cell experiments (Figure S12). Not much can be said in terms of comparisons, but the polymer solution with higher viscosity showed the better oil sweeping efficiency, as expected. There might be a positive effect on oil recovery due to the surface activity of such polymers, but the current investigation does not allow drawing any conclusions in this respect, and further studies would be required.

Finally, cytotoxicity of one polymer sample was tested, showing low toxicity of the polymer solution (Table S1,

Figures S13−S15). This, if confirmed for multiple samples by further experiments, would enable an investigation for biomedical applications.

■

In this work, we showed the possibility to prepare partially bio-based amphiphilic polymers from a lignin derivative and acrylamide. The starting lignin, obtained by a mild butanosolv extraction, presents favorable characteristics for the preparation of amphiphilic polymers, in particular, preservation of structural features of the lignin backbone, solubility in organic solvents, and the specific distribution of reactive OH groups.

By varying the synthetic method used and the lignin/ acrylamide ratio, we obtained polymers showing very different solution behavior, which can lead to potential use in different applications of industrial interest, for example, as emulsifiers or as viscosifiers for enhanced oil recovery. The use of different synthetic pathways affords materials with significantly different structures and properties.

The use of ATRP, in particular, the ARGET variant, seems to be more appealing for the production of well-defined structures. Blank experiments suggest that no acrylamide homopolymers are formed during the process. The water

solutions of the polymers obtained by ATRP remain clear and show no precipitation after several months. On the contrary, in the case of free radical polymerization, the solutions are not stable, as unreacted lignin precipitates with time. Moreover, these polymers certainly contain chains of the acrylamide homopolymer.

All prepared polymers display a thickening ability in water and surface activity typical for polymeric surfactants. Free radical polymerization provides polymers with superior properties in terms of the thickening ability in water, most likely due to the higher molecular weights. The polymers with relatively higher amounts of lignin are the ones presenting better surface activity.

Very preliminary tests, aimed at specific applications, showed that more hydrophobic polymers are promising emulsion stabilizers, while polymers with higher molecular weights are potentially more suitable as systems for enhanced oil recovery. The results obtained in this respect are very preliminary and did not provide much insight in precise structure−properties relationships; however, they constitute a solid ground for further investigations. Future work should be aimed at a more complete evaluation of the prepared system in real-life applications, which was only very preliminarily studied here. Additionally, achieving better structural control in the synthesis of these lignin-derived amphiphilic polymers could enable the optimization of such structures for a specific application, which on the long term could lead to the definition of a new-generation of bio-based amphiphilic polymers with tunable properties in water.

■

*

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acsapm.0c01006.

(Figure S1) P-NMR of lignin butanosolv; (Figure S2) 2D-HSQC spectra of the macro initiator model compound; (Figure S3) 2D-HSQC overlay of the macro initiator model compound with LigBr; (Figure S4) HSQC NMR of pristine LigBu and different Lignin derivate polymers; (Figure S5) comparison of FT-IR spectra between lignin-based-g-PAM polymers synthe-sized in this work normalized at 1502 cm−1 correspond-ing to the benzenes signals; (Figure S6) SEC analysis of butanosolv lignin precursor performed in THF as eluent, with linear polystyrene standard; (Figure S7) kinetic plot for the synthesis of LigBr-g-PAM ARGET2; (Figure S8) polymer solutions in water at different concentrations; (Figure S9) viscosity as a function of the share rate of lignin-based polymers; (Figure S10) cryo-TEM images of lignin-based amphiphilic copolymer solutions; (Figure S11) 1 wt % water solutions, emulsion 40:50 v/v with n-hexane; (Figure S12) flow cell experiment to evaluate the sweep efficiency of LigBu-g-PAM AIBN2; (Table S1) effects of polymer solutions on macrophages and fibroblasts; (Figure S13) XTT conversion (metabolic activity) by L929fibroblasts; (Figure S14) cells exposed for 24 h; and (Figure S15) cells exposed for 72 h (PDF)

■

Corresponding Author

Patrizio Raffa − Department of Chemical Engineering (ENTEG), University of Groningen, 9747 AG Groningen,

The Netherlands; Department of Biomedical Engineering, University of Groningen and University Medical Center Groningen, 9713 AV Groningen, The Netherlands;

orcid.org/0000-0003-0738-3393; Email:p.raffa@rug.nl

Authors

Nicola Migliore − Department of Chemical Engineering (ENTEG), University of Groningen, 9747 AG Groningen, The Netherlands

Douwe S. Zijlstra − Department of Chemical Engineering (ENTEG), University of Groningen, 9747 AG Groningen, The Netherlands

Theo G. Van Kooten − Department of Biomedical

Engineering, University of Groningen and University Medical Center Groningen, 9713 AV Groningen, The Netherlands Peter J. Deuss − Department of Chemical Engineering

(ENTEG), University of Groningen, 9747 AG Groningen, The Netherlands; orcid.org/0000-0002-2254-2500

Complete contact information is available at:

https://pubs.acs.org/10.1021/acsapm.0c01006

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to thefinal version of the manuscript.

Notes

The authors declare no competingfinancial interest.

■

The authors are grateful to Albert J.J Woortman for GPC analysis and Marc C. A. Stuart for cryo-TEM analysis during various stages of this work.

■

RDRP, reversible deactivation radical polymerization; BIBB, α-bromoisobutyryl; ATRP, atom transfer radical polymerization; hb, hyperbranched; EOR, enhanced oil recovery; AM, acrylamide; PMDETA, pentamethyldiethylenetriamine; AIBN 2,2′, azobis(2-methylpropionitrile); MeOH, methanol; THF, tetrahydrofuran; HCl, hydrochloric acid; HBr, hydrobromic acid; DMF N,N, dimethylformamide; DMSO, dimethyl sulfoxide; D₂O, deuterium oxide; LigBu, butoxylated lignin; LigBr, lignin-bromide; FT-IR, Fourier transform infrared spectroscopy; NMR, nuclear magnetic resonance; GPC, gel permeation chromatography; LigBr-g-PAM, lignin-bromide-g-polyacrylamide; wt%, weight percentage; M_w, weight average molecular weight; Mn, number average molecular weight;

TEM, transmission electron microscopy

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