High strength hydrogel using phenolated lignin

Abstract


Introduction
Lignin, a byproduct in the pulping and biorefinery industry, is the second most abundant biopolymer after cellulose.
Although this product has a number of distinctive properties, including diverse functional groups, renewable, biocompatible, biodegradable and non-toxic characteristics, and antioxidant properties etc, only a small amount of lignin is used commercially.Lignin is a cost-efficient feedstock with a large potential for hydrogel synthesis as a starting material because of its free phenolic and aliphatic hydroxyl groups (Wu et al. 2019a).Due to the presence of high amount of phenolic groups in lignin structure, it has a large potential to be a starting material for many biobased materials like hydrogel (Kubo and Kadla 2003).
Hydrogels are a cross-linked three-dimensional polymer network that is capable of retaining a high amount of water.
Hydrogels have received attention over the past two decades of its many application fields such as water purification, biomedical healing systems, biomimetic scaffolds, drug delivery devices, agriculture and permselective membranes (Varaprasad et al. 2017).Hydrogels are generally capable of swelling without dissolving in water and retaining a lot of water.The majority of commercial hydrogels are petroleum-based products that are difficult to degrade and may cause secondary pollution (Wu et al. 2019b).
Bio-based hydrogels offer an environmentally friendly and biocompatible alternative to fossil fuel-based hydrogels (Huang et al. 2019).Moreover, biobased hydrogels have the additional benefit of being easily degradable under aqueous conditions and showing tailored properties (Zhu Ryberg et al. 2011).These hydrogels are also able to mimic natural tissues as a result of their huge water-holding capacity, excellent biocompatibility and unique surface characteristic (Zhang et al. 2005).
From biomedical to automobile applications, bio renewable polymers are of growing interest in the research community.There are many studies have been conducted on lignocellulose based hydrogel (Thakur and Thakur 2015).
Hydrogels based on kraft or ionic-liquid lignin exhibited low swelling and poor mechanical properties but showed a high degree of crosslinking when crosslinked with epichlorohydrin (Shen et al. 2016).Hydrogels produced from various technical lignins crosslinked with poly methyl vinyl ether co-maleic acid displayed a water swelling ratio of 1300% to 13000% (Wu et al. 2019b).The kraft lignin isolated from oil palm empty fruit bunches was blended with agarose at a ratio of 1:1, and then a hydrogel was produced by chemical cross-linking with ECH.The hydrogel showed excellent mechanical strength (Sathawong et al. 2018).Using lignin as a base material, Dai et al. (Dai et al. 2020) prepared a low-cost, pH-responsive hydrogel that can be applied to sensors, actuators, and separation applications.
Feng et al. (Feng et al. 2011) produced a temperature-sensitive hydrogel by grafting organic acid lignin with Nisopropyl acrylamide using N, N'-Methylenebisacrylamide as a crosslinker and H2O2 as an initiator.The lower critical solution temperature of the hydrogel was approximately 31°C, and its thermal decomposition occurred between 400°C and 410°C.Ciolacu et al. (Ciolacu Diana and Cazacu Georgeta 2018) obtained green hydrogel using lignin with epoxyresin mixed with poly(vinyl alcohol).Recently, Ahmad et al. (Ahmad et al. 2022) fabricated smart lignin-hydrogels by increasing the number of cycles at the crosslinking stage.Lignin hydrogels exhibited high levels of crosslinking, lower swelling, and unsatisfactory mechanical properties when crosslinking agents included epichlorohydrin, glutaraldehyde, and glycidyl ether.
Phenolation is a new modification concept which directly enriches the phenolic hydroxyl group in lignin and it ultimately helps in the formation of cross-linked hydrogel (Kong et al. 2015).So the high valued utilization of technical lignin would help to reduce the excessive dependence on petrochemical resources and to improve the economy of pulping and biofuels industries.Technical lignin in general contains a relatively low amount of hydroxyl groups, which makes cross-linking difficult.Generally, Lignin is modified through a chemical process for a wide range of applications.It is well known that phenolation is an effective method for functionalizing lignin with the increase in its phenolic hydroxyl group (Meng et al. 2019).Lignin is used in polymer composites and hydrogels, either modified or unmodified.
In our earlier investigation, phenolated lignin provided higher in copolymerization acrylic acid, and the prepared copolymer with phenolated lignin coating on steel showed higher corrosion inhibition efficiency (Rahman et al. 2023).
Therefore, it is expected that phenolated lignin can perform better hydrogel production through the formation of crosslinked.Jiang et al. (Jiang et al. 2018a) synthesized thermosensitive gel by phenolated lignin and observed thermal and swelling behavior of prepared hydrogel.This type of gel widely is used in biomedical application and water purification system.So the high valued utilization of technical lignin would help to reduce the excessive dependence on petrochemical resources and to improve the economy of pulping and biofuels industries.
Therefore, in this investigation, lignin was phenolated using H2SO4 catalyst.The phenolated lignin was characterized by FTIR and yield.The hydrogel was prepared by lignin (L) and phenolated lignin (PhL) with PVA using ECH.The prepared hydrogels after this modification process was characterized by Fourier Transformed Infrared Radiation (FT-IR), Thermogravimetric Analysis (TGA) and Compression Tests.A mechanically strong conductive PhL-PVA hydrogel was prepared using ferric chloride solution.

Raw materials
Lignin was extracted from the locally grown wood in the laboratory through the kraft pulping process.The kraft lignin dissolved in black liquor was precipitated from the black liquor by means of acid precipitation.Precipitated lignin was separated and washed with hot distilled water in order to reach a pH value of over 7.After washing the lignin, it was dried at 60°C and stored in a glass bottle, ready to be used for subsequent uses.

Phenolation
The phenolation process involved dissolving 1 g of lignin in 3-7 g of phenol (different ratio of the L/P) in and placing it into the flask.In order to liquefy the phenol, the temperature was increased to 60 °C.A solution of concentrated sulfuric acid based on 10% of lignin was added to the homogeneous mixture and then refluxed.The flask was submerged in a heated oil bath at a controlled temperature.The reaction was performed at 120°C for 2 h with continuous stirring.After the phenolation was completed, the mixture was rapidly cooled to room temperature using tap water to prevent further reactions.And immediately, ethyl acetate (20 ml) was mixed to the solution.Then the insoluble fraction was filtered out and washed thoroughly by a 10-fold amount of petroleum ether to remove excess phenol.Then, the precipitated lignin was washed with distilled water until it was neutral, and then it was dried at 60°C.This lignin was referred to as phenolated lignin.

Synthesis of hydrogel
A 2.5% (w/v) alkaline solution i.e. 0.5 g of PVA to 20 mL of a 4% (w/v) NaOH aqueous solution was prepared that was magnetically stirred at 400 rpm and heated to 80-90 °C, ensuring the complete dissolution of PVA pellets.Once the solution was cooled, lignin was added and stirred at room temperature until a homogeneous mixture was obtained.
The homogeneous and stable L-PVA hydrogel was then produced by adding 0.5 mL of ECH to the solution and the reaction was done at 75 °C for 20-25 minutes while continuous stirring.Finally, lignin based-PVA hydrogel was synthesized.Thereafter, immediately the hydrogel was poured into mold and placed at room temperature for 24 h.All other hydrogel preparation such as PhL-PVA, hydrogels were performed using the same procedure as those used to produce L-PVA hydrogels.The hydrogels containing different concentration of lignin were labelled as PVA, Lig/PVA1:1, Lig/PVA2:1, Lig/PVA1:2 respectively.PhL-PVA hydrogels were synthesized following the same procedure as previous to obtain the L-PVA hydrogels.The hydrogels were named Ph-lig/PVA1:1, Ph-lig/PVA1:2, Ph-lig/PVA2:1, Ph-lig/PVA3:1, Ph-lig/PVA4:1, and Ph-lig/PVA5:1 according to the composition of the precursor solution, respectively.
Along with various composition of lignin and PVA, the reaction temperature as well as the concentrations of crosslinking agent were varied to prepare the hydrogel.

Characterization FT-IR analysis
FTIR spectroscopy has been used as a simple and practical method for analyzing structural changes quickly and accurately.To avoid moisture exposure, nonphenolated lignin and phenolated lignin were placed in a desiccator for seven days containing phosphorus pentaoxide.FT-IR analysis of the samples was performed directly on the samples.
A Perkin Elmer FT-IR spectrometer (Model: Frontier, Perkin Elmer, USA) with a GAAS detector was used for the analysis.The reflectance percentage was calculated by averaging 32 scans of a sample at a spectral resolution of 16 cm -1 with an interval of 4 cm -1 .Spectral data was processed using Perkin Elmer Spectrum (Version 10.4.4).The frequency range of each spectrum was 600-4000 cm −1 .

Calculation of swelling ratio and yield of hydrogels
The prepared hydrogels were subjected to swelling test by soaking them into 200 mL distilled water.Water swelling experiments were conducted at 25°C temperature.The swollen weight, Ms was measured after removing the hydrogel from the swelling media and wiping away the surface water.Hydrogel was dried until a constant weight was achieved, and the dry mass (Md) was determined by taking the weight of the dried hydrogel.Each water swelling test was carried out over a period of 48 hours.Yield and swelling ratio (SR) of the hydrogels were determined using Equations ( 1) and (2), respectively.

Thermogravimetric Analysis (TGA)
Thermogravimetric analysis was performed by a thermal analyzer of SII TG/DTA 6300.During the analysis the temperature raised to 600 0 C with a heating rate of 20 0 C min -1 .An inert atmosphere was maintained in the heating chamber by continuously flowing nitrogen gas at a rate of 100 ml min -1 .Approximately 10 mg of ground sample was placed in a platinum crucible for the test.

Conductivity of hydrogels
PhL-PVA hydrogels with and without FeCl3 were prepared and later washed with distilled water.Further, hydrogels were cut into half: one half was dried; another one was dipped into water for swelling.The conductivity was evaluated by measuring the resistance of these hydrogels in two different state: dry and hydrated state.The testing was completed by using ET4510 LRC Benchtop Desktop LCR Tester Meter.

Phenolation of lignin
Phenolation was intended to increase the number of reactive sites in lignin to enhance their functionality.Phenol was used not only as a reactant in the phenolation process but also as a solvent for lignin.The effect of lignin/phenol (L/P) ratio on the yield has been shown in Fig. 1.The phenolation was carried out by varying L/P ratio at a given temperature and acid catalyst.A variety of phenol contents were examined, ranging from L/P ratio of 1/3 to 1/7.The highest yield was found to be 123% at 10% sulfuric acid charge and L/P ratio of 1/4 at 120 °C.Although the effect of lignin ratio on the yield was marginal, the yield reduced with increasing L/P ratio higher than 1:4.Jing et al. studied acid catalyzed phenolation with pine kraft BioChoice TM Lignin (BCL) and Sweetgum biorefinery lignin (AER).They showed phenolation yielded 130% for BCL and 145% for AER lignin respectively (Jiang et al. 2018b).Compared with Jiang et al. results, the phenolated lignin obtained in our experiment showed a lower yield.It might be because of the difference of the experimental differences.

FTIR analysis of lignin
The FTIR spectra obtained for the nonphenolated and phenolated lignins are shown in Fig. 2. It was found that both lignins had the same pattern of spectra, except for the intensity of the peaks.The peak at 3356 and 3271 cm −1 assigned to the stretching vibration frequency of O-H, revealed group, the band around 2937 cm -1 was attributed to the C-H stretching vibration of aromatic ring.Carbonyl stretching band at 1700 and 1705 cm -1 was observed.The peaks at 1598 and 1594 cm −1 , 1509 and 1511 cm −1 were assigned to aromatic ring vibrations of C-C stretching.The C-H deformation combined with aromatic ring vibration at around 1452 cm −1 are common for all lignins, although the intensity of the bands differ (I F Fitigau et al. 2013).The increasing intensity of the band at 1356 cm −1 , assigned to the bending vibration of the phenolic hydroxyl group (O-H) was obvious.It indicated that lignin was successfully phenolated.A weak peak for aromatic in plane C−H vibrations appeared at 1117 cm -1 .A very sharp band around 1209 cm −1 was primarily due to C-O stretching vibrations of phenol.The phenolated lignin exhibits a strong band at 836 cm-1 that was characterized by O-H deformation, which was absent in the nonphenolated lignin.Besides, the peaks at 755 and 693 cm −1 were due to the para position substitution of phenol and confirmed the phenolation of lignin (Luo et al. 2020).

Hydrogel yields and lignin content
A series of lignin based PVA hydrogels was synthesized with various ratio of lignin and PVA by using a fixed amount of ECH crosslinker to the reaction mixture (Fig. 3).There was a remarkable difference observed between phenolated and nonphenolated lignin.Phenolated lignin produced much higher yield than the nonphnolated lignin.Phenolated lignin produced 98% yield whereas nonphenolated lignin yielded 88% at 1:1 L/PVA ratio.But, at 2:1 L/PVA yielded 98% and 76% hydrogel respectively.It was noticeable that at L/PVA 2:1, nonphenolated lignin did not form stable hydrogel.It remained as very soft galley like liquid after 24 h of swelling (Fig. 4).However, phenolated lignin form stable hydrogel up to 5:1 L/PVA ratio with 78% yield.

Hydrogel swelling ratio and lignin content
After completion the hydrogel formation reaction, it was molded and kept for 24 h at room temperature.Twenty-four hours later, the hydrogel was swelled into 200 times w/v distilled water.The hydrogel form with PVA only had the highest swelling than any composition of PVA and lignin or phenolated lignin (Fig. 5).L-PVA hydrogel had the higher swelling ratio in comparison with PhL-PVA hydrogel.The swelling ratio changed slightly with increasing the lignin ratio in the composition.However, the swelling ratio increased with increasing the PVA ratio.The swelling ratio of PhL-PVA hydrogel was increased dramatically from 1745% to 3101% as the Ph-lignin to PVA ratio increased from 1:1 to 1:2.Same trend was observed for lignin/PVA hydrogel and the swelling ratio increased from 3798% to 4417%.This behavior may be achieved due to the higher number of active site on phenolated lignin molecule participated in the cross linking which result in compact network of hydrogel.This fact would justify the lower swelling of the PhL-PVA hydrogel.Similar results have been reported by Ciolacu et al. for chemically crosslinked hydrogels containing PVA and lignin.(Roberto Rinaldi et al. 2016).Various concentrations of cross-linking were used to synthesize hydrogels.A decrease in the swelling ratio of the hydrogel occurred as ECH concentration increased, and yield initially increased, but then tended to remain unaffected as ECH concentration increased in the hydrogel.Based on the results, the swelling ratio and yield closely resembled those observed by Wu et al. (Wu et al. 2019b).The yield was only 69% and swelling ratio was 3169% when the ECH concentration was 2%; however, the swelled hydrogel resembled a liquid jelly.In the case of 9% ECH concentration, PhL and PVA cross-linked excessively, resulting in a tight polymer network with 98% yield and 1683% swelling ratio.A 5% concentration of ECH resulted in a swelling ratio of 1844% and a yield of 97% for the PhL-PVA hydrogel.

Effect of cross-linking agent
The hydrogel obtained under this condition was used for mechanical and thermal analysis.

Effect of temperature
As aforementioned, temperature is also an external stimulus to which hydrogels are usually responsive.Therefore, in order to evaluate the effect of the temperature, hydrogels were synthesized in three different temperature.As shown in Fig. 7, temperature enhanced the swelling capacity and yield at first.At a temperature of 75°C, hydrogel performed a swelling ratio of 1844 and the yield was about 96%.When the temperature was further increased above 75°C, it could be observed that temperature drastically reduced the swelling ability.

Effect of reaction time
The reaction time for the synthesis of lignin-based hydrogels revealed some important results.Approximately 30 minutes were required for the reaction to be completed to produce pure PVA hydrogel.By contrast, the reaction time of hydrogels containing lignin and PVA decreased noticeably from 22 to 18 minutes with increasing lignin concentrations.However, the results found with the hydrogel containing phenolated lignin and PVA were much more interesting.As compared to both PVA hydrogels and L-PVA hydrogels, the reaction time for the synthesis of PhL-PVA hydrogels was significantly reduced to 15 minutes.Phenolated lignin had a higher amount of phenolic groups, which may contribute to the faster formation of hydrogels than nonphenolated lignin.

Characterization of Hydrogels by FT-IR
The pattern of FTIR spectra were almost similar which indicated that both of the prepared hydrogels had the same kind of bonding present in them (Fig. 8).A broad band at 3302 cm −1 assigned to the O-H stretching vibration.Both phenolated and nonphenolated lignin based hydrogels showed a band at around 2930 cm −1 due to C-H bond present on the skeleton of hydrogels.Nonphenolated hydrogel showed peak at 1585 cm −1 and 1508 cm −1 for the presence of aromatic skeleton (Morales et al. 2020) similar peaks were observed at 1585 cm −1 and 1510 cm −1 for PhL-PVA hydrogel.Due to the crosslinking among lignin, PVA and crosslinker C-O was formed [13].The peak at 1263 cm -1 and 1232 cm -1 were responsible for C-O stretching vibrations of nonphenolated and phenolated lignin hydrogel.The bands at 1419 cm −1 was found due to methoxyl groups in nonphenolated lignin hydrogel whereas it was little shifted to 1408 cm −1 in pheholated lignin hydrogel.Primary and secondary hydroxyl group provided bands at 1122 and 1032 cm-1 (Sathawong et al. 2018).A little shifted similar band was observed in both hydrogels at 1084 cm -1 & 1039 cm -1 and 1100 cm -1 & 1034 cm -1 respectively.Aromatic ring C-H of syringly unit of lingin molecule gives a characteristic peak at 829 cm -1 .The band peak at 841 cm -1 was obtained for nonphenolated lignin hydrogel and 832 cm -1 for phenolated lignin hydrogel.The peak shifting occurred due to irregular crosslinking among the polymer units.lignin gives a weak band for C-S bond vibration at around 630 cm -1 (Tejado et al. 2007).The L-PVA hydrogel provided a weak peak at 640 cm -1 however it was very weak in PhL-PVA hydrogel.In phenolated lignin hydrogel a comparatively strong band was observed at 707 cm -1 for para position substitution phenol.

Mechanical property and Thermal analysis
The compression test of a hydrogel is an important parameter to demonstrate its mechanical properties.We compared the mechanical properties of hydrogels produced from PhL-PVA and L-PVA.As shown in Fig 9, the hydrogel compressibility is expressed as a stress-strain curve.It was found that phenolated hydrogels were more compressible than nonphenolated hydrogels.A phenolated hydrogel was compressed by up to 61% from its original swollen state under a stress of 13.4 MPa and returned to its original position as soon as the pressure was released.In comparison, the nonphenolated hydrogel showed a 49% compression at the same stress, however, it lost its shape as a result.
In Fig. 10, the thermal degradation of hydrogels over a temperature range of 600 °C is shown.It has been found that hydrogel degrades slowly with increasing temperature.Both hydrogels did not show any sharp degradation of the compound over the course of the experiment.Hydrogel based on phenolated lignin has been found to have better thermal stability than those based on nonphenolated lignin.It was found that the majority of decomposition for phenolated hydrogel took place at 280 °C, while for nonphenolated hydrogel it took place at 270 °C.As a result of heating to 600 °C, phenolated lignin hydrogel left 45.65% mass, while nonphenolated lignin hydrogel left 37.97% mass.The mechanically more stable PhL-PVA hydrogel shows better thermal stability.

Conductance performance of the hydrogel
The PhL-PVA hydrogel demonstrated greater mechanical strength and flexibility than a L-PVA hydrogel.A conductive hydrogel has been produced by combining phenolated lignin with PVA and ferric chloride.Hydrogel conductivity was measured by comparing the resistance between hydrogels incorporating ferric chloride and hydrogels without ferric chloride (Fig. 11).Hydrogel without ferric chloride showed a high resistance value of 2.38 MΩ when dry, which can be considered a low conductor of electricity.While hydrated, the resistance decreased to 1.06 MΩ.
Hydrogel with ferric chloride was less resistant in both the dry and hydrated states with resistance values of 8.912 kΩ and 5.942 kΩ , respectively.According to Mondal et al (Mondal et al. 2022) a high conductive lignin hydrogel has been developed for use as a superconductor.Due to the flexibility of phenolated lignin-based hydrogel, a stronger and more flexible superconductor can be produced.Using hydrated hydrogel with or without ferric chloride, an electric circuit was formed (Fig. 12).The LED bulb attached to the hydrogel without ferric chloride showed very faint light, while the LED bulb attached to the hydrogel with ferric chloride showed very bright light.

Conclusion
Pulp mills produce a large quantity of industrial lignin as a by-product.Therefore, lignin is a suitable feedstock for biobased materials both economically and ecologically.By phenolation, Kraft lignin has been chemically activated to overcome the reactivity and applicability obstacles.The phenolated lignin was characterized by its yield and FTIR.
The phenolated lignin showed characteristic peak at 755 and 693 cm -1 for the para substitution of phenol.Both the nonphenolated and phenolated lignin were subjected to hydrogel formation with PVA.Nonphenolated lignin provided lower yield and mechanical strength than the pnenolated lignin hydrogel.The contribution of lignin in the hydrogel was remarkable in the phenolated lignin based hydrogel.PhL-PVA ratio of 5:1 would produce stable hydrogel whereas 2:1 ratio of L-PVA could not form solid hydrogel.The hydrogel formation was more rapid for the PhL-PVA than the L-PVA system.Phenolated lignin based hydrogel was mechanically stronger that the nonphenolated lignin based hydrogel.Moreover, the effect of cross-linker on hydrogel synthesis was also noteworthy as increasing amount have resulted in tight polymer network and poor swelling ratio.
(%) =   −    × 100 ………………………….. (2) Md = mass of the dry hydrogel Ms = mass of the swollen hydrogel M = total mass of lignin and PVA used in the synthesis process Compression test of hydrogels The hydrogels were subjected to a compression test in order to determine their mechanical strength.The compression gear was set up on a single column UTM manufactured by Test Resource, USA, model no.100P250.It was equipped with 10 kgf loadcell operated at 10 mm/min speed.Approximately 20 mm x 10 mm cylindrical swelled hydrogel samples were allowed for compression test.Approximately 60% of the initial thickness of the samples was compressed during compression.

Fig 6
Fig 6 illustrates the effects of epichalohydrin (ECH) cross-linker on PhL-PVA hydrogel swelling ratio and yield.

Figures
Figures

Fig. 3
Fig.3Effect of lignin content on the yield of the hydrogels.

Fig. 5
Fig.5Effect of the lignin content on the swelling ratio of the hydrogels.

Fig. 6
Fig.6Effect of ECH on the swelling ratio and yield of the PhL-PVA hydrogels.

Fig. 7
Fig. 7Effect of temperature on the swelling ratio and yield of the PhL-PVA hydrogels.

Fig. 9
Fig. 9 Stress versus strain curve of hydrogels