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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
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Review
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Recent advances in green hydrogels from lignin: a review
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Vijay Kumar Thakur a,∗ , Manju Kumari Thakur b,∗ a b
School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, USA Division of Chemistry, Government Degree College Sarkaghat, Himachal Pradesh University Shimla-171005, India
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Article history: Received 22 July 2014 Accepted 22 September 2014 Available online xxx
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Keywords: Lignin hydrogel biomedical applications
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Contents
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4. 5.
Recently, biorenewable polymers from different natural resources have attracted a greater attention of the research community for different applications starting from biomedical to automotive. Lignin is the second most abundant non-food biomass next to cellulose in the category of biorenewable polymers and is abundantly available as byproduct of several industries involved in paper making, ethanol production, etc. The development of various green materials from lignin, which is most often considered as waste, is therefore of prime interest from environmental and economic points of view. Over the last few years, little studies have been made into the use of lignin as an indispensable component in the hydrogels. This article provides an overview of the research work carried out in the last few years on lignin based hydrogels. This article comprehensively reviews the potential efficacy of lignin in biopolymer based green hydrogels with particular emphasis on synthesis, characterization and applications. In this article, several examples of hydrogels synthesized using different types of lignin are discussed to illustrate the state of the art in the use of lignin. © 2014 Published by Elsevier B.V.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties and advantages of lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. General background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Preparation of hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lignin based hydrogels and applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Polymers exhibit very high promising potential as suitable 30 alternative to traditional metallic/inorganic materials for diverse 31 applications owing to their certain user-friendly/industrial advan32 tages such as low density, low abrasiveness, facile synthesis, 33 ease of modification and low environmental impact, to name a few [1–3]. Among various polymers recently being used for a 34 number of applications, biorenewable polymers from different 35 resources have attracted a greater attention of the research com36 37Q2 munity due to the advantages such as eco-friendliness, low cost, 29
∗ Corresponding author. Tel.: +1 509 335 8491; fax: +1 509 335 4662. E-mail addresses: [email protected] (V.K. Thakur), [email protected] (M.K. Thakur).
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biodegradability, etc. Biorenewable polymers such as lignocelluloses are showing the tremendous potential to reduce energy consumption/pollution by replacing traditional synthetic materials [1,4]. The prime constituents of any lignocellulosic material include cellulose, hemicelluloses, lignin, organic extractives and inorganic minerals [5,6]. However, the exact composition of the constituents depends upon their origin and species [7]. Generally, lignocellulosic materials from hardwood stems contain 40–55% cellulose, 24–40% hemicellulose, and 18–25% lignin and the softwood-stem-based materials contain 45–50% cellulose, 25–35% hemicellulose, and 25–35% lignin [8–11]. In any lignocellulosic material, the cellulose chains are bonded together by hydrogen bonding in elementary microfibrils that have been found to be attached to each other by hemicelluloses along with lignin [8,9,12]. One of the great- Q3 est assets of using lignocellulosic materials is their availability in very large amount at relatively low prices [13–15]. However,
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lignocellulosic materials also suffer from few disadvantages (e.g. poor hydrolysis, poor chemical resistance) depending upon the targeted applications and these materials are frequently being modified with various pretreatments [10,13,16]. Among numerous biorenewable polymer based materials, biopolymeric hydrogels are one of the most popular materials for biological/biomedical/environmental applications [17,18]. Biopolymer based hydrogels have also an added advantage in that these are easily degraded in an aqueous environment as well as exhibit tunable properties [19,20]. These hydrogels also exhibit the ability to simulate natural tissues because of their high water content, good biocompatibility and special surfaces properties [21,22]. Very recently lignin based hydrogels have attracted the interest of research community for a number of applications due to the inherent advantages of lignin. The research on lignin based hydrogels is still in its infancy and limited information is available in the existing literature of lignin based hydrogels [23]. So in this article, we comprehensively review lignin based hydrogels with particular emphasis on the different aspects of physico-chemical/applications of lignin based hydrogels.
2. Properties and advantages of lignin Along with cellulose and hemicellulose, lignin is an important constituent of the natural lignocellulosic polymers [24–29]. Fig. 1 shows the schematic concept of different component of lignocellulosic biomass [26]. Similar to other biorenewable polymers, lignin offers a number of advantages such as being antioxidant, being antimicrobial, frequent availability in huge amount as byproduct of industrial waste, being biodegradable; being CO2 neutral, etc. Lignin provides the strength to the cell walls of the cellulosic materials as well as protects the cell wall from biochemical stresses by inhibiting enzymatic degradation of other components [26]. Compared to cellulose/hemicellulose and other polysaccharides, lignin has been reported to be resistant to most of biological attacks [27]. Lignin is primarily composed of three different phenylpropane units, namely, p-coumaryl, coniferyl and sinapyl alcohols. Different types of carbon–carbon/and carbon–oxygen bonds are formed between different monomer units in lignin [28]. The amount of lignin in a given material varies upon the cultivation of the parent cellulosic materials as well the types of the species (soft wood/hard wood) [29]. Fig. 2 shows some of the main characteristics peaks of a section of softwood lignin [26]. The abundant functional groups available in lignin make it susceptible for chemical modification as well as suggest the possible potential prime role as a new chemical feedstock with particular application in the formation of supramolecular architecture/aromatic chemicals [26,30]. Compared to other constituents of the cell wall, lignin has been found to be hydrophobic in nature and prevents the penetration of water into the cell wall. It is also known as one of the most widely available aromatic renewable resource [26]. Lignin is available in large quantities all around the globe as one of the byproduct of the paper and bio refinery industry [16]. It is generally considered as waste material and recently some efforts are being made to utilize it in composites/hydrogel applications [31]. However, some of the disadvantages such as the unclear defined structure and versatility in the variation in the properties depending upon the sources from which it is derived limit some of its applications [26]. Generally the mass of lignin varies in the range 1000–20,000 g/mol. However, one of the biggest hurdles lies in determining the degree of polymerization of lignin because during extraction process it is invariably fragmented [27]. Table 1 shows the molecular weight and functional groups present in some of the lignins [27]. Lignin has been found to behave as a thermoplastic materials and exhibit a glass transition temperature [26] that is especially difficult to Please cite this article in press as: http://dx.doi.org/10.1016/j.ijbiomac.2014.09.044
V.K.
Table 1 Molecular weight and functional groups of lignins. Lignin type
Mn (g/mol)
COOH (%)
OH phenolic (%)
Methoxy (%)
Soda (bagasse) Soda (wheat straw) Kraft (softwood) Organosolv (hardwood) Organosolv (bagasse)
2160 1700 3000 800 2000
13.6 7.2 4.1 3.6 7.7
5.1 2.6 2.6 3.7 3.4
10.0 16.0 14.0 19.0 15.1
Adopted with permission from Ref. [27]. Copy right 2011 Elsevier.
determine in case of isolated lignin. The glass transition temperature (Tg ) varies depending upon the molecular weight and some other factors including method of extraction and thermal history [26]. It has been reported that Tg of lignin generally increases with increase in the molecular weight. Lignin from different resources is processed employing different chemical techniques and the commonly used techniques involve acid/base catalyzed mechanism [29]. During the processing of lignin (e.g. extraction) it is broken down to lower molecular weight fragments. These fragments also considerably affect the physicochemical properties of lignin. Lignin is commonly processed using three processes, namely, (a) sulfite process, (b) kraft process and (c) soda lignin process [32]. Efforts are being made to extract lignin using some green roots that also involve the use of ionic liquids as solvent during the processing of lignin. Different processes used in the extraction of lignin have been recently reviewed [26]. The advantages of lignin, including presence of several functional groups and availability in huge amount as discussed in the preceding section, make lignin a potential low cost environmental friendly material for multifunctional applications. Lignin is being used as such or after modification for a number of applications with particular attention in the field of polymer composites and hydrogels. This article has been restricted to the synthesis, characterization and discussion of relevant work in terms of the use of pristine lignin as a backbone polymer for hydrogel synthesis.
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3. Hydrogels
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3.1. General background
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Hydrogels are commonly known as the polymer based networks having the intrinsic ability to swell in water/aqueous solvents 10–20% (an arbitrary lower limit) up to thousands of times their dry weight in water without being dissolved [33]. In hydrogels, the networks are generally cross-linked and are rich in hydrophilic groups/domains [34]. Every hydrogel has a great affinity toward water and can absorb several times the water than their parent weight without being dissolved as a result of chemical/physical bonding between the numerous polymer chains [35,36]. The facile penetration of water in the hydrogels causes the swelling and gives the hydrogel its forms [34]. Hydrogels exhibit muco-adhesive as well as bio-adhesive characteristics, making them novel candidates in enhancing the drug residence time and tissue permeability [34]. Some of the properties of hydrogels especially in swollen state have been found to be common to living tissues [34,37]. Depending upon the targeted applications, the properties of hydrogels are frequently tailored [38]. One of the most common strategies is to alter the properties by changing the chemical structure to include groups that are susceptible to cleavage through different means [39]. The size of hydrogels also varies from nanometers to centimeters in width depending upon the synthesis methods and they exhibit the tendency to adopt the shape of any space to which they are confined [34].
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Fig. 1. Schematic concept of biorefinery based on lignocellulosic biomass. Reprinted with permission from Ref. [26]. Copyright 2013, Elsevier Ltd.
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3.2. Preparation of hydrogels Hydrogels can be synthesized by a number of methods. Free radical polymerization, irrdaditaion cross-linking of polymers, chemical cross-linking of polymers and physical cross-linking of polymers are some of the commonly used methods frequently employed for the preparation of polymeric hydrogels [40–43]. Among all the parameters that are involved in the synthesis of hydrogels, method of binding the polymer chains in the gel network as well as the amount of water plays a major role [44]. Depending upon the monomers used in the synthesis of hydrogels or density of the backbone polymer, each hydrogel can absorb a particular amount of water [34]. Some of the commonly used polymer/monomer in the preparation of hydrogel are poly (2-hydroxyethyl methacrylate) (PHEMA), poly(ethyl methacrylate) (PEMA) polyacrylamide (PAAm), poly (methacrylic acid) (PMA), poly (acrylic acid) (PAA), poly(glucosylethyl methacrylate) (PGEMA) and poly(hydroxypropyl methacrylamide) (PHPMA) [33]. Among various categories of hydrogels, physically cross-linked and chemically cross-linked hydrogels are of much importance due to their intrinsic properties [45]. In physical hydrogels, the cross-linking between different units includes hydrogen bonding, polyelectrolyte complexation, hydrophobic association, molecular entanglement and secondary forces including ionic [33]. Due to the weak nature of the forces between different networks, these hydrogels are also known as reversible hydrogels and
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sometimes these hydrogels disintegrate and dissolve in water [46,47]. However, in case of chemically cross-linked hydrogels, there is strong covalent bonding between the different polymeric networks of the hydrogels and these are also referred to as permanent hydrogels [48]. Depending upon the different constituents of physically and chemically cross-linked hydrogels, there may be different macromolecular structures for them, e.g. linear co-polymers, block co-polymers, graft copolymers, crosslinked/entangled networks of linear homopolymers, hydrophilic networks stabilized by hydrophobic domains, polyion–multivalent ion, polyion–polyion or H-bonded complexes and IPNs or physical blends [21,49]. Fig. 3a and b summarizes some of the roots commonly used to synthesize hydrogels [33]. 4. Lignin based hydrogels and applications
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As discussed in the preceding section, lignin is a biorenewable polymer, which has the potential to be used as a starting material for hydrogel applications. The development of hydrogels in the last few years has witnessed a rapid shift from synthetic to natural polymers as one of the indispensable component [50]. The prime interest in natural polymer based hydrogels arises due to their inherent properties such as biocompatibility, biodegradability, low toxicity, eco-friendliness, and susceptibility to enzymatic degradation, to name a few [40]. Different applications of the biopolymer based hydrogels are in the fields of water purification, biomedical,
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Fig. 2. Main linkages in a softwood lignin. Reprinted with permission from Ref. [26]. Copyright 2013, Elsevier Ltd.
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healing systems, biomimetic scaffolds, and drug delivery devices [51,52]. Among different natural polymers, lignin is an attractive candidate in hydrogels because of its advantages as discussed earlier that make it a versatile materials for extensive applications in different fields. There are a very limited number of research papers available on the use of lignin as backbone polymer in hydrogel applications. So in the following, we will be discussing different types of lignin based hydrogels along with their potential applications in detail. Green polymer hydrogels were prepared using the alkaline or kraft lignin [53]. Both types of lignin used in this study were isolated from pulping liquor and characterized by UV/FTIR spectroscopy. The hydrogels in this study were synthesized using graft copolymerization technique. The graft copolymerization reaction onto lignin was carried out in the presence of acrylamide (AM) and poly (vinyl alcohol) (PVA) followed by further addition of acrylamide monomer. These hydrogels were then characterized using UV and FTIR spectroscopy. The hydrogels were then subjected to different water uptake and deswelling study. It was observed from the study that the alkaline lignin hydrogel had very high swelling ratios, slower water uptake and deswelling rates as compared to the kraft lignin hydrogel. The slower deswelling rate behavior was attributed to the compatible network structure of the alkaline lignin while the high swelling ratio was attributed to the collaborative interactions between the sponge particles and the bulk matrix. Both types of hydrogels were subsequently used to study the influence
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of sodium chloride on the absorption capacity at room temperature and swelling ratios at different temperatures. It was observed that the addition of NaCl resulted in a continuous absorption capacity of the hydrogels and this behavior resulted due to the decrease in the osmotic pressure difference between the gel and the external solution [53]. Lignin based hydrogels were also prepared employing black liquor as the backbone material by the same research group. Black liquor is a mixture of lignin/carbohydrates and is one of the major industrial wastes that result from the pulping method [54]. The black liquor in this study was procured from alkaline pulping of rice straw. The hydrogels were prepared by chemical cross-linking method. In the synthesis process, two different crosslinking techniques, namely, cross-linking by radical polymerization and cross-linking by addition reaction, were used. In cross-linking by radical polymerization, ceric ammonium sulfate was used as initiator in the presence of N,N -methylenebisacrylamide to carry out graft copolymerization of poly vinyl alcohol (PVA) and polyacrylamide (PAAm). However, during the cross-linking by the addition reaction, the polymerization of hydrogels was carried out in the absence of the initiator. The hydrogels prepared using the different techniques were characterized by FT-IR spectroscopy and scanning electron microscopy (SEM). The hydrogels prepared using radical polymerization were found to exhibit a high swelling capacity, 60.00%, compared to 27.27% obtained from the hydrogels that were prepared by the addition reaction. The influence of sodium chloride on the absorption capacity of these hydrogels was also
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Fig. 3 . (a) Schematic of methods for formation of two types of ionic hydrogels. Reproduced with permission from Ref. [33]. Copyright 2012, Elsevier Ltd. (b) Schematic of methods for formation of hydrogels by chemical modification of hydrophobic polymers. Reproduced with permission from Ref. [33].Copyright 2012, Elsevier Ltd.
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studied at different temperatures and pH. From the study it was concluded that lignin containing black liquor can be employed as green hydrogel precursors [54]. Temperature sensitive lignin based hydrogels had also been prepared employing the acetic acid lignin (AAL) [55]. In this study graft copolymerization reaction was employed to prepare the hydrogels. The copolymerization reaction was carried out using acetic acid lignin and N-isopropyl acrylamide (NIPAAm) in the presence of N,N -methylenebisacrylamide (MBBAm) as the cross-linker and H2 O2 as the initiator. The synthesized hydrogels were subsequently studied for their thermal, morphological and swelling behaviors. The pore size in the hydrogels was found to increase with the increase in the acetic acid lignin and was confirmed from the SEM study. The thermogravimetric study of these hydrogels revealed their decomposition in the temperature range of 400–410◦ C, while the differential scanning calorimetry (DSC) study indicated the
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lower critical solution temperature (LCST) of 31◦ C. The detailed investigation of these hydrogels confirmed their temperature sensitive nature. Lignin based hydrogels were also prepared for their subsequent use as aerogel and cryogels [56]. In this interesting study, lignin–phenol–formaldehyde (LPF) hydrogels were prepared by dissolving certain amount of lignin in water and subsequently stirring and heating at 85◦◦ C for 1 h. Certain amount of sodium hydroxide (on dry basis) was also added in the meantime in the form of 30 wt.% aqueous solution to make the final pH of the resulting solution 12. As a result of the stirring, a dark homogeneous solution of lignin (20 wt.% concentration) was obtained. The solution was then cooled and used to prepare a number of hydrogels by the suitable addition of solid phenol and 37 wt.% aqueous solution formaldehyde. All the hydrogels in this study were prepared by keeping the fraction of solid (sum of dry lignin + dry phenol + dry formaldehyde) always equal to 26 wt.%. Up to 80% of lignin was
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Fig. 4. SEM pictures of: (a), (c), (e) ALPF0.25/1.7 aerogel and (b), (d), (f) CLPF0.25/1.7 cryogel at different magnifications (g) and (h) correspond to CLPF0.43/1.25 and CLPF1.5/1.25 cryogels, respectively. Reproduced with permission from Ref. [56].Copyright 2013, Elsevier Ltd.
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incorporated in the fractions and that was much higher than the other reported results, thus successfully replacing most of the toxic phenolic content. These hydrogel mixture solutions after complete homogenization had a pH close to 10, and were transferred into glass tubes. These tubes were instantly sealed and placed vertically for a period of five days in an oven at 85 ◦ C followed by their repeated inspections to determine their gelation time. After the
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predetermined period of five days, the hydrogels were taken out from the tube by breaking them and were cut into cylindrical pieces of diameter and typical thickness 13 and 6 mm, respectively. Subsequently aerogels and cryogels were prepared using these hydrogels. All the aerogels and cryogels formed from these hydrogels were found to be opaque and crack-free having different colors. The color of these aerogel and cryogels was found to depend upon the
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Fig. 5. Pore texture characteristics of TLF aerogels: (a) BET surface area; (b) mesopore volumes; (c) macropore volumes. Reproduced with permission from Ref. [57]. Copyright 2013, Elsevier Ltd.
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phenol/lignin mass ratio. These aerogels were then characterized using different techniques including FTIR, SEM, BET surface area, etc. Fig. 4 shows the SEM image of some of the aerogel and cryogel samples. Among both the gels, cryogels were found to be less porous than aerogels. These presented lower surface areas, micropore/mesopore volumes. The higher incorporation of lignin in the formulation was found to exhibit a higher macroporosity. However, compared to the other studied aerogel/cryogel systems the effect on micro- and mesoporosity was very complex. Thermal conductivity of some of these samples was also studied and the results supported their expected insulating character [56]. Tannin-lignin formaldehyde hydrogels were also prepared for their use in the preparation of aerogels by the same research group [57]. In this work first of all hydrogels were prepared by the procedure described in the preceding section at constant solid weight fraction/constant pH with different tannin/lignin and (tannin + lignin)/formaldehyde weight ratios. Subsequently aerogels were prepared from these hydrogels and these aerogels were studied for their different structural and surface characteristics. As an example, the porosity of the aerogels dried with supercritical CO2 was investigated in terms of surface area, macro (pore width > 50 nm), meso (2–50 nm) and microporosity (
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
1
Review
2
Recent advances in green hydrogels from lignin: a review
3 4 5
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Vijay Kumar Thakur a,∗ , Manju Kumari Thakur b,∗ a b
School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, USA Division of Chemistry, Government Degree College Sarkaghat, Himachal Pradesh University Shimla-171005, India
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Article history: Received 22 July 2014 Accepted 22 September 2014 Available online xxx
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Keywords: Lignin hydrogel biomedical applications
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Contents
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1. 2. 3.
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4. 5.
Recently, biorenewable polymers from different natural resources have attracted a greater attention of the research community for different applications starting from biomedical to automotive. Lignin is the second most abundant non-food biomass next to cellulose in the category of biorenewable polymers and is abundantly available as byproduct of several industries involved in paper making, ethanol production, etc. The development of various green materials from lignin, which is most often considered as waste, is therefore of prime interest from environmental and economic points of view. Over the last few years, little studies have been made into the use of lignin as an indispensable component in the hydrogels. This article provides an overview of the research work carried out in the last few years on lignin based hydrogels. This article comprehensively reviews the potential efficacy of lignin in biopolymer based green hydrogels with particular emphasis on synthesis, characterization and applications. In this article, several examples of hydrogels synthesized using different types of lignin are discussed to illustrate the state of the art in the use of lignin. © 2014 Published by Elsevier B.V.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties and advantages of lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. General background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Preparation of hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lignin based hydrogels and applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Polymers exhibit very high promising potential as suitable 30 alternative to traditional metallic/inorganic materials for diverse 31 applications owing to their certain user-friendly/industrial advan32 tages such as low density, low abrasiveness, facile synthesis, 33 ease of modification and low environmental impact, to name a few [1–3]. Among various polymers recently being used for a 34 number of applications, biorenewable polymers from different 35 resources have attracted a greater attention of the research com36 37Q2 munity due to the advantages such as eco-friendliness, low cost, 29
∗ Corresponding author. Tel.: +1 509 335 8491; fax: +1 509 335 4662. E-mail addresses: [email protected] (V.K. Thakur), [email protected] (M.K. Thakur).
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biodegradability, etc. Biorenewable polymers such as lignocelluloses are showing the tremendous potential to reduce energy consumption/pollution by replacing traditional synthetic materials [1,4]. The prime constituents of any lignocellulosic material include cellulose, hemicelluloses, lignin, organic extractives and inorganic minerals [5,6]. However, the exact composition of the constituents depends upon their origin and species [7]. Generally, lignocellulosic materials from hardwood stems contain 40–55% cellulose, 24–40% hemicellulose, and 18–25% lignin and the softwood-stem-based materials contain 45–50% cellulose, 25–35% hemicellulose, and 25–35% lignin [8–11]. In any lignocellulosic material, the cellulose chains are bonded together by hydrogen bonding in elementary microfibrils that have been found to be attached to each other by hemicelluloses along with lignin [8,9,12]. One of the great- Q3 est assets of using lignocellulosic materials is their availability in very large amount at relatively low prices [13–15]. However,
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lignocellulosic materials also suffer from few disadvantages (e.g. poor hydrolysis, poor chemical resistance) depending upon the targeted applications and these materials are frequently being modified with various pretreatments [10,13,16]. Among numerous biorenewable polymer based materials, biopolymeric hydrogels are one of the most popular materials for biological/biomedical/environmental applications [17,18]. Biopolymer based hydrogels have also an added advantage in that these are easily degraded in an aqueous environment as well as exhibit tunable properties [19,20]. These hydrogels also exhibit the ability to simulate natural tissues because of their high water content, good biocompatibility and special surfaces properties [21,22]. Very recently lignin based hydrogels have attracted the interest of research community for a number of applications due to the inherent advantages of lignin. The research on lignin based hydrogels is still in its infancy and limited information is available in the existing literature of lignin based hydrogels [23]. So in this article, we comprehensively review lignin based hydrogels with particular emphasis on the different aspects of physico-chemical/applications of lignin based hydrogels.
2. Properties and advantages of lignin Along with cellulose and hemicellulose, lignin is an important constituent of the natural lignocellulosic polymers [24–29]. Fig. 1 shows the schematic concept of different component of lignocellulosic biomass [26]. Similar to other biorenewable polymers, lignin offers a number of advantages such as being antioxidant, being antimicrobial, frequent availability in huge amount as byproduct of industrial waste, being biodegradable; being CO2 neutral, etc. Lignin provides the strength to the cell walls of the cellulosic materials as well as protects the cell wall from biochemical stresses by inhibiting enzymatic degradation of other components [26]. Compared to cellulose/hemicellulose and other polysaccharides, lignin has been reported to be resistant to most of biological attacks [27]. Lignin is primarily composed of three different phenylpropane units, namely, p-coumaryl, coniferyl and sinapyl alcohols. Different types of carbon–carbon/and carbon–oxygen bonds are formed between different monomer units in lignin [28]. The amount of lignin in a given material varies upon the cultivation of the parent cellulosic materials as well the types of the species (soft wood/hard wood) [29]. Fig. 2 shows some of the main characteristics peaks of a section of softwood lignin [26]. The abundant functional groups available in lignin make it susceptible for chemical modification as well as suggest the possible potential prime role as a new chemical feedstock with particular application in the formation of supramolecular architecture/aromatic chemicals [26,30]. Compared to other constituents of the cell wall, lignin has been found to be hydrophobic in nature and prevents the penetration of water into the cell wall. It is also known as one of the most widely available aromatic renewable resource [26]. Lignin is available in large quantities all around the globe as one of the byproduct of the paper and bio refinery industry [16]. It is generally considered as waste material and recently some efforts are being made to utilize it in composites/hydrogel applications [31]. However, some of the disadvantages such as the unclear defined structure and versatility in the variation in the properties depending upon the sources from which it is derived limit some of its applications [26]. Generally the mass of lignin varies in the range 1000–20,000 g/mol. However, one of the biggest hurdles lies in determining the degree of polymerization of lignin because during extraction process it is invariably fragmented [27]. Table 1 shows the molecular weight and functional groups present in some of the lignins [27]. Lignin has been found to behave as a thermoplastic materials and exhibit a glass transition temperature [26] that is especially difficult to Please cite this article in press as: http://dx.doi.org/10.1016/j.ijbiomac.2014.09.044
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Table 1 Molecular weight and functional groups of lignins. Lignin type
Mn (g/mol)
COOH (%)
OH phenolic (%)
Methoxy (%)
Soda (bagasse) Soda (wheat straw) Kraft (softwood) Organosolv (hardwood) Organosolv (bagasse)
2160 1700 3000 800 2000
13.6 7.2 4.1 3.6 7.7
5.1 2.6 2.6 3.7 3.4
10.0 16.0 14.0 19.0 15.1
Adopted with permission from Ref. [27]. Copy right 2011 Elsevier.
determine in case of isolated lignin. The glass transition temperature (Tg ) varies depending upon the molecular weight and some other factors including method of extraction and thermal history [26]. It has been reported that Tg of lignin generally increases with increase in the molecular weight. Lignin from different resources is processed employing different chemical techniques and the commonly used techniques involve acid/base catalyzed mechanism [29]. During the processing of lignin (e.g. extraction) it is broken down to lower molecular weight fragments. These fragments also considerably affect the physicochemical properties of lignin. Lignin is commonly processed using three processes, namely, (a) sulfite process, (b) kraft process and (c) soda lignin process [32]. Efforts are being made to extract lignin using some green roots that also involve the use of ionic liquids as solvent during the processing of lignin. Different processes used in the extraction of lignin have been recently reviewed [26]. The advantages of lignin, including presence of several functional groups and availability in huge amount as discussed in the preceding section, make lignin a potential low cost environmental friendly material for multifunctional applications. Lignin is being used as such or after modification for a number of applications with particular attention in the field of polymer composites and hydrogels. This article has been restricted to the synthesis, characterization and discussion of relevant work in terms of the use of pristine lignin as a backbone polymer for hydrogel synthesis.
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3. Hydrogels
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3.1. General background
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Hydrogels are commonly known as the polymer based networks having the intrinsic ability to swell in water/aqueous solvents 10–20% (an arbitrary lower limit) up to thousands of times their dry weight in water without being dissolved [33]. In hydrogels, the networks are generally cross-linked and are rich in hydrophilic groups/domains [34]. Every hydrogel has a great affinity toward water and can absorb several times the water than their parent weight without being dissolved as a result of chemical/physical bonding between the numerous polymer chains [35,36]. The facile penetration of water in the hydrogels causes the swelling and gives the hydrogel its forms [34]. Hydrogels exhibit muco-adhesive as well as bio-adhesive characteristics, making them novel candidates in enhancing the drug residence time and tissue permeability [34]. Some of the properties of hydrogels especially in swollen state have been found to be common to living tissues [34,37]. Depending upon the targeted applications, the properties of hydrogels are frequently tailored [38]. One of the most common strategies is to alter the properties by changing the chemical structure to include groups that are susceptible to cleavage through different means [39]. The size of hydrogels also varies from nanometers to centimeters in width depending upon the synthesis methods and they exhibit the tendency to adopt the shape of any space to which they are confined [34].
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Fig. 1. Schematic concept of biorefinery based on lignocellulosic biomass. Reprinted with permission from Ref. [26]. Copyright 2013, Elsevier Ltd.
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3.2. Preparation of hydrogels Hydrogels can be synthesized by a number of methods. Free radical polymerization, irrdaditaion cross-linking of polymers, chemical cross-linking of polymers and physical cross-linking of polymers are some of the commonly used methods frequently employed for the preparation of polymeric hydrogels [40–43]. Among all the parameters that are involved in the synthesis of hydrogels, method of binding the polymer chains in the gel network as well as the amount of water plays a major role [44]. Depending upon the monomers used in the synthesis of hydrogels or density of the backbone polymer, each hydrogel can absorb a particular amount of water [34]. Some of the commonly used polymer/monomer in the preparation of hydrogel are poly (2-hydroxyethyl methacrylate) (PHEMA), poly(ethyl methacrylate) (PEMA) polyacrylamide (PAAm), poly (methacrylic acid) (PMA), poly (acrylic acid) (PAA), poly(glucosylethyl methacrylate) (PGEMA) and poly(hydroxypropyl methacrylamide) (PHPMA) [33]. Among various categories of hydrogels, physically cross-linked and chemically cross-linked hydrogels are of much importance due to their intrinsic properties [45]. In physical hydrogels, the cross-linking between different units includes hydrogen bonding, polyelectrolyte complexation, hydrophobic association, molecular entanglement and secondary forces including ionic [33]. Due to the weak nature of the forces between different networks, these hydrogels are also known as reversible hydrogels and
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sometimes these hydrogels disintegrate and dissolve in water [46,47]. However, in case of chemically cross-linked hydrogels, there is strong covalent bonding between the different polymeric networks of the hydrogels and these are also referred to as permanent hydrogels [48]. Depending upon the different constituents of physically and chemically cross-linked hydrogels, there may be different macromolecular structures for them, e.g. linear co-polymers, block co-polymers, graft copolymers, crosslinked/entangled networks of linear homopolymers, hydrophilic networks stabilized by hydrophobic domains, polyion–multivalent ion, polyion–polyion or H-bonded complexes and IPNs or physical blends [21,49]. Fig. 3a and b summarizes some of the roots commonly used to synthesize hydrogels [33]. 4. Lignin based hydrogels and applications
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As discussed in the preceding section, lignin is a biorenewable polymer, which has the potential to be used as a starting material for hydrogel applications. The development of hydrogels in the last few years has witnessed a rapid shift from synthetic to natural polymers as one of the indispensable component [50]. The prime interest in natural polymer based hydrogels arises due to their inherent properties such as biocompatibility, biodegradability, low toxicity, eco-friendliness, and susceptibility to enzymatic degradation, to name a few [40]. Different applications of the biopolymer based hydrogels are in the fields of water purification, biomedical,
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Fig. 2. Main linkages in a softwood lignin. Reprinted with permission from Ref. [26]. Copyright 2013, Elsevier Ltd.
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healing systems, biomimetic scaffolds, and drug delivery devices [51,52]. Among different natural polymers, lignin is an attractive candidate in hydrogels because of its advantages as discussed earlier that make it a versatile materials for extensive applications in different fields. There are a very limited number of research papers available on the use of lignin as backbone polymer in hydrogel applications. So in the following, we will be discussing different types of lignin based hydrogels along with their potential applications in detail. Green polymer hydrogels were prepared using the alkaline or kraft lignin [53]. Both types of lignin used in this study were isolated from pulping liquor and characterized by UV/FTIR spectroscopy. The hydrogels in this study were synthesized using graft copolymerization technique. The graft copolymerization reaction onto lignin was carried out in the presence of acrylamide (AM) and poly (vinyl alcohol) (PVA) followed by further addition of acrylamide monomer. These hydrogels were then characterized using UV and FTIR spectroscopy. The hydrogels were then subjected to different water uptake and deswelling study. It was observed from the study that the alkaline lignin hydrogel had very high swelling ratios, slower water uptake and deswelling rates as compared to the kraft lignin hydrogel. The slower deswelling rate behavior was attributed to the compatible network structure of the alkaline lignin while the high swelling ratio was attributed to the collaborative interactions between the sponge particles and the bulk matrix. Both types of hydrogels were subsequently used to study the influence
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of sodium chloride on the absorption capacity at room temperature and swelling ratios at different temperatures. It was observed that the addition of NaCl resulted in a continuous absorption capacity of the hydrogels and this behavior resulted due to the decrease in the osmotic pressure difference between the gel and the external solution [53]. Lignin based hydrogels were also prepared employing black liquor as the backbone material by the same research group. Black liquor is a mixture of lignin/carbohydrates and is one of the major industrial wastes that result from the pulping method [54]. The black liquor in this study was procured from alkaline pulping of rice straw. The hydrogels were prepared by chemical cross-linking method. In the synthesis process, two different crosslinking techniques, namely, cross-linking by radical polymerization and cross-linking by addition reaction, were used. In cross-linking by radical polymerization, ceric ammonium sulfate was used as initiator in the presence of N,N -methylenebisacrylamide to carry out graft copolymerization of poly vinyl alcohol (PVA) and polyacrylamide (PAAm). However, during the cross-linking by the addition reaction, the polymerization of hydrogels was carried out in the absence of the initiator. The hydrogels prepared using the different techniques were characterized by FT-IR spectroscopy and scanning electron microscopy (SEM). The hydrogels prepared using radical polymerization were found to exhibit a high swelling capacity, 60.00%, compared to 27.27% obtained from the hydrogels that were prepared by the addition reaction. The influence of sodium chloride on the absorption capacity of these hydrogels was also
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Fig. 3 . (a) Schematic of methods for formation of two types of ionic hydrogels. Reproduced with permission from Ref. [33]. Copyright 2012, Elsevier Ltd. (b) Schematic of methods for formation of hydrogels by chemical modification of hydrophobic polymers. Reproduced with permission from Ref. [33].Copyright 2012, Elsevier Ltd.
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studied at different temperatures and pH. From the study it was concluded that lignin containing black liquor can be employed as green hydrogel precursors [54]. Temperature sensitive lignin based hydrogels had also been prepared employing the acetic acid lignin (AAL) [55]. In this study graft copolymerization reaction was employed to prepare the hydrogels. The copolymerization reaction was carried out using acetic acid lignin and N-isopropyl acrylamide (NIPAAm) in the presence of N,N -methylenebisacrylamide (MBBAm) as the cross-linker and H2 O2 as the initiator. The synthesized hydrogels were subsequently studied for their thermal, morphological and swelling behaviors. The pore size in the hydrogels was found to increase with the increase in the acetic acid lignin and was confirmed from the SEM study. The thermogravimetric study of these hydrogels revealed their decomposition in the temperature range of 400–410◦ C, while the differential scanning calorimetry (DSC) study indicated the
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lower critical solution temperature (LCST) of 31◦ C. The detailed investigation of these hydrogels confirmed their temperature sensitive nature. Lignin based hydrogels were also prepared for their subsequent use as aerogel and cryogels [56]. In this interesting study, lignin–phenol–formaldehyde (LPF) hydrogels were prepared by dissolving certain amount of lignin in water and subsequently stirring and heating at 85◦◦ C for 1 h. Certain amount of sodium hydroxide (on dry basis) was also added in the meantime in the form of 30 wt.% aqueous solution to make the final pH of the resulting solution 12. As a result of the stirring, a dark homogeneous solution of lignin (20 wt.% concentration) was obtained. The solution was then cooled and used to prepare a number of hydrogels by the suitable addition of solid phenol and 37 wt.% aqueous solution formaldehyde. All the hydrogels in this study were prepared by keeping the fraction of solid (sum of dry lignin + dry phenol + dry formaldehyde) always equal to 26 wt.%. Up to 80% of lignin was
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Fig. 4. SEM pictures of: (a), (c), (e) ALPF0.25/1.7 aerogel and (b), (d), (f) CLPF0.25/1.7 cryogel at different magnifications (g) and (h) correspond to CLPF0.43/1.25 and CLPF1.5/1.25 cryogels, respectively. Reproduced with permission from Ref. [56].Copyright 2013, Elsevier Ltd.
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incorporated in the fractions and that was much higher than the other reported results, thus successfully replacing most of the toxic phenolic content. These hydrogel mixture solutions after complete homogenization had a pH close to 10, and were transferred into glass tubes. These tubes were instantly sealed and placed vertically for a period of five days in an oven at 85 ◦ C followed by their repeated inspections to determine their gelation time. After the
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predetermined period of five days, the hydrogels were taken out from the tube by breaking them and were cut into cylindrical pieces of diameter and typical thickness 13 and 6 mm, respectively. Subsequently aerogels and cryogels were prepared using these hydrogels. All the aerogels and cryogels formed from these hydrogels were found to be opaque and crack-free having different colors. The color of these aerogel and cryogels was found to depend upon the
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Fig. 5. Pore texture characteristics of TLF aerogels: (a) BET surface area; (b) mesopore volumes; (c) macropore volumes. Reproduced with permission from Ref. [57]. Copyright 2013, Elsevier Ltd.
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phenol/lignin mass ratio. These aerogels were then characterized using different techniques including FTIR, SEM, BET surface area, etc. Fig. 4 shows the SEM image of some of the aerogel and cryogel samples. Among both the gels, cryogels were found to be less porous than aerogels. These presented lower surface areas, micropore/mesopore volumes. The higher incorporation of lignin in the formulation was found to exhibit a higher macroporosity. However, compared to the other studied aerogel/cryogel systems the effect on micro- and mesoporosity was very complex. Thermal conductivity of some of these samples was also studied and the results supported their expected insulating character [56]. Tannin-lignin formaldehyde hydrogels were also prepared for their use in the preparation of aerogels by the same research group [57]. In this work first of all hydrogels were prepared by the procedure described in the preceding section at constant solid weight fraction/constant pH with different tannin/lignin and (tannin + lignin)/formaldehyde weight ratios. Subsequently aerogels were prepared from these hydrogels and these aerogels were studied for their different structural and surface characteristics. As an example, the porosity of the aerogels dried with supercritical CO2 was investigated in terms of surface area, macro (pore width > 50 nm), meso (2–50 nm) and microporosity (
