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Green semi-IPN hydrogels by direct utilization of crude wood hydrolysates Laleh Maleki, Ulrica M Edlund, and Ann-Christine Albertsson ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.6b00938 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 18, 2016

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ACS Sustainable Chemistry & Engineering

Green semi-IPN hydrogels by direct utilization of crude wood hydrolysates

Laleh Maleki, Ulrica Edlund, Ann-Christine Albertsson* Fiber and Polymer Technology, Royal Institute of Technology (KTH), Teknikringen 56, SE-100 44 Stockholm, Sweden *[email protected]; +46-8-7908274

Synopsis Semi-interpenetrating hydrogel networks were developed from crude and unmodified hemicellulose-rich forestry biomass using a facile synthetic pathway.

Abstract Crude and unmodified acetylated galactoglucomannan containing wood hydrolysate (WH) was directly incorporated into a semi-interpenetrating network (semi-IPN) composed of up to 60 % renewable polymers. Semi-IPNs were produced by a facile and green synthetic pathway through cross-linking modified carboxymethylcellulose in the presence of at least 30 %(w/w) WH, via free radical co-polymerization with acrylic acid and N-N’-methylenebisacrylamide. FTIR verified the presence of WH interlaced with CMC in the semi-IPN, and the highly porous microscopic structure of the networks was confirmed by SEM. The gelation process of these networks was monitored in situ and their individual “gel point” (the G′-G″ cross-over) was determined by in situ rheological measurements. Semi-IPN hydrogels with various swelling ratios (Q = 20-225) were obtained within 2.8-20 min of cross-linking reaction.

Keywords Interpenetrating network, Hemicellulose, Biomass, Wood hydrolysate, Hydrogel

ACS Paragon Plus Environment


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Introduction Growing environmental awareness about the excessive use of fossil-based products and the need to evolve into a more sustainable global society, call for a rather urgent turnaround in the field of material and energy technology. The utilization of natural polymers as a renewable feedstock rich in macromolecules ready to be extracted and transformed into valuable products, has recently regained momentum. Polysaccharides, particularly lignocellulosic biomass, have been identified as of great potential among the available sources of naturally occurring macromolecules, thanks to their abundance, diversity, and biodegradability.1 In line with the biorefinery concept, efforts are dedicated to implementing biomass into products of everyday use2 (e.g. hydrogels) by realizing the inherent characteristics of its constituents for instance, hydrophilicity, versatility in chemistry, and abundance of chemical functionalities (e.g. hydroxyls, carboxylic acids, etc.). Polysaccharide-derived hydrogels are biopolymeric networks capable of absorbing and retaining large quantities of liquids, specifically water, thus they have found numerous applications as absorbent agents, drug delivery systems, water treatment agents, and many others. These networks although, extensively used in various applications, often suffer from low mechanical strength and in some cases even slow swelling response.3 In recent years, in line with the efforts dedicated to enhancing the mechanical strength of hydrogels and tailor-making their properties, multi-component polymeric networks known as interpenetrating polymer networks (IPNs) have been widely investigated.3 IPNs can be described as “alloys” of cross-linked polymer networks that are synthesized in juxtaposition so that the polymeric alloy is comprised of two or more partially interlaced polymeric networks.3,4 A semi-IPN is a multi-component network made by molecular scale penetration of a linear or branched polymer into a cross-linked network.5 IPNs and semi-IPNs offer superior control and versatility over single network hydrogels with possibilities to tailor-make products with desired properties by mixing countless combinations of polymers with complementing characteristics.6-9 Polysaccharides have received special attention in the design of polymeric networks, IPNs, and semi-IPNs, including hyaluronic acid,5,10,11 alginate,5,12 cellulose,13 chitosan,14,15 and others.3 Another suitable, yet hardly utilized class of polysaccharides that represents an ample resource for IPN production is hemicellulose. Hemicelluloses belong to a class of heterogeneous noncellulosic polysaccharides that interconnected with cellulose and lignin are the main cell wall


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constituents of higher plants.16,17 Thanks to their intrinsic properties, hemicelluloses have been proposed to be valuable raw materials as matrices for hydrogels18-20 and gas barrier films.21-23 Large quantities of wood hemicelluloses such as xylan and galactoglucomannan can potentially be recovered as by-products of forestry processes. The extraction of wood hemicelluloses is normally achieved by different methods depending on the employed pulping technique such as cooking of wood chips under alkaline or acidic conditions,16 hydrothermal treatment,24,25 or pretreatment of wood chips.26 Hemicelluloses in the poly-, oligo- and/or monosaccharide states (depending on the extraction severity) are then liberated from the wood matrix and solubilized or dispersed in the medium. Side-stream process liquors rich in hemicelluloses and containing a fair amount of lignin fractions are produced, which are collectively known as wood hydrolysate (WH). Conventionally, hemicelluloses are subjected to upgrading strategies and chemical modification to be converted into desired products, e.g. hydrogels.27-29 To date, many crosslinking chemistries for hemicellulose conversion into hydrogels have been developed typically relying on the use of organic solvents,30 multi-step procedures,31 and/or purified hemicellulose reagents.32 However, such chemical modifications, tedious purifications, and upgrading strategies counteract the sustainability efforts and render biobased products less green and economically feasible. In addition, the vast majority of hydrogels from polysaccharides in general and hemicelluloses in particular require pure reagents while robust pathways that accommodate cruder reagents are desirable but much scarcer amongst reported methods. Our aim was to develop a green pathway for the fabrication of highly swelling semi-IPN hydrogels, by direct utilization of crude wood hydrolysates (WH), a by-product produced in wood pulping processes. Our hypothesis was that O-acetyl-galactoglucomannan chains of WH could be directly incorporated, without chemical modification, into a semi-IPN matrix through a benign synthesis route involving carboxymethylcellulose as a precursor for the network formation, meeting the swelling and mechanical properties demands on such hydrogels for many applications.



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Experimental Materials Sodium carboxymethylcellulose (CMC) (Mw 90000 g mol-1, 0.7 carboxymethyl groups per anhydroglucose unit, Sigma Aldrich, Sweden), glycidyl methacrylate (GMA) (97%, Sigma Aldrich, Sweden), sodium hydroxide, acrylic acid (AA) (anhydrous, Aldrich), N-N’methylenebisacrylamide (MBA) (99%, Sigma Aldrich, Sweden) potassium persulfate (KPS) (99%, Sigma Aldrich, Sweden), sodium sulfite (anhydrous, ≥ 98%), acetic acid (99.7%, Sigma Aldrich, Sweden), and sodium acetate trihydrate (Fluka) were all used as received except for AA, which was distilled prior to use. The softwood hydrolysate (SWH) utilized in this work was obtained from the hydrothermal treatment of industrially screened spruce chips provided by Södra Cell AB, Sweden. The hydrothermal treatment was performed in a laboratory circulation digester at 165 °C.33 The wood hydrolysates generated in the hydrothermal treatment step were subsequently exposed to ultrafiltration using ceramic membranes with a 1000 g mol-1 cut-off (Orélis), followed by diafiltration and finally lyophilization. The carbohydrate composition of this SWH fraction containing O-acetyl-galactoglucomannan (AcGGM) as the main component was determined according to a procedure described in TAPPI standard method T249 coupled with ion-exchange chromatography (IC).34 The polymeric monosugar composition of SWH was determined to be 15 % glucose, 52.3% mannose, 5.2% galactose, and 0.7% arabinose.33 Furthermore, a Klason lignin content of 15% for SWH was verified based on TAPPI standard method T222.35 SWH had a molecular weight of 2400 g mol-1 and a dispersity (Ɖ) of 1.3 as determined by aqueous SEC. Synthesis of methacrylated CMC Methacrylated CMC (MA-CMC) was synthesized by first dissolving CMC (1 mmol hexose units) in 20 mL 0.5 M NaOH solution, under vigorous stirring and by placing the flask in a 50 °C oil bath. After obtaining a homogeneous solution, 5 mmol GMA (0.71 mL, with respect to the repeating unit of CMC) was introduced into the solution and the reaction was allowed to proceed for 2 h. The reaction was then terminated by precipitating the product into 300 mL acetone, prior to collecting the product by centrifugation. This was followed by a second dissolving and precipitation, and eventually the final product was obtained after drying under vacuum.


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Synthesis of semi-IPN hydrogels from SWH and methacrylated CMC A number of SWH semi-IPN hydrogels were synthesized from crude SWH and MA-CMC using the following procedure: 30/40/50 mg SWH corresponding to 30/40/50 %(w/w) of the final hydrogel composition, and 20/10/10 mg MA-CMC corresponding to 20/10/10 %(w/w) of the final hydrogel composition were dissolved in 250 µL deionized water in glass vials with an inner diameter of 12 mm. Subsequent to their complete dissolution, either 50.4/50.4/39.9 µL AA (corresponding to 48/48/38 %(w/w) of the final hydrogel, respectively) and 2 mg equal to 2 %(w/w) MBA, or 51.45/51.45/40.3 µL AA (respectively corresponding to 49/49/39 %(w/w) of the final hydrogel) and 1 mg corresponding to 1 %(w/w) MBA were added under stirring, Table 1. Upon obtaining a homogeneous mixture, 3 mol% KPS and sodium sulfite were added to each individual mixture under stirring for 3 min. Subsequently, the cross-linking reaction was carried out by placing each vial in an oven at 65 °C for 1 h until complete gelation was achieved. Cylindrically shaped hydrogels were then retrieved by breaking the glass vials. Remaining initiator and unreacted monomers were extracted from hydrogels by placing these gels in 100 mL deionized water for 48 h, and replacing the water with fresh water frequently. Finally, dried semi-IPN hydrogels were obtained by drying the purified gels under a gentle stream of air for 4 days. Hereinafter, SWH semi-IPNs are denoted SWH-Gx-y in which “x” refers to the weight fraction of SWH in the network composition, and “y” stands for the weight percentage of MBA used as a cross-linker in the system, Table 1. Table 1 Composition and denotation of SWH semi-IPNs

Semi-IPN composition

SWH %(w/w)

MA-CMC %(w/w)

MBA %(w/w)

SWH-G3-1

30

20

1

49

3

SWH-G4-1

40

10

1

49

3

SWH-G5-1

50

10

1

39

3

SWH-G3-2

30

20

2

48

3

SWH-G4-2

40

10

2

48

3

SWH-G5-2

50

10

2

38

3


AA %(w/w)

KPS mol%


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Spectrometry Nuclear magnetic resonance (NMR) was used to determine the chemical composition of CMC after methacrylation. CMC (15 mg) was dissolved in 0.8 mL D2O and the solution was transferred into NMR tubes with 5 mm outer diameter. The analysis was performed on a Bruker Avance DMX-400 NMR operating at 400 MHz at room temperature. The NMR spectra were analyzed using a Mestrelab Research MestReNova software version 9.0.0. Fourier transform infrared spectroscopy (FTIR) was used to analyze the chemical structures of CMC, MA-CMC, crude SWH and the produced semi-IPNs. The analyses were recorded using a Perkin-Elmer Spectrum 100 spectrometer equipped with an attenuated total reflectance (ATR) accessory. A mean of 16 scans with atmospheric water and carbon dioxide correction were obtained in the range of 4000 cm-1 to 600 cm-1 at a 4 cm-1 resolution. The data was then analyzed using Spectrum 10.5.0 software. Characterization Swelling behavior of semi-IPNs in deionized water was assessed by immersing 20-25 mg dry semi-IPN samples of varying compositions, enclosed within stainless steel baskets with a mesh opening of 0.20 mm and an inner diameter of 30 mm, in 100 mL deionized water. The weight gain of each sample was observed gravimetrically over a period of 48 h until a constant weight was reached. The equilibrium swelling ratio was then calculated as the mean of three individual measurements and according to Eq. 1: =

Eq. 1

where m0 refers to the initial weight of a semi-IPN hydrogel in dry state and mt defines the weight of each swollen network at t = 48 h. Swelling behavior of semi-IPNs in saline solution was determined based on the EDANA standard test 241.2.R336 however, instead of the teabags suggested in the standard test, the aforementioned baskets were used. For this purpose, a similar procedure to the previously described method for the determination of swelling ratio of semi-IPNs in deionized water was adopted in which the dry hydrogels were immersed in 0.9% saline solution instead. The mean


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equilibrium swelling ratio in saline solution (QNaCl) was defined as the average of swelling ratios obtained from three parallel measurements, and was calculated according to Eq. 2: =

Eq. 2

where mt refers to the weight of swollen semi-IPN hydrogel after 48 h. Swelling behavior of these semi-IPNs was also studied at pH 5.0 by placing each dry sample in approximately 100 mL of an acetate buffer solution prepared from acetic acid (0.1 M, 59 mL) and sodium acetate tri-hydrate (0.1M, 141 mL). (Figure S4) Field Emission Scanning Electron Microscopy (FE-SEM) was employed to study the topography and the cross-section of SWH semi-IPN hydrogels using a Hitachi s-4800 fieldemission scanning electron microscope operating with an acceleration voltage of 1.5 kV. SemiIPN samples in their dry and swollen states were submerged in liquid nitrogen prior to being freeze-dried for 2 days. The freeze-dried samples were then mounted on carbon tape coated stubs and were sputter coated with a thin layer of Au/Pt using a Cressington 208HR High Resolution Au/Pt sputter. Rheological characterization of semi-IPN hydrogels Gelation kinetics and rheological properties of SWH semi-IPNs were investigated via an in situ dynamic rheology test, using a TA Discovery Hybrid 2 (DHR-2) rheometer equipped with a Peltier plates set-up for temperature regulation, and Ø 20 mm stainless steel Peltier plate-plate configuration. To perform these experiments, semi-IPN precursor solutions with desired compositions were prepared in glass vials prior to rheology test. The initiator was added to the solutions within 2 min before starting each rheology test. Subsequently, approximately 0.15 mL of each sample solution was placed on the temperature regulated bottom plate, and each sample was exposed to a gap size of 300 µm at room temperature. The kinetic of gelation for each semiIPN composition was then followed by small amplitude oscillatory shear measurements in a timesweep mode at a frequency (ω) of 1 Hz, subjected to 1% strain at 65 °C. To ensure all analyzes were performed in the linear viscoelastic region, the obtained hydrogel networks subsequently underwent a frequency-sweep test at 65 °C, using a strain of 1% over an angular frequency range of 0.1-100 rad/s.



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Results and discussion The substitution of components derived from fossil-based resources by renewable raw materials, and developing less energy consuming and greener approaches to design products with competitive property profiles are demands of today and keys to a sustainable future. Here, a facile method for the fabrication of semi-interpenetrating (semi-IPN) hydrogels, with desirable swelling and rheological properties, was designed through direct utilization of crude and unmodified hemicellulose-rich WH factions obtained from wood pulping. The cross-linking technique and co-components used in the semi-IPNs were carefully selected to afford a greener synthetic strategy. Synthesis of methacrylated CMC CMC is a cellulose adduct and was chosen as a functional co-component in the semi-IPNs for its renewability, structural integrity, and high hydrophilicity. To allow for the chemical incorporation of CMC during the cross-linking of SWH containing semi-IPN hydrogels, MACMC was synthesized via a straightforward approach through the epoxide ring-opening of GMA in aqueous solution under alkaline conditions, Scheme 1. For this purpose, pendant hydroxyl groups on the backbone of CMC were activated by dissolving the polymer in 0.5 M NaOH solution at 50 °C. As a result of this activation, the ring-opening of epoxide moieties in GMA was mediated and methacrylate functionalities were introduced onto CMC chains.37


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Scheme 1 Schematic outline of the synthesis of SWH-based semi-IPN hydrogels

The 1H NMR spectra of CMC before and after methacrylation are shown in Figure 1. The characteristic peaks corresponding to the sugar backbone of CMC are visible in the range between 3.0 ppm and 4.5 ppm, in both spectra. In the 1H NMR spectrum of MA-CMC, three additional peaks are seen, which stem from the protons of the methacryloyl-glyceryl esters pendant to the CMC backbone. The signals at 5.23 and 5.54 ppm arise from the hydrogens linked to the vinyl carbon in pendant methacryloyl-glyceryl esters, and verify the presence of unsaturated sites on the modified CMC. The existence of these unsaturated sites was further confirmed by the peak at 1.76 ppm, attributed to the methyl protons linked to the vinyl group.



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Figure 1 1H NMR spectra of unmodified CMC (bottom) and MA-CMC (top) in D2O.

Synthesis of SWH based semi-IPNs Multicomponent semi-IPN hydrogel networks containing up to 60 % renewable polymers were synthesized without any post-production chemical modification, by direct utilization of the hemicellulose-rich spruce wood hydrolysate (SWH) liquor generated in the pre-hydrolysis step developed to precede pulping in a converted kraft dissolving pulp mill.33 A series of said networks with varying chemical compositions were synthesized by radical cross-linking MACMC using AA as co-monomer and MBA as a cross-linker in the presence of SWH, leading to the entrapment of polymeric AcGGM chains in SWH within a covalently cross-linked network. The representative FTIR spectrum of SWH-G5-1 is compared to the corresponding FTIR spectra of SWH, MA-CMC and unmodified CMC in Figure 2. In the FTIR spectra of CMC before and after methacrylation (Figure 2a and 2b), the emergence of two sharp peaks at 2917 and 2850 cm-1 after methacrylation is attributed to the C-H stretching vibrations of the methylene groups in the pendant methacryloyl-glyceryl esters on the backbone of CMC. Stronger C-H bending vibration bands at approximately 1580 cm-1 in the spectrum of MA-CMC in comparison with the low


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absorbance intensity of this peak in the spectrum of unmodified CMC also verifies this attachment. After methacrylation of CMC, a shoulder appears in the spectrum of MA-CMC at around 1635 cm-1, assigned to the vinyl groups. However, the unsaturated C-H stretching vibrations of pendant vinyl moieties, expected above 3000 cm-1, are overlaid by the –OH vibrations corresponding to the polymer backbone and bound water molecules. Furthermore, the band at approximately 1750 cm-1 in the spectra of unmodified CMC and MA-CMC, represents the carbonyl groups of the carboxylic acid moieties present on the backbone of CMC.38

Figure 2 FTIR spectra of a) Unmodified CMC, b) MA-CMC, c) crude SWH, and d) corresponding semi-IPN

The resemblance of the SWH-G5-1 hydrogel spectrum to that of MA-CMC indicates the presence of CMC chains in the final semi-IPN structure. Moreover, traces of SWH in the final network structure is noticeable by the appearance of a peak at approximately 1240 cm-1 arising from the C-O stretching vibrations of acetyl-groups pendant to the backbone of AcGGM in SWH (Figure 2c and 2d). In addition, a sharp increase in the intensity of the vibration band in carbonyl region

(1700 cm-1) perceived that indicates the formation of AA oligomeric chains during cross-linking of the hydrogel. The FTIR spectrum of SWH-G5-1 cannot verify the presence of MBA in the network as its characteristic bands are overlaid by the vibration bands arising from other network components, thus the presence of MBA was confirmed by observing the swelling and rheological properties of semi-IPNs.



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An advantage of incorporating polysaccharides into semi-IPNs arises from the fact that every hydrophilic polymeric chain, regardless of its chemical composition, can easily be interlaced with a cross-linked network, creating a versatile platform for the utilization of hemicelluloses (AcGGM, and Xylan) or polysaccharides in general, without the need for chemical functionalizations. Rheological behavior of SWH semi-IPN hydrogels Investigating the kinetics of gelation provides valuable insight into the nature of a cross-linking reaction and macroscopic formation of a hydrogel. Dynamic rheology has been widely used as a powerful technique to monitor the gelation process of a gel in situ. Dynamic rheology offers the possibility of tracing changes in the rheological properties of a cross-linking system, such as storage (G′) and loss (G″) moduli in real-time experiments,39 reflecting the status of a network structure obtained by recording small amplitude oscillatory shear as a function of cross-linking time, with minimal disruption to the sol-gel transition and network formation process.40,41 The gelation kinetics of SWH semi-IPN hydrogels with varying amounts of cross-linker, comonomer and SWH was monitored in situ by small amplitude oscillatory shear measurements in a time-sweep mode at a frequency of 1 Hz and a strain of 1%. The “gel point” (GP), the intermediate state of transition from liquid to solid,41 was determined for each hydrogel formulation under isothermal conditions. An approximate GP is defined as the point at which G′ and G″ cross over each other and tan δ (= G″/G′) equals 1.40,42 Furthermore, the effect of network composition and weight fraction of the cross-linker on the kinetics of gelation was investigated. Variations in G′ and G″ over 30 min of cross-linking time for semi-IPN hydrogel formulations containing the maximum and minimum amounts of SWH together with either 1 or 2 %(w/w) MBA are shown in Figure 3. By comparing the rheological behaviors of SWH-G3-1 and SWHG3-2, both containing 30 %(w/w) SWH and 20 %(w/w) MA-CMC, it is evident that upon doubling the amount of MBA in case of SWH-G3-2, GP is reached at approximately half of the reaction time required for SWH-G3-1 to arrive upon G′ and G″ cross-over point, Table 2. A similar trend was observed by comparing the results obtained from small amplitude oscillatory shear measurements of SWH-G5-1 and SWH-G5-2, Figure 3 and Table 2. In each case the gelation process of these semi-IPNs, regardless of their composition, undergo three stages which


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is a characteristic behavior of cross-linking gels.43,44 The first stage is an incubation period in which the system behaves like a viscous liquid. The second stage is the sol-gel transition period that is evidenced by a rapid increase in G′. The final plateau stage is a period during which G′ reaches equilibrium and the network formation is complete.43 All semi-IPNs in this work reached the plateau stage within 30 min of reaction at 65 °C, Figure 3 and S2, except SWH-G5-1. This could be attributed to the lower concentration of unsaturated double bonds in the reaction mixture of SWH-G5-1, as compared to other semi-IPN formulations, in combination with low amount of MBA. This was also further confirmed by observing the swelling behavior of SWH-G5-1, and extendedly discussed in the following section.

SWH-G3-1

1000

100

G''

G' (Pa), G'' (Pa)

G' (Pa), G'' (Pa)

G'

SWH-G3-2

G'

1000

G''

100

10

10

1

1 500

1000

1500

500

1000

Time (s)

Time (s)

SWH-G5-1

SWH-G5-2

1500

G' 1000

1000

100

G' G''

10

1

G' (Pa), G'' (Pa)

G' (Pa), G'' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

G'' 10

1 500

1000

1500

500

Time (s)

1000

1500

Time (s)

Figure 3 Variations of storage modulus (G′) and loss modulus (G″) as a function of cross-linking reaction time.

Equilibrium G′ and G″ of SWH containing semi-IPNs are listed in Table 2. SWH-G3-2 and SWHG4-2 exhibited the highest G′ values among all semi-IPN formulations. SWH-G3-2 and SWH-G4-



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2 show very similar values of equilibrium G′, attributed to their identical contents of AA and MBA, which results in a higher cross-link density than for the other SWH semi-IPNs. The existence of higher cross-link density in these samples is further validated by the swelling behavior as elaborated in the next section. SWH-G5-1 exhibited the lowest G′, related to its higher SWH content and lower weight percentage of MBA and AA. The rheological behavior of hydrogels (e.g. G′ and G″) is a function of their chemical structure, cross-linking technique, and molecular weight between cross-links hence, the comparison between the results obtained in this work with G′ and G″ reported in literature may not provide extensive information. Nevertheless, the G′ obtained for semi-IPNs in this work lie within the same range as that for a library of hydrogels previously fabricated in our group,30 through a sequential synthetic pathway by using pure hemicellulose. Table 2 Gel point (GP), storage modulus (G′) and loss modulus (G″) of semi-IPN hydrogels

Semi-IPN composition

Time to reach GP (s)

Max G′ (kPa)

Max G″ (kPa)

SWH-G3-1

353

4.102

0.180

SWH-G4-1

697

0.691

0.069

SWH-G5-1

1222

0.169

0.051

SWH-G3-2

169

7.402

0.198

SWH-G4-2

361

7.516

0.132

SWH-G5-2

676

1.402

0.067

To further confirm the results obtained from in situ rheological measurements, the gelation times of two representative semi-IPN formulations were also determined via an inverted vial test. For this purpose, SWH-G3-2 with the shortest time to reach GP, and SWH-G5-1 with the slowest gelling were selected to exemplify the gelation process. The gelation process was visually followed over time, and the samples were photographed prior to starting the cross-linking reaction and at 5 min intervals subsequent to the start of the reaction (Figure 4). Before the cross-linking reaction (time 00:00), the liquid state of the reaction mixtures is clearly noticeable. However, after only 5 min of reaction at 65 °C the gelation of SWH-G3-2 was


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complete, and after 10 min a hydrogel with dimensional integrity could be retrieved by breaking the vial (Figure 4). This is in consistent with the dynamic rheology result that defines the GP for SWH-G3-2 at approximately 2.8 min. In recent years there have been many efforts to design fast cross-linking hydrogels, for instance, Bi et al has reported a library of fast cross-linking dendrimer-based hydrogels with gelation time in the range of 0.5-6.0 min.45 The gelation process of SWH-G5-1is also illustrated in Figure 4 showing that the viscosity of the SWH-G5-1 reaction mixture increases over time. The gelation process is considered complete after 20 min of reaction as verified by the inversion of the vial. This is also in accordance with the data acquired from rheology test that suggests a GP of 20.37 min. Also in this case, the reaction was allowed to proceed further for 5 additional minutes. Finally, the hydrogel was retrieved after 25 min of cross-linking.

Figure 4 Images illustrating the gelation processes of SWH-G5-1 and SWH-G3-2 using an inverted vial test.

Rheology and inverted vial test results combined suggest that all these SWH containing semiIPNs could be considered rapid-forming hydrogels, although their gelation times vary in accordance with their compositions. From the standpoint of a more sustainable industry, the renewable nature of the precursors and their rapid gelation capacity merit these semi-IPN



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hydrogels as potentially valuable for a relatively fast production of renewable hydrogel products that are today dependent on fossil-fuel based precursors. Swelling behavior of SWH semi-IPN hydrogels The semi-IPN swelling behavior was evaluated by placing each synthesized hydrogel formulation into a large excess of the desired swelling medium, and following the weight gain gravimetrically over a period of 48 h. Swelling curves in deionized water of samples prepared from the lower amount of cross-linker, are presented in Figure 5-a. Among these hydrogels, SWH-G5-1 shows a superior equilibrium swelling ratio in deionized water attributed to the larger amount of SWH and the lower amount of AA present in this formulation that leads to a lower degree of crosslinking, and consequently a higher degree of swelling. This finding further supports the previously discussed results obtained from in situ rheological analysis of SWH-G5-1. Although, an identical amount of AA was used to produce SWH-G3-1 and SWH-G4-1, the higher amount of MA-CMC in SWH-G4-1 resulted in a higher degree of cross-linking thereby lowering the swelling capacity. The swelling curves in water of hydrogels synthesized using 2 %(w/w) MBA are shown in Figure 5-b. Clearly, an increase in cross-linker amount from 1 to 2 %(w/w) leads to a dramatic decrease

in equilibrium swelling ratios, which is in accordance with a greater degree of cross-linking and higher storage moduli of the latter. While the cross-link density is higher in samples SWH-G3-2, SWH-G4-2, and SWH-G5-2 their slight variations of the weight ratio of SWH did not affect the swelling capacity to any significant extent. The current semi-IPNs show swelling ratios within a range of Q =20-225 that is rather similar to the swelling properties (Q =50-270)31 of single network SWH hydrogels previously reported by our group, verifying that the multicomponent nature of semi-IPNs has not exerted an adverse effect on their swelling behavior. The water retention capacity of hydrogels containing ionic groups (polyelectrolytes) varies depending on the polarity and ion strength of the swelling medium.46 The swelling behavior of hydrogels in the presence of external ions is of great significance for specific applications such as hydrogels in hygiene products, membranes, and agricultural applications. A commonly used method to evaluate the swelling behavior of hydrogels in the presence of external ions is to perform the swelling test in salt solutions with predefined concentration and ionic strength.47,48


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Swelling under saline conditions was assessed in 0.9% saline solution, according to an EDANA standard test.36 (Figures 5c, 5d, and S3)

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SWH semi-IPNs were consistently much less prone to swell in saline, decreasing the equilibrium swelling ratio by a factor 10 or more compared to swelling in water. This can be explained by taking the main driving forces for the swelling of polyelectrolyte hydrogels into consideration: Swelling of such hydrogels occurs due to a combination of osmosis phenomenon, and electrostatic interactions within the hydrogel structure and at the interface created between the hydrogel and surrounding medium. The surface of a hydrogel containing dissociable ionic moieties behaves like a semi-permeable membrane; allowing water to enter the hydrogel



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structure in order to balance the ion concentration within and outside of the hydrogel.49 However when such networks are placed in saline solution, low molecular weight electrolytes can migrate into the gel structure, where the interactions between the counterions and the carboxylate anions of the oligomeric acrylic acid chains can introduce a shielding effect. This screening effect diminishes the difference in salt concentration between the vicinity of the hydrogel and the remainder of the surrounding medium, resulting in a much lower osmotic pressure and eventually decreasing the equilibrium swelling ratio.49,50 Morphological studies of SWH hydrogels To better investigate the network formation, the porous structure, and the effect of the crosslinker content on the microstructure of SWH based semi-IPNs, the morphology of two representative hydrogel formulations was studied using SEM. For this purpose, SWH-G5-1 and SWH-G5-2, in dry and swollen states, were first lyophilized and subsequently examined using SEM (Figure 6). Figures 6a and 6b show the topography of SWH-G5-1 and SWH-G5-2 hydrogels in the dry state,

respectively and reveal compact and solid-like morphologies, with small differences attributed to how well each formulation retained its cylindrical structure upon drying. That, in turn, is a function of the rigidity of each hydrogel, which is related to their degree of cross-linking. The morphology and the porous structures of SWH-G5-1 and SWH-G5-2 in their swollen state are presented by micrographs of their respective cross-sections in Figure 6c, 6d, 6e and 6f. A highly porous morphology with well-oriented pores possessing thin walls is noticeable in case of SWHG5-1 (Figures 6c and 6e), consistent with its high equilibrium swelling ratio (Q = 225). SWH-G52 has a denser structure with larger and fewer pores (Figures 6d and 6f) expected considering the higher degree of cross-linking, the lower degree of swelling, and the higher storage moduli exhibited by the semi-IPN formulations prepared using 2 %(w/w) cross-linker.


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Figure 6 FE-SEM micrographs (x100 magnification) of the surfaces of a) dry SWH-G5-1, b) dry SWH-G5-2, cross-sections of c) Swollen SWH-G5-1, d) Swollen SWH-G5-2, and SEM micrographs (x300 magnification) of the cross-sections of e) Swollen SWH-G5-1, and f) Swollen SWH-G5-2



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Conclusions Unrefined and lignin containing hemicellulose-rich water soluble biomass was directly converted into highly swelling semi-interpenetrating (semi-IPN) hydrogels containing 30-50 %(w/w) crude SWH, a hemicellulose-rich side stream generated in the hydrothermal treatment of spruce wood chips. A renewable co-component, up to 20 %(w/w) of methacrylated carboxymethylcellulose was used as a precursor in the gelation process. The cross-linking was achieved under thermally initiated free radical polymerization condition by using acrylic acid as a co-monomer and N-N’methylenebisacrylamide (MBA) as a cross-linker. A series of semi-IPN hydrogel formulations were prepared by altering the weight ratios of the components, and introducing either 1 or 2 %(w/w) MBA into the system. FTIR analysis proved the successful incorporation of all precursors and components into the gel structure. Gelation kinetics was assessed by in situ dynamic rheological measurements and inverted vial tests and the gel points detected to occur rapidly within 2.8 to 20 min, depending on the semi-IPN composition. Swelling and rheological analysis combined show that renewable semi-IPN hydrogels possessing appreciable mechanical performance, and offering swelling ratios as high as Q = 225 can be obtained by direct exploitation of hemicellulose-rich by-products from wood pulping.

Acknowledgements We thank Formas (project number 2011-1542) for financial support.

Supporting Information 1

H NMR spectrum of MA-CMC, calculation of degree of methacrylation, rheological behavior of

SWH-G4-1 and SWH-G4-2, swelling behavior of semi-IPNs in 0.9% saline solution, and swelling of semi-IPNs at pH 5.0

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For Table of Contents Use Only

Green semi-IPN hydrogels by direct utilization of crude wood hydrolysates

Laleh Maleki, Ulrica Edlund, Ann-Christine Albertsson* Fiber and Polymer Technology, Royal Institute of Technology (KTH), Teknikringen 56, SE-100 44 Stockholm, Sweden *[email protected]; +46-8-7908274

Synopsis Semi-interpenetrating hydrogel networks were developed from crude and unmodified hemicellulose-rich forestry biomass using a facile synthetic pathway.


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Figure 1. 1H NMR spectra of unmodified CMC (bottom) and MA-CMC (top) in D2O. 84x65mm (300 x 300 DPI)



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Figure 2 FTIR spectra of a) Unmodified CMC, b) MA-CMC, c) crude SWH, and d) corresponding semi-IPN


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Figure 3 Variations of storage modulus (G′) and loss modulus (G″) as a function of cross-linking reaction time. 76x54mm (300 x 300 DPI)



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Figure 4. Images illustrating the gelation processes of SWH-G5-1 and SWH-G3-2 using an inverted vial test. 167x127mm (300 x 300 DPI)


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Figure 5 Swelling curves of a) semi-IPNs containing 1% MBA and b) semi-IPNs containing 2% MBA in deionized water, and swelling curves of c) semi-IPNs containing 1% MBA and d) semi-IPNs containing 2% MBA in 0.9% saline solution 82x61mm (300 x 300 DPI)



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Figure 6. SEM micrographs (x100 magnification) of the surfaces of a) dry SWH-G5-1, b) dry SWH-G5-2, cross-sections of c) Swollen SWH-G5-1, d) Swollen SWH-G5-2, and SEM micrographs (x300 magnification) of the cross-sections of e) Swollen SWH-G5-1, and f) Swollen SWH-G5-2. 167x185mm (300 x 300 DPI)


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Scheme 1 Schematic outline of the synthesis of SWH-based semi-IPN hydrogels



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Table of Content graphic 83x42mm (300 x 300 DPI)


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Green Semi-IPN Hydrogels by Direct Utilization of ... - ACS Publications

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