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Jan 21, 2017 - Faculty of Natural System, Institute of Science and Engineering, Kanazawa University, Kakuma-machi,. Kanazawa 920-1192, Japan; ...
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Nano-Structural Investigation on Cellulose Highly Dissolved in Ionic Liquid: A Small Angle X-ray Scattering Study Takatsugu Endo 1, *, Shota Hosomi 1 , Shunsuke Fujii 1 , Kazuaki Ninomiya 2 and Kenji Takahashi 1, * 1 2

*

Faculty of Natural System, Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan; [email protected] (S.H.); [email protected] (S.F.) Institute for Frontier Science Initiative, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan; [email protected] Correspondence: [email protected] (T.E.); [email protected] (K.T.); Tel.: +81-76-234-4807 (T.E.); +81-76-234-4828 (K.T.)

Academic Editor: Hua Zhao Received: 21 December 2016; Accepted: 18 January 2017; Published: 21 January 2017

Abstract: We investigated nano-structural changes of cellulose dissolved in 1-ethyl-3methylimidazolium acetate—an ionic liquid (IL)—using a small angle X-ray scattering (SAXS) technique over the entire concentration range (0–100 mol %). Fibril structures of cellulose disappeared at 40 mol % of cellulose, which is a significantly higher concentration than the maximum concentration of dissolution (24–28 mol %) previously determined in this IL. This behavior is explained by the presence of the anion bridging, whereby an anion prefers to interact with multiple OH groups of different cellulose molecules at high concentrations, discovered in our recent work. Furthermore, we observed the emergence of two aggregated nano-structures in the concentration range of 30–80 mol %. The diameter of one structure was 12–20 nm, dependent on concentration, which is ascribed to cellulose chain entanglement. In contrast, the other with 4.1 nm diameter exhibited concentration independence and is reminiscent of a cellulose microfibril, reflecting the occurrence of nanofibrillation. These results contribute to an understanding of the dissolution mechanism of cellulose in ILs. Finally, we unexpectedly proposed a novel cellulose/IL composite: the cellulose/IL mixtures of 30–50 mol % that possess liquid crystallinity are sufficiently hard to be moldable. Keywords: ionic liquids; cellulose; small angle X-ray scattering; dissolution; composites

1. Introduction Cellulose—the major component of wood—is renewable, and is the most abundant biopolymer on Earth; therefore, its advanced unitization is crucial for a sustainable society. However, the severe limiting factor is that cellulose does not dissolve in any conventional solvents. A new class of cellulose solvents—ionic liquids (ILs)—was discovered in 2002 [1]. ILs are defined as salts that are liquid around room temperature, and some of them are able to efficiently dissolve cellulose [2,3]. ILs have some outstanding properties as cellulose solvents compared with those used previously: high designability (any appropriate physicochemical properties can be introduced by modifying the cation/anion structures or altering their combination), single component, practically nonvolatile and nonflammable, and highly thermally/chemically/electrochemically stable. Since these green solvents can be a breakthrough for cellulose science, understanding how cellulose dissolves in ILs is a very important issue. Many studies have been conducted thus far to contribute to this aim [2–5]. The most crucial finding is that IL anions play a major role in the dissolution by disrupting the

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inter- and intramolecular hydrogen-bonding network of cellulose. These anions mainly interact with the hydrogen of the OH groups in a 1:1 ratio [6], although it was suggested by molecular dynamics (MD) simulations that some anions participate in multiple interactions with the OH groups [7,8]. The role of the cations seems to be less significant; nevertheless, they unquestionably have some Molecules 2017, 22, 178 2 of 9 impact on the dissolution. Although the definitive role is still controversial, MD simulations have intramolecular network of cellulose. between These anions with the indicated the presence hydrogen-bonding of weak hydrophobic interactions the mainly cationsinteract and the glucose ring of hydrogen of the OH groups in aaromatic 1:1 ratio [6],cations althoughtend it wastosuggested molecular dynamics (MD) [2,3]. cellulose [4,5], which explains why exhibit by high dissolution ability simulations that some anions participate in multiple interactions with the OH groups [7,8]. The role It is well known that the concentration of a solution governs the state of dissolution of solute of the cations seems to be less significant; nevertheless, they unquestionably have some impact on molecules.theParticularly at high (e.g., lyotropic liquid dissolution. Although the concentrations, definitive role is still self-assembly controversial, MDphenomena simulations have indicated the weak hydrophobic interactions However, between the cations and the glucose ring of cellulose crystal or presence micelleofformation) are observed. a concentration regime of more[4,5], than 30 mol why as aromatic cations tend to exhibit high ability [2,3]. % (mol % which here explains is defined glucose unit/(glucose unitdissolution + IL))—which is considered to exceed the It is well known that the concentration of a solution governs the state of dissolution of solute maximummolecules. solubilityParticularly because at the ratio of IL to the OH groups of cellulose is greater than 1:1—has been high concentrations, self-assembly phenomena (e.g., lyotropic liquid crystal completely overlooked. Recently, we performed a structural investigation on in or micelle formation) are observed. However, a concentration regime of more than 30cellulose mol % (moldissolved % here is defined as glucoseacetate unit/(glucose unit + IL))—which is considered to exceed the maximum 1-ethyl-3-methylimidazolium ([Emim][OAc])—as the most representative IL for cellulose solubility because the ratio of IL to the OH groups of cellulose is greater than 1:1—has been dissolution—over the entire concentration range (0–100 mol %) by means of wide-angle X-ray completely overlooked. Recently, we performed a structural investigation on cellulose dissolved in 13 scattering 1-ethyl-3-methylimidazolium with the aid of quantumacetate chemical calculations and C solid-state NMR spectroscopy [9]. ([Emim][OAc])—as the most representative IL for cellulose We demonstrated that the concentrations ≥30 mol % showed dissolution—over thestates entire at concentration range (0–100 mol %) by means anomalous of wide-angle deconstruction X-ray scattering with the aid of quantum chemical calculations and 13C solid-state NMR and restructuring of cellulose, as schematically summarized in Figure 1. spectroscopy Even at 40[9]. mol %, the We demonstrated that the states at concentrations ≥30 mol % showed anomalous deconstruction and original crystalline structure of cellulose is absent. Furthermore, a new cellulose/IL complex with high restructuring of cellulose, as schematically summarized in Figure 1. Even at 40 mol %, the original structural crystalline periodicity emerges at approximately 35 mol %. It was revealed that these characteristic structure of cellulose is absent. Furthermore, a new cellulose/IL complex with high features originated from theemerges anionatbridging phenomenon, is when one characteristic anion interacts with structural periodicity approximately 35 mol %. It which was revealed that these features originated from thecellulose anion bridging phenomenon, which is when anion with multiple OH groups of different chains. At concentrations ≤25one mol %, interacts the anion/OH group OH groups of different cellulose chains. At concentrations ≤25 mol the anion/OH group bridging interactionmultiple is mainly 1:1 (non-bridging), as previously reported [6–8]. In%,contrast, the anion interaction is mainly 1:1 (non-bridging), as previously reported [6–8]. In contrast, the anion bridging state is energetically preferred at the higher concentrations, which induces intriguing structural changes state is energetically preferred at the higher concentrations, which induces intriguing structural in cellulose. changes in cellulose.

Figure 1. Schematic summary of Reference [9]. Below 25 mol %, the anions mainly interact with the

Figure 1. Schematic Reference [9]. Below 25 some mol %, the of anions mainly interact with the OH groups ofsummary cellulose inof a 1:1 ratio (non-bridging), while portion the anions have multiple interactions with in theaOH In the concentration range of 25–40 mol %, state multiple OH groups of cellulose 1:1groups ratio [7,8]. (non-bridging), while some portion ofthe theinteraction anions have of with the anion-OH group gradually from non-bridging to 25–40 bridging, induces the interactions the OH groups [7,8]. In switches the concentration range of molwhich %, the interaction state of emergence of the cellulose/[Emim][OAc] (1-ethyl-3-methylimidazolium acetate) complex structure. the anion-OH group gradually switches from non-bridging to bridging, which induces the emergence The anions bridge cellulose chains, forming a sheet structure. The sheets are stacked upon each other, of the cellulose/[Emim][OAc] (1-ethyl-3-methylimidazolium acetate) structure. The anions intercalated by the cations, forming a layered structure. Note that the exactcomplex structure is less ordered bridge cellulose forming a sheet chains structure. The sheets are sheets stacked each other, intercalated than thechains, schematic; i.e., the cellulose and the cellulose/anion are upon not completely straight and flat, respectively. The transformation the bridging state is completed at is 40 less mol %, which than the by the cations, forming a layered structure. toNote that the exact structure ordered enables the IL to thoroughly deconstruct the cellulose crystalline structure at this high concentration. schematic; i.e., the cellulose chains and the cellulose/anion sheets are not completely straight and flat, The bridging state is maintained above 40 mol %, where undissolved cellulose co-exists. Note that in respectively. transformation the bridging statemixtures is completed 40 mol %, which enables the The concentration range ofto 10–70 mol %, the exhibit at a lyotropic cholesteric liquid the IL to thoroughlycrystallinity, deconstruct the cellulose structure at this high concentration. The bridging state associated with the crystalline chain alignment. is maintained above 40 mol %, where undissolved cellulose co-exists. Note that in the concentration Although these findings provided a new aspect of cellulose dissolution state in IL, knowledge is range of 10–70 mol %, the mixtures exhibit a lyotropic cholesteric liquid crystallinity, associated with limited at the molecular level. It is well known that cellulose exhibits a hierarchical structure [10–12]: the chain alignment.

Although these findings provided a new aspect of cellulose dissolution state in IL, knowledge is limited at the molecular level. It is well known that cellulose exhibits a hierarchical structure [10–12]:

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Molecules 2017, 22, 178 3 ofform 9 nano-level, forms microfibrils thethe diameter of several nm, which gathergather together to at at thethe nano-level, itit forms microfibrilswith with diameter of several nm, which together to larger bundles. From From this standpoint, this study is devoted investigating the structuralthe changes in form larger bundles. this standpoint, this study istodevoted to investigating structural at the nano-level, it forms microfibrils with the diameter of several nm, which gather together to form cellulose/[Emim][OAc] at the nano-level—particularly focusing onfocusing the high on concentration regime— changes in bundles. cellulose/[Emim][OAc] at the nano-level—particularly the highchanges concentration larger From this standpoint, this study is devoted to investigating the structural in by means means of smallofangle X-ray scattering (SAXS) measurements, supportedsupported by scanning regime—by small angle X-ray scattering (SAXS) measurements, byelectron scanning cellulose/[Emim][OAc] at the nano-level—particularly focusing on the high concentration regime— microscopy. electron microscopy. by means of small angle X-ray scattering (SAXS) measurements, supported by scanning electron

microscopy.

2. Results and Discussion 2. Results and Discussion 2. Results and Discussion 2 versus Figure shows SAXSpatterns patternsofofcellulose/[Emim][OAc] cellulose/[Emim][OAc] in q2 qversus q) q) Figure 2 2shows SAXS in the theKratky Kratkyplot plot(Intensity (Intensity× × 2 0–100 mol%% concentration. In scattering from nanostructures appears as a× peak Figure 2concentration. shows SAXS patterns ofplot, cellulose/[Emim][OAc] in nanostructures the Kratky plot (Intensity qasversus q)For at at 0–100 mol Inthis this plot, scattering from appears a[13]. peak [13]. pure cellulose (100 mol %), only a shoulder is observable in the plot, indicating the presence of of at 0–100 mol % concentration. In this plot, scattering from nanostructures appears as a peak [13]. For For pure cellulose (100 mol %), only a shoulder is observable in the plot, indicating the presence structures larger than the detectable of theiscurrent SAXSinsetup (>50 nm). shoulder scattering pure cellulose (100 mol %), only alimit shoulder observable the plot, the presence of structures larger than the detectable limit of the current SAXS setup (>50indicating nm).The The shoulder scattering structures the detectable limitmicrofibrils, of the currentthe SAXS setup (>50 nm). The scattering would comelarger fromthan bundles of cellulose presence of which wasshoulder confirmed by SEM would come from bundles of cellulose microfibrils, the presence of which was confirmed by SEM would come from bundles of cellulose presence of which was confirmed by SEM observation (far-right in Figure 3). Thismicrofibrils, shoulder isthenot observed for the cellulose/[Emim][OAc] observation (far-right in in Figure 3).3).This shoulder is not observed observedfor forthe thecellulose/[Emim][OAc] cellulose/[Emim][OAc] observation (far-right Figure This shoulder is not mixtures up to a concentration of 40 mol %, even though the upper limit of dissolution at room mixtures up to a concentration ofof4040mol %, even though though theupper upper limit of dissolution at room mixtures up a concentration molto %, limit of dissolution at room temperature fortothis system was reported beeven 26–28 mol %the [14,15]. This behavior is consistent with temperature for this system was reported mol%%[14,15]. [14,15]. This behavior is consistent with for this system reportedto tobe be 26–28 26–28 mol This behavior consistent thetemperature findings obtained at thewas molecular level [9], and can be explained by theis concept ofwith anion thebridging. findings obtained at the molecular level [9], and can be explained by the concept of anion bridging. the findings obtained at the molecular level [9], and can be explained by the concept of anion Namely, the anion bridging form—which is energetically preferred around a concentration Namely, the anion bridging form—which is energetically preferred around a concentration Namely, the anion bridging form—which is energetically preferred around a concentration of 40 mol of bridging. 40 mol % (and above)—can completely deconstruct the crystalline structure of cellulose, and of 40 mol % (and above)—can completely deconstruct the crystalline structure of cellulose, and % (and above)—can completely deconstruct the crystalline structure of cellulose, and consequently, consequently, the fiber structures at this concentration. These SAXS results do not contradict the SEM consequently, the structures at this concentration. These SAXS results do not contradict the SEM theobservations fiber structures atfiber this concentration. Theseonly SAXS results do not contradict the observations shown in Figure 3, while SEM detects surface morphology. TheSEM bundled fibers observations shown in Figure 3, while SEM only detects surface morphology. The bundled fibers shown Figure at 3, 70–100 while SEM only detects Thethan bundled fibers were observed were in observed mol %. At 60 mol %,surface surfacemorphology. roughness rather fiber structures was seen. were observed at 70–100 mol %. At 60 mol %, surface roughness rather than fiber structures was seen. at 70–100 mol %. At 60 smoother mol %, surface roughness cellulose rather than fiber structures seen. The The surface became with decreasing concentration, and was eventually no surface clear The surface became smoother with decreasing cellulose concentration, and eventually no clear became smoother with decreasing cellulose concentration, and eventually no clear structure was structure was detected at 30 mol %. structure was detected at 30 mol %. detected at 30 mol %.

Figure 2. Small angle X-ray scattering (SAXS) patterns of the cellulose/[Emim][OAc] mixtures in the

Figure 2. 2. Small angle X-ray mixtures Figure Small angle X-rayscattering scattering(SAXS) (SAXS)patterns patterns of of the the cellulose/[Emim][OAc] cellulose/[Emim][OAc] mixtures in in thethe Kratky plot. Kratky plot. Kratky plot.

Figure 3. SEM micrographs. White scale bars for the top (100×) and bottom (100,000×) figures represent 300 μm and 300 nm,White respectively. Figure SEM micrographs. scale bars bars for for the the top top (100 (100×) Figure 3. 3.SEM micrographs. White scale ×) and and bottom bottom(100,000×) (100,000×figures ) figures represent 300 μm and 300 nm, respectively. represent 300 µm and 300 nm, respectively.

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There is another characteristic feature in Figure 1, which is the emergence of two new peaks in the concentration range of 30–80 mol %. This clearly indicates the presence of two nanostructures (or aggregations) which are not present in either intact cellulose or [Emim][OAc]. To derive their size and conduct a detailed discussion, the peaks at 1.1 and 0.3 nm−1 were fitted with the generalized Guinier–Porod function [16]. The generalized Guinier–Porod function experimentally combines the Guinier and Porod regions, and is expressed as: ! −q2 R2g I (0) I (q) = s exp for q ≤ q1 , q 3−s I (q) = where

D for q ≥ q1 , qd

1  1 (d − s)(3 − s) 2 q1 = , Rg 2  2 2 −q R (d−s) D = I (0) exp 3−1 s g q1 h i d−s , s) (d−s)(3−s) 2 = I((d0−)s) exp − (d− 2 2

(1)

(2)

(3)

(4)

Rg

and where I(0) is the scattered intensity at q = 0 and Rg is the radius of gyration. A value of s = 0 occurs when the scatterer is a three-dimensional globular object (such as a sphere). For one- (rod) and two-dimensional (lamellae or platelet) objects, the values are defined as 1 and 2, respectively. The fitted results are displayed in Figure 4a. Note that the Porod function was employed for the shoulder scatterer; therefore, the sum of two generalized Guinier–Porod functions and one Porod function was utilized to reproduce the overall patterns. All experimental patterns at the concentrations of 30–80 mol % were well reproduced in the entire q range by the combination of the functions. The derived parameters are summarized in Table 1. The s values for the aggregates were mostly zero, strongly implying that the shape of these structures was close to a sphere. The diameter of a sphere (R) can be determined from Rg as 5 R2 = R2g (5) 3 The concentration dependence of the R values is shown in Figure 4b. The behavior of the two structures with varying concentration is quite different. The structure with smaller R exists only at 50–80 mol %, and its size is independent of the concentration. In contrast, the structure with larger R is observed at 30–80 mol %, exhibiting concentration dependency. The invariant 4.1 nm diameter for the smaller scatterer is reminiscent of the microfibril of cellulose, whose size was reported as 4–5 nm for Avicel [17,18]. It is well known that cellulose fibrillation occurs with mechanical treatment in the presence of solvents [19]. The association of the 4.1-nm-diameter scatterer with the cellulose microfibril is in line with the fact that the shoulder scatterer associated with large bundles also does not exist at concentrations below 50 mol %. If so, then the result demonstrates that the interfibril breakages occur more easily than the intrafibril ones. The dynamic dissolution process can proceed similarly.

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Figure 4. (a) SAXS patterns of the cellulose/[Emim][OAc] mixtures in the Kratky plot for the

Figure 4. (a) SAXS patterns of the cellulose/[Emim][OAc] mixtures in the Kratky plot for the concentrations of 30–80 mol %. Black curves are the observed patterns, while the red curves are the concentrations of 30–80 mol %. Black curves are the observed patterns, while the red curves are theoretical patterns comprising the sum of two generalized Guinier–Porod functions and one Porod the theoretical patterns comprising the sum of two generalized Guinier–Porod functions and one Porod function; (b) Red: Diameter changes of the aggregated structures, assuming a sphere. Filled and open function; (b) Red: Diameter changes of the aggregated structures, assuming a sphere. and open symbols are for large and small aggregates, respectively. Black: The normalized I2(0)/I1Filled (0) change, symbols are for large and small aggregates, respectively. Black: The normalized I (0)/I (0) change, 2 1 and 1 2 are where I1(0) and I2(0) are the scattered intensities of the aggregates at q = 0. The subscripts whereforI1the (0) and I (0) are the scattered intensities of the aggregates at q = 0. The subscripts 1 and large 2and small aggregates, respectively. The ± signs represent standard deviation produced 2 are in large the fitting based on a nonlinear regression method. for the and procedures small aggregates, respectively. The ± signs represent standard deviation produced in the fitting procedures based on a nonlinear regression method. Table 1. SAXS parameters obtained from Figure 4a. The subscripts 1, 2, and s are for the large aggregates, small aggregates, and shoulder scattering, respectively. The ± signs represent standard Table 1. SAXS parameters obtained from Figure 4a. The subscripts 1, 2, and s are for the large deviation produced in the fitting procedures based on a nonlinear regression method.

aggregates, small aggregates, and shoulder scattering, respectively. The ± signs represent standard deviationCellulose produced in the Rfitting procedures based on a nonlinear regression method. g1/nm d1 s1 Rg2/nm d2 s2 ds

Concentration/mol % 7.9 ± 0.4 Cellulose30 Rg1 /nm 35 % 7.5 ± 0.2 Concentration/mol 40 7.1 ± 0.1 30 7.9 ± 0.4 50 35 7.5 ±5.5 0.2± 1.5 60 40 7.1 ±5.9 0.1± 0.5 70 50 5.5 ±4.9 1.5± 0.2 60 5.9 ±4.7 0.5± 0.0 80 70 4.9 ± 0.2 80 4.7 ± 0.0

4.0 ± 0.0 d1 ± 0.0 3.8 3.9 ± 0.0 4.0 ± 0.0 0.3 3.83.0 ± ±0.0 0.0 3.94.0 ± ±0.0 0.8 3.03.6 ± ±0.3 4.03.8 ± ±0.0 0.4 3.6 ± 0.8 3.8 ± 0.4

0.0 ± 0.1 0.0 s±1 0.1 0.0 ± 0.1 0.0 ± 0.1 0.0 ± ± 0.8 0.0 0.1 0.0 ± ± 0.0 0.0 0.1 0.1 ± ± 0.1 0.0 0.8 0.0 0.0 0.1 ± ± 0.1 0.1 ± 0.1 0.1 ± 0.1

n/a Rn/a g2 /nm n/a n/a 1.6 n/a ± 0.5 1.6 n/a ± 0.0 1.6 1.6±±0.0 0.5 1.6±±0.1 0.0 1.5 1.6 ± 0.0 1.5 ± 0.1

n/a n/ad2 n/a n/a 3.5 ±n/a 0.4 4.0 ±n/a 0.0 4.03.5 ± 0.0 ± 0.4 ± 0.0 4.04.0 ± 0.0 4.0 ± 0.0 4.0 ± 0.0

n/a n/a n/a s2 n/a ds n/a n/a n/a n/a 0.0 ± 0.9 4.0 ± 1.5n/a n/a 0.1 ± 0.1 n/a 4.0 ± 0.0n/a 0.1 0.0 ± 0.1 0.0 ± 1.5 ± 0.9 4.0 ± 4.0 ± 0.1 4.0 ± 4.0 0.1 0.1 ± 0.1 0.0 ± 0.0 0.1 ± 0.1 4.0 ± 0.0 0.1 ± 0.1 4.0 ± 0.0

The next question is on the origin of the larger scatterer. During the dissolution process, MD simulations observe the peeling of cellulose chains on the fibril surface from the chain edges by intercalation of IL ions Since of thethe intramolecular hydrogen bonding of peeled chains is MD The next question is on[20,21]. the origin larger scatterer. During the dissolution process, considerably disrupted by the IL ions, chain bending and twisting occur [22,23]. This flexibility of the simulations observe the peeling of cellulose chains on the fibril surface from the chain edges by terminal parts such Since chains the to entangle, which could correspond to the larger aggregate. intercalation of ILmay ionsenable [20,21]. intramolecular hydrogen bonding of peeled chains is This structure disappears at 25 mol %, because all OH groups can interact with an anion in a 1:1 ratio considerably disrupted by the IL ions, chain bending and twisting occur [22,23]. This flexibility (non-bridging) at this concentration, which unravels the entanglement. Another piece of indirect of the terminal parts may enable such chains to entangle, which could correspond to the larger evidence for the assignment of this structure is the moldability of the cellulose/IL mixture. After aggregate. structure disappears at 25 mol %, because all OH groups can%interact withbecame an anion in severalThis hours of mixing cellulose/[Emim][OAc], the samples of 30–50 mol intriguingly a 1:1 mechanically ratio (non-bridging) at this unravels Another piece sufficiently hardconcentration, to be moldedwhich (Figure 5), and the thisentanglement. moldability disappeared at of indirect evidence for the assignment of idea this that structure is the moldability cellulose/IL mixture. concentrations below 30 mol %. The cellulose entanglement is of thethe origin of mechanical well-established cellulose hydrogels [24,25].the Although exploring themol potential of this Afterstrength severalishours of mixingfor cellulose/[Emim][OAc], samples of 30–50 % intriguingly cellulose/IL composite film is beyond of this paper, it is that among reported became mechanically sufficiently hardthe to scope be molded (Figure 5),worth and noting this moldability disappeared cellulose/IL composite materials the that emergence of the moldability in is cellulose/IL with at concentrations below 30 mol %.[26–29], The idea cellulose entanglement the originsystem of mechanical liquid crystallinity [9] (in a 10–70 mol % regime) has been discovered here for the first time. The strength is well-established for cellulose hydrogels [24,25]. Although exploring the potential of this aggregate size decreases with increasing cellulose concentration (Figure 4b), probably because of the cellulose/IL composite film is beyond the scope of this paper, it is worth noting that among reported decrease in the numbers of glucose units in the chains and the IL ions that participate in the aggregates.

cellulose/IL composite materials [26–29], the emergence of the moldability in cellulose/IL system with liquid crystallinity [9] (in a 10–70 mol % regime) has been discovered here for the first time. The aggregate size decreases with increasing cellulose concentration (Figure 4b), probably because of the decrease in the numbers of glucose units in the chains and the IL ions that participate in the aggregates.

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Figure 5. Cellulose/IL (ionicliquid) liquid)composite composite films %)%) with a diameter of 25ofmm and and Figure 5. Cellulose/IL (ionic films(30–50 (30–50mol mol with a diameter 25 mm thickness of 1 mm. opacity thefilm filmofof40 40 mol mol % micro airair bubbles [9] and surface thickness of 1 mm. TheThe opacity of of the % isisattributed attributedtoto micro bubbles [9] and surface (or interfacial) roughness, which were confirmed infilms the 30(30–50 and theaSEM images 3and Figure roughness, 5. Cellulose/IL (ionicwere liquid) composite %) with of(Figure 25 mm (or interfacial) which confirmed in the 30mol mol%mol %film film and thediameter SEM images (Figure 3 top).thickness Conversely, the presence of undissolved crystals is also a reason for the opacity at 50 mol %. of 1 mm. The opacity of the film of 40 mol % is attributed to micro air bubbles [9] and surface top). Conversely, the presence of undissolved crystals is also a reason for the opacity at 50 mol %. (or interfacial) roughness, which were confirmed in the 30 mol % film and the SEM images (Figure 3

Thetop). mixture with 25 % cellulose—which gives is some no peak Conversely, themol presence of undissolved crystals also ascattering reason for but the opacity at in 50 the mol Kratky %. The mixture be with 25 mol gives some scattering butwould no peak the Kratky plot—cannot fitted with % thecellulose—which generalized Guinier–Porod model, and thus needinanother theoretical etthe al.mol [15] the SAXS gives profilesome of the transparent gel-like of plot—cannot bemodel. fittedRein with generalized Guinier–Porod model, and thus would need another The mixture with 25 %reported cellulose—which scattering but no peak mixture in the Kratky cellulose/[Emim][OAc] at 28 mol %, which was prepared by rapid evaporation of a volatile plot—cannot be fitted with the generalized Guinier–Porod model, and thus would need another theoretical model. Rein et al. [15] reported the SAXS profile of the transparent gel-like mixture of co-solvent. They applied aet combination of prepared the [30] and Ornstein–Zernike [31] of theoretical model. al. reported the Debye–Bueche SAXS by profile the transparent cellulose/[Emim][OAc] atRein 28 mol %,[15] which was rapidofevaporation of agel-like volatilemixture co-solvent. equations for the pattern to gain correlation lengths of the scatterer. The former and latter equations cellulose/[Emim][OAc] at the 28 Debye–Bueche mol %, which was by rapid evaporation of a volatile They applied a combination of [30] prepared and Ornstein–Zernike [31] equations for the express the scattering from a long-range inhomogeneity and a fluctuation in polymer chains, and[31] co-solvent. They applied a combination of the Debye–Bueche [30] and Ornstein–Zernike pattern to gain correlation lengths of the scatterer. The former and latter equations express the scattering were employed to describe major and minor components in theThe pattern, The equations for the pattern tothe gain correlation lengths of the scatterer. formerrespectively. and latter equations from Debye–Bueche a long-range inhomogeneity and a fluctuation in polymer chains, and were employed to describe equation is express the scattering from a long-range inhomogeneity and a fluctuation in polymer chains, and the major andemployed minor components themajor pattern, The Debye–Bueche is The were to describeinthe and respectively. minor components in the pattern,equation respectively. A ( ) I q = 2 Debye–Bueche equation is (6) 1 + ξ 2 q 2A , , (6) I (q) = 2 A I (q ) =(1 + ξ 2 q22) (6) where ξ is the correlation length. The Ornstein–Zernike is 1 + ξ 2 q 2equation ,

(

)

(

)

where ξ is the correlation length. The Ornstein–Zernike equation is (q) = A2 2 where ξ is the correlation length. TheIOrnstein–Zernike equation is (7) 1+ ξ q . A A I (qI)(q= . (7) ) =1require 2 2 The quality of our data for 25 mol % does not + 1 + ξξ2 qq2 .the Ornstein–Zernike equation, and yields(7) a correlation length of 4.8 nm with the Debye–Bueche equation (Figure 6). This value is smaller than

The quality of ourof data for 25for mol does not require thethe Ornstein–Zernike equation, yields the reported value nm [15], possibly owing to slightly lower cellulose concentration of our The quality of18 our data 25% mol % does notthe require Ornstein–Zernike equation,and and yields a correlation length 4.8 nm with the Debye–Bueche equation (Figure 6).6).This issmaller smaller than sample and theoflength differences in sample preparation procedures, whereas the value differences in the a correlation of 4.8 nmthe with the Debye–Bueche equation (Figure This value is than fitting of of the SAXS pattern andpossibly dataowing quality S/N ratio for our results)concentration could contribute the reported value 18 nm [15], to(lower the slightly lower cellulose our the range reported value of 18 nm possibly [15], owing to the slightly lower cellulose concentration ofof our to the gap thethe correlation length. In any case, preparation it is indicated that the 25 mol % sample does not form sample differences in the sample procedures, whereas the differences infitting the sample and theofand differences in the sample preparation procedures, whereas the differences in the structures, which seen in the samples with higher cellulose range of the SAXS pattern and data quality (lower S/N ratio forconcentrations. our results) could contribute rangedefinite offitting the SAXS pattern andwere data quality (lower S/N ratio for our results) could contribute to the gap to the gap of the correlation length. In any case, it is indicated that the 25 mol % sample notdefinite form of the correlation length. In any case, it is indicated that the 25 mol % sample does notdoes form definite structures, which were seen in the samples with higher cellulose concentrations. structures, which were seen in the samples with higher cellulose concentrations.

Figure 6. A single logarithmic SAXS pattern of the mixture at 25 mol %. The Debye–Bueche equation was applied (red line) to reproduce the experimental pattern. 6. Alogarithmic single logarithmic pattern of the mixture at 25 mol TheDebye–Bueche Debye–Bueche equation Figure 6.Figure A single SAXSSAXS pattern of the mixture at 25 mol %.%.The equation was applied (red line) to reproduce the experimental pattern. was applied (red line) to reproduce the experimental pattern.

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3. Materials and Methods Microcrystalline cellulose (Avicel PH-101, Sigma-Aldrich, St. Louis, MO, USA) and [Emim][OAc] (>95%, IoLiTec, Heilbronn, Germany) were dried by heating under vacuum (1 Pa) before use. Avicel PH-101 contains approximately 5 wt % of water, and after drying at 343 K for 3 h, the level of water detectable by a moisture analyzer (Sartorius, model MA35) was zero. The water content in the IL was measured using 1 H-NMR (JEOL JNM-ECS400 or JNM-ECA600, Tokyo, Japan), and no water peak signal was detected after one day of drying at 323 K. The typical sample-mixing procedure was as follows: 0.1 g of Avicel was added to a certain amount of the IL in a vial and mixed with a spatula until the mixture appeared visually homogenous (normally after 5 min). The mixtures were prepared in the range of 10–90 mol % at every 5 or 10 mol %. After one day (which is considered a sufficient period to reach an experimental equilibrium state [9]), the mixture was packed within a Teflon cell with 1 mm thickness. The cell was sandwiched between two sheets of Kapton film (5 µm thickness) and sealed with grease (Apiezon N). These procedures were performed in an inert-atmosphere glove box (the water content was less than 10 ppm). In the mixtures prepared with the current condition, no detectable chemical reactions or cellulose degradation occurred [9]. SAXS measurements were conducted with a NANO-Viewer (IP system, Rigaku Co., Ltd, Tokyo, Japan) with a Cu Kα radiation source (λ = 0.154 nm) at an applied voltage of 40 kV and a filament current of 30 mA. The distance between the sample and the detector was set at 700 mm (q = 0.142–2.844 nm−1 ). The samples were irradiated for 1 h. The obtained two-dimensional image was transformed into a one-dimensional pattern for theoretical analyses. It should be noted that anisotropic images were not observed in any of our data because the cellulose sample used here was powder. The scattering experiments were conducted three times with a cellulose concentration of more than 40 mol % at different spots in the same sample, and the averaged patterns were used for analyses. This is because there exists inhomogeneity in the sample with such high concentrations due to the presence of undissolved (intact cellulose) regions [9]. Backgrounds including scattering from Kapton films were subtracted from the scattering patterns. A field emission scanning electron microscope (FE-SEM, JEOL JSM-7100F) was operated at 15 kV. The samples were coated with Au/Pt using an ion sputter coater for SEM observation. 4. Conclusions We have investigated structural changes of cellulose/[Emim][OAc] at the nano-level over the entire concentration range of 0–100 mol % by SAXS measurements with the aid of SEM observations. The shoulder scattering in the Kratky plot ascribed to bundles of cellulose microfibrils does not exist up to 40 mol %. This finding is well explained by the fact that at high concentrations, the anions are energetically preferred to have multiple interactions with OH groups of cellulose chains (anion bridging), which enables the deconstruction of the cellulose crystallinity, and consequently fibril structures at 40 mol %. We observed two aggregated structures that are not observed either in intact cellulose or in the IL at 30–80 mol %. The smaller structure—which is reminiscent of a microfibril with an average diameter of 4.1 nm—does not show concentration dependency. Conversely, the larger one depends on the concentration, and its size is in the range of 12–20 nm. The structure originates from cellulose chain entanglement, which is the origin of the moldability of the cellulose/IL composite films that possess liquid crystallinity. Since the local concentration of cellulose in ILs at the early stage of the dissolution process should be much higher than that of the consequent solution, the nano-structures observed in the high concentrations can be intermediate states for the dissolution process. Therefore, our findings contribute to an understanding of the dissolution mechanism of cellulose in the IL; inter-fibrillation occurs more easily than intra-fibrillation, and cellulose chains are entangled with each other after IL ions peel the chains from fibrils, which may be one reason for the slow dissolution process of cellulose in ILs.

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Acknowledgments: This study was supported in part by Advanced Low Carbon Technology Research and Development Program (ALCA) (Grant number 2100040), Cross-ministerial Strategic Innovation Promotion Program (SIP), and Center of Innovation Program “Construction of next-generation infrastructure using innovative materials: Realization of safe and secure society that can coexist with the Earth for centuries” from Japan Science and Technology Agency, and also by JSPS KAKENHI Grant Number 15H04193. Author Contributions: T.E. and K.T. conceived and designed the experiments; S.H. and S.F. performed the experiments; T.E. analyzed the data; T.E. wrote the paper in consultation with K.T. and K.N. Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of the compounds are commercially available from the company mentioned in Section 3 Materials and Methods. © 2017 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).