Communication Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
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A Novel Cellulose/Ionic Liquid Complex Crystal Guangjie Song, Jian Yu, Meichun Ding, and Jun Zhang* CAS Key Laboratory of Engineering Plastics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
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S Supporting Information *
ABSTRACT: Controlling water vapor diffusion into cellulose−ionic liquid solution induced a novel cellulose complex crystal. Its unit cell consists of one glucopyranoside unit and one molecule of ionic liquid as an asymmetric unit, revealed by the results of 13C solid-state NMR spectroscopy. The complex crystals exhibited exceptionally large size as indicated by the narrow peak width of X-ray diffraction. This crystal reported in this work represents a new class of cellulose complexes after caustic alkali and amine complexes of cellulose. The present study is potentially significant in understanding the molecular interactions of cellulose and leads to possible applications.
T
chains and strongly interact with cellulose chains. Highly concentrated solution of high-DP cellulose (DP = degree of polymerization), up to 25 wt % for DP ≈ 1000, can be obtained in such solvents.9 We recently reported on the crystallization behavior of cellulose in ionic liquid solution through water-vapor-diffusion precipitation.16,17 These studies mainly focused on crystal morphology. Effect of several factors such as concentration of cellulose, temperature, and fillers has been discussed in detail on formation of cellulose spherulites. However, the crystal structure of the spherulite has not been examined, and it is not known whether cellulose already forms cellulose II in the spherulite. In this research, we studied the structure of spherulites by Xray diffraction and solid-state NMR. The effect of water or alcohol on the crystal structure of cellulose spherulites was also studied and is presented as preliminary information. Cellulose spherulites were prepared in microcrystalline cellulose (MCC)/AmimCl solution.16,17 The detailed experimental procedures are described in the Supporting Information. The spherulites under polarized optical microscope show a strong birefringence (Figure 1). The matrix part appears dark without any birefringence and is thus isotropic. X-ray diffraction pattern of an individual spherulite is presented in Figure 2A (Supporting Information). Figure 2B shows the circularly averaged intensity profile of Figure 2A, diffraction of pure ionic liquid crystals, and MCC. The diffraction angles were converted for a wavelength of 1.542 Å
he complexation of polymers with low molecular weight compounds by noncovalent interactions is found in many polymers.1 It brings the polymers novel structures and properties, which are important in fundamental and practical studies. As the most abundant biomaterials on the Earth, cellulose has drawn increasing attention.2−4 One of the challenges for cellulose utilization is the difficulty of processing, since it can not be melted and is hardly soluble in common solvents due to the strong interaction of inter- and intrachain hydrogen bonds.5−10 Complexation of cellulose with small molecules is an effective means to control the cellulose structures for chemical processing.11−14 The complex crystals of cellulose have been discovered in several solvent systems, playing an essential role in conversion of cellulose polymorphs.11−15 Cellulose−NaOH complex forms in mercerization process.11,12 Cellulose II can be obtained by washing with water and drying. Cellulose−amine complex is obtained by the incorporation of ammonia molecules13,14 or certain amines such as ethylene diamine (EDA)15 into crystal lattices of cellulose I or II. Cellulose IIII and IIIII can be obtained, respectively, by removal of amine with alcohol. The elucidation of the complex crystal structure allows better understanding of molecular interaction between the solvent molecules and the polymer. Recently room-temperature ionic liquids (ILs) show great potential as green solvents for cellulose dissolution due to their excellent dissolving capability, negligible vapor pressure, high thermal stability, and ease of recycling. Some ILs, such as 1allyl-3-methylimidazolium chloride (AmimCl)6 and 1-butyl-3methylimidazolium chloride (BmimCl),9 are able to effectively break the extensive hydrogen-bond networks between cellulose © XXXX American Chemical Society
Received: May 15, 2018 Revised: July 10, 2018 Published: July 11, 2018 A
DOI: 10.1021/acs.cgd.8b00754 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Communication
The structural features of the spherulites were further studied by 13C cross polarization/magic-angle spinning (CP/ MAS) solid-state NMR (Supporting Information). Figure 3A shows the spectra of different samples (spherulite, matrix including amorphous cellulose and ionic liquid, crystal in spherulite, and AmimCl crystal). For comparison in detail, spectra of IL regime of the different samples are shown separately on the right side as Figure 3B. In the cellulose regime between 60 and 110 ppm in Figure 3A (spherulite), the narrow line width from cellulose carbon signal is in agreement with the high structural regularity of the crystal in the spherulites as was evidenced by the X-ray diffraction. In this regime, each carbon atom gives rise only to a singlet, so a single glucosyl residue of cellulose appears to be the asymmetric unit of the structure in the unit cell in this crystalline form. In contrast, a doublet appears in the spectra for cellulose Iα, Iβ, II, and IIIII due to the two nonequivalent glucosyl moieties in the unit cell (Table I).19,24,26,27 The characteristic chemical shifts of cellulose in the spherulite is a shift toward high magnetic field of C4 compared to other allomorphs. The C4 environment, due to the twofold screw symmetry of the chain, is constrained in most cellulose allomorphs. This constraint leads to the displacement of the corresponding chemical shift to low magnetic field, typically between 88 and 90 ppm, while it is below 80 ppm in solution.11,14,24,26,27 In amorphous or surface chains, the molecular conformation is slightly relaxed giving the resonance at ∼84 ppm,19,28 which is close to the value in the spherulite. A threefold helical structure seen in highly concentrated sodium hydroxide solution gives the C4 below 80 ppm,11 so the molecular conformation is still close to twofold helix. The chemical shift of C6 is sensitive to the O6 conformation,29 and the 62 ppm corresponds to a gt conformation as was confirmed for structures of different cellulose allomorphs determined. In Figure 3A (matrix), the ionic liquid in amorphous matrix gives rise to sharp peaks because of the high mobility of small molecules. But the cellulose in the matrix exhibits very weak and broad peaks possibly resulting from the relatively low concentration of cellulose chains. Compared with the 13C CP/ MAS NMR spectrum of the amorphous matrix (Figure 3A matrix), the spectrum of Figure 3A (spherulite) shows a combination of peaks from amorphous fraction and crystalline fraction indicating the presence of ordered (crystalline) and disordered (amorphous) components in the spherulite. The
Figure 1. Polarized optical microscopy image of cellulose spherulites. Scale bar 50 μm.
to be comparable with literature data. The ionic liquid (AmimCl) can be in a crystalline state at room temperature, since the melting point of the salt is 52 °C.18 The overall aspect remains liquid due to the supercooling and absorption of humidity. The diffraction peak positions from the spherulites do not correspond to those of either neat ionic liquid crystals or MCC, indicating that this spherulite does not contain the components of pure ionic liquid crystals or MCC. The crystal structure of the spherulites and cellulose polymorphs are further compared in the X-ray diffraction plots. 2θ plots of the spherulites and cellulose II family using an experimental profile in the literature19 are shown in Figure 2C. It can be seen that there is no crystal plane with a distance of 9 Å in these cellulose polymorphs. The crystal structure of the spherulite is different from those of the well-known cellulose polymorphs13,19−25 implying the crystal in the spherulites is a new form. The diffraction peaks coming from the spherulites are exceptionally sharp for a polymer crystal. The typical peak width at half-maximum of the inner peak (dspacing of 9 Å) corresponds to a crystallite size of ca. 0.22− 0.36 μm according to the Scherrer equation without taking into account the instrument broadening. The estimated size is thus a lower bound. Underneath the sharp diffraction peaks, a broad amorphous halo centered at ∼23.5° for spherulites can be seen. Despite the height of the diffraction peaks, the integrated area of the diffraction peaks is ∼40% of the total integrated area due to the halo scattering. Thus, significant part of spherulite is amorphous.
Figure 2. (A) X-ray diffraction pattern of an initial spherulite; (B) 2θ plots extracted from (A), diffraction of AmimCl crystal and MCC; (C) 2θ plots of spherulites and cellulose II family crystals from literature (ref 19.). All plots are converted for wavelength 1.542 Å. The inner ring marked by an arrow in (A) corresponds to d-spacing of 9 Å. B
DOI: 10.1021/acs.cgd.8b00754 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Communication
Figure 3. (A) 13C solid-state NMR spectra of different samples: spherulite, matrix part including amorphous cellulose and ionic liquid, crystal in spherulites, and AmimCl crystal. (B) IL regime was shown separately on the right side for comparison in detail.
Table I.
13
positions and splitting of crystallized ionic liquid in the spherulites are different from those of the pure ionic liquid crystal, due to difference in molecular packing and electrostatic interactions when cellulose is present. Therefore, cellulose/ ionic liquid complex crystals are formed in the spherulite. The unit cell of the cellulose/ionic liquid complex consists of a single magnetically independent glucosyl moiety of cellulose and a moiety of ionic liquid. The ionic liquid in the cellulose spherulites can be washed out with water to regenerate cellulose. X-ray measurement was performed on this water-washed sample, shown in Figure 4A.
C Chemical Shifts of Cellulose chemical shift (ppm) C1
spherulite cellulose Iα
104.6 106.9
cellulose Iβ
106.1 104.0 107.1 105.0 104.8 106.5 105.7 ca. 105
cellulose II cellulose IIII cellulose IIIII amorphous
C4 85.1 91.6 90.8 88.9 88.0 88.7 87.5 87.8 88.5 87.3 ca. 84
C6
ref
62.2 67.1
expt 26
65.6 65.0 63.0 62.6 62.3 62.5 62.1 ca. 63
27 27 14 24 19
arrow-marked peaks in Figure 3B (spherulite) are not observed in the spectrum of pure AmimCl crystals (Figure 3B AmimCl crystal), indicating that crystalline fraction in the spherulites is not the superposition of crystalline state of ionic liquid and cellulose but a cellulose complex crystal by incorporation of ionic liquid in the unit cell. The carbons from ionic liquid and cellulose in the amorphous matrix both have fast spin−lattice relaxation, compared with those in cellulose/ionic liquid complex crystals. The difference in relaxation behavior can be applied to resolve the spectrum of complex crystals in spherulites. The peaks from amorphous fraction in the spherulites were completely absent in Figure 3A,B (crystal in spherulite) by the modified CP inversion recovery (CPRX) experiment (Supporting Information). Each carbon atom in the ionic liquid from the crystalline fraction also gives rise only to a singlet, so a single moiety of ionic liquid appears to be the magnetically independent residue of the structure. Moreover, the peak
Figure 4. (A) X-ray diffraction pattern of water-washed spherulite. (B) 13C CP/MAS solid-state NMR spectrum of dry spherulites after water wash.
A diffuse ring is shown in this X-ray diffraction pattern, indicating the low crystallinity (∼37%) and small crystal sizes of this regenerated cellulose. The 13C CP/MAS solid-state NMR spectrum on these water-washed spherulites (Figure 4B) confirms the low crystallinity, as the resonance peak from C4 is centered at ∼85 ppm, and the low magnetic field component of crystalline cellulose is hardly resolvable. C
DOI: 10.1021/acs.cgd.8b00754 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Communication
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A new type of cellulose complex crystals forming spherulites was reported. X-ray diffraction and 13C CP/MAS solid-state NMR measurements indicate that complex crystals grew to exceptionally large sizes and that the unit cell of complex crystals consists of a single magnetically independent glucosyl residue of cellulose and a single magnetically independent ionic liquid molecule. After ionic liquid incorporated in the complex crystal is removed with water or alcohol, regenerated cellulose shows low crystallinity. Inspection into this novel complex crystal may be significant in understanding molecular interactions of cellulose and possible applications.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b00754. Experimental procedures for spherulite growth, X-ray diffraction, and 13C solid-state NMR measurements (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Guangjie Song: 0000-0002-3329-4629 Jian Yu: 0000-0003-0591-0524 Jun Zhang: 0000-0003-4824-092X Funding
This work was supported by the National Key Research and Development Program of China (2017YFA0403103) and the National Natural Science Foundation of China (Nos. 21704107 and 51425307). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the staffs from BL17B beamline of National Facility for Protein Science Shanghai (NFPS) at Shanghai Synchrotron Radiation Facility, for assistance during data collection.
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REFERENCES
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DOI: 10.1021/acs.cgd.8b00754 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Communication
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DOI: 10.1021/acs.cgd.8b00754 Cryst. Growth Des. XXXX, XXX, XXX−XXX
